Decimeter-scale mapping of carbonate-controlled trace element distribution in Neoarchean cuspate stromatolites

Abstract

Ancient stromatolites can provide key insights into the early evolution of life on Earth. Neoarchean fenestrate stromatolites from the ~2520 Ma Upper Nauga Formation (Transvaal Supergroup, South Africa) preserve cuspate morphologies. They possess clearly delineated support and drape structures interpreted as dolomitized microbial mat material. Petrographic observations show that the biogenic structures are composed of planar-s to non-planar ferroan dolomite, encased in ferroan calcite, including herringbone calcite textures. The cuspate stromatolites were analyzed using Synchrotron Rapid Scanning X-Ray Fluorescence (SRS-XRF) and more conventional techniques to determine: (i) whether element distributions could be distinguished in ancient stromatolites at both cm to dm scales, (ii) whether element distributions show variation between biogenic and abiogenic textures, and (iii) the sample's paragenesis. The distributions of Ca, Fe, Mn, Pb, Cu, As, Br, Al, Si, P, and S directly correspond to dolomitized stromatolitic structures and show trace element distributions are principally controlled by calcite and dolomite occurrence. Dolomite formation was mainly driven by seawater-derived fluids given the high concentrations of Fe and retention of marine shale-normalized rare earth element and yttrium (REY SN) patterns, however the spatial association of dolomite to stromatolite structures may reflect microbially-influenced mineral nucleation. Given the complexity of this sample's paragenetic evolution, trace metal distributions cannot be conclusively tied to specific metabolic pathways, bioaccumulation or passive binding, however, the results show SRS-XRF can be used for quantifiable, spatial, in-situ investigation of ancient microbialites.

A microfacies analysis of arid continental carbonates from the Cedar Mesa Sandstone Formation, Utah, USA

“…The features are similar to those observed from stromatolites (see Warke et al, 2019) which form under conditions of generally deeper, and longer-lived, standing water than features observed within MF1. The features are similar to those observed from stromatolites (see Warke et al, 2019) which form under conditions of generally deeper, and longer-lived, standing water than features observed within MF1.…”

“…The presence of clotted and laminar microbial growth also indicate an environment that was wetter than that for MF1. The features are similar to those observed from stromatolites (see Warke et al, 2019) which form under conditions of generally deeper, and longer-lived, standing water than features observed within MF1.…”

A copper isotope investigation of methane cycling in Late Archaean sediments

“…Fabric destructive dolomitsation, driven by interaction of precursor carbonate phases with a burial brine, is also known to affect the Campbellrand Subgroup (Sumner, 1996). Dolomite precipitating from such a fluid could enrich Cu host dolostones up to concentrations of 150 -1500 ppm (Warke et al, 2019). As we do not observe concentrations above 126 ppm in this study, and this portion of GKF01 retains discernible microbial fabrics (Schröder et al, 2006; Therefore, there is no evidence -either from considering the sulphides themselves or from considering the susceptible host sediments -to suggest that Cu-sulphides have been significantly altered.…”

“…Synchrotron X-Ray Fluorescence has shown that microbial fabric-retentive ferroan dolomite in GKF01 can contain up to 20 ppm Cu, although generally concentrations in the ferroan dolomite are lower than detection (~1 ppm; Warke et al, 2019). This form of dolomitisation, driven by seawater derived fluids (Warke et al, 2019), is common in the studied interval and is therefore unlikely to account for the higher Cu concentrations measured in this study.…”

“…Nonetheless, lower temperature diagenetic processes can reset or alter carbonate trace metal and isotopic inventories. The carbonate geochemistry of the Campbellrand Subgroup, including core GKF01, has been well studied in relation to primary seawater vs secondary alteration signatures (Schröder et al, 2006;Fischer et al, 2009;Eroglu et al, 2015;2017;Warke et al, 2019). Here, we consider diagenetic processes (dolomitisation styles, fluid interaction) that may have altered the upper Nauga Formation and factors that may affect Cu systematics.…”