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In this work, the biogeochemistry of marine sediments from the Kiel Bight, coastal SW Baltic Sea, is studied based on the abundance and isotopic composition of organic carbon and different forms of sedimentary sulphur. Active bacterial sulphate reduction, partly under sulphate-limiting conditions, is evident from paired δ(34)S and δ(18)O values of pore water sulphate. The resulting pore water sulphide is partly precipitated as acid-volatile iron sulphide and subsequently forms sedimentary pyrite, partly serves in later diagenetic sulphurisation of organic matter, or remains dissolved in the pore water, all evident from the respective δ(34)S values. Microbial sulphate turnover is associated with an apparent isotopic fractionation between dissolved sulphate and dissolved sulphide (Δ(34)S) that varies between 46 and 66‰.

This study examines the representativeness of low-temperature hydrothermal fluid samples with respect to their chemical and microbiological characteristics. Within this scope, we investigated short-term temporal chemical and microbial variability of the hydrothermal fluids. For this purpose we collected three fluid samples consecutively from the same spot at the Clueless field near 5 degrees S on the southern Mid-Atlantic Ridge over a period of 50 min. During sampling, the temperature was monitored online. We measured fluid chemical parameters, characterized microbial community compositions and used statistical analyses to determine significant differences between the samples. Overall, the three fluid samples are more closely related to each other than to any other tested habitat. Therefore, on a broad scale, the three collected fluid samples can be regarded as habitat representatives. However, small differences are apparent between all samples. One of the Clueless samples even displayed significant differences (P-value < 0.01) to the other two Clueless samples. Our data suggest that the observed variations in fluid chemical and microbial compositions are not reflecting sampling artefacts but are related to short-term fluid variability due to dynamic subseafloor fluid mixing. Recorded temporal changes in fact reflect spatial heterogeneity found in the subsurface as the fluid flows through distinctive pathways. While conservative elements (Cl, Si, Na and K) indicate variable degrees of fluid-seawater mixing, reactive components, including Fe(II), O(2) and H(2)S, show that chemical and microbial reactions within the mixing zone further modify the emanating fluids on short-time scales. Fluids entrain microorganisms, which modify the chemical microenvironment within the subsurface biotopes. This is the first study focusing on short-term microbial variability linked to chemical changes in hydrothermal fluids.

The interface between the hypersaline brine and the overlying sea-water (brine-seawater interface) of the Shaban Deep, northern Red Sea was investigated for the presence of microorganisms using the 16S rRNA gene as a molecular marker. Samples of the south and east basin (depth: 1331 m and 1332 m respectively) were selected to ascertain the microbial diversity of this extreme and, so far, unexplored environment. Phylogenetic analysis revealed novel lineages within the Bacteria, the Crenarchaeota and the Euryarchaeota. Novel representatives of the KB1 sequence group (Eder et al., 1999 Arch Microbiol 172: 213-218) were detected indicating a widespread distribution of the corresponding Bacteria in Deep Sea brine pools. Our results contribute to the understanding of the hitherto unknown microbial diversity at the chemical gradient of the Shaban Deep, and suggest the presence of novel Bacteria and Archaea thriving under extreme environmental conditions.

