Search

Environmental DNA (eDNA) can be used to identify macroorganisms and describe biodiversity, and thus has promise to supplement biological monitoring in marine ecosystems. Despite this promise, scaling sample acquisition to the spatial and temporal scales needed for effective monitoring would require prohibitively large investments in time and human resources. To address this challenge, we evaluated the efficacy of an autonomous eDNA sampling system and compare results obtained to traditional eDNA sampling methods. The autonomous sampling instrument consisted of the Environmental Sample Processor (ESP) coupled to an autonomous underwater vehicle (AUV). We tested equivalency between the ESP and traditional eDNA sampling techniques by comparing the quantification of eDNA across a broad range of taxa, from microbes (SAR11), phytoplankton (Pseudo-nitzschia spp.), and invertebrates (krill: Euphausia pacifica) to vertebrates (anchovy: Engraulis mordax). No significant differences in eDNA densities were observed between the two sample collection and filtration methods. eDNA filters collected by the ESP were preserved and stable for 21 days, the typical deployment length of the instrumentation. Finally, we demonstrated the unique capabilities of an autonomous, mobile ESP during a deployment near Monterey Bay, CA, by remotely and repeatedly sampling a water mass over 12 h. The development of a mobile ESP demonstrates the promise of utilizing eDNA measurements to observe complex biological processes in the ocean absent a human presence.

The overarching logistical challenge in microbial oceanography is acquiring enough samples to provide meaningful scientific interpretation. The number of samples collected during ship expeditions is limited by weather, time on station, and budget. Here we describe a robotic, autonomous vehicle platform equipped with a unique sampling instrument that mitigates some of these constraints. In a joint cruise on the R/V Falkor, the Monterey Bay Aquarium Research Institute and the University of Hawaii deployed two of these vehicles in a mesoscale eddy north of the island of Maui. One vehicle collected contextual measurements while circling a freely drifting sampling vehicle. On the sampling vehicle we implemented several behaviors, including sampling every three hours for a 4-day underwater drift while maintaining position within the deep chlorophyll maximum layer (~100m). Results demonstrate the ability to remain with features of interest and point to an exciting future of long-term, directed, persistent sampling.

The documented presence of the pesticide DDT (1,1,1-trichloro-2,2-bis( p -chlorophenyl)ethane) and its metabolites DDE (1,1-dichloro-2,2-bis( p -chlorophenyl)ethylene), and DDD (1,1-dichloro-2,2-bis( p -chlorophenyl)ethane), collectively referred to as ΣDDT, in sediments from the Monterey Submarine Canyon, Monterey Fan Channel and Monterey Fan indicates that during the present sea-level highstand the Monterey Fan has been an active depositional system. The presence of ΣDDT in marine sediment is an indication of the recent arrival of terrigenous fine-grained sediment because DDT was only applied on land in California between 1944 and 1972, and is being transported to the ocean in the fine-grained sediment fraction of rivers. ΣDDT was found in 81 out of 82 cores collected in water depths deeper than 3000 m in the Monterey Fan Channel and in the Monterey Fan, within a geographical extent of 250 km to the south and 210 km to the west of the Monterey Canyon head, with surface concentrations averaging 6 and 3 ng g − 1 , in the upper and lower Monterey Fan, respectively. This is the first documentation of ΣDDT presence in ocean sediments below 3000 m water depth worldwide. The estimated total amount of ΣDDT in the fan is consistent with the amount of ΣDDT discharged by the Salinas and Pajaro Rivers since DDT started being used in California, suggesting that current fine-grained sediment discharge from these rivers has been accumulating in the fan. The ratio DDE/ΣDDT, an indication of the extent of DDT degradation, provides a first-order proxy for the relative age of the sediments. DDT is found along the thalweg of the Monterey Fan channel as far as 215 km from the canyon head, indicating recent fine-grained sediment transport along the channel. In contrast, only DDE was detected in sediments on the fan, which suggests that the fan does not accumulate fine-grained sediments at a constant rate.

Despite years of research into microbial activity at diffuse flow hydrothermal vents, the extent of microbial niche diversity in these settings is not known. To better understand the relationship between microbial activity and the associated physical and geochemical conditions, we obtained co-registered metatranscriptomic and geochemical data from a variety of different fluid regimes within the ASHES vent field on the Juan de Fuca Ridge. Microbial activity in the majority of the cool and warm fluids sampled was dominated by a population of Gammaproteobacteria (likely sulfur oxidizers) that appear to thrive in a variety of chemically distinct fluids. Only the warmest, most hydrothermally-influenced flows were dominated by active populations of canonically vent-endemic Epsilonproteobacteria. These data suggest that the Gammaproteobacteria collected during this study may be generalists, capable of thriving over a broader range of geochemical conditions than the Epsilonproteobacteria. Notably, the apparent metabolic activity of the Gammaproteobacteria—particularly carbon fixation—in the seawater found between discrete fluid flows (the intra-field water) suggests that this area within the Axial caldera is a highly productive, and previously overlooked, habitat. By extension, our findings suggest that analogous, diffuse flow fields may be similarly productive and thus constitute a very important and underappreciated aspect of deep-sea biogeochemical cycling that is occurring at the global scale.

Episodic 13C depletions in the carbon isotopic composition of benthic foraminiferal tests preserved in the stratigraphic record have been interpreted as an active incorporation of methane-derived carbon. Understanding the extent to which these isotope excursions reflect basin-wide fluxes of methane carbon to bottom waters versus a local supply of methane carbon within the sediments in which benthic foraminifera live, or a postmortem diagenetic imprint is critical to the interpretation of δ13C paleoceanographic proxies. Here we evaluate the impact of chemical gradients measured in pore waters adjacent to active methane vents on carbon assimilation by living benthic foraminifera and show that those living near methane vents do not assimilate the distinctly 13C depleted methane-derived dissolved inorganic carbon into their tests from the pore water in which they were found. Our observations can be explained by the recently articulated physiological limits imposed on deep-sea fauna by low-oxygen and high-pCO2 environments. Understanding the importance of the different processes involved in the observed disequilibrium between the carbon isotopic composition of the benthic forams and the pore waters where they were found has important implications on the reliability of carbon isotopic composition of benthic foraminifera for paleoceanographic reconstructions. In particular, the observation on the inhospitability of these environments for benthic foraminifera at least for reproduction and growth raises the issue on the overprint either in the late adult stages of foraminifera that grew in a different neighboring environment or during early diagenesis in these geochemically active environments.

