Erwin Suess

Authigenic carbonates from the Cascadia subduction zone and their relation to gas hydrate stability

Authigenic carbonates are intercalated with massive gas hydrates in sediments of the Cascadia margin. The deposits were recovered from the uppermost 50 cm of sediments on the southern summit of the Hydrate Ridge during the RV Sonne cruise SO110. Two carbonate lithologies that differ in chemistry, mineralogy, and fabric make up these deposits. Microcrystalline high-magnesium calcite (14 to 19 mol% MgCO3) and aragonite are present in both semiconsolidated sediments and carbonate-cemented clasts. Aragonite occurs also as a pure phase without sediment impurities. It is formed by precipitation in cavities as botryoidal and isopachous aggregates within pure white, massive gas hydrate. Variations in oxygen isotope values of the carbonates reflect the mineralogical composition and define two end members: a Mg-calcite with δ18O =4.86‰ PDB and an aragonite with δ18O =3.68‰ PDB. On the basis of the ambient bottom-water temperature and accepted equations for oxygen isotope fractionation, we show that the aragonite phase formed in equilibrium with its pore-water environment, and that the Mg-calcite appears to have precipitated from pore fluids enriched in 18O. Oxygen isotope enrichment probably originates from hydrate water released during gas-hydrate destabilization.

Archaea mediating anaerobic methane oxidation in deep-sea sediments at cold seeps of the eastern Aleutian subduction zone

Cold seeps in the Aleutian deep-sea trench support prolific benthic communities and generate carbonate precipitates which are dependent on carbon dioxide delivered from anaerobic methane oxidation. This process is active in the anaerobic sediments at the sulfate reduction-methane production boundary and is probably performed by archaea working in syntrophic co-operation with sulfate-reducing bacteria. Diagnostic lipid biomarkers of archaeal origin include irregular isoprenoids such as 2,6,11,15-tetramethylhexadecane (crocetane) and 2,6,10,15,19-pentamethylicosane (PMI) as well as the glycerol ether lipid archaeol (2,3-di-O-phytanyl-sn-glycerol). These biomarkers are prominent lipid constituents in the anaerobic sediments as well as in the carbonate precipitates. Carbon isotopic compositions of the biomarkers are strongly depleted in 13C with values of δ13C as low as −130.3‰ PDB. The process of anaerobic methane oxidation is also reflected in the carbon isotope composition of organic matter with δ13C-values of −39.2 and −41.8‰ and of the carbonate precipitates with values of −45.4 and −48.7‰. This suggests that methane-oxidizing archaea have accumulated within the microbial community, which is active at the cold seep sites. The dominance of crocetane in sediments at one station indicates that, probably due to decreased methane venting, archaea might no longer be growing, whereas high amounts of crocetenes found at other more active stations may indicate recent fluid venting and active archaea. Comparison with other biomarker studies suggests that various archaeal assemblages might be involved in the anaerobic consumption of methane. The assemblages are apparently dependent on specific conditions found at each cold seep environment. Selective conditions probably include water depth, temperature, degree of anoxia, and supply of free methane.

Massive barite deposits and carbonate mineralization in the Derugin Basin, Sea of Okhotsk: precipitation processes at cold seep sites

An area of massive barite precipitations was studied at a tectonic horst in 1500 m water depth in the Derugin Basin, Sea of Okhotsk. Seafloor observations and dredge samples showed irregular, block- to column-shaped barite buildups up to 10 m high which were scattered over the seafloor along an observation track 3.5 km long. High methane concentrations in the water column show that methane expulsion and probably carbonate precipitation is a recently active process. Small fields of chemoautotrophic clams (Calyptogena sp., Acharax sp.) at the seafloor provide additional evidence for active fluid venting. The white to yellow barites show a very porous and often layered internal fabric, and are typically covered by dark-brown Mn-rich sediment; electron microprobe spectroscopy measurements of barite sub-samples showa Ba substitution of up to 10.5 mol% of Sr. Rare idiomorphic pyrite crystals ( V1%) in the barite fabric imply the presence of H2S. This was confirmed by clusters of living chemoautotrophic tube worms (1 mm in diameter) found in pores and channels within the barite. Microscopic examination showed that micritic aragonite and Mg-calcite aggregates or crusts are common authigenic precipitations within the barite fabric. Equivalent micritic carbonates and barite carbonate cemented worm tubes were recovered from sediment cores taken in the vicinity of the barite build-up area. Negative N 13 C values of these carbonates (s343.5x PDB) indicate methane as major carbon source; N 18 O values between 4.04 and 5.88x PDB correspond to formation temperatures, which are certainly below 5‡C. One core also contained shells of Calyptogena sp. at different core depths with 14 C-ages ranging from 20 680 to s 49 080 yr. Pore water analyses revealed that fluids also contain high amounts of Ba; they also show decreasing SO 2�

