Abstract Reconstructing the environmental conditions that supported early life on Earth relies on well-preserved geochemical archives in the rock record. However, many geochemical tracers either lack specificity or they are… Click to show full abstract
Abstract Reconstructing the environmental conditions that supported early life on Earth relies on well-preserved geochemical archives in the rock record. However, many geochemical tracers either lack specificity or they are affected by post-depositional alteration. We present a data set of major and trace element abundances and Cd isotope compositions of dome-shaped and conophyton-type stromatolites of the Late Mesoproterozoic Paranoa Group (Brazil), showing distinct values with unprecedented resolution at the lamina scale. The studied stromatolites consist of dolomite with a high purity and a negligible content of immobile elements (e.g., Two endmember compositions can be distinguished by multiple proxy analysis, reflecting the contrasting depositional environments of the two types of stromatolites: Shale-normalized rare earth elements including yttrium (REYSN) patterns of domal stromatolites show a light REYSN (LREY) enrichment (YbSN/PrSN In contrast, REYSN patterns of the conophyta are parallel those of modern seawater with an LREYSN depletion relative to HREYSN (YbSN/PrSN = 2.1 to 3.9), positive GdSN anomalies (1.1 to 1.4) and strong super-chondritic Y/Ho ratios (37.9 to 46.2), suggesting a microbial habitat that was dominated by seawater. Cd isotopes correlate negatively with Cd and U, but positively with Mn and Ce concentrations, reflecting authigenic carbonate formation at different depths within a redox gradient of the ancient microbial mat. e112/110Cddol values increase from −3.52 at the mat surface to +3.46 in the interior of the mat, due to the effect of kinetic fractionation during Cd-uptake, e.g. by adsorption onto organic matter or by precipitation of sulfides, in addition to incorporation into carbonate minerals. Hence, our multi-proxy approach including Cd isotopes bears a high potential to shed light on environmental conditions in ancient microbial habitats and the activity of microbial life on Early Earth.
               
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