Oxygen isotopic composition of Paleoproterozoic seawater revealed by clumped isotope analysis of dolomite, Vempalle Formation, Cuddapah, India

The Paleoproterozoic (-2.0-1.9 Ga) Vempalle Formation, in the south-western part of the Cuddapah Basin, is a well-studied Paleoproterozoic succession. Detailed descriptions of sedimentological features, micro-textures, and isotopic compositions of dolomite in the stromatolites and associated lithologies suggest that the succession was deposited on a carbonate ramp. Early burial diagenesis of the dolomite modified the original isotopic signatures. Nonetheless, here we discern evidence for two distinct depositional events based on clumped isotopic signatures preserved within dolomitic stromatolite and the underlying dolomite mud. Dolomitic muds within cherty precipitate record original thermometry signatures from these rocks. Clumped isotope analyses suggest that the lowest temperature of the Paleoproterozoic seawater was 21.8 +/- 2 degrees C, and that its isotopic composition (delta 18OVSMOW) during dolomite precipitation at equilibrium was -9.6 +/- 0.1%o, which is lighter than that of present-day ocean water. The overlying stromatolitic dolomite exhibits a temperature range of 70.7-109.8 degrees C, reflecting late-stage diagenetic processes and hydrothermal activity. Hydrothermal fluids were isotopically enriched (up to 4.9%o) suggesting their mantle origin and interaction with evaporative seawater. We evaluated the degree of water-rock interaction with water of heavier isotopic composition that was responsible for the precipitation of late phase dolomite found in the strata overlying carbonate mud.