The keeper of Chronicles - Investigating calcium carbonate archives using clumped thermometry

Abstract

Calcium carbonates (CaCO3) are the most abundant mineral archive for reconstructing past climate owing to their tendency to register the environmental condition during their formation. From biogenically mediated carbonates (molluscs, foraminifers, coccolithophores) to abiogenic precipitates (speleothem, tufa and pedogenic carbonates), the geochemistry of carbonates is exploited for the reconstruction of temperatures and hydrological conditions during sedimentation. Oxygen isotope (δ18O) thermometry in carbonates is a conventional tool used as a proxy for temperature estimation but with the caveat of assuming δ18O value of the palaeowater. A most advanced analytical method for examining CO2 isotopologues sourced from carbonate minerals enables the determination of heavier carbon and oxygen bonds (13C-18O), also known as "clumped isotope" abundance (denoted as Δ47), which trace directly to the carbonate precipitation temperature. In this work, we investigated the reliability of Δ47 in temperature reconstruction in a myriad of modern carbonates with independent knowledge of growth temperature to validate this methodology and used their fossil counterparts to deduce palaeo-environmental conditions. The first part of the thesis includes a study on biologically precipitated carbonate mollusc shells from inter-tidal and freshwater setups. Gastropod shells of different species were collected from various locations in India, exhibiting a range of Δ47 values; some provided the expected temperature, while others showed an offset from the true value. Biogenically produced carbonates are prone to vital effect, which affect the Δ47 value, leading to erroneous temperature estimation, with an observation that the prosobranch gastropods are suitable mollusc species for environmental or climate investigations as compared to pulmonated gastropods. Interestingly, Telescopium sp. demonstrates δ18O and Δ47 consistent with the instrumental record. We used fossil specimens of the same species to investigate temperature during the Little Ice Age. Here we analyse carbonate from the freshwater bivalves (Unionidae) originating from the bank of the river Ganges in the alluvial plain near Varanasi. Live specimens were collected during the post-monsoon period of 2018 and 2020. Analysis of Δ47 values in these samples revealed different mean temperature values for both years, which exhibit the disequilibrium precipitation process owing to the change in river water chemistry observed in the different sampling periods. A fossil specimen belonging to the same species was analysed to infer monsoon dynamics during the Late Miocene. In the second part of the thesis, present Δ47 observations on the modern speleothem carbonate precipitate from caves of South India and North-East India caves. Previous studies have reported the effect of kinetic isotope fractionation in speleothem associated with CO2 degassing from cave drip water during rapid growth, which contributed to observed anomalous Δ47. The speleothems from the South Indian cave formed under high relative humidity (~98%) condition displayed Δ47 values near similar to the observed temperature. The results show that speleothem geometry, particularly surface area and water film thickness, is crucial for accurate prediction of temperature analysis Δ47. In contrast, the Northeastern cave carbonates formed under variable relative humidity and lower temperature conditions reported Δ47 values registering a strong disequilibrium isotopic fractionation proportionate to the rate of carbonate precipitation. Cave-specific environmental factors, such as ventilation-driven humidity changes and diurnal temperature variations, exacerbate non-equilibrium isotope effects. This study confirms that speleothem in the South Indian cave is a reliable candidate for carbonate-clumped thermometry studies. Further, we reported temperature from Δ47 values measured in the Southern Indian Belum cave for hydroclimate reconstruction during interglacial Marine Isotopic Stage 9. We have demonstrated the importance of understanding microclimatic conditions when applying carbonate-clumped thermometry in cave carbonate records. Besides its use as thermometry, the sensitivity of Δ47 to multiple physical parameters makes it an ideal proxy to identify processes contributing to disequilibrium signatures. We illustrate the application of carbonate-clumped thermometry coupling with fluid inclusion isotope analysis in speleothems to ascertain kinetic fractionation across several stages of its growth. We further propose a simultaneous analysis of triple oxygen isotope (∆′17O ) and clumped isotope in speleothems to understand precipitation kinetics. Triple oxygen isotopes displayed in the Δʹ17O and δʹ18O space for speleothems allowed for constraining the cave kinetics, Rayleigh distillation, temperature effects, and evaporation/seasonality. This chapter demonstrates how the complementary use of fluid inclusion and triple oxygen isotope analyses can further enhance the scope for interpretation of fossil speleothem records, enabling the reconstruction of cave microclimates, shifts in moisture sources, and precipitation patterns over varying timescales. Overall, the thesis encapsulates the utility and constraints of carbonate- clumped thermometry in biogenic molluscs and abiogenic speleothems records.