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  • 1
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    Composition of hydrothermal fluids and mineralogy of associated chimney material on the East Scotia Ridge back-arc spreading centre (2014)
    Elsevier
    Details
    Publication Date: 2022-05-26
    Description: © The Author(s), 2014. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Geochimica et Cosmochimica Acta 139 (2014): 47-71, doi:10.1016/j.gca.2014.04.024.
    Description: The East Scotia Ridge is an active back-arc spreading centre located to the west of the South Sandwich island arc in the Southern Ocean. Initial exploration of the ridge by deep-tow surveys provided the first evidence for hydrothermal activity in a back-arc setting outside of the western Pacific, and we returned in 2010 with a remotely operated vehicle to precisely locate and sample hydrothermal sites along ridge segments E2 and E9. Here we report the chemical and isotopic composition of high- and low-temperature vent fluids, and the mineralogy of associated high-temperature chimney material, for two sites at E2 (Dog’s Head and Sepia), and four sites at E9 (Black & White, Ivory Tower, Pagoda and Launch Pad). The chemistry of the fluids is highly variable between the ridge segments. Fluid temperatures were ∼350 °C at all vent sites except Black & White, which was significantly hotter (383 °C). End-member chloride concentrations in E2 fluids (532–536 mM) were close to background seawater (540 mM), whereas Cl in E9 fluids was much lower (98–220 mM) indicating that these fluids are affected by phase separation. Concentrations of the alkali elements (Na, Li, K and Cs) and the alkaline earth elements (Ca, Sr and Ba) co-vary with Cl, due to charge balance constraints. Similarly, concentrations of Mn and Zn are highest in the high Cl fluids but, by contrast, Fe/Cl ratios are higher in E9 fluids (3.8–8.1 × 10−3) than they are in E2 fluids (1.5–2.4 × 10−3) and fluids with lowest Cl have highest Cu. Although both ridge segments are magmatically inflated, there is no compelling evidence for input of magmatic gases to the vent fluids. Fluid δD values range from 0.2‰ to 1.5‰, pH values (3.02–3.42) are not especially low, and F concentrations (34.6–54.4 μM) are lower than bottom seawater (62.8 μM). The uppermost sections of conjugate chimney material from E2, and from Ivory Tower and Pagoda at E9, typically exhibit inner zones of massive chalcopyrite enclosed within an outer zone of disseminated sulphide, principally sphalerite and pyrite, in an anhydrite matrix. By contrast, the innermost part of the chimneys that currently vent fluids with lowest Cl (Black & White and Launch Pad), is dominated by anhydrite. By defining and assessing the controls on the chemical composition of these vent fluids, and associated mineralisation, this study provides new information for evaluating the significance of hydrothermal processes at back-arc basins for ocean chemistry and the formation of seafloor mineral deposits.
    Description: This work was funded by the Natural Environment Research Council consortium grant NE/D01249X/1. C.R.G. acknowledges further support from the National Science Foundation’s Office of Polar Programs grant ANT-0739675. N.R.B. acknowledges funding from the National Sciences and Engineering Research Council of Canada, Ontario Ministry of Research and Innovation, and the Academic Development Fund at Western University.
    Repository Name: Woods Hole Open Access Server
    Type: Article
    Format: application/pdf
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  • 2
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    Detection and quantification of a release of carbon dioxide gas at the seafloor using pH eddy covariance and measurements of plume advection (2022)
    Elsevier
    Details
    Publication Date: 2022-05-27
    Description: © The Author(s), 2021. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Koopmans, D., Meyer, V., Schaap, A., Dewar, M., Farber, P., Long, M., Gros, J., Connelly, D., & Holtappels, M. Detection and quantification of a release of carbon dioxide gas at the seafloor using pH eddy covariance and measurements of plume advection. International Journal of Greenhouse Gas Control, 112, (2021): 103476, https://doi.org/10.1016/j.ijggc.2021.103476.
