Evolution of paleo-weathering during the late Neoproterozoic in South China: Implications for paleoclimatic conditions and organic carbon burial

摘要

The late Neoproterozoic was a key transition time for Earth, involving turbulent fluctuations of the atmosphere-ocean system including Snowball Earth events, supercontinent reorganisation, and revolution of the nutrient cycle, all of which could have potentially triggered the development of increasingly complex life forms. To further investigate the temporal variations of paleoclimatic conditions and to better understand the burial of organic carbon during this interval, comprehensive geochemical and mineralogical analyses were performed on upper Neoproterozoic samples from the Cryogenian Datangpo, the Marinoan-age Nantuo, and the Ediacaran Doushantuo formations in South China. Geochemical compositions indicate that sedimentary rocks from the Doushantuo Formation have a provenance from granodiorite-like rocks, and the Datangpo samples likely have a mixed provenance of basalt and granite. The chemical index of alteration (CIA) values of black shales from the Datangpo and Doushantuo formations are in a similar range (65-85), indicating moderate chemical weathering under a relatively warm and humid climate. However, conspicuous low Rb/Sr ratios and enrichment of chemically immature minerals for the uppermost Doushantuo samples, are indicative of elevated physical erosion during the terminal Ediacaran. Furthermore, a negative correlation between corrected CIA (CIA(corr)) and total organic carbon (TOC), and a positive correlation between organic carbon isotopic compositions (delta C-13(org)) and TOC, imply that supercontinent-related nutrient supply and concomitantly elevated marine primary productivity played an important role in organic carbon burial during the Ediacaran-Cambrian transition. In contrast, elevated chemical weathering under increased temperatures and intensity of the hydrological cycle occurred during the deposition of post-glacial shales from the Datangpo Formation and Member II of the Doushantuo Formation. Enhanced primary productivity fostered by chemical weathering-released nutrients and sequestration of organic carbon by pedogenic clay minerals generated under such climatic conditions, and probably redox condition variations, are inferred to have been the main control on organic carbon burial during this interval.