Decadal, sediment-driven sulfur isotope evolution in Lake Cadagno

The sulfur isotope composition (δ34S) of sedimentary pyrite has been widely used to reconstruct the past global marine sulfur cycle, yet local diagenetic conditions may exert a greater influence than previously recognised. In this work, we present new dissolved H2S and SO42− isotope data alongside microscale sulfur isotope analyses of sedimentary pyrite from the redox stratified Lake Cadagno, Switzerland. A comparison with previously published data reveals a ∼15 ‰ shift in the δ34S values of water column H2S over the past two decades. Constant δ34S values of sulfate in the water column over the same time interval and mass balance calculations indicate that this shift is likely driven by the diffusion into the water column of highly 34S enriched H2S, microbially produced in the sediment porewaters. This 34S enrichment of H2S was recorded by sedimentary pyrite that formed during this period. Additionally, the near-equilibrium microbial isotope fractionation observed in incubations of Lake Cadagno sediment is poorly recorded in pyrite, possibly due to isotopic homogenisation of the H2S pool within the sediment. Our findings offer a complementary perspective on the conventional model of euxinia, where H2S production primarily occurs in the water column, by highlighting the importance of sedimentary processes in shaping the dynamics of the sulfur pool and pyrite isotope composition.