Um Sohryngkew River Section, Meghalaya: Re-evaluating stress and recovery in the Cretaceous-Paleogene interval

The Cretaceous-Paleogene (K/Pg) boundary marks a pivotal moment in Earth's history, defined by one of the five major mass extinction events and significant disruptions in global biogeochemical cycles, including shifts in carbon cycling and ocean chemistry. The Um Sohryngkew River section in Meghalaya, India, represents the most complete K/Pg sequence in the region and is comparable to global shallow marine sites. Its proximity to the Deccan volcanic province makes it a critical location for investigating the environmental stressors associated with Large Igneous Province (LIP) volcanism and their role in mass extinction and biotic recovery.

Late Maastrichtian planktic foraminiferal assemblages at Meghalaya reveal persistent stress, with species richness ranging from 6 to 13 taxa and consistently low absolute abundances (~14 individuals/g). A significant increase in abundance is observed 2.5 meters below the K/Pg boundary, from ~14/g to ~250/g. However, this interval is characterized by a population dominated by opportunistic taxa, particularly Guembelitria spp., which account for over 90% of individuals. While similar patterns are reported at coeval sites like Egypt, the prolonged dominance of Guembelitria spp. at Um Sohryngkew highlights the intense and sustained environmental stress, likely exacerbated by its proximity to the Deccan volcanic province. Assemblages are further characterized by small test sizes (<120 µm), with genera such as Planoheterohelix (8-12k µm²) and Rugoglobigerina (10-20k µm²) being 2–4 times smaller than counterparts at sites like Bidart. Test sizes of Guembelitria, Planoheterohelix, and Rugoglobigerina show further reductions of 10.6%, 10.2%, and 29.2%, respectively, around 60 cm below the K/Pg boundary. This decline aligns with increased test fragmentation, leaching, and elevated mercury levels, strongly indicating intensified volcanic activity and associated calcification stress. The absolute abundance of planktic foraminifera collapses at the K/Pg boundary, marked by a one cm-thick red clay layer enriched in Platinum Group Elements and a pronounced negative carbon isotope excursion. A brief recovery episode occurs ~2.75 meters above the boundary, with a significant increase in species richness and a rise in absolute abundance from ~2 to ~70 individuals/g. However, recovery can be tracked at 13 meters above the boundary, based on greater species richness, higher abundance (~2500 individuals/g), improved test preservation, and larger foraminiferal test sizes. Future studies, including detailed biostratigraphy, chemostratigraphy, and age modeling, will be crucial in refining the timing of extinction triggers, environmental changes, and the pace of recovery.