    Description: We detected a controlled release of CO2 (g) with pH eddy covariance. We quantified CO2 emission using measurements of water velocity and pH in the plume of aqueous CO2 generated by the bubble streams, and using model predictions of vertical CO2 dissolution and its dispersion downstream. CO2 (g) was injected 3 m below the floor of the North Sea at rates of 5.7–143 kg d − 1. Instruments were 2.6 m from the center of the bubble streams. In the absence of injected CO2, pH eddy covariance quantified the proton flux due to naturally-occurring benthic organic matter mineralization (equivalent to a dissolved inorganic carbon flux of 7.6 ± 3.3 mmol m − 2 d − 1, s.e., n = 33). At the lowest injection rate, the proton flux due to CO2 dissolution was 20-fold greater than this. To accurately quantify emission, the kinetics of the carbonate system had to be accounted for. At the peak injection rate, 73 ± 13% (s.d.) of the injected CO2 was emitted, but when kinetics were neglected, the calculated CO2 emission was one-fifth of this. Our results demonstrate that geochemical techniques can detect and quantify very small seafloor sources of CO2 and attribute them to natural or abiotic origins.
    Description: This project received funding from the European Union's Horizon 2020 research and innovation program under grant agreement No. 654462 (STEMM-CCS), it also received funding from the Max Planck Society and the Helmholtz Society. MHL was supported by US NSF grant # OCE-1657727.
    Keywords: CO2 vent ; Offshore CCS ; Leakage detection and quantification ; Marine sediment ; Proton flux
    Repository Name: Woods Hole Open Access Server
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  • 3
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    Behaviour of chromium isotopes in the eastern sub-tropical Atlantic Oxygen Minimum Zone (2018)
    Elsevier
    In:  Geochimica et Cosmochimica Acta, 236 . pp. 41-59.
    Details
    Publication Date: 2021-04-23
    Description: Constraints on the variability of chromium (Cr) isotopic compositions in the modern ocean are required to validate the use of Cr isotopic signatures in ancient authigenic marine sediments for reconstructing past levels of atmospheric and ocean oxygenation. This study presents dissolved Cr concentrations (Cr-T, where Cr-T = Cr(VI) + Cr(III)) and Cr isotope data (delta Cr-53) for shelf, slope and open ocean waters within the oxygen minimum zone (OMZ) of the eastern sub-tropical Atlantic Ocean. Although dissolved oxygen concentrations were as low as 44-90 mu mol kg(-1) in the core of the OMZ, there was no evidence for removal of Cr(VI). Nonetheless, there was significant variability in seawater delta Cr-53, with values ranging from 1.08 to 1.72 parts per thousand. Shelf Cr-T concentrations were slightly lower (2.21 +/- 0.07 nmol kg(-1)) than in open ocean waters at the same water depth (between 0 and 160 m, 2.48 +/- 0.07 nmol kg(-1)). The shelf waters also had higher delta Cr-53 values (1.41 +/- 0.14 parts per thousand compared to 1.18 +/- 0.05 parts per thousand for open ocean waters shallower than 160 m). This is consistent with partial reduction of Cr(VI) to Cr(III), with subsequent removal of isotopically light Cr(III) onto biogenic particles. We also provide evidence for input of relatively isotopically heavy Cr from sediments on the shelf. Intermediate and deep water masses (AAIW and NADW) show a rather limited range of delta Cr-53 values (1.19 +/- 0.09 parts per thousand) and inputs of Cr from remineralisation of organic material or re-oxidation of Cr (III) appear to be minimal. Authigenic marine precipitates deposited in deep water in the open ocean therefore have the potential to faithfully record seawater delta Cr-53, whereas archives of seawater delta Cr-53 derived from shelf sediments must be interpreted with caution.
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.1016/j.gca.2018.03.004
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  • 4
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    A high performance microfluidic analyser for phosphate measurements in marine waters using the vanadomolybdate method (2013)
    Elsevier
    In:  Talanta, 116 . pp. 382-387.
