Bibliography - Michael Bender

1. Hamme, Roberta C., N. Cassar, Veronica P. Lance, Robert D. Vaillancourt, Michael Bender, Peter G. Strutton, Tommy S. Moore, Michael D. DeGrandpre, C. L. Sabine, D. T. Ho, and Bruce R. Hargreaves, 2012: Dissolved O₂/Ar and other methods reveal rapid changes in productivity during a Lagrangian experiment in the Southern Ocean. Journal of Geophysical Research, American Geophysical Union, 117(C00F12), doi:10.1029/2011JC007046
   [ Abstract ]

   We use continuous and discrete measurements of the dissolved O₂/Ar ratio in the mixed layer to investigate the dynamics of biological productivity during the Southern Ocean Gas Exchange Experiment in March and April 2008. Injections of SF₆ defined two water masses (patches) that were followed for up to 2 weeks. In the first patch, dissolved O₂/Ar was supersaturated, indicating net biological production of organic carbon. In the second patch, rapidly decreasing O₂/Ar could only be reasonably explained if the mixed layer was experiencing a period of net heterotrophy. The observations rule out dominant contributions from vertical mixing, lateral dilution, or respiration in the ship's underway seawater supply lines. We also compare nine different estimates of net community, new, primary, or gross production made during the experiment. Net community and new production estimates agreed well in the first patch but disagreed in the second patch, both during an initial net heterotrophic period but also during the apparently autotrophic period at the end of the observations. Rapidly changing productivity during the second patch complicated the comparison of methods that integrate over daily and several week timescales. Primary productivity values from on-deck 24 h ¹⁴C incubations and gross carbon production values from photosynthesis-irradiance experiments were nearly identical even during highly dynamic periods of net heterotrophy, while gross oxygen production measurements were 3.5—4.2 times higher but with uncertainties in that ratio near ±2. These comparisons show that the photosynthesis-irradiance experiments based on 1—2 h ¹⁴C incubations underestimated gross carbon production.

2. Huang, Kuan, Hugh Ducklow, Maria Vernet, N. Cassar, and Michael Bender, 2012: Export production and its regulating factors in the West Antarctica Peninsula region of the Southern Ocean. Global Biogeochemical Cycles, American Geophysical Union, 26(GB2005), doi:10.1029/2010GB004028
   [ Abstract ]

   In connection with the Palmer LTER program, mixed layer water samples were collected during the cruise of the L.M. Gould in Jan., 2008 at 49 stations on a 20 x 100 km grid in the West Antarctica Peninsula (WAP) region of the Southern Ocean. In this study, [O₂]/[Ar] ratios and the triple isotope composition of dissolved O₂ were measured, and were used to estimate net community O₂ production (NCP) and gross primary O₂ production (GPP), respectively. These estimates are further converted to carbon export production, primary production and the f-ratio. Our measurements give NCP ranging from -3 to 76 mmol O₂ m^-2 day ^-1 (-25 to 650 mg C m ^-2 day ^-1), and GPP from 40 to 220 mmol O₂ m ^-2 day ^-1 (180 to 1010 mg C m ^-2 day ^-1). The O₂ NCP/GPP ratios range from -0.04 to 0.43, corresponding to f-ratios of -0.08 to 0.83. NCP and the NCP/GPP ratio are highest in the northern coastal areas, and decrease to lower values toward the southern coastal area and the open ocean. The inshore-offshore gradient appears to be regulated primarily by iron availability, as supported by the positive correlation between NCP and F_v/F_m ratios (r² = 0.22, p < 0.05). Mixed layer depth (MLD) is inversely correlated with NCP (r² = 0.21, p < 0.002) and NCP/GPP (r² = 0.21, p < 0.02), and highest NCP occurred in the fresh water lenses probably formed from melted coastal glaciers. These results suggest that export production and the f-ratio increase where water stratification is intensified by input of fresh meltwater, and that mixed layer stratification is the major factor regulating NCP in the inner-shelf and coastal regions. Along-shelf variability of phytoplankton community composition is highly correlated with NCP, i.e., NCP increases when the diatom-dominated community in the south transitions to the cryptophyte-dominated one in the north. A high correlation is also observed between NCP and the logarithm of the surface chlorophyll concentration (r² = 0.72, p < 0.0001), which makes it possible to estimate carbon export as a function of Chl a concentration in this region.

