Dynamic deep-water oxygenation of marginal seas in the aftermath of the Sturtian Snowball Earth: Insights from redox-hydrological reconstructions of the Nanhua basin (South China)

Abstract

The Cryogenian interglacial period, separated by the Sturtian and Marinoan icehouses, witnessed early radiations of eukaryotic algae and the first emergence of putative metazoans. To date, the detailed oceanic redox state in the aftermath of the Sturtian glaciation remains obscure, limiting our understanding of the causal linkage, if any, between environmental changes and biological evolution during this critical interval. In order to gain further insights into the marine redox framework following the Sturtian Snowball glaciation, here we present high-resolution geochemical results of an integrated study on the basal Datangpo Formation from the Daotuo area (South China), and also provide a systematically compiled dataset of counterparts in other profiles from the Nanhua Basin. The basal Datangpo Formation is characterized by black shales intercalated with Mn carbonate ore layers in the lower (Member I), and only black shales in the upper (Member II). The Member I black shales have relatively high abundances of Mo and Re, and display a MoEF-UEF pattern typical of Mn-oxide shuttle, indicating that the Nanhua basin was redox-stratified with the oxic-euxinic chemocline fluctuating above the deep-basin floor. Multiple lines of evidence, including positive PAAS-normalized Ce anomalies, negative δ13Ccarb values, and marked authigenic enrichments of manganophile elements (Co, Ni, Cu), imply that the Mn carbonate ore beds were generated through the diagenetic transformation of Mn oxides originally deposited under oxic bottom waters. Therefore, these Mn ore layers hosted by the Member I black shales reflect episodic benthic oxygenation. By contrast, the Nanhua basin was more stagnant during deposition of Member II black shales, as supported by their rather low Mo/TOC ratios, as well as the relatively lower abundances of Mo and Re compared to Member I black shales. Moreover, the Mo isotope data of the basal Datangpo Formation indicate that the global ocean remained pervasively anoxic. Taken together, this study reveals a more holistic redox landscape in the wake of the Sturtian Snowball Earth—that is, extensive anoxia in the global ocean with episodic deep-water oxygenation in the marginal seas such as the Nanhua Basin. These epicratonic seas with dynamic redox variations might promote the ecological revolution during the interglacial period immediately after the Sturtian glaciation.