Bibliography - Daniel Sigman

Brookshire, E. N. Jack, L.O. Hedin, J. Denis Newbold, Daniel Sigman, and John K. Jackson, January 2012: Sustained losses of bioavailable nitrogen from montane tropical forests. Nature Geosciences, Nature Publishing Group, doi:10.1038/ngeo1372
[ Abstract ]
Tropical forests account for one third of terrestrial primary production and contribute significantly to the land carbon sink^{1, 2}. The future of this sink relies critically on forest interactions with nutrient cycles^{3, 4, 5}. Humid montane tropical forests are often thought to be rich in phosphorus, but to contain low levels of bioavailable nitrogen⁶. Here, we examine the concentration of dissolved nitrogen compounds and the isotopic composition of nitrate in streams in six well-characterized and phosphorus-rich montane forests⁷ in Costa Rica, and in 55 montane forests across Central America and the Caribbean, using data collected between 1990 and 2008. We found high levels of nitrate in these streams, indicative of large losses of bioavailable nitrogen from these forests. We detected no trend in the concentration and isotopic signature of nitrate over the measurement period, implying that high levels of export are neither recent nor episodic. An analysis of the oxygen isotopic signature of stream nitrate showed that exports are sourced from the plant-soil system, rather than from atmospheric deposition that bypasses forest biota. Our findings indicate that nitrogen-rich conditions can develop irrespective of phosphorus availability at the ecosystem scale. We suggest that nitrogen availability may not limit plant growth, or its response to increasing atmospheric carbon dioxide levels, in many montane tropical forests.
Martinez-Garcia, Alfredo, A. Rosell-Mele, S. L. Jaccard, Walter Geibert, Daniel Sigman, and G. H. Haug, 2011: Southern Ocean dust-climate coupling over the past four million years. Nature, Macmillan Publishers Limited, 476, doi:10.1038/nature10310 312-316
[ Abstract ]
Dust has the potential to modify global climate by influencing the radiative balance of the atmosphere and by supplying iron and other essential limiting micronutrients to the ocean^1,2. Indeed, dust supply to the Southern Ocean increases during ice ages, and 'iron fertilization' of the subantarctic zone may have contributed up to 40 parts per million by volume (p.p.m.v.) of the decrease (80-100 p.p.m.v.) in atmospheric carbon dioxide observed during late Pleistocene glacial cycles^3-7. So far, however, the magnitude of Southern Ocean dust deposition in earlier times and its role in the development and evolution of Pleistocene glacial cycles have remained unclear. Here we report a high-resolution record of dust and iron supply to the Southern Ocean over the past four million years, derived from the analysis of marine sediments from ODP Site 1090, located in the Atlantic sector of the subantarctic zone. The close correspondence of our dust and iron deposition records with Antarctic ice core reconstructions of dust flux covering the past 800,000 years (refs 8, 9) indicates that both of these archives record large-scale deposition changes that should apply to most of the Southern Ocean, validating previous interpretations of the ice core data. The extension of the record beyond the interval covered by the Antarctic ice cores reveals that, in contrast to the relatively gradual intensification of glacial cycles over the past three million years, Southern Ocean dust and iron flux rose sharply at the Mid-Pleistocene climatic transition around 1.25 million years ago. This finding complements previous observations over late Pleistocene glacial cycles^5,8,9, providing new evidence of a tight connection between high dust input to the Southern Ocean and the emergence of the deep glaciations that characterize the past one million years of Earth history.
Brunelle, B. G., Daniel Sigman, S. L. Jaccard, L. D. Keigwin, B. Plessen, G. Schettler, M. S. Cook, and G. H. Haug, 2010: Glacial/interglacial changes in nutrient supply and stratification in the western. Quaternary Science Reviews, Elsevier, (29), doi:10.1016/j.quascirev.2010.03.010 2579-2590
[ Abstract ]
In piston cores from the open subarctic Pacific and the Okhotsk Sea, diatom-bound δ¹⁵N (δ¹⁵N_db), biogenic opal, calcium carbonate, and barium were measured from coretop to the previous glacial maximum (MIS 6). Glacial intervals are generally characterized by high δ¹⁵N_db (w8&) and low productivity, whereas interglacial intervals have a lower δ¹⁵N_db (5.7-6.3 per mil) and indicate high biogenic productivity. These data extend the regional swath of evidence for nearly complete surface nutrient utilization during glacial maxima, consistent with stronger upper water column stratification throughout the subarctic region during colder intervals. An early deglacial decline in δ¹⁵N_db of 2 per mil at ~17.5 ka, previously observed in the Bering Sea, is found here in the open subarctic Pacific record and arguably also in the Okhotsk, and a case can be made that a similar decrease in δ¹⁵N_db occurred in both regions at the previous deglaciation as well. The early deglacial δ¹⁵N_db decrease, best explained by a decrease in surface nutrient utilization, appears synchronous with southern hemisphere-associated deglacial changes and with the Heinrich 1 event in the North Atlantic. This δ¹⁵N_db decrease may signal the initial deglacial weakening in subarctic North Pacific stratification and/or a deglacial increase in shallow subsurface nitrate concentration. If the former, it would be the North Pacific analogue to the increase in vertical exchange inferred for the Southern Ocean at the time of Heinrich Event 1. In either case, the lack of any clear change in paleoproductivity proxies during this interval would seem to require an early deglacial decrease in the iron-to-nitrate ratio of subsurface nutrient supply or the predominance of light limitation of phytoplankton growth during the deglaciation prior to B�lling-Aller�d warming.
DiFiore, P. J., Daniel Sigman, K. L. Karsh, T. W. Trull, Robert B. Dunbar, and R. S. Robinson, 2010: Poleward decrease in the isotope effect of nitrate assimilation across the Southern Ocean. Geophysical Research Letters, American Geophysical Union, 37(L17601), doi:10.1029/2010GL044090 1-5
[ Abstract ]
Recent studies provide seasonally and spatially resolved information on the isotopic characteristics of nitrate supply and N cycling in Southern Ocean surface waters. The new data improve our understanding of the nitrate supply to the Antarctic surface and its isotopic characteristics, especially with regard to the summertime subsurface minimum temperature (T_min) layer in the Antarctic. We use these findings to update and compile estimates of the N isotope effect of nitrate assimilation, ε, in the Southern Ocean near Australia. A poleward decrease in ε emerges, from 8-9% in the Subantarctic Zone (SAZ, 40-52°S) to ~5% in the Polar Antarctic Zone (PAZ, ~66°S). ε is strongly correlated with mixed layer depth at the time of sampling. We hypothesize that the correlation is driven by the physiological response of diatoms to light availability, with light limitation leading to higher cellular efflux of nitrate and thus higher ε.
Hain, M. P., Daniel Sigman, and G. H. Haug, 2010: Carbon dioxide effects of Antarctic stratification, North Atlantic Intermediate Water formation, and subantarctic nutrient drawdown during the last ice age: Diagnosis and synthesis in a geochemical box model. Global Biogeochemical Cycles, American Geophysical Union, 24, GB4023, doi:10.1029/2010GB003790
[ Abstract ]
In a box model synthesis of Southern Ocean and North Atlantic mechanisms for lowering CO₂ during ice ages, the CO₂ changes are parsed into their component geochemical causes, including the soft�]tissue pump, the carbonate pump, and whole ocean alkalinity. When the mechanisms are applied together, their interactions greatly modify the net CO₂ change. Combining the Antarctic mechanisms (stratification, nutrient drawdown, and sea ice cover) within bounds set by observations decreases CO₂ by no more than 36 ppm, a drawdown that could be caused by any one of these mechanisms in isolation. However, these Antarctic changes reverse the CO₂ effect of the observed ice age shoaling of North Atlantic overturning: in isolation, the shoaling raises CO₂ by 16 ppm, but alongside the Antarctic changes, it lowers CO₂ by an additional 13 ppm, a 29 ppm synergy. The total CO₂ decrease does not reach 80 ppm, partly because Antarctic stratification, Antarctic sea ice cover, and the shoaling of North Atlantic overturning all strengthen the sequestration of alkalinity in the deepest ocean, which increases CO₂ both by itself and by decreasing whole ocean alkalinity. Increased nutrient consumption in the sub�]Antarctic causes as much as an additional 35 ppm CO₂ decrease, interacting minimally with the other changes. With its inclusion, the lowest ice age CO₂ levels are within reach. These findings may bear on the two-stepped CO₂ decrease of the last ice age.
