Blake Ridge Research Articles

Late Holocene foraminifera of Blake Ridge diapir: Assemblage variation and stable-isotope record in gas-hydrate bearing sediments

The presence of gas hydrates on the Blake Ridge diapir , northeastern Atlantic Ocean, offers an opportunity to study the impact of methane seepage on the ecology and geochemistry of benthic foraminifera in the late Holocene . Three push cores, covering a time span of ~ 1000 yrs, were retrieved from three distinct microhabitats at the top of the diapir at a water depth of ~ 2150 m: (i) sediments away from seepage (control core), (ii) sediments overlain by clusters of methanotrophic and thiotrophic bivalves, and (iii) chemoautotrophic microbial mats . The foraminiferal assemblages at the two seep sites are marked by a reduction in benthic foraminiferal species diversity, coupled with a near-absence of agglutinated species. However, an opportunistic population rise in CH 4 - or H 2 S-tolerant calcareous species (e.g., Globocassidulina subglobosa and Cassidulina laevigata ) that utilize the abundant trophic resources at the seeps has led to an increase in the overall assemblage density there. The δ 18 O and δ 13 C values of three species of benthic foraminifera – Gyroidinoides laevigatus , Globocassidulina subglobosa , and Uvigerina peregrina – and the planktonic species Globorotalia menardii were acquired from all three cores. The benthic species from methane seeps yield δ 13 C values of 0.1 to − 4.2 (‰VPDB), that are distinctly more 13 C-depleted relative to the δ 13 C of 0.4 to − 1.0 (‰VPDB) at the control (off seep) site. The species from a mussel-bed site exhibit more negative δ 13 C values than those from microbial mats, possibly reflecting different food sources and higher rate of anaerobic oxidation of methane. The positive δ 13 C values in the paired planktonic species suggest that authigenic carbonate precipitation did not overprint the observed 13 C depletions. Hence the probable cause of negative δ 13 C of benthic foraminifera is primary calcification from Dissolved Inorganic Carbon (DIC) containing mixed carbon fractions from (a) highly 13 C-depleted, microbially-oxidized methane and (b) a seawater source. • Benthic species from methane seeps record δ 13 C negative values reflecting seepages. • The diversity is reduced at seeps by the elimination of species intolerant to gases. • The agglutinated group is decimated at the seeps.

Surface and deep-water variability at the Blake Ridge, NW Atlantic during the Plio-Pleistocene is linked to the closing of the Central American Seaway

The closing of the Central American Seaway (CAS) during the late Pliocene played an important role in global climate change as well as variation in surface and deep ocean paleoceanography. Restriction in the mixing of Pacific Ocean water with Atlantic Ocean water through the CAS ultimately led to the enhanced production of high salinity warm Gulf Stream water which flowed poleward and caused the Northern Hemisphere Glaciation (NHG) as well as increased production of deep Northern Component Water (NCW). The timing of the final closure of the CAS is still a topic of debate. The present study pursued on planktic foraminifera and carbon isotopic variation in planktic foraminifera from the Blake Ridge to understand the variation in surface and deep ocean paleoceanography and their linkages to the timing of the closing of the CAS. Planktic faunal turnover at around 2.2Ma along with the increased production of NCW at around 2.4Ma is considered as the ceasing period of the CAS. Prior to 2.2Ma, the dominance of Globigerina bulloides, Orbulina universa and Globigerinoides ruber indicates the existence of the opening between the two Americas and invasion of Pacific surface water into the Atlantic through this gateway. It is proposed that during this time the studied ODP Hole 994C was under the influence of highly productive gyre water as observed by the dominance of G. bulloides and depleted δ13C of Neogloboquadrina dutertrei (δ13CNd). The dominance of N. dutertrei and Globorotalia truncatulinoides with enriched δ13CNd from 2.2 to 1.2Ma indicates the presence of high salinity-less productive surface water over the studied Hole 994C. The closing of the CAS restricted the mixing of two ocean waters and thus the intensity of the Gulf Stream increased significantly with the dominance of NCW at the deeper level. Increased cooling and intensified Gulf Stream shifted the gyre water eastward and deepened the thermocline, and thus the studied hole was under gyre margin/Gulf Stream water during 2.2–1.2Ma. The Mid Pleistocene Transition (1.2 to 0.8Ma) is marked by the presence of high productive gyre water (depleted δ13CNd) over Hole 994C. The dominance of all the species with highly variable δ13CNd and δ18ONd during the last 0.4Ma indicates that fluctuations in surface water perhaps are linked to increased NHG.

