Bridging the (redox) gap: reconciling biomarker and inorganic proxy records in the Middle Cambrian Currant Bush Formation

Abstract

Redox conditions have exerted a strong control upon organic matter (OM) preservation throughout time, determining oxidant availability and heterotrophic activity, and thus controlling OM remineralisation rates across different marine environments. Despite this, redox effects upon ancient OM at a molecular level are comparatively poorly understood. To elucidate redox effects upon biomarkers, this thesis investigates ~103m of the middle Cambrian Currant Bush Formation (CBF), a unit of cyclical outer ramp carbonates, siltstones and black shales, which preserve biomarkers throughout and indicate redox variability. This work undertook high-resolution multiproxy analysis of the CBF (n=109), combining biomarkers with elemental abundances (U, Mo, V, Re), Fe speciation, organic C and bulk N isotopes, mineralogy, sedimentology, and palaeontology. First, this thesis refines the characterisation of CBF euxinia, identifying S limitation from intermediate Fe speciation values (Fepy/FeHR 0.6-0.8) and Fe limitation (FeHR/FeT vs FeT (wt.%) R2=0.9). S limitation in the CBF is additionally indicated by tracers of low sulphide (DBT/Phen xbar=0.2, HHI xbar=3.2) and sulphate (3-MHI xbar=3.0, max.=6.8). These data plug an important Cambrian gap in 3-MHI records. Second, this thesis describes, in addition to evidence for euxinia (delta15N <1.0permil, TOC/N 20-40, Mn <250ppm) contradictory evidence for oxia throughout CBF deposition (AIR xbar=1.6, Pr/Ph mostly >2, TOC/P mostly < 50, UEF xbar=6 and MoEF xbar=10). Together this indicates rapid oscillation of benthic oxic-anoxic sulphidic redox conditions in the CBF, likely driven by frequent storm incursion and relating to oscillating environmental energy levels evident in alternating silty and muddy laminae. Deposition rate estimates gained from laminae counts, along with constraints upon the maximum possible depositional duration of the CBF, require a high frequency, possibly seasonal or decadal, for lamination depositional events, and indicatively for benthic redox oscillations. This work constitutes a highly detailed characterisation of an ancient episodically anoxic system. Third, this thesis investigates sedimentary cyclicity in the CBF, finding that the carbonate-siltstone-black shale cycles observed describe gradual deepening upwards trends with rapid reversions, most likely produced by tectonic subsidence effects. This work investigates 2 particular cycles (92.2-95.6m depth, n=51) identifying parallel cycling of benthic redox conditions driven by increasing up-cycle limitation upon benthic oxygen (U 1-16ppm, Mo 1-13ppm). A wide variety of biomarker parameters demonstrate cyclicity and strong correlations with U and Mo (C30/C31 hopane vs Mo R2=0.7, diastimgastanes/stigmastanes vs U R2=0.7), identifying an overarching control of benthic redox upon molecular distributions. Observed redox responses of biomarkers include systematic covariation in short/long homologue ratios in steranes and aromatic steroids, hopanes, benzohopanoids, and 2,3,6-trimethylarylisoprenoids (C27/C29 steranes vs C27/C33-35 hopanes R2=0.7), and systematic covariation in hopane and diasterane isomer ratios (C27 hopane Ts/Tm vs C30 hopane/moretane R2=0.9). These responses identify distinct redox effects upon biomarkers in the CBF, with homologue relative abundances controlled by oxidation 'clipping' of side-chains determined at the sediment surface and proportional to seafloor residence time, and isomer ratios controlled by double bond preservation determined by redox conditions during early burial. This represents a major advancement in the understanding of redox responses of biomarkers. Show more