Authigenic Mg-calcite at a cold methane seep site in the Laptev Sea

Authigenic minerals were studied in Holocene shelf sediments of the Laptev Sea (cold methane seep site, water depth 71 m). The study presents the first finds of large hard carbonate concretions with Mg-calcite cement in recent sediments of the Arctic shelf seas. These concretions differ from previously reported glendonites and concretions from bottom sediments of the White Sea, Kara Sea, Sea of Okhotsk, etc. A study of the morphology, microstructure, and composition of these newly reported concretions revealed the multistage formation of carbonates (structural varieties of Mg-calcite and aragonite). It was shown that organic matter played an important role in the formation of authigenic carbonates, i.e., in the formation of sedimentary–diagenetic Mg-calcite. The role of methane as a possible source for authigenic carbonate formation was estimated. It was found that methane-derived Mg-calcite accounts for 17–35% of concretion materials. Mg-calcite had δ¹³С-С_carb values between–24 and–23‰ and δ¹³С-С_org values between–44.5 and–88.5‰.     

Authigenic carbonates related to active seepage of methane-rich hot brines at the Cheops mud volcano,Menes caldera (Nile deep-sea fan,eastern Mediterranean Sea)

On the passive margin of the Nile deep-sea fan, the active Cheops mud volcano (MV; ca. 1,500 m diameter, ~20–30 m above seafloor, 3,010–3,020 m water depth) comprises a crater lake with hot (up to ca. 42 °C) methane-rich muddy brines in places overflowing down the MV flanks. During the Medeco2 cruise in fall 2007, ROV dives enabled detailed sampling of the brine fluid, bottom lake sediments at ca. 450 m lake depth, sub-surface sediments from the MV flanks, and carbonate crusts at the MV foot. Based on mineralogical, elemental and stable isotope analyses, this study aims at exploring the origin of the brine fluid and the key biogeochemical processes controlling the formation of these deep-sea authigenic carbonates. In addition to their patchy occurrence in crusts outcropping at the seafloor, authigenic carbonates occur as small concretions disseminated within sub-seafloor sediments, as well as in the bottom sediments and muddy brine of the crater lake. Aragonite and Mg-calcite dominate in the carbonate crusts and in sub-seafloor concretions at the MV foot, whereas Mg-calcite, dolomite and ankerite dominate in the muddy brine lake and in sub-seafloor concretions near the crater rim. The carbonate crusts and sub-seafloor concretions at the MV foot precipitated in isotopic equilibrium with bottom seawater temperature; their low δ¹³C values (–42.6 to –24.5‰) indicate that anaerobic oxidation of methane was the main driver of carbonate precipitation. By contrast, carbonates from the muddy lake brine, bottom lake concretions and crater rim concretions display much higher δ¹³C (up to –5.2‰) and low δ¹⁸O values (down to –2.8‰); this is consistent with their formation in warm fluids of deep origin characterized by ¹³C-rich CO₂ and, as confirmed by independent evidence, slightly higher heavy rare earth element signatures, the main driver of carbonate precipitation being methanogenesis. Moreover, the benthic activity within the seafloor sediment enhances aerobic oxidation of methane and of sulphide that promotes carbonate dissolution and gypsum precipitation. These findings imply that the coupling of carbon and sulphur microbial reactions represents the major link for the transfer of elements and for carbon isotope fractionation between fluids and authigenic minerals. A new challenge awaiting future studies in cold seep environments is to expand this work to oxidized and reduced sulphur authigenic minerals.     

Particulate organic carbon (POC) in surface sediments of the Baltic Sea

In this study, particulate organic carbon (POC) contents and their distribution pattern in surficial sediments of the Baltic Sea are presented for 1,471 sampling stations. POC contents range from approx. 0.1% in shallow sandy areas up to 16% in deep muddy basins (e.g. Gotland Basin). Some novel relationships were identified between sediment mass physical properties (dry bulk density (DBD), grain size) and POC levels. Notably, the highest POC concentrations (about 10–17 mg cm^–3) occur in sandy mud to mud (60–100% mud content) with intermediate POC contents of about 3–7% and DBDs of 0.1–0.4 g cm^–3. Areas with this range in values seem to represent the optimum conditions for POC accumulation in the Baltic Sea. The maximum POC contents (8–16%) are found in fluid mud of the central Baltic Sea characterized by extremely low DBDs (<0.1 g cm^–3) and moderate POC concentrations (4–7 mg cm^–3). Furthermore, sediment mass accumulation rates (MAR), based on ²¹⁰Pb and ¹³⁷Cs measurements and available for 303 sites of the Baltic Sea, were used for assessing the spatial distribution of POC burial rates. Overall, these vary between 14 and 35 g m^–2 year^–1 in the mud depositional areas and, in total, at least 3.5 (±2.9) Mt POC are buried annually. Distribution patterns of POC contents and burial rates are not identical for the central Baltic Sea because of the low MAR in this area. The presented data characterize Baltic Sea sediments as an important sink for organic carbon. Regional differences in organic carbon deposition can be explained by the origin and transport pathways of POC, as well as the environmental conditions prevailing at the seafloor (morphology, currents, redox conditions). These findings can serve to improve budget calculations and modelling of the carbon cycle in this large brackish-water marginal sea.     