ABUNDANCE BIOZONES AND COMMUNITY STRUCTURES IN THE UPPER QUATERNARY OF ODP HOLE 994C (BLAKE RIDGE, WESTERN ATLANTIC OCEAN) COMPARED WITH MARINE ISOTOPE STAGES, GLACIAL CYCLES, AND TERMINATIONS

Bathyal and abyssal benthonic foraminiferal assemblages have been thought to undergo community- and species-level regime shifts between glacials and interglacials. Comparisons between communities and oxygen isotope curves are, however, few. Comparisons between multiple aspects of the benthonic foraminiferal community and the oxygen isotope curve are made here for the abyssal Upper Quaternary of ODP Hole 994C (Blake Ridge, western Atlantic Ocean). Presumed allochthonous species were removed from the dataset to give an in situ abyssal assemblage dominated by Brizalina lowmani with subordinate Cibicides wuellerstorfi and other species. The total number N of benthonic foraminifera/20 cm3 sediment sample, the point diversity measured using the information function H, and max(pi) (the proportional abundance of the most abundant species in each sample) all fluctuated between samples. These variables apparently mirrored the oxygen-isotope curve based on planktonic foraminifera, and so show potential for tracking marine isotope stages (MIS). However, neither abundance biozone boundaries nor species-level regime shifts (i.e., points of change in percentage carrying capacity, Kp) coincided or showed consistent associations with terminations (pronounced boundaries between isotopic maxima and minima). Cibicides wuellerstorfi was most abundant towards the base of the section, which might indicate stronger current action. Most abundance biozones (ABs) had a Type 0 community structure, but Type 1 community structure was recorded in ABs from the upper part of Glacial Cycle E (glacial MIS 10) and the lower part of Glacial Cycle D (interglacial MIS 9), for which this community structure may mark an extended period of stress. Peaks in percentage abundance of Uvigerina spp. occurred in glacial MIS 2, 6, and 8, but not in MIS 4, 10, and 12. It is, therefore, recommended that workers be cautious when using glacial-interglacial cycles to explain ecostratigraphic events in abyssal Pleistocene successions, especially in the absence of an oxygen-isotope curve.

Methane Hydrate and Two-Dimensional Fluid Transport Model: Comparison with Blake Ridge Chlorinity Measurements

The potential benefits and risks of natural methane hydrate occurrences have raised the need to understand the processes governing hydrate formation and dissociation over the last decades, and models of increasing complexity have been developed for this purpose. We propose a formulation of a multi-dimensional methane hydrate model that couples the established chemistry of hydrate formation to the fluid flow in marine sediments undergoing compaction. The numerical model applies the Finite Volume Method to construct a segregated solver for the coupled system. The solution of the sequence of individual processes is stabilised with an adaptive Picard iteration in each time step. We implement the model based on the OpenFOAM library and extend the functionality by generalising the diffusion term to take into account variable porosity of the sediment matrix. The advantage of this robust and efficient scheme is that the formulation is conservative by construction, allowing an accurate solution of mass transport when strong concentration gradients develop in the vicinity of phase boundaries. We validate the model using data from the Blake Ridge hydrate province. Good agreement is found for the pore water chlorinity, a proxy for hydrate formation, except for regions where heterogeneous hydrate formation results in highly variable measurements.