In situ measurement of fluid flow from cold seeps at active continental margins

In situ measurement of fluid flow rates from active margins is an important parameter in evaluating dissolved mass fluxes and global geochemical balances as well as tectonic dewatering during developments of accretionary prisms. We have constructed and deployed various devices that allow for the direct measurement of this parameter. An open bottom barrel with an exhaust port at the top and equipped with a mechanical flowmeter was initially used to measure flow rates in the Cascadia accretionary margin during an Alvin dive program in 1988. Sequentially activated water bottles inside the barrel sampled the increase of venting methane in the enclosed body of water. Subsequently, a thermistor flowmeter was developed to measure flow velocities from cold seeps. It can be used to measure velocities between 0.01 and 50 cm s−1, with a response time of 200 ms. It was deployed again by the submersible Alvin in visits to the Cascadia margin seeps (1990) and in conjunction with sequentially activated water bottles inside the barrel. We report the values for the flow rates based on the thermistor flowmeter and estimated from methane flux calculations. These results are then compared with the first measurement at Cascadia margin employing the mechanical flowmeter. The similarity between water flow and methane expulsion rates over more than one order of magnitude at these sites suggests that the mass fluxes obtained by our in situ devices may be reasonably realistic values for accretionary margins. These values also indicate an enormous variability in the rates of fluid expulsion within the same accretionary prism. n nFinally, during a cruise to the active margin off Peru, another version of the same instrument was deployed via a TV-controlled frame within an acoustic transponder net from a surface ship, the R.V. Sonne. The venting rates obtained with the thermistor flowmeter used in this configuration yielded a value of 4411 m−2 day−1 at an active seep on the Peru slope. The ability for deployment of deep-sea instruments capable of measuring fluid flow rates and dissolved mass fluxes from conventional research vessels will allow easier access to these seep sites and a more widespread collection of the data needed to evaluate geochemical processes resulting from venting at cold seeps on a global basis. Comparison of the in situ flow rates from steady-state compactive dewatering models differ by more than 4 orders of magnitude. This implies that only a small area of the margin is venting and that there must be recharge zones associated with venting at convergent margins.

Geophysical constraints on the surface distribution of authigenic carbonates across the Hydrate Ridge region, Cascadia margin

On active tectonic margins methane-rich pore fluids are expelled during the sediment compaction and dewatering that accompany accretionary wedge development. Once these fluids reach the shallow subsurface they become oxidized and precipitate cold seep authigenic carbonates. Faults or high-porosity stratigraphic horizons can serve as conduits for fluid flow, which can be derived from deep within the wedge and/or, if at seafloor depths greater than ∼300 m, from the shallow source of methane and water contained in subsurface and surface gas hydrates. The distribution of fluid expulsion sites can be mapped regionally using sidescan sonar systems, which record the locations of surface and slightly buried authigenic carbonates due to their impedence contrast with the surrounding hemipelagic sediment. Hydrate Ridge lies within the gas hydrate stability field offshore central Oregon and during the last 15 years several studies have documented gas hydrate and cold seep carbonate occurrence in the region. In 1999, we collected deep-towed SeaMARC 30 sidescan sonar imagery across the Hydrate Ridge region to determine the spatial distribution of cold seep carbonates and their relationship to subsurface structure and the underlying gas hydrate system. High backscatter on the imagery is divided into three categories, (I) circular to blotchy with apparent surface roughness, (II) circular to blotchy with no apparent surface roughness, and (III) streaky to continuous with variable surface roughness. We interpret the distribution of high backscatter, as well as the locations of mud volcanoes and pockmarks, to indicate variations in the intensity and activity of fluid flow across the Hydrate Ridge region. Seafloor observations and sampling verify the acoustic signals across the survey area and aid in this interpretation. Subsurface structural mapping and swath bathymetry suggest the fluid venting is focused at the crests of anticlinal structures like Hydrate Ridge and the uplifts along the Daisy Bank fault zone. Geochemical parameters link authigenic carbonates on Hydrate Ridge to the underlying gas hydrate system and suggest that some of the carbonates have formed in equilibrium with fluids derived directly from the destabilization of gas hydrate. This suggests carbonates are formed not only from the methane in ascending fluids from depth, but also from the shallow source of methane released during the dissociation of gas hydrate. The decreased occurrence of high-backscatter patches and the dramatic reduction in pockmark fields, imaged on the eastern part of the survey, suggest gas hydrate near its upper stability limit may be easily destabilized and thus, responsible for these seafloor features. High backscatter along the left-lateral Daisy Bank fault suggests a long history of deep-seated fluid venting, probably unrelated to destabilized gas hydrate in the subsurface.

