Near-offset vertical seismic experiments during leg 204

Three successful vertical seismic profiles (VSPs) were acquired during Ocean Drilling Program (ODP) Leg 204 at South Hydrate Ridge. The data confirm earlier results from ocean bottom seismometer data and analysis of moveout from common midpoint reflection data that the average velocity between the seafloor and the bottom-simulating reflector (BSR) is <1600 m/s throughout the region and is lowest near the summit, where the amount of hydrate is greatest. This result supports the conclusions that free gas and hydrate coexist beneath the summit and that the average amount of gas hydrate present elsewhere is low. The data also indicate that low-velocity zones (LVZs) resulting from free gas beneath the BSR must be thin and stratigraphically controlled. The only LVZ resolvable from traveltime analysis of the VSP data is associated with Horizon A, which has been interpreted to be the primary conduit transporting free gas to vents at the summit of South Hydrate Ridge. Thin LVZs associated with Horizons B and B', however, are indicated by sonic logs as well as by strong negative polarity reflections in the multichannel seismic data. This limited distribution of sub-BSR free gas contrasts with previous results at North Hydrate Ridge (Leg 146) and Blake Ridge (Leg 164), which indicate the presence of free gas zones several hundred meters thick that result in distinct LVZs in the VSP data from those earlier ODP legs

Upward shifts in the southern Hydrate Ridge gas hydrate stability zone following postglacial warming, offshore Oregon

High‐resolution three‐dimensional (3‐D) seismic reflection data acquired on the R/V Thomas G. Thompson in 2000 reveal a pair of bottom simulating reflections (BSRs) across a broad region of southern Hydrate Ridge, offshore Oregon. The primary BSR (BSRp) is a regionally extensive reflection that lies 120–150 m below seafloor and exhibits typical characteristics of a gas hydrate BSR. We also imaged a second weaker BSR (BSRs), 20–40 m below BSRp, with similar characteristics. BSRs is interpreted as a remnant of a BSR that probably formed during the Last Glacial Maximum 18,000 years ago, when the base of the gas hydrate stability zone (GHSZ) was deeper. An increase in bottom water temperatures of 1.75°–2.25° and a corresponding sea level rise of 120 m could have produced the BSR shift. The preservation of BSRs for at least 5000 years, which is the time since subseafloor temperatures stabilized following ocean warming after the Last Glacial Maximum, implies very slow upward advective and diffusive flow of methane (<1 m/1000 years in the vicinity of BSRs). BSRs appears where there are no resolvable steeply dipping faults and fractures, consistent with very low advective flow rates, and has dispersed where vertical fractures are visible. Free gas released by the shift in the BSR either migrates so slowly that it remains stable beneath the GHSZ or is directed upward along fractures to reform as hydrate in the GHSZ. There is no evidence for release of this free gas into the ocean or atmosphere.Keywords: Hydrate Ridge, bottom simulating reflection, gas hydrat

Seismic sequence stratigraphy and tectonic evolution of Southern Hydrate Ridge

This paper presents a seismic sequence and structural analysis of a high-resolution three-dimensional seismic reflection survey that was acquired in June 2000 in preparation for Ocean Drilling Program (ODP) Leg 204. The seismic data were correlated with coring and logging results from nine sites drilled in 2002 during Leg 204. The stratigraphic and structural evolution of this complex accretionary ridge through time, as inferred from seismic-stratigraphic units and depositional sequences imaged by the seismic data, is presented as a series of interpreted seismic cross sections and horizon time or isopach maps across southern Hydrate Ridge. Our reconstruction starts at ~1.2 Ma with a shift of the frontal thrust from seaward to landward vergent and thrusting of abyssal plain sediments over the older deformed and accreted units that form the core of Hydrate Ridge. From ~1.0 to 0.3 Ma, a series of overlapping slope basins with shifting depocenters was deposited as the main locus of uplift shifted northeastward. This enigmatic landward migration of uplift may be related to topography on the subducted plate, which is now deeply buried beneath the upper slope and shelf. The main locus of uplift shifted west to its present position at ~0.3 Ma, probably in response to a change to a seaward-vergent frontal thrust and related sediment underplating and duplexing. This structural and stratigraphic history has influenced the distribution of gas hydrate and free gas by causing variable age and permeability of sediments beneath and within the gas hydrate stability zone, preferential pathways for fluid migration, and varying amounts of decompression and gas dissolution

North-south variability in the history of deformation and fluid venting across Hydrate Ridge, Cascadia margin