While submarine canyons are the major conduits through which sediments are transported from the continents out into the deep sea, the time it takes for sediment to pass down through a submarine canyon system is poorly constrained. Here we report on the first study to couple optically stimulated luminescence (OSL) ages of quartz sand deposits and accelerator mass spectrometry 14 C ages measured on benthic foraminifera to examine the timing of sediment transport through the axial channel of Monterey Submarine Canyon and Fan, offshore California. The OSL ages date the timing of sediment entry into the canyon head while the 14 C ages of benthic foraminifera record the deposition of hemipelagic sediments that bound the sand horizons. We use both single-grain and small (∼2 mm area) single-aliquot regeneration approaches on vibracore samples from fining-upward sequences at various water depths to demonstrate relatively rapid, decadal-scale sand transport to at least 1.1 km depth and more variable decadal- to millennial-scale transport to a least 3.5 km depth on the fan. Significant differences between the time sand was last exposed at the canyon head (OSL age) and the timing of deposition of the sand (from 14 C ages of benthic foraminifera in bracketing hemipelagic sediments) are interpreted as indicating that the sand does not pass through the entire canyon instantly in large individual events, but rather moves multiple times before emerging onto the fan. The increased spread in single-grain OSL dates with water depth provides evidence of mixing and temporary storage of sediment as it moves through the canyon system. The ages also indicate that the frequency of sediment transport events decreases with distance down the canyon channel system. The amalgamated sands near the canyon head yield OSL ages that are consistent with a sub-decadal recurrence frequency while the fining-upward sand sequences on the fan indicate that the channel is still experiencing events with a 150–250 year recurrence frequency out to 3.5 km water depths.

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 34 S occur in the interstitial sulfate pool. These isotopic enrichments are transmitted to the dissolved sulfide pool (∑HS − ) and subsequently into sulfide minerals (So, ∼FeS, FeS 2 ). 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. δ 34 S values of bulk sulfide minerals tend to be more enriched in 34 S 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 34 S enrichments in sulfide minerals. Similar associations of AOM and large δ 34 S enrichments (>0‰) occur in other gas hydrate terranes (Cascadia margin) but their exact origin is equivocal at present. An analysis of δ 34 S data from freshwater and marine sedimentary environments reveals that 34 S 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 34 S 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, 34 S-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.

Diverse copper-containing membrane-bound monooxygenase-encoding sequences (Cu-MMOs) have recently been described from the marine environment, suggesting widespread potential for oxidation of reduced substrates. Here, we used the well-defined oxygen and methane gradients associated with the Costa Rican oxygen minimum zone (OMZ) to gain insight into the physico-chemical parameters influencing the distribution and abundance of Cu-MMO-encoding marine microorganisms. Two Methylococcales-related Cu-MMO-encoding lineages, termed groups OPU1 and OPU3, demonstrated differences in their relative abundance, with both pmoA and candidate 16S rRNA genes correlating significantly with reduced environmental oxygen concentrations and depth. In contrast, a newly identified Cu-MMO-encoding lineage, Group C, was primarily associated with the oxygenated euphotic zone. An updated phylogenetic analysis including these sequences, a marine pxmABC gene cluster, ethylene-utilizing Cu-MMO-encoding lineages and previously reported planktonic Cu-MMOs (Groups W, X, Z and O) demonstrates the breadth of diversity of Cu-MMO-encoding marine microorganisms. Groups C and X affiliated phylogenetically with ethane- and ethylene-oxidizing Cu-MMOs, Groups W and O affiliated phylogenetically with the recently described Cu-MMO ‘pXMO’, and Group Z clustered with Cu-MMOs recovered from soils. Collectively, these data demonstrate widespread genetic potential in ocean waters for the oxidation of small, reduced molecules and advance our understanding of the microorganisms involved in methane cycling in the OMZ environment.

Upon their initial discovery, hydrothermal vents and methane seeps were considered to be related but distinct ecosystems, with different distributions, geomorphology, temperatures, geochemical properties and mostly different species. However, subsequently discovered vents and seep systems have blurred this distinction. Here, we report on a composite, hydrothermal seep ecosystem at a subducting seamount on the convergent Costa Rica margin that represents an intermediate between vent and seep ecosystems. Diffuse flow of shimmering, warm fluids with high methane concentrations supports a mixture of microbes, animal species, assemblages and trophic pathways with vent and seep affinities. Their coexistence reinforces the continuity of reducing environments and exemplifies a setting conducive to interactive evolution of vent and seep biota.

Crescent-shaped bedforms with wavelengths from 20 to 80 m, amplitudes to 2.5 m, and concave down-canyon crests occur in the axial channel of Monterey Canyon (offshore California, USA) in water depths from 11 to more than 350 m. The existence of these features may be an important new clue as to how sediment moves through submarine canyons. Three complementary studies were initiated in 2007 to understand the origin and evolution of these bedforms. (1) Vibracoring. Three transects of closely spaced remotely operated vehicle–collected vibracores were obtained across these bedforms. The seafloor underneath these features is composed of gravity-flow deposits. (2) Acoustic array. Three boulder-sized concrete monuments containing acoustic beacons were buried just below the surface of the canyon floor in ∼290 m water depth and their locations were redetermined on 17 subsequent occasions. Although the beacons became more deeply buried >0.6 m below the seafloor, they still could be tracked acoustically. Over a 26-month period the position of 1 or more of the beacons moved down-canyon during at least 6 discrete transport events for a total displacement of 994–1676 m. The movement and burial of the monuments suggest that the seabed was mobilized to >1 m depth during gravity-flow events. (3) Autonomous underwater vehicle (AUV) repeat mapping. AUV-acquired high-resolution multibeam mapping, and CHIRP (compressed high-intensity radar pulse) subbottom profiling surveys of the seafloor in the active channel were repeated four times in the first half of 2007. In addition, the movement of large instrument frames deployed in 2001–2003 within the axis of Monterey Canyon in the area now known to be associated with the crescent-shaped bedforms is documented. The fate of the frames has helped elucidate the frequency, transport potential, and processes occurring within the axis of Monterey Canyon associated with these bedforms. The crescent-shaped bedforms appear to be produced during brief gravity-flow events that occur multiple times each year, commonly coincident with times of large significant wave heights. Whether the bedforms are generated by erosion associated with cyclic steps in turbidity flows or internal deformation associated with slumping during gravity-flow events remains unclear.

Deposits within the floor of the Norwegian Basin were sampled to characterize the deposition from the Storegga Slide, the largest known Holocene-aged continental margin slope failure complex. A 29 to 67 cm thick veneer of variable-coloured, finely layered Holocene sediment caps a homogeneous, extremely well-sorted, poorly consolidated, very fine-grained, grey-coloured sediment section that is >20 m thick on the basin floor. This homogeneous unit is interpreted to represent the uppermost deposits generated by a gravity flow associated with the last major Storegga Slide event. Sediments analogous to the inferred source material of the slide deposits were collected from upslope on the Norwegian Margin. Sediments sampled within the basin are distinguishable from the purported source sediments, suggesting that size sorting has significantly altered this material along its flow path. Moreover, the very fine grain size (3·1 ± 0·3 μm) suggests that the >20 m thick homogeneous unit which was sampled settled from suspension after the turbulent flow was over. Although the turbulent phase of the gravity flow that moved material out into the basin may have been brief (days), significantly more time (years) is required for turbid sediments to settle and dewater and for the new sea floor to be colonized with a normal benthonic fauna. Pore water sulphate concentrations within the uppermost 20 m of the event deposit are higher than those normally found in sea water. Apparently the impact of microbial sulphate reduction over the last ca 8·1 cal ka bp since the re-deposition of these sediments has not been adequate to regenerate a typical sulphate gradient of decreasing concentration with sub-bottom depth. This observation suggests low rates of microbial reactions, which may be attributed to the refractory carbon composition in these re-deposited sediments.

