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

Abstract

Reconstructing the environmental conditions that supported early life on Earth relies on 32 well-preserved geochemical archives in the rock record. However, many geochemical tracers 33 either lack specificity or they are affected by post-depositional alteration. We present a data 34 set of major and trace element abundances and Cd isotope compositions of dome-shaped 35 and conophyton-type stromatolites of the Late Mesoproterozoic Paranoá Group (Brazil), 36 showing distinct values with unprecedented resolution at the lamina scale. 37 The studied stromatolites consist of dolomite with a high purity and a negligible content of 38 immobile elements (e.g., < 0.66 ppm Zr), indicating that elemental compositions are not 39 influenced by detrital contamination. Even though the carbonates have experienced different 40 extent of recrystallization, the measured elemental and isotopic compositions do not 41 correlate with fluid mobile elements. The stromatolites thus represent prime archives for 42 geochemical proxies to reconstruct paleo-environmental conditions. 43 Two endmember compositions can be distinguished by multiple proxy analysis, reflecting 44 the contrasting depositional environments of the two types of stromatolites: Shale-normalized 45 rare earth elements including yttrium (REY SN) patterns of domal stromatolites show a light 46 REY SN (LREY) enrichment (Yb SN /Pr SN < 0.84), slightly super-chondritic Y/Ho ratios (33.6-47 39.3) and unfractionated Cd isotopes relative to upper continental crust. This indicates that 48 the stromatolites formed in a shallow-water environment that was episodically influenced by 49 seawater. Their REY and Cd compositions are dominated by dissolved elements that were 50 delivered via weathering and erosion processes from the ambient continent. 51 In contrast, REY SN patterns of the conophyta are parallel those of modern seawater with 52 an LREY SN depletion relative to HREY SN (Yb SN /Pr SN = 2.1 to 3.9), positive Gd SN anomalies 53 (1.1 to 1.4) and strong super-chondritic Y/Ho ratios (37.9 to 46.2), suggesting a microbial 54 habitat that was dominated by seawater. Cd isotopes correlate negatively with Cd and U, but 55 positively with Mn and Ce concentrations, reflecting authigenic carbonate formation at 56 different depths within a redox gradient of the ancient microbial mat. ε 112/110 Cd dol values 57 increase from-3.52 at the mat surface to +3.46 in the interior of the mat, due to the effect of 58 kinetic fractionation during Cd-uptake, e.g. by adsorption onto organic matter or by 59 precipitation of sulfides, in addition to incorporation into carbonate minerals. Hence, our 60 multi-proxy approach including Cd isotopes bears a high potential to shed light on 61 environmental conditions in ancient microbial habitats and the activity of microbial life on 62 Early Earth.

High‐Precision Cd Isotope Measurements of Soil and Rock Reference Materials by MC‐ICP‐MS with Double Spike Correction

“…With advances in instrumentation, increasingly high precision Cd isotope measurement results have been reported using different chemical purification procedures and different instrumental analysis methods (e.g., Wombacher et al , Cloquet et al , Ripperger and Rehkämper , Gao et al , Schmitt et al , Xue et al , Li et al ). The Cd isotopes of different types of samples with low Cd mass fractions including organic materials, plants, carbonates, silicate rocks, river sediments, seawater and meteorites have been determined precisely and accurately (e.g., Rosman and De Laeter , Cloquet et al , Ripperger and Rehkämper , Gao et al , Wombacher et al , Schmitt et al , Abouchami et al , Pallavicini et al , Georgiev et al , Wei et al , Wiggenhauser et al , Wen et al , Hohl et al , John et al , Imseng et al , Li et al , Viehman et al ). The mass bias produced by the instrument has been corrected by different methods including sample‐calibrator bracketing, and the Ag doping and double spike method (Wombacher et al , Cloquet et al , Ripperger and Rehkämper , Schmitt et al , Li et al ).…”

Trace Metal and Cd Isotope Systematics of the Basal Datangpo Formation, Yangtze Platform (South China) Indicate Restrained (Bio)Geochemical Metal Cycling in Cryogenian Seawater

“…The concept of mean ocean residence time (MORT) includes all processes involved in the modern ocean trace element cycling as the residence time of an element, i.e., the average time from source to sink, depending on how fast the element can be adsorbed on, or incorporated into, a phase that is available for sedimentary deposition [29]. However, in the Precambrian oceans the MORT of most of the elements is unknown and availability of metals and coincident (bio)geochemical cycling may have been very different from modern environments [30][31][32]. During the Proterozoic and Archean, the oceans had different source contributions [33,34] and the Precambrian oceans were most likely stratified in terms of their oxygen availability with an oxidized surface layer and anoxic to euxinic (containing free H 2 S) deep waters [12,35].…”

“…Cadmium isotopes in marine environments are a promising new tool for studying (bio)geochemical cycling of micronutrients in the oceans [53][54][55] and in sedimentary archives, such as Fe-Mn crusts [56], carbonates [57,58], microbialites [30], mudstones, and black shales [57][58][59]. Cd concentrations in seawater show a nutrient-like behaviour, with surface depletion and deep water enrichments.…”

“…Under incomplete oxidative OM recycling, biological uptake may act as an effective Cd sink and authigenic minerals may store the ambient seawater Cd isotope composition. Consequently, stable Cd isotope compositions show broad variations in ancient sediments that may be linked to variations in paleo productivity and redox state [30,57,58,[61][62][63]. Many more processes govern the Cd isotopic compositions of sediments than in seawater.…”

“…Under truly anoxic (pO 2 <10 µmol) conditions, OM will not be recycled anymore and deposited in OM-rich shales showing common enrichments of the redox sensitive elements U and Mo [41,42]. By contrast, carbonates will be generally enriched in Mn and Ce due to the breakdown of Mn-oxides [30]. Further, redox sensitive trace metals, such as U, Re, V, and Mo but also Cd, have been shown to be relatively enriched in authigenic carbonate under reducing and anoxic conditions possibly due to supressed oxidative biogeochemical metal recycling [43].…”