Clumped isotope geochemistry of British Middle and Upper Jurassic sedimentary archives

Paxton, Richmal (2022) Clumped isotope geochemistry of British Middle and Upper Jurassic sedimentary archives. Doctoral thesis, University of East Anglia.

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Clumped isotope (Δ47) data from British Middle and Upper Jurassic diagenetic and biogenic (skeletal) carbonates is presented. Δ47 provides an assumption-free measure of carbonate formation temperature, which, when used in paleotemperature equations, allow derivation of the precipitating fluid δ18O.

The earliest microspar cements, from the bodies of early diagenetic, mudstone-hosted, septarian concretions (British Middle and Upper Jurassic), formed at temperatures between 9 ± 5°C and 18 ± 5 °C, from pore fluids with δ18O between 0.2 ± 1.1‰ and -2.2 ± 1.1‰ VSMOW. The results confirm synsedimentary concretion growth from marine pore fluids. Concretion body cements are proposed as robust archives of benthic palaeotemperatures.

Metre-scale, sandstone-hosted calcite concretions from the Bathonian Valtos Sandstone Formation (Great Estuarine Group) initiated at temperatures of 45 ± 6 °C from isotopically depleted, meteoric pore waters (-12.4 ± 1.1‰ VSMOW); the fluid compositions unequivocally identify cementation as an early Paleocene event. Temperatures rise in the outer part of the concretions to 73 ± 8 °C, and δ18OFLUID compositions become enriched (2.1 ± 1.1‰ VSMOW). This heating records interaction between meteoric-water and hot lavas formed during the earliest Hebridean Igneous Event. The high temperature signal is found in Valtos Sandstone Formation concretions basin-wide, implying that the porous sandstone was a conduit for hot water in the early Paleocene.

Δ47 values in ‘pristine’ aragonite shells (Praemytilus strathairdensis) from the lagoonal, Lealt Shale Formation (Great Estuarine Group), give a range of temperatures (23-58 °C), mostly too high to represent growth environment. Values above 35 °C are interpreted to have undergone partial solid-state resetting of Δ47 under burial temperatures >50 °C but <100 C. The lowest temperatures (20-30 °C) and lagoon water δ18OFLUID compositions (0‰ to -2‰ VSMOW) are plausible palaeoenvironmental values. Wider reassessment of preservation criteria for Δ47 studies of biogenic aragonite is recommended.

Item Type: Thesis (Doctoral)
Faculty \ School: Faculty of Science > School of Environmental Sciences
Depositing User: Chris White
Date Deposited: 31 May 2023 13:38
Last Modified: 31 May 2023 13:38


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