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Abstract

The Mozambique Channel has recently been identified as a hotspot for nitrogen fixation in the modern ocean; this process imparts a clear signature of low nitrogen isotopic values (δ15N) to the available surface ocean nitrate pool provided by the subsurface ocean nitrate supply. In turn, the nitrogen preserved within the CaCO3 lattice of foraminifera can be analyzed to understand the variability over geological time. Here we present a high resolution sedimentary record of planktonic foraminiferal-bound δ15N to document the timing of changes in regional nitrogen fixation over the last 45 kyr. The sediment sequence, IODP Site U1477, is ideally situated to test the relationship between the nitrogen fixation and the hydroclimate variability of the adjacent Zambezi watershed. Prior radiocarbon work demonstrates that sedimentation rates average ~1 m/kyr throughout the last ice age interval, affording bi-decadal resolution. The δ15N results from three species of planktonic foraminifera show a small (1 ‰) glacial-interglacial shift to lower values during the Last Glacial Maximum and an equally subtle maximum during the early Holocene. However, the multi-species results are dominated by prominent excursions to low δ15N during the Heinrich Stadial (HS) events of the past 40 ka. The temporal connection between δ15N decreases and the abrupt ocean and hydroclimate changes associated with HS events offers strong clues for the controls on nitrogen fixation in the Mozambique Channel and other analogous environments. Furthermore, we can use the full δ15N record to quantify the Mozambique Channel water endmember, as the foraminiferal bound δ15N offers one potential tracer for monitoring the leakage of Agulhas Current fauna into the Atlantic over abrupt and orbital scale climatic events.