Quantifying Assemblage Turnover and Species Contributions at Ecologic Boundaries

Not all boundaries, whether stratigraphical or geographical, are marked by species-level changes in community composition. For example, paleodata for some sites do not show readily discernible glacial-interglacial contrasts. Rather, the proportional abundances of species can vary subtly between glacials and interglacials. This paper presents a simple quantitative measure of assemblage turnover (assemblage turnover index, ATI) that uses changes in species' proportional abundances to identify intervals of community change. A second, functionally-related index (conditioned-on-boundary index, CoBI) identifies species contributions to the total assemblage turnover. With these measures we examine benthonic foraminiferal assemblages to assess glacial/interglacial contrasts at abyssal depths. Our results indicate that these measures, ATI and CoBI, have potential as sequence stratigraphic tools in abyssal depth deposits. Many peaks in the set of values of ATI coincide with terminations at the end of glaciations and delineate peak-bounded ATI intervals (PATIs) separated by boundaries that approximate to glacial terminations and to transgressions at neritic depths. These measures, however, can be used to evaluate the assemblage turnover and composition at any defined ecological or paleoecological boundary. The section used is from Ocean Drilling Program (OPD) Hole 994C, drilled on the Blake Ridge, offshore SE USA.

High-resolution record of the Laschamp geomagnetic excursion at the Blake-Bahama Outer Ridge

SUMMARY Geomagnetic excursions are brief deviations of the geomagnetic field from behaviour expected during ‘normal secular’ variation. The Laschamp excursion at ∼41ka was one such deviation. Previously published records suggest rapid changes in field direction and a concurrent substantial decrease in field intensity associated with this excursion. Accurate dating of excursions, and determination of their durations from multiple locations, is vital to our understanding of global field behaviour during these deviations. We present here high-resolution palaeomagnetic records of the Laschamp excursion obtained from two Ocean Drilling Program (ODP) Sites, 1061 and 1062 on the Blake-Bahama Outer Ridge (ODP Leg 172). High sedimentation rates (∼30‐40cmkyr −1 ) at these locations allow determination of transitional field behaviour during the excursion. Palaeomagnetic measurements of discrete samples from four cores reveal a single excursional feature, across an interval of 30cm, associated with a broader palaeointensity low. We determine the age and duration of the Laschamp excursion using a stratigraphy linked to the δ 18 O record from the Greenland ice cores. This chronology dates the Laschamp excursion at the Blake Ridge to 41.3ka. The excursion is characterized by rapid transitions (less than 200yr) between stable normal polarity and a partially reversed polarity state. The palaeointensity record is in good agreement between the two sites, revealing two prominent minima. The first minimum is associated with the Laschamp excursion at 41ka and the second corresponds to the Mono Lake excursion at ∼35.5ka. We determine that the directional excursion during the Laschamp at this location was no longer than ∼400yr, occurring within a palaeointensity minimum that lasted 2000yr. The Laschamp excursion at this location is much shorter in duration than the Blake and Iceland Basin excursions.

Millennial-scale versus long-term dynamics in the surface and subsurface of the western North Atlantic Subtropical Gyre during Marine Isotope Stage 5

Subtropical Gyres are an important constituent of the ocean–atmosphere system due to their capacity to store vast amounts of warm and saline waters. Here we decipher the sensitivity of the (sub)surface North Atlantic Subtropical Gyre with respect to orbital and millennial scale climate variability between ~140 and 70ka, Marine Isotope Stage (MIS) 5. Using (isotope) geochemical proxy data from surface and thermocline dwelling foraminifers from Blake Ridge off the west coast of North America (ODP Site 1058) we show that the oceanographic development at subsurface (thermocline) level is substantially different from the surface ocean.Most notably, surface temperatures and salinities peak during the penultimate deglaciation (Termination II) and early MIS 5e, implying that subtropical surface ocean heat and salt accumulation might have resulted from a sluggish northward heat transport. In contrast, maximum thermocline temperatures are reached during late MIS 5e when surface temperatures are already declining. We argue that the subsurface warming originated from intensified Ekman downwelling in the Subtropical Gyre due to enhanced wind stress.During MIS 5a-d a tight interplay of the subtropical upper ocean hydrography to high latitude millennial-scale cold events can be observed. At Blake Ridge, the most pronounced of these high latitude cold events are related to surface warming and salt accumulation in the (sub)surface. Similar to Termination II, heat accumulated in the Subtropical Gyre probably due to a reduced Atlantic Meridional Overturning Circulation. Additionally, a southward shift and intensification of the subtropical wind belts lead to a decrease of on-site precipitation and enhanced evaporation, coupled to intensified gyre circulation. Subsequently, the northward advection of this warm and saline water likely contributed to the fast resumption of the overturning circulation at the end of these high latitude cold events.