Authigenic barites and fluxes of barium associated with fluid seeps in the Peru subduction zone

Large deposits of barite were discovered in association with biological communities, indicative of active fluid seepage on the middle slope of Paita and in the Chiclayo Canyon, in the Peru margin. We postulate that the barium source for the deposits is associated with the high concentration of non-detrital barite buried in sediments from this high productivity region. Barite is remobilized within the sediment column due to sulfate depletion. Subsequent flushing of the barium-rich fluids from the sediment to the bottom water, leads to the formation of barite deposits at the cold vent sites. High barium concentrations measured in pore fluids of sediments are consistent with remobilization of barium sulfate below the zone of sulfate depletion. Fluid samples—collected in a time sequence using a benthic chamber in the Paita middle slope vent sites — document a contemporaneous release of barium to the bottom water at a rate of 23 μmol cm−2 yr−1. n nFluid seepage in the Peru margin is not restricted to the middle slope of Paita and the Chiclayo Canyon where barite deposits occur, but is also evident in the upper and lower slopes of Paita and in the Chimbote upper slope. Deployment of a benthic chamber on the Chimbote upper slope site show no measurable release of barium; even though the dissolved barium concentration in the pore fluids is high. These observations indicate that the barite deposits associated with fluid seepage in the Peru margin are restricted to areas where slope failure has exposed sequences deep enough such that the barium-rich fluids do not encounter sulfate-bearing pore fluids before emanating at the seafloor.

A new "schlieren" technique application for fluid flow visualization at cold seep sites

A new optical instrument for the investigation of submarine fluid flows based on a ‘schlieren’ technique was developed and successfully deployed at cold seep sites. With this application it is possible to visualize the discharge and distribution of fluids in the ambient bottom water and to resolve microstructures and mixing processes at a scale of centimeters. The system is sensitive to small refractive index anomalies caused by temperature and salinity variations. Density anomalies of Δσt=0.049 are detectable evaluated by in-situ temperature variations of ΔT=0.1°C and salinity variations of ΔS=0.045 psu. In flume experiments the smallest detectable density variation was even lower with Δσt=0.023. The technique has been successfully applied in two different environments. First field experiments were performed to observe submarine groundwater discharge in Eckernforde Bay (western Baltic Sea) at shallow water depths. Subsequently, the ‘schlieren’ technique was successfully brought to a cold seep location at the Cascadia convergent margin (800 m water depth). The discharge of fluids was recorded in both field experiments which enabled a qualitative seep site identification.

Geochemistry of a sealed deep-sea borehole on the Cascadia Margin

The deep-sea borehole seal CORK was deployed for the first time on a modern accretionary prism during ODP Leg 146 to the Cascadia Margin. Ten months after the deployment the fluid flow and geochemistry of the borehole fluids was investigated during several dives by DSRV Alvin. The chemical analysis of the borehole fluids revealed methane concentrations of more than 3.5 mM, whereas oxygen and dissolved ions as Cl, NO3, or PO4 are still close to the ambient seawater composition. The exceedingly high methane content measured at the top of the sealed borehole and the observed degassing during the ascent of the submersible indicates that the sampled fluid was initially saturated or close to saturation with respect to CH4. The hydrocarbons are characterized by ratios of 170–200 and δ13C values of − 59.5 to − 62.4%o which indicates a considerable admixture of thermogenic hydrocarbon gases. The occurrence of methane of partly thermogenic origin demonstrates that CH4 enters the sealed borehole in the lower, perforated section (94–178 mbsf) and accumulates at the top of the borehole. This suggests the occurrence of free gas within the encapsulated borehole. Considering the stability field of CH4-hydrates, the formation of these ice-like structures may take place and potentially results in a clogging of the top of the borehole. Such precipitates could result in a decoupling of the top of the borehole from the hydraulic and geochemical regime of the accretionary complex, an important aspect for future plans of CORK deployments.