Hydrate Ridge is an accretionary thrust ridge located on the lower slope of the central Cascadia convergent margin. Structural mapping based on two-dimensional and three-dimensional multichannel seismic reflection profiles and gridded bathymetry coupled with deep-towed sidescan sonar data and Ocean Drilling Program (ODP) biostratigraphy suggests that seafloor fluid venting patterns are likely controlled by the seaward-vergent (SV) structural style at northern Hydrate Ridge (NHR) and by the dominantly landward-vergent (LV) structural style at southern Hydrate Ridge (SHR). North-south structural variability across Hydrate Ridge is coincident with the seafloor authigenic carbonate distribution, which varies from aerially extensive authigenic carbonate crusts at NHR to a minor focused occurrence of authigenic carbonate at SHR. The older stratigraphy exposed at the seafloor at NHR (>1.6–1.7 Ma) has likely been subjected to a longer history of sediment compaction, dewatering, and deformation than the younger slope basin strata preserved at SHR (1.7 Ma to recent), suggesting the extent of carbonates at NHR may result from a longer history of fluid flow and/or more intense venting through a more uplifted, lithified, and fractured NHR sequence. Furthermore, recent work at SHR shows that the major seafloor fluid venting site there is fed by fluid flow through a volcanic ash–bearing turbidite sequence, suggesting stratigraphic conduits for fluid flow may be important in less uplifted, LV-dominated portions of Hydrate Ridge. In addition, the variability in structural style observed at Hydrate Ridge may have implications for the distributions and concentrations of fluids and gas hydrates in other accretionary settings and play a role in the susceptibility of accretionary ridges to slope failure

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

Feeding methane vents and gas hydrate deposits at south Hydrate Ridge

Log and core data document gas saturations as high as 90% in a coarse-grained turbidite sequence beneath the gas hydrate stability zone (GHSZ) at south Hydrate Ridge, in the Cascadia accretionary complex. The geometry of this gas-saturated bed is defined by a strong, negative-polarity reflection in 3D seismic data. Because of the gas buoyancy, gas pressure equals or exceeds the overburden stress immediately beneath the GHSZ at the summit. We conclude that gas is focused into the coarse-grained sequence from a large volume of the accretionary complex and is trapped until high gas pressure forces the gas to migrate through the GHSZ to seafloor vents. This focused flow provides methane to the GHSZ in excess of its proportion in gas hydrate, thus providing a mechanism to explain the observed coexistence of massive gas hydrate, saline pore water and free gas near the summit

A summary of ODP leg 141 hydrogeologic, geochemical and thermal results

The subduction of the oceanic spreading center at the Chile Triple Junction is marked by a substantial thermal perturbation and marked changes in the hydrogeologic and aqueous geochemical regimes in the overthrust plate. Ridge subduction substantially changes the fluid chemistry in the wedge through variably hydrating the oceanic basement, accretionary wedge, and continental backstop. This generates positive anomalies in salinity and chloride values with respect to sea water. The wedge immediately above the subducted ridge also experiences greatly enhance diagenesis and cementation together with the influx of primordial mantle derived ⁴He. Linear temperature and pore fluid chemistry profiles suggest a predominantly diffusive/conductive regime predominates in the interior eastern portion of the wedge and continental backstop region. In contrast, a vigorous and transient hydrogeolgic system within 5 km of the toe of the wedge at both Sites 859 and 863 generates spatially narrow, large, and complex anomalies in temperature and fluid chemistry. At the toe the vigorous hydrogeologic system may be variably influenced by the episodic expulsion of fluid from both the deeper parts of the wedge and oceanic basement driven convection systems. Structural and diagenetic observations are also consistent with a hydrogeologic regime that both evolves with time and that is dominated by episodic processes. In particular, studies of cements, mineralized veins, deformation bands, and Fe sulfide distribution suggest that above the subducting ridge (i.e., Site 863) the lithification in the wedge is greatly enhanced and that and periods of enhanced fluid expulsion are associated with local hydrofracture and dilation episodes

Little Mac's double feat of equitation

During the 1864 presidential campaign a popular analogy was drawn between Democratic presidential candidate George B. McClellan and an acrobat uneasily straddling two horses at once. The artist here portrays the McClellan candidacy as trying to combine two seemingly irreconcilable causes, peace and war. In a circus ring McClellan stretches between horses marked "Letter of Acceptance" and "Chicago Platform." In Chicago on August 29, 1864, McClellan was nominated the Democratic candidate on a peace platform. In his letter of acceptance, however, the candidate expressed his support for the war. At left his running mate George H. Pendleton, wearing a dunce cap "PEACE!," eggs him on, saying, "I "say" Mac! Can't you hold on to "both" 'till the 8th of Nov.? [i.e., election day]" In the background a huge audience watches the spectacle. Among the instruments in the band is a drum inscribed "N.Y. World," one of McClellan's most vocal supporters.Entered . . . 1864 . . . by N. Bangs Williams, Providence.Title appears as it is written on the item.Murrell, p. 225.Weitenkampf, p. 143.Published in: American political prints, 1766-1876 / Bernard F. Reilly. Boston : G.K. Hall, 1991, entry 1864-25