Last glacial emplacement of methane-derived authigenic carbonates in the Sea of Japan constrained by diatom assemblage,carbon-14, and carbonate content

We studied diatom assemblages and CaCO₃ contents of methane-derived authigenic carbonates from the eastern margin of the Sea of Japan and assessed the formation time of these samples. Radioactive ¹⁴C date was determined in selected samples to obtain the maximum age of the time. The results of our study suggest mass formation of carbonate nodules in a glacial period within ∼40 ky, consistent with a published U/Th dating result of carbonate nodules in the study area. Diatom assemblages and contents in the carbonate nodules (abundance of ∼10⁶/g, dominance of neritic-littoral species, warm/cold water species ratio lower than ∼25) differ from the near-seafloor sediments in the study area, which have characteristics of Holocene sediments in the Sea of Japan, and suggest cementation of glacial sediments. Laminated sediments in some nodule samples are glacial sediments because laminations are records of a low sea level period in the semi-enclosed ocean. Similarity of diatom assemblages and contents in all carbonate samples is another evidence of glacial sediments in nodules. Glacial sediments with oceanic cold water species as low as Holocene sediments restricts the sediment age to before 20 cal. ky BP. Carbonate contents higher than 78 wt% suggest the cementation of poorly compacted sediments near the seafloor, and the date of carbonate cementation is, therefore, close to that of the cemented sediments. Most carbonate nodule samples in this study were formed in a glacial period and detection of ¹⁴C restricts this period to within ∼40 ky.     

Authigenic carbonates from active methane seeps offshore southwest Africa

The southwest African continental margin is well known for occurrences of active methane-rich fluid seeps associated with seafloor pockmarks at water depths ranging broadly from the shelf to the deep basins, as well as with high gas flares in the water column, gas hydrate accumulations, diagenetic carbonate crusts and highly diverse benthic faunal communities. During the M76/3a expedition of R/V METEOR in 2008, gravity cores recovered abundant authigenic carbonate concretions from three known pockmark sites—Hydrate Hole, Worm Hole, the Regab pockmark—and two sites newly discovered during that cruise, the so-called Deep Hole and Baboon Cluster. The carbonate concretions were commonly associated with seep-benthic macrofauna and occurred within sediments bearing shallow gas hydrates. This study presents selected results from a comprehensive analysis of the mineralogy and isotope geochemistry of diagenetic carbonates sampled at these five pockmark sites. The oxygen isotope stratigraphy obtained from three cores of 2–5?m length indicates a maximum age of about 60,000–80,000?years for these sediments. The authigenic carbonates comprise mostly magnesian calcite and aragonite, associated occasionally with dolomite. Their very low carbon isotopic compositions (–61.0?<?δ¹³C ‰ V-PDB?<?–40.1) suggest anaerobic oxidation of methane (AOM) as the main process controlling carbonate precipitation. The oxygen isotopic signatures (+2.4?<?δ¹⁸O ‰ V-PDB?<?+6.2) lie within the range in equilibrium under present-day/interglacial to glacial conditions of bottom seawater; alternatively, the most positive δ¹⁸O values might reflect the contribution of ¹⁸O-rich water from gas hydrate decomposition. The frequent occurrence of diagenetic gypsum crystals suggests that reduced sulphur (hydrogen sulphide, pyrite) from sub-seafloor sediments has been oxidized by oxygenated bottom water. The acidity released during this process can potentially induce the dissolution of carbonate, thereby providing enough Ca²⁺ ions for pore solutions to reach gypsum saturation; this is thought to be promoted by the bio-irrigation and burrowing activity of benthic fauna. The δ¹⁸O–δ¹³C patterns identified in the authigenic carbonates are interpreted to reflect variations in the rate of AOM during the last glacial–interglacial cycle, in turn controlled by variably strong methane fluxes through the pockmarks. These results complement the conclusions of Kasten et al. in this special issue, based on authigenic barite trends at the Hydrate Hole and Worm Hole pockmarks which were interpreted to reflect spatiotemporal variations in AOM related to subsurface gas hydrate formation–decomposition.     

