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Highlights: • Clay dehydration water expelled from buried sediments drives mud volcanism. • Rise of fluids mediated by crustal-scale strike-slip faults cross-cutting wedge. • On active accretionary wedge, petroleum accumulations were dismantled in Neogene. • 4He enrichment and δ13C-CH4 ~−50‰ in fluids reflect an open hydrocarbon system. • Petroleum pools remain on shallow margin. Microbial gas vented out of active wedge. Abstract: A geochemical study of the composition of hydrocarbon gases and helium isotopes (3He/4He) in fluids from Mud Volcanoes (MVs) located on and out of the active accretionary wedge of the Gulf of Cadiz (GoC) provides information on fluid sources and migrations in deeply buried sediments. The GoC is a tectonically active segment of the Africa-Iberia plate boundary occluded beneath the thick sediments of an accretionary wedge dissected by crustal-scale strike-slip faults. Initially built during the Miocene Gibraltar Arc subduction, the wedge has since developed toward the W-NW in an oblique convergent setting. Interstitial water expelled from clays undergoing diagenesis in buried sediments drives mud volcanism on the wedge, with MVs located along strike-slip faults mediating fluid ascent. The large excess of radiogenic helium (4He) in all GoC fluids agrees with a clay mineral dehydration source of water. Hydrocarbon gases from all deepwater MVs bear methane having similar stable carbon isotope compositions of ~−50‰VPDB whether fluids are highly enriched in methane relative to heavier homologues (C2+) or not (Methane / (Ethane + Propane) ~10 to 10,000). We suggest that methane with −50‰VPDB was largely diffused out of early generating source rocks, and became dissolved in the water expelled by the buried sediments. Consistently, low 3He/4He ratios suggest an open hydrocarbon system: Petroleum accumulations and 3He dissolved in the original sedimentary pore water have mostly escaped into the water column during the major Late Neogene compressional events. At present, some MVs vent CH4-rich fluids from dewatering sediments, while other structures located on active thrusts additionally vent C2+-rich gases generated by active Cretaceous source intervals. By contrast, evaporitic seals preserved petroleum accumulations on the shallow Moroccan Margin, while the westernmost MVs located out of the accretionary wedge vent microbial gas.

The formation of mud volcanoes in the Gulf of Cadiz is closely linked to diapirism in the deep subsurface. The Mercator mud volcano (MMV) is a rare example where diapiric emplacement, in addition to being key for upward fluid migration, is also an important zone for fluid and mineral diagenesis. The most intriguing findings in the near-surface muds of the MMV are extremely high salinities of up to 5.2 M of NaCl from diapiric and evaporitic halite dissolution and the occurrence of authigenic gypsum and anhydrite crystals, both of which have not been observed to date in the Gulf of Cadiz. Employing a thermodynamic model we elucidate how the interplay of temperature pulses, strong salinity gradients, and fluid flow dynamically drive mineral dissolution and re-formation. The strong increase in salinity in the pore fluids has important implications for thermodynamic equilibria by significantly lowering the activity of water, thereby raising the gypsum–anhydrite transition zone from >1 km to about 400 m sediment depth at the MMV. This transition is further shifted to immediately below the seafloor during intervals of active mud and fluid expulsion when the MV surface temperature is heated up to at least 30 °C. As a consequence, precipitation of authigenic gypsum near the sediment surface (1–2 mbsf) has been linked to the dissolution of evaporites below the MMV. More precisely, the mechanisms generating supersaturation in the ascending gypsum-saturated MMV fluids are (1) the slow and constant cooling of these fluids along the geothermal gradient during their ascent leading to formation of ubiquitous micro-crystals and (2) the more rapid cooling after a heat pulse or transport from greater and warmer depth during an active mud volcano phase leading to the precipitation of cm-scale gypsum crystals or even fist-size concretions. The MMV fluids approaching the salt diapir from farther below have experienced a genesis similar to those of other mud volcanoes in the Gulf of Cadiz located above deep-rooted faults. These processes include clay mineral dewatering, thermogenic degradation of organic matter and deep high-temperature leaching of terrigenous sediments or continental crust.

