The Ocean Chemistry Changes During Meso-to Neoproterozoic

The Proterozoic(2500-641 Ma,million years ago)witnessed the change of the early Earth from an anoxic to an oxygenated state.It also records the development of various eukaryotic communities and the onset of animals,which was closely related to the atmospheric oxygen level.The Mesoproterozoic is thought to have experienced important changes in ocean chemistry and eukaryote evolution.In this study,samples from the Yanshan Basin in North China were collected,and the iron species,redox sensitive trace metal concentrations and multi-S isotope were analyzed to investigate the ocean chemistry and its implications to the evolution of eukaryotes during the Mesoproterozoic.The late Neoproterozoic was considered to record the second significant oxygenation event,which coincided with the evolution of eukaryotic biota and the rise of animals.But the exact time of the oxygenation and how it happened is still under debate.We analyzed TOC and TS content as well as multiple S-isotope of the samples from the Datangpo and Doushantuo formations in the Nanhua Basin,South China,to explore the changes of S cycling and the ocean chemistry before and after the Marinoan Glaciation,as well as its implication to the evolutions of the early metazoans.1.We report analyses of Fe species and redox-sensitive trace metals of U and Mo from the Yanshan Basin in North China.The Fe speciation data from the Wumishan,Hongshuizhuang,and Tieling formations first reported in this study help bridge the global data gap of Fe species data during 1500-1400 Ma.Our data suggest likely oxic bottom water conditions for the Wumishan Formation and ferruginous bottom water conditions for the Tieling Formation.The Fe species data can distinguish ferruginous conditions for the upper Hongshuizhuang from sulfidic depositional conditions for the lower-middle Hongshuizhuang Formation.Based on Fe species and trace metal data,redox conditions likely varied from anoxic conditions with transient euxinia in the lower-middle Xiamaling Formation to oxic conditions in the upper Xiamaling Formation.The new Fe speciation data from the Yanshan Basin,when combined with previously published data,show a surprisingly high proportion of middle Proterozoic samples with very low highly reactive to total iron ratios.This suggests either a relatively large extent of oxic seafloor,or non-uniformitarian iron cycling.Resolving this discrepancy will be key to understanding the extent of suitable habitats for complex eukaryotes.2.Pyrite ?34S for the Yanshan Basin varies extensively from-16.91‰ to+48.21‰with A33S ranging from-0.0829‰ to+0.083‰,which are characterized by heavy 834S.Modeling suggests the significance of microbial sulfur disproportionation in Mesoproterozoic S cycling.Our data highlight the important role of Rayleigh Fractionation in the S cycling of the Yanshan Basin and suggest the low sulfate concentrations in Mesoproterozoic consistent with the previous studies.3.We present high-resolution paired ?34S and A33S from the Datangpo and Doushantuo Formation in the Nanhua Basin,South China,as well as their TOC and TS contents,to investigate the variations of sulfate concentrations from pre-to post-Marinoan Glaciation as well as more details of the oxygenation of the Earth's surface environment.Our results suggest low sulfate concentrations and a key role of Rayleigh Fractionation process during the deposition of Datangpo Formation.Comparing sections in different depths,more negative A33S in our study suggests more intense S reservoir effects in Xiangtan section,possibly due to high organic matter concentration.The variation of rising A33S with decreasing ?34S from Datangpo to Doushantuo Formation suggests a signature from sulfate-limited system to open system,which reflects an increase in sulfate concentrations from pre-to post-Marinoan Glaciation,consistent with the hypothesis of the growth of Ediacaran sulfate reservoir.The results for Doushantuo Formation within the dissimilatory S fractionation model suggest the important role of microbial sulfur disproportionation,although its significance to the sulfate concentrations for Ediacaran in South China is still needed to be discussed.The enhanced sulfate concentrations suggest increasing atmospheric oxygen levels in Ediacaran,which could remove the barrier of animal evolution,allow larger body sizes,support more complex food webs and increase the marine ecosystem complexity.