Evidence for extensive methane venting on the southeastern U.S. Atlantic margin

We present the first evidence for widespread seabed methane venting along the southeastern United States Atlantic margin beyond the well-known Blake Ridge diapir seep. Recent ship- and autonomous underwater vehicle (AUV)–collected data resolve multiple water-column anomalies (>1000 m height) and extensive new chemosynthetic seep communities at the Blake Ridge and Cape Fear diapirs. These results indicate that multiple, highly localized fluid conduits punctuate the areally extensive Blake Ridge gas hydrate province, and enable the delivery of significant amounts of methane to the water column. Thus, there appears to be an abundance of seabed fluid flux not previously ascribed to the Atlantic margin of the United States.

Use of cosmogenic 129I to constrain numerical models of fluid flow in marine sediments: Application to the Blake Ridge Hydrate Province

AbstractConcentrations of cosmogenic iodine, 129I, in the pore fluid of marine sediments often indicate that the pore fluid is much older than the host sediment, even when vertical flow due to sediment compaction is taken into account. Old pore fluid has been used in previous studies to argue for pervasive upward fluid flow and a deep methane source for hydrate deposits. Alternatively, old pore fluid age may reflect more complex flow patterns. We use a two‐dimensional numerical transport model to account for the effects of topography and fractures on pore fluid pathlines when sediment permeability is anisotropic. We find that fluid focusing can cause significant lateral migration as well as regions where downward flow reverses direction and returns toward the seafloor. Longer pathlines can produce pore fluid ages much older than that expected with a one‐dimensional compaction model. For steady‐state models with geometry representative of Blake Ridge (USA), a well‐studied hydrate province, we find pore fluid ages beneath regions of topography and within fractured zones that are up to 70 Ma old. Our results suggest that the measurements of 129I/127I reflect a mixture of new and old pore fluid. However, old pore fluid need not originate at great depths. Methane within pore fluids can travel laterally several kilometers, implying an extensive source region around the deposit. This type of focusing should aid hydrate formation beneath topographic highs.

Cold-seep habitat mapping: High-resolution spatial characterization of the Blake Ridge Diapir seep field

Relationships among seep community biomass, diversity, and physiographic controls such as underlying geology are not well understood. Previous efforts to constrain these relationships at the Blake Ridge Diapir were limited to observations from piloted deep-submergence vehicles. In August 2012, the autonomous underwater vehicle (AUV) Sentry collected geophysical and photographic data over a 0.131km2 area at the Blake Ridge Diapir seeps. A nested survey approach was used that began with a regional or reconnaissance-style survey using sub-bottom mapping systems to locate and identify seeps and underlying conduits. This survey was followed by AUV-mounted sidescan sonar and multibeam echosounder systems mapping on a mesoscale to characterize the seabed physiography. At the most detailed survey level, digital photographic imaging was used to resolve sub-meter characteristics of the biology. Four pockmarks (25–70m diameter) were documented, each supporting chemosynthetic communities. Concentric zonation of mussels and clams suggests the influence of chemical gradients on megafaunal distribution. Data collection and analytical techniques used here yield high-resolution habitat maps that can serve as baselines to constrain temporal evolution of seafloor seeps, and to inform ecological niche modeling and resource management.

Are 34S-enriched authigenic sulfide minerals a proxy for elevated methane flux and gas hydrates in the geologic record?

