Bibliography - M. B. Hendricks

1. 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.

2. 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.

3. 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.

4. 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.

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