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  • Other Sources  (2,314)
  • AGU (American Geophysical Union)  (1,976)
  • Inter Research  (334)
  • American Institute of Physics (AIP)
  • 1
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    16S rDNA-based phylogeny of sulfur-oxidizing bacterial endosymbionts in marine bivalves from cold-seep habitats (2003)
    Inter Research
    In:  Marine Ecology Progress Series, 249 . pp. 39-51.
    Details
    Publication Date: 2018-05-30
    Description: The phylogenetic relationship of sulphur-oxidising endosymbiotic bacteria from bivalves of the families Vesicomyidae (Calyptogena sp. C1, Calyptogena sp. C3), Solemyidae (Acharax sp.) and Thyasiridae (Conchocele sp.) from cold-seep habitats were determined by 16S rDNA nucleotide sequence analyses. The endosymbiotic bacteria form distinct groups within the gamma-Proteobacteria and are well separated from each other and from free-living sulphur-oxidising bacteria of the genera Beggiatoa, Halothiobacillus and Thiomicrospira. The endosymbiotic bacteria of Acharax sp. from cold seeps off Oregon, Indonesia and Pakistan have sequences highly similar to each other but quite distinct from other thiotrophic endosymbionts. This includes endosymbionts from Solemya spp., to which they are distantly related. Symbiotic bacteria of Conchocele sp. from a cold seep in the Sea of Okhotsk are similar to those of Bathymodiolus thermophilus and related species, as shown by their overall sequence similarity and by signature sequences. The endosymbiotic bacteria of Calyptogena spp. from cold seeps off Oregon and Pakistan are closely related to those of other vesicomyids. Endosymbiont species found off Oregon corresponded to 2 different clusters of Calyptogena spp. symbionts in the same samples. The results corroborate the hypothesis of a monophyletic origin of the symbionts in vesicomyid clams, and support the existence of deeply branching groups in solemyid symbionts and of divergent lines and distribution for thyasirid symbionts. The results also indicate that certain symbiont species cluster according to the depth distribution of their hosts, and that in consequence host species together with their symbionts may have undergone depth-specific adaptation and evolution.
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  • 2
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    2‐D V p and V s Models of the Indian Oceanic Crust Adjacent to the NinetyEast Ridge (2023)
    Wiley | AGU (American Geophysical Union)
    Details
    Publication Date: 2024-02-07
    Description: Until now, few offshore seismic studies have acquired simultaneously P- and S- wave data to derive in detail the seismic structure of the oceanic crust. We present 2-D Vp and Vs models using wide-angle seismic data at the Indian basin adjacent to the NinetyEast Ridge. Here, an outcrop basement located at the middle of the seismic line presents uppermost crustal Poisson's ratios (ν) of 0.28–0.29 (Vp ∼ 4.2 km/s and Vs ∼ 2.3 km/s). At the flanks of the outcrop basement, the sediment cover is 200–300 m thick and ν values are similar (0.28–0.3), but Vp and Vs values are higher (4.5–4.8 and 2.4–2.6 km/s, respectively). We interpret the relatively lower Vp and Vs around the basement outcrop in terms of hydrothermal alteration, while at the flanks of the basement outcrop, hydrothermal alteration has most likely ceased by sedimentation and compaction processes. Across the seismic layer 2, the Vp–Vs trend is linear and follows a ν value of 0.28–0.29, however, at the seismic layer 2/3 transition, the Vp–Vs trend abruptly changes following a ν value of 0.25–0.26. These reduced observed ν values at the layer 2/3 transition are lower than those reported by laboratory measurements for gabbro (ν ∼ 0.293) and are interpreted in terms of epidotization at the dike-gabbro contact and/or crack-change properties around the lower part of the intrusive sheeted dike section. Key Points We obtain 2-D Vp and Vs models from active seismic data for the Indian oceanic crust The seismic models suggest hydrothermal alteration near a basement outcrop Poisson's ratios change at the layer 2/3 transition from 0.28–0.29 to 0.25–0.26
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  • 3
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    20th Century δ 18 O Seawater and Salinity Variations Reconstructed From Paired δ 18 O and Sr/Ca Measurements of a La Reunion Coral (2019)
    AGU (American Geophysical Union) | Wiley
    In:  Paleoceanography and Paleoclimatology, 34 . pp. 2183-2200.