International audience; Four seep sites located within an ∼20 km2 area offshore Georgia (Batumi seep area, Pechori Mound, Iberia Mound, and Colkheti Seep) show characteristic differences with respect to element concentrations, and oxygen, hydrogen, strontium, and chlorine isotope signatures in pore waters, as well as impregnation of sediments with petroleum and hydrocarbon potential. All seep sites have active gas seepage, near surface authigenic carbonates and gas hydrates. Cokheti Seep, Iberia Mound, and Pechori Mound are characterized by oil-stained sediments and gas seepage decoupled from deep fluid advection and bottom water intrusion induced by gas bubble release. Pechori Mound is further characterized by deep fluid advection of lower salinity pore fluids. The Pechori Mound pore fluids are altered by mineral/water reactions at elevated temperatures (between 60 and 110 °C) indicated by heavier oxygen and lighter chlorine isotope values, distinct Li and B enrichment, and K depletion. Strontium isotope ratios indicate that fluids originate from late Oligocene strata. This finding is supported by the occurrence of hydrocarbon impregnations within the sediments. Furthermore, light hydrocarbons and high molecular weight impregnates indicate a predominant thermogenic origin for the gas and oil at Pechori Mound, Iberia Mound, and Colkheti Seep. C15+ hydrocarbons at the oil seeps are allochtonous, whereas those at the Batumi seep area are autochthonous. The presence of oleanane, an angiosperm biomarker, suggests that the hydrocarbon source rocks belong to the Maikopian Formation. In summary, all investigated seep sites show a high hydrocarbon potential and hydrocarbons of Iberia Mound, Colkheti Seep, and Pechori Mound are predominantly of thermogenic origin. However, only at the latter seep site advection of deep pore fluids is indicated.

Gulf of Mexico cold seeps characterized by variable compositions and magnitudes of hydrocarbon seepage were sampled in order to investigate the effects of natural oils, methane, and non-methane hydrocarbons on microbial activity, diversity, and distribution in seafloor sediments. Though some sediments were characterized by relatively high quantities of oil, which may be toxic to some microorganisms, high rates of sulfate reduction (SR, 27.9714.7 mmol m

In February 2008, cruise P362/2 was undertaken aboard R/V Poseidon to the Giza and North Alex mud volcanoes (MVs) on the upper slope of the western Nile deep-sea fan. Emitted fluids were strongly depleted in chloride and rich in hydrocarbons, predominantly of thermogenic origin. In-situ sediment temperature measurements indicate extremely high and moderate levels of activity for the North Alex MV and Giza MV, respectively, and suggest rapid changes from dormant to active stages. Both the physical properties of core sediments (e.g., color and magnetic susceptibility), and their assemblages of micro- and nannofossils point to different sources for the two mud volcanoes. Biostratigraphic dating suggests source depths of 2,100–2,450 mbsf for the Giza MV and 1,150–1,550 mbsf for the North Alex MV. Very high temperatures of up to 70°C in shallow sediments at the North Alex MV can be explained only if the fluid source were warmer and deeper than the sediment source.

Vodyanitskii mud volcano is located at a depth of about 2070 m in the Sorokin Trough, Black sea. It is a 500-m wide and 20-m high cone surrounded by a depression, which is typical of many mud volcanoes in the Black Sea. 75 kHz sidescan sonar show different generations of mud flows that include mud breccia, authigenic carbonates, and gas hydrates that were sampled by gravity coring. The fluids that flow through or erupt with the mud are enriched in chloride (up to similar to 650 mmol L-1 at similar to 150-cm sediment depth) suggesting a deep source, which is similar to the fluids of the close-by Dvurechenskii mud volcano. Direct observation with the remotely operated vehicle QUEST revealed gas bubbles emanating at two distinct sites at the crest of the mud volcano, which confirms earlier observations of bubble-induced hydroacoustic anomalies in echosounder records. The sediments at the main bubble emission site show a thermal anomaly with temperatures at similar to 60 cm sediment depth that were 0.9 degrees C warmer than the bottom water. Chemical and isotopic analyses of the emanated gas revealed that it consisted primarily of methane (99.8%) and was of microbial origin (delta D-CH4 = -170.8 parts per thousand (SMOW), delta C-13-CH4 = -61.0 parts per thousand (V-PDB), delta C-13-C2H6 = -44.0 parts per thousand (V-PDB)). The gas flux was estimated using the video observations of the ROV. Assuming that the flux is constant with time, about 0.9 +/- 0.5 x 106 mol of methane is released every year. This value is of the same order-of-magnitude as reported fluxes of dissolved methane released with pore water at other mud volcanoes. This suggests that bubble emanation is a significant pathway transporting methane from the sediments into the water column. (C) 2009 Elsevier Ltd. All rights reserved.