    Details
    Publication Date: 2014-01-06
    Description: We report a high performance autonomous analytical system based on the vanadomolybdate method for the determination of soluble reactive phosphorus in seawater. The system combines a microfluidic chip manufactured from tinted poly (methyl methaaylate) (PMMA), a custom made syringe pump, embedded control electronics and on-board calibration standards. This "lab-on-a-chip" analytical system was successfully deployed and cross-compared with reference analytical methods in coastal (south west England) and open ocean waters (tropical North Atlantic). The results of the miniaturized system compared well with a reference bench-operated phosphate auto-analyser and showed no significant differences in the analytical results (student's t-test at 95% confidence level). The optical technology used, comprising of tinted PMMA and polished fluidic channels, has allowed an improvement of two orders of magnitude of the limit of detection (52 nM) compared to currently available portable systems based on this method. The system has a wide linear dynamic range 0.1-60 mu M, and a good precision (13.6% at 0.4 mu M, n=4). The analytical results were corrected for silicate interferences at 0.7 mu M, and the measurement frequency was configurable with a sampling throughput of up to 20 samples per hour. This portable micro-analytical system has a low reagent requirement (340 mu L per sample) and power consumption (756J per sample), and has allowed accurate high resolution measurements of soluble reactive phosphorus in seawater
    Type: Article , PeerReviewed
    DOI: 10.1016/j.talanta.2013.05.004
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  • 5
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    Seawater-pH measurements for ocean-acidification observations (2012)
    Elsevier
    In:  TrAC - Trends in Analytical Chemistry, 40 . pp. 146-157.
    Details
    Publication Date: 2020-05-04
    Description: The uptake of anthropogenic CO2 by the oceans since the onset of the industrial revolution is considered a serious challenge to marine ecosystems due to ensuing carbonate-chemistry changes (ocean acidification). Furthermore, the CO2 uptake is reducing the ocean's capacity to absorb future CO2 emissions. In order to follow the changes in the ocean's carbonate system, high-quality analytical measurements with good spatial and temporal resolution are necessary. High-precision and accurate pH measurements are now possible, and allow us to determine the progression of ocean acidification. The spectrophotometric pH technique is now widely used and capable of the required high-quality measurements. Spectrophotometric pH systems are deployed on ships and in situ on remote platforms. Smaller and more rugged instruments are nevertheless required for more widespread in situ application to allow routine high-resolution measurements, even in the most remote regions. We critically review oceanic pH measurements, and focus on state-of-the-art spectrophotometric pH measurement techniques and instrumentation. We present a simple microfluidic design integrated in a shipboard instrument featuring high accuracy and precision as a key step towards a targeted pH microsensor system
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.1016/j.trac.2012.07.016
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  • 6
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    Authigenic carbonates from the Darwin Mud Volcano, Gulf of Cadiz : a record of palaeo-seepage of hydrocarbon bearing fluids (2012)
    Elsevier
    In:  Chemical Geology, 300-301 . pp. 24-39.
    Details
    Publication Date: 2017-12-07
    Description: Hydrocarbon-rich fluids expelled at mud volcanoes (MVs) may contribute significantly to the carbon budget of the oceans, but little is known about the long-term variation in fluid fluxes at MVs. The Darwin MV is one of more than 40 MVs located in the Gulf of Cadiz, but it is unique in that its summit is covered by a thick carbonate crust that has the potential to provide a temporal record of seepage activity. In order to test this idea, we have conducted petrographic, chemical and isotopic analyses of the carbonate crust. In addition a 1-D transport-reaction model was applied to pore fluid data to assess fluid flow and carbonate precipitation at present. The carbonate crusts mainly comprise of aragonite, with a chaotic fabric exhibiting different generations of cementation and brecciation. The crusts consist of bioclasts and lithoclasts (peloids, intraclasts and extraclasts) immersed in a micrite matrix and in a variety of cement types (microsparite, botryoidal, isopachous acicular, radial and splayed fibrous). The carbonates are moderately depleted in 13C (δ13C = − 8.1 to − 27.9‰) as are the pore fluids (δ13C = − 19.1 to − 28.7‰), which suggests that their carbon originated from the oxidation of methane and higher hydrocarbons, like the gases that seep from the MV today. The carbonate δ18O values are as high as 5.1‰, and it is most likely that the crusts formed from 18O-rich fluids derived from dehydration of clay minerals at depth. Pore fluid modelling results indicate that the Darwin MV is currently in a nearly dormant phase (seepage velocities are 〈 0.09 cm yr− 1). Thus, the thick carbonate crust must have formed during past episodes of high fluid flow, alternating with phases of mud extrusion and uplift. Highlights ► Results of pore fluid modelling indicate low seepage activity at localised sites. ► Pore fluids are supersaturated with respect to hydrocarbons of thermogenic origin. ► AOM supports vent fauna and results in the formation of authigenic carbonates. ► The carbonate crust has a brecciated appearance and mainly consists of aragonite. ► The crust formation seems to be regulated by changes in fluid and mudflow activity.
