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

A. Sarkar, S. Sarangi, Mitsuru Ebihara, S. K. Bhattacharya, A. K. Ray

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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.

Original languageEnglish
Pages (from-to)281-293
Number of pages13
JournalChemical Geology
Volume201
Issue number3-4
DOIs
Publication statusPublished - 2003 Nov 28
Externally publishedYes

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Keywords

  • Carbonate
  • Eocene/Oligocene boundary
  • Extinction
  • Geochemistry
  • India

ASJC Scopus subject areas

  • Geology
  • Geochemistry and Petrology

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