Geochemical proxies archived in banded iron formations (BIFs) can provide information on the physico-chemical conditions in the Precambrian atmosphere-hydrosphere system and the element sources to ambient Early Earth's seawater. We here report results of the first investigation of the Ga-Al systematics of BIFs and use adjacent magnetite and metachert bands of the well-studied and very pure 2.7 Ga-old Temagami BIF in Ontario, Canada, in an attempt to test the suitability of the Ga/Al ratio as a geochemical proxy. Due to very low Ga concentrations in BIFs and multiple mass interferences on the stable isotopes 69Ga and 71Ga, we conducted comparative analyses of dissolved samples with sector-field (SF-)ICP-MS and tandem ICP-MS/MS as well as laser-ablation (LA-)SF-ICP-MS on nano-particulate pressed powder tablets (NP tablets). The unexpected results indicate uniform Ga/Al weight ratios between 2.0×10−4 and 7×10−4 for the Temagami BIF, i.e. Ga/Al ratios that are slightly above those of potential clastic detritus but well below those of modern seawater or marine hydrothermal fluids. Consistent results from solution SF- and ICP-MS/MS and a two-component mixing experiment with BIF reference material IF-G and synthetic pure quartz sand corroborate the high analytical quality of these Ga and Al data. The Ga/Al ratios of the metachert bands show the same range of Ga/Al ratios as the magnetite bands and appear to be dominated by small amounts of finely dispersed Fe oxide particles that were deposited together with the chert. The Ga/Al ratios of the magnetite bands either suggest that (in contrast to other trace elements such as the rare earth elements and Y) Ga and Al were fractionated from each other during scavenging from Archean seawater by Fe (oxyhydr)oxides or that ambient Temagami seawater had significantly lower Ga/Al ratios than modern seawater, possibly due to different weathering, riverine, and estuarine processes in the Archean. While these first results demonstrate the potential of Ga-Al systematics as an additional geochemical tool for studies of Early Earth, they also reveal the knowledge gaps that still need to be addressed in future investigations. [ABSTRACT FROM AUTHOR]

Seawater Mg:Ca and Sr:Ca ratios are biogeochemical parameters reflecting the Earth-ocean-atmosphere dynamic exchange of elements. The ratios' dependence on the environment and organisms' biology facilitates their application in marine sciences. Here, we present a measured single-laboratory dataset, combined with previous data, to test the assumption of limited seawater Mg:Ca and Sr:Ca variability across marine environments globally. High variability was found in open-ocean upwelling and polar regions, shelves/neritic and river-influenced areas, where seawater Mg:Ca and Sr:Ca ratios range from ~4.40 to 6.40 mmol:mol and ~6.95 to 9.80 mmol:mol, respectively. Open-ocean seawater Mg:Ca is semiconservative (~4.90 to 5.30 mol:mol), while Sr:Ca is more variable and nonconservative (~7.70 to 8.80 mmol:mol); both ratios are nonconservative in coastal seas. Further, the Ca, Mg, and Sr elemental fluxes are connected to large total alkalinity deviations from International Association for the Physical Sciences of the Oceans (IAPSO) standard values. Because there is significant modern seawater Mg:Ca and Sr:Ca ratios variability across marine environments we cannot absolutely assume that fossil archives using taxa-specific proxies reflect true global seawater chemistry but rather taxa- and process-specific ecosystem variations, reflecting regional conditions. This variability could reconcile secular seawater Mg:Ca and Sr:Ca ratio reconstructions using different taxa and techniques by assuming an error of 1 to 1.50 mol:mol, and 1 to 1.90 mmol:mol, respectively. The modern ratios' variability is similar to the reconstructed rise over 20 Ma (Neogene Period), nurturing the question of seminonconservative behavior of Ca, Mg, and Sr over modern Earth geological history with an overlooked environmental effect. [ABSTRACT FROM AUTHOR]

