Marine porewater dissolved organic matter linked to the paleoclimate and archaeal communities on glacial-interglacial timescales

Abstract

Although dissolved organic matter (DOM) in marine sediments is one of the most important carbon reservoirs, its distribution, characterization, and associated biogeochemical processes in marine sediments have not been fully investigated, especially in the deep ocean. In this study, we sampled a deep-sea gravity core from the northern South China Sea (SCS) to investigate the characterization and distribution of porewater DOM, which is the most dynamic part of the sedimentary DOM pool, as well as its correlations with other environmental components in sediments during glacial-interglacial cycles. The gravity core covers a period from Marine Isotope Stage (MIS) 3 to Holocene. Overall, the fluorescence intensity of porewater DOM increased with depth along the core expect the sample belonging to MIS 2, indicating the accumulation of fluorescence DOM (FDOM) with geological age. The abnormally high value of FDOM intensity occurred in MIS 2, which might be attributed to the enhanced marine primary productivity and favorable preservation conditions during this glacial period. Despite a longer degradation time since MIS 3, the molecular weight (MW) of porewater DOM remained relative constant among all stages. Compared to MIS 1, higher paleo-productivity and stronger upper marine mixing in MIS 3 may be responsible for this relatively constancy in MW, leading to an increase in marine autochthonous labile DOM with high MW. The composition of the sedimentary archaeal communities as well as archaeal functions were significantly different between MIS 1 and MIS 3. These differences implied that there were variations in DOM-microbe interactions along the core, thus could influence the vertical profile of porewater DOM. The findings in this study elucidate the close connection between porewater DOM and geological events as well as microbial communities in deep-sea sediments.