    Details
    Publication Date: 2022-01-31
    Description: Sea surface salinity (SSS) is an important variable in the global ocean circulation. However, decadal to interdecadal changes in SSS are not well understood due to the lack of instrumental data. Here we reconstruct SSS from a paired, bimonthly resolved coral δ18O and Sr/Ca record from La Reunion Island that extends from 1913 to 1995. Coral Sr/Ca correlates with regional sea surface temperature (SST) back to 1966, when instrumental coverage is good, while coral δ18O does not. The slope of the monthly (annual mean) coral Sr/Ca-SST regression is −0.040 mmol/mol per 1 °C (−0.068 mmol/mol per 1 °C) consistent with published estimates of the Sr/Ca-SST relationship. Coral Sr/Ca suggest a warming of 0.39 °C since 1913. δ18O seawater is calculated by subtracting the temperature component from measured coral δ18O, using coral Sr/Ca as well as historical SST products. The derived δ18O seawater reconstructions are correlated (r 〉 0.6), and all show a significant shift in the midtwentieth century (−0.17‰ to −0.19‰), indicating a freshening of SSS by 0.7 psu. However, the timing of this shift depends on the temperature component and varies from 1947 (δ18O seawater calculated with historical SST) to the late 1950s (δ18O seawater calculated with coral Sr/Ca). Coral Sr/Ca shows warm temperature anomalies in the mid-1950s, while historical SST products show warm anomalies from 1940 to 1945 followed by cooling in the 1950s, a pattern typical for the World War II bias. This suggests that historical SST may bias reconstructions of δ18O seawater and SSS from corals.
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  • 4
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    230Th-normalization: an essential tool for interpreting sedimentary fluxes during the late Quaternary (2004)
    AGU (American Geophysical Union)
    In:  Paleoceanography, 19 (PA1018).
    Details
    Publication Date: 2017-01-23
    Description: There is increasing evidence indicating that syndepositional redistribution of sediment on the seafloor by bottom currents is common and can significantly affect sediment mass accumulation rates. Notwithstanding its common incidence, this process (generally referred to as sediment focusing) is often difficult to recognize. If redistribution is near synchronous to deposition, the stratigraphy of the sediment is not disturbed and sediment focusing can easily be overlooked. Ignoring it, however, can lead to serious misinterpretations of sedimentary fluxes, particularly when past changes in export flux from the overlying water are inferred. In many instances, this problem can be resolved, at least for sediments deposited during the late Quaternary, by normalizing to the flux of 230Th scavenged from seawater, which is nearly constant and equivalent to the known rate of production of 230Th from the decay of dissolved 234U. We review the principle, advantages and limitations of this method. Notwithstanding its limitations, it is clear that 230Th normalization does provide a means of achieving more accurate interpretations of sedimentary fluxes and eliminates the risk of serious misinterpretations of sediment mass accumulation rates.
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  • 5
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    234 Th-Derived Particle Fluxes and Seasonal Variability: When Is the SS Assumption Reliable? Insights From a Novel Approach for Carbon Flux Simulation (2018)
    AGU (American Geophysical Union) | Wiley
    In:  Geophysical Research Letters, 45 (24). 13,414-13,426.
