LOWER EOCENE HYPERTHERMAL AND TRANSGRESSIVE EVENTS AS REVEALED FROM STABLE ISOTOPIC ANALYSIS OF LIGNITE BEARING COASTAL DEPOSITS FROM KUTCH BASIN, WESTERN INDIA

The Palaeocene-Eocene thermal maximum (PETM) represents one of the largest hyperthermal events in the earth’s history, followed by similar pulses of lower magnitudes during lower Eocene as documented by the negative carbon isotopic excursions (CIE) from the organosedimentary strata around the world (e.g., Zachos et. al. 2010). The lower Eocene organo-sedimentary succession of Umarsar mine area Kutch basin, western India can be broadly divided into three distinct lithounits - lower grey shale (26.5 meter), middle lignite (22 meter) and upper fossiliferous green and grey shales (25.5 meter). The succession reflects transgressive systems tract. Here, in this study, we have measured stable carbon isotopic ratios from bulk organic matter (δ¹³C_OM) from the entire succession. δ¹³C was also determined from amber (δ¹³C_AMB), wherever found from the succession, and from bulk carbonate derived from fossil shell fragments (δ¹³C_BC) of the upper lithounit. Stable oxygen isotopic ratio (δ¹⁸O_BC) was also determined from shell fragments of the fossiliferous green and grey shales.

Our study shows presence of seven negative CIEs across the entire section where the lowermost CIE with a δ¹³C_OM magnitude of -30.6‰ can be attributed to the PETM. δ¹³C_AMB followed a similar pattern like that of the δ¹³C_OM. The δ¹³C_BC from the shell carbonate shows the variation in the productivity whereas δ¹⁸O_BC reflects fluctuations in terms of oxygen isotopic ratios of water composition as a result of variations in temperature, and mixing between seawater and freshwater.

The lignite bearing succession with profuse plant debris and pyrite represents a sheltered environment. The upper part of the succession bearing well-preserved marine molluscs indicates incursion of the sea in this coastal lagoon. It was preceded by a CIE. This suggests the presence of a causal link between the hyperthermal and the transgression.

Reference:

Zachos, J. C., McCarren, H., Murphy, B., Röhl, U., & Westerhold, T. (2010). Tempo and scale of late Paleocene and early Eocene carbon isotope cycles: Implications for the origin of hyperthermals. Earth and Planetary Science Letters, 299(1-2), 242-249.