High metal and rare-earth element (REE) concentrations with unusual ( ‘atypical’ ) normalized REE patterns are documented in fluids from active hydrothermal vent fields on the Mid-Atlantic Ridge, 5°S and the East Scotia Ridge. Those fluids show relative enrichment of middle heavy REEs and almost no Eu anomalies in chondrite-normalized patterns. To understand the processes that produce such atypical REE patterns we ran a series of experiments, in which natural bottom seawater or aqueous solutions (NaCl, NaCl–MgCl 2 , or NaCl–CaCl 2 ) were reacted with gabbro and gabbro mineral assemblages from 300 to 475 °C and 40 and 100 MPa. These P–T conditions are representative for water–rock interactions in hydrothermal root and discharge zones. Fluid flux variability and kinetics were addressed in the experiments by varying the water-to-rock mass ratio (w/r) from 0.5–10 and using different run durations from 3–720 h. Only seawater and synthetic MgCl 2 -bearing fluid mobilized significant amounts of REEs, Si, Ca, Fe, and Mn from gabbro, from clinopyroxene, and from plagioclase. At 425 °C and 40 MPa, fluids were initially acidic with pH (25 °C) of ∼2 increasing to values between ∼4 and 7 upon progressing reactions. Rare earth element and Fe contents peaked within 3–6 h after interaction with gabbroic mineral grains (125–500 μm) at w/r of 5 (REEs) and 2–5 (Fe) but decreased with continuing reaction without strong REE fractionation. Most of the REEs that were leached from primary minerals and dissolved in the fluids early became redeposited into solid reaction products after 720 h. Contents of dissolved SiO 2 were pressure-dependent, being about twofold higher at 100 MPa than at 40 MPa (425 °C) and were below quartz saturation with gabbro and clinopyroxene as solid starting material and close to quartz saturation with plagioclase reactant. However, Si in fluids from the rock-dominated experiments at 100 MPa with gabbro (w/r 0.5–1) dropped to very low contents. A concomitant decrease in chlorinity suggests that these changes may be due to the breakdown of olivine and the formation of serpentine and Fe-hydroxy chlorides. Regardless of the starting solid reactants, fluid REE patterns were dominantly controlled by w/r. Atypical fluid REE patterns and high fluid REE contents were obtained at high w/r (⩾5). Whereas typical REE patterns known from many mid-ocean ridge vent fluids, showing relative enrichments of light REEs and a positive Eu anomaly, were obtained at low w/r of 0.5–1. Our results hence clearly show that REE contents and patterns of vent fluids are sensitive to variations in the w/r. [ABSTRACT FROM AUTHOR]

Germanium (Ge) and silicon (Si) are an element pair which shows rather coherent behaviour in many geochemical systems. In aqueous systems and in the critical zone, however, Ge and Si may fractionate from each other, leading to slight differences between the Ge/Si ratios of continental crust, river water and seawater. We here show that even stronger fractionation of the Ge Si couple occurs within seawater during the formation of hydrogenetic ferromanganese (Fe Mn) crusts and nodules. We determined Ge/Si ratios of hydrogenetic Fe Mn crusts from the Pacific Ocean following a new analytical approach using laser-ablation ICP-MS on nano-particulate tablets prepared from the Fe Mn crust material. Data quality was monitored and demonstrated by repetitive measurements of Fe Mn nodule reference material (RM) NOD-P1 from the Pacific Ocean (Ge/Si wt : 16.8 × 10−6 g/g; Ge/Si mol : 6.47 × 10−6 mol/mol), which is the only RM for which high-quality Ge Si reference data are available. We also report Ge/Si ratios for Fe Mn nodule RMs NOD-A1 from the Atlantic Ocean (Ge/Si wt : 24.9 × 10−6; Ge/Si mol : 9.6 × 10−6) and for JMn-1 from the Pacific Ocean (Ge/Si wt : 17.9 × 10−6; Ge/Si mol : 6.9 × 10−6). The Ge/Si wt ratios of the hydrogenetic Fe Mn crusts from the Pacific Ocean range from 13.2 × 10−6 to 33.5 × 10−6 (Ge/Si mol : 5.1 × 10−6 to 13 × 10−6). The Ge/Si ratios of both Fe Mn crusts and nodules are significantly higher than those of ambient seawater (ca. Ge/Si wt : 1.7 × 10−6; Ge/Si mol : 0.7 × 10−6). Their Ge/Si ratios are also well-above those of potential (alumino)silicate detritus. Hence, these high Ge/Si ratios reveal fractionation of Ge and Si during scavenging from seawater by the iron (oxy)hydroxide component in Fe Mn crusts, and do not result from the presence of detrital material in the chemical precipitate. Their high Ge/Si ratios relative to seawater also suggest that despite higher complexation constants with organic ligands for Ge than Si, dissolved organic ligands do not control marine Ge Si fractionation via preferential retention of Ge in seawater. [ABSTRACT FROM AUTHOR]