    Details
    Publication Date: 2021-02-08
    Description: 234Th measurements are widely used to estimate the downward carbon flux of particles via the oceanic Biological Pump. Carbon export is evaluated from 234Th-238U disequilibrium assuming either steady state (SS) conditions, or including a non-SS (NSS) correction. We use a novel stochastic simulation to quantify the temporal variation of vertical carbon and 234Th (dissolved and particulate) concentration profiles with high temporal resolution. We calculate seasonal export as if in situ measured with sediment trap and SS- and NSS-234Th approaches and quantify the periods of validity for SS/NSS conditions defined in previous works. The SS approach is valid throughout the entire season in oligotrophic regions. In temperate regions, the SS introduces a bias in the export if sampling takes place outside specific temporal windows. Windows of validity range from days in short blooms of ~15-day duration to weeks in blooms longer than ~30 days.
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  • 6
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    3-D active source tomography around Simeulue Island offshore Sumatra: thick crustal zone responsible for earthquake segment boundary (2013)
    AGU (American Geophysical Union) | Wiley
    In:  Geophysical Research Letters, 40 (1). pp. 48-53.
    Details
    Publication Date: 2017-06-20
    Description: We present a detailed 3-D P-wave velocity model obtained by first-arrival travel-time tomography with seismic refraction data in the segment boundary of the Sumatra subduction zone across Simeulue Island, and an image of the top of the subducted oceanic crust extracted from depth-migrated multi-channel seismic reflection profiles. We have picked P-wave first arrivals of the air-gun source seismic data recorded by local networks of ocean-bottom seismometers, and inverted the travel-times for a 3-D velocity model of the subduction zone. This velocity model shows an anomalous zone of intermediate velocities between those of oceanic crust and mantle that is associated with raised topography on the top of the oceanic crust. We interpret this feature as a thickened crustal zone in the subducting plate with compositional and topographic variations, providing a primary control on the upper plate structure and on the segmentation of the 2004 and 2005 earthquake ruptures.
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  • 7
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    3-D basin-scale reconstruction of natural gas hydrate system of the Green Canyon, Gulf of Mexico (2017)
    AGU (American Geophysical Union) | Wiley
    In:  Geochemistry, Geophysics, Geosystems, 18 (5). pp. 1959-1985.
    Details
    Publication Date: 2020-02-06
    Description: Our study presents a basin-scale 3D modeling solution, quantifying and exploring gas hydrate accumulations in the marine environment around the Green Canyon (GC955) area, Gulf of Mexico. It is the first modeling study that considers the full complexity of gas hydrate formation in a natural geological system. Overall, it comprises a comprehensive basin re-construction, accounting for depositional and transient thermal history of the basin, source rock maturation, petroleum components generation, expulsion and migration, salt tectonics and associated multi-stage fault development. The resulting 3D gas hydrate distribution in the Green Canyon area is consistent with independent borehole observations. An important mechanism identified in this study and leading to high gas hydrate saturation (〉 80 vol. %) at the base of the gas hydrate stability zone (GHSZ), is the recycling of gas hydrate and free gas enhanced by high Neogene sedimentation rates in the region. Our model predicts the rapid development of secondary intra-salt mini-basins situated on top of the allochthonous salt deposits which leads to significant sediment subsidence and an ensuing dislocation of the lower GHSZ boundary. Consequently, large amounts of gas hydrates located in the deepest parts of the basin dissociate and the released free methane gas migrates upwards to recharge the GHSZ. In total, we have predicted the gas hydrate budget for the Green Canyon area that amounts to ∼3,256 Mt of gas hydrate which is equivalent to ∼340 Mt of carbon (∼7 x 1011 m3 of CH4 at STP conditions), and consists mostly of biogenic hydrates.
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  • 8
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    A 3‐D Model of Gas Generation, Migration, and Gas Hydrate Formation at a Young Convergent Margin (Hikurangi Margin, New Zealand) (2019)
    AGU (American Geophysical Union) | Wiley
    In:  Geochemistry, Geophysics, Geosystems, 20 (11). pp. 5126-5147.
