TY - JOUR A1 - Sigman, Daniel M. A1 - DiFiore, Peter J. A1 - Hain, Mathis P. A1 - Deutsch, Curtis A1 - Wang, Yi A1 - Karl, David M. A1 - Knapp, Angela N. A1 - Lehmann, Moritz F. A1 - Pantoja, Silvio T1 - The dual isotopes of deep nitrate as a constraint on the cycle and budget of oceanic fixed nitrogen N2 - 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 delta N-15 and delta O-18 in the ocean interior. The delta O-18 of nitrate is our focus because it has been explored less in previous work. Denitrification raises the delta N-15 and delta O-18 of mean ocean nitrate by equal amounts above their input values for N-2 fixation (for delta N-15) and nitrification (for delta O-18), generating parallel gradients in the delta N-15 and delta O-18 of deep ocean nitrate. Partial nitrate assimilation in the photic zone also causes equivalent increases in the delta N-15 and delta O-18 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 delta N-15 of the regenerated nitrate is equivalent to that originally consumed, whereas the regeneration replaces nitrate previously elevated in delta O-18 due to denitrification or nitrate assimilation with nitrate having the delta O-18 of nitrification. This lowers the delta O-18 of mean ocean nitrate and weakens nitrate delta O-18 gradients in the interior relative to those in delta N-15. 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 O-18 and delta N-15 within the ocean interior and a mean ocean nitrate delta O-18 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 delta N-15 and delta O-18 of preformed nitrate and/or too little cycling of intermediate-depth nitrate through the low latitude photic zone. Y1 - 2009 UR - http://www.sciencedirect.com/science/journal/09670637 U6 - https://doi.org/10.1016/j.dsr.2009.04.007 SN - 0967-0637 ER - TY - JOUR A1 - Sigman, Daniel M. A1 - DiFiore, Peter J. A1 - Hain, Mathis P. A1 - Deutsch, Curtis A1 - Karl, David M. T1 - Sinking organic matter spreads the nitrogen isotope signal of pelagic denitrification in the North Pacific N2 - Culture studies of denitrifying bacteria predict that denitrification will generate equivalent gradients in the delta N-15 and delta O-18 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 O-18 than in delta N-15 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-delta N-15 signal of pelagic denitrification into waters well below and beyond the suboxic zone, whereas the nitrate delta O-18 signal of denitrification can only be transmitted by circulation in the interior. Y1 - 2009 UR - http://www.agu.org/journals/gl/ U6 - https://doi.org/10.1029/2008gl035784 SN - 0094-8276 ER - TY - JOUR A1 - Ren, Haojia A1 - Sigman, Daniel M. A1 - Meckler, Anna Nele A1 - Plessen, Birgit A1 - Robinson, Rebecca S. A1 - Rosenthal, Yair A1 - Haug, Gerald H. T1 - Foraminiferal isotope evidence of reduced nitrogen fixation in the Ice Age Atlantic ocean N2 - 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 N-15/N-14 ratio from the last ice age is higher than that from the current interglacial, indicating a higher nitrate N-15/N-14 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. Y1 - 2009 UR - http://www.sciencemag.org/ U6 - https://doi.org/10.1126/science.1165787 SN - 0036-8075 ER - TY - JOUR A1 - Jaccard, Samuel Laurent A1 - Galbraith, Eric D. A1 - Sigman, Daniel M. A1 - Haug, Gerald H. A1 - Francois, Roger A1 - Pedersen, Thomas F. A1 - Dulski, Peter A1 - Thierstein, Hans R. T1 - Subarctic Pacific evidence for a glacial deepening of the oceanic respired carbon pool N2 - 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 redox- sensitive 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 deep-water 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 deleted in ox en 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. Y1 - 2009 UR - http://www.sciencedirect.com/science/journal/0012821X U6 - https://doi.org/10.1016/j.epsl.2008.10.017 SN - 0012-821X ER - TY - JOUR A1 - Haug, Gerald H. A1 - Sigman, Daniel M. T1 - Palaeoceanography : polar twins N2 - 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. Y1 - 2009 UR - http://www.nature.com/ngeo U6 - https://doi.org/10.1038/Ngeo423 SN - 1752-0894 ER -