ALBERT

Description: The dataset contains B, Li, Mg and Sr concentrations and isotopic compositions of black-smoker, acid-sulfate fluids and "hybrid-types" as well as of fresh and altered rocks from different vent fields (DESMOS and North Su) within the Manus Basin, Papua New Guinea. The data is used to understand the controls of their compositional variability. In particular, the formation of acid-sulfate and hybrid smoker fluids is still poorly understood, and their high Mg concentrations are explained either by dissolution of Mg-bearing minerals in the basement or by mixing between unmodified seawater and magmatic fluids. Mg isotope ratios of the acid-sulfate fluids from Manus Basin are seawater-like, which supports the idea that acid-sulfate fluids in this study area predominantly form by mixing between unmodified seawater and a Mg-free magmatic fluid. Changes in the B, Li, and Sr isotope ratios relative to seawater indicate water-rock interaction in all acid-sulfate fluids. Further, the combination of δ7Li with B concentrations of the same fluids links changes in δ7Li to changes in (1) basement alteration, (2) water-to-rock ratios during water-rock interaction and/or (3) the reaction temperature. These isotope systems, thus, allow tracing of basement composition and acid-sulfate driven alteration of the backarc crust, and help increase our understanding of hydrothermal fluid-rock interactions and the behavior of fluid-mobile elements.

Description: The dataset contains boron (B) concentrations and isotope ratios of hydrothermal vent fluids and volcanic rocks from different vent fields within the Manus Basin, Papua New Guinea, and Nifonea volcano, New Hebrides back-arc. The fluids from these settings show a range of salinities, gas contents, acidities, and host rock compositions; many of them are influenced by phase separation and by addition of magmatic volatiles (both CO2 and SO2). Previous studies of hydrothermal vents in arc/back-arc settings suggest that B contents and isotopic composition of vent fluids are controlled by interactions between seawater, basement and sediments, and propose that phase separation and magmatic fluids play only a subordinate role. In our study, we demonstrate that vent fluids with minor magmatic input indeed reflect the interaction between seawater and oceanic crust. In contrast, the low-salinity Nifonea fluids and some of the acid-sulfate fluids from the Manus Basin have higher B contents as expected, whereas other volatile-rich fluids from the Manus Basin show B depletions. The lack of correlation between B contents and the intensity of magmatic fluid influx (CO2 and SO2) may indicate that magma degassing is not responsible for the B enrichments or depletions in these vent fluids. B enrichments might be related to preferential partitioning of B into the vapour phase during phase separation under PT-conditions well above the two-phase curve and critical line (i.e. T 〉〉 Tcritical, P 〉〉 Pcritical). However, this cannot explain the low B concentrations in the vapour-rich vent fluids from the Manus Basin and the low B isotope ratios in the Nifonea fluids. Instead, we propose that B concentrations and isotope ratios in submarine vent fluids largely depend on the residence and reaction time of the vent fluid in the subsurface. In general, all vent fluids are still influenced by water-rock interaction during hydrothermal circulation. However, vent fluids with short residence times define a trend towards lower B concentrations and isotope ratios, which can be explained by mixing between hydrothermal and magmatic fluid, which is similar to the composition of the host rock. In contrast, the B signature of the magmatic fluid can be overprinted due to preferential mobilization of B from the oceanic crust into vapour-rich fluids at longer reaction times. Thus, B may provide a tool for estimating the extent of B leaching and hence hydrothermal alteration in the subseafloor.

Description: In this study, microbial communities in hydrothermal fluids from four different venting areas (White Flames, Cage Site, Woody and Babylon) within the Menez Gwen hydrothermal field (Mid-Atlantic Ridge) were analyzed. Samples were taken along mixing gradients, including diffuse fluid discharge points, their immediate surroundings, and the buoyant parts of hydrothermal plumes, selected based on visible venting, temperature readings and gas concentrations as indicated by in situ mass spectrometry (ISMS) spectra. High throughput 16S rRNA gene amplicon sequences obtained by Illumina paired-end sequencing using the primer combination Bakt_341F and Bakt_805R were analyzed. Total cell counts were determined and fluorescence in situ hybridization was conducted using probes specific for Archaea and Bacteria as well as for different bacterial subgroups. Additionally, three metagenomes were sequenced (Illumina MiSeq paired-end shotgun), assembled, binned, compared and analyzed for key metabolic pathways. Molecular analyses were combined with geochemical analyses and thermodynamic calculations. The study revealed that close to diffuse venting orifices dominated by chemolithoautotrophic Epsilonproteobacteria, in areas where environmental conditions still supported chemolithoautotrophic processes, microbial communities enriched for versatile heterotrophic Alpha- and Gammaproteobacteria were present. The potential for alkane degradation could be shown for several genera and yet uncultured clades. It was proposed that hotspots of chemolithoautotrophic life support a "belt" of heterotrophic bacteria significantly different from the dominating oligotrophic microbiota of the deep sea.

Description: Two records of seawater sulfate isotope composition from the Early Jurassic demonstrate that large isotopic gradients existed between the European epicontinental sea and the open Tethys Ocean. These differences can be explained by the modification of open-ocean sulfate isotopic compositions by water-mass isolation, sea-level rise, and the effects of changing regional weathering and pyrite burial fluxes, during a time of rapid environmental change. Both records contain large positive isotopic excursions. In the section from Europe (Yorkshire, UK), a 6{per thousand} excursion begins in the early exaratum subzone of the Toarcian in the middle of the organic-rich shale representing a well-known oceanic anoxic event. An open Tethyan margin record from Tibet records a much larger 19{per thousand} excursion, but the section is less well dated. Two age interpretations are possible: sparse biostratigraphic evidence places this excursion in the Aalenian, but we suggest that it may correlate with the positive excursion in Yorkshire. Hence these records may document both a Toarcian event and an Aalenian sulfur isotope event, or the early Toarcian anoxic event alone. Conservative estimates of the rate of isotopic change with time based on the Tibetan section suggest that Early Jurassic seawater sulfate concentrations were between 1 and 5 mM, much lower than previously thought.