Jaccard, S. L., E. D. Galbraith, Daniel Sigman, and G. H. Haug, 2010: A pervasive link between Antarctic ice core and subarctic Pacific sediment records over the past 800 kyrs. Quaternary Science Reviews, 29, doi:10.1016/j.quascirev.2009.10.007 206-212
[ Abstract ]
Recently developed XRF core-scanning methods permit paleoceanographic reconstructions on timescales similar to those of ice-core records. We have investigated the distribution of biogenic barium (Ba/Al), opal and carbonate (Ca/Al) in a sediment core retrieved from the abyssal subarctic Pacific (ODP 882, 50°N, 167°E, 3244 m) over an interval that spans the full length of the EPICA Dome C (EDC) ice-core record. Ba/Al and biogenic opal show a strong resemblance to the EDC δD and CO₂, with generally high concentrations during interglacials and lower values during ice ages of the past 800 kyrs. The sedimentary Ba/Al and biogenic opal are most easily interpreted as indicating a reduced sinking flux of organic matter from the surface ocean during cold periods. The Ba/Al maxima during peak interglacials are accompanied by transient Ca/Al peaks in these otherwise carbonate-devoid sediments, which are best explained by a deepening of the calcite lysocline, presumably due to reduced storage of respired CO₂ in the deep North Pacific. For most of the ��luke-warm�� interglacials noted between 420 and 750 ka in EDC, the Ba/Al peaks in ODP 882 are also lower, further strengthening the evidence for a simple physical link between global climate and the biogeochemistry of the subarctic Pacific.
Sigman, Daniel, M. P. Hain, and G. H. Haug, July 2010: The polar ocean and glacial cycles in atmospheric CO₂ concentration. Nature, 466, doi:10.1038/nature09149 47�55
[ Abstract ]
Global climate and the atmospheric partial pressure of carbon dioxide (p CO₂ ^atm) are correlated over recent glacial cycles, with lower p CO₂ ^atm)during ice ages, but the causes of the p CO₂ ^atm) changes are unknown. The modern Southern Ocean releases deeply sequestered CO₂ to the atmosphere. Growing evidence suggests that the Southern Ocean CO₂ �leak� was stemmed during ice ages, increasing ocean CO₂ storage. Such a change would also have made the global ocean more alkaline, driving additional ocean CO₂ uptake. This explanation for lower ice-age p CO₂ ^atm), if correct, has much to teach us about the controls on current ocean processes.
DiFiore, P. J., Daniel Sigman, and Robert B. Dunbar, November 2009: Upper ocean nitrogen fluxes in the Polar Antarctic Zone: Constraints from the nitrogen and oxygen isotopes of nitrate. Geochemistry Geophysics Geosystems, Washington, D.C., American Geophysical Union, doi:10.1029/2009GC002468
[ Abstract ]
[1] We report nitrate nitrogen (N) and oxygen (O) isotope measurements from the seasonally sea ice covered Polar Antarctic Zone (PAZ) south of the Southern Antarctic Circumpolar Front. The ¹⁵N/¹⁴N and ¹⁸O/¹⁶O ratios of nitrate both increase into the summertime surface mixed layer, in strong correlation with the upward decrease in nitrate concentration, the expected result of nitrate assimilation by phytoplankton. Culture studies indicate that algal assimilation of nitrate fractionates the nitrate N and O isotopes equally, while previous field studies suggest that nitrate N and O isotope behavior can be decoupled by euphotic zone nitrification. Our data for the PAZ show strong coupling of the dual isotopes of nitrate, and a numerical model of Antarctic summertime surface layer N cycling fits our observations (including isotopic compositions of both nitrate and suspended particulate N) if the nitrification rate is no more than 6% of the nitrate assimilation rate by phytoplankton. The model estimates that the N isotope effect of nitrate assimilation is 5.0 � 0.7�. This estimate lacks some of the uncertainties associated with previous studies within the Antarctic Circumpolar Current, and it is at the low end of most recent estimates from the Southern Ocean, the range of which we speculatively attribute to an effect of mixed layer depth on the amplitude of isotope discrimination./
Haug, G. H., and Daniel Sigman, 2009: Palaeoceanography: Polar Twins. Nature Geosciences, 2, doi:10.1038/ngeo423 91-92
[ Abstract ]
Ice ages in the North Pacific Ocean and the Southern Ocean were marked by low productivity. Accumulating evidence indicates that strong stratification restricted the supply of nutrients from the deep ocean to the algae of the sunlit surface in these regions.
Jaccard, S. L., E. D. Galbraith, Daniel Sigman, G. H. Haug, R. Francois, T. F. Pedersen, P. Dulski, and H. R. Thierstein, 2009: Subarctic Pacific evidence for a glacial deepening of the oceanic respired carbon pool. Earth and Planetary Science Letters, 277, doi:10.1016/j.epsl.2008.10.017 156-165
[ Abstract ]
Measurements of benthic foraminiferal cadmium: calcium (Cd/Ca) have indicated that the glacial�interglacial change in deep North Pacific phosphate (PO₄) concentration was minimal, which has been taken by some workers as a sign that the biological pump did not store more carbon in the deep glacial ocean. Here we present sedimentary redoxsensitive trace metal records from Ocean Drilling Program (ODP) Site 882 (NW subarctic Pacific, water depth 3244 m) to make inferences about changes in deep North Pacific oxygenation � and thus respired carbon storage � over the past 150,000 yr. These observations are complemented with biogenic barium and opal measurements as indicators for past organic carbon export to separate the influences of deepwater oxygen concentration and sedimentary organic carbon respiration on the redox state of the sediment. Our results suggest that the deep subarctic Pacific water mass was depleted in oxygen during glacial maxima, though it was not anoxic. We reconcile our results with the existing benthic foraminiferal Cd/Ca by invoking a decrease in the fraction of the deep ocean nutrient inventory that was preformed, rather than remineralized. This change would have corresponded to an increase in the deep Pacific storage of respired carbon, which would have lowered atmospheric carbon dioxide (CO₂) by sequestering CO₂ away from the atmosphere and by increasing ocean alkalinity through a transient dissolution event in the deep sea. The magnitude of change in preformed nutrients suggested by the North Pacific data would have accounted for a majority of the observed decrease in glacial atmospheric pCO₂.
Ren, H., Daniel Sigman, , B. Plessen, R. S. Robinson, Y. Rosenthal, and G. H. Haug, 2009: Foraminiferal isotope evidence of reduced nitrogen fixation in the ice age Atlantic Ocean. Science, 323, doi:10.1126/science.1165787 244-248
[ Abstract ]
Fixed nitrogen (N) is a limiting nutrient for algae in the low-latitude ocean, and its oceanic inventory may have been higher during ice ages, thus helping to lower atmospheric CO₂ during those intervals. In organic matter within planktonic foraminifera shells in Caribbean Sea sediments, we found that the ¹⁵N/¹⁴N ratio from the last ice age is higher than that from the current interglacial, indicating a higher nitrate ¹⁵N/¹⁴N ratio in the Caribbean thermocline. This change and other species-specific differences are best explained by less N fixation in the Atlantic during the last ice age. The fixation decrease was most likely a response to a known ice age reduction in ocean N loss, and it would have worked to balance the ocean N budget and to curb ice age�interglacial change in the N inventory.
Sigman, Daniel, P. J. DiFiore, M. P. Hain, C. Deutsch, and D. Karl, 2009: Sinking organic matter spreads the nitrogen isotope signal of pelagic denitrification in the North Pacific. Geophysical Research Letters, 36(L08605), doi:10.1029/2008GL035784
[ Abstract ]
Culture studies of denitrifying bacteria predict that denitrification will generate equivalent gradients in the δ¹⁵N and &delta:¹⁸O of deep ocean nitrate. A depth profile of nitrate isotopes from the Hawaii Ocean Time-series Station ALOHA shows less of an increase in &delta:¹⁸O than in δ¹⁵N as one ascends from abyssal waters into the denitrification impacted mid-depth waters. A box model of the ocean nitrate N and O isotopes indicates that this is the effect of the low latitude nitrate assimilation/regeneration cycle: organic N sinking out of the surface spreads the high -δ¹⁵N signal of pelagic denitrification into waters well below and beyond the suboxic zone, whereas the nitrate &delta:¹⁸O signal of denitrification can only be transmitted by circulation in the interior.
Sigman, Daniel, P. J. DiFiore, M. P. Hain, C. Deutsch, Y. Wang, D. Karl, T. R. Knutson, K. K. Lehman, and S. Pantoja, 2009: The dual isotopes of deep nitrate as a constraint on the cycle and budget of oceanic fixed nitrogen. Deep Sea Research I, 56(9), doi:10.1016/j.dsr.2009.04.007 1419-1439
[ Abstract ]
We compare the output of an 18-box geochemical model of the ocean with measurements to investigate the controls on both the mean values and variation of nitrate δ¹⁵N and &delta:¹⁸O in the ocean interior. The &delta:¹⁸O of nitrate is our focus because it has been explored less in previous work. Denitrification raises the δ¹⁵N and &delta:¹⁸O of mean ocean nitrate by equal amounts above their input values for N₂ fixation (for δ¹⁵N) and nitrification (for &delta:¹⁸O), generating parallel gradients in the δ¹⁵N and &delta:¹⁸O of deep ocean nitrate. Partial nitrate assimilation in the photic zone also causes equivalent increases in the δ¹⁵N and &delta:¹⁸O of the residual nitrate that can be transported into the interior. However, the regeneration and nitrification of sinking N can be said to decouple the N and O isotopes of deep ocean nitrate, especially when the sinking N is produced in a low latitude region, where nitrate consumption is effectively complete. The δ¹⁵N of the regenerated nitrate is equivalent to that originally consumed, whereas the regeneration replaces nitrate previously elevated in &delta:¹⁸O due to denitrification or nitrate assimilation with nitrate having the &delta:¹⁸O of nitrification. This lowers the &delta:¹⁸O of mean ocean nitrate and weakens nitrate &delta:¹⁸O gradients in the interior relative to those in δ¹⁵N. This decoupling is characterized and quantified in the box model, and agreement with data shows its clear importance in the real ocean. At the same time, the model appears to generate overly strong gradients in both &delta:¹⁸O and δ¹⁵N within the ocean interior and a mean ocean nitrate &delta:¹⁸O that is higher than measured. This may be due to, in the model, too strong an impact of partial nitrate assimilation in the Southern Ocean on the δ¹⁵N and &delta:¹⁸O of preformed nitrate and/or too little cycling of intermediate-depth nitrate through the low latitude photic zone.
