Helium isotope concentrations and ratios in sediments from ODP Hole 130-806B

We have determined the helium abundance and isotopic composition of seafloor carbonate sediments from the flanks of the Ontong Java Plateau, western equatorial Pacific Ocean (ODP Site 806). These results provide a two million year record of the burial flux of extraterrestrial 3He, which we believe is a proxy for the terrestrial accretion rate of interplanetary dust particles. The 3He burial flux prior to ~700 ka was relatively low, ~0.5 pcc/cm**2/kyr, but from 700 ka to the present, the burial flux gradually increased to a value of ~1.0 pcc/cm**2/kyr. 100 kyr periodicity in the 3He burial flux is apparent over the last 700 kyr and correlates with the oxygen isotope record of global climate, with high 3He burial fluxes associated with interglacial periods. This periodicity and phase are consistent with previous 3He measurements in North Atlantic sediments. Although 100 kyr periodicity in 3He burial flux is in agreement with recent predictions of the accretion rate of interplanetary dust based on a model of the orbital evolution of asteroidal debris, the measurements and predictions differ by one half cycle in phase. Nevertheless, our observations suggest the terrestrial accretion rate of interplanetary dust is controlled by orbital eccentricity and/or inclination relative to the solar-system invariable plane. Such control is a necessary but not sufficient condition for the hypothesis of that variations in extraterrestrial dust accretion modulates terrestrial climate with a 100 kyr period. We also identify several brief (<25 kyr) intervals of strongly enhanced 3He burial, possibly related to random and transient fluctuations in the accretion rate of asteroidal or cometary dust particles.

Mineralogy of ODP Site 105-645

Cores from the upper 70 meters below seafloor (mbsf) (upper Pleistocene) at Ocean Drilling Program (ODP) Site 645 in Baffin Bay show dramatic meter-scale changes in color and mineralogy. Below this interval, mineralogical changes are more gradual to the top of the Miocene at about 550 mbsf. The Pliocene-Pleistocene section can be divided into five facies: Facies 1 - massive, poorly sorted, gravel-bearing muds; Facies 2 - gray silty clays and silty muds; Facies 3 - laminated detricarbonate silty muds; Facies 4 - silty sand and sandy silt; and Facies 5 - poorly sorted muddy sands and silty muds. Facies 4 and 5 are restricted to the Pliocene section below depths of about 275 mbsf. The mineralogical/color cycles in the upper 70 mbsf are the result of alternations between Facies 2 and three lithotypes of Facies 1: lithotype A - tan-colored, carbonate-rich, gravel-bearing mud; lithotype B - weak, red-colored, gravel-bearing mud rich in sedimentary rock fragments; and lithotype C - gray, gravel-bearing mud. A fourth lithotype, D, is restricted to depths of 168-275 mbsf and is dark gray, carbonate-poor, gravel-bearing mud. We believe that all lithotypes of Facies 1 and the sand and gravel fractions of Facies 2 and 3 were deposited by ice rafting. Depositional processes for Facies 4 and 5 probably include ice rafting and bottom- and turbidity-current transport. Data from petrographic analyses of light and heavy sand-sized grains and X-ray analyses of silt- and clay-size fractions suggest that tan-colored sediments (lithotype A of Facies 1; Facies 3) were derived mainly from Paleozoic carbonates of Ellesmere, Devon, and northern Baffin islands. Weak red sediments (lithotype B) contain significant red sedimentary clasts, reworked quartzarenite grains and clasts, and rounded colorless garnets, all derived from Proterozoic sequences of the Borden and Thule basins, and from minor Mesozoic red beds. Other sediments in the upper 335 mbsf at Site 645 contain detritus from a heterogeneous mixture of sources, including Precambrian shield terranes around Baffin Bay. Sediments from 335 to 550 mbsf (Facies 5) are rich in friable sedimentary clasts and detrital micas and contain glauconite and, in a few samples, reworked diatoms. These components suggest derivation from poorly consolidated Mesozoic-Tertiary sediments in coastal outcrops and beneath the modern shelves of northeastern Baffin Island and western Greenland. For the upper Pleistocene section (about 0-100 mbsf), marked mineralogical cyclicity is attributed to fluctuating glacial margins, calving rates, and iceberg melting rates, particularly around the northern end of Baffin Bay. Tan-colored, carbonate-rich units were derived at times of maximum advance of glaciers on Ellesmere and Devon islands, during relatively warm intervals induced by incursion of warm Atlantic surface water into the bay. At the beginning of these warmer episodes, most icebergs were contributed by glaciers near sea level around the Arctic channels, which resulted in deposition of weak red, ice-rafted units rich in Proterozoic sedimentary clasts.

