Paleoenvironment reconstruction of the eastern Tethys during the pre-onset excursion preceding the PETM

A rapid warming event known as the pre-onset excursion (POE) occurred prior to the Paleocene-Eocene Thermal Maximum (PETM). It is less well studied compared to the PETM due to the limited number of well-resolved records globally despite its importance in shaping the subsequent PETM. Shallow marine environments, particularly in low latitudes, are insufficiently documented despite their significant influence on heat and moisture distribution. Here we report high temporal resolution geochemical and magnetic susceptibility records from a well-preserved shallow marine Kuzigongsu section in the Tarim Basin in China to reconstruct the paleoenvironment of the eastern Tethys during the POE. Changes in chemical weathering proxies (e.g., chemical index of alteration [CIA] and Rb/Sr) suggest chemical weathering intensification occurred after the POE. Higher enrichment factors of P, Ba, Ni, and Cu support increased primary productivity due to enhanced river runoff and nutrient input, which aligns with the observed rise in magnetic susceptibility. Concurrently, the eastern Tethys experienced bottom water deoxygenation, which is supported by redox-sensitive proxies (e.g., elevated enrichment factors of U and V, and low Mn* values). Increased δ¹⁵N_org values provide additional evidence of enhanced reduction processes due to increased productivity, attributed to enhanced denitrification and ¹⁵N enrichment in seawater's NO₃⁻ reservoir. The carbon isotope excursion during the POE recovers significantly faster than that during the PETM yet coincides with lingering environmental impacts that are also globally reported for the PETM. These paleoenvironmental proxies reflect notable changes in chemical weathering, ocean productivity, and redox conditions of the eastern Tethys region, potentially setting the stage for the anomalously impactful PETM.