Reconstructing Glacial Nitrogen and Carbon Cycling Using Isotopes

Description

Measurements of nitrogen and carbon isotopes from deep-sea sediments document dramatic changes between the last ice age and the modern ocean. However, both nitrogen and carbon isotopes are influenced by different physical and biogeochemical processes, which makes their interpretation ambiguous. In this research, a process-based coupled model of climate, ocean circulation and biogeochemistry that includes carbon and nitrogen isotopes has been used in conjunction with sediment data in order to reconstruct glacial nitrogen and carbon cycles. The model was amended with explicit iron cycling and compared with global databases of nitrogen and carbon isotopes from Last Glacial Maximum and modern ocean sediments. This approach allows testing of hypotheses concerning changes in the total ocean nitrogen inventory, polar nutrient consumption due to iron fertilization and their effects on atmospheric CO2 concentrations.

Top: Observed core-top δ15N distribution from NICOPP. Bottom: as above but results from best fitting model. From Somes et al. (2013).

Funded by

the National Science Foundation's Marine Geology and Geophysics Program

Project Report 2015 Project Report 2014 Project Report 2013

Results

Nitrogen cycle and isotopes: The study by Somes et al. (2013) provides new estimates of global nitrogen fluxes for the pre-industrial ocean using the nitrogen isotope model and the Late Holocene data set assembled by NICOPP (figure). We estimate nitrogen fixation, water column denitrification, and benthic denitrification to be between 195-350 (225), 65-80 (76), and 130-270 (149) TgN/yr, respectively, with the best esimate in parenthesis. This paper also emphasizes and quantifies uncertainties for these estimates due to nutrient utlilization during water column denitrification and the fractionation factor for benthic denitrification.

Carbon isotopes: The paper by Schmittner et al. (2013) provides new insights on how biological fractionation and air sea gas exchange affect the preindustrial and modern distributions of carbon isotopes in the ocean. We quantify, for the first time, the effects of air-sea gas exchange on biologically created gradients in δ13C and the temperature dependent fractionation during air-sea gas exchange separately. We find they are both about equally important.

Nitrogen and Carbon Isotopes During the Last Glacial Maximum (LGM): A manuscript by Schmittner and Somes (submitted manuscript) presents the first, to our knowledge, global model simulations of the LGM that include both isotopes. It presents idealized simulations in which the efficiency of the biological pump has been changed by increasing phytoplankton growth rates, its effects on carbon and nitrogen cycles are analyzed, and its carbon and nitrogen isotope distributions have been compared with reconstructions from LGM sediments. The results demonstrate that carbon and nitrogen isotopes provide complementary constraints on reconstructions of the biological pump. Model simulations that fit both isotopes best have moderately increased phytoplankton growth rates (15-30%), increased surface nutrient utilization, a more efficient biological pump, similar inventories and global fluxes of fixed nitrogen and a smaller ocean carbon inventory. The latter suggests that whole ocean alkalinity changes and interactions with sediments (calcium carbonate compensation), which were not included in the simulations, are important to explain the glacial carbon inventory.

Data

Publications
  • Schmittner, A., and C. J. Somes (submitted)
    Complementary Constraints from Carbon (13C) and Nitrogen (15N) Isotopes on the Efficiency of the Glacial Ocean's Biological Pump
    Paleoceanography.
  • Lund, D. C., Tessin, A. C., Hoffman, J. L., and Schmittner, A. (2015)
    Southwest Atlantic watermass evolution during the last deglaciation
    Paleoceanography. 30, doi:10.1002/2014PA002657
    html pdf
  • Schmittner, A., and Lund, D. C. (2015)
    Early deglacial Atlantic overturning decline and its role in atmospheric CO2 rise inferred from carbon isotopes (δ13C)
    Climate of the Past, 11, 135-152.
    html pdf data press release
  • Schmittner, A., Gruber, N., Mix, A. C., Key, R. M., Tagliabue, A., and Westberry, T. K. (2013)
    Biology and air-sea gas exchange controls on the distribution of carbon isotope ratios (δ13C) in the ocean
    Biogeosciences, 10, 5793-5816, doi:10.5194/bgd-10-5793-2013.
  • Somes, C. J., Oschlies, A., and Schmittner, A. (2013)
    Isotopic constraints on the pre-industrial oceanic nitrogen budget
    Biogeosciences, 10, 5889-5910, doi:10.5194/bgd-10-5889-2013.
  • Galbraith, E. D., Kienast, M., Albuquerque, A. L., Altabet, M., Batista, F., Bianchi, D., Calvert, S. E., Contreras Quintana, S., Crosta, X., De Pol Holz, R., Dubois, N., Etourneau, J., Francois, R., Hsu, T.-C., Ivanochko, T., Jaccard, S. L., Kao, S.-J., Kiefer, T., Kienast, S., Lehmann, M. F., Martinez, P., McCarthy, M., Meckler, A. N., Mix, A. C., Mobius, J., Pedersen, T. F., Quan, T. M., Robinson, R. S., Ryabenko, E., Schmittner, A., Schneider, R., Schneider-Mor, A., Shigemitsu, M., Sinclair, D., Somes, C., Studer, A. S., Tesdal, J.-E., Thunell, R., and Yang, J.-Y. T. (2012)
    The acceleration of oceanic denitrification during deglacial warming,
    Nature Geosc., 6, 579-584, doi:10.1038/ngeo1832.
  • Robinson, R. S., Kienast, M., Albuquerque, A. L., Altabet, M., Contreras Quintana, S., De Pol Holz, R., Dubois, N., Francois, R., Galbraith, E., Hsu, T.-C., Ivanochko, T., Jaccard, S., Kao, S.-J., Kiefer, T., Kienast, S., Lehmann, M., Martinez, P., McCarthy, M., Mobius, J., Pedersen, T., Quan, T. M., Ryabenko, E., Schmittner, A., Schneider, R., Schneider-Mor, A., Shigemitsu, M., Sinclair, D., Somes, C., Studer, A., Thunell, R., and Yang, J.-Y. (2012)
    A review of nitrogen isotopic alteration in marine sediments
    Paleoceanogr., 24, PA4203, doi: 10.1029/2012PA002321.

