Trace and rare earth element systematics of cold-seep carbonates from the Krishna-Godavari basin: A comparison between isotopically distinct carbonate deposits

Abstract

Mineralogical and isotopically distinct authigenic carbonates from the zones of methane oxidation and methanogenesis in the cold-seep environment, from the Krishna-Godavari basin are investigated for major, trace, and rare earth element compositions. For this study, the elemental compositions of the ¹³C-depleted carbonate phase (δ¹³C < −45‰) developed under the influence of anaerobic methane oxidation in the shallow parts (<115mbfs) of a sediment core, are compared with those in isotopically heavier siderite (δ¹³C > +5‰) from the zone of microbial methanogenesis at the deeper sediment depths (115-197mbfs). Results showed both types of cold-seep carbonate in two geochemical regimes have a comparable range of concentrations of redox-sensitive (e.g. V, Cr, Co, Ni, Zn) and refractory elements (e.g. Sc, Cs, Ga, Zr, Hf, Ta, Th), indicating limited impacts of mineralogy and their genetic processes on the distribution of these elements. In contrast, the degree of enrichments of elements like Si, Al, Sr, Y, HREE, and U in those isotopically discrete seep carbonates were distinctly different. Such compositional variability in two types of authigenic carbonates is described in terms of their diverse formation mechanisms, mineralogical controls, and variable contributions of terrigenous materials.