Submarine mud volcanism represents an important pathway for methane from deeper reservoirs to the surface, where it enters the benthic carbon cycle. To quantify overall methane release from the Captain Arutyunov mud volcano (CAMV) and to assess the contribution of mac- robenthic seep organisms to the regulation of the benthic methane flux, we linked water column methane concentrations, seabed methane emission and pore water geochemistry to the spatial distri- bution of seep biota. Prominent organisms of the CAMV seep biota were 3 different species of frenu- late tubeworms. Seabed methane emission ranged from 0.001 to 0.66 mmol m -2 d -1 . Dense patches of tubeworms were associated with the lowest seabed methane emission. Elevated methane emission was associated with a sporadic distribution of tubeworms and the occurrence of numerous mud clasts. Despite the presence of a large subsurface methane reservoir, the estimated total methane release from CAMV was low (0.006 × 10 6 mol yr -1 ). In addition to direct methane consumption by Siboglinum poseidoni, the tubeworms likely contribute to the retention of methane carbon in the sed- iment by affecting bacterial communities in the proximity of the tubes. The siboglinids create new meso-scale habitats on the sediment surface, increasing habitat heterogeneity and introducing niches for bacterial communities.

Based upon the molecular and isotopic composition of hydrocarbons it has been proposed that the source of CH4 in Gulf of Cadiz mud volcanoes (MV) is a mixture of deep sourced thermogenic CH4 and shallow biogenic CH4. We directly investigated this possibility by comparing porewater CH4 concentrations and their δ13C values with the potential for Archaeal methanogenesis in Gulf of Cadiz mud volcano (MV) sediments (Captain Arutyunov, Bonjardim, Ginsburg and Porto) using 14C-rate measurements. The CH4 has a deep sourced thermogenic origin (δ13C ∼ -49) but becomes 13C-depleted in and beneath the zone of anaerobic oxidation of methane (AOM) where the rates of hydrogenotrophic methanogenesis increase. Thus we infer that a portion of AOM-produced CO2 is being recycled to CH4 by methanogens yielding further 13C-depleted CH4, which might be misinterpreted as indicative of a fully shallow biogenic origin for this gas. Production of H2 is related to compositional changes in sedimentary organic matter, or to upward flux of substrate-enriched fluids. In contrast to other MVs in the Gulf of Cadiz, Ginsburg MV fluids are enriched in SO42- and contain very high concentrations of acetate (2478 μM below 150 cmbsf); however, the high levels of acetate did not stimulate methanogenesis but instead were oxidized to CO2 coupled to sulphate reduction. Both anaerobic oxidation of thermogenic CH4 linked to shallow methanogenesis and fluid geochemistry control the recycling of deep-sourced carbon at Gulf of Cadiz MVs, impacting near-surface δ13C-CH4 values.

