Early Earth microbial metallome: Insights from Cd, Ba, and Ni isotopes in stromatolites

Abstract

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 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. This study uses stromatolites from the ~3-billion-year-old Pongola Supergroup (S.-Africa) as a 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, Cd, P, Fe and Mn in laminae; in contrast, Ba shows a more heterogeneous distribution. 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 of δ112Cd with δ13C and the nutrients Cu, Mo, and P argue for primarily photosynthetic over methanogenic metal uptake. 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 the microbial habitat. We show that the combination of Cd and Ni isotopes has unique potential as novel isotope biomarker for early Earth's bio-chemical sediment record where traditional lipid biomarkers are not applicable due to fragmentary preservation of organic material.