Metal cycling in Mesoproterozoic microbial habitats: Insights from trace elements and stable Cd isotopes in stromatolites

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., <0.66 ppm Zr), indicating that elemental compositions are not influenced by detrital contamination. Even though the carbonates have experienced different extent of recrystallization, the measured elemental and isotopic compositions do not correlate with fluid mobile elements. The stromatolites thus represent prime archives for geochemical proxies to reconstruct paleo-environmental conditions. 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 (Yb-SN/Pr-SN < 0.84), slightly super-chondritic Y/Ho ratios (33.6-39.3) and unfractionated Cd isotopes relative to upper continental crust. This indicates that the stromatolites formed in a shallow-water environment that was episodically influenced by seawater. Their REY and Cd compositions are dominated by dissolved elements that were delivered via weathering and erosion processes from the ambient continent. In contrast, REYSN patterns of the conophyta are parallel those of modern seawater with an LREYSN depletion relative to HREYSN (Yb-SN/Pr-SN = 2.1 to 3.9), positive Gd-SN 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. epsilon Cd-112/110(dol) 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. (C) 2018 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.