Carbonate precipitates recovered from 2,000 m water depth at the Dolgovskoy Mound (Shatsky Ridge, north eastern Black Sea) were studied using mineralogical, geochemical and lipid biomarker analyses. The carbonates differ in shape from simple pavements to cavernous structures with thick microbial mats attached to their lower side and within cavities. Low δ13C values measured on carbonates (−41 to −32‰ V-PDB) and extracted lipid biomarkers indicate that anaerobic oxidation of methane (AOM) played a crucial role in precipitating these carbonates. The internal structure of the carbonates is dominated by finely laminated coccolith ooze and homogeneous clay layers, both cemented by micritic high-magnesium calcite (HMC), and pure, botryoidal, yellowish low-magnesium calcite (LMC) grown in direct contact to microbial mats. δ18O measurements suggest that the authigenic HMC precipitated in equilibrium with the Black Sea bottom water while the yellowish LMC rims have been growing in slightly 18O-depleted interstitial water. Although precipitated under significantly different environmental conditions, especially with respect to methane availability, all analysed carbonate samples show lipid patterns that are typical for ANME-1 dominated AOM consortia, in the case of the HMC samples with significant contributions of allochthonous components of marine and terrestrial origin, reflecting the hemipelagic nature of the primary sediment.

Suboxic sapropel S1 sediments of the eastern Mediterranean carry an untypically light δ98/95Mo signal, being even lighter than oxic Mn-enriched sediments and Mn-crusts/nodules, which were previously thought to carry the lightest Mo isotope values. The evaluation and comparison of oxic S1, diagenetically overprinted oxic S1, and suboxic S1 sediments indicates that this light isotope signal is primarily an oxic signal that has been overprinted by secondary diagenetic processes occurring after sediment burial. Such secondary processes bear the potential of additional Mo isotope fractionation in particular in non-steady state diagenetic environments that involve the discontinuous re-location of the redox boundary.

Pore fluids from the Green Canyon Block in the northern Gulf of Mexico show distinct differences with respect to element concentrations and oxygen, hydrogen, and strontium isotope signatures. The shallowest of the three investigated sites (GC185 or Bush Hill at 540 m water depth) is interpreted as a seafloor piercing mud mound and the two deeper areas (GC415 East and West at 950 and 1050 m water depth) as gas vent and oil seep sites. All three locations accommodate near-surface gas hydrates and the sediment surface is populated with chemosynthetic communities. They are characterized by a distinct increase in salinity with depth. However, the origin of this increasing salinity is different for the GC415 sites and Bush Hill and the depth source of the fluids is considerably different for all sites. The more saline fluids of the GC415 sites result from the dissolution of halite by formation water from two different sources. The fluids of GC415 East have most likely a deeper origin (early Cenozoic or even Mesozoic) and experienced elevated temperatures leading to mineral/water reactions including mineral transformations (e.g. smectite to illite transformation) and dissolution (e.g. feldspar dissolution). This process is expressed by the heavier oxygen isotope values and distinct Li, Sr, and Ca enrichments. The fluids of GC415 West have a shallower origin which is expressed by a smaller enrichment in Li, Sr, and Ca and lighter oxygen isotopes. The fluids from Bush Hill are less saline and its fluid signature indicates intensive water/mineral interaction. Oxygen and hydrogen isotope values as well as Na/Cl and Br/Cl molar ratios suggest that the salt enrichment was caused by phase separation under sub-critical conditions. A simple heat flow model simulation suggests that sub-critical phase separation may have occurred at a depth of ∼ 1650 m at ∼ 350 °C.

CaCO3 contents in eastern Mediterranean sapropels are generally lower than in the enclosing marl sediments. In sediments of a transect from ∼ 650 to 2100 m water depth at the Sirte continental slope, the opposite feature was observed. Although the increased Sr/Ca ratios observed in these sediments agree with such feature observed in other eastern Mediterranean S1 sediments, their absolute magnitude is much higher. The enhanced Sr/Ca ratio in these sediments is associated with their aragonite content, which for the Sirte transect reaches levels of up to ∼ 40 wt.%. The aragonite content and Sr/Ca ratios in the S1 sediments of this transect decrease with increasing water depth and decrease with distance to the African coast for all eastern Mediterranean cores. In view of the lack of a coherent relationship with sedimentary reduced sulphur contents and porosity, and considering the major amount of aragonite found specifically in the sediments of the Sirte transect, authigenic precipitation can contribute only a limited fraction at most. SEM observation and electron-microprobe analyses showed that the needles and needle clusters are morphologically aragonite, contain the highest Sr/Ca ratio, and are similar to skeleton fragments of the green alga Halimeda. Consequently, a detrital/biogenic source is the likely mechanism for the major part of the aragonite enrichments found in S1 sediments. Possibly, offshore-directed surface water flows related to wind stress and/or enhanced run-off (consistent with enhanced precipitation during sapropel S1) may have assisted in the transport of near-coastal aragonitic organisms to more coast-remote areas.

