15 research outputs found
Clinoptilolite as a new proxy of enhanced biogenic silica productivity in lower Miocene carbonate sediments of the Bahamas platform: Isotopic and thermodynamic evidence.
International audienc
Fluid-Sediment Interactions in the Nankai Subduction Zone (NanTroSEIZE): Pore Fluid and Mineralogical Records
International audienceThe Nankai Trough Seismogenic Zone Experiment (IODP), SW Japan, aimed at unraveling hydrologic processes at subduction megathrusts and in situ rock and fluid properties to understand the fault zone behavior during earthquake nucleation and rupture propagation. At different sites, characterization of the sediments at high resolution allows tracing fluid-rock reactions that are either associated to early transformations (hydration or dehydration of clay minerals) during the accretion phase or to mineral recrystallization processes linked to deformation phases in gouge zones. In the sedimentary strata at the front of the accretionary prism changes in clay composition are inferred from chemical trends observed in pore fluids, which further indicate the influence of volcanic ash alteration (zeolite formation). Along the megasplay fault, recrystallization processes (e.g. pyrite) are observed in foliated gouge zones at the contact with non-foliated silty clay. The data highlight the importance of small-scale observations for process and reactive phases such as clays in such variable fluid-sediment reactions
Strong impact of ion filtration on the isotopic composition of chlorine in young clay-rich oceanic sediment pore fluids
International audienc
Fluid circulation along an oceanic detachment fault: insights from fluid inclusions in silicified brecciated fault rocks (Mid-Atlantic Ridge at 13°20'N)
International audienceThe MAR 13°20âČN corrugated detachment fault is composed of pervasively silicified mafic cataclastic breccias, instead of ultramafics and gabbros commonly found at other detachments. These breccias record overplating of hangingwall diabases, with syntectonic silicification due to important influx of silicaâironârich fluids, able to leach alkalis and calcium. Fluids trapped in quartz inclusions show important salinity variations (2.1â10 wt.% NaCl eq.) indicating supercritical phase separation. Fluid inclusions also contain minor amounts of H2 ± CO2 ± CH4 ± H2S, with high H2/CO2 and H2/H2S ratios, signatures typical of ultramaficâhosted vent fluids. We propose that seawater infiltrated the hangingwall upper crust at the axis adjacent to the active detachment, reaching a reaction zone at the dyke complex base (âŒ2 km). At >500°C, fluids become Siârich during diabase alteration (amphiboliteâfacies alteration in clasts), and undergo phase separation. Brines, preferentially released in the nearby detachment fault during diabase brecciation, mix with serpentiniteâderived fluids bearing H2 and CH4. Cooling during detachment deformation and fluid upward migration triggers silica precipitation at greenschistâfacies conditions (quartz + Feârichâchlorite ± pyrite). Important variations in fluid inclusion salinity and gas composition at both sample and grain scales record heterogeneous fluid circulation at small spatial and short temporal scales. This heterogeneous fluid circulation operating at <2 km depth, extending both alongâaxis and over time, is inconsistent with models of fluids channeled along detachments from heat sources at the base of the crust at the fault root. Presentâday venting at detachment footwall, including Irinovskoe, is instead likely underlain by fluid circulation within the footwall, with outflow crossing the inactive detachment fault nearâsurface
Crustal fluid and ash alteration impacts on the biosphere of Shikoku Basin sediments, Nankai Trough, Japan
We present data from sediment cores collected from IODP Site C0012 in the Shikoku Basin. Our site lies at the Nankai Trough, just prior to subduction of the 19 Ma Philippine Sea plate. Our data indicate that the sedimentary package is undergoing multiple routes of electron transport and that these differing pathways for oxidant supply generate a complex array of metabolic routes and microbial communities involved in carbon cycling. Numerical simulations matched to pore water data document that Ca2+ and Cl1â are largely supplied via diffusion from a highâsalinity (44.5 psu) basement fluid, which supports the presence of halophile Archean communities within the deep sedimentary package that are not observed in shallow sediments. Sulfate supply from basement supports anaerobic oxidation of methane (AOM) at a rate of ~0.2 pmol cmâ3 dayâ1 at ~400 mbsf. We also note the disappearance of ÎŽâProteobacteria at 434 mbsf, coincident with the maximum in methane concentration, and their reappearance at 463 mbsf, coinciding with the observed deeper increase in sulfate concentration toward the basement. We did not, however, find ANME representatives in any of the samples analyzed (from 340 to 463 mbsf). The lack of ANME may be due to an overshadowing effect from the more dominant archaeal phylotypes or may indicate involvement of unknown groups of archaea in AOM (i.e., unclassified Euryarchaeota). In addition to the supply of sulfate from a basement aquifer, the deep biosphere at this site is also influenced by an elevated supply of reactive iron (up to 143 ÎŒmol gâ1) and manganese (up to 20 ÎŒmol gâ1). The effect of these metal oxides on the sulfur cycle is inferred from an accompanying sulfur isotope fractionation much smaller than expected from traditional sulfateâreducing pathways. The detection of the manganeseâ and ironâreducer ÎłâProteobacteria Alteromonas at 367 mbsf is consistent with these geochemical inferences
Fossil black smoker yields oxygen isotopic composition of Neoproterozoic seawater
Uncertainty regarding the evolution of the oxygen isotopic composition of seawater casts doubt on past temperature reconstructions. Here, the authors present a new, precise ÎŽ18O value for the Neoproterozoic, and propose that ocean temperatures on the eve of the Sturtian glaciation were 15â30â°C warmer than present