Brauer, A., G. H. Haug, P. Dulski, Daniel Sigman, and F. W. Negendank, 2008: An abrupt wind shift in Western Europe at the onset of the Younger Dryas cold period. Nature Geosciences, 1, doi:10.1038/ngeo263 520-523
[ Abstract ]
The Younger Dryas cooling 12,700 years ago is one of the most abrupt climate changes observed in Northern Hemisphere palaeoclimate records. Annually laminated lake sediments are ideally suited to record the dynamics of such abrupt changes, as the seasonal deposition responds immediately to climate, and the varve counts provide an accurate estimate of the timing of the change. Here, we present sub-annual records of varve microfacies and geochemistry from Lake Meerfelder Maar in western Germany, providing one of the best dated records of this climate transition. Our data indicate an abrupt increase in storminess during the autumn to spring seasons, occurring from one year to the next at 12,679 yr BP, broadly coincidentwith other changes in this region. We suggest that this shift in wind strength represents an abrupt change in the North Atlantic westerlies towards a stronger and more zonal jet. Changes in meridional overturning circulation alone cannot fully explain the changes in European climate; we suggest the observed wind shift provides the mechanism for the strong temporal link between North Atlantic Ocean overturning circulation and European climate during deglaciation.
de Boer, A. M., J. R. Toggweiler, and Daniel Sigman, 2008: Atlantic dominance of the meridional overturning circulation. Journal of Physical Oceanography, 38, doi:10.1175/2007JPO3731.1 435-450
[ Abstract ]
North Atlantic (NA) deep-water formation and the resulting Atlantic meridional overturning cell is generally regarded as the primary feature of the global overturning circulation and is believed to be a result of the geometry of the continents. Here, instead, the overturning is viewed as a global energy�driven system and the robustness of NA dominance is investigated within this framework. Using an idealized geometry ocean general circulation model coupled to an energy moisture balance model, various climatic forcings are tested for their effect on the strength and structure of the overturning circulation. Without winds or a high vertical diffusivity, the ocean does not support deep convection. A supply of mechanical energy through winds or mixing (purposefully included or due to numerical diffusion) starts the deep-water formation. Once deep convection and overturning set in, the distribution of convection centers is determined by the relative strength of the thermal and haline buoyancy forcing. In the most thermally dominant state (i.e., negligible salinity gradients), strong convection is shared among the NA, North Pacific (NP), and Southern Ocean (SO), while near the haline limit, convection is restricted to the NA. The effect of a more vigorous hydrological cycle is to produce stronger salinity gradients, favoring the haline state of NA dominance. In contrast, a higher mean ocean temperature will increase the importance of temperature gradients because the thermal expansion coefficient is higher in a warm ocean, leading to the thermally dominated state. An increase in SO winds or global winds tends to weaken the salinity gradients, also pushing the ocean to the thermal state. Paleoobservations of more distributed sinking in warmer climates in the past suggest that mean ocean temperature and winds play a more important role than the hydrological cycle in the overturning circulation over long time scales.
Robinson, R. S., and Daniel Sigman, 2008: Nitrogen isotopic evidence for a poleward decrease in surface nitrate within the ice age Antarctic. Quaternary Science Reviews, 27(9-10), doi:10.1016/j.quascirev.2008.02.005 1076-1090
[ Abstract ]
Surface sediment diatom-bound δ¹⁵N along a latitudinal transect of 170�W shows a previously unobserved increase to the South of the Antarctic Polar Front. The southward δ¹⁵N increase is best explained by the combination of two changes toward the South, a decrease in the isotope effect of nitrate assimilation (ε) and an increase in the degree of nitrate consumption, both associated with shoaling of the mixed layer into the seasonal ice zone (SIZ). New downcore records show high amplitude changes in diatom-bound δ¹⁵N during the last ice age, with intervals of higher δ¹⁵N, including the last glacial maximum, the transition between marine isotope stages 5 and 4, and marine isotope stage 6, while other intervals are similar in δ¹⁵N to interglacial sediments. Variation in the range of 0�3%, as seen in previously published records, may be entirely due to changes in ε. However, the observed magnitude of the change of 4�10% in the three new records and the locations of these records relative to the modern meridional gradient in mixed layer depth appear to require increased nitrate consumption to explain the high-δ¹⁵N intervals. The new sites are near the modern Southern Antarctic Circumpolar Current Front (SACCF), and one of the sites has been shown to be associated with sporadic summer sea ice during the LGM. As with other Antarctic sites, the available proxy data suggest that they were characterized by lower export production. Based on these and other observations, we propose that the weak southward nitrate decrease in the modern Antarctic surface was a fully developed ��nutrient front�� in the glacial Antarctic, associated with the SACCF. Both modern ocean and paleoceanographic work is needed to test this hypothesis, which would have major implications for atmospheric CO₂.
Brunelle, B. G., Daniel Sigman, M. S. Cook, L. D. Keigwin, G. H. Haug, B. Plessen, G. Schettler, and S. L. Jaccard, 2007: Evidence from diatom-bound nitrogen isotopes for Subarctic Pacific stratification during the last ice age and a link to North Pacific denitrification changes. Paleoceanography, 22(PA1215), doi:10.1029/2005PA001205
[ Abstract ]
In a piston core from the central Bering Sea, diatom microfossil-bound N isotopes and the concentrations of opal, biogenic barium, calcium carbonate, and organic N are measured over the last glacial/interglacial cycle. Compared to the interglacial sections of the core, the sediments of the last ice age are characterized by 3% higher diatom-bound δ¹⁵N, 70 wt % lower opal content and 1200 ppm lower biogenic barium. Taken together and with constraints on sediment accumulation rate, these results suggest a reduced supply of nitrate to the surface due to stronger stratification of the upper water column of the Bering Sea during glacial times, with more complete nitrate consumption resulting from continued iron supply through atmospheric deposition. This finding extends the body of evidence for a pervasive link between cold climates and polar ocean stratification. In addition, we hypothesize that more complete nutrient consumption in the glacial age subarctic Pacific contributed to the previously observed ice age reduction in suboxia and denitrification in the eastern tropical North Pacific by lowering the nutrient content of the intermediate-depth water formed in the subpolar North Pacific. In the deglacial interval of the Bering Sea record, two apparent peaks in export productivity are associated with maxima in diatom-bound and bulk sediment δ¹⁵N. The high δ¹⁵N in these intervals may have resulted from greater surface nutrient consumption during this period. However, the synchroneity of the deglacial peaks in the Bering Sea with similar bulk sediment δ¹⁵N changes in the eastern Pacific margin and the presence of sediment lamination within the Bering Sea during the deposition of the productivity peaks raise the possibility that both regional and local denitrification worked to raise the δ¹⁵N of the nitrate feeding Bering Sea surface waters at these times.
de Boer, A. M., Daniel Sigman, J. R. Toggweiler, and J. L. Russell, 2007: The Effect of global ocean temperature change on deep ocean ventilation. Paleoceanography, 22(PA2210), doi:10.1029/2005PA001242
[ Abstract ]
A growing number of paleoceanographic observations suggest that the ocean�s deep ventilation is stronger in warm climates than in cold climates. Here we use a general ocean circulation model to test the hypothesis that this relation is due to the reduced sensitivity of seawater density to temperature at low mean temperature; that is, at lower temperatures the surface cooling is not as effective at densifying fresh polar waters and initiating convection. In order to isolate this factor from other climate-related feedbacks we change the model ocean temperature only where it is used to calculate the density (to which we refer below as ��dynamic�� temperature change). We find that a dynamically cold ocean is globally less ventilated than a dynamically warm ocean. With dynamic cooling, convection decreases markedly in regions that have strong haloclines (i.e., the Southern Ocean and the North Pacific), while overturning increases in the North Atlantic, where the positive salinity buoyancy is smallest among the polar regions. We propose that this opposite behavior of the North Atlantic to the Southern Ocean and North Pacific is the result of an energy-constrained overturning.