3. Bender, Michael, Saul Kinter, N. Cassar, and R. Wanninkhof, 2011: Evaluating gas transfer velocity parameterizations using upper ocean radon distributions. Journal of Geophysical Research, American Geophysical Union, 116(C02010), doi:10.1029/2009JC005805
   [ Abstract ]

   Sea-air fluxes of gases are commonly calculated from the product of the gas transfer velocity (k) and the departure of the seawater concentration from atmospheric equilibrium. Gas transfer velocities, generally parameterized in terms of wind speed, continue to have considerable uncertainties, partly because of limited field data. Here we evaluate commonly used gas transfer parameterizations using a historical data set of ²²²Rn measurements at 105 stations occupied on Eltanin cruises and the Geosecs program. We make this evaluation with wind speed estimates from meteorological reanalysis products (from National Centers for Environmental Prediction and European Centre for Medium-Range Weather Forecasting) that were not available when the ²²Rn data were originally published. We calculate gas transfer velocities from the parameterizations by taking into account winds in the period prior to the date that ²²²Rn profiles were sampled. Invoking prior wind speed histories leads to much better agreement than simply calculating parameterized gas transfer velocities from wind speeds on the day of sample collection. For individual samples from the Atlantic Ocean, where reanalyzed winds agree best with observations, three similar recent parameterizations give k values for individual stations with an rms difference of ~40% from values calculated using ²²²Rn data. Agreement of basin averages is much better. For the global data set, the average difference between k constrained by ²²²Rn and calculated from the various parameterizations ranges from -0.2 to +0.9 m/d (average, 2.9 m/d). Averaging over large domains, and working with gas data collected in recent years when reanalyzed winds are more accurate, will further decrease the uncertainties in sea-air fluxes.

4. 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.

5. Dreyfus, G., J. Jouzel, Michael Bender, and Amaelle Landais, et al., 2010: Firn processes and δ¹⁵N: potential for a gas-phase climate proxy. Quaternary Science Reviews, 29(28-42), doi:10.1016/j.quascirev.2009.10.012
   [ Abstract ]

   In order to quantify the sequence of events between changes in atmospheric composition and climate changes recorded in ice cores, we must accurately account for the age difference between ice and gas at a given depth. This gas age–ice age difference depends on the age of the ice at the bottom of the firn layer, where the bubbles are closed-off. Firn densification models are used to calculate how this age difference varied in the past, but have an uncertainty on the order of 1000 years for central Antarctic sites. Here we explore the possibility that δ¹⁵N of N² is a gas phase proxy of climate, which can be used to synchronize gas and ice records. We present the δ¹⁵N record from the EPICA Dome C (EDC) ice core covering the last three glacial terminations and five glacial-interglacial cycles between 300 and 800 ka. Previous studies have shown that gravitational settling enriches δ¹⁵N as a function of the diffusive column height in the firn. If densification models’ prediction of deeper firn close-off under glacial conditions is correct, then we would expect heavier δ¹⁵N during glacial periods, and a negative correlation with temperature. Instead, EDC δ¹⁵N is positively correlated with the ice deuterium content, a proxy for temperature, as previously reported at Vostok, Dome Fuji, and EPICA Dronning Maud Land. We propose a mechanism that links accumulation rate, firn permeability, and convective mixing in the top meters of the firn to explain this correlation between δ¹⁵N and ice deuterium content. The tightest correlation is observed over glacial terminations, supporting the idea that δ¹⁵N is a property in the gas phase that records changes in surface conditions linked to deglacial warming.

6. Cassar, N., B. Barnett, Michael Bender, J. Kaiser, Roberta C. Hamme, and Bronte Tilbrook, 2009: Continuous High-Frequency Dissolved O₂/Ar Measurements by Equilibrator Inlet Mass Spectrometry. Analytical Chemistry, 81, doi:10.1021/ac802300u 1855-1864
   [ Abstract ]

