Abstract
AbstractCarbonates that exhibit obvious diagenetic alteration are usually excluded as archives in palaeoenvironmental studies. However, the potential impact of microbial alteration during early diagenesis is still poorly explored. To investigate the sensitivity of sulphur concentration, distribution, oxidation state and isotopic composition in marine aragonite to microbial alteration, Arctica islandica bivalves and Porites sp. corals were experimentally exposed to anaerobic microbial activity. The anoxic incubation media included a benthic bacterial strain Shewanella sediminis and a natural anoxic sediment slurry with a natural microbial community of unknown species. Combined fluorescence microscopy and synchrotron‐based analysis of the sulphur distribution and oxidation state enabled a comparison of organic matter and sulphur content in the two materials. Results revealed a higher proportion of reduced sulphur species and locally stronger fluorescence within the pristine bivalve shell compared to the pristine coral skeleton. Within the pristine bivalve specimen, reduced sulphur was enriched in layers along the inner shell margin. After incubation in the anoxic sediment slurry, this region revealed rust‐brown staining and a patchy S2‐ distribution pattern rather than S2‐‐layers. Another effect on sulphur distribution was rust‐brown coloured fibres along one growth line, revealing a locally higher proportion of sulphur. The δ34S value of carbonate‐associated sulphate remained largely unaffected by both incubation media, but a lower δ34S value of water‐soluble sulphate reflected the degradation of insoluble organic matter by microbes in both experiments. No significant alteration was detected in the coral samples exposed to microbial alteration. The data clearly identified a distinct sensitivity of organically bound sulphur in biogenic aragonite to microbial alteration even when ‘traditional’ geochemical proxies such as δ18OCARB or δ13CCARB in the carbonate did not show any effect. Differences in the intensity of microbial alteration documented are probably due to inherent variations in the concentration and nature of original organic compositions in the samples.
Highlights
Under favourable conditions, marine biogenic carbonates may serve as archives for their ambient seawater properties
The δ34S value of carbonate-associated sulphate remained largely unaffected by both incubation media, but a lower δ34S value of water-soluble sulphate reflected the degradation of insoluble organic matter by microbes in both experiments
The sterile seawater used for the incubation media contained 2.6 g/l of dissolved sulphate, which is close to concentrations typically observed in marine waters (2.7 g/l), including that of the North Sea
Summary
Marine biogenic carbonates may serve as archives for their ambient seawater properties. One of the many processes that affect this proxy data is microbial colonisation of skeletal hardparts and related metabolisms, a factor rarely addressed in previous work (cf Glover & Kidwell, 1993; Lange et al, 2018; Sejrup & Haugen, 1994). These studies demonstrated that microbial activity has the potential to alter the elemental composition of these archives, and can affect any conclusions drawn regarding ambient seawater conditions. Explanations for the different effects of microbial alteration in A. islandica and Porites sp. samples are discussed
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
