Description: Dissolved sulfate sulfur and oxygen isotope results and carbonate associated sulfate sulfur isotope results from IODP Site U1553 (166°11.4801′E, 52°13.4294′S) located on the Campbell Plateau are presented. Pore water collected during IODP Expedition 378 at Site U1553 in January 2020 were used for this study. These samples were processed using a procedure modified from that in Wotte et al. (2012, doi:10.1016/j.gca.2012.02.013) to extract the sulfate from the aqueous phase. The dissolved sulfate in the pore water was extracted by first adding 10% HCl to acidify the pore water, then the sulfate was removed from solution as barium sulfate using a 10% BaCl2 solution. Sulfur isotopes in dissolved sulfate were analyzed using an EA-Isolink elemental analyzer connected in continuous flow to a MAT253 IRMS and the oxygen isotopes in dissolved sulfate were measured using a thermal conversion elemental analyzer connected in continuous flow to a Delta V IRMS. Analytical precision was measured using repeated measurements of a blind standard with an average 2-sigma of 0.4‰ for sulfur isotopes (IAEA-SO-5) and 0.3‰ for oxygen isotopes (NBS-127). These data were collected to examine the impact of a long-duration (~26-million-year) unconformity has on the fidelity of the carbonate geochemical archive.

Description: Carbonate associated sulfate sulfur isotope results from IODP Site U1553 (166°11.4801′E, 52°13.4294′S) located on the Campbell Plateau are presented. Pore water and bulk carbonate samples collected during IODP Expedition 378 at Site U1553 in January 2020 were used for this study. The sediment samples were prepared using a 4-day digestion outlined in Wotte et al., 2012 which sequentially removed the water-soluble sulfate until only the carbonate associated sulfate (CAS) remained in the sediment. The CAS was extracted using a 10% HCl solution and then precipitated out of solution by adding 10% BaCl2 to precipitate the CAS as barite. Sulfur isotopes in from the CAS samples were analyzed using an EA-Isolink elemental analyzer connected in continuous flow to a MAT253 IRMS and the oxygen isotopes in dissolved sulfate were measured using a thermal conversion elemental analyzer connected in continuous flow to a Delta V IRMS. Analytical precision was measured using repeated measurements of a blind standard with an average 2-sigma of 0.4‰ for sulfur isotopes (IAEA-SO-5). These data were collected to examine the impact of a long-duration (~26-million-year) unconformity has on the fidelity of the carbonate geochemical archive.

Description: The earliest diagenetic post-mortem exposure of biogenic carbonates at the sea floor and in the uppermost sediment column results in the colonization of hard-part surfaces by bacterial communities. Some of the metabolic redox processes related to these communities have the potential to alter carbonate shell properties, and hence affect earliest diagenetic pathways with significant consequences for archive data. During a three-month in vitro study, shell subsamples of the ocean quahog Arctica islandica (Linnaeus, 1767) were incubated in natural anoxic sediment slurries and bacterial culture medium of the heterotrophic Shewanella sediminisHAW-EB3. Bulk analyses of the liquid media from the Shewanella sediminis incubation revealed an over ten-fold increase in total alkalinity, dissolved inorganic carbon and ΩAragonite, and the alteration of the Mg/Ca, Mg/Sr and Sr/Ca ratios relative to control incubations without cultures. Ion ratios were most affected in the incubation with anoxic sediment, depicting a 25% decrease in Mg/Ca relative to the control. Shell sample surfaces that were exposed to both incubations displayed visible surface dissolution features, and an 8 wt% loss in calcium content. No such alteration features were detected in control shells. Apparently, alteration of shell carbonate properties was induced by microbially driven decomposition of shell intercrystalline organic constituents and subsequent opening of pathways for pore fluid-crystal exchange. This study illustrates the potential influence of benthic bacterial metabolism on biogenic carbonate archives during the initial stages of diagenetic alteration within a relatively short experimental duration of only three months. These results suggest that foremost the biological effect of bacterial cation adsorption on divalent cation ratios has the potential to complicate proxy interpretation. Results shown here highlight the necessity to consider bacterial metabolic activities in marine sediments for the interpretation of palaeo-environmental proxies from shell carbonate archives.

Description: Carbonates 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 didn’t show any effect. Differences in the intensity of microbial alteration documented are likely due to inherent variations in the concentration and nature of original organic compositions in the samples.