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Low-latitude nitrogen fixation (N2 fixation) plays a crucial role in supplying bioavailable nitrogen to the global ocean, which acts as a major nutrient for phytoplankton growth. Previous foraminifera-bound (FB-) δ15N studies in the Caribbean Sea suggested a decline in N2 fixation during ice ages 1,2, yet the underlying cause remains uncertain. Here, we present three new Atlantic FB-δ15N records from the subtropical North and South Atlantic gyres (MSM58-50 and DSDP Site 516) as well as the equatorial Atlantic (ODP Site 662). Relatively steady FB-δ15N values in the equatorial Atlantic over the last 160 thousand years indicate a stable δ15N for nitrate supply to the thermocline throughout glacial and interglacial cycles. In the North and South Atlantic, we observe a consistent rise in FB-δ15N during glacials, with the northern gyre recording a more pronounced rise than the southern gyre. This FB-δ15N data seems to mirror fluctuations in regional shelf area influenced by glacial/interglacial sea level changes, similar to a previous study in the South China Sea3. Especially in the North Atlantic, the drop in sea level during glacials caused a reduction in highly productive submersed shelf areas, which in turn led to reduced shelf sedimentary denitrification. As a result, lower phosphorus excess was supplied to the surface waters downstream, leading to decreased N2 fixation rates in the gyre and thus elevated the FB-δ15N. Overall, our study highlights the importance of regional factors, like shelf sedimentary denitrification, in influencing oceanic N2 fixation.
References: [1] Straub, M. et al. Changes in North Atlantic nitrogen fixation controlled by ocean circulation. Nature 501, 200 (2013). [2] Ren, H., Sigman, D. M., Chen, M. T. & Kao, S. J. Elevated foraminifera‐bound nitrogen isotopic composition during the last ice age in the South China Sea and its global and regional implications. Global Biogeochemical Cycles 26 (2012). [3] Ren, H. et al. Impact of glacial/interglacial sea level change on the ocean nitrogen cycle. Proceedings of the National Academy of Sciences 114, E6759-E6766 (2017).
