Last glacial organic carbon burial patterns in contourites of the lower Shenhu slope, northern South China Sea

Organic carbon (OC) burial is a critical process in regulating atmospheric CO₂ concentrations and is fundamental to understanding past and future climate change. Elucidating the factors and processes controlling OC burial is essential for clarifying its relationship with climate variability. In this study, we examined sediment core C1-7 from the lower Shenhu slope of the South China Sea through radiocarbon dating, grain size analysis, total organic carbon (TOC) and nitrogen content, organic carbon isotopic composition, and normal alkanes (n-alkanes) analysis. These analyses enabled us to characterize their sedimentary nature, to reconstruct the relative intensity of the South China Sea Deep Water (SCSDW), and to determine the sources of OC and the factors affecting burial and preservation. Our results reveal that the sediments in core C1-7 are predominantly contourites, deposited by the SCSDW since 32 cal ka BP. Current intensity was greater during Marine Isotope Stages (MIS) 2 and 3 than during MIS 1. Despite elevated marine primary productivity during MIS 2-3, terrestrial OC contributions exceeded those of marine OC, likely due to the proximity of the core to a canyon mouth. Comparisons of OC burial flux and short-chain even-carbon n-alkanes with bottom current intensity and oxygen levels indicate that higher current intensity, coupled with lower oxygen content, enhanced OC burial, preservation, and flux. The combination of a stronger, oxygen-depleted SCSDW and increased inputs of both terrestrial and marine OC resulted in higher TOC burial fluxes during MIS 3 and MIS 2 (1,181.9 and 1,187.7 g m^-2 ka^-1, respectively) compared to MIS 1 (235.1 g m^-2 ka^-1). Elevated primary productivity and enhanced terrestrial OC transport to the deep South China Sea were widespread during the last glacial stage throughout most of the region. Together with the vigorous and oxygen-depleted SCSDW, these conditions suggest that deep marine contourites in the South China Sea acted as a significant carbon sink, potentially contributing to atmospheric CO₂ sequestration during this period.