Geochemistry of basal dolomitic and Fe-Mn sediments from the Tyrrhenian Sea

In the Tyrrhenian Sea (Western Mediterranean), unusual reddish, soft to lithified, dolomitic sediments up to 45 m thick overlie igneous crust at the base of thick Pliocene-Quaternary deep-sea sediment successions in the Marsili (Site 650) and Vavilov (Site 651) basins. These sediments also overlie the Gortani Ridge, a basaltic Seamount near the base of the Sardinian continental margin (Site 655). At both basinal sites (650, 651), the lowest sediments are dolomitic, with manganese oxide (MnO) segregations. Whole-rock X-ray diffraction indicates abundant dolomite and quartz, with subordinate calcite, illite (authigenic), feldspar and minor kaolinite, chlorite, and anhydrite. Chemical analyses show strong enrichment in magnesium oxide (MgO) and MnO relative to shale or deep-sea clay. Mg and Mn correlate positively and exhibit decreasing concentrations up the succession in the Marsili Basin (Site 650). The following scenario is proposed: peridotites were exposed on the seafloor in the Vavilov Basin (Site 651) and then eroded, depositing talc in local fine-grained dolomitic sediments within the igneous basement. After local magmatism ended, the igneous basement at each site subsided rapidly (about 800 m/m.y.) and was blanketed with calcareous and clay-rich oozes. During early diagenesis (from isotopic evidence; McKenzie et al., this volume) tepid fluids, of modified seawater composition, reacted with and dolomitized the overlying deep-sea sediments. At Site 651 additional Mg may have been extracted from asthenosphere peridotite cored at shallow depths (about 100 m). One can hypothesize that fluids rich in Mg and Mn were flushed from the igneous basement, triggered by extensional faulting and local tilting during subsidence of the basement, and that these fluids then dolomitized the base of the overlying sediment succession. Late tectonic movements in the Vavilov Basin (Site 651) fractured already lithified dolomitic sediments and more reducing (? hydrothermal) fluids locally remobilized Fe and Mn and corroded dolomite crystals.

Petrography and geochemical composition of the Atlantis II Fracture Zone

SeaBeam echo sounding, seismic reflection, magnetics, and gravity profiles were run along closely spaced tracks (5 km) parallel to the Atlantis II Fracture Zone on the Southwest Indian Ridge, giving 80% bathymetric coverage of a 30- * 170-nmi strip centered over the fracture zone. The southern and northern rift valleys of the ridge were clearly defined and offset north-south by 199 km. The rift valleys are typical of those found elsewhere on the Southwest Indian Ridge, with relief of more than 2200 m and widths from 22 to 38 km. The ridge-transform intersections are marked by deep nodal basins lying on the transform side of the neovolcanic zone that defines the present-day spreading axis. The walls of the transform generally are steep (25°-40°), although locally, they can be more subdued. The deepest point in the transform is 6480 m in the southern nodal basin, and the shallowest is an uplifted wave-cut terrace that exposes plutonic rocks from the deepest layer of the ocean crust at 700 m. The transform valley is bisected by a 1.5-km-high median tectonic ridge that extends from the northern ridge-transform intersection to the midpoint of the active transform. The seismic survey showed that the floor of the transform contains up to 0.5 km of sediment. Piston-coring at two locations on the transform floor recovered more than 1 m of sand and gravel, which appears to be turbidites shed from the walls of the fracture zone. Extensive dredging showed that more than two-thirds of the crust exposed in the transform valley and its walls were plutonic rocks, principally gabbros and residual mantle peridotites. In contrast, based on dredging and seafloor morphology, only relatively undisrupted pillow basalt flows have been exposed on crust of the same age spreading away from the transform. Magnetic anomalies are well defined out to 11 m.y. over the flanking transverse ridges and transform valley, even where layer 2 appears to be absent. The total opening rate is 1.6 cm/yr, but the arrangement of the anomalies indicates that the spreading for each ridge is asymmetric, with the ridge flanks facing the transform spreading at a rate of 1.0 cm/yr. Such an asymmetric spreading pattern requires that both the northern and southern ridges migrate away from each other at 0.2 cm/yr, thus lengthening the transform at 0.4 cm/yr for the last 11 m.y. To the north, the fracture zone valley is oriented differently from the present-day transform, indicating a paleospreading direction change at 17 m.y. from N10°E to due north-south. This change placed the transform into extension for the 11-m.y. period required for simple orthogonal ridge-transform geometry to be reestablished and produced a large transtensional basin within the transform valley. This basin was split by continued transform slip after 11 m.y., with the larger half moving to the north with the African Plate.

