Elevated foraminifera-bound nitrogen isotopic composition during the last ice age in the South China Sea and its global and regional implications

Abstract

We report a new foraminifera‐boundδ15N (FB‐δ15N) record from the South China Sea (SCS) extending back to 42 ka. This record shows a ∼1.2‰ glacial‐to‐interglacialδ15N decrease, with a deglacial δ15N maximum similar to that observed in many bulk sedimentary δ15N records and in a Caribbean FB‐δ15N record. The glacial‐to‐interglacialδ15N decrease is smaller than in the Caribbean record, indicating that at least half of the Caribbean δ15N decrease into the Holocene was regional, not global, supporting the interpretation of a Holocene increase in Atlantic nitrogen fixation. At the same time, the glacial‐to‐interglacialδ15N decrease observed in the SCS may also be explained as a regional signal of increasing nitrogen fixation into the Holocene. Other aspects of the SCS record suggest an effect of physical circulation on FB‐δ15N. FB‐δ15N starts to increase toward its deglacial maximum before the last ice age ends, and it continues to decrease through the later Holocene, in contrast to the more pure glacial/deglacial/interglacial steps in the Caribbean record. These changes have parallels in other Pacific δ15N records, precessionally driven East Asian monsoon records, and the slow Holocene warming of SCS surface waters. In addition, the SCS record exhibits two one‐point high‐δ15N spikes in multiple foraminiferal species that coincide with apparent hydrographic events. Thinning or weakening of the thermocline, in the late glacial and early Holocene as well as during the noted events, may have yielded a higher δ15N for thermocline nitrate and thus a higher FB‐δ15N.