UC Riverside

By about 2.0 billion years ago (Ga), there is evidence for a period best known for its extended, apparent geochemical stability expressed famously in the carbonate-carbon isotope data. Despite the first appearance and early innovation among eukaryotic organisms, this period is also known for a rarity of eukaryotic fossils and organic biomarker fingerprints, suggesting low diversity and relatively small populations compared to the interval that followed. Nevertheless, the search for diagnostic biomarkers has not been performed within an independent paleoenvironmental context that should reveal the facies that were most likely hospitable to these oxygen-requiring organisms. Shales and mudstones obtained from drill core of the ca. 1.45 Ga Roper Group from the McArthur Basin of northern Australia provide one of our best windows into the mid-Proterozoic redox landscape. The group is well dated and minimally metamorphosed, and previous geochemical data show a relatively strong connection to the open ocean compared to other mid-Proterozoic records. Consequently, conditions captured in the Roper Group may better reflect the redox state of the ocean margin and may reveal eukaryote-favoring setting. Despite this potential, a comprehensive, multi-proxy study of the Roper Group had not been undertaken. Here we present one of the first integrated investigations of Precambrian biomarkers performed within a strict inorganic proxy context. Results show a textured paleoredox structure for the Velkerri Formation, vacillating between oxic and anoxic, even euxinic conditions in the water column. Despite this variability, including the likely presence of oxic bottom waters, we see no evidence for the sterane compounds that would point convincingly to the presence of eukaryotes in this marine basin roughly 1.45 Ga. Also missing are the aryl isoprenoids that would delineate shallow photic zone euxinia and thus the likelihood of appreciable anoxygenic primary production, even though the iron and trace metal data are consistent with the presence of euxinia deeper in the water column. The absence of eukaryotic signals--despite our search across oxic and anoxic facies that should favor their habitability and the preservation of their records, respectively--suggests that other controls such as long-term nutrient and oxygen deficiencies may have throttled the distributions and diversity of early complex life.