Previous stable oxygen isotope records of calcitic fossils (δ18Oshell) from Europe have been interpreted to reflect strong climatic perturbations throughout the Jurassic, but it is unknown whether they reflect global trends because data from other regions are sparse. Here, we use bivalve and brachiopod shells from western Asia and northern Africa to examine seawater temperatures at low latitudes as well as latitudinal temperature gradients in the Middle Jurassic. Our results include the first absolute temperature estimates for most of the study areas and this time interval. Furthermore, we acknowledge a latitudinal gradient in δ18O values of seawater (δ18Osea) that leads to more realistic temperature reconstructions of tropical water temperatures, which were underestimated by earlier methods. Following this approach, δ18Oshell values translate into average equatorial water temperatures during the Middle Jurassic several degrees higher than today (as high as 34-35 °C). Such high temperatures cause environmental stress, probably explaining previously documented low diversities of low-latitudinal ecosystems (e.g., coral reefs) during this time interval. A comparison with literature data raises doubts over whether previously published European temperature curves are truly representative of global patterns. Instead, they might reflect the complex paleogeography of European basins influenced by regional and/or short-term changes in δ18Osea values. [ABSTRACT FROM AUTHOR]

Over the last decades extensive research has been carried out on changes in the Atlantic Meridional Overturning Circulation and its role during the last deglacial period. At present, only a few high-resolution data sets from the deep eastern North Atlantic exist for this period. It therefore remains uncertain whether deepwater changes in the Eastern North Atlantic Basin were governed by alternating contributions of northern and southern deepwater or whether the Eastern North Atlantic Basin reflects changes in the initial composition and source of North Atlantic deepwater. Furthermore, it is still debated whether such changes are triggered by Northern or Southern Hemisphere climatic changes. In this centennial to decadal scale resolution study we investigate deepwater composition changes in the Eastern North Atlantic Basin over the last 15 ka BP. We used sediment cores GEOFAR KF16 and MD08-3180 (37.984° N; 31.118° W, wd 3050 m/37.999° N; 31.134° W, wd 3064 m), obtained from a small basin at the eastern flank of the Mid Atlantic Ridge south of the Azores Islands. Under modern conditions the coring site is situated at the boundary between southern Lower Deep Water and northern Eastern North Atlantic Deep Water consisting of Iceland Scotland Overflow Water and Labrador Sea Water. Distinct differences between the three water masses in terms of ventilation state, temperature and salinity signatures are ideal for tracking changes in the deglacial North Atlantic deepwater distribution using paired benthic foraminiferal stable isotopes ( δ 13 C , δ 18 O ) and Mg/Ca bottom water temperature reconstructions. The results show a close coupling of low bottom water temperature (1.5 °C) and δ 13 C ( 0 – 0.5 ‰ ) values during cold Heinrich event 1, the Younger Dryas and the Preboreal, that were coeval with a major δ 18 O depletion of deepwater. The strong similarities between subtropical Eastern North Atlantic Deep Water and subpolar North Atlantic surface and deepwater changes indicate that deglacial changes in Eastern North Atlantic Deep Water distribution were triggered in the North Atlantic. Consequently, this would imply that changes in the North Atlantic also contributed to deglacial changes in Antarctic bottom water composition. During the early Holocene stepwise increasing δ 13 C values suggest increasing Eastern North Atlantic Deep Water production and/or increasing Eastern North Atlantic Deep Water ventilation with minor but distinct interruptions (at 10.8, 10.6, 9.1, 8.4, 8.1 and 7.2 ka BP), which were most probably also triggered in the subpolar North Atlantic. [ABSTRACT FROM AUTHOR]