   The oxygen (O₂) concentration in the surface ocean is influenced by biological and physical processes. With concurrent measurements of argon (Ar), which has similar solubility properties as oxygen, we can remove the physical contribution to O₂ supersaturation and determine the biological oxygen supersaturation. Biological O₂ supersaturation in the surface ocean reflects the net metabolic balance between photosynthesis and respiration, i.e., the net community productivity (NCP). We present a new method for continuous shipboard measurements of O₂/Ar by equilibrator inlet mass spectrometry (EIMS). From these measurements and an appropriate gas exchange parametrization, NCP can be estimated at high spatial and temporal resolution. In the EIMS configuration, seawater from the ship’s continuous intake flows through a cartridge enclosing a gas-permeable microporous membrane contactor. Gases in the headspace of the cartridge equilibrate with dissolved gases in the flowing seawater. A fused-silica capillary continuously samples headspace gases, and the O₂/Ar ratio is measured by mass spectrometry. The ion current measurements on the mass spectrometer reflect the partial pressures of dissolved gases in the water flowing through the equilibrator. Calibration of the O₂/Ar ion current ratio (32/40) is performed automatically every 2 h by sampling ambient air through a second capillary. A conceptual model demonstrates that the ratio of gases reaching the mass spectrometer is dependent on several parameters, such as the differences in molecular diffusivities and solubilities of the gases. Laboratory experiments and field observations performed by EIMS are discussed. We also present preliminary evidence that other gas measurements, such as N₂/Ar and pCO₂ measurements, may potentially be performed with EIMS. Finally, we compare the characteristics of the EIMS with the previously described membrane inlet mass spectrometry (MIMS) approach.

7. Bender, Michael, B. Barnett, G. Dreyfus, J. Jouzel, and D. Porcelli, 2008: The contemporary degassing rate of ⁴⁰Ar from the solid Earth. Proceedings of the National Academy of Sciences of the United States of America, 105(24), doi:10.1073/pnas.0711679105
   [ Abstract ]

   Knowledge of the outgassing history of radiogenic ⁴⁰Ar, derived over geologic time from the radioactive decay of ⁴⁰K, contributes to our understanding of the geodynamic history of the planet and the origin of volatiles on Earth’s surface. The ⁴⁰Ar inventory of the atmosphere equals total ⁴⁰Ar outgassing during Earth history. Here, we report the current rate of ⁴⁰Ar outgassing, accessed by measuring the Ar isotope composition of trapped gases in samples of the Vostok and Dome C deep ice cores dating back to almost 800 ka. The modern outgassing rate (1.1 ± 0.1 x 10⁸ mol/yr) is in the range of values expected by summing outgassing from the continental crust and the upper mantle, as estimated from simple calculations and models. The measured outgassing rate is also of interest because it allows dating of air trapped in ancient ice core samples of unknown age, although uncertainties are large (±180 kyr for a single sample or ±11% of the calculated age, whichever is greater).

8. Cassar, N., G. McKinley, Michael Bender, R. Mika, and M. Battle, 2008: An improved comparison of atmospheric Ar/N₂ time series and paired ocean-atmosphere model predictions. Journal of Geophysical Research – Atmosphere, 113(D21122), doi:10.1029/2008JD009817
   [ Abstract ]

   Ar/N₂ variations in the atmosphere reflect ocean heat fluxes, air-sea gas exchange, and atmospheric dynamics. Here atmospheric Ar/N₂ time series are compared to paired ocean-atmosphere model predictions. Agreement between Ar/N₂ observations and simulations has improved in comparison to a previous study because of longer time series and the introduction of automated samplers at several of the atmospheric stations, as well as the refinement of the paired ocean-atmosphere models by inclusion of Ar and N₂ as active tracers in the ocean component. Although analytical uncertainties and collection artifacts are likely to be mainly responsible for observed Ar/N₂ outliers, air parcel back-trajectory analysis suggests that some of the variability in Ar/N₂ measurements could be due to the low-altitude history of the air mass collected and, by extension, the local oceanic Ar/N₂ signal. Although the simulated climatological seasonal cycle can currently be evaluated with Ar/N₂ observations, longer time series and additional improvements in the signal-to-noise ratio will be required to test other model predictions such as interannual variability, latitudinal gradients, and the secular increase in atmospheric Ar/N₂ expected to result from ocean warming.

