Sea level modulation of Atlantic nitrogen fixation over glacial cycles

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-)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.

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ORCID for Alexandra Auderset: orcid.org/0000-0002-6316-4980

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