CRYOGENIAN PALEOCLIMATE FLUCTUATIONS AND ITS INFLUENCE ON MANGANESE DEPOSITS IN SOUTH CHINA

The Nanhua Basin of South China records a nearly complete Cryogenian stratigraphic succession, including manganese-ore-bearing cap carbonates of the Datangpo Formation. In this study, we report high-resolution chemical index of alteration (CIA) values to reconstruct paleoclimate variation during formation of these cap carbonates and related units. The CIA profile of drillcore ZK2115, which shows no relationship to core lithology, exhibits a progressive upsection increase, from values of 45-67 (mean 58) in the upper Tiesi’ao Formation (syn-Sturtian Glaciation) to 60-72 (mean 67) in the basal Datangpo Formation cap carbonate and to 64-70 (mean 68) in the overlying Datangpo Formation black shales (interglaciation). This pattern reflects a slowly warming climate during the final stage of the Sturtian Glaciation and the subsequent deglacial/early interglacial intervals. Co-fluctuations in CIA and Mn-carbonate precipitation during the deglacial interval can be interpreted as evidence of stepwise disintegration of the Sturtian icesheet, releasing pulses of cold, dense, well-oxygenated waters that episodically ventilated the deeper parts of the Nanhua Basin. These hyperpycnal flows transferred large quantities of dissolved Mn(II) to the sediment through precipitation of Mn(IV)-oxyhydroxides that were later reductively dissolved to promote diagenetic formation of rhodochrosite, the main Mn-carbonate mineral, under high-alkalinity porewater conditions. Our CIA dataset thus provides support for the “episodic ventilation” model of Yu et al. (2016; Palaeo-3, v. 459, p. 321-337).