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.1016/j.chemgeo.2012.01.006
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  • 7
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    Rare earth elements as indicators of hydrothermal processes within the East Scotia subduction zone system (2014)
    Elsevier
    In:  Geochimica et Cosmochimica Acta, 140 . pp. 20-38.
    Details
    Publication Date: 2017-06-19
    Description: The East Scotia subduction zone, located in the Atlantic sector of the Southern Ocean, hosts a number of hydrothermal sites in both back-arc and island-arc settings. High temperature (〉348 °C) ‘black smoker’ vents have been sampled at three locations along segments E2 and E9 of the East Scotia back-arc spreading ridge, as well as ‘white smoker’ (〈212 °C) and diffuse (〈28 °C) hydrothermal fluids from within the caldera of the Kemp submarine volcano. The composition of the endmember fluids (Mg = 0 mmol/kg) is markedly different, with pH ranging from 〈1 to 3.4, [Cl−] from ∼90 to 536 mM, [H2S] from 6.7 to ∼200 mM and [F−] from 35 to ∼1000 μM. All of the vent sites are basalt- to basaltic andesite-hosted, providing an ideal opportunity for investigating the geochemical controls on rare earth element (REE) behaviour. Endmember hydrothermal fluids from E2 and E9 have total REE concentrations ranging from 7.3 to 123 nmol/kg, and chondrite-normalised distribution patterns are either light REE-enriched (LaCN/YbCN = 12.8–30.0) with a positive europium anomaly (EuCN/Eu∗CN = 3.45–59.5), or mid REE-enriched (LaCN/NdCN = 0.61) with a negative Eu anomaly (EuCN/Eu∗CN = 0.59). By contrast, fluids from the Kemp Caldera have almost flat REE patterns (LaCN/YbCN = 2.1–2.2; EuCN/Eu∗CN = 1.2–2.2). We demonstrate that the REE geochemistry of fluids from the East Scotia back-arc spreading ridge is variably influenced by ion exchange with host minerals, phase separation, competitive complexation with ligands, and anhydrite deposition, whereas fluids from the Kemp submarine volcano are also affected by the injection of magmatic volatiles which enhances the solubility of all the REEs. We also show that the REE patterns of anhydrite deposits from Kemp differ from those of the present-day fluids, potentially providing critical information about the nature of hydrothermal activity in the past, where access to hydrothermal fluids is precluded.
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.1016/j.gca.2014.05.018
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  • 8
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    Methane in shallow subsurface sediments at the landward limit of the gas hydrate stability zone offshore western Svalbard (2017)
    Elsevier
    In:  Geochimica et Cosmochimica Acta, 198 . pp. 419-438.
    Details
    Publication Date: 2020-02-06
    Description: Offshore western Svalbard plumes of gas bubbles rise from the seafloor at the landward limit of the gas hydrate stability zone (LLGHSZ; ∼400 m water depth). It is hypothesized that this methane may, in part, come from dissociation of gas hydrate in the underlying sediments in response to recent warming of ocean bottom waters. To evaluate the potential role of gas hydrate in the supply of methane to the shallow subsurface sediments, and the role of anaerobic oxidation in regulating methane fluxes across the sediment–seawater interface, we have characterised the chemical and isotopic compositions of the gases and sediment pore waters. The molecular and isotopic signatures of gas in the bubble plumes (C1/C2+ = 1 × 104; δ13C-CH4 = −55 to −51‰; δD-CH4 = −187 to −184‰) are similar to gas hydrate recovered from within sediments ∼30 km away from the LLGHSZ. Modelling of pore water sulphate profiles indicates that subsurface methane fluxes are largely at steady state in the vicinity of the LLGHSZ, providing no evidence for any recent change in methane supply due to gas hydrate dissociation. However, at greater water depths, within the GHSZ, there is some evidence that the supply of methane to the shallow sediments has recently increased, which is consistent with downslope retreat of the GHSZ due to bottom water warming although other explanations are possible. We estimate that the upward diffusive methane flux into shallow subsurface sediments close to the LLGHSZ is 30,550 mmol m−2 yr−1, but it is 〈20 mmol m−2 yr−1 in sediments further away from the seafloor bubble plumes. While anaerobic oxidation within the sediments prevents significant transport of dissolved methane into ocean bottom waters this amounts to less than 10% of the total methane flux (dissolved + gas) into the shallow subsurface sediments, most of which escapes AOM as it is transported in the gas phase.