Pockmark G11 is the most spectacular one among the pockmarks located at the southern border of Voring Plateau and 1–2 km away from the northern flank of the Storegga Slide, mid-Norwegian continental margin. For the first time, detailed pore-water geochemical studies were conducted to address methane hydrate occurrence, methane seepage and associated geochemical processes, and methane characteristics in the pockmark. Pore-waters collected from five sediments cores inside and one sediment core outside the pockmark, were analyzed for dissolved Cl−, sulfate (SO42−), total hydrogen sulfide (ΣH2S), Ca2+, Mg2+, dissolved inorganic carbon (DIC), δ13CDIC and δ18O. Methane hydrates were recovered in all sediments below 0.75 m in a core inside pockmark G11, which is in good accordance with heavy oxygen isotope (1.9 to 2.3‰SMOW) and low Cl−concentrations (84.9 to 16.1 mM) in pore-waters. Pore-water profiles indicate that upward-migrating methane fluids are spatially variable in the pockmark, with methane fluxes ranging from below detection in the center and outside, to 0.30–0.54 mol m− 2 a− 1 inside. In the cores with active methane fluxes, maximum DIC concentrations (19.4 to 21.5 mM) and corresponding minimum δ13CDIC values (− 52.3 to − 54.6 ‰ PDB) occur within sulfate-methane-transition (SMT) zones from ∼ 0.40 to 0.50 m below seafloor (mbsf), close to the seafloor. Simple mass balance modeling and δ13CDIC measurements within the SMT zones suggest that methane in shallow sediments within pockmark G11 is microbial in origin. Pore-water geochemistry and seabed observations suggest that methane fluxes inside pockmark G11 are episodic, and derived mostly from the recycling of methane hydrate at depth during sediment burial.

Video analysis of a whale-fall discovered in the northeast Pacific Ocean, off Vancouver Island at a depth of 1288 m during ROV diving operations has identified 26 taxa of deep-sea benthic organisms inhabiting the seafloor immediately surrounding remnants of the whale skeleton. A photo-mosaic derived from high-definition video provides a quantitative visual record of the present condition of the site, the species richness, and substrate preference. Only the skull and caudal vertebrae remains of this large whale skeleton are estimated to have been approximately 16.5 m in length. Most organisms identified near the whale-fall are common benthic deep-sea fauna, typical of this water depth and seafloor composition. Much of this species richness comes from sessile suspension feeding cnidarians attached to the numerous glacial dropstones found throughout the area rather than the presence of the whale skeleton. Seep and bone specialists are rare (4 taxa) and may be, in part, a remnant population from a sulphophilic stage of whale-fall decomposition. Evidence of past colonization by Osedax sp. is visible on the remaining bones and we conclude that rapid degradation of the missing bones has occurred at this site as has been observed at whale-falls off central California in Monterey Canyon.

We have developed, deployed, and tested a novel probe for study of the geochemistry of sediment pore waters based upon Raman spectroscopy. The Raman technique has already been used successfully for in situ measurements of targets of scientific interest including gas and hydrothermal vents and complex gas hydrates, but sediment geochemistry has so far been an intractable problem since the sediments themselves are strongly fluorescent and typically only very small sample volumes are obtainable. The 35 cm long probe extracts pore fluids through a 10 mu m sintered metallic frit and draws the sample through a 2 mm diameter channel into a sapphire windowed optical cell within which the laser beam is focused and the spectrum recorded. The dead volume of the system is similar to 1 ml and the instrument is ROV deployable with activation of probe insertion and sample withdrawal under direct operator control. The unique features of this mode of detection include observation of the sulfate gradient in marine Pore waters as an indicator of diagenesis, direct measurement of the dissolved sulfide species H(2)S and HS(-), and measurement of dissolved methane; all of which are of primary geochemical interest. Quantitative analysis is achieved by area ratio to known water peaks and from standard calibration curves with a precision of +/- 5%. We find only very small fluorescence from Pore waters measured in situ, but observe rapid increases in fluorescence from cores returned to the Surface and exposed to oxygen. (C) 2009 Elsevier Ltd. All rights reserved.

Erosional and depositional bedforms have been imaged at outcrop scale in the upper Redondo Fan, in the San Pedro Basin of offshore Southern California in ≥600 m water depths, using an Autonomous Underwater Vehicle developed by the Monterey Bay Aquarium Research Institute. The Autonomous Underwater Vehicle is equipped with multibeam and chirp sub-bottom sonars. Sampling and photographic images using the Monterey Bay Aquarium Research Institute Remotely Operated Vehicle Tiburon provide groundtruth for the Autonomous Underwater Vehicle survey. The 0·3 m vertical and 1·5 m lateral bathymetric resolution and 0·1 m sub-bottom profile resolution provide unprecedented detail of bedform morphology and structure. Multiple channels within the Redondo Fan have been active at different times during the Late Holocene (0 to 3000 yr bp). The currently active channel extending from Redondo Canyon makes an abrupt 90° turn at the canyon mouth before resuming a south-easterly course along the east side of the Redondo Fan. This channel is floored by sand and characterized by small steps generally

Methane vents are of significant geochemical and ecological importance. Notable progress has been made toward understanding anaerobic methane oxidation in marine sediments; however, the diversity and distribution of aerobic methanotrophs in the water column are poorly characterized. Both environments play an essential role in regulating methane release from the oceans to the atmosphere. In this study, the diversity of particulate methane monooxygenase ( pmoA ) and 16S rRNA genes from two methane vent environments along the California continental margin was characterized. The pmoA phylotypes recovered from methane-rich sediments and the overlying water column differed. Sediments harbored the greatest number of unique pmoA phylotypes broadly affiliated with the Methylococcaceae family, whereas planktonic pmoA phylotypes formed three clades that were distinct from the sediment-hosted methanotrophs and distantly related to established methanotrophic clades. Water column-associated phylotypes were highly similar between field sites, suggesting that planktonic methanotroph diversity is controlled primarily by environmental factors rather than geographical proximity. Analysis of 16S rRNA genes from methane-rich waters did not readily recover known methanotrophic lineages, with only a few phylotypes demonstrating distant relatedness to Methylococcus . The development of new pmo primers increased the recovery of monooxygenase genes from the water column and led to the discovery of a highly diverged monooxygenase sequence which is phylogenetically intermediate to Amo and pMMO. This sequence potentiates insight into the amo/pmo superfamily. Together, these findings lend perspective into the diversity and segregation of aerobic methanotrophs within different methane-rich habitats in the marine environment.

Seafloor blister-like mounds, methane migration and gas hydrate formation were investigated through detailed seafloor surveys in Santa Monica Basin, offshore of Los Angeles, California. Two distinct deep-water (≥800 m water depth) topographic mounds were surveyed using an autonomous underwater vehicle (carrying a multibeam sonar and a chirp sub-bottom profiler) and one of these was explored with the remotely operated vehicle Tiburon. The mounds are N10 m high and N100 m wide dome-shaped bathymetric features. These mounds protrude from crests of broad anticlines (~20 m high and 1 to 3 km long) formed within latest Quaternary-aged seafloor sediment associated with compression between lateral offsets in regional faults. No allochthonous sediments were observed on the mounds, except slumped material off the steep slopes of the mounds. Continuous streams of methane gas bubbles emanate from the crest of the northeastern mound, and extensive methane-derived authigenic carbonate pavements and chemosynthetic communities mantle the mound surface. The large local vertical displacements needed to produce these mounds suggests a corresponding net mass accumulation has occurred within the immediate subsurface. Formation and accumulation of pure gas hydrate lenses in the subsurface is proposed as a mechanism to blister the seafloor and form these mounds. © 2008 Elsevier B.V. All rights reserved.