9. Suwa, M., and Michael Bender, 2008: Chronology of the Vostok ice core constrained by O₂/N₂ ratios of occluded air, and its implication for the Vostok climate records. Quaternary Science Reviews, 27(11-12), doi:10.1016/j.quascirev.2008.02.017 1093-1106
   [ Abstract ]

   We present a timescale for the Vostok ice core that is derived by orbitally tuning to O₂/N₂ ratios in occluded air for depths deeper than 1550m (>112 ka), and by gas correlation to the GISP2 chronology for the section shallower than 1422m (<102 ka). Our chronology of the deeper section rests on the assumption that, during the bubble close off process, local summer insolation indirectly controls the extent of O₂ exclusion and hence the O₂/N₂ ratio in trapped gases. The newly derived O₂/N₂ chronology is consistent with absolutely dated speleothem records. The O₂/N₂ chronology differs from previously published orbital tuning chronologies (CH₄ and δ¹⁸O_atm) by up to ~ ±6 kyr, and from the original GT4 chronology by up to ~15 kyr. The difference between the O₂/N₂ chronology and the δ¹⁸O_atm chronology varies in time with strong signals centered at 1/100 and 1/41 kyr^-1. The ages for the last four glacial terminations in Vostok correspond to high obliquity (>23.7° at terminations’ midpoints). They also correspond with decreasing precession index, corresponding to increasing boreal summer insolation. The Vostok temperature record, boreal summer insolation, and the rate of change of the SPECMAP property (reflecting planktonic foram δ¹⁸O) with respect to time are highly coherent at precession and obliquity periods. These three properties vary almost synchronously, with the possibility that Vostok temperature lags behind the other two. Our new timescale supports the widespread view that boreal summer insolation played an important role in glacial–interglacial cycles.

10. Suwa, M., and Michael Bender, 2008: O₂/N₂ ratios of occluded air in the GISP2 ice core. Journal of Geophysical Research – Atmosphere, 113(D1119), doi:10.1029/2007JD009589
    [ Abstract ]

    We present and discuss the record of O₂/N₂ ratios in air occluded in the GISP2 ice core retrieved from Summit, Greenland. In this study, we examine results for 601 samples from 331 depths measured by 1994, and 92 samples from 46 depths newly measured in 2006. Poorly replicated samples, samples with no replicates and samples falling more than three standard deviations from the mean of each δO₂/N₂ data set are excluded from the analysis. The majority of poorly replicated samples are from the depth zone associated with the transition between gas in bubbles and gas present as clathrate hydrates. We found that the O₂/N₂ ratio of samples is depleted by an average value of 7.3% during 11 years of storage at -35°C O₂/N₂ ratios measured at various times were corrected for depletion during storage and combined to form a single data set. The stacked GISP2 δO₂/N₂ record shows strong spectral power at the orbital frequencies, and δO₂/N₂ is in antiphase with local summer insolation. This observation is consistent with the earlier findings for the Vostok and Dome Fuji ice cores from East Antarctica. It validates previous conclusions that fractionation during bubble close-off depends on ice grain properties set at the surface by solar insolation. In addition, the GISP2 δO₂/N₂ record shows millennial duration signals that are in phase with the local temperature record of rapid climate change. The exact mechanisms by which local summer insolation induces orbital variability in δO₂/N₂, and by which temperature (or related properties) induce high frequency signals in δO₂/N₂, remain to be identified.

11. Reuer, M. K., B. Barnett, Michael Bender, P. G. Falkowski, and M. B. Hendricks, 2007: New estimates of Southern Ocean biological production rates from O₂/Ar ratios and the triple isotope composition of O₂. Deep Sea Research I, 54(6), doi:10.1016/j.dsr.2007.02.007
    [ Abstract ]