    Type: Article , PeerReviewed
    Format: text
    Format: text
    DOI: 10.1016/j.gca.2016.11.015
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  • 9
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    Spatial variation in fluid flow and geochemical fluxes across the sediment–seawater interface at the Carlos Ribeiro mud volcano (Gulf of Cadiz) (2011)
    Elsevier
    In:  Geochimica et Cosmochimica Acta, 75 (4). pp. 1124-1144.
    Details
    Publication Date: 2017-09-27
    Description: Submarine mud volcanism is an important pathway for transfer of deep-sourced fluids enriched in hydrocarbons and other elements into the ocean. Numerous mud volcanoes (MVs) have been discovered along oceanic plate margins, and integrated elemental fluxes are potentially significant for oceanic chemical budgets. Here, we present the first detailed study of the spatial variation in fluid and chemical fluxes at the Carlos Ribeiro MV in the Gulf of Cadiz. To this end, we combine analyses of the chemical composition of pore fluids with a 1-D transport-reaction model to quantify fluid fluxes, and fluxes of boron, lithium and methane, across the sediment–seawater interface. The pore fluids are significantly depleted in chloride, but enriched in lithium, boron and hydrocarbons, relative to seawater. Pore water profiles of sulphate, hydrogen sulphide and total alkalinity indicate that anaerobic oxidation of methane occurs at 34–180 cm depth below seafloor. Clay mineral dehydration, and in particular the transformation of smectite to illite, produces pore fluids that are depleted in chloride and potassium. Profiles of boron, lithium and potassium are closely related, which suggests that lithium and boron are released from the sediments during this transformation. Pore fluids are expelled into the water column by advection; fluid flow velocities are 4 cm yr−1 at the apex of the MV but they rapidly decrease to 0.4 cm yr−1 at the periphery. The associated fluxes of boron, lithium and methane vary between 7–301, 0.5–6 and 0–806 mmol m−2 yr−1, respectively. We demonstrate that fluxes of Li and B due to mud volcanism may be important on a global scale, however, release of methane into the overlying water column is suppressed by microbial methanotrophy.
    Type: Article , PeerReviewed
    Format: text
    Format: text
    DOI: 10.1016/j.gca.2010.11.017
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  • 10
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    Developments in marine pCO2 measurement technology; towards sustained in situ observations (2017)
    Elsevier
    In:  TrAC - Trends in Analytical Chemistry, 88 . pp. 53-61.
    Details
    Publication Date: 2021-04-23
    Description: Highlights • Evaluation of currently available marine pCO2 sensors with respect to in situ deployment potential. • In depth review of novel optode technology for pCO2. • A pCO2 optode was calibrated using modified time-domain dual lifetime referencing, over the range 280–480 μatm, with a precision of 0.8 μatm. Abstract The oceanic uptake of anthropogenic CO2 causes pronounced changes to the marine carbonate system. High quality pCO2 measurements with good temporal and spatial coverage are required to monitor the oceanic uptake, identify regions with pronounced carbonate system changes, and observe the effectiveness of CO2 emission mitigation strategies. There are currently several instruments available, but many are unsuitable for autonomous deployments on in situ platforms such as gliders, moorings and Argo floats. We assess currently available technology on its suitability for in situ deployment, with a focus on optode technology developments. Optodes for pCO2 measurements provide a promising new technological approach, and were successfully calibrated over the range of 280–480 μatm applying modified time-domain dual lifetime referencing. A laboratory precision of 0.8 μatm (n = 10) and a response time (τ90) of 165 s were achieved, and with further development pCO2 optodes may become as widely used as their oxygen counterparts.
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.1016/j.trac.2016.12.008
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