The Fate and Mobility of Heavy Metals in East Anglian Salt Marsh Sediments
Measuring the evolution of redox conditions in Earth’s atmosphere and oceans through time is a prominent challenge in Earth Sciences. Geochemical proxies in sedimentary rocks used to reconstruct past redox conditions—such as the speciation of iron mineralogy, δ98Mo, and δ53Cr—rely on the assumption that the geochemical signature acquired at the sediment surface is translated directly into the geologic record. In this thesis, I have used East Anglian salt marsh anoxic pond sediments as a modern analogue to test the effect of diagenesis on these paleoredox proxies. This site is particularly applicable because of an intriguing geochemical dichotomy that exists; the anoxic sedimentary porewater beneath ponds either contains high concentrations of ferrous iron or aqueous sulfide, which are somewhat comparable to the ferruginous and euxinic ocean sediments of the mid-Proterozoic respectively. I found that proximity of tidal creeks imparts a first order control on the spatial distribution of sulfide-rich and iron-rich pond sediments. Evidence is provided of a geochemical "switching" from iron-rich pond sediment chemistry to sulfide-rich pond chemistry which is hypothesised to be driven by an accumulation of organic carbon over time. High resolution porewater and sedimentary δ98Mo samples are used to infer the behaviour of Mo within these sediments. The speciation of iron mineralogy is the primary control on Mo distribution in iron-rich pond sediment. Two separate sulfide-rich pond sediments with similar sulfide concentrations have very different δ98Mo profiles; it is hypothesised that these differences reflect the amount of time since that pond sediment was iron-rich. This suggests that sedimentary δ98Mo can be altered during a diagenetic redox change. A method to measure δ53Cr using thermal ionisation mass spectrometry is developed and used to determine the Cr isotope composition of these pond sediments. The δ53Cr in iron-rich and sulfide-rich pond sediment occupies a narrow isotope range which is more similar to oxic marine settings than anoxic marine settings. This implies that the redox conditions of the water column, rather than the sediment porewater, dictates the δ53Cr which is recorded. The reaction of aqueous sulfide with iron minerals is a fundamental diagenetic reaction observed in these salt marsh pond sediments. I used a simple diagenetic model to illustrate how a significant proportion of mid-Proterozoic sedimentary rock samples classified as euxinic using conventional iron speciation classification could be explained by solely diagenetic conversion of iron mineralogy. The work in this thesis addresses the hitherto underappreciated importance of early diagenetic reactions on paleoredox proxies. A better understanding of these reactions will help deconvolve what the actual paleoredox conditions were in the geological past.
Geochemistry, Biogeoscience, Chromium, Molybdenum, Salt marsh
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