Abstract
Cap dolostones of the Ediacaran Doushantuo Formation (Yangtze Platform, South China) from various palaeo-water depths were studied to evaluate the extent of their diagenetic alteration and to assess temporal and spatial variations of seawater chemistry in the aftermath of the Marinoan glaciation. Diagenetic fluid overprint is common in cap dolostone lithologies. However, the mobilization of trace elements and the modification of Sr and O isotopic compositions are variable and were controlled by multiple stages of fluid overprint. The highest 87 Sr/ 86 Sr ratios (up to 0.7246) occur in cap dolostones from the palaeo-slope environment at the Huanglianba section, whereas cap dolostones deposited in platform settings and in one of the distal basinal settings reveal 87 Sr/ 86 Sr ratios close to the proposed late Neoproterozoic seawater composition (0.7077). Shale-normalized REE + Y patterns of carbonate leachates display enrichments of heavy over light REE and superchondritic Y/Ho ratios, typical of seawater. However, Y/Ho ratios in the cap dolostones are always lower than modern seawater values, which is interpreted to reflect dilution of the seawater signal by continent-derived meltwater influx during deglaciation. Negative Ce anomalies in carbonate leachates from platform and slope sections suggest that oxidized conditions existed in shallow marine environments shortly after the Marinoan glaciation, whereas positive or no resolvable Ce anomalies in basin settings indicate that the latter remained anoxic. Redox stratification of the Yangtze margin at the beginning of the Ediacaran is further supported by the relative enrichment of redox-sensitive trace metals in basinal sections. These data may indicate moderately anoxic (less than 10µM dissolved O 2 ) and presumably manganous conditions during the deposition of cap dolostones in the deeper realms of the Yangtze basin. Supplementary material: Supplementary data for this paper are available at https://doi.org/10.6084/m9.figshare.c.3770957
Highlights
Trace elements such as Sr, Rb and Al, as well as Sr, O and C isotopic compositions, are used to infer whether the variations in the dataset are of pristine origin, for example owing to source effects, or the result of post-sedimentary diagenetic overprint by fluid infiltration
To fit the unusual Sr evolution pathways determined in cap dolostone from basin and slope water environments from the Yangtze Platform we modelled two brines leading to a late-stage fluid overprint of the carbonate lithologies
Data on fluid-mobile elements and their isotopes indicate that Ediacaran cap dolostones from platform, slope and basin depositional environments of the Yangtze Platform, South China, underwent variable fluid–rock interaction and mixing with fluids from different sources
Summary
Radiogenic Sr and stable C and O isotopic compositions, the purpose of this study is to (1) evaluate diagenetic alteration processes on cap dolostones from different geological settings and their impact on the isotopic and elemental compositions of the carbonates, (2) obtain insights into variations of early Ediacaran seawater oxygenation from platform to basin sections, and (3) aim for a better understanding of redox condition changes during the deposition of Ediacaran cap dolomites within the water and sediment bodies at the respective localities
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