Geoacoustic and physical properties of carbonate sediments of the Lower Florida Keys

 Near-surface sediment geoacoustic and physical properties were measured from a variety of unconsolidated carbonate sediments in the Lower Florida Keys. Surficial values of compressional and shear speed correlate with sediment physical properties and near-surface acoustic reflectivity. Highest speeds (shear 125–150 m s^-1; compressional 1670–1725 m s^-1) are from sandy sediments near Rebecca Shoal and lowest speeds (shear 40–65 m s^-1; compressional 1520–1570 m s^-1) are found in soft, silty sediments which collect in sediment ponds in the Southeast Channel of the Dry Tortugas. High compressional wave attenuation is attributed to scattering of acoustic waves from heterogeneity caused by accumulation of abundant shell material and other impedance discontinuities rather than high intrinsic attenuation. Compared to siliciclastic sediments, carbonate sediment shear wave speed is high for comparable values of sediment physical properties. Sediment fabric, rather than changes due to the effects of biogeochemical processes, is responsible for these differences.     

Authigenic minerals from the Paola Ridge (southern Tyrrhenian Sea): Evidences of episodic methane seepage

Paola Ridge, along the NW Calabrian margin (southern Tyrrhenian Sea), is one of the few reported deep sea sites of precipitation of authigenic carbonates in the Tyrrhenian Sea. Here, the changing composition of the seeping fluids and the dynamic nature of the seepage induced the precipitation of pyrite, siderite and other carbonate phases. The occurrence of this array of authigenic precipitates is thought to be related to fluctuation of the sulfate-methane transition zone (SMTZ).Concretions of authigenic minerals formed in the near sub-bottom sediments of the Paola Ridge were investigated for their geochemical and isotopic composition. These concretions were collected in an area characterized by the presence of two alleged mud volcanoes and three mud diapirs. The mud diapirs are dotted by pockmarks and dissected by normal faults, and are known for having been a site of fluid seepage for at least the past 40 kyrs. Present-day venting activity occurs alongside the two alleged mud volcanoes and is dominated by CO₂-rich discharging fluids. This discover led us to question the hypothesis of the mud volcanoes and investigate the origin of the fluids in each different domed structure of the study area.In this study, we used stable isotopes (carbon and oxygen) of carbonates coupled with rare earth element (REE) composition of different carbonate and non-carbonate phases for tracing fluid composition and early diagenesis of authigenic precipitates. The analyses on authigenic precipitates were coupled with chemical investigation of venting gas and sea-water.Authigenic calcite/aragonite concretions, from surficial sediments on diapiric structures, have depleted ¹³C isotopic composition and slightly positive δ¹⁸O values. By contrast, siderite concretions, generally found within the first 6 m of sediments on the alleged mud volcanoes, yielded positive δ¹³C and δ¹⁸O values. The siderite REE pattern shows consistent LREE (light REE) fractionation, MREE (medium REE) enrichment and positive Gd and La anomalies. As shown by the REE distribution, the ¹³C-depleted composition and their association with chemosymbiotic fauna, calcite/aragonite precipitated at time of moderate to high methane flux close to the seafloor, under the influence of bottom seawater. Authigenic siderite, on the other hand, formed in the subseafloor, during periods of lower gas discharges under prolonged anoxic conditions within sediments in equilibrium with ¹³C-rich dissolved inorganic carbon (DIC) and ¹⁸O-rich water, likely related to methanogenesis and intermittent venting of deep-sourced CO₂.     