A high-resolution study of an active Fe 2+ /Fe 3+ redox horizon has been carried out on sediments of the central equatorial Atlantic. The multidisciplinary approach combining geochemical and rock-magnetic parameters gives evidence of the interrelation of the redox horizon with the last change from glacial to interglacial conditions (T1). Distinct enrichments of redox-sensitive elements (Mn, Fe, V, and U) reveal a characteristic depth-sequence, indicating non-steady-state diagenetic conditions caused by a decrease in productivity and an increase in oxygen content in the bottom water during T1 and a reversal during the Holocene. Thus, the redox boundaries first moved down into the sediment and subsequently upwards. The movement of the redox boundaries led to the development of conspicuous double peaks for Fe and Mn. The reconstruction based on geochemical data is supported by susceptibility and magnetization measurements of the sediments displaying well-defined anomalies in the vicinity of active and paleo redox boundaries. The combination of geochemical and rock-magnetic parameters, like the ferrimagnetic susceptibility, and total Fe concentration ( χ f /Fe) are suitable for characterizing distinct zones of dissolution and precipitation of ferromagnetic mineral phases (e.g. magnetite). Moreover, the small scale movement of the Fe 2+ /Fe 3+ redox-boundary, could be traced in detail by overlaying the peak-shape pattern of V/Al and of χ f / χ tot .

Sediments deposited on deep-sea fans are an excellent geological archive to reconstruct past changes in fluvial discharge. Here we present a reconstruction of changes in the regime of the Nile River during the Holocene obtained using bulk elemental composition, grain-size analyses and radiogenic strontium (Sr) and neodymium (Nd) isotopes from a sediment core collected on the Nile deep-sea fan. This 6-m long core was retrieved at View the MathML source water-depth and is characterized by the presence of a 5-m thick section of finely laminated sediments, which were deposited between 9.5 and 7.3 ka BP and correspond to the African Humid Period (AHP). The data show distinct changes in eolian dust inputs as well as variations in discharge of the Blue Nile and White Nile. Sedimentation was mainly controlled by changes in fluvial discharge during the Holocene, which was predominantly forced by low-latitude summer insolation and by the location of the eastern African Rain Belt. The changes in relative contribution from the Blue Nile and White Nile followed changes in low-latitude spring/autumn insolation, which highlights the role of changes in seasonality of the precipitation on the Nile River regime. The relative intensity of the Blue Nile discharge was enhanced during the early and late Holocene at times of higher spring insolation (with massive erosion and runoff during the AHP at times of high summer insolation), while it was reduced between 8 and 4 ka at times of high autumn insolation. The gradual insolation-paced changes in fluvial regime were interrupted by a short-term arid event at 8.5–7.3 ka BP (also associated with rejuvenation of bottom-water ventilation above the Nile fan), which was likely related to northern hemisphere cooling events. Another arid event at 4.5–3.7 ka BP occurred as the apex of a gradually drier phase in NE Africa and marks the end of the AHP.