The sulfate–methane transition (SMT) zone is a diagenetic transition within anoxic marine sediments created by the metabolic activity of a consortium of sulfate-reducing bacteria and methane-oxidizing Archaea. As interstitial dissolved sulfate is consumed by microbially mediated sulfate reduction of sedimentary organic matter (SOM) and anaerobic oxidation of methane (AOM) large enrichments of 34S occur in the interstitial sulfate pool. These isotopic enrichments are transmitted to the dissolved sulfide pool (∑HS−) and subsequently into sulfide minerals (So, ∼FeS, FeS2).We investigate the sulfur isotopic composition of pore-water sulfate and sulfide minerals at three sites underlain by gas hydrates at the Blake Ridge. The isotopic composition of sulfate-sulfur is most positive at the SMT showing maximum values of +29.1, 49.6, 51.6‰ VCDT at each of the respective sites. δ34S values of bulk sulfide minerals tend to be more enriched in 34S at and below the SMT ranging from −12.7 to +23.6‰, corresponding to enrichments of 26.7–62.4‰ relative to the mean value of −38.8‰ in the sulfate reduction zone. Both enhanced delivery of methane to the SMT, and non-steady-state sedimentation appear necessary to create large 34S enrichments in sulfide minerals. Similar associations of AOM and large δ34S enrichments (>0‰) occur in other gas hydrate terranes (Cascadia margin) but their exact origin is equivocal at present. An analysis of δ34S data from freshwater and marine sedimentary environments reveals that 34S enrichments within sulfide minerals occur under a range of conditions, but are statistically associated with AOM and systems not limited by dissolved interstitial iron.In methane-rich sediments, methane delivery to the SMT increases the role of AOM in sulfate depletion that impacts the formation and isotopic composition of authigenic sulfide minerals. We hypothesize that under certain diagenetic conditions large 34S enrichments within sulfide minerals in the geologic record potentially identify: (1) the former occurrence of AOM (2) present-day and “fossil” locations of the sulfate–methane transition zone; and (3) a diagenetic terrane, today characteristic of deep-water, methane-rich, marine sediments conducive to gas hydrate formation. Thus, 34S-enriched sulfide minerals preserved in modern and ancient continental-margin sediments may allow for the identification of AOM-related processes that occur in methane-rich sediments.

The Distribution of Larval Fishes of the Charleston Gyre Region off the Southeastern United States in Winter Shaped by Mesoscale, Cyclonic Eddies

AbstractSerial, cyclonic, mesoscale eddies arise just north of the Charleston Bump, a topographical rise on the continental slope and Blake Plateau, and characterize the U.S. outer shelf and upper slope in the region of the Charleston Gyre. This region was transected during the winters of 2000, 2001, and 2002, and hydrographic data and larval fishes were collected. The hydrodynamics of the cyclonic eddies of the Charleston Gyre shape the distribution of larval fishes by mixing larvae from the outer continental shelf and the Gulf Stream and entraining them into the eddy circulation at the peripheral margins, the wrap‐around filaments. Over all years and transects (those that intercepted eddies and those that did not), chlorophyll a concentrations, zooplankton displacement volumes, and larval fish concentrations were positively correlated. Chlorophyll a concentrations were highest in filaments that wrapped around eddies, and zooplankton displacement volumes were highest in the continental shelf–Gulf Stream–frontal mix. Overall, the concentration of all larval fishes declined from inshore to offshore with highest concentrations occurring over the outer shelf. Collections produced larvae from 91 fish families representing continental shelf and oceanic species. The larvae of shelf‐spawned fishes—Atlantic Menhaden Brevoortia tyrannus, Round Herring Etrumeus teres, Spot Leiostomus xanthurus, and Atlantic Croaker Micropogonias undulatus—were most concentrated over the outer shelf and in the continental shelf–Gulf Stream–frontal mix. The larvae of ocean‐spawned fishes—lanternfishes, bristlemouths, and lightfishes—were more evenly dispersed in low concentrations across the outer shelf and upper slope, the highest typically in the Gulf Stream and Sargasso Sea, except for lightfishes that were highest in the continental shelf–Gulf Stream–frontal mix. Detrended correspondence analysis rendered groups of larval fishes that corresponded with a gradient between the continental shelf and Gulf Stream and Sargasso Sea. Eddies propagate northeastward with a residence time on the outer shelf and upper slope of ∼1 month, the same duration as the larval period of most fishes. The pelagic habitat afforded by eddies and fronts of the Charleston Gyre region can be exploited as nursery areas for feeding and growth of larval fishes within the southeastern Atlantic continental shelf ecosystem of the U.S. Eddies, and the nursery habitat they provide, translocate larvae northeastward.Received March 18, 2013; accepted June 21, 2013