Authigenic barite nodules and carbonate concretions in the Upper Devonian shale succession of western New York – A record of variable methane flux during burial

Authigenic barite nodules associated with modestly ¹³C-depleted calcium carbonate concretions and ³⁴S-enriched pyrite at the bottom of the Upper Devonian Hanover Shale of western New York provide evidence of sulfate reduction coupled with anaerobic oxidation of methane (AOM). The methane, much of it biogenic in origin, may have diffused upward from Middle Devonian Marcellus Shale and perhaps the Upper Ordovician Utica Shale. Strong ³⁴S enrichment and high δ³⁴S/δ¹⁸O values of the barite nodules reflect: (1) substantial kinetic fractionation induced by microbial sulfate reduction perhaps intensified by a low seawater sulfate recharge rate and (2) upward delivery of Ba²⁺- and CH₄- bearing pore fluid sourced within underlying sulfate-depleted deposits. However, the association of authigenic calcium carbonate and barite in the same stratigraphic interval, especially the presence of barite overgrowths on carbonate concretions, is not consistent with what is known of AOM-related mineralization of a sediment column passing downward through the sulfate–methane transition (SMT). The documented early formation of authigenic carbonate followed by barite observed relations may reflect a diminished rate of methanogenesis and/or CH₄ supply. The tempered methane flux would have induced the SMT to descend the sediment column enabling barite to form within the same stratigraphic horizon that ¹³C-depleted calcium carbonate had most recently precipitated. Diminished methane flux may have been caused by burial-related passage of the organic-rich Marcellus Shale below the depth of peak biogenic methane generation and its replacement at that depth interval by organic-lean deposits of the upper part of the Hamilton Group. Subsidence of the SMT would have increased the preservation potential of authigenic barite. However, continued survival of the labile barite as it eventually moved through the SMT suggests that the underlying sulfate-depleted zone was strongly enriched in Ba²⁺.     

Methane-derived authigenic carbonates from active hydrocarbon seeps of the St. Lawrence Estuary,Canada

Nearly 2000 pockmarks with diameters ranging from a few tens of meters up to 700 m are present on the seafloor of the St. Lawrence Estuary in eastern Canada. Coring of some pockmarks resulted in the recovery of various-sized and shaped carbonate concretions in a predominantly silty mud matrix. Petrographic and geochemical data on four authigenic carbonate concretions are reported as well as data from shell material in the unconsolidated sediment. Video observations and echo-sounder images indicate that the sampled pockmarks are actively gas venting. The video images show significant look-alike microbial mats in areas where gas is venting. The carbonate concretions are primarily made up of carbonate cements with varying percentage of shell fragments, micrite particles and fine-grained clastics. Orthorhombic crystal morphology and diagenetic fabrics including isopachous layers and botryoids characterize the aragonite cement. Oxygen isotopes ratios for the cement crusts do not record any thermal anomaly at the site of precipitation with δ¹⁸O_VPDB ratios (+3‰) in equilibrium with cold (5 °C) deep marine waters, whereas significant negative δ¹³C_VPDB ratios (−9.9 to −33.5‰) for cement and shell material within concretions indicate that the carbonates largely derive from the microbial oxidation of methane. The δ¹³C_VPDB ratios of aragonite shells (−2.7 to −5.6‰) taken from unconsolidated sediments at some distance from the concretions/vents show variable dilution of HCO₃⁻ with negative δ¹³C_VPDB ratios derived from microbial oxidation of methane with isotopically normal (0‰) marine bicarbonate. These results are in agreement with other lines of evidence suggesting that pockmarks formed through the recent and still active release of gas from a reservoir within the Paleozoic sedimentary succession.     

Authigenic carbonates from methane seeps of the Congo deep-sea fan

Submersible investigations with the ROV Victor 6000 of some pockmark structures on the seafloor of the Congo deep-sea fan have shown that they are active venting sites of methane-rich fluids, associated with abundant fauna and carbonate crusts. Moreover, methane hydrates have been observed both outcropping and deep in the sediments in the centre of the “Regab” giant pockmark. Authigenic carbonates, mostly calcite sometimes mixed with aragonite, are cementing the sedimentary matrix components and fauna; diatoms are abundant but only as moulds, indicating that biogenic silica dissolution occurred in situ synchronous with carbonate precipitation. The occurrence of diagenetic barite and pyrite in some carbonate crusts demonstrates that they can be formed either within the sulphate/methane transition zone or deeper in sulphate-depleted sediments. The oxygen isotopic compositions of the diagenetic carbonates (3.17–6.01‰ V-PDB) indicate that precipitation occurred with bottom seawater mixed with a variable contribution of water from gas hydrate decomposition. The very low carbon isotopic compositions of the diagenetic carbonates (−57.1 to −27.75‰ V-PDB) demonstrate that carbon derives mostly from the microbial oxidation of methane.     