Ar and U-Th-Pb age determinations for minerals from ODP Holes of ODP Legs 173 and 210

We report U-Pb and 39Ar-40Ar measurements on plutonic rocks recovered from the Ocean Drilling Program (ODP) Legs 173 and 210. Drilling revealed continental crust (Sites 1067 and 1069) and exhumed mantle (Sites 1070 and 1068) along the Iberia margin and exhumed mantle (Site 1277) on the conjugate Newfoundland margin. Our data record a complex igneous and thermal history related to the transition from rifting to seafloor spreading. The results show that the rift-to-drift transition is marked by a stuttering start of MORB-type magmatic activity. Subsequent to initial alkaline magmatism, localized mid-oceanic ridge basalts (MORB) magmatism was again replaced by basin-wide alkaline events, caused by a low degree of decompression melting due to tectonic delocalization of deformation. Such "off-axis" magmatism might be a common process in (ultra-) slow oceanic spreading systems, where "magmatic" and "tectonic" spreading varies in both space and time.

Consolidation properties, permeabilities, grain sizes and age of samples from ODP Holes 194-1194A and 194-1198A

Uniaxial strain consolidation experiments were conducted to determine elastic and plastic properties and to estimate the permeability of sediments from 0 to 200 meters below seafloor at Ocean Drilling Program Sites 1194 and 1198. Plastic deformation is described by compression indices, which range from 0.19 to 0.37. Expansion indices, the elastic deformation measured during unload/reload cycles on samples, vary from 0.02 to 0.029. Consolidation experiments provide lower bounds on permeability between 5.4 x 10**-16 m**2 and 1.9 x 10**-18 m**2, depending on the consolidation state of the sample.

Diatom distribution in sediments of the Peruvian coastal upwelling

Two shelf and upper-slope sites (681 and 686) near present cores of active coastal upwelling on the Peruvian margin were sampled continuously at 7.5 cm intervals down to approximately 25 meters below the seafloor (mbsf). Sedimentary structures were subdivided into homogeneous/bioturbated, laminated, and laminated-varved intervals. Diatom analysis and statistical treatment of data revealed almost persistent upwelling over the last 400,000 yr, with slightly increased upwelling during oxygen-isotope Stages 3, 5, 7, and 8 that resulted in anoxic bottom-water conditions and deposition of laminated/undisturbed primary sedimentary structures.

Paleomagnetic and rock magnetic characterization of ODP Leg 210 cores

We conducted an integrated paleomagnetic and rock magnetic study on cores recovered from Ocean Drilling Program Sites 1276 and 1277 of the Newfoundland Basin. Stable components of magnetization are determined from Cretaceous-aged sedimentary and basement cores after detailed thermal and alternating-field demagnetization. Results from a series of rock magnetic measurements corroborate the demagnetization behavior and show that titanomagnetites are the main magnetic carrier. In view of the normal polarity of magnetization and radiometric dates for the sills at Site 1276 (~98 and ~105 Ma, both within the Cretaceous Normal Superchron) and for a gabbro intrusion in peridotite at Site 1277 (~126 Ma, Chron M1), our results suggest that the primary magnetization of the Cretaceous rocks is likely retained in these rocks. The overall magnetic inclination of lithologic Unit 2 in Hole 1277A between 143 and 180 meters below seafloor is 38°, implying significant (~35° counterclockwise, viewed to the north) rotation of the basement around a horizontal axis parallel to the rift axis (010°). The paleomagnetic rotational estimates should help refine models for the tectonic evolution of the basement. The mean inclinations for Sites 1276 and 1277 rocks imply paleolatitudes of 30.3° ± 5.1° and 22.9° ± 12.0°, respectively, with the latter presumably influenced by tectonic rotation. These values are consistent with those inferred from the mid-Cretaceous reference poles for North America, suggesting that the inclination determinations are reliable and consistent with a drill site on a location in the North America plate since at least the mid-Cretaceous. The combined paleolatitude results from Leg 210 sites indicate that the Newfoundland Basin was some 1800 km south of its current position in the mid-Cretaceous. Assuming a constant rate of motion, the paleolatitude data would suggest a rate of 12.1 mm/yr for the interval from ~130 Ma (Site 1276 age) to present, and 19.6 mm/yr for the interval from 126 Ma (Site 1277 age) to recent. The paleolatitude and rotational data from this study are consistent with the possibility that Site 1276 may have passed over the Canary and Madeira hotspots that formed the Newfoundland Seamounts in the mid-Cretaceous.