Tomography of diffraction-based focusing operators

Diffractions carry the same kinematic information provided by common focus point operators (CFPOs). Thus CFPO and diffraction time trajectories may be used separately, or combined into a single unified tomography for velocity analysis. Velocity estimation by tomography of CFPOs reduces the depth-velocity ambiguity compared to two-way time tomography. CFPO estimation is complicated where there are event discontinuities and diffractions. This problem is overcome by using the kinematic information in diffractions in near-offset common-offset gathers. The procedure is illustrated using synthetic data, and a single-channel field seismic profile from the Blake Ridge (off the east coast of the United States). The results show the effectiveness of the proposed method for estimation of velocity from single channel seismic data, and for refinement of the velocity field from multichannel data. Both applications are cost-competitive.

Estimation of free gas saturation from seismic reflection surveys by the genetic algorithm inversion of a P-wave attenuation model

Many previously proposed methods of estimating free gas saturation from seismic survey data rely on calibration to invasively collected, in situ measurements. Typically, such in situ measurements are used to parameterize or calibrate rock-physics models, which can then be applied to seismic data to achieve saturation estimates. We tested a technique for achieving estimates of the spatial distribution of gas saturation solely from shipboard seismic surveys. We estimated the quality factor from seismic reflection surveys using the spectral ratio method, and then inverted a mesoscopic-scale P-wave attenuation model to find the parameters that matched the modeled attenuation to our estimates of observed attenuation within the range of seismic frequencies. By using a genetic algorithm for this inversion, we not only searched efficiently for a global solution to the nonlinear set of equations that compose the model, but also constrain the search to a relatively broad set of realistic parameter values. Thus, our estimates do not rely on in situ measurements of these parameters, but on distributions of their possible values, many of which may be referenced from literature. We first tested this method at Blake Ridge, offshore North and South Carolina, where an approximately 400-m-deep gas-saturated zone underlies a field of methane hydrates. The extensive field work and subsequent studies at this site make it ideal for validating our method. We also demonstrated the applicability of our method to shallower deposits by presenting results from Finneidfjord, Norway, where the inversion of the P-wave attenuation model recognizes very small gas saturations.

Factors influencing the porosity of gas hydrate bearing sediments

Porosity is a key parameter in calculating the velocity of gas hydrate bearing sediments and quantifying the amount of gas hydrate. The variation of porosity is affected by many factors. The influences of different factors on porosity are distinct. The purpose of this paper is to analyze the main factors that affect the overall and local change of porosity in marine sediments where gas hydrate was sampled. Porosity logs were collected from ODP Leg 164, Blake Ridge, ODP Leg 204, Hydrate Ridge, and IODP expedition 311, Cascadia Margin. Based on the characteristic of porosity variation in depth, porosity was divided into three components: low frequency component, middle frequency component, and high frequency component. The factors influencing each component were discussed. From the analysis, we observed that the porosity of unconsolidated sediment was very high, and the decreasing trend of low frequency component versus depth was affected by compaction. In addition, the initial porosity and slope of low frequency component variation were affected by the content of fine grain and geothermal gradient respectively. The middle component could reflect the variation of lithology, which was affected by the content variation of different sized grains and gas hydrate. The high frequency component was affected by the frequent change of grain size. The existence of volcanic ash-rich sand caused a high value to the high frequency component. The results are applicable to porosity evaluation in gas hydrate bearing sediments.