    We report O₂/Ar ratios (a constraint on net community production) and the triple isotopic composition of dissolved O₂ (a constraint on gross primary production) in samples collected from the surface mixed layer on 23 Southern Ocean transits. Samples were collected at 1–2° meridional resolution during the austral summer. Methodological limitations notwithstanding, the results constrain the net/gross production ratio, net O₂ production, and gross O₂ production at unprecedented resolution throughout the Southern Ocean mixed layer. Gross O₂ production rates inferred from the oxygen triple isotopes are greater than production rates calculated from a model based on remotely sensed chlorophyll. This result agrees with previous ¹⁸O and ¹⁴C incubations along 170°W. O₂/Ar ratios exceeding saturation are consistently observed within the Subantarctic and Polar Frontal Zones south of New Zealand and Australia, showing that a net autotrophic community predominates during austral summer. Lower O₂/Ar values are observed within the Drake Passage and Antarctic Zone, suggesting unresolved influences of low net community production, net heterotrophy, and upwelling of O₂-undersaturated waters. In autotrophic waters of the austral summer mixed layer, ratios of net community production/gross O₂ production scatter about 0.13, corresponding to f ratios of ˜0.25. Net community/gross O₂ production ratios show no meridional gradient across the Antarctic Circumpolar Current, suggesting that an approximately constant fraction of gross primary productivity is regenerated or exported. Our calculated net O₂ production rates are in satisfactory agreement with comparable published estimates. Net and gross O₂ production rates are highest in the Subantarctic and decline to the south, paralleling the well-known trend of chlorophyll a concentrations. In an analysis of variance of net O₂ production and gross O₂ production with other environmental variables, the strongest correlations are between net O₂ production and sea surface temperature (SST) (direct correlation), climatological [NO₃] (inverse correlation), and estimates of primary productivity derived from a remote sensing (direct correlation). These trends are as expected if aerosol iron input is the most important influence on production. They are unexpected if upwelling-derived SiO₂ and iron are the leading influence or if lower SSTs promote greater export in this region.

12. Battle, M., S. E. Mikaloff-Fletcher, Michael Bender, R. F. Keeling, A. C. Manning, N. Gruber, P. P. Tans, M. B. Hendricks, D. T. Ho, C. Simonds, R. Mika, and B. Paplawsky, 2006: Atmospheric potential oxygen: New observations and their implications for some atmospheric and oceanic models. Global Biogeochemical Cycles, 20(GB1010), doi:10.1029/2005GB002534
    [ Abstract ]

    Measurements of atmospheric O₂/N₂ ratios and CO₂ concentrations can be combined into a tracer known as atmospheric potential oxygen (APO ≈ O₂/N₂ + CO₂) that is conservative with respect to terrestrial biological activity. Consequently, APO reflects primarily ocean biogeochemistry and atmospheric circulation. Building on the work of Stephens et al. (1998), we present a set of APO observations for the years 1996–2003 with unprecedented spatial coverage. Combining data from the Princeton and Scripps air sampling programs, the data set includes new observations collected from ships in the low-latitude Pacific. The data show a smaller interhemispheric APO gradient than was observed in past studies, and different structure within the hemispheres. These differences appear to be due primarily to real changes in the APO field over time. The data also show a significant maximum in APO near the equator. Following the approach of Gruber et al. (2001), we compare these observations with predictions of APO generated from ocean O₂ and CO₂ flux fields and forward models of atmospheric transport. Our model predictions differ from those of earlier modeling studies, reflecting primarily the choice of atmospheric transport model (TM3 in this study). The model predictions show generally good agreement with the observations, matching the size of the interhemispheric gradient, the approximate amplitude and extent of the equatorial maximum, and the amplitude and phasing of the seasonal APO cycle at most stations. Room for improvement remains. The agreement in the interhemispheric gradient appears to be coincidental; over the last decade, the true APO gradient has evolved to a value that is consistent with our time-independent model. In addition, the equatorial maximum is somewhat more pronounced in the data than the model. This may be due to overly vigorous model transport, or insufficient spatial resolution in the air-sea fluxes used in our modeling effort. Finally, the seasonal cycles predicted by the model of atmospheric transport show evidence of an excessive seasonal rectifier in the Aleutian Islands and smaller problems elsewhere.

13. Bender, Michael, D. T. Ho, M. B. Hendricks, R. Mika, M. Battle, P. P. Tans, T. J. Conway, B. Sturtevant, and N. Cassar, 2005: Atmospheric O₂/N₂ changes, 1993–2002: Implications for the partitioning of fossil fuel CO₂ sequestration. Global Biogeochemical Cycles, 19(GB4017), doi:10.1029/2004GB002410
    [ Abstract ]

