Breaking the Boring Billion: A Case for Solid-Earth Processes as Drivers of System-Scale Environmental Variability During the Mid-Proterozoic

Abstract

Emplacement and weathering of large igneous provinces (LIPs) can have dramatic effects on global climate and biogeochemical systems. In the Phanerozoic, these events are often linked to expanding oceanic anoxia, widespread deposition of organic-rich shales, and metazoan extinctions. In the Precambrian, LIP activity would have had similar potential to perturb global systems, though the outcome may have differed given different starting conditions. Specifically, against a backdrop of mostly low oxygen conditions, Precambrian LIP emplacement may have stimulated oxygenation of the atmosphere and shallow oceans, which could have favored innovations among eukaryotic life. Here, we review the records of several oceanic-atmosphere redox proxies, summarize their utility, and compare them with the record of continental LIPs. While the mid- Proterozoic (1.8-0.8 Ga) is famous for long-term environmental stability, several independent proxies suggest the possibility that conditions were transiently more oxygenated at ~1.4 Ga. This time coincides roughly with a period of increased LIP activity that has been recently linked with widespread deposition of black shales. We explore the possibility that LIP-induced impacts on productivity might have favored organic matter burial, transient oxygenation, and potential advantages for aerobic life, focusing in particular on the feedbacks that maintained environmental balance for nearly a billion years. Our understanding of the evolution of Proterozoic environmental conditions is still nascent, but as records improve and we search for mechanisms that underpinned long-term stability or drove secular change, it is critical that the whole Earth system be considered.