Congratulations to Kathryn Rico who defended her dissertation on Friday, April 10, 2019
Advisor: Nathan Sheldon
Identifying how nutrient availability has impacted the evolution of the biosphere requires a comprehensive understanding of the oxygen availability (e.g. redox chemistry) of ancient environments. However, the sediments of ancient oceans are the only relics of their existence, making sediment geochemistry necessary for constraining ancient biogeochemical cycling. In particular, the geochemistry of elements that are sensitive to oxygen (e.g. redox-sensitive metals, such as Fe, Mn, and Mo) 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 (~0.55–2.35) oceans, wherein microbial life diversified and eukaryotic life evolved. This dissertation uses sediment geochemistry of a low-oxygen analogue for Proterozoic oceans—the Middle Island Sinkhole (MIS)—in order to better characterize biogeochemical cycling in the Proterozoic by 1) testing various metal redox proxies in a ferruginous setting, 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 confirming that MIS is ferruginous and that 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 identify whether or not trace metals can serve as biosignatures for microbial mat presence. I establish that the burial of trace metals in MIS and LH is not consistent with our expectations based on their use as paleoredox and paleoproductivity proxies. Therefore, this work has major implications for how we use these proxy metals to interpret redox chemistry within the fossil record. Additionally, there is no indication that bulk sediment trace metal abundance can be used as a biosignature for the community composition of microbial mats.
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 Mo and U covariation confirming that euxinia is not necessary for moderate Mo burial (in contrast to how Mo enrichments are conventionally interpreted). Altogether, the comparison of Nonesuch Formation to the modern analogue data indicates that elemental abundance is unlikely to constrain atmospheric oxygen, with no clear indicator for abundant biological productivity. Taken together, these results demonstrate that the burial mechanisms for metals in modern ferruginous environments need to be better understood. Only then can confidently use these metals as proxies for paleoenvironmental biogeochemical cycling.