Delayed carbon-cycle stabilization and ecological recovery across the K/Pg boundary: evidence from the Um Sohryngkew River section, Meghalaya (India)

The Cretaceous–Paleogene (K/Pg) mass extinction represents one of the most severe crises in Earth history, with marked regional variations in the tempo of pre- and post-extinction environmental stress and ecological recovery. The Um Sohryngkew River (USR) section of Meghalaya (NE India) provides a unique perspective on stress and recovery dynamics in a marine setting proximal to the Deccan Traps. This study integrates planktonic foraminiferal assemblage data with sedimentological observations and bulk-carbonate δ¹³C measurements to reconstruct the nature and duration of marine stress and to constrain the timing of ecological and carbon-cycle recovery in the eastern Tethyan realm. This integrated, high-resolution multi-proxy approach was previously lacking for this Deccan-proximal archive, and provides a critical constraint on how volcanogenic forcing modulated K/Pg stress and recovery at regional to global scales.

The late Maastrichtian record at USR indicates highly stressed surface-ocean conditions. Planktonic assemblages are dominated by small opportunistic taxa, particularly Guembelitria cretacea (>80%), with strong dwarfing, dominance of thin-walled morphotypes, poor preservation, and a near absence of heavily calcified taxa (e.g., Pseudotextularia spp., Globotruncana spp.). These assemblage and preservation features point to sustained calcification stress and unfavourable conditions for carbonate production in surface waters, consistent with enhanced nutrient input and surface-water acidification under intensified continental weathering/runoff and volcanogenic CO₂ emissions. Following the K/Pg boundary, planktonic foraminiferal abundance (4 tests/g) and diversity remained markedly suppressed through the early Danian. The post-boundary interval is similarly characterised by persistent dominance of small opportunistic taxa (>30%; e.g., Guembelitria spp. and Chiloguembelina spp.) and continued dwarfing, indicating sustained calcification stress and hindered ecosystem rebuilding. Bulk-carbonate δ¹³C indicates delayed carbon-cycle recovery, beginning only after ~750 kyr at USR compared to ~200–300 kyr at many distal sites. Ecological recovery lagged further, with low diversity and small test sizes persisting for ~2 Myr until biozone P1c, indicating decoupling between carbon-cycle recovery and biological reorganization under continued environmental forcing.

The first robust evidence for ecological improvement appears in planktonic foraminiferal biozone P1c, where assemblages become more diverse and better preserved, test sizes increase, and morphogroup proportions stabilise. These changes suggest improved conditions for calcification, progressive strengthening of the pelagic carbonate system, and a more efficient biological pump. By biozones P1c–P2, community structure indicates that ecological balance was largely restored, and carbonate production increased steadily towards a better-developed carbonate-factory environment. Comparison with global K/Pg records suggests that recovery mechanisms in the USR section broadly mirror global ecological and biogeochemical feedbacks, but their timing is substantially delayed relative to distal sections. Importantly, similar evidence for prolonged stress and delayed recovery has also been documented from the Krishna–Godavari Basin of southern India, supporting a coherent regional pattern in marine environments proximal to the Deccan Traps. Together, these Deccan-proximal records highlight strong spatial heterogeneity in post-K/Pg recovery trajectories, including a delayed return to stable carbon cycling, carbonate production, and ecosystem structure.