Chemostratigraphic insights on the evolution of surface environments and life over Earth's history

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SUMMARY Since the appearance of microbial life on Earth, as early as 4.1 billion years ago (Ga), its evolution has been tied directly to that of Earth's surface environments. This coupled evolution is the combined result of geological and biological processes that control trace element cycling. A number of trace elements are required by microbial lineages as micronutrients necessary for metabolic activity or as components in enzymes. Variations in trace element abundances in the ancient marine sedimentary record may therefore be tied to biological evolution. Iron formations, iron- and silica-rich chemical sedimentary rocks that precipitated in the Archean (4.0 to 2.5 Ga) and Paleoproterozoic (2.5 to 1.6 Ga) oceans, have been used extensively to reconstruct trace element abundances. These are especially important records, as the bulk of iron formation deposition occurs in the immediate lead up, and aftermath, of the Great Oxidation Event (2.48 to 2.32 Ga); the first permanent rise of oxygen in surface environments and a fundamental biological transition. To date, however, trace element records and chemostratigraphic profiles for Archean to Paleoproterozoic iron formations have been primarily constructed at coarse sampling resolutions and inferences regarding ancient seawater composition have been drawn from empirical models of trace element partitioning to precursor mineral phases. These factors have hindered the identification of environmental or biological controls that drive short-term trace element variability and have led to contrasting estimates for ancient seawater composition. Reconciling such uncertainties is critical to the long-term objective of my research programme: gaining a more robust understanding of coupled marine geochemical and biological evolution over Earth's history. Accordingly, in this NSERC Discovery Grant proposal, I have outlined three short-term objectives that seek to improve our understanding of paleomarine geochemical conditions and the controls on trace element cycling in ancient marine settings. Objectives 1 and 2 will generate new chemostratigraphic records including a chert record that spans 3.8 Ga of Earth's history and high-resolution records from 2.5 Ga iron formations deposited close to the Great Oxidation Event. These novel records will generate insights into trace element availability, which can be integrated with existing records, and constraints on the mechanisms that led to decadal to millennial scale trace element variability during increasing global oxygenation. Objective 3 will use these records alongside thermodynamically grounded surface complexation models for the precursor sediments to iron formations, iron oxyhydroxides and amorphous silica, to elucidate robust estimates for the composition of ancient seawater. These projects are designed to support my long-term goal of constraining how life and Earth's surface environments have evolved in tandem over the last 4 Ga.