Auderset, Alexandra, Fripiat, François, Creel, Roger C., Oesch, Lukas, Studer, Anja S., Repschläger, Janne, Hathorne, Ed C. , Vonhof, Hubert, Schiebel, Ralf, Gordon, Laura, Lawrence, Kira, Ren, Haojia Abby, Haug, Gerald H., Sigman, Daniel M. and Martínez‐García, Alfredo (2024) Sea Level Modulation of Atlantic Nitrogen Fixation Over Glacial Cycles. Paleoceanography and Paleoclimatology, 39 (8). Art.Nr. e2024PA004878. DOI 10.1029/2024PA004878.

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Abstract

N2 fixation in low-latitude surface waters dominates the input of fixed nitrogen (N) to the global ocean, sustaining ocean fertility. In the Caribbean Sea, higher foraminifera-bound (FB-)delta 15N indicates a decline in N2 fixation during ice ages, but its cause and broader implications are unclear. Here, we report three additional Atlantic FB-delta 15N records, from the subtropical North and South Atlantic gyres (MSM58-50 and DSDP Site 516) and the equatorial Atlantic (ODP Site 662). Similar glacial and interglacial delta 15N in the equatorial Atlantic suggests a stable delta 15N for the nitrate below the gyre thermoclines. The North Atlantic record shows a FB-delta 15N rise during the ice ages, resembling a previously published FB-delta 15N record from the South China Sea. The commonality among the FB-delta 15N records is that they resemble sea level-driven variation in regional shelf area, with high FB-delta 15N (inferred reduction in N2 fixation) during periods of low shelf area. The South China Sea shows the largest delta 15N signal, the subtropical North Atlantic shows less, and the South Atlantic shows the least, the same ordering as the ice age reductions in continental shelf area in the different regions. Reduced shelf sedimentary denitrification would have increased the nitrogen-to-phosphorus ratio of the nutrient supply to open ocean surface waters, leading to decreased N2 fixation and thus higher gyre thermocline nitrate delta 15N, explaining the higher FB-delta 15N of peak ice ages. These observations identify shelf sediment denitrification as an important regional driver of modern N2 fixation and imply strong basin-scale coupling of fixed nitrogen losses and inputs.

Nitrogen fixation plays the crucial role in the ocean of supplying bioavailable nitrogen (N), a major nutrient for phytoplankton growth. Variations in nitrogen fixation over time can, therefore, significantly impact ocean productivity and, consequently, carbon sequestration in the ocean interior. To infer past changes in nitrogen fixation during ice ages, we measured the nitrogen isotope (15N-to-14N) ratio of organic matter preserved within the carbonate skeleton of planktic foraminifera. Our study reveals a substantial reduction in nitrogen fixation during ice ages in the low-nutrient regions of the North Atlantic, with only minor variations in the South Atlantic. The basin-dependent changes are attributed to sea level-driven reductions in regional continental shelf area during ice ages, resulting in diminished sedimentary denitrification and subsequently lower phosphorus excess in the surface waters downstream of the continental shelves. Overall, our study highlights the importance of regional factors, like shelf sediment denitrification, in influencing oceanic nitrogen fixation within a given ocean basin. Furthermore, it suggests that changes in nitrogen fixation cannot explain the decline in atmospheric carbon dioxide concentration during ice ages.

Reduction of nitrogen fixation in the Atlantic Ocean during ice ages, proportional to sea-level variations in regional shelf area Regional coupling between nitrogen fixation and shelf sedimentary denitrification in the subtropical gyres Change in nitrogen fixation cannot explain the lowering of atmospheric CO2 during ice ages

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