Deutsch, C., Jorge Sarmiento, Daniel Sigman, N. Gruber, and J. P. Dunne, 2007: Spatial coupling of nitrogen inputs and losses in the ocean. Nature, 445, doi:10.1038/nature05392
[ Abstract ]
Nitrogen fixation is crucial for maintaining biological productivity in the oceans, because it replaces the biologically available nitrogen that is lost through denitrification. But, owing to its temporal and spatial variability, the global distribution of marine nitrogen fixation is difficult to determine from direct shipboard measurements. This uncertainty limits our understanding of the factors that influence nitrogen fixation, which may include iron, nitrogen-to-phosphorus ratios, and physical conditions such as temperature. Here we determine nitrogen fixation rates in the world�s oceans through their impact on nitrate and phosphate concentrations in surface waters, using an ocean circulation model. Our results indicate that nitrogen fixation rates are highest in the Pacific Ocean, where water column denitrification rates are high but the rate of atmospheric iron deposition is low. We conclude that oceanic nitrogen fixation is closely tied to the generation of nitrogen-deficient waters in denitrification zones, supporting the view that nitrogen fixation stabilizes the oceanic inventory of fixed nitrogen over time.
Galbraith, E. D., S. L. Jaccard, T. F. Pedersen, Daniel Sigman, G. H. Haug, M. S. Cook, J. R. Southon, and R. Francois, 2007: Carbon dioxide release from the North Pacific abyss during the last deglaciation. Nature, doi:10.1038/nature06227 890-893
[ Abstract ]
Atmospheric carbon dioxide concentrations were significantly lower during glacial periods than during intervening interglacial periods, but the mechanisms responsible for this difference remain uncertain. Many recent explanations call on greater carbon storage in a poorly ventilated deep ocean during glacial periods, but direct evidence regarding the ventilation and respired carbon content of the glacial deep ocean is sparse and often equivocal. Here we present sedimentary geochemical records from sites spanning the deep subarctic Pacific that - together with previously published results - show that a poorly ventilated water mass containing a high concentration of respired carbon dioxide occupied the North Pacific abyss during the Last Glacial Maximum. Despite an inferred increase in deep Southern Ocean ventilation during the first step of the deglaciation (18,000�15,000 years ago), we find no evidence for improved ventilation in the abyssal subarctic Pacific until a rapid transition 14,600 years ago: this change was accompanied by an acceleration of export production from the surface waters above but only a small increase in atmospheric carbon dioxide concentration. We speculate that these changes were mechanistically linked to a roughly coeval increase in deep water formation in the North Atlantic, which flushed respired carbon dioxide from northern abyssal waters, but also increased the supply of nutrients to the upper ocean, leading to greater carbon dioxide sequestration at mid-depths and stalling the rise of atmospheric carbon dioxide concentrations. Our findings are qualitatively consistent with hypotheses invoking a deglacial flushing of respired carbon dioxide from an isolated, deep ocean reservoir, but suggest that the reservoir may have been released in stages, as vigorous deep water ventilation switched between North Atlantic and Southern Ocean source regions.
Houlton, B. Z., Daniel Sigman, E.A.G. Schuur, and L.O. Hedin, 2007: A climate-driven switch in plant nitrogen acquisition within tropical forest communities. Proceedings of the National Academy of Sciences of the United States of America, 104(21), doi:10.1073/pnas.0609935104 8902-8906
[ Abstract ]
The response of tropical forests to climate change will depend on individual plant species� nutritional strategies, which have not been defined in the case of the nitrogen nutrition that is critical to sustaining plant growth and photosynthesis. We used isotope natural abundances to show that a group of tropical plant species with diverse growth strategies (trees and ferns, canopy, and subcanopy) relied on a common pool of inorganic nitrogen, rather than specializing on different nitrogen pools. Moreover, the tropical species we examined changed their dominant nitrogen source abruptly, and in unison, in response to precipitation change. This threshold response indicates a coherent strategy among species to exploit the most available form of nitrogen in soils. The apparent community-wide flexibility in nitrogen uptake suggests that diverse species within tropical forests can physiologically track changes in nitrogen cycling caused by climate change.
Meckler, A. N., G. H. Haug, Daniel Sigman, B. Plessen, L. C. Peterson, and H. R. Thierstein, 2007: Detailed sedimentary N isotope records from Cariaco Basin for Terminations I and V: Local and global implications. Global Biogeochemical Cycles, 21(GB4019), doi:10.1029/2006GB002893
[ Abstract ]
For the last deglaciation and Termination V (the initiation of MIS 11 at around 430 ka) we report high-resolution sedimentary nitrogen isotope (δ¹⁵N) records from Cariaco Basin in the Caribbean Sea. During both terminations the previously reported interglacial decrease in δ¹⁵N clearly lags local changes such as water column anoxia as well as global increases in denitrification by several thousand years. On top of the glacialinterglacial change, several δ¹⁵N peaks were observed during the last deglaciation. The deglacial signal in Cariaco Basin can be best explained as a combination of (1) local variations in suboxia and water column denitrification as the reason for the millennialscale peaks, (2) a deglacial maximum in mean ocean nitrate δ¹⁵N, and (3) increasing N₂ fixation in response to globally increased denitrification causing the overall deglacial δ¹⁵N decrease. In the Holocene, much of the decrease in δ¹⁵N occurred between 6 and 3 ka, coinciding with an expected precession-modulated increase in African dust transport to the tropical North Atlantic and the Caribbean. This begs the hypothesis that N₂ fixation in this region increased in response to interglacial maxima in denitrification elsewhere but that this response strengthened with increased mid-Holocene iron input. It remains to be seen whether the data for MIS 11 support this interpretation.
Sigman, Daniel, A. M. de Boer, and G. H. Haug, 2007: Antarctic stratification, atmospheric water vapor, and Heinrich events: A hypothesis for late Pleistocene deglaciations. AGU Geophysical Monograph, http://department.princeton.edu/geosciences/people/sigman/pdf/Sigman07_OCMI.pdf, 173, 335-350
[ Abstract ]
We have previously argued that the Antarctic and subarctic North Pacific are stratified during ice ages, causing to a large degree the observed low CO₂ levels of ice age atmospheres by sequestering respired CO₂ in the ocean abyss. Here, we suggest a mechanism for the major deglaciations of the late Pleistocene. The mechanism begins with freshwater discharge to the North Atlantic, as evidenced by a Heinrich event, that shuts down North Atlantic overturning. Because of a global requirement for deep ocean ventilation, the North Atlantic shutdown drives overturning in the Antarctic, which, in turn, releases CO₂ to the atmosphere and reduces Antarctic sea ice extent. The resulting increase in atmospheric CO₂ and decrease in albedo then drive global warming and deglaciation. As a control on the timing of deglaciations, we look to the sensitivity of atmospheric freshwater transport to low latitude temperature, which is a natural antagonist to Antarctic stratification under cold climates. While Antarctic stratification is proposed to develop early in a glacial period, continued cooling through the glacial period may reduce the poleward atmospheric freshwater transport and thus may prepare the Antarctic halocline for collapse. Deglaciations may coincide with obliquity maxima because a reduced low-to-high latitude insolation gradient decreases the net poleward freshwater transport and perhaps also because increased polar insolation can warm the deep ocean and shift the westerly winds poleward, all of which should work to weaken Antarctic stratification. Precession minima may encourage Antarctic destratification by biasing tropical water vapor transport toward the northern hemisphere. Finally, obliquity and precession may work together to encourage the circum-North Atlantic freshwater discharge event that initiates the deglacial sequence.
Yancheva, G., N. R. Nowaczyk, J. Mingram, P. Dulski, G. Schettler, F. W. Negendank, J. Liu, Daniel Sigman, L. C. Peterson, and G. H. Haug, 2007: Influence of the Intertropical Convergence Zone on the East Asian Monsoon. Nature, 445, doi:10.1038/nature05431 74-77
[ Abstract ]
The Asian�Australian monsoon is an important component of the Earth�s climate system that influences the societal and economic activity of roughly half the world�s population. The past strength of the rain-bearing East Asian summer monsoon can be reconstructed with archives such as cave deposits, but the winter monsoon has no such signature in the hydrological cycle and has thus proved difficult to reconstruct. Here we present high-resolution records of the magnetic properties and the titanium content of the sediments of Lake Huguang Maar in coastal southeast China over the past 16,000 years, which we use as proxies for the strength of the winter monsoon winds. We find evidence for stronger winter monsoon winds before the B�lling�Aller�d warming, during the Younger Dryas episode and during the middle and late Holocene, when cave stalagmites suggest weaker summer monsoons. We conclude that this anticorrelation is best explained by migrations in the intertropical convergence zone. Similar migrations of the intertropical convergence zone have been observed in Central America for the period AD 700 to 900, suggesting global climatic changes at that time. From the coincidence in timing, we suggest that these migrations in the tropical rain belt could have contributed to the declines of both the Tang dynasty in China and the Classic Maya in Central America.