We measured significant activities of short-lived radium isotopes, 223 Ra (half-life = 11 days) and 224 Ra (half-life = 3.7 days), around the margins of the Hawaiian Islands to water depths of 3500 m. These measurements suggest fluid inputs from the basalt to the surrounding ocean. In general 223 Ra activities were considerably greater than 224 Ra in spite of the expected higher production rate of 224 Ra activity in basalt. The 223 Ra was not supported by dissolved 227 Ac. The highest enrichments of 223 Ra were measured over the Puna Ridge (2100 m depth) east of Hawaii. Here 223 Ra activities reached 19 dpm/m 3 , similar to activities measured near sites of active submarine groundwater discharge in the South Atlantic Bight. To explain the high activities of 223 Ra unaccompanied by 224 Ra, we postulate that thermally-driven circulation of seawater through the Puna Ridge deposits 231 Pa on basalt surfaces. With time the 231 Pa produces 227 Ac and 223 Ra; and 223 Ra desorbs into the circulating fluids. These fluids then transport 223 Ra into the overlying ocean. Based on the inventory of 223 Ra above the Puna Ridge, we estimate the flow of fluids through the ridge to be on the order of 20–60 cm 3 cm − 2 day − 1 . In less than 1000 years the incoming seawater could provide enough 231 Pa to basalt surfaces to balance the inventory of 223 Ra above the ridge if only 8% of the 223 Ra was transported to the overlying water. These observations on the flanks of a volcanically-active ocean island have significant implications for quantifying fluid fluxes from the flanks of the mid-ocean ridge system. By mapping 223 Ra inventories in the ocean above ridge flanks and measuring the activity of 223 Ra in the emerging fluids, the fluid flux can be obtained.

Seafloor pockmarks and subsurface chimney structures are common on the Norwegian continental margin north of the Storegga Slide scar. Such features are generally inferred to be associated with fluid expulsion, and imply overpressures in the subsurface. Six long gravity and piston cores taken from the interior of three pockmarks were compared with four other cores taken from the same area but outside the pockmarks, in order to elucidate the origins and stratigraphy of these features and their possible association with the Storegga Slide event. Sulfate gradients in cores from within pockmarks are less steep than those in cores from outside the pockmarks, which indicates that the flux of methane to the seafloor is presently smaller within the pockmarks than in the adjacent undisturbed sediments. This suggests that these subsurface chimneys are not fluid flow conduits lined with gas hydrate. Methane-derived authigenic carbonates and Bathymodiolus shells obtained from a pockmark at >6.3 m below the seafloor indicate that methane was previously available to support a chemosynthetic community within the pockmark. AMS 14C measurements of planktonic Foraminifera overlying and interlayered with the shell-bearing sediment indicate that methane was present on the seafloor within the pockmark prior to 14 ka 14C years b.p., i.e., well before the last major Storegga Slide event (7.2 ka 14C years b.p., or 8.2 ka calendar years b.p.). These observations provide evidence that overpressured fluids existed within the continental margin sediments off Norway during the last major advance of Pleistocene glaciation.

Authigenic carbonates were sampled in piston cores collected from both the Tunica Mound and the Mississippi Canyon area on the continental slope of the northern Gulf of Mexico during a Marion Dufresne cruise in July 2002. The carbonates are present as hardgrounds, porous crusts, concretions or nodules and shell fragments with or without carbonate cements. Carbonates occurred at gas venting sites which are likely to overlie gas hydrates bearing sediments. Electron microprobe, X-ray diffraction (XRD) and thinsection investigations show that these carbonates are high-Mg calcite (6–21 mol% MgCO3), with significant presence of framboidal pyrite. All carbonates are depleted in 13C (δ13C = − 61.9 to − 31.5‰ PDB) indicating that the carbon is derived mainly from anaerobic methane oxidation (AMO). Age estimates based on 14C dating of shell fragments and on regional sedimentation rates indicate that these authigenic carbonates formed within the last 1000 yr in the Mississippi Canyon and within 5500 yr at the Tunica Mound. The oxygen isotopic composition of carbonates ranges from + 3.4 to + 5.9‰ PDB. Oxygen isotopic compositions and Mg2+ contents of carbonates, and present in-situ temperatures of bottom seawater/sediments, show that some of these carbonates, especially from a core associated with underlying massive gas hydrates precipitated in or near equilibrium with bottom-water. On the other hand, those carbonates more enriched in 18O are interpreted to have precipitated from 18O-rich fluids which are thought to have been derived from the dissociation of gas hydrates. The dissociation of gas hydrates in the northern Gulf of Mexico within the last 5500 yr may be caused by nearby salt movement and related brines.

Authigenic carbonates from five continental margin locations, the Eel River Basin, Monterey Bay, Santa Barbara Basin, the Sea of Okhotsk, and the North Sea, exhibit a wide range of mineralogical and stable isotopic compositions. These precipitates include aragonite, low- and high-Mg calcite, and dolomite. The carbon isotopic composition of carbonates varies widely, ranging from −60‰ to +26‰, indicating complex carbon sources that include ^(13)C-depleted microbial and thermogenic methane and residual, 13C-enriched, bicarbonate. A similarly large variability of δ^(18)O values (−5.5‰ to +8.9‰) demonstrates the geochemical complexity of these sites, with some samples pointing toward an ^(18)O-enriched oxygen source possibly related to advection of ^(18)O-enriched formation water or to the decomposition of gas hydrate. Samples depleted in ^(18)O are consistent with formation deeper in the sediment or mixing of pore fluids with meteoric water during carbonate precipitation. A wide range of isotopic and mineralogical variation in authigenic carbonate composition within individual study areas but common trends across multiple geographic areas suggest that these parameters alone are not indicative for certain tectonic or geochemical settings. Rather, the observed variations probably reflect local controls on the flux of carbon and other reduced ions, such as faults, fluid conduits, the presence or absence of gas hydrate in the sediment, and the temporal evolution of the local carbon reservoir. Areas with seafloor carbonates that indicate formation at greater depth below the sediment–water interface must have undergone uplift and erosion in the past or are still being uplifted. Consequently, the occurrence of carbonate slabs on the seafloor in areas of active hydrocarbon seepage is commonly an indicator of exhumation following carbonate precipitation in the shallow subsurface. Therefore, careful petrographic and geochemical analyses are critical components necessary for the correct interpretation of processes related to hydrocarbon seepage in continental margin environments and elsewhere.

