Description: Mid-Oceanic ridge hydrothermal systems are in general significantly enriched in Ca due to leaching from the oceanic basement (albitization of anorthite), while Mg and SO4 are quantitatively removed by Mg-rich smectite and anhydrite formation respectively. During Meteor expedition 141 in September 2017 we sampled, close to a volcanic outcrop in the Terceira Rift (Azores, Central North Atlantic Ocean), pore fluids with a significantly different composition compared to typical Mid-Oceanic hydrothermal systems. Pore water Mg, SO4, and total alkalinity (TA) concentrations are significantly higher compared to seawater and a nearby reference core, while Ca concentrations stay at low values. The most straightforward way of interpreting the observed deviations is the dissolution of the prior hydrothermally formed high temperature (〉 240°C) mineral caminite (MgSO4·0.25Mg(OH)2·0.2H2O). This interpretation is corroborated by a thorough investigation of fluid isotope systems (δ26Mg, δ30Si, δ34S, δ44/42Ca, and 87Sr/86Sr). Caminite is known from mineral assemblages with e.g. anhydrite and forms only under specific conditions such as high fluid temperatures and in altered oceanic crust with only little fresh basaltic glass present, which are generally met in the Terceira Rift. To date, no signs of extensive caminite formation and/or dissolution have been reported, caminite has only been once described before in the nature by Haymon and Kastner (1986). Our study is the first indication of an abundant occurrence of caminite. The results imply that element recycling through caminite might play a presently unrecognized role in element budgets of hydrothermal systems. Haymon, R. M., and Kastner, M., 1986, Caminite; a new magnesium-hydroxide-sulfate-hydrate mineral found in a submarine hydrothermal deposit, East Pacific Rise, 21 degrees N: American Mineralogist, v. 71, no. 5-6, p. 819-825.

Description: Typical Mid-Oceanic ridge hydrothermal systems are in general significantly enriched in Ca due to leaching from the magmatic basement (albitization of anorthite), while Mg and SO4 are quantitatively removed because of Mgrich smectite and anhydrite formation and further processes. In the Terceira Rift (TR), a hyper-slow spreading center in the Central North Atlantic Ocean, hydrothermal fluid venting is known to occur only at shallow intertidal water depths around the volcanogenic Azores Archipelago. Here, we show for the first time that hydrothermal fluid venting is active in the eastern TR at water depths of 2800 m. Pore fluids of a sediment core taken close to a volcanic cone, however, show that the fluid composition is significantly different from typical Mid-Oceanic hydrothermal systems. Pore water Mg, SO4, and total alkalinity (TA) concentrations are significantly higher compared to seawater and a nearby reference core. The most straightforward way of interpreting these excursions is the re-dissolution of the metastable mineral caminite (MgSO4 0.4Mg(OH)2 0.2H2O). Caminite is known from mineral assemblages with e.g. anhydrite and forms only under specific conditions such as high fluid temperatures and in altered oceanic crust with only few fresh basaltic glass present, which are generally met at the TR. Isotope measurements of �34S, �26Mg, 87Sr/86Sr, �88=86Sr, �44=42Ca and �30Si provide additional evidence for caminite as a source for Mg, SO4 and TA. The redissolution of the caminite is interpreted as a sign of cooling temperatures, which may indicate a waning state of the hydrothermal system. To date, no signs of extensive caminite formation and/or dissolution have been reported. Our study implies that element recycling through caminite might play a presently unrecognized role in element budgets of hydrothermal systems.

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.