Hastings, M. G., Daniel Sigman, and E. J. Steig, 2005: Glacial/interglacial changes in the isotopes of nitrate from the Greenland Ice Sheet Project 2 (GISP2) ice core. Global Biogeochemical Cycles, 7, doi:10.1029/2005GB002502 611-625
[ Abstract ]
The ¹⁵N/¹⁴N and ¹⁸O/¹⁶O ratios of nitrate in the Greenland Ice Sheet Project 2 (GISP2) (Summit, Greenland) ice core are much higher in ice from the last glacial period than in the pre-industrial Holocene, despite the lack of a significant glacial/interglacial change in nitrate concentration. While both the ¹⁵N/¹⁴N and ¹⁸O/¹⁶O records are anticorrelated with snow accumulation rate, neither is satisfactorily explained by accumulation changes or post-depositional processes. The similarity in the glacial/ interglacial change in ¹⁵N/¹⁴N from several different Greenland ice cores and the large amplitude of this change relative to observed seasonal variation raise the possibility that the isotopes of nitrate in ice cores indicate a large-scale glacial/interglacial change in the isotopic composition of atmospheric NO_x. The glacial/interglacial change in ¹⁸O/¹⁶O is best explained by a greater contribution of HNO₃ production from hydrolysis of N₂O₅, which has implications for reconstruction of past atmospheric oxidant levels. Although isotope effects associated with NO_x photochemistry and nitrate scavenging have not been fully characterized, the ¹⁵N/¹⁴N data may indicate glacial/interglacial changes in the relative contributions from different natural sources of NO_x on a hemispheric or global scale.
Haug, G. H., A. Ganopolski, Daniel Sigman, A. Rosell-Mele, G.E.A. Swann, R. Tiedemann, S. L. Jaccard, J. Bollman, R. J. Matear, M. J. Leng, and G. Eglinton, 2005: North Pacific seasonality and the glaciation of North America 2.7 million years ago. Nature, 433, doi:10.1038/nature03332 821-825
[ Abstract ]
In the context of gradual Cenozoic cooling, the timing of the onset of significant Northern Hemisphere glaciation 2.7 million years ago is consistent with Milankovitch�s orbital theory, which posited that ice sheets grow when polar summertime insolation and temperature are low. However, the role of moisture supply in the initiation of large Northern Hemisphere ice sheets has remained unclear. The subarctic Pacific Ocean represents a significant source of water vapour to boreal North America, but it has been largely overlooked in efforts to explain Northern Hemisphere glaciation. Here we present alkenone unsaturation ratios and diatom oxygen isotope ratios from a sediment core in the western subarctic Pacific Ocean, indicating that 2.7 million years ago latesummer sea surface temperatures in this ocean region rose in response to an increase in stratification. At the same time, winter sea surface temperatures cooled, winter floating ice became more abundant and global climate descended into glacial conditions. We suggest that the observed summer warming extended into the autumn, providing water vapour to northern North America, where it precipitated and accumulated as snow, and thus allowed the initiation of Northern Hemisphere glaciation.
Jaccard, S. L., G. H. Haug, Daniel Sigman, T. F. Pedersen, H. R. Thierstein, and U. Röhl, 2005: Glacial/interglacial changes in subarctic North Pacific stratification. Science, 308, doi:10.1126/science.1108696 1003-1006
[ Abstract ]
Since the first evidence of low algal productivity during ice ages in the Antarctic Zone of the Southern Ocean was discovered, there has been debate as to whether it was associated with increased polar ocean stratification or with sea-ice cover, shortening the productive season. The sediment concentration of biogenic barium at Ocean Drilling Program site 882 indicates low algal productivity during ice ages in the Subarctic North Pacific as well. Site 882 is located southeast of the summer sea-ice extent even during glacial maxima, ruling out sea-ice-driven light limitation and supporting stratification as the explanation, with implications for the glacial cycles of atmospheric carbon dioxide concentration.
Robinson, R. S., Daniel Sigman, P. J. DiFiore, M. M. Rohde, T. A. Mashiotta, and D. W. Lea, 2005: Diatombound ¹⁵N/¹⁴N: New support for enhanced nutrient consumption in the ice age Subantarctic. Paleoceanography, 20(PA303), doi:10.1029/2004PA001114
[ Abstract ]
Diatom-bound ¹⁵N/¹⁴N was used to reconstruct the glacial nutrient status of the Subantarctic Zone in the Southern Ocean. Down-core records from both the Pacific and Indian sectors show δ¹⁵N of 5 to 6% during the Last Glacial Maximum and a decrease, coincident with the glacial termination, to values as low as 2%. The effect of either diatom assemblage or physiological change on the diatom-bound ¹⁵N/¹⁴N is unknown and cannot yet be ruled out as a possible explanation for the observed change. However, the consistency between Indian and Pacific sector records and with other paleoceanographic data suggests that the glacial-interglacial difference in diatom-bound ¹⁵N/¹⁴N was driven by higher consumption of nitrate in the subantarctic surface during the last ice age. Such a change in nutrient consumption may have resulted from atmospheric iron fertilization and/or decreased glacial mixed layer depths associated with sea ice melting. Enhanced nutrient consumption in the glacial subantarctic would have worked to lower the concentration of CO₂ in the ice age atmosphere. It also would have reduced the preformed nutrient content of the low-latitude thermocline, leading to decreases in lowlatitude productivity, suboxia, and denitrification.
Sigman, Daniel, J. Granger, P. J. DiFiore, K. K. Lehman, R. Ho, G. Cane, and A. van Geen, 2005: Coupled nitrogen and oxygen isotope measurements of nitrate along the eastern North Pacific margin. Global Biogeochemical Cycles, 19(GB4022), doi:10.1029/2005GB002458
[ Abstract ]
Water column depth profiles along the North Pacific margin from Point Conception to the tip of Baja California indicate elevation of nitrate (NO₃) ¹⁵N/¹⁴N and ¹⁸O/¹⁶O associated with denitrification in the oxygen-deficient thermocline waters of the eastern tropical North Pacific. The increase in δ¹⁸O is up to 3% greater than in δ¹⁵N, whereas our experiments with denitrifier cultures in seawater medium indicate a 1:1 increase in NO₃ δ¹⁸O and δ¹⁵N during NO₃ consumption. Moreover, the maximum in NO₃ δ¹⁸O is somewhat shallower than the maximum in NO₃ δ¹⁵N. These two observations can be summarized as an ��anomaly�� from the 1:1 δ¹⁸O-to-δ¹⁵N relationship expected from culture results. Comparison among stations and with other data indicates that this anomaly is generated locally. The anomaly has two plausible interpretations: (1) the addition of low-δ¹⁵N NO₃ to the shallow thermocline by the remineralization of newly fixed nitrogen, or (2) active cycling between NO₃ and NO₂ (coupled NO₃ reduction and NO₂ oxidation) in the suboxic zone.
Deutsch, C., Daniel Sigman, R. C. Thunell, A. N. Meckler, and G. H. Haug, 2004: Isotopic constraints on glacial/interglacial changes in the oceanic nitrogen budget. Global Biogeochemical Cycles, 18(GB4012), doi:10.1029/2003GB002189
[ Abstract ]
We investigate the response of the ¹⁵N/¹⁴N of oceanic nitrate to glacial/interglacial changes in the N budget, using a geochemical box model of the oceanic N cycle that includes N₂ fixation and denitrification in the sediments and suboxic water column. This model allows us to quantify the isotopic response of different oceanic nitrate pools to deglacial increases in water column and sedimentary denitrification, given a range of possible feedbacks between nitrate concentration and N₂ fixation/denitrification. This response is compared to the available paleoceanographic data, which suggest an early deglacial maximum in nitrate ¹⁵N/¹⁴N in suboxic zones and no significant glacialto- late Holocene change in global ocean nitrate ¹⁵N/¹⁴N. Consistent with the work of Brandes and Devol [2002], we find that the steady state ¹⁵N/¹⁴N of oceanic nitrate is controlled primarily by the fraction of total denitrification that occurs in the water column. Therefore a deglacial peak in the ratio of water column-to-sediment denitrification, caused by either a strong feedback between water column denitrification and the N reservoir or by an increase in sediment denitrification due to sea level rise, can explain the observed deglacial ¹⁵N/¹⁴N maximum in sediments underlying water column denitrification zones. The total denitrification rate and the mean ocean nitrate concentration are also important determinants of steady state nitrate ¹⁵N/¹⁴N. For this reason, modeling a realistic deglacial ¹⁵N/¹⁴N maximum further requires that the combined negative feedbacks from N₂ fixation and denitrification are relatively strong, and N losses are relatively small. Our results suggest that the glacial oceanic N inventory was at most 30% greater than today�s and probably less than 10% greater.
