Controls on the Sediment Geochemistry of a Low-Oxygen Precambrian Analogue

Abstract

With early life presumed to have evolved in ancient oceans and lakes, identifying the availability of oxygen (e.g. redox chemistry) in these environments is necessary to establish the environmental conditions required for early biosphere evolution. However, the sediments of ancient oceans are the only relics of their existence, making sediment geochemical analyses critical tools for analyzing ancient biogeochemistry. In particular, the geochemistry of elements that are sensitive to oxygen (e.g. redox-sensitive metals, such as iron, manganese, and molybdenum) have proven useful for considering the biogeochemical cycling of modern environments. Subsequently, these geochemical tools have been considered robust proxies for identifying the redox chemistry of ancient systems such as Proterozoic (~0.5–2.3) oceans, wherein microbial life diversified and eukaryotic life evolved with limited atmospheric and aquatic oxygen. This dissertation uses a Proterozoic ocean analogue—the Middle Island Sinkhole (MIS)— to characterize the sediment geochemistry of a modern low-oxygen aquatic environment, and consider implications for the biogeochemical cycling of ancient oceans and lakes. This is achieved by 1) testing various metal redox proxies in an iron-rich environment (such as is inferred for Proterozoic oceans), and 2) assessing how the presence of a cyanobacterial microbial mat in MIS impacts macronutrient and metal burial in sediments. In Chapter II, I explore macronutrient and iron geochemistry in MIS sediments and a fully oxygenated Lake Huron control site (LH). Differences in redox between the two locales drive the enhanced burial of macronutrients in MIS, with iron speciation results consistent with known water chemistry: MIS is ferruginous and LH is oxic. Given that iron speciation in MIS is only recording a small portion of the water column, these results indicate that we must take caution when using iron geochemistry to interpret water column redox in the fossil record. In Chapter III, I use sediment redox-sensitive trace metal contents and microbial community composition in MIS and LH to consider how trace metals do or do not reflect the presence of a microbial mat. Results indicate that bulk sediment trace metal abundance cannot be used as a biosignature for the community composition of microbial mats in the analogue site. Additionally, I establish that the relationships between trace metals and organic carbon in MIS and LH are not consistent with our expectations based on their use as paleo-redox and paleo-productivity proxies. Therefore, this work impacts how we use proxy metals to interpret redox chemistry within the fossil record. In Chapter IV, I compare the sediment geochemistry of MIS to that of Proterozoic lake sediment—the Nonesuch Formation (~1.1 billion years old)—in order to determine the redox chemistry of this Proterozoic lake, and to gauge whether or not the abundance of redox-sensitive metals can help to elucidate biological productivity or atmospheric oxygen levels. Iron geochemistry describes fluctuating oxic and anoxic redox chemistry within the Nonesuch Formation, with molybdenum and uranium covariation confirming that euxinia is not necessary for moderate molybdenum burial. Altogether, the comparison of Nonesuch Formation to the modern analogue data indicates that elemental abundance is unlikely to record atmospheric oxygen, with no clear indicator for abundant biological productivity. Taken together, these results demonstrate that we need critical evaluations of metal burial mechanisms in modern ferruginous environments before we can confidently use these metals as proxies to constrain paleoenvironmental biogeochemical cycling.