Strong coupling between the carbon and nitrogen cycles, environmental change, and organic carbon enrichment during the early–middle Eocene

Abstract

The early–middle Eocene constitutes a key stage of the Phanerozoic because it represents the last long-lived greenhouse period in Earth's history. The identification of the mechanisms underlying the associated geochemical carbon and nitrogen cycles can help us understand the environmental effects of global warming during the early–middle Eocene. There is currently notable interest in marine systems, but little is known regarding the geochemical cycling of carbon and nitrogen in lake systems during the early–middle Eocene. To address this problem, we systematically analyzed carbon and nitrogen isotopes in organic-rich lacustrine mudstones of the lower–middle Eocene Wenchang Formation in the Pearl River Mouth Basin (South China). We then determined the paleoclimate disturbances and historical paleoenvironmental information recorded in the sediments and established a conceptual environmental and biogeochemical model of the Wenchang Formation on the basis of the carbon and nitrogen cycles. The carbon isotope data revealed that the geochemical carbon cycle was affected mainly by the concentration of dissolved CO₂ and input from chemoautotrophs in lake water and that negative δ¹³C_org values provide evidence for positive feedback to the warm paleoclimate and high atmospheric pCO₂ levels during the early–middle Eocene. Enhanced chemical weathering promoted an increase in primary productivity and intensified the lacustrine reducing environment, thereby forming a feedback loop between productivity and hypoxia. This mechanism driving the warm and humid climate was particularly significant in Units I and III of the Wenchang Formation. With decreasing atmospheric pCO₂ level, the positive excursions of δ¹³C_org suggest the existence of a cooling interval in the climate (during the deposition of Units II and IV). A salinization event led to the occurrence of an anoxic and brackish water column under the influence of a strongly restricted environment during these periods, while the positive excursion of δ¹⁵N suggested an increased denitrification rate. The positive excursions of δ¹⁵N and δ¹³C_carb were significantly associated with increasing total organic carbon (TOC) contents, which suggests that the increase in nitrate concentration was synchronous with the intensification of the lacustrine reducing environment and the increase in bioproductivity. Elucidating the strong coupling among the carbon and nitrogen cycles, environmental changes, and organic carbon enrichment during the early–middle Eocene can provide key constraints for understanding the mechanisms driving the carbon and nitrogen cycles and organic matter enrichment in lakes during the paleoclimate warming period.