Hastings, M. G., E. J. Steig, and Daniel Sigman, 2004: Seasonal variations in N and O isotopes of nitrate in snow at Summit, Greenland: Implications for the study of nitrate in snow and ice cores. Journal of Geophysical Research � Atmosphere, 109(D20306), doi:10.1029/2004JD004991
[ Abstract ]
Nitrogen and oxygen isotopes of NO₃ have been measured in snow and firn from Summit, Greenland. The ¹⁵N/¹⁴N and ¹⁸O/¹⁶O ratios of NO₃ in recently fallen snow are similar to those of surface snow. Diurnal variation is observed in ¹⁵N/¹⁴N of NO₃, and possibly ¹⁸O/¹⁶O, suggesting fractionating loss of NO₃ from snow during the day, which is subsequently recovered at night. A larger seasonal variation is observed, with higher ¹⁵N/¹⁴N and lower ¹⁸O/¹⁶O of NO₃ in summer than winter, which cannot be explained by postdepositional fractionation. The generally high ¹⁸O/¹⁶O of NO₃ in Greenland snow (δ¹⁸O versus VSMOW = 65.2 to 79.6%) indicates that oxygen atoms from ozone have been incorporated into NO_x that was subsequently deposited as HNO₃. The lower mean δ¹⁸O of NO₃ in summer snow relative to winter (68.9% in summer 2000 and 70.5% in summer 2001 versus 77.5% in winter 2000�01) is a result of summertime HNO₃ production via NO₂ reaction with hydroxyl radical (OH), which dilutes the high δ¹⁸O imparted on NO₂ from ozone. The higher mean ¹⁵N/¹⁴N of NO₃ observed in snow from spring (δ¹⁸O versus air N₂ = +5.9% in 2000 and -1.4% in 2001) and summer (+0.1% in 2000 and -0.8% in 2001) than fall (-9.2% in 2000) and winter( -10.0% in 2000�01) is more difficult to explain with seasonal photochemistry, given current knowledge. The seasonal ¹⁵N/¹⁴N change may reflect NO_X sources, with a greater fall and wintertime contribution from fossil fuel emissions relative to other inputs of NO_X (i.e., biogenic soil emissions, biomass burning, and lightning).
Robinson, R. S., B. G. Brunelle, and Daniel Sigman, 2004: Revisiting nutrient utilization in the glacial Antarctic: Evidence from a new method for diatom-bound N isotopic analysis. Paleoceanography, 19(PA3001), doi:10.1029/2003PA000996
[ Abstract ]
Isotopic measurements of diatom-bound nitrogen, using a wet chemical oxidation combined with the ��denitrifier�� method for nitrate analysis, show significant offsets from previously published combustion-based measurements. This offset is attributed to a gaseous nitrogen blank associated with the diatom�s opal frustule. Moreover, experimentation with multiple chemical cleaning protocols demonstrates that diatom microfossils from the clay-rich sediments of the glacial Antarctic are more difficult to clean than Holocene materials. New downcore profiles from the Antarctic show no change in the diatom-bound N ¹⁵N/¹⁴N between the last glacial and the Holocene in the Atlantic sector, and the elevation of glacial diatom-bound ¹⁵N/¹⁴N relative to the Holocene in the Indian sector is smaller than in previous measurements. These data suggest no change in the degree of nitrate utilization in the Atlantic sector and at most a 20% increase (from ~ 25 to 45%) in the Indian sector. The new measurements suggest that, during the last ice age in the Atlantic sector of the Antarctic, the atmospheric source of biologically available iron was not so great as to become significant relative to the iron supply from below. Given the apparent spatial variability in the degree of nitrate drawdown, more work is required to develop an adequate picture of the glacial Antarctic nutrient field.
Sigman, Daniel, S. L. Jaccard, and G. H. Haug, 2004: Polar ocean stratification in a cold climate. Nature, 428, doi:10.1038/nature02357 59-63
[ Abstract ]
The low-latitude ocean is strongly stratified by the warmth of its surface water. As a result, the great volume of the deep ocean has easiest access to the atmosphere through the polar surface ocean. In the modern polar ocean during the winter, the vertical distribution of temperature promotes overturning, with colder water over warmer, while the salinity distribution typically promotes stratification, with fresher water over saltier. However, the sensitivity of seawater density to temperature is reduced as temperature approaches the freezing point, with potential consequences for global ocean circulation under cold climates1,2. Here we present deep-sea records of biogenic opal accumulation and sedimentary nitrogen isotopic composition from the Subarctic North Pacific Ocean and the Southern Ocean. These records indicate that vertical stratification increased in both northern and southern high latitudes 2.7 million years ago, when Northern Hemisphere glaciation intensified in association with global cooling during the late Pliocene epoch. We propose that the cooling caused this increased stratification by weakening the role of temperature in polar ocean density structure so as to reduce its opposition to the stratifying effect of the vertical salinity distribution. The shift towards stratification in the polar ocean 2.7 million years ago may have increased the quantity of carbon dioxide trapped in the abyss, amplifying the global cooling.
Hastings, M. G., Daniel Sigman, and R. Lipshultz, 2003: Isotopic evidence for source changes of nitrate in rain at Bermuda. Journal of Geophysical Research � Atmosphere, 108(D24), doi:10.1029/2003JD003789
[ Abstract ]
Rainwater collected on the island of Bermuda between January 2000 and January 2001 shows pronounced seasonal variation in the nitrogen and oxygen isotopic composition of nitrate. Higher ¹⁵N/¹⁴N and lower ¹⁸O/¹⁶O ratios are observed in the warm season (April�September) in comparison to the cool season (October�March): The mean d15N of nitrate for the warm and cool seasons is -2.1% and -5.9% (versus air N2), respectively, while the mean δ:¹⁸O is 68.6% and 76.9% (versus Vienna Standard Mean Ocean Water). The few cool season rain events that had high ¹⁵N/¹⁴N and low ¹⁸O/¹⁶O exhibited trajectory paths originating from the south, similar to those of warm season samples. Accordingly, the region from which air is transported to the island determines the ¹⁵N/¹⁴N and ¹⁸O/¹⁶O of the nitrate. The source region provides precursor nitrogen oxides (NO_x), influencing the ¹⁵N/¹⁴N of nitrate, and contributes to the chemistry that produces nitrate from NO_x, which determines the ¹⁸O/¹⁶O of nitrate. While the range in nitrate ¹⁵N/¹⁴N observed during the cool season is consistent with anthropogenic emissions from North America, the higher warm season ¹⁵N/¹⁴N suggests that lightning is a significant source of nitrate to Bermuda. The isotopic evidence for a significant southern source of nitrate to Bermuda helps to explain the previous observation of unexpectedly high nitrate concentrations in warm season rain. The ¹⁸O/¹⁶O of nitrate in rain at Bermuda is high throughout the year (δ:¹⁸O = 60.3 to 86.5%) as a result of interactions of precursor NO_x with ozone, which has a high ¹⁸O/¹⁶O ratio. The lower nitrate ¹⁸O/¹⁶O in the warm season and in cool season air masses from the south is consistent with elevated concentrations of hydroxyl radical (OH), which dilutes the isotopic signal of ozone. Our limited data set suggests that the relative importance of the OH sink for NO_x during the cool season varies spatially over as large a range as is observed between the warm and cool seasons.
Karsh, K. L., T. W. Trull, M. J. Lourey, and Daniel Sigman, 2003: Relationship of nitrogen isotope fractionation to phytoplankton size and iron availability during the SOIREE Southern Ocean Iron Release Experiment. Limnology and Oceanography, http://www.aslo.org/lo/toc/vol_48/issue_3/1058.pdf, 48(3), 1058-1068
[ Abstract ]
The ¹⁵N composition of sediments has been used as a proxy for nitrate utilization in surface waters to assess the role of Southern Ocean export production in glacial/interglacial changes in atmospheric CO₂ concentration. Interpretation has relied on a temporally constant isotope effect (ε) associated with uptake and assimilation of nitrate by phytoplankton. To investigate the reliability of this approach, we examined the relationships between the ¹⁵N compositions of dissolved nitrate, bulk and size-fractionated (200, 70, 20, 5, 1 �m) suspended particulate organic nitrogen (PON), and sinking particles obtained from sediment traps during the Southern Ocean iron release experiment (SOIREE). We found variations in phytoplankton nitrogen isotopic compositions with both cell size and iron availability. δ¹⁵N_PON increased by .2� with increasing size, both within and outside the iron enriched patch. In comparison to unfertilized waters, δ¹⁵N_PON within the iron-fertilized patch was a further 3�4� higher in those size fractions dominated by large diatoms (20�70, 70�200 �mm). We speculate that this iron response might result from (1) variation in � of nitrate utilization or (2) an iron-stimulated shift from ammoniumbased to nitrate-based production. Comparing the δ¹⁵N_PON of the large diatom�dominated size fractions to the δ¹⁵N_PON of nitrate suggests relatively low ε values of 4�5�, in contrast to estimated values of 7�10� from seasonal nitrate depletion and export production. This suggests that higher glacial δ¹⁵N in Southern Ocean sediments could, in part, reflect increases in iron availability, dominant cell size, and possibly growth rates, and these effects must be considered in any quantitative scaling of δ¹⁵N variations, including those of diatom-bound δ¹⁵N, to the extent of nitrate utilization.
