Detailed sedimentary N isotope records from Cariaco Basin for terminations I and V: Local and global implications

For the last deglaciation and Termination V (the initiation of MIS 11 at around 430 ka) we report high-resolution sedimentary nitrogen isotope (δ¹⁵N) records from Cariaco Basin in the Caribbean Sea. During both terminations the previously reported interglacial decrease in δ¹⁵N clearly lags local changes such as water column anoxia as well as global increases in denitrification by several thousand years. On top of the glacial-interglacial change, several δ¹⁵N peaks were observed during the last deglaciation. The deglacial signal in Cariaco Basin can be best explained as a combination of (1) local variations in suboxia and water column denitrification as the reason for the millennial-scale peaks, (2) a deglacial maximum in mean ocean nitrate δ¹⁵N, and (3) increasing N₂ fixation in response to globally increased denitrification causing the overall deglacial δ¹⁵N decrease. In the Holocene, much of the decrease in δ¹⁵N occurred between 6 and 3 ka, coinciding with an expected precession-modulated increase in African dust transport to the tropical North Atlantic and the Caribbean. This begs the hypothesis that N₂ fixation in this region increased in response to interglacial maxima in denitrification elsewhere but that this response strengthened with increased mid-Holocene iron input. It remains to be seen whether the data for MIS 11 support this interpretation.