Sea Level Modulation of Atlantic Nitrogen Fixation Over Glacial Cycles

• Published in: Paleoceanography and paleoclimatology 2024-08, Vol.39 (8), p.n/a
• Main Authors: Auderset, Alexandra, Fripiat, François, Creel, Roger C., Oesch, Lukas, Studer, Anja S., Repschläger, Janne, Hathorne, Ed, Vonhof, Hubert, Schiebel, Ralf, Gordon, Laura, Lawrence, Kira, Ren, Haojia Abby, Haug, Gerald H., Sigman, Daniel M., Martínez‐García, Alfredo
• Format: Article
• Language: English
• Subjects: Atlantic Ocean; Bioavailability; Carbon dioxide; Carbon dioxide concentration; Carbon sequestration; Carbonates; Commonality; Continental shelves; Denitrification; Fertility; Foraminifera; Glacial periods; glacial/interglacial; Gyres; Ice ages; Interglacial periods; Nitrates; Nitrogen; Nitrogen fixation; Nitrogen isotopes; Nitrogenation; Nutrient cycles; Nutrients; Ocean basins; Ocean surface; Oceans; Organic matter; Phosphorus; Phytoplankton; Sea level; Sea level changes; Sea level variations; Sediment; sedimentary denitrification; Shelf sedimentation; Surface water; Thermocline; Thermoclines; Variation
• ISSN: 2572-4517; 2572-4525
• DOI: 10.1029/2024PA004878

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‐)δ15N indicates a decline in N2 fixation during ice ages, but its cause and broader implications are unclear. Here, we report three additional Atlantic FB‐δ15N 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 δ15N in the equatorial Atlantic suggests a stable δ15N for the nitrate below the gyre thermoclines. The North Atlantic record shows a FB‐δ15N rise during the ice ages, resembling a previously published FB‐δ15N record from the South China Sea. The commonality among the FB‐δ15N records is that they resemble sea level‐driven variation in regional shelf area, with high FB‐δ15N (inferred reduction in N2 fixation) during periods of low shelf area. The South China Sea shows the largest δ15N 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 δ15N, explaining the higher FB‐δ15N 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. Plain Language Summary 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