The Paleoproterozoic marine biosphere: Impacts on the ancient Earth surface system

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Throughout much of the oceans, microbial life is not lacking carbon, nitrogen or oxygen; instead, it is the scarcity of phosphorous and essential trace metals, e.g., iron, molybdenum, nickel, cobalt and zinc, that limits microbial proliferation and productivity. This has likely been true since the dawn of life over 3.8 billion years ago (Ga) when microbes began evolving elaborate metal scavenging strategies to incorporate solutes from their aqueous milieu. Trace metal availability in seawater is, in part, controlled by long-term geological processes, yet is also affected by the activity of the marine biosphere that controls their speciation and distribution in a myriad of ways. Over the last decade, I have developed an extensive database of trace metal concentrations in both banded iron formations (BIF) and shales that have revealed secular trends in their availability over the course of Earth's history. Critically, however, our ability to interpret this data further is hampered by uncertainties about how microbes affected trace metal cycling in the water column, how this influence may have evolved over geological timescales, and how the evolution of microbial lineages may be recorded in ancient sedimentary records. Resolving these uncertainties is a new trajectory of my research program which I consider critical to fulfilling my long-term aim of better understanding the co-evolution of life and the Precambrian environment. Therefore, in this NSERC DG proposal, I have detailed 4 short-term objectives focused on experimentally determining how phytoplankton may have impacted trace metal availability in the microbially-dominated marine ecosystems of our ancient planet. I will then apply this knowledge to critical Paleoproterozoic sedimentary deposits that record fundamental changes to the Earth surface system, such as the 2.45-2.32 Ga Great Oxidation Event and the 2.22-2.06 Ga Lomagundi-Jatuli Event. Objectives (1-3) will experimentally determine the partitioning of trace metals to marine phytoplankton, cell lysates and clay; if extant marine photoautotrophs possess diagnostic trace metal compositions that can be used as biosignatures in the rock record; and whether the relatively more abundant dissolved organic matter in ancient oceans impacted metal speciation differently from today. Objective (4) will then use those results to better understand the controls on the chemical composition of marine sediments from the Paleoproterozoic. These projects will draw heavily on my extensive geochemical database of BIF and shales from that time, including rocks from Australia, South Africa, North America, Brazi, Chinal and Fennoscandia. The ultimate goal will be to improve our understanding of the nutrient landscape and bacterial diversity of Earth's ancient oceans.