Reconstruction of temperature for Cenozoic and Proterozoic Ocean water using clumped isotope thermometry

Abstract

Clumped isotope thermometry is a novel tool which is used for the determination of formation temperature for carbonates of diverse origin and understanding different marine and terrestrial processes. The primary interest of this study is to develop use clumped as well as stable isotope as the tool to deduce temperature of ocean water from Cenozoic and Proterozoic time window. Stable and clumped isotope studies have previously shown that both of these isotopes experience different degrees of fractionation for different mineralogy of carbonates (e.g., calcite, dolomite, aragonite etc.) during the sample preparation procedures conducted at different temperature conditions. Major highlights of my research work include development of analytical procedure and revisiting the numerical equation for thermometry using aragonite and dolomite minerals. Exclusive, experiments are conducted at different temperatures to understand the isotopic fractionation during sample preparation and analysis. As a part of the present study, the natural aragonite specimens with well characterized temperature information are analysed in order to study the fractionation during acid digestion and clumped isotope thermometry calibration using sealed vessel or Break-seal method developed at IISc. Calcite and aragonite react or fractionate in a similar manner during phosphoric acid digestion which is evident from the previous theoretical and experimental studies. However, the protocol specific variation of temperature sensitivity and intercept values of the clumped isotope thermometric calibrations demarcates the existence of inconsistencies associated with the acid digestion protocols. This study proposes revised calibration for aragonite clumped isotope thermometry using the otoliths. As an application of the calibration study, palaeo-otoliths and travertine are analysed for temperature estimates infer about the freshwater availability near coastal United States covering the time from early Cretaceous, Eocene, Oligocene and Pleistocene time. Further, we validated the temperature of travertine formation analysing modern day tufa with known temperature record extending the thermometry to 40°C. A second analytical accomplishment includes designing and executing a new isotope dilution method for the clumped isotope analysis for small carbonate powder (2-3 mg). We have also established acid fractionation for dolomite during reaction with the phosphoric acid digestion at 25℃ using seal vessel method. We proposed acid fractionation correction factor of 0.069 for the break seal method, suitable for our experimentation. Using this approach, we determined clumped isotope ratio in the pristine relic carbonate mud from the Vempalle Formation of Proterozoic Ocean. Our observation revealed lowest temperature value of 21.7℃ and δ18OVSMOW value of water as -9.96‰. This is the first evidence documenting lighter oxygen isotopic composition of Proterozoic Ocean The overlying stromatolites with dolomites registered consistently higher temperatures (72.8-106.2℃) that might suggests precipitation from hydrothermal fluid or early phase burial diagenesis promoting water-rock interaction. Our observations resolved long-standing dispute about existence of mild and lighter composition of water in the Proterozoic Ocean.