A Three-Dimensional Model of the Marine Nitrogen Cycle during the Last Glacial Maximum Constrained by Sedimentary Isotopes

Abstract

Nitrogen is a key limiting nutrient that influences marine productivity and carbon sequestration in the ocean via the biological pump. In this study, we present the first estimates of nitrogen cycling in a coupled 3D ocean-biogeochemistry-isotope model forced with realistic boundary conditions from the Last Glacial Maximum (LGM) ∼21,000 years before present constrained by nitrogen isotopes. The model predicts a large decrease in nitrogen loss rates due to higher oxygen concentrations in the thermocline and sea level drop, and, as a response, reduced nitrogen fixation. Model experiments are performed to evaluate effects of hypothesized increases of atmospheric iron fluxes and oceanic phosphorus inventory relative to present-day conditions. Enhanced atmospheric iron deposition, which is required to reproduce observations, fuels export production in the Southern Ocean causing increased deep ocean nutrient storage. This reduces transport of preformed nutrients to the tropics via mode waters, thereby decreasing productivity, oxygen deficient zones, and water column N-loss there. A larger global phosphorus inventory up to 15% cannot be excluded from the currently available nitrogen isotope data. It stimulates additional nitrogen fixation that increases the global oceanic nitrogen inventory, productivity, and water column N-loss. Among our sensitivity simulations, the best agreements with nitrogen isotope data from LGM sediments indicate that water column and sedimentary N-loss were reduced by 17-62% and 35-69%, respectively, relative to preindustrial values. Our model demonstrates that multiple processes alter the nitrogen isotopic signal in most locations, which creates large uncertainties when quantitatively constraining individual nitrogen cycling processes. One key uncertainty is nitrogen fixation, which decreases by 25-65% in the model during the LGM mainly in response to reduced N-loss, due to the lack of observations in the open ocean most notably in the tropical and subtropical southern hemisphere. Nevertheless, the model estimated large increase to the global nitrate inventory of 6.5-22% suggests it may play an important role enhancing the biological carbon pump that contributes to lower atmospheric CO 2 during the LGM.

Glacial–interglacial changes and Holocene variations in Arabian Sea denitrification

“…Deep water was evidently less oxygenated than at present (Duplessy, 1982;Kallel et al, 1988;Schmiedl and Leuschner, 2005;Waelbroeck et al, 2006). This was reproduced by models that showed a generally more sluggish bottom water ventilation from the Antarctic (Rickaby and Elderfield, 2005) with reduced oxygen contents due to the increase in sea ice cover (Buchanan et al, 2016;Somes et al, 2017). A better ventilation of the upper water column in the glacial ocean was explained by the better oxygen solubility in the colder water (Somes et al, 2017).…”

“…This was reproduced by models that showed a generally more sluggish bottom water ventilation from the Antarctic (Rickaby and Elderfield, 2005) with reduced oxygen contents due to the increase in sea ice cover (Buchanan et al, 2016;Somes et al, 2017). A better ventilation of the upper water column in the glacial ocean was explained by the better oxygen solubility in the colder water (Somes et al, 2017). Studies from the southern Arabian Sea further suggest that there was much stronger formation of AAIW during the Heinrich events.…”

“…It carried more oxygen due to less mixing with IIW and RSW, and an accelerated circulation (Böning and Bard, 2009) led to a lower residence time in the Arabian Sea. Moreover, it is quite possible that glacial SAMW carried fewer preformed nutrients due to the better nutrient utilization related to increased eolian supply of phosphate and trace metals to the region of SAMW formation (Somes et al, 2017). The lower amount of preformed nutrients further reduced productivity in the Arabian Sea.…”

“…Enhanced N fixation has been suggested as an alternative reason for the low δ 15 N found, especially during stadials, in the Arabian Sea (Altabet et al, 1995;Emeis et al, 1995;Suthhof et al, 2001). It could have been stimulated by the supply of excess phosphate and iron from the more arid continents (Prins, 1999;Sirocko et al, 2000). In this case, N fixation in surface waters provided N with low δ 15 N that may have masked the high δ 15 N signal from denitrification.…”

“…Models suggest -albeit with many uncertainties and unknowns -that both denitrification and N 2 fixation were lower during the glacial (Deutsch et al, 2004;Eugster et al, 2013;Galbraith et al, 2013;Schmittner and Somes, 2016;Somes et al, 2017). However, due to a stronger reduction of denitrification than of N 2 fixation, total export production was higher and increased the glacial oceanic N inventory by 10-50 % over that of the Holocene, also enhancing the carbon storage in the ocean (Deutsch et al, 2004;Eugster et al, 2013;Schmittner and Somes, 2016;Somes et al, 2017). Distinct changes in sedimentary δ 15 N values during deglaciation are interpreted to reflect the major changes in the N inventory (e.g., Galbraith et al, 2013).…”