Integrated stratigraphy of the upper Neoproterozoic succession in Yunnan Province of South China: Re-evaluation of global correlation and carbon cycle

This study provides a chemostratigraphic and sequence stratigraphic framework of a relatively unfossiliferous sub-Cambrian marine sedimentary succession in Yunnan Province of South China. Our results allow its global stratigraphic correlation and provide an improved understanding of marine carbon cycle during the late Neoproterozoic. The succession has a basal glacial diamictite (“Nantuo” Formation) and comprises two large-scale depositional cycles towards the Precambrian-Cambrian (PC-C) boundary, each of which changes from siliciclastic to carbonate sediments with environments ranging from shallow subtidal to evaporitic supratidal. In the absence of biostratigraphic control, attribution of the tillite has remained equivocal between the ∼710 Ma Sturtian and ∼635 Ma Marinoan glaciations. A compilation of known paleomagnetic and other geochronologic constraints suggests that the Nantuo glaciation in South China was at least diachronous, in which the “Nantuo” tillite in Yunnan belongs to the older glaciation. Stratigraphic patterns of the diagenetically-screened δ13C variation and depositional sequences during the carbonate-dominated interval show a striking similarity with those of the upper Cryogenian and entire terminal Proterozoic carbonates at widely-separated other localities rather than the terminal Proterozoic alone, supporting the view that deposition of the “Nantuo” tillite in Yunnan reflects the Sturtian rather than Marinoan glaciation. Further considerations of the δ13C results provide three important implications for the late Neoproterozoic marine carbon cycle. First, comparison of different δ13C profiles suggests considerable variability in the absolute values of carbonate on both intra- and inter-basinal scales, which differs profoundly from their relative homogeneity in marine Phanerozoic counterparts and may therefore reflect lateral isotopic heterogeneity in the oceanic carbonate reservoir during the late Neoproterozoic. Second, an empirical correlation between the δ13C and relative sea-level changes was applied to evaluate hypothesized explanations for δ13C temporal variation in surface sea, yielding short-term inverse and long-term positive relationships. The short-term trend marks periods of the negative δ13C excursions and is compatible to most of the existing arguments for the δ13C events. On the other hand, mechanisms causing the long-term trend remain enigmatic in the absence of comparable data from other localities. Our observation, however, suggests that at least two different mechanisms operated marine carbon cycle during the late Neoproterozoic. Third, the negative δ13C anomaly, characterizing the period following the Marinoan glaciation worldwide, was not coincident between kerogen and carbonate but occurs as a lagged drop in Yunnan, where a ∼7‰-depletion in kerogen preceded an ∼3‰-depletion in carbonate. Their lagged trends likely reflect different sources, as kerogen and carbonate signals are considered to monitor surface and deeper waters, respectively. The negative δ13C event would therefore have occurred in surface water, followed by a smaller 13C-depletion through the entire water column due to vertical mixing between surface and deeper waters at the studied site. This mechanism supports the proposed hypothesis that injection of the 13C-depleted atmospheric CO2 into the surface sea, deriving ultimately from quantitative dissociation of methane hydrate in terrestrial permafrost during the post-glacial warming, contributed to the negative δ13C anomaly.