Weak and segmented bottom simulating reflections on the Hikurangi Margin, New Zealand — Implications for gas hydrate reservoir rocks

The quality of reservoir rocks, in particular their permeability, is likely to be a key factor for the economic viability of future gas production from gas hydrates. As for conventional gas resources, high-permeability sands are considered the economically most promising gas hydrate reservoirs. Studies of subsurface lithology however, are difficult without calibration from boreholes. We investigated seismic data from the Hikurangi Margin, a subduction zone east of New Zealand and New Zealand's largest gas hydrate province. We suggest that the strength of bottom simulating reflections (BSRs) from the base of the gas hydrate stability zone may support lithologic interpretations on this margin.BSRs along large parts of this margin are exceptionally weak. Absolute reflection coefficients of a weak BSR on Puke Ridge, a thrust ridge in the accretionary wedge, are roughly between 0.01 and 0.02, an order of magnitude lower than those observed for many BSRs globally. A combination of rock physics modelling and seismic amplitude-versus-offset analysis leads to the conclusion that these weak BSRs are primarily caused by low saturation of gas with patchy distribution, i.e., gas that is only present in pores or fractures of some mesoscopic (i.e., larger than pore sizes but smaller than seismic wavelengths) sediment patches while other patches are fully water saturated. This type of distribution, combined with observed high seismic velocities, is compatible with lithified fine-grained reservoir rocks, similar to indurated mudstones dredged from a submarine outcrop close to the study area. We therefore suggest that weak BSRs may mark fine-grained reservoir rocks with usually low primary permeability. Even though these reservoir rocks may exhibit enhanced secondary permeability from fracturing, they would currently not be considered prime candidates for potential gas production from hydrates.We identified several high amplitude bright spots along weak BSRs. Two possible lithologic explanation for this reflection pattern are that (1) the bright spots mark higher saturation of gas in high-permeability, probably sand-dominated layers, as found in the Gulf of Mexico, and (2) evenly distributed networks of pores may result in gas to be distributed more homogenously in the sediments even though permeability may still be relatively low, as suggested for highly reflective layers beneath the Blake Ridge. Elevated gas hydrate saturations above layers with high saturations of gas would be expected to lead to highly reflective layers in the gas hydrate stability zone and thus, reflection patterns above the BSR may allow distinguishing between both causes.

Geochemical characteristics of pore water in shallow sediments from north continental slope of South China Sea and their significance for natural gas hydrate occurrence

Northern slope of the South China Sea is a potential area of gas hydrates and research focus areas. Analyzed geochemical parameters about pore water of the anions and cations of major components and δ 13 C DIC in the sediments from Shenhu sea area, Dongsha sea area and Southwest Taiwan Basin and collected by gravity piston. The results showed that Cl − concentration were no significant changes with depth at the three stations and their values were consistent with the normal sea water, SO 4 2- concentration showed significant changes in gradient descent from core top to bottom at three stations and Ca 2+ , Mg 2+ , Ca 2+ /Mg 2+ showed a similar downward trend with SO 4 2− concentration, but the amplitude is inconsistent. The SMI of Shenhu sea area, Dongsha sea area, Southwest Taiwan Basin were separately 11m, 8m, 6m and δ 13 C DIC of pore water in the sediments range from -10 ‰ to -27 ‰, which were similar with geochemistry characteristics of pore water in the sediments at other international regions of being found gas hydrate, such as Blake Ridge and Mexico Gulf. These geochemistry characteristics may suggest that natural gas hydrate reservoir released methane in deep sediments and methane was mixture gas as main causes of pyrolysis. Summary a series anomaly indicators and identification methods that use these geochemical anomalies characteristics about pore water of shallow sediments to trace gas hydrate in deep stratum.