Extensive ROV-based sampling and exploration of the seafloor was conducted along an eroded transform-parallel fault scarp on the northeastern side of the Guaymas Basin in the Gulf of California to observe the nature of fluids venting from the seafloor, measure the record left by methane-venting on the carbonates from this area, and determine the association with gas hydrate. One gas vent vigorous enough to generate a water-column gas plume traceable for over 800 m above the seafloor was found to emanate from a ∼10-cm-wide orifice on the eroded scarp face. Sediment temperature measurements and topography on a sub-bottom reflector recorded in a transform-parallel seismic reflection profile identified a subsurface thermal anomaly beneath the gas vent. Active chemosynthetic biological communities (CBCs) and extensive authigenic carbonates that coalesce into distinct chemoherm structures were encountered elsewhere along the eroded transform-parallel scarp. The carbon isotopic composition of methane bubbles flowing vigorously from the gas vent (−53.6±0.8‰ PDB) is comparable to methane found in sediment cores taken within the CBCs distributed along the scarp (−51.9±8.1‰ PDB). However, the δ13C value of the CO2 in the vent gas (+12.4±1.1‰ PDB) is very distinct from those for dissolved inorganic carbon (DIC) (−35.8‰ to −2.9‰ PDB) found elsewhere along the scarp, including underneath CBCs. The δ13C values of the carbonate-rich sediments and rocks exposed on the seafloor today also span an unusually large range (−40.9‰ to +12.9‰ PDB) and suggest two distinct populations of authigenic carbonate materials were sampled. Unconsolidated sediments and some carbonate rocks, which have lithologic evidence for near-seafloor formation, have negative δ13C values, while carbonate rocks that clearly formed in the subsurface have positive δ13C values (up to +23.0‰) close to that measured for CO2 in the vent gas. There appears to be two carbon sources for the authigenic carbonates: (1) deeply-sourced, isotopically heavy CO2 (∼+12‰); and (2) isotopically light DIC derived from local anaerobic oxidation of methane at the sulfate–methane interface in the shallow subsurface. Addition of isotopically light methane-derived carbon at the seafloor may completely mask the isotopically heavy CO2 signature (+12.4‰) in the underlying sediments. Thus, the authigenic carbonates may have formed from the same methane- and carbon dioxide-bearing fluid, but under different migration and alteration conditions, depending on how it migrated through the sediment column.

Laminated fine-grained sediments drape the flanks and terraces on the sides of upper Monterey Canyon, where rapid sedimentation occurs in the relatively calm environment sandwiched between the wave-washed continental shelf and the sediment gravity-flow-scoured axial channel on the canyon floor. The isotopic composition of organic matter contained in these accumulating sediments shows that they are primarily of terrestrial origin and are apparently supplied by the Salinas and Pajaro Rivers that drain the adjacent agricultural valleys and discharge near the canyon head. The presence of alien pollen grains within these fine-grained sediments indicates that > 5 m of sediment was deposited within historical times (post-1700) on the canyon flanks. Two trends are seen in the 14 C content of the sedimentary organic carbon. The uppermost sediments on the canyon flanks contain pesticide residues indicating post-1945 deposition and have an average 14 C content that is 82 ± 3.2 percent modern carbon. This is similar to 14 C values for suspended sediments discharged from the Salinas River during floods. Disseminated bulk organic matter and carbonate sampled immediately below pesticide-bearing sediments on the flanks of Monterey Canyon are distinctly more 14 C-depleted. The proportion of 14 C in the disseminated bulk organic matter increases somewhat erratically with further depth in all nine cores with multiple measurements. These discordant 14 C-stratigraphies are developed within sediments that are pesticide-free (pre-1945), but still of historical age based on the occurrence of alien pollen grains. Two factors may contribute to the discordant 14 C-stratigraphies observed in upper Monterey Canyon: (1) the addition of bomb-produced 14 C in post-1945 sediments, and (2) changes in the proportion of older soil carbon that was washed off agricultural land. Land use practices associated with the early stages of modern development of the Salinas and Pajaro Valleys caused rapid loss of virgin soils and a subsequent increase in the proportion of older soil carbon exported as deeper soil horizons were exhumed. A sharp decrease in the amount of older carbon being flushed off the land has occurred since the construction of dams and impoundment structures on the Salinas and Pajaro Rivers by the middle of the 20th century.

During Ocean Drilling Program Leg 164, an unprecedented effort was made to determine the amounts of gas and gas hydrate in the sediments from Sites 994, 995, and 997. For the first time in the history of academic drilling, a pressure core sampler (PCS) worked well enough to generate an independent stratigraphy of in situ gas concentrations and compositions with depth. Here, gas concentrations and composition data produced by routine shipboard gas sampling techniques are compared with PCS data.

We investigated hydrothermal gas venting associated with a coastal fault zone along the western margin of Bahia Concepcion, B.C.S., Mexico. Copious discharge of geothermal liquid (≈ 90 °C) and gas is occurring in the intertidal and shallow subtidal zones (to a depth of 13 m) through soft sediments and fractures in rocks along a ∼750 m linear trend generally sub-parallel to an onshore fault near Punta Santa Barbara. Hydrothermal activity shows negative correlation with tidal height; temperatures in the area of hydrothermal activity were up to 11.3 °C higher at low tide than at high tide (measured tidal range ≈ 120 cm). Gas samples were collected using SCUBA and analyzed for chemical composition and stable isotope values. The main components of the gas are N2 (≈ 53%; 534 mmol/mol), CO2 (≈ 43%; 435 mmol/mol), and CH4 (≈ 2.2%; 22 mmol/mol). The δ13C values of the CH4 (mean = − 34.3‰), and the ratios of CH4 to C2H6 (mean = 89), indicate that the gas is thermogenic in origin. The carbon stable isotopes and the δ15N of the N2 in the gas (mean = 1.7‰) suggest it may be partially derived from the thermal alteration of algal material in immature sedimentary organic matter. The He isotope ratios (3He / 4He = 1.32 RA) indicate a significant mantle component (16.3%) in the gas. Here, we suggest the name El Requeson fault zone for the faults that likely formed as a result of extension in the region during the late Miocene, and are currently serving as conduits for the observed hydrothermal activity.

Gas hydrates are common within near-seafloor sediments immediately surrounding fluid and gas venting sites on the continental slope of the northern Gulf of Mexico. However, the distribution of gas hydrates within sediments away from the vents is poorly documented, yet critical for gas hydrate assessments. Porewater chloride and sulfate concentrations, hydrocarbon gas compositions, and geothermal gradients obtained during a porewater geochemical survey of the northern Gulf of Mexico suggest that the lack of bottom simulating reflectors in gas-rich areas of the gulf may be the consequence of elevated porewater salinity, geothermal gradients, and microbial gas compositions in sediments away from fault conduits.

Geomorphic, stratigraphic, and faunal observations of submarine slide scars that occur along the flanks of Monterey Canyon in 2.0–2.5 km water depths were made to identify the processes that continue to alter the surface of a submarine landslide scar after the initial slope failure. Deep-sea chemosynthetic biological communities and small caves are common on the sediment-free surfaces of the slide scars, especially along the headwall. The chemosynthetic organisms observed on slide scars in Monterey Canyon undergo a faunal succession based in part on their ability to maintain their access to the redox boundaries in the sediment on which they depend on as an energy source. By burrowing into the seafloor, these organisms are able to follow the retreating redox boundaries as geochemical re-equilibration occurs on the sole of the slide. As these organisms dig into the seafloor on the footwall, they often generate small caves and weaken the remaining seafloor. While chemosynthetic biological communities are typically used as indicators of fluid flow, these communities may be supported by methane and hydrogen sulfide that are diffusing out of the fresh seafloor exposed at the sole of the slide by the slope failure event. If so, these chemosynthetic biological communities may simply mark sites of recent seafloor exhumation, and are not reliable fluid seepage indicators.