    Improvements made to an established mass spectrometric method for measuring changes in atmospheric O₂/N₂ are described. With the improvements in sample handling and analysis, sample throughput and analytical precision have both increased. Aliquots from duplicate flasks are repeatedly measured over a period of 2 weeks, with an overall standard error in each flask of 3–4 per meg, corresponding to 0.6–0.8 ppm O₂ in air. Records of changes in O₂/N₂ from six global sampling stations (Barrow, American Samoa, Cape Grim, Amsterdam Island, Macquarie Island, and Syowa Station) are presented. Combined with measurements of CO₂ from the same sample flasks, land and ocean carbon uptake were calculated from the three sampling stations with the longest records (Barrow, Samoa, and Cape Grim). From 1994–2002, We find the average CO₂ uptake by the ocean and the land biosphere was 1.7 ± 0.5 and 1.0 ± 0.6 GtC yr^-1 respectively; these numbers include a correction of 0.3 Gt C yr^-1 due to secular outgassing of ocean O₂. Interannual variability calculated from these data shows a strong land carbon source associated with the 1997–1998 El Nin˜o event, supporting many previous studies indicating that high atmospheric growth rates observed during most El Ninõ events reflect diminished land uptake. Calculations of interannual variability in land and ocean uptake are probably confounded by non-zero annual air sea fluxes of O₂. The origin of these fluxes is not yet understood.

14. Kaiser, J., M. K. Reuer, B. Barnett, and Michael Bender, 2005: Marine productivity estimates from continuous oxygen/argon ratio measurements by membrane inlet mass spectrometry. Geophysical Research Letters, 32(L19605), doi:10.1029/2005GL023459
    [ Abstract ]

    Dissolved oxygen/argon (O₂/Ar) ratios in the oceanic mixed layer are indicative of net community production (NCP) because O₂ and Ar share similar physical solubility properties, but only O₂ is biologically produced and consumed. We describe a membrane inlet mass spectrometer (MIMS) that allows continuous high-precision shipboard analysis of O₂/Ar ratios and eventually other gases, calibrated with discrete samples analyzed in the laboratory. We also present O₂/Ar data from the eastern equatorial Pacific. Shortterm reproducibilities of 0.05% were achieved. Meridional gradients and small-scale phenomena were clearly resolved. O₂/Ar undersaturations around the equator reflect the interaction of biological and physical forcings. Mixed-layer NCP estimated from wind speed-gas exchange parameterizations was near zero north of 2.75°N, and about 12 mmol m^-2 d^-1 south of 6.75°S. Ar supersaturations, calculated from MIMS O₂/Ar measurements and accompanying O₂ concentration measurements, ranged from -0.8 to +3.0%.

15. Bender, Michael, 2003: Climate-biosphere interactions on glacial-interglacial timescales. Global Biogeochemical Cycles, 17(3), doi:10.1029/2002GB001932
    [ Abstract ]

    Potential positive feedbacks of the biosphere on glacial-interglacial climate change have been extensively investigated in recent years. In this paper, we summarize these feedbacks and the evidence that they may play a quantitatively significant role. We then attempt to assess the role of biosphere feedbacks in glacial/interglacial climate change by evaluating five lines of empirical evidence: (1) synchroneity of warming during the last glacial termination (expected if the biosphere is important, because of short response times); (2) changes in the δ¹⁸O of O₂, which may reflect the relative fertility of the land and ocean biospheres, (3) changes in the triple isotope composition of O₂, which constrain global rates of photosynthesis in the past; (4) the relationship between atmospheric CO₂ and dust accumulation at Vostok, and (5) indications for the occurrence or absence of Pleistocene-style glacial cycles before the evolution of the land biosphere. The evidence is compatible with a significant role for the biosphere in driving glacial-interglacial change, but unambiguous empirical support is not yet in hand.

16. Bender, Michael, 2002: Orbital tuning chronology for the Vostok climate record supported by trapped gas composition. Earth and Planetary Science Letters, 204(1-2), doi:10.1016/S0012-821X(02)00980-9 275-289
    [ Abstract ]