    • Presentations
  • Schmittner, A., Egbert, G., and Green, M. (2014)
    Modeling Tidal Mixing, Past, Present, and Future (Oral)
    Ocean Sciences Meeting, Feb. 27, Honolulu pdf

  • Muglia, J., and Schmittner, A. (2013)
    Atlantic Circulation During the Last Glacial Maximum Simulated by PMIP3 Climate Models (Poster)
    PMIP Ocean Workshop 2013, Understanding Changes since the Last Glacial Maximum, Dec. 4-6, Corvallis, Oregon, USA.

  • Somes, C., Schmittner, A., and Oschlies, A. (2013)
    Isotopic constraints on the global oceanic nitrogen inventory in the modern and glacial oceans (Poster)
    11th International Conference on Paleoceanography, Sep. 1-6, Barcelona-Sitges, Catalonia-Spain.

  • Muglia, J., and Schmittner, A. (2013)
    Ocean Circulation During the Last Glacial Maximum Simulated by PMIP3 Climate Models (Poster)
    Key Uncertainties in the Global Carbon Cycle: Perspectives across terrestrial and ocean ecosystems, Aug. 6-10, 2013, NCAR, Boulder, Colorado, USA.

  • Somes, C., Oschlies, A., and Schmittner, A. (2012)
    Isotopic constraints on the pre-industrial fixed nitrogen budget (Poster)
    American Geophysical Union Fall Meeting, Dec. 3-7, San Francisco, California, USA.

  • Breeden, M., and Schmittner, A. (2012)
    Last Glacial Maximum Ocean Circulation Simulated by PMIP3 Climate Models (Poster)
    American Geophysical Union Fall Meeting, Dec. 3-7, San Francisco, California, USA.

  • Somes, C., Oschlies, A., and Schmittner, A. (2012)
    Constraining rates of N2 fixation and denitrification in the ocean using NO3:PO4 ratios (Oral)
    Ocean Sciences Meeting, Feb. 20-24, Salt Lake City, USA.

  • Somes, C., Schmittner, A., and Oschlies, A. (2012)
    Isotopic constraints on the global oceanic nitrogen inventory in the modern and glacial oceans (Poster)
    International Conference on Earth System Modelling, Sep. 17-21, Hamburg, Germany.

  • Somes, C. (2011)
    Changes in the nitrogen cycle in the modern and glacial ocean (Poster)
    XVIII International Union for Quaternary Research Congress, Jul. 21-27, Bern, Switzerland.

  • Somes, C. (2011)
    Using nitrogen isotopes and xsP to constrain the fixed-N budget in the past and present ocean (Oral)
    2nd NICOPP Meeting: Sedimentary δ15N - data synthesis, analysis and modeling, Jul. 8-10, Zürich, Switzerland.

  • Somes, C. (2011)
    Using nitrogen isotopes and xsP to constrain the fixed-N budget in the past and present ocean (Oral)
    2nd NICOPP Meeting: Sedimentary δ15N - data synthesis, analysis and modeling, Jul. 8-10, Halifax, Canada.

  • Somes, C. (2011)
    Changes in the nitrogen cycle in the modern and glacial ocean (Oral)
    43th International Liège Colloquium on Ocean Dynamics Tracers of physical and biogeochemical processes, past changes and ongoing anthropogenic impacts, May. 2-6, Liège, Belgium.

  • Somes, C. (2010)
    Modeling the distribution of nitrogen isotopes in the global ocean (Oral)
    FB2 Seminar, Leibnitz-Institute of Marine Sciences (IFM-GEOMAR), Kiel, Germany.

    • Outreach

    People