Sack Lunch Seminar (SLS)

Quantitative and mechanistic understanding of the changes in atmospheric CO2 concentrations and ocean carbon content between glacial and interglacial periods remains lacking. An increase in the efficiency of the biological pump has been hypothesized to contribute to higher ocean carbon storage during glacial periods. Here we use a model of the ocean's biogeochemical cycles that includes both carbon (13C) and nitrogen (15N) isotopes but no sediment interactions. We present results from one present day simulation and six simulations of the Last Glacial Maximum (LGM, ~20 ka before the present). The LGM simulations are idealized sensitivity experiments that explore effects of changes in maximum phytoplankton growth rates (mmax). Increasing mmax in the model leads to a more efficient biological pump, more carbon storage, and lower d13CDIC and oxygen concentrations. The resulting increase of denitrification stimulates additional nitrogen fixation and increases the spatial variance of d15NNO3, while decreasing the ocean’s fixed nitrogen inventory. Increased nitrogen fixation lowers surface d15NNO3 in most of the tropics. In the model’s Southern Ocean modest increases in mmax result in higher d15NNO3 due to enhanced local nutrient utilization, consistent with reconstructions, but larger mmax cause declining values there owing to the poleward transport of low tropical d15NNO3. Comparison to reconstructions from LGM sediments indicates that models with moderately increased mmax (by 16 - 33 %) fit both isotope data best, whereas large increases are inconsistent with nitrogen isotopes although they still fit the carbon isotopes reasonably well. The best fitting models reproduce major features of the glacial d13CDIC, d15N, and oxygen reconstructions, while simulating reduced carbon storage, compared with the pre-industrial ocean, due to lower preformed carbon concentrations. We conclude that the biological pump was more efficient during the LGM. However, sediment interactions and whole ocean alkalinity changes may be required to increase ocean carbon storage. Our analysis illustrates interactions between the carbon and nitrogen cycles as well as the complementary constraints provided by their isotopes.