    We present data on the O₂/N₂ ratios of trapped gas samples for the entire length of the Vostok climate record. As in other cores, O₂/N₂ ratios in these samples are less than the atmospheric ratio, by a small and variable amount, because O₂ is selectively excluded during the gas trapping process, and because O₂ is also preferentially lost in poorly preserved core samples. Samples younger than 150ka have large and variable O₂ depletions. Samples older than 200 ka have O₂/N₂ ratios that replicate well and vary smoothly with depth. We plot O₂/N₂ ratios of well replicated samples older than 160ka, using a chronology derived by matching the δ¹⁸O of paleoatmospheric O₂ (δ¹⁸O_atm) to northern hemisphere June insolation. On this timescale, O₂/N₂ varies coherently with local (78±S) summertime insolation. Based on time series analysis of the O₂/N₂ record and the dynamics of snow metamorphism at the surface, we conclude that summertime insolation influences physical properties of ice grains that control the degree of O₂ exclusion during bubble closeoff. O₂/N₂ in Vostok is thus arguably a property that records local summertime insolation and can be used to test independent chronologies for the core. We show that the δ¹⁸O_atm chronology, supported by the coincidence of O₂/N₂ ratios with insolation, is also compatible with recent radiometric dating of corals from high sea stands. We further successfully test the N18Oatm tuning chronology by showing that it predicts a chronology for the GISP2 core which is essentially indistinguishable from the standard GISP2 chronology and, therefore, in excellent agreement with the radiometric chronology of Hulu Cave, China. An accurate chronology for the Vostok ice core is now in place.

17. Blunier, T., B. Barnett, Michael Bender, and M. B. Hendricks, 2002: Biological oxygen productivity during the last 60,000 years from triple oxygen isotope measurements. Global Biogeochemical Cycles, 16(3), doi:10.1029/2001GB001460
    [ Abstract ]

    The oxygen isotope signature of atmospheric O₂ is linked to the isotopic signature of seawater (H₂O) through photosynthesis and respiration. Fractionation during these processes is mass dependent, affecting δ ¹⁷O about half as much as δ ¹⁸O. An ‘‘anomalous’’ fractionation process, which changes δ ¹⁷O and δ ¹⁸O of O₂ about equally, takes place during isotope exchange between O₂ and CO₂ in the stratosphere. The relative rates of biologic O₂ production and stratospheric processing determine the relationship between δ ¹⁷O and δ ¹⁸O of O₂ in the atmosphere. Variations of this relationship thus allow us to estimate changes in the rate of mass-dependent O₂ production by photosynthesis versus the rate of O₂-CO₂ exchange in the stratosphere with about equal fractionations of δ ¹⁷O and δ ¹⁸O. In this study we reconstruct total oxygen productivity for the last glacial, the last glacial termination, and the early Holocene from the triple isotope composition of atmospheric oxygen trapped in ice cores.With a box model we estimate that total biogenic productivity was only ~76–83% of today for the glacial and was probably lower than today during the glacial-interglacial transition and the early Holocene. Depending on how reduced the oxygen flux from the land biosphere was during the glacial, the oxygen flux from the glacial ocean biosphere was 88–140% of its present value.

18. Keller, Klaus, R. D. Slater, Michael Bender, and R. M. Key, 2001: Possible biological or physical explanations for decadal scale trends in North Pacific nutrient concentrations and oxygen utilization. Deep Sea Research II, 49(103), doi:10.1016/S0967-0645(01)00106-0 345-362
    [ Abstract ]

    We analyze North Pacific GEOSECS (1970s) and WOCE (1990s) observations to examine potential decadal trends of the marine biological carbon pump. Nitrate concentrations {[NO₃]} and apparent oxygen utilization (AOU) decreased significantly in intermediate waters (by - 0.6 and - 2.9 μmol kg^-1; respectively, at σ_θ = 27.4 kg m^-3; corresponding to ≈ 1050 m). In shallow waters (above roughly 750 m) [NO₃] and AOU increased, though the changes were not statistically significant. A sensitivity study with an ocean general circulation model indicates that reasonable perturbations of the biological carbon pump due to changes in export production or remineralization efficiency are insufficient to account for the intermediate water tracer trends. However, changes in water ventilation rates could explain the intermediate water tracer trends and would be consistent with trends of water age derived from radiocarbon. Trends in AOU and [NO₃] provide relatively poor constraints on decadal scale trends in the marine biological carbon pump for two reasons. First, most of the expected changes due to decadal scale perturbations of the marine biota occur in shallow waters, where the available data are typically too sparse to account for the strong spatial and temporal variability. Second, alternative explanations for the observed tracer trends (e.g., changes in the water ventilation rates) cannot be firmly rejected. Our data analysis does not disprove the null hypothesis of an unchanged biological carbon pump in the North Pacific.

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