Geochemical composition of olivine, spinel and amphibole in ODP Hole 209-1271B

Dunite and gabbroic materials recovered from Hole 1271B, Ocean Drilling Program (ODP) Leg 209, were examined for mineral chemistry to understand melt flow and melt-mantle reactions in the shallowest upper mantle of the Mid-Atlantic Ridge near the 15°20' Fracture Zone. Hole 1271B was drilled to 103.8 meters below seafloor on the inner corner high along the south wall of the 15°20' Fracture Zone. The total length of core collected was 15.9 m (recovery = ~15%). The dominant rock type in Hole 1271B is dunite, followed by brown amphibole gabbro, olivine gabbro, and troctolite, along with minor amounts of harzburgite and olivine gabbronorite. A large proportion of the dunite is associated with gabbroic rocks in Hole 1271B, similar to those observed in the Mohorovicic (Moho) transition zone of the Oman ophiolite, indicating significant magmatic activity in this region near the 15°20' Fracture Zone. Olivine Fo content varies from 89.2 to 91.2 in impregnated dunite and from 85.6 to 88.6 in troctolite, olivine gabbro, and olivine gabbronorite. Spinel Cr# (= 100 x Cr/[Cr + Al] molar ratio) ranges from 38.9 to 62.7 in dunite and from 46.3 to 57.6 in troctolites, olivine gabbro, and olivine gabbronorite. Compositional trends for spinel from dunite through troctolite toward olivine gabbro/gabbronorite are characterized by increases in TiO2, Cr#, and Fe3+#, very similar to those reported from Hess Deep Site 895. Olivine gabbro, olivine gabbronorite, and troctolite in Hole 1271B are considered to have formed as hybrid rocks between dunite and an evolved melt in the walls of a melt channel in the shallowest upper mantle that is tens of meters wide. The melt trapped in the wall rock crystallized plagioclase and clinopyroxene. On the other hand, dunite in the center of the melt channel became more refractory by melt-mantle reactions, increasing spinel Cr# to 62.5.

Isotopic composition and chemistry of pore waters from ODP Site 195-1201 sediments, West Philippine Basin

This report summarizes chemical and isotopic data from Ocean Drilling Program Leg 195 Site 1201. Pore water is divided into three intervals based on the rate of chemical change with depth. The shallowest interval is the red clay unit between 1.26 and 56.40 meters below seafloor (mbsf). In this section, there are overall decreases in the concentrations of alkalinity, boron, lithium, magnesium, potassium, sodium, and sulfate, whereas concentrations of calcium and chloride increase. Values of d18O and dD plot near standard mean ocean water to the right of the global meteoric water line (GMWL). Five samples from 72.60 and 83.33 mbsf yielded pore water for analyses. These samples help define a trend in the second interval, which is between 56.4 and 238.98 mbsf. Here, concentrations of magnesium, potassium, sodium, and sulfate decease, whereas concentrations of boron, calcium, and chloride increase. Concentrations of alkalinity and lithium remain roughly constant. The deepest interval, between 238.04 and 504.8 mbsf, has comparatively slower decreases of sodium and sulfate, increases of calcium and chloride, slow increases of alkalinity and lithium, and roughly constant concentrations of magnesium, potassium, and boron. Values of d18O and dD in pore water between 146.98 and 504.80 mbsf plot in a linear trend to the right of the GMWL.

