Middle-Late Quaternary planktonic foraminifera and the upper-column sea water changes in the northern SCS during the δ13C maximum events

The long-term ocean carbon isotope (δ13C) curve is characterized by a series of δ13C maxima (δ13Cmax) events, exhibiting ∼400-kyr cycles during the Pliocene and prolonged to ∼500-kyr cycles after 1.6 Ma. Although the dissolved organic carbon (DOC) hypothesis and the effect of the internal feedback of the Earth's climate system have been proposed to explain the extended δ13C cycles, the mechanism remains incompletely understood, partly due to limited studies on the variations in productivity and its controlling factors. This study presents the planktonic foraminiferal assemblage and the stable isotopes of benthic foraminifers of Site U1505 of IODP Expedition 368 from the northern South China Sea. First, an oxygen isotope stratigraphy was established for the upper ∼72 m of Holes U1505C and U1505D. Subsequently, sea surface temperature (SST), upper water structure, and productivity variations during the δ13Cmax events in the mid- and late Quaternary were reconstructed. It was found that productivity was higher during interglacial periods in the δ13Cmax-V, IV and III events, due to increased chemical weathering during warm and humid interglacial climate. The δ13Cmax-I event exhibited consistently high productivity, attributed to the strong weathering of shelf sediments during sea-level lowstands from MIS 3 to early-MIS 1, the intensified East Asian winter monsoon during MIS 3-MIS 2, and strong East Asian summer monsoon rainfall during MIS 3 and MIS 1. According to the DOC hypothesis, we speculate that the high productivity in δ13Cmax-V, δ13Cmax-IV and δ13Cmax-I events suppressed the refractory DOC reservoir, resulting in more 12C being released into seawater and a less pronounced peak in δ13CDIC, thereby obscuring the 400-kyr δ13C cycles. Furthermore, comparisons among benthic δ13C records in the South China Sea reveal a highly fluctuating ocean carbon reservoir after ∼1.6 Ma. The significant decrease in benthic δ13C coincided with periods of high productivity. It is inferred that the reorganization of the global ocean around 1.6 Ma potentially intensified the influence of the fluctuating polar ice sheet on the ocean carbon reservoir by affecting ocean productivity through monsoon, sea level change, weathering, nutrient input.