Dodd, Matthew S; (2018) Biosignatures in Precambrian sedimentary rocks. Doctoral thesis (Ph.D), UCL (University College London).

Abstract

The overarching goal of this work was to trace the origins of organic matter as a possible biosignature for early life on Earth. This work documented the petrological, structural, isotopic, elemental and molecular properties of organic matter, in a suite of sediments ranging from the Eoarchean to the Phanerozoic. Particular attention was paid to iron formations which form a major part of the Precambrian sedimentological record. Optical and micro-Raman spectroscopy were used to document the petrological occurrences of organic matter and its associated diagenetic features. Focused ion beam and transmission electron microscopy were used to detail the nanoscale structures of the organic matter. Mass spectroscopic techniques were utilised to measure the isotopic, elemental and molecular properties of organic matter and its petrographically associated carbonate. The timing and possible mechanisms of organic matter formation were investigated and evaluated, against the observed attributes of the organic matter. This work has documented organic matter forming key mineral associations with apatite and carbonate, which point to the diagenetic recycling of biogenic carbon. The diagenetic cycling of organic matter is proposed, to fuel the formation of granules during organic matter, iron and sulphur cycling. This cycling is hypothesised to lead to heterogeneity in organic matter structure and preservation in sediments. Additionally, this work has demonstrated that fluid deposited graphite in metamorphosed sediments may represent redistributed, syngenetic organic matter, shedding light on the controversial origins of graphite biosignatures in Earths oldest rocks. Finally, a detailed study of microfossil structures and their associations with organic matter and biominerals in iron formations was conducted to provide evidence for the preservation of Proterozoic microfossils, and the earliest signs of life on Earth.

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