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  • 1
    Publication Date: 2019-10-24
    Description: We report on newly discovered mud volcanoes located at ∼4500 m water depth ∼90 km west of the deformation front of the accretionary wedge of the Gulf of Cadiz, and thus outside of their typical geotectonic environment. Seismic data suggest that fluid flow is mediated by a 〉400-km-long strike-slip fault marking the transcurrent plate boundary between Africa and Eurasia. Geochemical data (Cl, B, Sr, 87Sr/86Sr, δ18O, δD) reveal that fluids originate in oceanic crust older than 140 Ma. On their rise to the surface, these fluids receive strong geochemical signals from recrystallization of Upper Jurassic carbonates and clay-mineral dehydration in younger terrigeneous units. At present, reports of mud volcanoes in similar deep-sea settings are rare, but given that the large area of transform-type plate boundaries has been barely investigated, such pathways of fluid discharge may provide an important, yet unappreciated link between the deeply buried oceanic crust and the deep ocean.
    Type: Article , PeerReviewed
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  • 2
    Publication Date: 2019-10-24
    Description: During opening of a new ocean magma intrudes into the surrounding sedimentary basins. Heat provided by the intrusions matures the host rock creating metamorphic aureoles potentially releasing large amounts of hydrocarbons. These hydrocarbons may migrate to the seafloor in hydrothermal vent complexes in sufficient volumes to trigger global warming, e.g. during the Paleocene Eocene Thermal Maximum (PETM). Mound structures at the top of buried hydrothermal vent complexes observed in seismic data off Norway were previously interpreted as mud volcanoes and the amount of released hydrocarbon was estimated based on this interpretation. Here, we present new geophysical and geochemical data from the Gulf of California suggesting that such mound structures could in fact be edifices constructed by the growth of black-smoker type chimneys rather than mud volcanoes. We have evidence for two buried and one active hydrothermal vent system outside the rift axis. The vent releases several hundred degrees Celsius hot fluids containing abundant methane, mid-ocean-ridge-basalt (MORB)-type helium, and precipitating solids up to 300 m high into the water column. Our observations challenge the idea that methane is emitted slowly from rift-related vents. The association of large amounts of methane with hydrothermal fluids that enter the water column at high pressure and temperature provides an efficient mechanism to transport hydrocarbons into the water column and atmosphere, lending support to the hypothesis that rapid climate change such as during the PETM can be triggered by magmatic intrusions into organic-rich sedimentary basins.
    Type: Article , PeerReviewed
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  • 3
    Publication Date: 2019-10-24
    Description: Sulfate-reducing bacteria are known to mediate dolomite formation under hypersaline conditions, but details of the crystal nucleation process are still poorly constrained. Our laboratory study demonstrates for the first time that Desulfobulbus mediterraneus, a marine sulfate-reducing bacterium, mediates primary precipitation of Mg-rich dolomite under anoxic conditions in media replicating modern seawater chemistry at low temperature (21 °C). Precipitation of crystals was associated with extracellular polymeric substances in a monospecific biofilm, providing templates for nucleation by altering the molar Mg/Ca ratio. After initial nucleation of single nanospherulites (∼50 nm), growth was mediated by aggregation, resulting in spherulites of ∼2–3 μm in diameter. Nucleation led to differences in Mg/Ca ratios and δ44/40Ca values among the organic material (i.e., biofilm including cells and extracellular polymeric substances; 0.87 ± 0.01 [2 SD] and 0.48‰ ± 0.11‰ [2 SE], respectively), the crystals (1.02 ± 0.11 [2 SD] and 〈−0.08‰ ± 0.24‰ [2 SE], respectively), and the liquid bulk medium after mineral precipitation (4.53 ± 0.04 [2 SD] and 1.10‰ ± 0.24‰ [2 SE], respectively). These data indicate a two-step fractionation process involved in the sequestration of Ca from the solution into the crystal lattice of the mineral precipitated. Our results demonstrate the capability of extracellular polymeric substances to overcome kinetic inhibition, fostering the formation of kinetically less favorable Mg-rich dolomite, and they also question the applicability of the Ca isotopic system as a proxy for paleogeochemistry of seawater.
