The ambition of this project is to provide the very first realistic assessment of the contribution of microbes to the chemical alteration of silicate rocks, a process that contributes to the geochemical cycles of most chemical elements, and partly controls atmospheric CO2 uptake and Earth’s climate over geologic timescales. If successful, the knowledge collected in this project will help improve existing models of rock alteration at the Earth’s surface, and ultimately contribute provide a more reliable estimate of the feedback between global warming and rock weathering.
The motivation for such a research project is simple: ~70% of Earth’s microbes live underground, and microbes have long been suspected to impact the alteration rate of silicate rocks. However, the rates and mechanisms of microbially-mediated silicate alteration essentially remain unknown and not accounted for in existing models of rock alteration. This project particularly focuses on basalt settings, as these environments are suggested to be a potential host for early life and represent prime targets for massive injections of CO2 to fight against global warming, whose success strongly relies on silicate reactivity. Having a deep insight into the respective biotic and abiotic contributions to subsurface silicate weathering rates in these settings is therefore both fundamental and urgent.
Providing such estimates requires to overcome a twofold challenge: 1) to quantify silicate dissolution rates directly in the environment (as opposed to in vitro estimates from laboratory experiments) and at the same time and 2) to identify features at the mineral surface that can be unambiguously be attributed to the impact of microbial life, to eventually estimate the respective contribution of abiotic and biotic processes.
This proposal offers a solution: An interdisciplinary and non-conventional approach to assess the contribution of microbes to silicate weathering rates in complex environmental media. It consists in measuring dissolution rates using non-invasive nanotopography measurements of the silicate substrates reacted in soil profiles and/or environmental fluids. These substrates are treated beforehand to get surface properties that mimic various stages of aging. These measurements are combined with studies of the microbial diversity associated with the substrates, innovative nanoscale characterizations of the reacted surfaces, and modeling of the dissolution process based on parameters derived independently from quantum mechanics. If successful, this strategy will not only provide an unprecedented and timely picture of the functioning and rates of microbially-mediated subsurface silicate bioweathering: it will also pave the way to the definition of criteria for biosignatures of microbially-mediated silicate dissolution, of prime interest for the search for life in the Earth’s geological record and beyond.
