Microbial alteration of geochemical proxies in finegrained carbonate sediments during early diagenesis

Fine-grained carbonate sediments play a crucial role in the global carbon cycle and are an exceptional record of past environmental conditions. Major paleoenvironmental constraints, derived from stable isotope and trace element records in these carbonate sediments, include ancient ocean temperatures, -seawater salinities and global elemental circulation patterns. Besides the inferences of past conditions at the Earth´s surface, the understanding of these environmental archives enables the parameterization of models of future scenarios. Early diagenesis, involving carbonate phase dissolution and recrystallization, can potentially compromise these archives and, hence, distort the outcome of scientific models. For decades, the impact of changing physicochemical pore water gradients on contemporaneous carbonate phases during the earliest stages of diagenesis has been subject to controversial discussion. However, the role of specific bacterial communities within these processes is still not completely understood and bacterial genes are still not fully established as an additional proxy in descriptive in diagenetic concepts. This thesis aims to track such bacterial effects on early diagenesis at shallow sediment depths. It shall provide a valuable feature for the evaluation of traditional isotope- and trace element signals in the fossil record. The primary objectives of this work include (i) assessment of the effects of changing physicochemical pore water gradients on contemporary fine-grained carbonate phases, (ii) the main impacts of the nearby environment on calcium carbonate alteration, (iii) the role of specific bacteria in the establishment of pore water gradients, and (iv) the potential use of multi-proxy studies, i. e. the combination of sediment geochemical-, pore water-, and bacterial 16S rRNA datasets, to trace complex diagenetic reaction paths. The first and second case study focus on authigenic calcium-magnesium carbonate mud in the shallow alkaline Lake Neusiedl, located in eastern Austria. The cored sediments featured 20 cm of homogeneous greyish mud, containing 60 wt% carbonate, on top of dark laminated, semi-consolidated mud with 50 wt% carbonate and plant debris. The oxic water column with a pH of 9 changed to anoxic sediment pore water with a pH of 7.5. In addition, a decrease in sulfate, coinciding with an increase of total sulfur and ammonia between 0 and 15 cm core depth indicated an anaerobic heterotrophic decomposition of organic matter (including sulfate reduction) in the sediment. Saturation indices indicated supersaturation of disordered dolomite and calcite in the water column, compared to undersaturated conditions in the sediment. However, despite significant physicochemical changes in the pore water, the stochiometric compositions and relative proportions of authigenic carbonate phases remained constant with increasing sediment depth. Hence, no evidence for calciummagnesium carbonate formation or diagenetic alteration to more stable dolomite is apparent within the anoxic, sulfidic sediment of Lake Neusiedl. In contrast, the analytical data suggest authigenic carbonate formation in the supersaturated, well-mixed aerobic water column. These findings support an alternative concept to dolomite formation in anoxic sediments and potentially indicate low diagenetic reaction rates in such oxygen depleted deposits. The third case study investigates shallow marine diagenetic environments on Aldabra, a remote coral atoll in the western Indian Ocean. The lagoon of Aldabra has previously been proposed to be a recent analogue of Jurassic lagoons that formed lithographic limestones. Thus, it should have represented a suitable setting to retrace diagenesis within fine-grained carbonate mud. Three different locations were sampled and analyzed within the 34 x 14.5 km sized lagoon. One each in the western- (WL), northern- (NL), and southern (SL) part. Overall, three different types of sediments and pore water environments were sampled by plexiglass push cores. The west lagoon cores consisted of intertidal, bioclastic carbonate sand with an oxygenated and porous pore space. Sediments in the north lagoon occurred in supratidal karstic depressions and were characterized by anoxic shell detritus and a highly porous pore space. The cores from the south lagoon contained fine-grained, greyish carbonate mud with low porosity. Furthermore, the time scale for diagenetic processes was limited by a maximum radiocarbon age of 2357 ± 70 years. However, diagenetic alteration was solely noticeable in oxic sediments from the west lagoon and occurred in form of marine vadose, cryptocrystalline calcium carbonate cements. The exact reaction paths leading to this cement formation are not entirely clear. It may be either induced by abiogenic evaporation of pore waters during low tide, or biologically influenced by the presence of phototrophic and sulfate reducing bacteria. No significant traces of geochemical alteration were found in the sedimentary record of the north and south lagoon. Yet, pore water gradients indicate minor dissolution of metastable carbonate phases, possibly induced by acidifying sulfur oxidizing bacteria of the order Campylobacterales. The outcomes of this study suggest that diagenetic reactions are faster in oxygenated than in anoxic pore water environments. Moreover, sulfur oxidizing bacteria appear to play a fundamental role in the dissolution of metastable carbonates during early diagenesis. The fourth case study includes a comparative pore water-sediment study of the Cinq Cases pool system, a shallow and saline water body in the southeast of Aldabra. The pools are characterized by occasional marine influx and short-time diagenetic reactions between sediment and pore water. However, the cored sediments provide radiocarbon ages of up to 3760 ± 30 years and indicated three environmental stages. An initial stage, characterized by least temporary anoxic conditions in a palustrine environment, with meteoric diagenesis (Unit III), a stage of slow marine flooding, including cyanobacteria and sponge blooms (Unit II), and a stage of lagoon flooding, with oxic conditions within the sediment (Unit I). These frequent changes in the depositional and diagenetic environments were retraced by three different proxies, which represent partially overlapping but different time scales. (i) Sediment data reflect ancient processes (ii) pore waters are influenced by recent processes, and (iii) bacterial communities mirror an overlay of ancient and recent processes. Hence, the decoupling of the different datasets used in this study exemplifies the feasibility and necessity of the application of such multiproxy studies in unstable depositional environments. In summary, the results presented in this thesis indicate that only minor changes in the bulk geochemical record of fine-grained carbonate sediments can be expected during early diagenesis. If diagenetic changes occur in anoxic carbonate deposits, they are characterized by the dissolution of metastable phases, coupled to the activity of sulfur oxidizing bacteria of the order Campylobacterales. The final metabolic products of these organisms include sulfuric acid, which lowers the pH value and favors carbonate phase dissolution. However, diagenesis likely occurs faster in oxygenated pore water environments, but the exact reaction pathways and the involvement of microorganisms are still unclear. Future studies should focus on these oxic, cemented diagenetic zones. Furthermore, a precise evaluation and quantification of the influence of sulfur oxidizing bacteria on metastable carbonates during early diagenesis would be desirable. This could be achieved via cultivation experiments with bacteria of the order Campylobacterales.2022-06-0