Stromatolitic carbonates are geochemical archives that allow studying the long-term interplay of the biosphere, atmosphere and hydrosphere through deep-time, with the unique potential to also investigate early life environments and the evolution of the metallome. Recently non-traditional stable isotopes of bioactive metals emerged as novel proxies to reconstruct the micronutrient cycling in stromatolitic microbial habitats.
In this study, we use stromatolites from the ~2.95 billion-year-old Pongola Supergroup (South Africa) as field laboratory for combined in-situ trace metal mapping and layer-specific novel stable metal isotope compositions to determine biogeochemical metal cycling in early Earth microbial habitats. LA ICP-MS maps reveal intrinsic bio-sedimentary enrichments of Ni and Cd in laminae; in contrast, Ba shows a more heterogeneous distribution throughout the stromatolite. Intra-laminae δ⁶⁰Ni and δ¹¹²Cd follow typical kinetic isotopic fractionation, i.e., the isotopic composition of Cd and Ni evolves to heavier values with decreasing respective element concentrations arguing for carbonate precipitation from a fractionated Ni and Cd pool heavier than ambient silicate rocks. Further correlations with δ¹³C and macronutrient P argue for co-existing methanogenetic, and photogenetic metal uptake responsible for pronounced isotopic fractionation. In contrast, δ¹³⁸Ba records isotope fractionation related to variable aragonite precipitation rates in the stromatolite, i.e., Ba evolves to isotopically heavier values with increasing concentrations under variable alkalinity in microbial habitats. We show that the combination of Cd and Ni isotopes has a unique potential as novel isotope biomarker for early Earths bio-chemical sediment record of where traditional biomarkers are not applicable due to fragmentary preservation of organic material.
 

叠层石,镍同位素,钡同位素,镉同位,古代生活