Lourey, M. J., T. W. Trull, and Daniel Sigman, 2003: Sensitivity of δ15N of nitrate, surface suspended and deep sinking particulate nitrogen to seasonal nitrate depletion in the Southern Ocean. Global Biogeochemical Cycles, 17(3), doi:10.1029/2002GB001973
[ Abstract ]
We report measurements of the δ¹⁵N of nitrate, suspended particulate nitrogen (PN), and sinking PN from cruises and moored sediment traps in the Subantarctic Zone (SAZ) and Polar Frontal Zone (PFZ) south of Australia. As expected, surface water nitrate δ¹⁵N increased as nitrate was consumed during the spring/summer bloom. In contrast, the seasonal cycles of surface water suspended and sinking PN δ¹⁵N did not fit expectations from nitrate assimilation alone. Rather than increasing, the δ¹⁵N of surface suspended PN was relatively constant in the SAZ (at ~1%), and decreased during the summer in the PFZ (from ~0 to ~-4%), most likely due to the production of low ¹⁵N PN by summertime ammonium recycling. Deep sediment trap PN δ¹⁵N also displayed seasonal decreases (from ~4 to ~1% in the SAZ, and from ~3.5 to ~0.5% in the PFZ), which correlated with PON flux magnitude. During high-flux periods, exported PN δ¹⁵N values were close to expectations from nitrate-based export, but low-flux periods exhibited higher δ¹⁵N, consistent with either a reduction in the isotope effect of nitrate assimilation or more extensive isotopic alteration of the sinking material during low-flux periods. The mass balance between net nitrate supply and exported PN that links sinking flux δ¹⁵N to nitrate utilization requires only that the annually integrated (rather than the seasonally varying) sinking flux of PN δ¹⁵N correlates with nitrate depletion. While a correlation between annually integrated sinking PN δ¹⁵N to nitrate depletion was observed in both the SAZ and PFZ, the sensitivity of sinking PN δ¹⁵N to nitrate depletion was lower than expected. Moreover, the seasonal observations raise the possibility that loss of the summertime high-flux period represents an alternative explanation to increased nitrate utilization for the high sedimentary PN δ¹⁵N observed during glacial periods.
Sigman, Daniel, and G. H. Haug, 2003: Biological Pump in the Past. Treatise On Geochemistry, Oxford, Elsevier Science, 6, doi:10.1016/B0-08-043751-6/06118-1 492-528
[ Abstract ]
It is easy to imagine that the terrestrial biosphere sequesters atmospheric carbon dioxide; the form and quantity of the sequestered carbon, living or dead organic matter, are striking. In the ocean, there are no aggregations of biomass comparable to the forests on land. Yet biological productivity in the ocean plays a central role in the sequestration of atmospheric carbon dioxide, typically overshadowing the effects of terrestrial biospheric carbon storage on timescales longer than a few centuries. In an effort to communicate the ocean's role in the regulation of atmospheric carbon dioxide, marine scientists frequently refer to the ocean's biologically driven sequestration of carbon as the "biological pump." The original and strict definition of the biological (or "soft-tissue") pump is actually more specific: the sequestration of carbon dioxide in the ocean interior by the biogenic flux of organic matter out of surface waters and into the deep sea prior to decomposition of that organic matter back to carbon dioxide (Volk and Hoffert, 1985) ( Figure 1). The biological pump extracts carbon from the "surface skin" of the ocean that interacts with the atmosphere, presenting a lower partial pressure of carbon dioxide (CO₂) to the atmosphere and thus lowering its CO₂ content.
Sigman, Daniel, S. J. Lehman, and D. W. Oppo, 2003: Evaluating mechanisms of nutrient depletion and ¹³ C enrichment in the intermediate-depth Atlantic during the last ice age. Paleoceanography, 18(3), doi:10.1029/2002PA000818
[ Abstract ]
Using an ocean box model, we have studied the effect of altered circulation on the oceanic distributions of phosphate (PO₄^-3) and the ¹³C/¹²C and ¹⁴C/¹²C of dissolved inorganic carbon to evaluate competing hypotheses for the cause of observed nutrient depletion and ¹³C enrichment at intermediate depths of the Atlantic during the last ice age. Because of ��nutrient trapping�� and limited air-sea carbon isotopic equilibration, the simple imposition of an intense meridional overturning cell in the Atlantic fails to simultaneously lower nutrient concentrations and raise ¹³C/¹²C to observed glacial levels. Export of intermediate water out of the Atlantic causes a basin-to-basin nutrient transfer, thus providing a more efficient mechanism of intermediate-depth Atlantic nutrient depletion and improved carbon isotopic equilibration at low temperatures (i.e., ¹³C enrichment). Although this export adds nutrients to the intermediate depths of the Pacific and Indian Oceans, the simulated glacial intermediate-depth Indo-Pacific is nevertheless moderately depleted in PO₄^-3 relative to the model�s interglacial control, in agreement with consensus paleoceanographic evidence. This Indo-Pacific PO₄^-3 depletion results from our use of a ��glacial base case�� in which nutrient-rich Antarctic Intermediate Water formation is absent as part of the elimination of the modern North-Atlantic-Deep-Water-based ��conveyor�� circulation.
Brezeinski, M. A., C. J. Pride, V. M. Franck, Daniel Sigman, Jorge Sarmiento, K. Matsumoto, and N. Gruber, 2002: A switch from Si(OH)₄ to NO₃ depletion in the glacial Southern Ocean. Geophysical Research Letters, 29(12), doi:10.1029/2001GL014349 1564
[ Abstract ]
Phytoplankton in the Antarctic deplete silicic acid (Si(OH)₄) to a far greater extent than they do nitrate (NO₃). This pattern can be reversed by the addition of iron which dramatically lowers diatom Si(OH)₄: NO₃ uptake ratios. Higher iron supply during glacial times would thus drive the Antarctic towards NO₃ depletion with excess Si(OH)₄ remaining in surface waters. New δ³⁰SI and δ ¹⁵N records from Antarctic sediments confirm diminished Si(OH)₄ use and enhanced NO₃ depletion during the last three glaciations. The present low-Si(OH)₄ water is transported northward to at least the subtropics. We postulate that the glacial high-Si(OH)₄ water similarly may have been transported to the subtropics and beyond. This input of Si(OH)₄ may have caused diatoms to displace coccolithophores at low latitudes, weakening the carbonate pump and increasing the depth of organic matter remineralization. These effects may have lowered glacial atmospheric pCO₂ by as much as 60 ppm.
Carrillo, J. H., M. G. Hastings, Daniel Sigman, and B. J. Huebert, 2002: Atmospheric deposition of inorganic and organic nitrogen and base cations in Hawaii. Global Biogeochemical Cycles, 16(4), doi:10.1029/2002GB001892
[ Abstract ]
Atmospheric deposition of nitrogen (N) and base cations was measured for 5�7 years on the island of Hawaii and for 1.5 years on Kauai. On Hawaii, mean annual fluxes of K⁺, Mg²⁺, and Ca²⁺ were 15, 17, and 13 kg ha^-1 yr^-1, respectively. Fog interception was the largest deposition pathway. Sea salt contributed the majority of cations, although biomass burning and Asian dust were significant sources for some years. Total N deposition (inorganic and organic) averaged 17 kg N ha^-1 yr^-1. Fog interception was also the largest source of N, depositing 16 kg N ha^-1 yr^-1. Precipitation deposition was 1.0 and 0.2 kg N ha^-1 yr^-1, respectively on Hawaii and Kauai. Dry deposition on Hawaii was 0.1 kg N ha^-1 yr^-1. Organic N averaged 16 and 12% of total N in rain and fog, respectively. The δ¹⁵N values for NO₃-N are consistent with long-range transport of N from Asia in the spring/summer and from North America in the fall/winter as nonvolcanic sources. Atmospheric deposition on Hawaii may completely account for a previously identified soil N imbalance.
Casciotti, K. L., Daniel Sigman, M. G. Hastings, J. K. Böhlke, and A. Hilkert, 2002: Measurement of the oxygen isotopic composition of nitrate in seawater and freshwater using the denitrifier method. Analytical Chemistry, 74(19), doi:10.1021/ac020113w 4905-4912
[ Abstract ]
We report a novel method for measurement of the oxygen isotopic composition (¹⁸O/¹⁶O) of nitrate (NO₃ -) from both seawater and freshwater. The denitrifier method, based on the isotope ratio analysis of nitrous oxide generated from sample nitrate by cultured denitrifying bacteria, has been described elsewhere for its use in nitrogen isotope ratio (¹⁵N/¹⁴N) analysis of nitrate.1 Here, we address the additional issues associated with ¹⁸O/¹⁶O analysis of nitrate by this approach, which include (1) the oxygen isotopic difference between the nitrate sample and the N₂O analyte due to isotopic fractionation associated with the loss of oxygen atoms from nitrate and (2) the exchange of oxygen atoms with water during the conversion of nitrate to N₂O. Experiments with ¹⁸O labeled water indicate that water exchange contributes less than 10%, and frequently less than 3%, of the oxygen atoms in the N₂O product for Pseudomonas aureofaciens. In addition, both oxygen isotope fractionation and oxygen atom exchange are consistent within a given batch of analyses. The analysis of appropriate isotopic reference materials can thus be used to correct the measured ¹⁸O/¹⁶O ratios of samples for both effects. This is the first method tested for ¹⁸O/¹⁶O analysis of nitrate in seawater. Benefits of this method, relative to published freshwater methods, include higher sensitivity (tested down to 10 nmol and 1 μM NO₃ -), lack of interference by other solutes, and ease of sample preparation.
