Could persistent anoxia–euxinia enhance microbial resilience in the Southern Tethys upwelling ecosystem during the Cretaceous-Paleogene transition?

The Golan section of the southern Tethys upwelling belt preserves a continuous record of the Cretaceous-Paleogene (K-Pg) mass extinction event. This study investigates ecosystem function and community structure across the K-Pg boundary, from the P.han to P1b biozones, using an integrated approach combining bulk geochemical, molecular biomarker, and isotopic proxies. We investigated the response of a highly productive upwelling ecosystem to one of the most severe biotic crises in Earth's history. Contrary to expectations from major ecological disruption, productivity proxies (TOC and CaCO₃) show no significant changes across the K-Pg boundary. Similarly, sediment-water anoxia remained stable, as indicated by multiple independent proxies, including δ³⁴S of kerogen and pyrite, δ¹⁵N_org, and molecular proxies from the free bitumen fraction such as Pr/Ph <1 and homohopane index (HHI) >5, gammacerane index >1. These data indicate a persistently stratified, anoxic, and hypersaline water column that evolved into intermittent photic-zone euxinia, a condition already established before the extinction event. Molecular evidence reveals diverse microbial communities, including green sulfur bacteria adapted to sulfide-rich environments. Despite these anoxic–euxinic conditions, biomarker and isotopic records indicate minimal ecological disturbance. Stable sterane distributions, consistent sterane/(sterane + hopane), and C₂₈/C₂₉ sterane ratios, and negligible shifts in compound-specific δ¹³C values of low-molecular-weight n-alkanes demonstrate remarkable stability in microbial and algal communities throughout the studied interval. We hypothesize that persistent anoxic–euxinic conditions, together with diverse microbial communities and the characteristics of the semi-restricted, hypersaline upwelling basin, enhanced ecosystem stability and resilience, buffering environmental disturbances associated with the K-Pg mass extinction event.