Phytoplankton as main organism in the Eocene organic-rich turbidites of Jiyang Depression, China: Implication for organic matter accumulation mechanism

Persistent anoxia and still water condition is traditionally regarded as the prerequisite to deposit organic-rich sediments. Recent studies found that turbidity current can also carry and deposit terrigenous organic matter, forming organic-rich turbidites. However, it is still unclear whether the organic-rich property of turbidites can form by depositing phytoplankton. Here we found organic-rich turbidites (avg. 2.86 wt% in total organic carbon) in the Eocene Jiyang Depression, China, with the organic matter dominated by phytoplankton. Investigations show turbidity current and algae blooms coupled with each other, both of which induced by warm-humid climate. The turbidity current involved the living phytoplankton during motion, and then phytoplankton deposited along with turbiditic minerals. Meanwhile, the high depositional rate of turbidites prevents organic matter degradation by bottom-water oxidation. Organic-rich turbidites, phytoplankton as dominating organisms, consequently form in the lake basin, acting as a new mechanism for organic matter accumulation. This mechanism shows that turbidites can also act as source rock, as phytoplankton is ideal parent material for hydrocarbon.