Marine methane vents and cold seeps are common features along continental margins worldwide, serving as localized sites for methane release and colonization by microbial and chemosynthetic megafaunal communities. The Eel River Basin (ERB), located on the continental slope off Northern California, contains active methane vents and seep-associated chemosynthetic biological communities (CBC) on the crests of anticlines in ∼520-m water depth. Seep-related features on the seafloor have a patchy distribution and include active bubbling vents, chemosynthetic clam beds, and sulfide-oxidizing bacterial mats. Methane sources supplying local seeps are heterogeneous on all spatial scales and support a large and diverse microbial assemblage involved in the anaerobic oxidation of methane (AOM). To develop a comprehensive understanding of the complex biological, geochemical and physical processes associated with, and influencing seafloor methane seepage, a multidisciplinary approach is required. Here we present an integrative, multidisciplinary study that illustrates the diverse processes associated with seafloor methane seepage within the Eel River Basin and the complex interactions defining the geochemistry, mineralogy and microbiology within this environment.

Measurements of fluid and gas properties associated with the sediments within and around the Belfast Bay, ME, pockmark field and in the overlying water column were conducted to determine whether fluid flow or methane gas venting is occurring through these pockmarks or has occurred recently. No clear geochemical, lithologic, or biological indications of methane and/or freshwater seepage, such as low salinity pore water, obvious authigenic carbonates, anomalously high water methane concentrations in pore water and the overlying water column, or unusual benthic fauna were observed. Thus, these pockmarks may be inactive or a process different from gas/and or freshwater seepage is responsible for creating and maintaining these features.

A sediment gravity flow descended through the axis of Monterey Canyon on 20 December 2001 at 13:35 Pacific standard time. The timing of this event is documented by a current-meter package which recorded an 11.9-dbar pressure increase in less than 10 min and was found 550 m down-canyon from its deployment site, buried completely within a >70-cm-thick gravity flow deposit. This event is believed to have started in less than 290 m of water because an instrument at this location was also lost at the same time. A 178-cm core collected after the event from the axis of the canyon at 1,297-m water depth contained fresh, greenish, chlorophyll-rich organic material at 32-cm sub-bottom depth, suggesting the event extended to this water depth. The only trigger identified for this mass movement event appears to be moderate sea and surf conditions. Thus, gravity flow events of this magnitude do not require an exceptional triggering event.

The scientific community is engaged in a lively debate over whether and how venting from the gas-hydrate reservoir and the Earth’s climate is connected. The various scenarios which have been proposed are based on the following assumptions: the inventory of methane gas-hydrate deposits is locally enormous, the stability of marine gas-hydrate deposits can easily be perturbed by temperature and pressure changes, enough methane can be released from these deposits to contribute adequate volumes of this isotopically distinct greenhouse gas to alter the composition of oceanic or atmospheric methane reservoirs, and the mechanisms exist for the transfer of methane from deeper geologic reservoirs to the ocean and/or atmosphere. However, some potential transfer mechanisms have been difficult to evaluate. Here, we consider the possibility of marine slumping as a mechanism to transfer methane carbon from gas hydrates within the seafloor into the ocean and atmosphere. Our analyses and field experiments indicate that large slumps could release volumetrically significant quantities of solid gas hydrates which would float upwards in the water column. Large pieces of gas hydrate would reach the upper layers of the ocean before decomposing, and some of the methane would be directly injected into the atmosphere.

A pockmark field was discovered during EM-300 multi-beam bathymetric surveys on the lower continental slope off the Big Sur coast of California. The field contains ∼1500 pockmarks which are between 130 and 260 m in diameter, and typically are 8–12 m deep located within a 560 km2 area. To investigate the origin of these features, piston cores were collected from both the interior and the flanks of the pockmarks, and remotely operated vehicle observation (ROV) video and sampling transects were conducted which passed through 19 of the pockmarks. The water column within and above the pockmarks was sampled for methane concentration. Piston cores and ROV collected push cores show that the pockmark field is composed of monotonous fine silts and clays and the cores within the pockmarks are indistinguishable from those outside the pockmarks. No evidence for either sediment winnowing or diagenetic alteration suggestive of fluid venting was obtained. 14C measurements of the organic carbon in the sediments indicate continuous sedimentation throughout the time resolution of the radiocarbon technique (∼45 000 yr BP), with a sedimentation rate of ∼10 cm per 1000 yr both within and between the pockmarks. Concentrations of methane, dissolved inorganic carbon, sulfate, chloride, and ammonium in pore water extracted from within the cores are generally similar in composition to seawater and show little change with depth, suggesting low biogeochemical activity. These pore water chemical gradients indicate that neither significant accumulations of gas are likely to exist in the shallow subsurface (∼100 m) nor is active fluid advection occurring within the sampled sediments. Taken together the data indicate that these pockmarks are more than 45 000 yr old, are presently inactive, and contain no indications of earlier fluid or gas venting events.

The traditional ocean chemical explanation for the emergence of suboxia is that once O 2 levels decline to about 10 µmol kg −1 then onset of NO 3 − reduction occurs. This piece of ocean chemical lore is well founded in observations and is typically phrased as a microbial choice and not as an obligate requirement. The argument based on O 2 levels alone could also be phrased as being dependent on an equivalent amount of NO 3 − that would yield the same energy gain. This description is based on the availability of the electron acceptor: but the oxidation reactions are usually written out as free energy yield per mole of organic matter, thus not addressing the oxidant availability constraint invoked by ocean scientists. Here we show that the argument can be phrased simply as competing rate processes dependent on the free energy yield ratio per amount of electron acceptor obtained, and thus the [NO 3 − ]:[O 2 ] molar ratio is the critical variable. The rate at which a microbe can acquire either O 2 or NO 3 − to carry out the oxidation reactions is dependent on both the concentration in the bulk ocean, and on the diffusivity within the microbial external molecular boundary layer. From the free energy yield calculations combined with the ~25% greater diffusivity of the O 2 molecule we find that the equivalent energy yield occurs at a ratio of about 3.8 NO 3 − :O 2 for a typical Redfield ratio reaction, consistent with an ocean where NO 3 − reduction onset occurs at about 10 μmol O 2 :40 μmol NO 3 − , and the reactions then proceed in parallel along a line of this slope until the next energy barrier is approached. Within highly localized microbial consortia intensely reducing pockets may occur in a bulk ocean containing finite low O 2 levels; and the local flux of reduced species from strongly reducing shelf sediments will perturb the large scale water column relationship. But all localized reactions drive towards maximal energy gain from their immediate diffusive surroundings, thus the ocean macroscopic chemical fields quite well approximate the net efficiency and operational mode of the ensemble microbial engine.