Multidisciplinary investigations exploring indicators of gas hydrate occurrence in the Krishna–Godavari Basin offshore, east coast of India

We report some main results of multidisciplinary investigations carried out within the framework of the Indian National Gas Hydrate Program in 2002–2003 in the Krishna–Godavari Basin offshore sector, east coast of India, to explore indicators of likely gas hydrate occurrence suggested by preliminary multi-channel seismic reflection data and estimates of gas hydrate stability zone thickness. Swath bathymetry data reveal new evidence of three distinct geomorphic units representing (1) a delta front incised by several narrow valleys and mass flows, (2) a deep fan in the east and (3) a WNW–ESE-trending sedimentary ridge in the south. Deep-tow digital side-scan sonar, multi-frequency chirp sonar, and sub-bottom profiler records indicate several surface and subsurface gas-escape features with a highly resolved stratification within the upper 50 m sedimentary strata. Multi-channel seismic reflection data show the presence of bottom simulating reflections of continuous to discrete character. Textural analyses of 76 gravity cores indicate that the sediments are mostly silty clay. Geochemical analyses reveal decreasing downcore pore water sulphate (SO₄ ²⁻) concentrations (28.7 to <4 mM), increasing downcore methane (CH₄) concentrations (0–20 nM) and relatively high total organic carbon contents (1–2.5%), and microbial analyses a high abundance of microbes in top core sediments and a low abundance of sulphate-reducing bacteria in bottom core sediments. Methane-derived authigenic carbonates were identified in some cores. Combined with evidence of gas-escape features in association with bottom simulating reflections, the findings strongly suggest that the physicochemical conditions prevailing in the study area are highly conducive to methane generation and gas hydrate occurrence. Deep drilling from aboard the JOIDES Resolution during 2006 has indeed confirmed the presence of gas hydrate in the Krishna–Godavari Basin offshore.     

Seafloor geological studies above active gas chimneys off Egypt (Central Nile Deep Sea Fan)

Four mud volcanoes of several kilometres diameter named Amon, Osiris, Isis, and North Alex and located above gas chimneys on the Central Nile Deep Sea Fan, were investigated for the first time with the submersible Nautile. One of the objectives was to characterize the seafloor morphology and the seepage activity across the mud volcanoes. The seepage activity was dominated by emissions of methane and heavier hydrocarbons associated with a major thermal contribution. The most active parts of the mud volcanoes were highly gas-saturated (methane concentrations in the water and in the sediments, respectively, of several hundreds of nmol/L and several mmol/L of wet sediment) and associated with significantly high thermal gradients (at 10 m below the seafloor, the recorded temperatures reached more than 40 °C). Patches of highly reduced blackish sediments, mats of sulphide-oxidizing bacteria, and precipitates of authigenic carbonate were detected, indicative of anaerobic methane consumption. The chemosynthetic fauna was, however, not very abundant, inhibited most likely by the high and vigorous fluxes, and was associated mainly with carbonate-crust-covered seafloor encountered on the southwestern flank of Amon. Mud expulsions are not very common at present and were found limited to the most active emission centres of two mud volcanoes, where slow extrusion of mud occurs. Each of the mud volcanoes is fed principally by a main narrow channel located below the most elevated areas, most commonly in the centres of the structures. The distribution, shape, and seafloor morphology of the mud volcanoes and associated seeps over the Central Nile Deep Sea Fan are clearly tectonically controlled.     

Carbon isotopic composition and lattice-bound carbonate of Peru-Chile margin phosphorites

New light-stable carbonate-carbon isotope and lattice-bound CO₂ data from Quaternary Peru-Chile margin phosphatic nodules, crusts and pelletal grains, and from associated dolomicritic concretions, are presented, which provide constraints on the timing and mechanisms of growth of these phases in organic carbon-rich sediments. Comparison of δ¹³C values from carbonate fluorapatite (CFA) nodules and pelletal grains (−4.8 to 0.0‰ and −2.9 to +1.0‰ PDB, respectively) with pore-water total dissolved δ¹³C values from these sediments suggests early authigenic CFA precipitation from pore waters within a few centimeters of the sediment-water interface in association with suboxic to perhaps anoxic microbial degradation of organic matter. In contrast, the dolomicritic cores of nodules recovered from about 12°S display both strongly negative to positive δ¹³C values (−10.8 to +6.1‰) characteristic of formation deeper in the sediments in association with methanogenic and perhaps sulfate reduction microbial processes.

