Rare earth element signatures of Doushantuo cap dolostones capture an increase in oxygen in the anoxic Ediacaran ocean

The Rare Earth Element (REE) systematics of the post-Marinoan cap dolostones reflect the marine redox conditions and chemistry in the immediate aftermath of the snowball Earth. Rare earth elements and yttrium (REY) compositions in the Doushantuo cap dolostones that directly overlie Nantuo glacial diamictites in south China are determined from the inner shelf to the slope. In general, shale-normalized REY patterns (REYSN) of the cap dolostones show significant fractionations that are characterized by light REE depletion, slight middle REE enrichment relative to the light and heavy REEs, positive Eu anomalies, and slightly super-chondritic Y/Ho ratios. These dolostones, however, show no significant shale-normalized negative Ce anomalies. Such REYSN patterns are consistent with an extensive anoxic Ediacaran ocean in the shallow-to-deep water columns. The REE concentrations and normalized patterns in the cap dolostones vary spatially and stratigraphically. When compared to the shelf edge and the slope, cap dolostones from the inner shelf are distinguished by uniformly less fractionated REYSN patterns with an absence of Eu anomalies, and suppressed light REE depletion and MREE enrichment. These marked differences in the REYSN patterns indicate significant continental influences on the seawater chemistry within the restricted inner shelf (lagoon) during glacial meltdown. The temporal trends in the REY dataset of the Doushantuo cap dolostone reflect shifts in marine redox conditions. Dolostones from two thick shelf margin sections (5 m and 7 m thick, respectively) show almost identical stratigraphic trends in the REY concentrations, and the Ce, Y, and middle REE anomalies. The overall stratigraphic decrease in the Ce anomaly, accompanied by the increase in the Y anomaly and decrease in the middle REE anomaly towards the top of the rock unit, suggests that dissolved oxygen was increasing in the shallow water column during the deposition of the cap dolostones. The REE data in this study reveal that the oxygenation of the Ediacaran oceans started in the immediate aftermath of the snowball Earth (635-632 Ma).(c) 2023 Elsevier B.V. All rights reserved.