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  • 1
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    Faulting and off-axis submarine massive sulfide accumulation at slow spreading mid-ocean ridges: A numerical modeling perspective (2017)
    AGU (American Geophysical Union) | Wiley
    In:  Geochemistry, Geophysics, Geosystems, 18 (6). 2305-2320 .
    Details
    Publication Date: 2020-02-06
    Description: The potential of mining seafloor massive sulfide deposits for metals such as Cu, Zn, and Au is currently debated. One key challenge is to predict where the largest deposits worth mining might form, which in turn requires understanding the pattern of subseafloor hydrothermal mass and energy transport. Numerical models of heat and fluid flow are applied to illustrate the important role of fault zone properties (permeability and width) in controlling mass accumulation at hydrothermal vents at slow spreading ridges. We combine modeled mass-flow rates, vent temperatures, and vent field dimensions with the known fluid chemistry at the fault-controlled Logatchev 1 hydrothermal field of the Mid-Atlantic Ridge. We predict that the 135 kilotons of SMS at this site (estimated by other studies) can have accumulated with a minimum depositional efficiency of 5% in the known duration of hydrothermal venting (58,200 year age of the deposit). In general, the most productive faults must provide an efficient fluid pathway while at the same time limit cooling due to mixing with entrained cold seawater. This balance is best met by faults that are just wide and permeable enough to control a hydrothermal plume rising through the oceanic crust. Model runs with increased basal heat input, mimicking a heat flow contribution from along-axis, lead to higher mass fluxes and vent temperatures, capable of significantly higher SMS accumulation rates. Nonsteady state conditions, such as the influence of a cooling magmatic intrusion beneath the fault zone, also can temporarily increase the mass flux while sustaining high vent temperatures.
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.1002/2017GC006880
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 2
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    Global rates of mantle serpentinization and H2 production at oceanic transform faults in 3-D geodynamic models (2017)
    AGU (American Geophysical Union) | Wiley
    In:  Geophysical Research Letters, 44 (13). 6726-6734 .
    Details
    Publication Date: 2020-02-06
    Description: Previous studies have estimated that mantle serpentinization reactions generate H2 at a rate of 1010–1012 mol/yr along the global mid-ocean ridge (MOR) system. Here we present results of 3-D geodynamic simulations that predict rates of additional mantle serpentinization and H2 production at oceanic transform faults (OTF). We find that the extent and rate of mantle serpentinization increases with OTF length and is maximum at intermediate slip rates of 5 to 10 cm/yr. The additional global OTF-related production of H2 is found to be between 6.1 and 10.7 × 1011 mol/yr, which is comparable to the predicted background MOR rate of 4.1–15.0 × 1011 mol H2/yr. This points to oceanic transform faults as potential sites of intense fluid-rock interaction, where chemosynthetic life could be sustained by serpentinization reactions.
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.1002/2017GL072893
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 3
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    The effect of obliquity-driven changes on paleoclimate sensitivity during the late Pleistocene (2018)
    AGU (American Geophysical Union) | Wiley
    In:  Geophysical Research Letters, 45 (13). pp. 6661-6671.
    Details
    Publication Date: 2021-02-08
    Description: We reanalyze existing paleodata of global mean surface temperature ΔTg and radiative forcing ΔR of CO2 and land ice albedo for the last 800,000 years to show that a state‐dependency in paleoclimate sensitivity S, as previously suggested, is only found if ΔTg is based on reconstructions, and not when ΔTg is based on model simulations. Furthermore, during times of decreasing obliquity (periods of land‐ice sheet growth and sea level fall) the multi‐millennial component of reconstructed ΔTg diverges from CO2, while in simulations both variables vary more synchronously, suggesting that the differences during these times are due to relatively low rates of simulated land ice growth and associated cooling. To produce a reconstruction‐based extrapolation of S for the future we exclude intervals with strong ΔTg‐CO2 divergence and find that S is less state‐dependent, or even constant (state‐independent), yielding a mean equilibrium warming of 2–4 K for a doubling of CO2.
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.1029/2018GL077717
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 4
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    Modeling fluid flow in sedimentary basins with sill intrusions: Implications for hydrothermal venting and climate change (2013)
    AGU (American Geophysical Union) | Wiley
    In:  Geochemistry, Geophysics, Geosystems, 14 (12). pp. 5244-5262.
