Mechanisms of terrestrial organic matter accumulation across the Paleocene-Eocene thermal maximum in the Jianghan Basin, Central China

Abstract

Marine settings provide major carbon sinks for organic carbon, and act as a key reservoir that can sequester excess carbon during times of high CO2 and global warming. Organic carbon burial in lake sediments also plays an important, but perhaps less appreciated, role in the global carbon cycle. However, the fate of organic carbon burial during times of increasing CO2 and rapid global warming in lacustrine settings is still unclear. This study investigates organic carbon accumulation and its mechanism during the globally significant Paleocene-Eocene Thermal Maximum (PETM, ∼56 Ma) hyperthermal event in the Jianghan Basin, China, based on organic and element geochemistry data. The PETM is identified in our studied rocks based on their broad age constraints and the presence of a 4.3‰ negative excursion in organic carbon isotope. Our study shows that organic carbon accumulation during the PETM in the Jianghan Basin might be mainly controlled by preservation conditions and sediment accumulation rates (SAR), which responded to changing hydrology/climate. A hot and humid climate during the PETM could lead to an increase in terrestrial input and SAR, which might dilute the overall organic matter concentration. Also, enhanced rainfall could contribute to the ventilation of the lake water and expedite the oxidation of organic matter, thus, contributing to relatively low organic matter concentration in the Jianghan Basin. In contrast, an arid and relatively low oxygen environment during the post-PETM interval of our studied section created conditions conducive to the retention of organic matter, resulting in high organic matter content. An examination of other terrestrial PETM records reveals that the poor preservation condition of the organic matter and higher SAR might also have been the primary factors for the low organic matter concentrations in PETM terrestrial sections at other subtropical and mid-latitude locations. Compared to marine settings, terrestrial settings may exhibit lower efficiency in sequestering excess CO2 from the atmosphere through organic carbon burial during the PETM.