Bibliography - P. J. DiFiore

1. Cassar, N., P. J. DiFiore, B. Barnett, Michael Bender, A. R. Bowie, Bronte Tilbrook, K. Petrou, K. J. Westwood, S. W. Wright, and D. Lefevre, 2011: The influence of iron and light on net community production in the Subantarctic and Polar Frontal Zones. Biogeosciences, Copernicus Publications, 8, doi:10.5194/bg-8-227-2011 227-237
   [ Abstract ]

   The roles of iron and light in controlling biomass and primary productivity are clearly established in the Southern Ocean. However, their influence on net community production (NCP) and carbon export remains to be quantified. To improve our understanding of NCP and carbon export production in the Subantarctic Zone (SAZ) and the northern reaches of the Polar Frontal Zone (PFZ), we conducted continuous onboard determinations of NCP as part of the Sub-Antarctic Sensitivity to Environmental Change (SAZ Sense) study, which occurred in January-February 2007. Biological O₂ supersaturation was derived from measuring O₂/Ar ratios by equilibrator inlet mass spectrometry. Based on these continuous measurements, NCP during the austral summer 2007 in the Australian SAZ was approximately 43 mmol O₂ m^-2 d^-1. NCP showed significant spatial variability, with larger values near the Subtropical front, and a general southward decrease. For shallower mixed layers (<50 m), dissolved Fe concentrations and Fe sufficiency, estimated from variable fluorescence, correlated strongly with NCP. The strong correlation between NCP and dissolved Fe may be difficult to interpret because of the correlation of dissolved Fe to MLD and because the concentration of iron may not be a good indicator of its availability. At stations with deeper mixed layers, NCP was consistently low, regardless of iron sufficiency, consistent with light availability also being an important control of NCP. Our new observations provide independent evidence for the critical roles of iron and light in mediating carbon export from the Southern Ocean mixed layer.

2. 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.

3. 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./

4. 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.

5. 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.

6. 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.

7. 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.

Direct link to page: http://cmi.princeton.edu/bibliography/results.php?author=4151