    Details
    Publication Date: 2018-02-28
    Description: Large volumes of magma emplaced within sedimentary basins have been linked to multiple climate change events due to release of greenhouse gases such as CH4. Basin-scale estimates of thermogenic methane generation show that this process alone could generate enough greenhouse gases to trigger global incidents. However, the rates at which these gases are transported and released into the atmosphere are quantitatively unknown. We use a 2D, hybrid FEM/FVM model that solves for fully compressible fluid flow to quantify the thermogenic release and transport of methane and to evaluate flow patterns within these systems. Our results show that the methane generation potential in systems with fluid flow does not significantly differ from that estimated in diffusive systems. The values diverge when vigorous convection occurs with a maximum variation of about 50%. The fluid migration pattern around a cooling, impermeable sill alone generates hydrothermal plumes without the need for other processes such as boiling and/or explosive degassing. These fluid pathways are rooted at the edges of the outer sills consistent with seismic imaging. Methane venting at the surface occurs in three distinct stages and can last for hundreds of thousands of years. Our simulations suggest that although the quantity of methane potentially generated within the contact aureole can cause catastrophic climate change, the rate at which this methane is released into the atmosphere is too slow to trigger, by itself, some of the negative δ13C excursions observed in the fossil record over short time scales (〈 10,000 years).
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.1002/2013GC005012
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 5
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    Interrelation between rifting, faulting, sedimentation, and mantle serpentinization during continental margin formation-including examples from the Norwegian Sea (2013)
    AGU (American Geophysical Union) | Wiley
    In:  Geochemistry, Geophysics, Geosystems, 14 (10). pp. 4351-4369.
    Details
    Publication Date: 2018-02-28
    Description: The conditions permitting mantle serpentinization during continental rifting are explored within 2-D thermotectonostratigraphic basin models, which track the rheological evolution of the continental crust, account for sediment blanketing effects, and allow for kinetically controlled mantle serpentinization processes. The basic idea is that the entire extending continental crust has to be brittle for crustal scale faulting and mantle serpentinization to occur. The isostatic and latent heat effects of the reaction are fully coupled to the structural and thermal solutions. A systematic parameter study shows that a critical stretching factor exists for which complete crustal embrittlement and serpentinization occurs. Increased sedimentation rates shift this critical stretching factor to higher values as sediment blanketing effects result in higher crustal temperatures. Sediment supply has therefore, through the temperature-dependence of the viscous flow laws, strong control on crustal strength and mantle serpentinization reactions are only likely when sedimentation rates are low and stretching factors high. In a case study for the Norwegian margin, we test whether the inner lower crustal bodies (LCB) imaged beneath the Møre and Vøring margin could be serpentinized mantle. Multiple 2-D transects have been reconstructed through the 3-D data set by Scheck-Wenderoth and Maystrenko (2011). We find that serpentinization reactions are possible and likely during the Jurassic rift phase. Predicted thicknesses and locations of partially serpentinized mantle rocks fit to information on LCBs from seismic and gravity data. We conclude that some of the inner LCBs beneath the Norwegian margin may be partially serpentinized mantle.
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.1002/ggge.20268
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 6
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    Modes of crustal accretion and their implications for hydrothermal circulation (2016)
    AGU (American Geophysical Union) | Wiley
    In:  Geophysical Research Letters, 43 (3). pp. 1124-1131.
    Details
    Publication Date: 2019-02-01
    Description: Hydrothermal convection at mid-ocean ridges links the ocean's long-term chemical evolution to solid earth processes, forms hydrothermal ore deposits, and sustains the unique chemosynthetic vent fauna. Yet the depth extent of hydrothermal cooling and the inseparably connected question of how the lower crust accretes remain poorly constrained. Here based on coupled models of crustal accretion and hydrothermal circulation, we provide new insights into which modes of lower crust formation and hydrothermal cooling are thermally viable and most consistent with observations at fast-spreading ridges. We integrate numerical models with observations of melt lens depth, thermal structure, and melt fraction. Models matching all these observations always require a deep crustal-scale hydrothermal flow component and less than 50% of the lower crust crystallizing in situ.
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.1002/2015GL067335
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 7
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    On subducting slab entrainment of buoyant asthenosphere (2007)
    Wiley
    In:  Terra Nova, 19 (3). pp. 167-173.
    Details
    Publication Date: 2019-09-23
    Description: Laboratory and numerical experiments and boundary layer analysis of the entrainment of buoyant asthenosphere by subducting oceanic lithosphere implies that slab entrainment is likely to be relatively inefficient at removing a buoyant and lower viscosity asthenosphere layer. Asthenosphere would instead be mostly removed by accretion into and eventual subduction of the overlying oceanic lithosphere. The lower (hot) side of a subducting slab entrains by the formation of a ∼10–30 km-thick downdragged layer, whose thickness depends upon the subduction rate and the density contrast and viscosity of the asthenosphere, while the upper (cold) side of the slab may entrain as much by thermal 'freezing' onto the slab as by mechanical downdragging. This analysis also implies that proper treatment of slab entrainment in future numerical mantle flow experiments will require the resolution of ∼10–30 km-thick entrainment boundary layers.
