Oxygenated oceans persisted after the Lomagundi Event: Evidence from the Zaonega Formation

Abstract

The Lomagundi-Jatuli Event (LJE; 2.22–2.06 Ga), the longest-lived C isotope excursion in Earth’s history, is commonly viewed as associated with increased burial of organic matter that lead to a Paleoproterozoic “Oxygen overshoot” (Bekker & Holland, 2012). The end of the LJE is, conversely, associated with a recovery of anoxic conditions that might have arrested the evolution of complex life for the next billion years. The post-LJE Zaonega Fm. (ZF; 2.0 Ga), an organic matter-rich carbonate-mudstone succession, is a linchpin in the study of Paloeproterozoic redox shifts. Geochemical data from the ZF has often been interpreted as prime evidence for an “O2 crash” (e.g., Scott et al., 2014), despite other authors including the ZF among well-oxygenated LJE-type successions (e.g., Partin et al., 2013). Here, we present trace element analyses from several recent drill cores in the ZF that contain some of the highest Mo, U and Re concentrations reported to date in pre-Neoproterozoic shales (1009 μg g−1; 238 μg g−1; and 516 ng g−1, respectively). These enrichments are most parsimoniously explained through the establishment of an efficient trace metal trap in a highly bio-productive semi-restricted basin that was intermittently, but reliably, connected to a trace metal-rich ocean. Given that U and Re can only accumulate in an oxic water column, these data imply oxic waters overlying continental shelves during the deposition of the ZF and, therefore, undiminished levels of O2 following the LJE. The temporal decoupling between the LJE and the O2 overshoot, in turn, places doubt on our understanding of the mechanisms underlying some of the most profound environmental shifts in the Paleoproterozoic.