Using Cd, Ba and Ni isotope systematics as novel biomarkers to decipher Archean microbial metal cycling in the ~3.0 Ga Pongola Supergroup stromatolites

Non-traditional stable isotope systematics of bioactive metals are emerging new tools for studying the biogeochemical cycling of micronutrients in sedimentary archives through deep time. In the case of Ni and Cd, their role as cofactors for biochemical reactions is directly related to enzymatic prosses essential for variable microbial communities. While methanotrophic bacteria incorporate Ni, Cd acts as a cofactor in carbonic anhydrase activity in phototrophic bacteria and matchesnutrient-like concentration patterns complementary to phosphorous in modern open ocean environments. Ba, although not acting as a cofactor in metabolic processes, is accumulated in sedimentary barite via respiration of organic matter and thus traces the extent of microbial activity and heterotrophic remineralization. All three metals follow kinetic isotopic fractionation systematics under trace element-influx limited environmental conditions, and microbial carbonates that form from a fractionated isotope pool in ambient waters have been shown to record these isotope variations through deep time.

We show a combined Ni, Cd and Ba isotope record from the ~3.0 Ga old Pongola supergroup stromatolites from the Kaapvaal Craton (S. Africa) as novel isotope-biomarkers for microbial communities. Strong positive correlated fractionation of authigenic δ¹³⁸Ba with Ba/Ca and δ¹¹²Cd with Cd/P ratios above igneous reference values reveal the presence of phototrophic and heterotrophic microbial communities building up the Pongola stromatolites.

In contrast, subdued δ⁶⁰Ni isotope fractionation close to igneous reference values either argues for less influence of methanotrophic communities or non-limited Ni influx into the microbial paleo-environment. The latter is likely linked to extensive weathering of a mafic and hence Ni rich upper crust in the Archean. Co-variation with C isotopes can only be observed with Cd isotopes underlining their potential as the most promising isotope biomarker for phototrophic activity in Archean microbial habitats. Our work aims to extend the use of heavy metals that work as cofactors in enzymatic processes and their isotope systematics in biogeochemical metal cycling in paleo-environmental reconstructions to understand better metallome evolution on Earth and beyond.