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.1111/j.1365-3121.2007.00737.x
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 8
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    Modeling the fate of methane hydrates under global warming (2015)
    AGU (American Geophysical Union) | Wiley
    In:  Global Biogeochemical Cycles, 29 (5). pp. 610-625.
    Details
    Publication Date: 2019-09-23
    Description: Large amounts of methane hydrate locked up within marine sediments are vulnerable to climate change. Changes in bottom water temperatures may lead to their destabilization and the release of methane into the water column or even the atmosphere. In a multimodel approach, the possible impact of destabilizing methane hydrates onto global climate within the next century is evaluated. The focus is set on changing bottom water temperatures to infer the response of the global methane hydrate inventory to future climate change. Present and future bottom water temperatures are evaluated by the combined use of hindcast high-resolution ocean circulation simulations and climate modeling for the next century. The changing global hydrate inventory is computed using the parameterized transfer function recently proposed by Wallmann et al. (2012). We find that the present-day world's total marine methane hydrate inventory is estimated to be 1146Gt of methane carbon. Within the next 100years this global inventory may be reduced by ∼0.03% (releasing ∼473Mt methane from the seafloor). Compared to the present-day annual emissions of anthropogenic methane, the amount of methane released from melting hydrates by 2100 is small and will not have a major impact on the global climate. On a regional scale, ocean bottom warming over the next 100years will result in a relatively large decrease in the methane hydrate deposits, with the Arctic and Blake Ridge region, offshore South Carolina, being most affected.
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.1002/2014GB005011
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 9
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    Anhydrite‐Assisted Hydrothermal Metal Transport to the Ocean Floor—Insights From Thermo‐Hydro‐Chemical Modeling (2020)
    AGU (American Geophysical Union) | Wiley
    Details
    Publication Date: 2023-02-08
    Description: High‐temperature hydrothermal venting has been discovered on all modern mid‐ocean ridges at all spreading rates. Although significant strides have been made in understanding the underlying processes that shape such systems, several first‐order discrepancies between model predictions and observations remain. One key paradox is that numerical experiments consistently show entrainment of cold ambient seawater in shallow high permeability ocean crust causing a temperature drop that is difficult to reconcile with high vent temperatures. We investigate this conundrum using a thermo‐hydro‐chemical model that couples hydrothermal fluid flow with anhydrite‐ and pyrite‐forming reactions in the shallow subseafloor. The models show that precipitation of anhydrite in warming seawater and in cooling hydrothermal fluids during mixing results in the formation of a chimney‐like subseafloor structure around the upwelling, high‐temperature plume. The establishment of such anhydrite‐sealed zones reduces mixing between the hydrothermal fluid and seawater and results in an increase in vent temperature. Pyrite subsequently precipitates close to the seafloor within the anhydrite chimney. Although anhydrite thus formed may be dissolved when colder seawater circulates through the crust away from the spreading axis, the inside pyrite walls would be preserved as veins in present‐day metal deposits, thereby preserving the history of hydrothermal circulation through shallow oceanic crust.
    Type: Article , PeerReviewed
    Format: text
    Format: text
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 10
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    Spontaneously exsolved free gas during major storms as an ephemeral gas source for pockmark formation (2022)
    AGU (American Geophysical Union) | Wiley
    Details
    Publication Date: 2022-08-02
    Description: Abrupt fluid emissions from shallow marine sediments pose a threat to seafloor installations like wind farms and offshore cables. Quantifying such fluid emissions and linking pockmarks, the seafloor manifestations of fluid escape, to flow in the sub-seafloor remains notoriously difficult due to an incomplete understanding of the underlying physical processes. Here, using a compositional multi-phase flow model, we test plausible gas sources for pockmarks in the south-eastern North Sea, which recent observations suggest have formed in response to major storms. We find that the presence of free gas in the subsurface effectively damps storm wave-induced pressure changes due to its high compressibility, so that the mobilization of pre-existing gas pockets is unlikely. Rather, our results point to spontaneous appearance of a free gas phase via storm-induced gas exsolution from pore fluids. This mechanism is primarily driven by the pressure-sensitivity of gas solubility. We show that in highly permeable sediments containing gas-rich pore fluids, wave-induced pressure changes result in the appearance of a persistent gas phase. This suggests that seafloor fluid escape structures are not always proxies for overpressured shallow gas and that periodic seafloor pressure changes can induce persistent free gas phase to spontaneously appear.
    Type: Article , NonPeerReviewed
    Format: text
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    OceanRep: Article in a Scientific Journal - without review
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