Stromatolites as geochemical archives to reconstruct microbial habitats through deep time: Potential and pitfalls of novel radiogenic and stable isotope systems

Abstract

Understanding the origin and evolution of life on Earth and potentially other planets in our solar system is of fundamental interest for humanity. For the longest time in Earth's history, life evolved in microbial communities. It is, however, still incompletely understood how, when and where such habitable environments formed and how microbial communities adopted to the drastic changes of the atmosphere-hydrosphere-lithosphere systems through deep time. Stromatolites, i.e., lithified microbial mats that occur in sedimentary successions from at least 3.4 billion years ago until today, hold the geochemical key to our understanding of the evolution of microbial life on Earth and may also provide a blueprint for planetary studies. This review targets the potential and pitfalls of emerging and established isotope applications to stromatolites based on improved and newly developed analytical and technical capabilities in the last decades. We provide a comprehensive overview of present data and the interpretation of applications of radiogenic (U–Pb, Rb–Sr, Sm–Nd) and stable (O, C-N-S, Fe, Mo, Cr, U, Cd) isotope systems in stromatolites. Although the behaviour and fractionation processes of different isotope systems in stromatolites and microbial mats are incompletely understood, the different isotope proxies are used to better understand and reconstruct microbial habitats in stromatolite-forming environments through deep time. Primarily, radiogenic isotopes are used to directly date stromatolites and determine the source of elements in ancient stromatolite environments, while stable isotopes are used to understand redox conditions, metal availability, and (biogenic) metal cycling processes in microbial habitats. We provide deep insights into each isotope application in stromatolites and show their unique potential and future perspectives to bridge the gap between geochemistry and microbiology and to better understand the evolution of microbial life on Earth and beyond.