Abstract: Marine nitrogen fixation is a key driver of the biological carbon pump, significantly influencing the global carbon cycle and climate. The long-term variability of nitrogen fixation over glacial–interglacial cycles has been extensively studied using nitrogen isotope value (δ¹⁵N). However, δ¹⁵N primarily reflects the relative contribution of nitrogen fixation in the euphotic zone versus upwelled nitrogen from the subsurface, leaving the absolute amount of nitrogen fixation poorly understood. Here, we present a 100-kyr record of heterocyst glycolipids (C5-HGs), biomarkers indicating diatom–diazotroph associations (DDAs), alongside δ¹⁵N_org, productivity, and physical mixing records from a sediment core in the northern South China Sea (SCS). The δ¹⁵N_org record, consistent with previous studies, shows a progressive decline in nitrogen fixation as sea level falls. However, C5-HGs exhibit an opposing trend, indicating increased DDAs production and, thus, increased nitrogen fixation. This apparent contradiction is explained by the intensified winter monsoon during the last glacial period, which promoted an increase in both diazotrophs and non-diazotrophs, with the latter outpacing the former. Our results of the increase in DDAs during the last glacial period in the SCS indicate that this marginal sea is a net carbon sequestration area that is not evident from the δ¹⁵N record alone.

Marine nitrogen fixation; Nitrogen fixation rate; diazotrophic abundance