The amount of structural carbonate in CFA suggests that carbonate substitution generally increases as δ¹³C in CFA decreases, a probable consequence of increasing carbonate and accompanying charge-balancing substitutions in the CFA lattice in response to increasing pore-water carbonate ion concentrations with depth below the sediment-water interface. In one buried upward-growing nodule, decreasing CFA δ¹³C and increasing structural CO₂ also correspond to decreasing CFA growth rates. These data suggest that in addition to other constraints such as pore-water phosphorus and fluoride availability, the lower limit of CFA precipitation in suboxic to anoxic sediments may be controlled by lattice poisoning due to excessive dissolved carbonate ion concentrations. In organic-rich Peru-Chile margin sediments this depth threshold appears to be at approximately 5–10 cm below the sediment-water interface where maximum CFA CO₂ contents of about 6 Wt.% occur; in less organic-rich settings, greater depths of precipitation of CFA may be anticipated. Below this relatively shallow depth of CFA precipitation on the Peru shelf, high pore-water alkalinity and associated elevated total dissolved carbon and carbonate ion concentration apparently favor the precipitation of authigenic carbonates.     

Fluid expulsion features associated with sand waves on Australia’s central North West Shelf

Multibeam swath bathymetric data collected in 95–120 m water depth on Australia’s North West Shelf revealed two distinct populations of sand waves: a laterally extensive, low-amplitude composite form comprising superimposed dunes and ripples, and a laterally restricted form which has unusually high bedform heights and slopes. These large subaqueous sand waves comprise bioclastic ooid/peloid sand. Significantly, evidence of seabed fluid flow was detected in association with the high-amplitude sand waves. This evidence includes seabed pockmarks approximately 2–15 m in diameter imaged with side-scan sonar, tubular and massive carbonate concretions dredged from the seabed, and potential active venting of a fluid plume from the seabed observed during an underwater camera tow. Molecular and isotopic analyses of carbonate concretions collected from within pockmarks associated with the high-amplitude sand waves indicate that the fluids from which they precipitated comprise modern seawater and are not related to thermogenic fluids or microbial gases. The fluid flow is interpreted to be driven by macrotidal currents flowing over the relatively steep slopes of the high-amplitude sand waves. Pockmarks and carbonate concretions then develop where the interstitial flows are confined and focused by subsurface ‘mounds’ in a shallow seismic reflector.     

Methane-derived carbonates and chemosymbiotic communities of Piedmont (Miocene,northern Italy): An update

Stable carbon isotope values of authigenic carbonate rocks in the Miocene terrigenous sediments of Piedmont indicate a methane-related origin. Some of these methane-derived carbonates (Lucina limestone) are characterized by the presence of abundant lucinid remains. Carbonate dissolution/precipitation and development of lucinid communities were related to bacterial methane oxidation, both aerobic and anaerobic. Anaerobic oxidation led to carbonate precipitation and production of sulfide, which sustained lucinid communities through chemosynthetic symbiotic bacteria. Aerobic oxidation of methane likely resulted in dissolution of carbonate skeletal grains. Several phases of carbonate precipitation, characterized by slightly different isotopic compositions, are recognizable in the limestones.     

Ground-truthing 11- to 12-kHz side-scan sonar imagery in the Norwegia–Greenland Sea: Part I: Pockmarks on the Vestnesa Ridge and Storegga slide margin

 Numerous small (50- to 300-m-diameter) strong-backscatter objects were imaged on the 1200- to 1350-m deep crest of Vestnesa Ridge (Fram Strait) and along the 900- to 1000-m deep northeast margin of the Storegga slide valley. Ground-truthing identified most of these objects as 2- to 10-m-deep pockmarks, developed within soft, acoustically stratified silty clays (typical wet bulk density: 1400–1600 kg m^-3; sound speed: 1480– 1505 m s^-1; porosity, 65–75%; shear strength: 5–10 kPa; water content: 80–120%; and thermal conductivity: 0.8–0.9 W m^-1 deg C^-1 in the top 3 m). Gas wipeouts, enhanced reflectors, and reflector discontinuities indicate recent or ongoing activity, but the absence of local heat flow anomalies suggests that any upward fluid flows are modest and/or local.     

