Geochemistry of Himalayan Black Shales From Krol-Tal Formation: Implications To Past Oceanic Redox State Around The Pc-C Boundary

Lipsita Mishra

Utkal University, Bhubaneswar 751004 India

Dr. Gyana Ranjan Tripathy

Assistant professor, Department of Earth and Climate Science, Indian Institute of Science Education and Research, Pune 411008 India

Abstract

This project work aims at constraining the oceanic redox state during this period using the chemistry of organic-rich shales from the Lesser Himalaya. Based on chemical, biostratigraphic and chronological investigation, the Krol-Tal Formation boundary of the Lesser Himalaya have been identified as the Pc-C boundary. Fresh and unmetamorphosed black shales with disseminated pyrites were collected from the Maldeota mines. These rocks overly immediate to the Krol-Tal boundary and belong to the Lower Tal Formation. Like any marine sediment, chemical composition of black shales is also regulated by its detrital, authigenic/biogenic components. Abundance of chemical constituents in their authigenic fraction can serve as a proxy for past oceanic condition. For this, we adopted a partial-leaching technique to extract mainly the authigenic part of the shales. Twenty four black shale samples were treated with inverse aqua regia (3:1 volume mixture of HNO3 and HCl) and this oxidizing solution dissolves the authigenic minerals present in the shales. These solutions were analyzed for their trace elements concentrations using a Quadrupole inductively coupled plasma mass spectrometry (Q ICP-MS). The total carbon of the shale samples were measured using a CHNS analyzer, whereas inorganic carbon content was analyzed using a CO2-coulometer. The difference of total and inorganic carbon concentrations yielded the organic carbon concentration for the samples. I have now been involved in interpreting the shale chemistry dataset to reconstruct the seawater chemistry at about 541 Ma.

Keywords: Krol-Tal Formation boundary, Lesser Himalaya, black shale, shale chemistry