Carbonate geochemistry across the Eocene/Oligocene boundary of Kutch, Western India: implications to oceanic O₂-poor condition and foraminiferal extinction

Abstract

Major, trace, and rare-earth element (REE) analyses of larger benthic foraminifera-bearing carbonates have been carried out across the Eocene/Oligocene boundary (EOB) of Kutch, western India. REEs of these carbonates display LREE-depleted-HREE-enriched patterns with low average Lan/Lun, and Lan/Ybn ratio (<1; 0.1-0.6), ∑REE values (average 10.4 ppm), and high Er/Nd ratio (mol/mol; 0.12-0.28). The data along with petrographic, Sr/Ca ratio, and Mn concentration, etc. indicate a rather pristine character of these carbonates. Depth profiles of several chalcophile elements, e.g., Fe, Ni, Mo, Co, Cr, Zn, As, V, and U show enrichment near the EOB, suggesting a serious oxygen-deficient (suboxic/anoxic) condition in the overlying water column with possible subsequent modification within pore water. The enrichments are almost synchronous to ~3°C δ¹⁸O cooling of ocean water, a positive Ce/Ce^∗ anomaly (maximum 1.1), and authigenic precipitation of glauconites and framboidal pyrites, all indicating slowly accumulating, sediment-starved, semiconfined, suboxic to anoxic depositional environment. The O₂-poor condition at the EOB probably developed due to a lowering of sea level and consequent cessation of open-ocean circulation. Simultaneously, several catastrophic climatic and environmental shifts occurred across the EOB, namely, decrease in sea surface temperature (SST), a regression and consequent O₂ deficiency, and reduced phytoplankton production which perturbed the existing life processes.