Organic matter burial and degradation in the southern South China Sea since the last glaciation

Abstract

Marine organic matter (OM) plays a crucial role in regulating global carbon cycling and climate change; however, its significance is often underestimated or even overlooked due to the relatively low proportion of organic carbon (OC) within marine carbon pool and the insufficient documentation of coupled relationships between marine OM processes and atmospheric CO₂ changes during major climatic events, periods and cycles. Here, we present a high-resolution record of bulk parameters, organic biomarkers and inorganic elements to explore the potential one-to-one connection between marine OM source-to-sink dynamics and atmospheric CO₂ variations over the last glacial periods. Our results reveal that sedimentary OM was mainly of marine origin throughout the last glaciation, albeit the increases in terrestrial-derived OM inputs during the low-sea-level Last Glacial Maximum (LGM) and the sea-level rapid-rise deglacial meltwater pulse events. In the LGM, the lower-oxygen (intermediate and deep) waters and higher sedimentation rates facilitated the deposition and preservation of OM in waters and sediments, hence leading to higher TOC contents and contributing to lower atmospheric CO₂ concentrations. On the contrary, during the deglaciation and Holocene, the higher-oxygen intermediate waters and lower sedimentation rates promoted the remineralization of OM in the upper water column, which correlated with the rise in atmospheric CO₂ levels. However, the oxygen-depleted intermediate waters and highest sedimentation rates in the Bølling-Allerød (B/A) time greatly accelerated the downward transportation of OM with insignificant degradation in upper waters. The sinking OM experienced further remineralization at water-sediment interface, as indicated by a marked negative excursion in bottom-water oxygenation. This process exemplifies the biological pump, thereby acting to slow down atmospheric CO₂ rise during the B/A warm interval. Our study presents a potential mechanism to interpret atmospheric CO₂ variability by invoking marine OM dynamics, with particular emphasis on the place where OM degradation takes.