Diagenetic formation of stevensite by replacement of diatom frustules in the sediments of the alkaline Lake Alchichica (Mexico)

International audience ; Better understanding the conditions of formation of authigenic Mg-silicates and their reactivity is key to interpret the palaeoenvironmental message carried by the sedimentary record and evaluate the effect of reverse weathering, a process involved in long-term climate evolution. Microbialites from most alkaline crater lakes in Mexico contain Mg-silicates except those in Lake Alchichica, where concentration of orthosilicic acid is low (m). This study investigated the first metre of sediments in Lake Alchichica in order to check how their mineralogy compared with that of shoreline microbialites. The mineralogy and chemistry of the sediment column were determined, together with the pore water chemistry, providing insights on the processes occurring during early diagenesis. Below ca 3 cm in depth, diatom frustules are progressively pseudomorphized into Al-poor Mg-silicates with a composition corresponding to stevensite. This diagenetic process is massive and the resulting silicate represents between 30 and 53 wt.% of the sediment content at all depths. This observation questions the possibility to infer lake palaeochemistry from the presence/absence of Mg-silicates in the sedimentary record. Moreover, it allowed refinement of the conditions under which Mg-silicates authigenesis occurs: the saturation of the solution should be higher or equal to the solubility of a Mg-silicate phase close to that of 'amorphous sepiolite'. Although the solubility of authigenic silicates is a key parameter of reverse weathering modelling during geological times, it is still debated. In this study, a solubility constant deduced from a natural system is proposed that should be considered when modelling the formation of Mg-silicates in a natural environment. The proportion of reverse weathering associated with this solubility constant could be higher than previously predicted based on experiments and thus have a greater impact on climate stability over geological timescales.