Microbial alteration of geochemical and isotope proxies in fine-grained carbonate sediments

点击翻译按钮获取中文摘要

While there is a strongly increasing knowledge on microbial effects on the primary precipitation of carbonates, the subsequent alteration of trace element and stable isotope signals in ordinary carbonate muds during early diagenesis is still poorly understood. Earlier studies provided partially contradicting results, with only minor diagenetic changes at shallow sediment depths despite of changes in pore water chemistry on the on hand side, or clear dissolution and recrystallization features on the other side. Likewise, stable isotope composition has been reported to change with changing pore waters, without a significant shift in bulk mineralogical composition of the sediment. Indeed, the bulk effect of microbial processes such as heterotrophic CO2 release, sulfide oxidation, sulfate reduction and ammonification on pore water carbonate mineral saturation states, and hence carbonate crystallites, is discussed controversially. One reason might be that sedimentological studies commonly rather focus solely on chemical and physicochemical changes of pore water, whereas geomicrobiological studies include, if at all, only fragmentary pore water data. In the present project proposal, a combined approach of pore water analyses (full and balanced analyses incl. trace elements), mineralogical and isotope analyses, plus NGS-based metagenomic and metatranscriptomic analyses of sediment microbial communities is developed to trace changes in fine-grained primary carbonates at shallow sediment depth in a tropical lagoonal setting on the atoll of Aldabra, a site that has been proposed as a potential analogue of lithographic limestone deposition. The phylogenetic and functional composition of microbial communities will be traced from the sediment/water interface to anoxic conditions at shallow sediment depth at cm to dm-scale, and correlated with exopolymer degradation and pore water saturation states. The sediments will be screened for corrosion and recrystallization of existing carbonate crystallites as well as syntaxial growth or even new precipitation, supplemented with delta 13C and delta 18O analyses. These traditional stable isotope measurements are complemented by delta 26Mg and delta 44Ca analyses (ZP 0 and TP 9) to assess their susceptibility with respect to pore water changes and microbial activity. The study provides an important basis to assess traditional stable isotope signals in the fossil record, which are widely used as palaeoenvironmental proxies, as well as a test for the use of non-traditional isotopes for tracing diagenetic pathways in a natural setting.