Palaeoenvironment of a Late Triassic microbialite: clumped isotopes in the Cotham Marble

Schinaia, Stefania Anastasia (2023) Palaeoenvironment of a Late Triassic microbialite: clumped isotopes in the Cotham Marble. Masters thesis, University of East Anglia.

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Abstract

The Cotham Marble is a late-Triassic microbialite of Rhaetian (late Triassic) age, present in parts of SW Britain (especially the Bristol area), at the top of the Upper Cotham Member (Lilstock Formation, Penarth Group). The Cotham Marble consists of cycles of microbialites growing principally with laminated or dendrolitic morphologies. The combined use in this study of clumped isotope and high-resolution stable isotope analyses, backed by sedimentological observations, provides insights into the palaeoenvironmental conditions that facilitated development of the microbialites and the underlying facies of the Upper Cotham Member.

The study focussed on material from Wickwar (Gloucestershire) and the ∆47-derived temperatures from clumped isotope analyses show that microbialite depositional temperatures were generally warmer (ca. +12 to +26 °C) than those in an underlying thrombolite and non-microbial micrites (+8 to +12 °C). Moreover, evidence of temperature seasonality in the laminar and dendrolitic microbialite facies identified in previous studies was not found in the specimen from Wickwar. A distinct negative inorganic carbon isotope excursion was discovered in the first laminar stromatolite and is consistent with earlier work that implicated a phase of prolific microbial mat growth as a source of isotopically light carbon to the Cotham lagoon water. The combined data
on modern microbial analogues, clumped isotopes and stable isotope analyses overall suggest that: (a) regressive conditions promoted deposition of the lower facies of the Upper Cotham Member in cool, coastal marginal lagoons of broadly non-marine character; (b) the Cotham Marble microbialites developed during sea-level stillstand in quasi-marine, warm lagoons where evaporation was common and water residence times relatively long; (c) alternating conditions of exposure and inundation created the hydrodynamic conditions under which laminar stromatolites and dendrolites developed, respectively; (d) longer exposure may have facilitated formation of desiccation surfaces
within the laminar stromatolites and transitory colonisation of burrowing organisms in the latter two laminar stromatolitic phases.

Item Type:	Thesis (Masters)
Faculty \ School:	Faculty of Science > School of Environmental Sciences
Depositing User:	Zoe White
Date Deposited:	25 Jun 2024 13:41
Last Modified:	25 Jun 2024 13:41
URI:	https://ueaeprints.uea.ac.uk/id/eprint/95668
DOI:

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