CARBON, SULFUR AND NITROGEN ISOTOPE CHEMOSTRATIGRAPHY OF EARLY-TO-MIDDLE EDIACARAN IN SOUTH CHINA

The coevolution of life and environment is highlighted in the recent debate on the possible relationship between ocean oxygenation and animal evolution in the Neoproterozoic. Ediacaran successions in South China are ideal targets for focused and integrated geochemical analysis to infer the impact of redox conditions on evolution and ecology. While previous studies have been focused on shelf sections in the Yangtze Gorges, it is necessary to gain insights from sections in deep-water slope and basinal facies, as well as shallow-water facies sections in western Zhejiang province. Integrated geochemical data, including δ¹³Cc_arb, δ¹³C_org, δ³⁴S_pyr, δ³⁴S_CAS and δ¹⁵N_sed, from the Lantian Formation deposited in deep-water environments are analyzed. A large spatial gradient of δ¹³C_carb is present in the lower Doushantuo/Lantian Formation. This gradient is largely due to the influence of ¹³C-depleted authigenic/diagenetic calcite cements in organic-rich and carbonate-poor sediments deposited in deep-water facies. A pronounced negative δ¹³C_carb excursion in the upper Doushantuo/Lantian Formation, however, is spatially consistent and does not display a > 10‰ depth gradient. The δ³⁴S_pyr record seems to show a spatial pattern, with more negative δ³⁴S_pyr values in deeper-water facies. The spatial pattern of δ³⁴S_pyr is interpreted as evidence for a chemically stratified basin, with predominantly euxinic deep waters where pyrite precipitated in the water column has lower δ³⁴S_pyr values. The Lantian and Doushantuo formations do not preserve highly positive δ¹⁵N_sed values characteristic of partial denitrification in suboxic environments of modern oxygen-minimum zone. Integrated TOC, TN, δ³⁴S_pyr, and Fe speciation data suggest that oscillation between oxic and euxinic conditions is characteristic of the Lantian Formation The prelimentary results established in shallow-water facies through western Yangtze platform keep well consistent with the previous studies in Lantian area.