Effective stress, porosity, p-wave velocity and mineral composition of ODP Hole 174A-1073A sediments

Porosity, permeability, and compressional (P-wave) velocity were measured as a function of stress on sediments from Ocean Drilling Program Site 1073, U.S. Mid-Atlantic continental slope. Thin sections, scanning electron microscopy, and X-ray diffraction analyses provided mineralogical characteristics of the samples. Uniaxial strain boundary conditions were imposed on the samples during consolidation tests with the maximum effective axial stress reaching 13 MPa. The maximum effective radial stress necessary to maintain uniaxial strain was 7.6 MPa. Over an effective axial stress interval of 0 to 5.2 MPa, Sample 174A-1073A-26X-2, 82-89 cm (226.65 meters below seafloor [mbsf]), exhibited the largest decrease in porosity (51% to 41%), whereas Sample 71X-1, 2-8 cm (644.70 mbsf), exhibited the smallest decrease in porosity (48% to 45%). All samples showed negligible porosity increases during unloading. The permeability (on the order of 1 x 10-17 m**2) of Sample 174A-1073A-71X-1, 2-8 cm, was twice that measured on Sample 8H-1, 23-26 cm (63.75 mbsf), even though the former was considerably deeper and older. The differences in porosity-stress behavior and permeability between shallow and deep samples is related to lithologic, mineralogic, and diagenetic differences between the sediments above and below the Pliocene-Pleistocene to Miocene unconformity. P-wave velocity for Samples 174A-1073A-41X-5, 97-103 cm (372.35 mbsf), and 71X-1, 2-8 cm, increased with decreasing porosity, but did not change significantly during unloading.

Grain size description of sediments from ODP Leg 174A sites

We drilled three sites (Sites 1071, 1072, and 1073) on the New Jersey shelf and slope at water depths between 88 and 664 m. Grain-size analyses from shelf sites (Sites 1071 and 1072) define five types of sediment: well-sorted fine sand, silty sand or sandy silt, clayey silt, poorly sorted sandy mud, and poorly sorted lag sediments. At slope Site 1073, a grain-size minimum of 3-6 µm is found at 300 meters below seafloor. These sediments are well sorted and lack sand- and clay-sized grains. Horizons of coarse-grained sediments are present in Unit I at Site 1073.

Composition and structural formulas of ODP Site 200-1224 Eocene ferrobasalt minerals

The ~46-m.y.-old igneous basement cored during Leg 200 in the North Pacific represents one of the few cross sections of Pacific oceanic crust with a total penetration into basalt of >100 m. The rocks, emplaced during the Eocene at a fast-spreading rate (~14 cm/yr; full rate) are strongly differentiated tholeiitic basalts (ferrobasalts) with 7-4.5 wt% MgO, relatively high TiO2 (2-3.5 wt%), and total iron as Fe2O3 (9.1-16.8 wt%). The differentiated character of these lavas is related to unusually large amounts of crystallization differentiation of plagioclase, clinopyroxene, and olivine. The lithostratigraphy of the basement (cored to ~170 meters below seafloor) is divided into three units. The deepest unit (lithologic Unit 3), is a succession of lava flows of no more that a few meters thickness each. The intermediate unit (lithologic Unit 2) is represented by intermixed thin flows and pillows, whereas the shallowest unit (lithologic Unit 1), comprises two massive flows. The rocks range from aphyric to sparsely clinopyroxene-plagioclase-phyric (phenocryst content = <3 vol%) and from holocrystalline to hypohyaline. Chilled margins of pillow fragments show holohyaline to sparsely vitrophyric textures. Site 1224 oxide minerals present a type of alteration not previously seen, where titanomagnetite is only partially destroyed and the pure magnetite component is partially removed from the mineral, leaving, in the most extreme case, a nearly pure ulvöspinel residuum. As a result of this dissolution, iron, mainly in the oxidized state, is added to the circulating solvent fluids. This means that a considerable metal source can result from low-temperature reactions throughout the upper ocean crust. The coarsest-grained lithologic Unit 1 rocks have interstitial myrmekitic intergrowths of quartz and sodic plagioclase (~An12), roughly similar in mineralogy and bulk composition to tonalite/trondhjemite veinlets in abyssal gabbros from the southwest Indian Ocean and Hess Deep, eastern equatorial Pacific. Based on idiomorphic relationships and projections into the simplified Q-Ab-Or-H2O granite ternary system, the myrmekitic intergrowths formed at the same time as, or just after, the oxide minerals coprecipitated and at low water vapor pressure (~0.5 kbar). Their compositions correspond to SiO2-oligoclase intergrowths that are considerably less potassic than dacitic glasses that erupt, although rarely, along the East Pacific Rise or that have been produced experimentally by partial melting of gabbro. Based on the crystallization history and comparison to experimental data, the original interstitial siliceous liquids resulted from late-stage immiscible separation of siliceous and iron-rich liquids. The rare andesitic lavas found along the East Pacific Rise may be hybrid rocks formed by mixing of these immiscible siliceous melts with basaltic magma.