    Details
    Publication Date: 2022-01-31
    Description: We present a three-dimensional gas hydrate systems model of the southern Hikurangi subduction margin in eastern New Zealand. The model integrates thermal and microbial gas generation, migration, and hydrate formation. Modeling these processes has improved the understanding of factors controlling hydrate distribution. Three spatial trends of concentrated hydrate occurrence are predicted. The first trend (I) is aligned with the principal deformation front in the overriding Australian plate. Concentrated hydrate deposits are predicted at or near the apexes of anticlines and to be mainly sourced from focused migration and recycling of microbial gas generated beneath the hydrate stability zone. A second predicted trend (II) is related to deformation in the subducting Pacific plate associated with former Mesozoic subduction beneath Gondwana and the modern Pacific-Australian plate boundary. This trend is enhanced by increased advection of thermogenic gas through permeable layers in the subducting plate and focused migration into the Neogene basin fill above Cretaceous-Paleogene structures. The third trend (III) follows the northern margin of the Hikurangi Channel and is related to the presence of buried strata of the Hikurangi Channel system. The predicted trends are consistent with pronounced seismic reflection anomalies related to free gas in the pore space and strength of the bottom-simulating reflection. However, only trend I is also associated with clear and widespread seismic indications of concentrated gas hydrate. Total predicted hydrate masses at the southern Hikurangi Margin are between 52,800 and 69,800 Mt. This equates to 3.4–4.5 Mt hydrate/km2, containing 6.33 × 108–8.38 × 108 m3/km2 of methane.
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  • 9
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    A 30 ‐Year Time Series of Transient Tracer‐Based Estimates of Anthropogenic Carbon in the Central Labrador Sea (2021)
    AGU (American Geophysical Union) | Wiley
    Details
    Publication Date: 2024-02-07
    Description: We use a 30-year time series (1986–2016) of dichlorodifluoromethane (CFC-12) concentrations with a refined transit time distribution (TTD) method, to estimate the temporal variation of anthropogenic carbon (Cant) in the Central Labrador Sea. We determined that the saturation of CFC-12 and sulfur hexafluroide (SF6) in newly-formed Labrador Sea Water had departed significantly from 100% and varied systematically with time. Multiple linear regression of the time-varying saturation, with the tracer's atmospheric growth rate and the wintertime mixed layer depth as independent variables, allowed reconstruction of the saturation history of CFC-12 and SF6 in wintertime surface waters, which was implemented in the TTD method. Use of the time-varying saturation for CFC-12 gave Cant concentrations ∼7 μmol kg−1 larger than estimates obtained assuming a constant saturation of 100%. The resulting Cant column inventories were ∼20% larger and displayed lower interannual variability compared to conventional TTD-based estimates. The column inventory of Cant increased at an average rate of 1.8 mol m−2 y−1 over the 30-year period. However, the accumulation rate of Cant was higher than this average in the early 1990s and since 2013, whereas inventories remained almost unchanged between 2003 and 2012. The variation in the Cant accumulation rate is shown to be linked to temporal variability in the relative layer thickness of the annually ventilated Labrador Sea Water and the underlying Deep Intermediate Water. The non-steady Cant accumulation highlights the importance of sampling frequency, especially in regions of variable deep mixing and high carbon inventories, and potential misinterpretation of Cant dynamics
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  • 10
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    A 500,000-year-long sediment archive drilled in eastern Anatolia (2011)
    AGU (American Geophysical Union)
    In:  Eos, Transactions American Geophysical Union, 92 (51). pp. 477-479.
    Details
    Publication Date: 2017-02-10
    Description: Sedimentary archives host a wealth of information that can be used to reconstruct paleoclimate as well as the tectonic and volcanic histories of specific regions. Long and continuous archives from the oceans have been collected in thousands of locations by scientific ocean drilling programs over the past 40 years. In contrast, suitable continental archives are rare because terrestrial environments are generally nondepositional and/or subject to erosion. Lake sediments provide ideal drilling targets to overcome this limitation if suitable lakes at key locations have existed continuously for a long time.
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