Underestimated organic carbon preservation of marine clay sediments

The long-term preservation of organic matter (OM) in the marine environment is crucial for Earth's climate regulation. Clay minerals are commonly believed to shield OC from degradation for millennium scale during its transport from terrestrial to marine environments. However, the complex interactions between clay minerals and OM during this land-ocean transport process and how clay mineral contributes to OM preservation and burial remains unclear. In this study, we examined the radiocarbon signatures of OM in clay fractions (< 2 &#956;m) and the clay mineral compositions in the Eastern China Marginal Sea (ECMS). Our biogeochemical analysis revealed a positive correlation between smectite content and &#9651;14C values in clay fractions from sediments transported along different pathways in the ECMS, indicating a strong association between smectite and fresh biomass-OM during sediment transport. Furthermore, our results suggest that intense hydrodynamic conditions may facilitate the continuous adsorption of new produced biomass-OM onto smectite surfaces. We performed molecular dynamics simulations to investigate adsorption mechanism of typical organic molecules on smectite structure surface under different environmental conditions, showing that OM may undergo a continuous desorption-adsorption cycle on smectite surfaces due to change in the binding forms of the mineral-OM assembly during land-ocean transport, which support our geochemical findings. We thus estimated that clay mineral adsorption of biomass onto smectite might generate on the order of approximately 6.6 Tg C yr&#8722;1 for OM preservation in marine sediments. We propose that variable smectite inputs to the ocean over geological time could exert a substantial but hitherto unexplored impact on the Earth&#8217;s long-term climate evolution.