Here we present trace element and Sr–Nd–Hf–Pb (double spike) isotopic data covering the entire igneous section of oceanic crust drilled at Ocean Drilling Program (ODP)/Integrated Ocean Drilling Program (IODP) Site 1256 on the Cocos Plate. The penetrated interval extends from the upper lavas through the sheeted dike complex to the gabbroic plutonic rocks, formed during superfast spreading at the mid-Miocene equatorial East Pacific Rise. The data are used to characterize the effects of chemical alteration, resulting from convection of seawater and hydrothermal fluids, on the trace element and isotopic composition of oceanic crust. Compared to normal mid-ocean-ridge basalt, the igneous basement of Site 1256 (Holes 1256C/D) is isotopically slightly enriched but shows only narrow downhole variations in Nd–Hf–Pb isotope ratios: 143 Nd/ 144 Nd = 0.513089 ± 0.000028 (2σ), 176 Hf/ 177 Hf = 0.283194 ± 0.000033 (2σ), 206 Pb/ 204 Pb = 18.61 ± 0.11 (2σ), 207 Pb/ 204 Pb = 15.521 ± 0.014 (2σ), 208 Pb/ 204 Pb = 38.24 ± 0.15 (2σ). We believe that this minor variability is mainly of primary (magmatic) origin. The Sr isotopic composition shows considerably larger variation and, as expected, serves as sensitive tracer of seawater influence, which is particularly pronounced in the lava–dike transition zone and the sheeted dikes. The seawater influence is most prominent in a highly metal sulfide-enriched breccia layer encountered in the transition zone with 87 Sr/ 86 Sr of ~ 0.706, indicating a maximum water–rock mixing ratio of ~ 12. However, compared to the igneous section drilled at Site 504 (Hole 504B), which formed at intermediate, i.e., slower spreading rates at the Galápagos Spreading Center and hosting a much thicker sulfide-rich stockwork zone, the average intensity of water–rock interaction is lower. This is expressed by lesser mobility of base metals, narrower variability of alteration-sensitive incompatible elements, and less radiogenic Sr isotopic compositions on average at Site 1256. The amount of metal sulfide precipitation seems to be positively correlated with the degree of hydrothermal overprint. The less intense alteration of the Site 1256 transition zone, compared to Site 504, most likely reflects the higher rate of spreading, eventually resulting in a shorter period of time of continuous exposure to hydrothermal convection at the ridge crest. The observed seafloor alteration, leading to modified radiogenic parent/daughter ratios in the Site 1256 rocks, is ultimately not sufficient to develop time-integrative high 206 Pb/ 204 Pb and moderate 87 Sr/ 86 Sr ratios, as being characteristic of the HIMU (high μ = high 238 U/ 204 Pb) mantle signature proposed to originate from hydrothermally altered, subducted oceanic crust. Therefore, additional modification during the subduction process must be taken into account. [ABSTRACT FROM AUTHOR]

Abstract: In this study, we present the first high-resolution data from coupled δ34S analyses of carbonate-associated sulfate (CAS) and chromium-reducible sulfur (CRS) from three Lower–Middle Cambrian sections in western Gondwana. CAS and CRS were extracted and analyzed from marine dolostone, limestone, and nodular limestone from Spanish and French successions. In parallel, carbonate samples were also analyzed for δ13Ccarb, δ18Ocarb, and major/trace element concentrations (Ca, Fe, Mg, Mn, Sr). δ34SCAS values vary between 17.6‰ and 33.2‰, with a maximum CAS concentration of ca. 900ppm. δ34SCRS data show a similar broad range between −5.1‰ and 29.7‰, with maximal CRS contents up to ca. 3700ppm. Notably, there is little stratigraphic variation in the δ34SCAS data in each of the sections confounding inter-basinal chemostratigraphic correlations. Nonetheless, the absolute differences in δ34SCAS between sections as well as variations in CAS and CRS concentrations are attributed to paleoenvironmental differences between proximal and distal parts of the carbonate ramp, as well as effects of subaerial exposure and riverine input. Thus, the generated δ34SCAS data deliver not only valuable paleoecological and paleoenvironmental information, they also illustrate a heterogeneity in the seawater sulfate sulfur isotopic composition of the western Gondwanan ocean. Consequently, the lack of correlation between our Gondwanan δ34SCAS data and time equivalent sections of Laurentia and Siberia is probably not only caused by the absence of an internationally accepted biostratigraphic correlation, but rather supports the view that sulfate was non-conservative anion in seawater during the Cambrian Period. [Copyright &y& Elsevier]

