The long-term ocean carbon isotope (S13C) curve is characterized by a series of S13C maxima (S13Cmax) 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 S13C 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 S13Cmax events in the mid- and late Quaternary were reconstructed. It was found that productivity was higher during interglacial periods in the S13Cmax-V, IV and III events, due to increased chemical weathering during warm and humid interglacial climate. The S13Cmax-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 S13Cmax-V, S13Cmax-IV and S13Cmax-I events suppressed the refractory DOC reservoir, resulting in more 12C being released into seawater and a less pronounced peak in S13CDIC, thereby obscuring the 400-kyr S13C cycles. Furthermore, comparisons among benthic S13C records in the South China Sea reveal a highly fluctuating ocean carbon reservoir after 1.6 Ma. The significant decrease in benthic S13C 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.