Changes in planktic foraminiferal faunas, temperature and salinity in the Gulf Stream during the last 30,000 years: influence of meltwater via the Mississippi River

Variations in the distribution of planktic foraminiferal faunas, temperature and salinity in the surface water of the Gulf Stream during the last 30,000 years have been reconstructed based on a sediment core from the Blake Ridge, subtropical western Atlantic Ocean. Temperatures and salinities were calculated using planktic foraminiferal transfer functions and previously published oxygen isotope values. Productivity was estimated using the >106 μm size fraction of planktic foraminifera. The reconstructed values show that temperature variations have been small during the investigated period with a total range of only 3 °C. The highest temperatures, about 1.5 °C above the present temperature in the area, occurred during the late Glacial, Heinrich event 1 and the mid Holocene time periods. The lowest temperatures, about 1.5 °C below the present temperatures, occurred during the Bølling interstadial. The early Holocene period was relatively cool. The low temperatures during the Bølling interstadial and during the early Holocene are in contrast to the northeastern Atlantic, where the Bølling interstadial was the warmest period of the deglaciation, and the early Holocene the warmest during the Holocene. We attribute the lower temperatures during the Bølling and early Holocene periods to colder meltwater from the Laurentide Ice Sheet flowing into the Gulf of Mexico through the Mississippi River system and carried to the Blake Ridge via the Gulf Stream.

Temperature dependence of organic matter remineralization in deeply-buried marine sediments

Assuming that the temperature dependence of sediment organic matter remineralization can be described by the Arrhenius equation, organic matter that is highly refractory at seafloor temperatures (~ 2–3 °C) should become more reactive at sediment depths of several hundred meters due to burial and heating by the natural geothermal gradient. Results obtained using a coupled non-linear reactive-transport model support this suggestion. For deeply-buried marine sediments (i.e., those found 100s of meters below the seafloor) model results predict the occurrence of a deep zone of methanogenesis that is separated by a relatively thick region in which methane diffuses upwards to be oxidized by downward diffusing sulfate. This depth zonation of biogeochemical processes in deeply-buried sediments is in sharp contrast to that observed in nearshore marine sediments where sulfate reduction and methanogenesis generally occur in much closer vertical proximity. Model results presented here also provide a simple mechanistic explanation for the occurrence of linear pore water sulfate profiles that are common in many continental margin sediments. Linear sulfate gradients are often taken as an indirect indicator of gas hydrate occurrence although results presented here suggest that this does not necessarily always have to be the case. Application of the model to gas hydrate-containing sediments on the Blake Ridge yields results that are in good agreement with sediment and pore water data, and microbial rate studies in these sediments. Model results also suggest that in situ methanogenesis in and/or near the gas hydrate stability zone may play an important role in supplying the methane found in Blake Ridge gas hydrates.

Capillary effects on hydrate stability in marine sediments

[1] We study the three-phase (Liquid + Gas + Hydrate) stability of the methane hydrate system in marine sediments by considering the capillary effects on both hydrate and free gas phases. The capillary pressure, a measure of the pressure difference across a curved phase interface, exerts a key control on the methane solubility in Liquid + Hydrate (L + H) and Liquid + Gas (L + G) systems. By calculating the L + H and L + G solubilities as a function of water depth (pressure) and pore size (interface curvature), we show how the solubility requirements for forming both gas hydrate and free gas can be met in a three-phase zone. The top of the three-phase zone shifts upward in sediments as the water depth increases and the mean pore size decreases. The thickness of the three-phase zone increases as the distribution of pore sizes widens. The top of the three-phase zone can overlie or underlie the bulk three-phase equilibrium depth. At Blake Ridge, we predict that the three-phase zone is 27.7 m thick and that the top of the three-phase zone lies 13 m above the predicted bulk equilibrium depth. This reconciles the observation of the bottom-simulating reflector (BSR) at Blake Ridge that is shallower than the predicted bulk equilibrium depth. In contrast, at Hydrate Ridge where water depth is shallower, we predict that the three-phase zone is 20.4 m thick and that the top of the three-phase zone lies 0.7 m below the predicted bulk equilibrium depth. Our model, which predicts an upward shift in the top of free gas occurrence with increasing water depth (pressure), is compatible with worldwide observations that the BSR is systematically shifted upward relative to the bulk equilibrium depth as water depth (pressure) is increased.