Abstract: We investigate the Logatchev Hydrothermal Field at the Mid-Atlantic Ridge, 14°45′N to constrain the calcium isotope hydrothermal flux into the ocean. During the transformation of seawater to a hydrothermal solution, the Ca concentration of pristine seawater ([Ca]SW) increases from about 10mM to about 32mM in the hydrothermal fluid endmember ([Ca]HydEnd) and thereby adopts a δ44/40CaHydEnd of −0.95±0.07‰ relative to seawater (SW) and a 87Sr/86Sr isotope ratio of 0.7034(4). We demonstrate that δ44/40CaHydEnd is higher than that of the bedrock at the Logatchev field. From mass balance calculations, we deduce a δ44/40Ca of −1.17±0.04‰ (SW) for the host-rocks in the reaction zone and −1.45±0.05‰ (SW) for the isotopic composition of the entire hydrothermal cell of the Logatchev field. The values are isotopically lighter than the currently assumed δ44/40Ca for Bulk Earth of −0.92±0.18‰ (SW) [Skulan J., DePaolo D. J. and Owens T. L. (1997) Biological control of calcium isotopic abundances in the global calcium cycle. Geochim. Cosmochim. Acta 61,(12) 2505–2510] and challenge previous assumptions of no Ca isotope fractionation between hydrothermal fluid and the oceanic crust [Zhu P. and Macdougall J. D. (1998) Calcium isotopes in the marine environment and the oceanic calcium cycle. Geochim. Cosmochim. Acta 62,(10) 1691–1698; Schmitt A. -D., Chabeaux F. and Stille P. (2003) The calcium riverine and hydrothermal isotopic fluxes and the oceanic calcium mass balance. Earth Planet. Sci. Lett. 6731, 1–16]. Here we propose that Ca isotope fractionation along the fluid flow pathway of the Logatchev field occurs during the precipitation of anhydrite. Two anhydrite samples from the Logatchev Hydrothermal Field show an average fractionation of about Δ44/40Ca=−0.5‰ relative to their assumed parental solutions. Ca isotope ratios in aragonites from carbonate veins from ODP drill cores indicate aragonite precipitation directly from seawater at low temperatures with an average δ44/40Ca of −1.54±0.08‰ (SW). The relatively large fractionation between the aragonite precipitates and seawater in combination with their frequent abundance in weathered mafic and ultramafic rocks suggest a reconsideration of the marine Ca isotope budget, in particular with regard to ocean crust alteration. [Copyright &y& Elsevier]

The δ 18 O value of seawater is presently buffered by high-temperature hydrothermal alteration and low-temperature weathering of lithospheric rocks. It is much debated whether the δ 18 O of seawater has been buffered to this steady-state value throughout Earth's history or, alternatively, whether it gradually increased towards the present value since the Archean. A third possibility is that the δ 18 O of seawater was buffered at a higher value before continents emerged, and has been buffered at its current value (δ 18 O ∼ −1‰) since the emergence of continents. In this contribution, we reconstruct the δ 18 O of Archean seawater from triple oxygen isotope (δ 17 O, δ 18 O) variations in >2.7 Ga ultramafic rocks that reacted with fluids that, in turn, had been derived from seawater in the Archean. The samples that were studied are peridotites from the Ivnartivaq complex, an ultramafic lens in the Archean Kuummiut terrane (Rae craton, southeast Greenland) that have unusually low δ 18 O values for peridotites (in olivine, 1.7 ‰ ≤ δ 18 O ≤ 4.6 ‰ , relative to VSMOW). Bulk rock trace element concentrations, mineral compositions and U-Pb dating of zircon grains in the peridotites indicate that these samples are metamorphic peridotites, that formed by the dehydration of serpentinites ∼2.7 Ga ago. The serpentinite protoliths, in turn, had formed by alteration of ultramafic cumulate rocks at high temperatures (250–450 °C) by fluids that were derived from seawater. Triple oxygen isotope variations in olivine from the peridotites indicate that the fluids could not have been derived from seawater with a significantly higher or lower δ 18 O value than that of seawater in the Phanerozoic, but could have been derived from seawater with a value of δ 18 O ∼ −1‰, i.e., the modern (ice-free) seawater composition. We conclude that the δ 18 O of seawater had reached its current steady state value by 2.7 Ga ago. • Archean peridotites from southeast Greenland contain olivine with 1.7‰ ≤ δ 18 O ≤ 4.6‰. • They are crystal cumulates that were altered by seawater to low- δ 18 O serpentinites. • The serpentinites became dehydrated during metamorphism ∼2.7 Ga ago. • We reconstruct the δ 18 O of >2.7 Ga seawater from triple O isotope variations in olivine. • Seawater had reached its current steady state value (δ 18 O ∼ −1‰) by 2.7 Ga ago. [ABSTRACT FROM AUTHOR]