Complementary Constraints from Carbon and Nitrogen Isotopes on the Glacial Ocean's Soft-Tissue Pump and Circulation

Andreas Schmittner and Christopher SomesCollege of Earth, Ocean, and Atmospheric Sciences, Oregon State University, Corvallis, USA.Contact information: aschmitt@coas.oregonstate.edu

Figure 1: Schematic diagram of MOBI showing prognostic tracers (biological on left and inorganic on right) and fluxes between them. Fractionation factors and typical delta values are also indicated.

Methods● Model of Ocean Biogeochemistry and Isotopes coupled to University of Victoria (UVic) model version 2.9. No sediments.● LGM orbital parameters, atmospheric CO

2 level (185 ppm) and ice

sheets. Wind changes and sea level effects on benthic denitrification not considered.● Physics not inconsistent with reconstructions: ΔSAT = -4.3K, sea ice area increased by 70%, AMOC is weaker (11 Sv) and shallower than PI (17 Sv), bottom waters are saltier, deep ocean has higher radiocarbon age Δ14Cage(z>2 km) = +600 a.● We evaluate the effect of idealized changes of the biological pump by varying maximum phytoplankton growth rates μ

max (mimics iron

fertilization).

Introduction● Was biological pump enhanced during LGM? ● If so, why? Changes in biology (e.g. iron fertilization) and in physics (e.g. ocean circulation and sea ice cover) have been implicated. ● Explore effects on ocean carbon cycle and isotope distributions.

Figure 3: Zonally averaged dissolved oxygen in piCtrl (top) and changes LGM-piCtrl (bottom color scale).

Results:

Figure 2: Global horizontally averaged profiles.

Figure 5: δ15N changes from reconstructions (colored circles; Galbraith et al., 2013) and simulated by the models.

Figure 6: LGM δ13CDIC

comparison with reconstructions (Peterson et al. 2014). Left: horizontal basin-wide

averages. Right: zonal averages of best fitting model.

Conclusions:● Carbon and nitrogen isotopes provide complementary constraints on glacial ocean's biological pump.● Carbon isotopes sensitive to circulation and Corg distribution, nitrogen isotopes sensitive to iron fertilization.● Best fitting models (Tab. 2) indicate increased biological carbon storage of 500-700 Pg (Tab. 1).

Figure 4: Zonally averaged δ13CDIC

.

Conclusions:● Carbon and nitrogen isotopes provide complementary constraints on glacial ocean's biological pump.● Carbon isotopes sensitive to circulation and Corg distribution, nitrogen isotopes sensitive to iron fertilization.● Best fitting models (Tab. 2) indicate increased biological carbon storage of 500-700 Pg (Tab. 1).

References: ● Schmittner and Somes (2016) Paleoceanogr., doi:10.1002/2015PA002905.● Galbraith et al. (2013) Nat. Geosci., doi:10.1038/ngeo1832Peterson et al. (2014) Paleoceanogr., doi:10.1002/2013PA002552.