Karl, D., A. Michaels, B. Bergman, D. Capone, E. Carpenter, R. M. Letelier, F. Lipschultz, H. Paerl, Daniel Sigman, and L. Stal, 2002: Dinitrogen fixation in the world�s oceans. Biogeochemistry, 57/58, doi:10.1023/A:1015798105851 47-98
[ Abstract ]
The surface water of the marine environment has traditionally been viewed as a nitrogen (N) limited habitat, and this has guided the development of conceptual biogeochemical models focusing largely on the reservoir of nitrate as the critical source of N to sustain primary productivity. However, selected groups of Bacteria, including cyanobacteria, and Archaea can utilize dinitrogen (N₂) as an alternative N source. In the marine environment, these microorganisms can have profound effects on net community production processes and can impact the coupling of C-N-P cycles as well as the net oceanic sequestration of atmospheric carbon dioxide. As one component of an integrated 'Nitrogen Transport and Transformations' project, we have begun to re-assess our understanding of (1) the biotic sources and rates of N₂ fixation in the world's oceans, (2) the major controls on rates of oceanic N₂ fixation, (3) the significance of this N_x fixation for the global carbon cycle and (4) the role of human activities in the alteration of oceanic N₂ fixation. Preliminary results indicate that rates of N₂ fixation, especially in subtropical and tropical open ocean habitats, have a major role in the global marine N budget. Iron (Fe) bioavailability appears to be an important control and is, therefore, critical in extrapolation to global rates of N₂ fixation. Anthropogenic perturbations may alter N₂ fixation in coastal environments through habitat destruction and eutrophication, and open ocean N₂ fixation may be enhanced by warming and increased stratification of the upper water column. Global anthropogenic and climatic changes may also affect N₂ fixation rates, for example by altering dust inputs (i.e. Fe) or by expansion of subtropical boundaries. Some recent estimates of global ocean N₂ fixation are in the range of 100-200 Tg N (1-2 x 10¹⁴ g N) yr^-1, but have large uncertainties. These estimates are nearly an order of magnitude greater than historical, pre-1980 estimates, but approach modem estimates of oceanic denitrification.
Pantoja, S., D. J. Repeta, J. P. Sachs, and Daniel Sigman, 2002: Stable isotope constraints on the nitrogen cycle of the Mediterranean Sea water column. Deep Sea Research I, 49(9), doi:10.1016/S0967-0637(02)00066-3 1609-1621
[ Abstract ]
We used the nitrogen isotope ratio of algae, suspended particles and nitrate in the water column to track spatial variations in the marine nitrogen cycle in the Mediterranean Sea. Surface PON (5�74 m) was more depleted in ¹⁵N in the eastern basin (-0.3 � 0.5%) than in the western basin (+2.4 � 1.4%), suggesting that nitrogen supplied by biological N₂ fixation may be an important source of new nitrogen in the eastern basin, where preformed nitrate from the Atlantic Ocean could have been depleted during its transit eastward. The δ¹⁵N of nitrate in the deep Mediterranean (~ 3%in the western-most Mediterranean and decreasing toward the east) is significantly lower than nitrate at similar depths from the North Atlantic (4.8�5%), also suggesting an important role for N2 fixation. The eastward decrease in the δ¹⁵N of surface PON is greater than the eastward decrease in the δ¹⁵N of the subsurface nitrate, implying that the amount of N2 fixation in the eastern Mediterranean is great enough to cause a major divergence in the δ¹⁵N of phytoplankton biomass from the δ¹⁵N of the nitrate upwelled from below. Variations in productivity associated with frontal processes, including shoaling of the nitracline, did not lead to detectable variations in the δ¹⁵N of PON. This indicates that no differential fertilization or productivity gradient occurred in the Almerian/Oran area. Our results are consistent with a lack of gradient in chlorophyll-a (chl-a) and nitrate concentration in the Alboran Sea. ¹⁵N enrichment in particles below 500m depth was detected in the Alboran Sea with respect to surface PON, reaching an average value of +7.470.7%. The δ¹⁵N in sinking particles caught at 100m depth (4.9�5.6%) was intermediate between suspended surface and suspended deep particles. We found a consistent difference in the isotopic composition of nitrogen in PON compared with that of chlorophyll (Δδ¹⁵N[PON-chlorin]=+6.4 � 1.4%) in the surface, similar to the offset reported earlier in cultures for cellular N and chl- a. This indicates that δ¹⁵N of phytoplankton biomass was retained in surface PON, and that alteration of the isotopic signal of PON at depth was due to heterotrophic activity.
Tortell, P. D., G. R. Di Tullio, Daniel Sigman, and Francois Morel, 2002: CO₂ effects on taxonomic composition and nutrient utilization in an Equatorial Pacific phytoplankton assemblage. Marine Ecology Progress Series, 236, doi:10.3354/meps236037 37-43
[ Abstract ]
We report the results of a field incubation experiment demonstrating a substantial shift in the taxonomic composition of Equatorial Pacific phytoplankton assemblages exposed to CO₂ levels of 150 and 750 ppm (dissolved CO₂ ~3 to 25 μM). By the end of the experiment, the phytoplankton community in all samples was dominated by diatoms and Phaeocystis sp. However, the relative abundance of these phytoplankton taxa differed significantly between CO₂ treatments. Taxonomic pigment analysis and direct microscopic examination of samples revealed that the abundance of diatoms decreased by ~50% at low CO₂ relative to high CO₂, while the abundance of Phaeocystis sp. increased by ~60% at low CO₂. This CO₂-dependent shift was associated with a significant change in nutrient utilization, with higher ratios of nitrate:silicate (N:Si) and nitrate:phosphate (N:P) consumption by phytoplankton in the low CO₂ treatment. Despite the significant changes in taxonomic composition and nutrient consumption ratios, total biomass and primary productivity did not differ significantly between the CO₂ treatments. Our results suggest that CO₂ concentrations could potentially influence competition among marine phytoplankton taxa and affect oceanic nutrient cycling.
Haug, G. H., K. A. Hughen, Daniel Sigman, L. C. Peterson, and U. Röhl, 2001: Southward migration of the Intertropical Convergence Zone through the Holocene. Science, 293, doi:10.1126/science.1059725 1304-1308
[ Abstract ]
Titanium and iron concentration data from the anoxic Cariaco Basin, off the Venezuelan coast, can be used to infer variations in the hydrological cycle over northern South America during the past 14,000 years with subdecadal resolution. Following a dry Younger Dryas, a period of increased precipitation and riverine discharge occurred during the Holocene "thermal maximum". Since ˜ 5400 years ago, a trend toward drier conditions is evident from the data, with high-amplitude fluctuations and precipitation minima during the time interval 3800 to 2800 years ago and during the "Little Ice Age". These regional changes in precipitation are best explained by shifts in the mean latitude of the Atlantic Intertropical Convergence Zone (ITCZ), potentially driven by Pacific-based climate variability. The Cariaco Basin record exhibits strong correlations with climate records from distant regions, including the high-latitude Northern Hemisphere, providing evidence for global teleconnections among regional climates.
Sigman, Daniel, and K. L. Casciotti, 2001: Nitrogen isotopes in the ocean. Encyclopedia of Ocean Sciences, London, Academic Press, doi:10.1006/rwos.2001.0172 1884-1894
[ Abstract ]
Nitrogen has two stable isotopes, ¹⁴ N and ¹⁵ N (atomic masses of 14 and 15, respectively). ¹⁴ N is the more abundant of the two, comprising 99.63% of the nitrogen found in nature. Physical, chemical, and biological processes discriminate between the two isotopes. This is known as isotopic fractionation, and it leads to subtle but measurable differences in the ratio of ¹⁵ N to ¹⁴ N among different forms of nitrogen found in the marine environment. Nitrogen is a central component of marine biomass and one of the major nutrients required by all phytoplankton. In this sense, biologically available (or �fixed�) N is representative of the fundamental patterns of biogeochemical cycling in the ocean. However, N differs from other nutrients in that its oceanic sources and sinks are dominantly internal and biological, with marine N₂ fixation supplying much of the fixed N in the ocean and marine denitrification removing it. The N isotopes provide a means of studying both the internal cycling and input/output budget of oceanic fixed N, yielding information on both its representative and unique aspects. This overview outlines the isotope systematics of N cycle processes and their impacts on the isotopic composition of the major N reservoirs in the ocean. This information provides a starting point for considering the wide range of questions in ocean sciences to which the N isotopes can be applied.
Sigman, Daniel, K. L. Casciotti, M. Andreani, C. Barford, M. Galanter, and J. K. Böhlke, 2001: A bacterial method for the nitrogen isotopic analysis of nitrate in seawater and freshwater. Analytical Chemistry, 73(17), doi:10.1021/ac010088e 4145-4153
[ Abstract ]
We report a new method for measurement of the isotopic composition of nitrate (NO₃ -) at the natural-abundance level in both seawater and freshwater. The method is based on the isotopic analysis of nitrous oxide (N₂O) generated from nitrate by denitrifying bacteria that lack N₂O-reductase activity. The isotopic composition of both nitrogen and oxygen from nitrate are accessible in this way. In this first of two companion manuscripts, we describe the basic protocol and results for the nitrogen isotopes. The precision of the method is better than 0.2� (1 SD) at concentrations of nitrate down to 1 μM, and the nitrogen isotopic differences among various standards and samples are accurately reproduced. For samples with 1 μM nitrate or more, the blank of the method is less than 10% of the signal size, and various approaches may reduce it further.

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