What controls the microbial fractionation

The isotopic composition of minerals can be used to reconstruct present and past microbial activity and physical processes of transport and mineralisation. To this respect, most of our knowledge is based on laboratory culture experiments and metabolic-isotopic network models, which show and predict large near-equilibrium microbial fractionation under environmentally relevant conditions (i.e., temperature, substrate concentration). 

We want to apply this knowledge to natural environments. Indeed, pyrite by capturing the isotopic composition of the products of microbial sulfur metabolisms (H2S, HS-) may serve as a good recorder of past microbial activity. However, by controlling the bulk δ34S values of pyrite preserved in marine sediments (see examples here), early diagenetic processes mask the true microbial fractionations. 

To overcome early diagenetic overprints,  we have developed projects that couple individual pyrite grain δ34S measurements and numerical modelling. Our goal is to use a microanalytical approach, in my case Secondary Ion Mass Spectrometry (SIMS), to recover the microbial fractionation expressed in the specific depositional environments.