Paleoenvironmental reconstruction preceding and during the early Aptian Oceanic Anoxic Event 1a in southern Tibet, eastern Tethys

Abstract

The early Aptian Oceanic Anoxic Event (OAE 1a, ∼120 Ma) represents an episode of severe paleoenvironmental disturbance during the Cretaceous, marked by the widespread deposition of organic matter (OM)-rich sediments. The OAE 1a has been extensively studied in the Pacific, Boreal, and western and southern Tethyan regions. However, limited studies of the OAE 1a were conducted in the eastern Tethys (especially in southern Tibet). Here, high-resolution organic and inorganic geochemical data are used to reconstruct paleoenvironmental and paleoclimatic conditions during the OAE 1a in the Gucuo area of southern Tibet, eastern Tethys. Enrichment factors of redox-sensitive Mo and U elements and the Th/U ratio revealed oxic-to-suboxic bottom water oscillations preceding and at the base of OAE 1a. A shift from oxic to suboxic condition occurred prior to the OAE 1a, followed by a change from suboxic to oxic at the base of OAE 1a. Based on multiple geochemical proxy indicators and plots, a warm-humid climate in response to global warming was prevalent during the early Aptian. The ratios of B/Ga, Sr/Ba, Rb/K, and CaO/(CaO + Fe) revealed freshwater-dominated salinity conditions prior to and at the base of the OAE 1a. Low salinity conditions during the deposition suggested that massive freshwater runoff took place into the ocean due to a warm-humid climate and related hydrological cycles at this time. OM production was controlled by a high surface water bioproductivity driven by enhanced freshwater input that promoted fluvial delivery of micronutrients and accelerated hydrological cycles. However, oxic-suboxic redox conditions played a negative role in oxidizing and decomposing available labile OM in the water column, which led to moderate organic carbon burial at this time.