Hydrocarbon seeps are ubiquitous at gas-prone Cenozoic deltas such as the Nile Deep Sea Fan (NDSF2) where seepage into the bottom water has been observed at several mud volcanoes (MVs3) including North Alex MV (NAMV4). Here we investigated the sources of hydrocarbon gases and sedimentary organic matter together with biomarkers of microbial activity at four locations of NAMV to constrain how venting at the seafloor relates to the generation of hydrocarbon gases in deeper sediments. At the centre, high upward flux of hot (70 °C) hydrocarbon-rich fluids is indicated by an absence of biomarkers of Anaerobic Oxidation of Methane (AOM) and nearly constant methane (CH4) concentration depth-profile. The presence of lipids of incompatible thermal maturities points to mixing between early-mature petroleum and immature organic matter, indicating that shallow mud has been mobilized by the influx of deep-sourced hydrocarbon-rich fluids. Methane is enriched in the heavier isotopes, with values of δ13C∼−46.6‰VPDB and δD ∼−228‰VSMOW, and is associated with high amounts of heavier homologues (C2+) suggesting a co-genetic origin with the petroleum. On the contrary at the periphery, a lower but sustained CH4 flux is indicated by deeper sulphate–methane transition zones and the presence of 13C-depleted biomarkers of AOM, consistent with predominantly immature organic matter. Values of δ13C-CH4∼−60‰VPDB and decreased concentrations of 13C-enriched C2+ are typical of mixed microbial CH4 and biodegraded thermogenic gas from Plio-Pleistocene reservoirs of the region. The maturity of gas condensate migrated from pre-Miocene sources into Miocene reservoirs of the Western NDSF is higher than that of the gas vented at the centre of NAMV, supporting the hypothesis that it is rather released from the degradation of oil in Neogene reservoirs. Combined with the finding of hot pore water and petroleum at the centre, our results suggest that clay mineral dehydration of Neogene sediments, which takes place posterior to reservoir filling, may contribute to intense gas generation at high sedimentation rate deltas.

A natural carbon dioxide (CO 2) seep was discovered during an expedition to the southern German North Sea (October 2008). Elevated CO 2 levels of ∼10–20 times above background were detected in seawater above a natural salt dome ∼30 km north of the East‐Frisian Island Juist. A single elevated value 53 times higher than background was measured, indicating a possible CO 2 point source from the seafloor. Measured pH values of around 6.8 support modeled pH values for the observed high CO 2 concentration. These results are presented in the context of CO 2 seepage detection, in light of proposed subsurface CO 2 sequestering and growing concern of ocean acidification. We explore the boundary conditions of CO 2 bubble and plume seepage and potential flux paths to the atmosphere. Shallow bubble release experiments conducted in a lake combined with discrete‐bubble modeling suggest that shallow CO 2 outgassing will be difficult to detect as bubbles dissolve very rapidly (within meters). Bubble‐plume modeling further shows that a CO 2 plume will lose buoyancy quickly because of rapid bubble dissolution while the newly CO 2 ‐enriched water tends to sink toward the seabed. Results suggest that released CO 2 will tend to stay near the bottom in shallow systems (

Lithium concentration and isotope data (δ7Li) are reported for pore fluids from 18 cold seep locations together with reference fluids from shallow marine environments, a sediment-hosted hydrothermal system and two Mediterranean brine basins. The new reference data and literature data of hydrothermal fluids and pore fluids from the Ocean Drilling Program follow an empirical relationship between Li concentration and δ7Li (δ7Li = −6.0(±0.3) · ln[Li] + 51(±1.2)) reflecting Li release from sediment or rocks and/or uptake of Li during mineral authigenesis. Cold seep fluids display δ7Li values between +7.5‰ and +45.7‰, mostly in agreement with this general relationship. Ubiquitous diagenetic signals of clay dehydration in all cold seep fluids indicate that authigenic smectite–illite is the major sink for light pore water Li in deeply buried continental margin sediments. Deviations from the general relationship are attributed to the varying provenance and composition of sediments or to transport-related fractionation trends. Pore fluids on passive margins receive disproportionally high amounts of Li from intensely weathered and transported terrigenous matter. By contrast, on convergent margins and in other settings with strong volcanogenic input, Li concentrations in pore water are lower because of intense Li uptake by alteration minerals and, most notably, adsorption of Li onto smectite. The latter process is not accompanied by isotope fractionation, as revealed from a separate study on shallow sediments. A numerical transport-reaction model was applied to simulate Li isotope fractionation during upwelling of pore fluids. It is demonstrated that slow pore water advection (order of mm a−1) suffices to convey much of the deep-seated diagenetic Li signal into shallow sediments. If carefully applied, Li isotope systematics may, thus, provide a valuable record of fluid/mineral interaction that has been inherited several hundreds or thousands of meters below the actual seafloor fluid escape structure.

