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

虽然微生物作用对碳酸盐原生沉淀的影响越来越多，但普通碳酸盐泥在早期成岩作用中微量元素和稳定同位素信号的后续变化仍然知之甚少。早期的研究提供了部分矛盾的结果，尽管一边孔隙水化学发生了变化，但在浅沉积深度只有轻微的成岩变化，或者另一边有明显的溶解和再结晶特征。同样，稳定同位素组成也随着孔隙水的变化而变化，而沉积物的总体矿物组成没有显著变化。事实上，微生物过程如异养CO2释放、硫化物氧化、硫酸盐还原和氨化作用对孔隙水碳酸盐矿物饱和状态的总体效应，从而对碳酸盐晶体进行了有争议的讨论。一个原因可能是，沉积学研究通常只关注孔隙水的化学和物理化学变化，而地球微生物学研究即使有，也只包括零碎的孔隙水数据。在目前的项目建议中，采用孔隙水分析（包括微量元素的全面和平衡分析）、矿物学和同位素分析，以及基于ngs的沉积物微生物群落宏基因组学和超转录组学分析相结合的方法，在阿尔达布拉环礁的热带泻湖环境中，在浅层沉积物深度追踪细粒原生碳酸盐的变化。阿尔达布拉环礁被认为是一个潜在的石灰岩沉积模拟点。微生物群落的系统发育和功能组成将从沉积物/水界面追溯到浅沉积物深度（cm至dm）的缺氧条件，并与外聚合物降解和孔隙水饱和状态相关。将对沉积物进行腐蚀和现有碳酸盐晶体的再结晶以及合成生长甚至新沉淀的筛选，并辅以δ 13C和δ 18O分析。这些传统的稳定同位素测量与δ 26Mg和δ 44Ca分析（zp0和TP 9）相辅相成，以评估它们对孔隙水变化和微生物活性的敏感性。该研究为评估化石记录中被广泛用作古环境代用指标的传统稳定同位素信号提供了重要依据，也为利用非传统同位素在自然环境中追踪成岩路径提供了试验。

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.