    Type: Article , PeerReviewed , info:eu-repo/semantics/article
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  • 4
    Publication Date: 2019-10-24
    Description: At convergent margins, fluids rise through the forearc in response to consolidation of the upper plate and dewatering of the subducting plate, and produce various cold-seep–related features on the seafloor (mud diapirs, mud mounds). At the Central American forearc, authigenic carbonates precipitated from rising fluids within such structures during active venting while typical mixed-mud sediments were ejected onto the surrounding seafloor where they became intercalated with normal pelagic background sediments, indicating that mud mounds evolved unsteadily through alternating active and inactive phases. Intercalated regional ash layers from Plinian eruptions at the Central American volcanic arc provide time marks that constrain the ages of mud ejection activity. U/Th dating of drill core samples of authigenic carbonate caps of mud mounds yields ages agreeing well with those constrained by ash layers and showing that carbonate caps grow inward rather than outward during active venting. Both dating approaches show that offshore Nicaragua and Costa Rica (1) active and inactive phases can occur simultaneously at neighboring mounds, (2) mounds along the forearc have individual histories of activity, but there are distinct time intervals when nearly all mounds have been active or inactive, (3) lifetimes of mounds reach several hundred thousand years, and (4) highly active periods last 10–50 k.y. with intervening periods of 〉10 k.y. of relative quiescence.
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  • 5
    Publication Date: 2023-02-08
    Description: The loss of carbonate production during the Toarcian Oceanic Anoxic Event (T-OAE, ca. 183 Ma) is hypothesized to have been at least partly triggered by ocean acidification linked to magmatism from the Karoo-Ferrar large igneous province (southern Africa and Antarctica). However, the dynamics of acidification have never been directly quantified across the T-OAE. Here, we present the first record of temporal evolution of seawater pH spanning the late Pliensbachian and early Toarcian from the Lusitanian Basin (Portugal) reconstructed on the basis of boron isotopic composition (δ11B) of brachiopod shells. δ11B declines by ~1‰ across the Pliensbachian-Toarcian boundary (Pl-To) and attains the lowest values (~12.5‰) just prior to and within the T-OAE, followed by fluctuations and a moderately increasing trend afterwards. The decline in δ11B coincides with decreasing bulk CaCO3 content, in parallel with the two-phase decline in carbonate production observed at global scales and with changes in pCO2 derived from stomatal indices. Seawater pH had declined significantly already prior to the T-OAE, probably due to the repeated emissions of volcanogenic CO2. During the earliest phase of the T-OAE, pH increased for a short period, likely due to intensified continental weathering and organic carbon burial, resulting in atmospheric CO2 drawdown. Subsequently, pH dropped again, reaching the minimum in the middle of the T-OAE. The early Toarcian marine extinction and carbonate collapse were thus driven, in part, by ocean acidification, similar to other Phanerozoic events caused by major CO2 emissions and warming.
    Type: Article , PeerReviewed , info:eu-repo/semantics/article
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  • 6
    Publication Date: 2024-02-07
    Description: We present pore-fluid geochemistry and heat-flow data along the SWIM1 fault in the Horseshoe Abyssal Plain (northeastern Atlantic Ocean). The SWIM1 fault is part of the transcurrent plate boundary between Africa and Eurasia and cuts through as much as 5-km-thick sediments overlying 〉140 Ma oceanic lithosphere. In a number of places, restraining segments (as long as 15 km) of the SWIM1 fault generate anticlines (positive flower structures) that protrude as ~100-m-high hills above the abyssal plain. Heat flow and gradients of dissolved constituents in pore water are enhanced at these seafloor highs. Transport-reaction modeling confirms that slow advection of deep-seated fluids, depleted in Mg and enriched in Sr and CH4, can explain the observations. The geochemical signature is similar to the one observed at deep-sea mud volcanoes located eastward on the SWIM1 fault. The upward-migrating fluids have interacted with carbonate rocks at maximum 5 km depth, which represent the oldest sedimentary unit on top of the basement. We argue that deep-rooted fluids can generally be mobilized and transported upward along flower structures that formed in restraining-bend segments of long strike-slip faults. Such tectonic settings represent largely unrecognized corridors for mass exchange between lithosphere and ocean.
    Type: Article , PeerReviewed
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