Ritwika Sengupta's seminar, Oxford University

An early cretaceous (Berriasian-Hauterivian) low latitude terrestrial temperature and hydrological reconstruction using D180 and clumped isotopes of sphaerosiderites

Ritwika Sengupta's seminar, 2020

Abstract

Terrestrial and marine carbon isotope records indicate a significant global carbon cycle perturbation during the Valanginian stage (the Valanginian positive Carbon Isotope Event, CIE). Although this event has typically been associated with decreased pCO2 and cooling, evidence of warm and invariant low latitude sea surface temperatures suggests complexity in the climate response. Therefore, the current constraints on temperature change during the Valanginian CIE are poor, whilst little attention has been paid to hydrological responses. The non-marine Wealden strata of Southern England (Berriasian – Hauterivian) span the Valanginian CIE and provide an ideal archive for reconstructing low latitude continental palaeoclimate during this event. These strata contain abundant sphaerosiderites, spherical iron carbonate concretions (FeCO3), which formed in waterlogged soils in wetland environments and are thought to record the regional precipitation δ18O signal and the atmospheric temperature signature. Sphaerosiderites from four cores spanning the entire Wealden stratigraphy were studied. Siderite clumped isotope compositions (Δ47) indicate terrestrial temperatures remained warm (33-39oC) throughout the Valanginian in the Wealden. Average siderite δ18O values are relatively enriched in 18O in the Upper Berriasian (-0.8 to 0.2 ‰) and depleted in the lower Valanginian (-2 ‰). Siderite δ18O increases to 0.3 ‰ in the mid Valanginian, just before the start of the Valanginian CIE, and then decreases to -1 ‰  by the Valanginian–Hauterivian boundary. These periodic δ18O changes are decoupled from the CIE, and likely reflect regional changes in hydrology independent of the global carbon cycle perturbation. The geochemistry of pure siderites and more complex samples (containing more than one carbonate phase) were assessed in light of modern sphaerosiderite data, our own experimental results, and climate models. This comparison implies that the Wealden sphaerosiderites record a seasonally biased palaeoenvironmental signal within a dynamic wetland, which correlates with sedimentological evidence and GCM models. This provides a better understanding of the paleoclimatic changes through the Early Cretaceous in the Weald Basin and further constrains siderite as a proxy for palaeohydrology.