Sections of the Florida–Hatteras Slope were investigated during dives of the Johnson Sea-Link I submersible and US Navy's submarine NR-1. Numerous calcareous coral-capped lithoherms, often exceeding 40 m in elevation, were encountered in the subphotic depths below 440 m. One huge lithoherm complex was explored that forms a long, slope-parallel ridge perched on the seaward edge of mid-slope terrace. This lithoherm belt is ∼400 m wide, more than 4.4 km long and ∼150 m high on its down slope flank. Direct observations and side-scan imaging indicate that the long axes of individual ridge crest mounds trend perpendicular to the regional current flow. Away from the ridge individual lithoherms trend more parallel with regional current flow and occur without obvious bedrock control. All of these structures have very steeply sloping flanks (frequently 30–60°) where cemented crusts are exposed by erosion. Thickets of living ahermatypic coral coat the crest and up-stream flanks of these topographic features. Living deep-water coral have modern 14C ages (700±80 year bp) and trap older (3250±100 year bp) sediment rich in fine-grained aragonite. Elsewhere, older deep-water coral rubble (20,230±230 year bp) traps younger sediment (13,760±140 year bp). The trapped sediments contain >50% aragonite fines (

Submarine pockmarks are common features on high-latitude continental shelves, yet the mechanisms for their formation are poorly understood. Here we propose an additional mechanism for the formation of high-latitude pockmarks involving freshwater ice rafting. Freshwater seeping upward through the sediments freezes at the sediment–water interface when bottom waters are below 0°C. Thin layers of the frozen water containing some sediment are buoyant and can rise off the bottom as small ice rafts, thereby excavating a seafloor depression over time. Sediment in the ice rafts becomes entrained in bottom currents and is dispersed over a wide area.

We test a hypothesis relating large pore water sulfate gradients to upward methane flux and the presence of underlying methane gas hydrate on continental rises by examining: (1) pore water geochemical data available from the global data set of Deep Sea Drilling Project–Ocean Drilling Program (DSDP–ODP) sites; (2) sulfate data from 51 coring sites located at the Carolina Rise and Blake Ridge (offshore southeastern United States); and (3) the relationship between the distribution of bottom-simulating reflectors (BSRs) and sulfate depletion patterns at the Carolina Rise–Blake Ridge (CR–BR) area. Within continental rise sediments, large sulfate gradients are correlative with marine methane gas hydrate settings (recognized by gas hydrate recovery and the presence of BSRs). This correlation is in part due to the rapid consumption of sedimentary organic matter by sulfate reduction and early microbial production of methane during burial and early diagenesis. However, detailed interstitial geochemical evidence from sediments of the CR–BR area strongly suggests that sulfate and methane co-consumption (anaerobic methane oxidation) at the sulfate–methane interface (SMI) is an important additional process in depleting interstitial sulfate, producing steep (and perhaps linear) sulfate gradients and shallow depths to the SMI. The presence of BSRs is currently the only routine technique used to identify gas hydrate localities. However, BSRs seem to represent an abrupt interface at the base of gas hydrate stability (BGHS) where methane gas bubbles occur, rather than being a direct indicator of gas hydrates in overlying sediments. Detailed comparisons between BSR distribution and geochemical data at the CR–BR show that BSRs are patchy in their occurrence, consistent with BSRs representing accumulations of free methane gas that pool within structural and stratigraphic traps near the crest and on the flanks of the Blake Ridge. In contrast, steep sulfate gradients (and proxy indicators of gas hydrate) are pervasive components of the CR–BR area, suggesting that steep sulfate gradients may be a better general indicator of gas hydrate potential. Steep sulfate gradients apparently identify large upward fluxes of methane, indicating conditions conducive to the formation of gas hydrates, given favorable pressure and temperature conditions. Global DSDP–ODP geochemical data identify many additional deep-water marine sites with large sulfate gradients that lack BSRs, perhaps suggesting the occurrence of previously unrecognized gas hydrate localities.

Resume Au cours de la campagne ODP 171 B, consacree a l'analyse de la marge du Blake Plateau au large de la Floride, 16 forages ont ete effectues. Repartis en cinq sites principaux, ils ont permis d'etablir une colonne sedimentaire allant de l'Aptien a l'Eocene. L'accent a ete mis sur une succession de periodes critiques comprenant la transition Paleocene-Eocene, la limite Cretace-Tertiaire carottee dans d'excellentes conditions, les extinctions du Maastrichtien moyen et les episodes anoxiques de PAlbien.

Data from piston cores collected from Carolina Rise and Blake Ridge, and from many DSDP/ODP sites indicate that extreme 13C-depletion of methane and ΣCO2 occurs within the uppermost methanogenic zone of continental rise sediments. We infer that 13C-depleted methane is generated near the top of the methanogenic zone when carbon of 13C-depleted ΣCO2, produced by microbially-mediated anaerobic methane oxidation, is recycled back to methane through CO2 reduction. Interstitial water and gas samples were collected in 27 piston cores, 16 of which penetrated through the sulfate reduction zone into methane-bearing sediments of the Carolina Rise and Blake Ridge. Isotopic measurements (δ13CCH4, δ13CCO2, δDCH4, and δDH2O) indicate that this methane is microbial in origin, produced by microbially-mediated CO2 reduction. Methane samples form two distinct isotopic pools. (1) Methane from a seafloor seep site shows a mean δ13CCH4 value of − 69 ± 2%., mirroring values found at ≥ 160 mbsf from a nearby DSDP site. (2) Twenty, areally-separated sites (sample depth, 10 to 25 mbsf) have δ13CCH4 values ranging from −85 to −103%., and δ13CCO2 as negative as −48%.. The very low δ13C values from the methane and CO2 pools highlight the importance of carbon cycling within continental rise sediments at and near the sulfate-methane boundary.

Recent advances in ocean observing systems and genomic technologies have led to the development of the deep-sea environmental sample processor (D-ESP). The D-ESP filters particulates from seawater at depths up to 4000 m and applies a variety of molecular assays to the particulates, including quantitative PCR (qPCR), to identify particular organisms and genes in situ. Preserved samples enable laboratory-based validation of in situ results and expanded studies of genomic diversity and gene expression. Tests of the D-ESP at a methane-rich mound in the Santa Monica Basin centered on detection of 16S rRNA and particulate methane monooxygenase (pmoA) genes for two putative aerobic methanotrophs. Comparison of in situ qPCR results with laboratory-based assays of preserved samples demonstrates the D-ESP generated high-quality qPCR data while operating autonomously on the seafloor. Levels of 16S rRNA and pmoA cDNA detected in preserved samples are consistent with an active community of aerobic methanotrophs near the methane-rich mound. These findings are substantiated at low methane sites off Point Conception and in Monterey Bay where target genes are at or below detection limits. Successful deployment of the D-ESP is a major step toward developing autonomous systems to facilitate a wide range of marine microbiological investigations.

In marine sediments gas hydrate growth removes water and gas molecules from the pore water while dissolved ions are excluded from the clathrate cage. As a consequence, the surrounding pore waters become more saline. Conversely, when gas hydrates decompose, water is released, causing pore water to become fresher. Ion exclusion produces distinctive geochemical signatures that are used to identify the presence of gas hydrate and to calculate the amount of gas hydrate in marine sediment samples. These calculations, however, require knowledge of the dissolved ion concentration of the in situ pore water prior to gas hydrate decomposition. Because arguably no useable pore water samples have been collected in situ from gas-hydrate-bearing sediments, the calculated amounts of gas hydrate are very dependent on the assumed in situ dissolved ion concentrations. In this paper, the principles behind using measured pore water chloride concentrations to estimate gas hydrate amounts are reviewed, along with data from selected sites around the world. These data clearly define local pore water anomalies in the sediment zones that are believed to currently contain gas hydrate, but also document the occurrence of thick zones of low chloride concentrations immediately below the base of gas hydrate stability. The potential causes for these low chloride anomalies below the base of gas hydrate stability are discussed.

