1,020 research outputs found
Anaerobic oxidation of methane in the Concepcion Methane Seep Area, Chilean continental margin
Within subduction zones of active continental margins, large amounts of methane can be mobilized by dewatering processes and transported to the seafloor along migration pathways. A recently discovered seep area located off Concepción (Chile) at water depth between 600 to 1100 mbsl is characterized by active methane vent sites as well as massive carbonates boulders and plates which probably are related to methane seepage in the past.
During the SO210 research expedition “Chiflux” (Sept-Oct 2010), sediment from the Concepción Methane Seep Area (CSMA) at the fore arc of the Chilean margin was sampled to study microbial activity related to methane seepage. We sampled surface sediments (0-30cm) from sulfur bacteria mats, as well as clam, pogonophoran, and tubeworm fields with push cores and a TV-guided multicorer system. Anaerobic oxidation of methane (AOM) and sulfate reduction rates were determined using ex-situ radioisotope tracer techniques. Additionally, porewater chemistry of retrieved cores as well as isotopic composition and age record of surrounding authigenic carbonates were analyzed.
The shallowest sulfate-methane-transition zone (SMTZ) was identified at 4 cm sediment depth hinting to locally strong fluid fluxes. However, a lack of Cl- anomalies in porewater profiles indicates a shallow source of these
fluids, which is supported by the biogenic origin of the methane (�13C -70h PDB). Sulfide and alkalinity was relatively high (up to 20 mM and 40 mEq, respectively). Rates of AOM and sulfate reduction within this area reached magnitudes typical for seeps with variation between different habitat types, indicating a diverse methane supply, which is affecting the depths of the SMTZ. Rates were highest at sulfur a bacteria mats (20 mmol m-2 d-1) followed by a large field of dead clams, a pogonophoran field, a black sediment spot, and a carbonate rich clam field. Lowest rates (0.2 mmol m-2 d-1) were measured in close vicinity to these hot spots.
Abundant massive carbonate blocks and plates hint to a very old seep system with a probably much higher activity in the past. The U-Th age record of these authigenic carbonates reach back to periods of venting activity with more
than 150 ka ago. Carbon isotopic signatures of authigenic carbonates (�13C -50 to -40hPDB) suggest a biogenic carbon source (i.e. methane), also in the past. We found several indications for the impact of recent earthquakes
within the seep area (cracks, shifted seafloor), which could be an important mechanism for the triggering of new seepage activity, change in fluid expulsion rates and colonization patterns of the cold seep fauna
Geochemistry of cold seeps - Fluid sources and systematics
Emanation of fluids at cold seeps, mud volcanoes, and
other types of submarine seepage structures is a typical
phenomenon occurring at continental margins worldwide.
They represent pathways along which volatiles and solutes are
recycled from deeply buried sediments into the global ocean,
and hence they may be considered as a potentially important
link in global geochemical cycles.
In this contribution we present geochemical data from
various geological and tectonic settings such as the Gulf of
Cadiz, the convergent margin off Central America, and/or the
Black Sea and provide approaches how to systemize available
data sets. Clay-mineral dewatering plays a central role in
terms of fluid-mobilization from greater depth, however,
resulting cold seep fluids are typically very different from
each other and cover a large range of geochemical signatures.
This is is due to variations in control parameters such as the
type and thickness of the sediment cover, thermal conditions,
extension of fluid pathways, and the potential for secondary
overprinting. For example, freshened fluids emanating at cold
seeps off Costa Rica indicate dewatering and related
geochemical reactions in subducting sediments, while fluids
sampled at mud volcanoes in the Gulf of Cadiz provide
evidence for a high-temperature fluid source originating in the
underlying oceanic basement. The latter finding provides
evidence for a hydrological connection between buried
oceanic crust and the water column even at old crustal ages.
Varius geochemical tracers were proposed in the past to
decipher relevant processes in the subsurface. In a recent
systematic study, Scholz et al. [1] demonstrated the general
use of Li, reflecting the temperature-dependent isotope
fractionation during early diagenetic Li uptake and burial
diagenetic Li release from sediments. However, additional
approaches are required in order to provide robust geochmical
interpretations of cold seep fluids
Paired Sr isotope (<sup>87</sup>Sr/<sup>86</sup>Sr, δ<sup>88/86</sup>Sr) systematic of pore water profiles: A new perspective in marine weathering and seepage studies
The simultaneous and independent determination of the
radiogenic (87Sr/86Sr) and the fractionation reflecting stable
(!88/86Sr) Sr isotope ratio on pore waters, sediments and
precipitates (e.g. carbonates and sulfates) opens a new
perspective in the field of submarine weathering and Sr
contribution to the ocean chemistry.
Four initial case studies covering (1.) CO2 seeps of the
Okinawa Trough (OT), (2.) mud volcanoes (MV) and mounds
in the Gulf of Cadiz (GoC) and the (3.) Central American Fore
Arc as well as first results from the (4.) Black Sea are
conducted and reflect a stable Sr perspective on seeps from a
broad range of geological settings.
Referred to NIST-SRM-987, in this study the IAPSO
seawater (SW) standard has a !88/86Sr of 0.39 ‰ (±0.03, 2SD).
As a prominent systematic deviation the OT pore water (PW)
data from a site with CO2 hydrate and liquid CO2 occurence
show values ranging from 0.27 to 0.59 ‰ (286 to 64 cm
sediment depth), accompanied by a weak inversely correlated
trend from 0.2 to 0.15 ‰ for the corresponding bulk sediment
(286 to 36 cm).
In contradiction to a simple fluid/SW-mixing approach as
driving mechanism for the PW stable Sr trend the 87Sr/86Sr
signature stays within analytical uncertainty constant with
depth (0.70980 (1)) and differs significantly from SW
(0.70917 (1)) and the more radiogenic, slightly heterogeneous
sediment (0.71892-0.71731).
Potential explanation for the observed !88/86Sr trend and
PW signatures heavier than SW are (a) strong fractionation
processes enriching light isotopes in secondary precipitates
and remineralisation products and heavier signatures in the
remaining fluid and/or (b) preferential dissolution of heavier
mineral phases.
Examples for the latter kind of sediment component are
determined in a detailed study of the Mercator MV (GoC) by
high !88/86Sr ratios of 0.72 for authigenic and 0.92 ‰ for
potentially extruded gypsum crystals.
Combined with PW data from the other seep settings (0.2
to 0.52 ‰) a broad range of Sr contribution and fractionation
processes becomes evident
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