Using iron sulphide as a marker to decipher taphonomic signals in fossil marine successions

Reedman, Christopher 2021. Using iron sulphide as a marker to decipher taphonomic signals in fossil marine successions. PhD Thesis, Cardiff University. Item availability restricted.

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Abstract

Iron sulphide is established as a marker to decipher taphonomic signals in the Early Jurassic of Dorset and East Devon. The extent to which pyrite replacement contributes to the representation of ecological skeletal diversity is evaluated. Variations in the preservation and composition of the shelly fossil record are analysed in relation to taphonomic processes associated with the Missing Molluscs effect. The replacement context for syn-sedimentary pyritisation broadly functioned as a taphonomic window in two scenarios: within the TAZ where aerobic decay of retained organic matter established localised reducing conditions inside ammonoid shells and enabled pyrite precipitation, or where the TAZ was elevated to within the water column such that molluscan spat and ammonoids were pyritised. Local buffering effects were imposed during bacterial sulphate reduction and pyrite precipitation. The fossil record was influenced by faunal recovery and anoxia at the T-J boundary interval; changing palaeoenvironmental conditions, notably fluctuations in the redox state; and taphonomic biases. Taphonomic distortion was significant in oxic environments where skeletal diversity was high, and moot within dysaerobic/anoxic environments where diversity was low. In open marine settings with a diverse biota, primary biomineralogical shell composition is significant in distortion; the type of bias affects different trophic levels whose presence is in part ecologically controlled. The application of iron palaeoredox proxies, specifically FeHR/FeT and FePy/FeHR ratios, to characterise changes in bottom water redox conditions within the local strata requires further work. Infrequent FeHR/FeT ratios reflect inferred oxygen conditions, but the range of lithology specific values shows extensive overlap and most inferred non-anoxic samples plot spuriously above the anoxic threshold. Masking of depositional redox signals relates to the validity of FeHR/FeT ratios in oxic carbonate-rich sediments, bioturbation that altered original FeHR/FeT ratios via physical mixing, and sampling resolution. Most data should be considered a time-averaged representation of palaeoenvironmental redox conditions.

Item Type:	Thesis (PhD)
Date Type:	Completion
Status:	Unpublished
Schools:	Earth and Environmental Sciences
Funders:	50% from Charles Wright Fund, 50% from School of Earth and Environmental Sciences, Cardiff University
Date of First Compliant Deposit:	16 August 2022
Last Modified:	06 Jan 2024 04:21
URI:	https://orca.cardiff.ac.uk/id/eprint/151972

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