Gas hydrate formation requires adequate concentrations of gas (usually methane) to be present in the sediments. Thus, methane-bearing gas hydrate occurrence is closely coupled with the distribution of methane in marine sediments. We review the history of gas sampling from bore holes in gas-rich continental margin sediment sequences that may be associated with gas hydrate. Gas measurements have been routinely made for both drilling safety and scientific investigations during the Deep Sea Drilling Project (DSDP) and the Ocean Drilling Program (ODP). Until recently, gas samples have come only from core voids and residual gas extracted from the cored sediments. However, recent successes with down-hole tools designed to maintain cores at their original pressure during core recovery have obtained independent data on in situ gas amounts, gas composition, and the process of core degassing. These data show that the residual core gas concentration represents only a tiny fraction of the total in situ gas inventory. Because the majority of the original gas escapes from the core during core recovery, caution must be used when trying to apply traditional core gas measurements to potentially gas hydrate-bearing sediment sequences.

We present a continuous, 2-year long record of carbon dioxide (CO 2 ) mixing ratio at three altitudes up to 496 m above the ground on a television transmitter tower in the southeastern United States. The data show strong diurnal and seasonal variations, and large vertical gradients. The diurnal cycles are modulated by surface uptake and release by vegetation and soils, emissions from fossil fuel combustion, and by the diurnal development of the planetary boundary layer. Gradients of 1-2 ppm between 496 m and 51 m are typically observed during summertime afternoons, due to vigorous photosynthetic uptake. With increasing altitude the magnitude of the diurnal cycle is damped, and daily average mixing ratios decrease, caused by coincident changes in the sign and magnitude of the surface flux, and changes in vertical stability of the boundary layer over the course of the day. Measurements at 496 m give an approximate measure (within a few tenths of a ppm) of the afternoon mean mixing ratio in the convective boundary layer. Vertical gradients between 51 m and 496 m are typically close to zero, and monthly mean mixing ratios increase slowly between November and April, indicating that biological activity is minimal during this period. The amplitude of the seasonal cycle of CO 2 is larger at the tower site than at marine boundary layer and mountaintop sites which are at nearly the same latitude, because of the proximity of the tower site to terrestrial sources and sinks. Comparison of our continental tower data with data from “background” sites should provide a strong constraint for regional and global models of terrestrial CO 2 fluxes. We also present data from weekly flask samples taken from the 496 m level and which have been analyzed for methane (CH 4 ), carbon monoxide (CO), and the stable isotopes of carbon in CO 2 ( δ 13 C). The flask data provide further information about the processes that drive observed changes in CO 2 mixing ratio at the tower. DOI: 10.1034/j.1600-0889.47.issue5.2.x

The cycling of methane across the soils of the tidally flooded bank margins in the tidal freshwater portion of the White Oak River estuary, North Carolina, was investigated from October 1990 to November 199 1. A comparison between these bank cdgcs and adjacent submerged stations was made. The bank stations showed large seasonal variations in methane production rates, with very little being produced in winter and large amounts (up to 1,000 mg m 2 d- I) during summer. This seasonality seems to be linked to the cycle of growth and senescence of vascular plants growing in these soils. In contrast, the submerged stations produced smaller amounts of methane year-round, with moderate increases during summer. Vascular plants are not rooted in these submerged sediments. Seasonal variations in methane production rates at the tidally flooded bank stations led to significant changes in the dissolved methane pool sizes. Although production rates and pore-water concentrations increased with seasonal increases in soil temperatures, the flux of methane to the atmosphere remained fairly low and variable. Methane flux varied with stage of the semidiurnal tidal cycle and exhibited a bimodal distribution, with fluxes greatest when the water level was nearest the soil surface. During summer months, methane production was up to 10 times greater than its flux to the atmosphere. This large difference between production and flux is attributed mainly to bacterially mediated methane oxidation at the soil surface during low-tide exposure and(or) in the root zone of plants. However, in midsummer, lateral export of dissolved methane from the bank soils, driven by tidal flushing, may account for almost 30% of this discrepancy between production and flux. Global budget estimates for methane, such as those of Cicerone and Orcmland (1988) and Tyler (1989), have shown that natural wetlands can contribute up to 50% of the methane emitted annually to the atmosphere. Thus, a major effort has been made to map the world’s wetlands (Matthews and Fung 1987; Aselman and Crutzen 1989) and to determine average fluxes from the major biomes. Grill (in press) divided the world’s wetlands into four major latitudinal zones, tropical (O-20”), temperate (2045”), boreal (45-65”), and arctic (>65”) and calculated that more than half (52.6%) of the yearly emission of methane from wetlands comes from the colder regions (boreal, 45.6%; arctic, 7.0%), with another 40.6% from the tropics and only 6.8% from temperate regions. Although temperate wetlands may contribute relatively little to the global methane flux (Crill in press), they can provide important opportunities to study factors that control methane production and flux. Given the east of access to these regions, they lend themselves to seasonal studies in which processes affecting the production of methane and its mode of transport to the atmosphere can be ascertained (c.g. Chanton and Martens 1988; Wilson

The effects of ion exclusion and isotopic fractionation associated with gas hydrate formation and decomposition in continental margin sediments are examined using simple mass balance calculations. In a closed system pore fluid salinity can be increased to brine levels and detectable changes in interstitial waterδ18O can be caused by formation of significant amounts of interstitial gas hydrate. Time- and mass-dependent models indicate that given appropriate geometries, the diffusion of dissolved salts is sufficiently rapid and their supply is large enough to establish dissolved ion gradients that can be measured in sediments obtained from piston cores or boreholes.

Diverse copper-containing membrane-bound monooxygenase-encoding sequences (Cu-MMOs) have recently been described from the marine environment, suggesting widespread potential for oxidation of reduced substrates. Here, we used the well-defined oxygen and methane gradients associated with the Costa Rican oxygen minimum zone (OMZ) to gain insight into the physico-chemical parameters influencing the distribution and abundance of Cu-MMO-encoding marine microorganisms. Two Methylococcales-related Cu-MMO-encoding lineages, termed groups OPU1 and OPU3, demonstrated differences in their relative abundance, with both pmoA and candidate 16S rRNA genes correlating significantly with reduced environmental oxygen concentrations and depth. In contrast, a newly identified Cu-MMO-encoding lineage, Group C, was primarily associated with the oxygenated euphotic zone. An updated phylogenetic analysis including these sequences, a marine pxmABC gene cluster, ethylene-utilizing Cu-MMO-encoding lineages and previously reported planktonic Cu-MMOs (Groups W, X, Z and O) demonstrates the breadth of diversity of Cu-MMO-encoding marine microorganisms. Groups C and X affiliated phylogenetically with ethane- and ethylene-oxidizing Cu-MMOs, Groups W and O affiliated phylogenetically with the recently described Cu-MMO 'pXMO', and Group Z clustered with Cu-MMOs recovered from soils. Collectively, these data demonstrate widespread genetic potential in ocean waters for the oxidation of small, reduced molecules and advance our understanding of the microorganisms involved in methane cycling in the OMZ environment.