Unchanged nitrogen isotopic composition of organic matter in the South China Sea during the last climatic cycle: Global implications

The δ 15 N of surface and down‐core sediments spanning the last 20–200 kyr from the entire South China Sea (SCS) ranges only from ∼3.0 to ∼6.5‰, with no correlation with discernible paleoclimatic/oceanographic changes. Detailed profiles of the uppermost sediment column, including fluff samples, indicate a minor diagenetic overprint of 0.3–1.2‰ at the sediment‐water interface. The absence of any correlation with reconstructed (glacial‐interglacial) changes in primary production, terrigenous input, and/or sea level related basin configuration is attributed to a complete consumption of nitrate during primary production in this marginal basin during at least the last 140,000 years. This, in turn, implies that the δ 15 N of the nitrate used during primary production remained approximately constant during the last climatic cycle. The proposed scenario infers an unchanged nitrogen isotopic composition of the western Pacific subsurface nitrate between glacial and interglacial stages as well as during terminations and thus constrains proposed changes in the oceanic N inventory.