EXPLORING CADMIUM AS A PROXY FOR PRIMARY PRODUCTIVITY BEFORE THE GREAT OXIDATION EVENT

Many independent lines of evidence suggest that molecular oxygen was present in small amounts at Earth’s surface before accumulating in the atmosphere during and after the Great Oxidation Event (~2.5 to 2.3 billion-years-ago). Although the presence of this molecular oxygen likely necessitates cyanobacterial production, corroborating evidence in the form of nutrient consumption remains elusive. Many of the micronutrient metals—such as Co, V, Mo—can be accumulated in sediments by redox-related processes, making it challenging to differentiate changes in primary production from those driven by preservation. However, in the modern oceans, dissolved [Cd] (cadmium concentrations) closely correlate with those of dissolved phosphorous, an important macronutrient. While there is some debate as to the nutritional status of Cd in marine microbes, the stable isotopic composition of Cd in surface waters generally traces primary production since its stable isotope are fractionated during biological uptake. Thus, the stable isotopic composition of Cd in organic-rich sediments can potentially discriminate whether trends in sedimentary [Cd] reflects changes nutrient utilization or Cd burial efficiency—production or preservation, respectively. Here, we examine whether increases in Cd concentrations across the dioxygen ‘whiff’ reflect increases in primary productivity or redox-driven preservation by applying Cd concentration and isotope measurements to the Mount McRae Shale from Western Australia. We augment these results with a factorial experiment to improve isolation of bioauthigenic sedimentary phases relative to detrital background. We will use these data to illustrate how paired records of Cd concentrations and stable isotopic compositions can be used to study productivity and redox on the nascent biosphere of Early Earth.