Benthic foraminifera and stable isotope record of ODP Leg 182 sites

Cores from Sites 1129, 1131, and 1132 (Ocean Drilling Program (ODP) Leg 182) on the uppermost slope at the edge of the continental shelf in the Great Australian Bight reveal the existence of upper Pleistocene bryozoan reef mounds, previously only detected on seismic lines. Benthic foraminiferal oxygen isotope data for the last 450,000 years indicate that bryozoan reef mounds predominantly accumulated during periods of lower sea level and colder climate since stage 8 at Sites 1129 and 1132 and since stage 4 at the deeper Site 1131. During glacials and interstadials (stages 2-8) the combination of lowered sea level, increased upwelling, and absence of the Leeuwin Current probably led to an enhanced carbon flux at the seafloor that favored prolific bryozoan growth and mound formation at Site 1132. At Site 1129, higher temperatures and downwelling appear to have inhibited the full development of bryozoan mounds during stages 2-4. During that time, favorable hydrographic conditions for the growth of bryozoan mounds shifted downslope from Site 1129 to Site 1131. Superimposed on these glacial-interglacial fluctuations is a distinct long-term paleoceanographic change. Prior to stage 8, benthic foraminiferal assemblages indicate low carbon flux to the seafloor, and bryozoan mounds, although present closer inshore, did not accumulate significantly at Sites 1129 and 1132, even during glacials. Our results show that the interplay of sea level change (eustatic and local, linked to platform progradation), glacial-interglacial carbon flux fluctuations (linked to local hydrographic variations), and possibly long-term climatic change strongly influenced the evolution of the Great Australian Bight carbonate margin during the late Pleistocene.

Chemical and isotope composition of poor fluid from gas-hydrate samples of ODP Site 146-892

Although the presence of extensive gas hydrate on the Cascadia margin, offshore from the western U.S. and Canada, has been inferred from marine seismic records and pore water chemistry, solid gas hydrate has only been found at one location. At Ocean Drilling Program (ODP) Site 892, offshore from central Oregon, gas hydrate was recovered close to the sediment-water interface at 2-19 m below the seafloor (mbsf) at 670 m water depth. The gas hydrate occurs as elongated platy crystals or crystal aggregates, mostly disseminated irregularly, with higher concentrations occurring in discrete zones, thin layers, and/or veinlets parallel or oblique to the bedding. A 2- to 3-cm thick massive gas hydrate layer, parallel to bedding, was recovered at ~17 mbsf. Gas from a sample of this layer was composed of both CH4 and H2S. This sample is the first mixed-gas hydrate of CH4-H2S documented in ODP; it also contains ethane and minor amounts of CO2. Measured temperatures of the recovered core ranged from 2 to -1.8°C and are 6 to 8 degrees lower than in-situ temperatures. These temperature anomalies were caused by the partial dissociation of the CH4-H2S hydrate during recovery without a pressure core sampler. During this dissociation, toxic levels of H2S (delta34S, +27.4?) were released. The delta13C values of the CH4 in the gas hydrate, -64.5 to -67.5? (PDB), together with deltaD values of -197 to -199? (SMOW) indicate a primarily microbial source for the CH4. The delta18O value of the hydrate H2O is +2.9? (SMOW), comparable with the experimental fractionation factor for sea-ice. The unusual composition (CH4-H2S) and depth distribution (2-19 mbsf) of this gas hydrate indicate mixing between a methane-rich fluid with a pore fluid enriched in sulfide; at this site the former is advecting along an inclined fault into the active sulfate reduction zone. The facts that the CH4-H2S hydrate is primarily confined to the present day active sulfate reduction zone (2-19 mbsf), and that from here down to the BSR depth (19-68 mbsf) the gas hydrate inferred to exist is a >=99% CH4 hydrate, suggest that the mixing of CH4 and H2S is a geologically young process. Because the existence of a mixed CH4-H2S hydrate is indicative of moderate to intense advection of a methane-rich fluid into a near surface active sulfate reduction zone, tectonically active (faulted) margins with organic-rich sediments and moderate to high sedimentation rates are the most likely regions of occurrence. The extension of such a mixed hydrate below the sulfate reduction zone should reflect the time-span of methane advection into the sulfate reduction zone.