ACTIVE BIOGEOCHEMICAL CYCLE DURING THE MARINOAN SNOWBALL EARTH

The Marinoan global glaciation represents the most extreme icehouse condition in Earth’s history. The ‘Snowball earth’ hypothesis proposed that the whole planet might have been covered by thick ice for tens of million years, resulting in a synglacial reduction of life activity and marine biogeochemical cycles. In order to investigate deglaciation impacting on marine biogeochemical cycles, here we measured carbon isotopic compositions of organic carbon (δ¹³C_org) and sulfur isotopic compositions of pyrites (δ³⁴S_py) of the Nantuo Formation in South China. The Nantuo Formation, equivalent to Marinoan glaciation, consists four lithological units, in ascending order, the lower diamictite (45 m), lower sandstone unit (60 m), upper diamictite unit (80 m), and upper sandstone unit (50 m). The diamictite unit is the deposition of glaciation, while sandstone unit, though containing thin diamictite layers, can be regarded as interglacial and deglacial depositions, respectively. In the early glaciation, δ³⁴S_py retains constant (~+29‰), but δ¹³C_org shows a gradually increase from -30‰ to -24‰. In the interglacial interval, δ³⁴S_py becomes more variable (ranging from -17‰ to +32‰), while δ¹³C_org show small range of variation (-27‰ to -24‰). Both δ³⁴S_py and δ¹³C_org remain more or less constant (+12~+17‰ and -25~-27‰) during the upper glacial interval. In the following deglaciation process, δ¹³C_org first show a slightly positive excursion followed by a 3‰ negative shift to the top of the Nantuo Formation, whereas δ³⁴S_py shows a gradual increase to ~+32‰, and become extreme variable at the top 5-m of the Nantuo Formation. Carbon and sulfur isotope data suggest that active biogeochemical cycles during the Marinoan glaciation, particularly the interglacial and deglacial processes, confirming active life activities as shown by the fossil record.