Contrasting mercury and manganese deposition in a mangrove-dominated estuary (Guaratuba Bay, Brazil)

Sediment cores were taken at seven sites along the mangrove-bound Guaratuba Bay estuary (southern Brazil), with the purpose of assessing conditions controlling Hg deposition along a horizontal estuarine sediment gradient. The data suggest contrasting depositional patterns for Hg and Mn in this relatively pristine setting. Total Hg contents of bulk sediments ranged from 12 to 36 ng/g along the estuary, the highest values being found in muddier organic-rich sediments of the upper estuary (the corresponding mud gradient is 12 to 42 wt.%, and the organic matter gradient 4 to 10 wt.%). Thus, the deposition of fine sediments relatively enriched in mercury occurs primarily in closer proximity to the freshwater source. The data also indicate a reverse gradient in reactive Mn contents, ranging from 29 to 81 μg/g, and increasing seaward. This implies that reactive Mn is mobilized from fine-grained reducing mangrove forest sediments in the upper estuary, and deposited downstream in sandier, oxygen-rich nearshore sediments. These results suggest that mangrove-surrounded estuaries may act as barriers to mercury transport to coastal waters, but as a source of manganese. The present findings also imply that reactive Mn may be used as an indication of Hg depositional patterns in other similar coastal sedimentary settings.     

Barium Increasing Prior to Opal during the Last Termination of Glacial Ages in the Okhotsk Sea Sediments

Ba and Ti in a sediment core (10 m long) from the Okhotsk Sea, covering the last 120 kyrs, were measured. The authigenic Ba (Ba_ex) contents were calculated and compared with the opal. The correlation coefficient between Ba_ex and opal was quite small (r = 0.34), but it greatly grew larger (r = 0.90), if the Ba_ex contents were multiplied by a simple function increasing with depth, except for two intervals. This may be due to the gradual change in the sedimentation environment during the glacial ages. One of the exceptional interval is found at 60–170 cm in depth, corresponding to 10–17 calendar kyr ago, the last termination period of glacial ages, where the Ba_ex began to increase prior to opal. Since the calcium carbonate contents similarly increased prior to the opal increase, the Ba_ex may be also related to calcareous organisms besides siliceous ones. The other is the last interglacial period around 120 kyr ago when the opal contents were high, but those of the Ba_ex were not increased. This can be explained, if the Ba_ex was reduced to sulfide and dissolved away in a strongly anoxic environment during the biologically productive period. During the glacial ages, the mass accumulation rate (MAR) of lithogenic Ti was about twice the amount of that during the interglacial ages. For opal, however, the contrast between the glacial and interglacial ages was more remarkable in its MAR than in its concentration in sediments, due to the larger variation in the bulk sedimentation rates. This revised version was published online in July 2006 with corrections to the Cover Date.     

Scale-dependent physical and geoacoustic property variability of shallow-water carbonate sediments from the Dry Tortugas, Florida

 Spatial variability of shallow-water carbonate sediments near Dry Tortugas, Florida, is scale-dependent. Wet bulk density, grain density, porosity, compressional wave velocity, and grain size variability generally increase down to 2.4 m vertically and 850 m laterally. Grain size is most variable, followed by porosity, wet bulk density, compressional wave velocity, and grain density bothvertically and laterally, consistent with Walther’s Law. Variability was empirically modeled by linear regression analysis to predict variability based on scale, characterize sediment property variability, and quantify sedimentisotropy.     

Exhumed hydrocarbon-seep authigenic carbonates from Zakynthos Island (Greece): Concretions not archaeological remains

In Zakynthos Island (Greece), authigenic cementation of marine sediment has formed pipe-like, disc and doughnut-shaped concretions. The concretions are mostly composed of authigenic ferroan dolomite accompanied by pyrite. Samples with >80% dolomite, have stable isotope compositions in two groups. The more indurated concretions have δ¹⁸O around +4‰ and δ¹³C values between −8 and −29‰ indicating dolomite forming from anaerobic oxidation of thermogenic methane (hydrocarbon seep), in the sulphate-methane transition zone. The outer surfaces of some concretions, and the less-cemented concretions, typically have slightly heavier isotopic compositions and may indicate that concretion growth progressed from the outer margin in the ambient microbially-modified marine pore fluids, inward toward the central conduit where the isotopic compositions were more heavily influenced by the seep fluid. Sr isotope data suggest the concretions are fossil features, possibly of Pliocene age and represent an exhumed hydrocarbon seep plumbing system. Exposure on the modern seabed in the shallow subtidal zone has caused confusion, as concretion morphology resembles archaeological stonework of the Hellenic period.