Taphonomic experiments on caridean shrimp elucidate the timing and influence of organic matter on diagenetic pyritization and sulfurization

Abstract

Sulfur is an element particularly valuable for preserving organisms in the fossil record. On one hand, sulfurization (i.e. incorporation of sulfur into organic matter) forms macromolecules more likely to be preserved over geological timescales than their low-weight molecular precursors. On the other hand, pyrite (FeS2) is one of the main minerals replicating organisms and contributes to the exceptional preservation of soft tissues. However, the incomplete understanding of the processes, controls, timing and relation (competition, succession …) of these reactions may conceal critical biases within the fossil record, used to reconstruct past biodiversity and environment. In particular, organic matter (OM) is critical in these fossilization modes, but the roles of its composition and abundance remain to be better assessed, in particular for pyritization. Therefore, laboratory experiments were conducted on crustaceans' carcasses in mesocosms containing natural water and sediment. Tanks were treated with variable conditions to test the relative roles on preservation potential of oxic perturbation, thermal degradation of OM, and internal soft tissues, or lack thereof. Various bio-physico-chemical parameters and sulfur speciation were then monitored. The precipitation of a macroscopic amorphous iron monosulfide was observed after 100 days, both in the sediment and within carcasses; sulfurization of carcasses was detected within tens of days. A periodic aeration of the sediment where the carcasses were decayed impeded neither pyritization nor sulfurization while thermal degradation of OM did. Further, absence of internal soft tissues led solely to sulfurization, which suggests (i) that labile OM may be the factor driving the balance between pyritization and sulfurization and (ii) that internal soft tissues may promote pyritization through an organomineralization process, with sulfur-bearing organic ligands acting as nucleation sites. These results invite further experiments to test additional factors, in order to help refine environmental and taxonomical controls, and further investigation of fossil material, to evaluate the significance of sulfurization in the fossil record by searching sulfurized organisms.