Widespread mud volcanism across the thick ( 7000 between sites. Mixing of shallow biogenic and deep thermogenic gases cannot account for the observed compositions which instead result mainly from extensive migration of thermogenic gases in the deeply-buried sediments, possibly associated with biodegradation of C2+ homologues and secondary methane production at Captain Arutyunov and Carlos Ribeiro MVs. At the deep-water Bonjardim, Olenin and Carlos Ribeiro MVs, generation of C2+-enriched gases is probably promoted by high heat flux anomalies which have been measured in the western area of the wedge. At Porto MV, gases are highly enriched in CH4 having delta C-13-CH4 similar to -50 parts per thousand, as at most sites, but markedly lower delta H-2-CH4 Values < -250 parts per thousand, suggesting that it is not generated by thermal cracking of n-alkanes but rather that it has a deep Archaeal origin. The presence of petroleum-type hydrocarbons is consistent with a thermogenic origin, and at sites where CH4 is predominant support the suggestion that gases have experienced extensive transport during which they mobilized oil from sediments similar to 2-4 km deep. These fluids then migrate into shallower, thermally immature muds, driving their mobilization and extrusion at the seafloor. At Porto MV, the limited presence of petroleum in mud breccia sediments further supports the hypothesis of a predominantly deep microbial origin of CH4. (C) 2009 Elsevier B.V. All rights reserved.

The chemical and isotopic composition of pore fluids is presented for five deep-rooted mud volcanoes aligned on a transect across the Gulf of Cadiz continental margin at water depths between 350 and 3860 m. Generally decreasing interstitial Li concentrations and Sr-87/Sr-86 ratios with increasing distance from shore are attributed to systematically changing fluid sources across the continental margin. Although highest Li concentrations at the near-shore mud volcanoes coincide with high salinities derived from dissolution of halite and late-stage evaporites, clayey, terrigenous sediments are identified as the ultimate Li source to all pore fluids investigated. Light delta Li-7 values, partly close to those of hydrothermal vent fluids (delta Li-7: +11.9 parts per thousand), indicate that Li has been mobilized during high-temperature fluid/sediment or fluid/rock interactions in the deep sub-surface. Intense leaching of terrigenous clay has led to radiogenic Sr-87/Sr-86 ratios (similar to 0.7106) in pore fluids of the near-shore mud volcanoes. In contrast, non-radiogenic Sr-87/Sr-86 ratios (similar to 0.7075) at the distal locations are attributed to admixing of a basement-derived fluid component, carrying an isotopic signature from interaction with the basaltic crust. This inference is substantiated by temperature constraints from Li isotope equilibrium calculations suggesting exchange processes at particularly high temperatures (>200 degrees C) for the least radiogenic pore fluids of the most distal location.Advective pore fluids in the off-shore reaches of the Gulf of Cadiz are influenced by successive exchange processes with both oceanic crust and terrigenous, fine-grained sediments, resulting in a chemical and isotopic signature similar to that of fluids in near-shore ridge flank hydrothermal systems. This suggests that deep-rooted mud volcanoes in the Gulf of Cadiz represent a fluid pathway intermediate between mid-ocean ridge hydrothermal vent and shallow, marginal cold seep. Due to the thicker sediment coverage and slower fluid advection rates, the overall geochemical signature is shifted towards the sediment-diagenetic signal compared to ridge flank hydrothermal environments. (C) 2009 Elsevier Ltd. All rights reserved.

Organic-rich sedimentary units called sapropels have formed repeatedly in the eastern Mediterranean Sea, in response to variations of solar radiation. Sapropel formation is due to a change either in the flux of organic matter to the sea floor from productivity changes or in preservation by bottom-water oxygen levels. However, the relative importance of surface-ocean productivity versus deep-water preservation for the formation of these organic-rich shale beds is still being debated, and conflicting interpretations are often invoked. Here we analyse at high resolution the differences in the composition of the most recent sapropel, S1, in a suite of cores covering the entire eastern Mediterranean basin. We demonstrate that during the 4,000 years of sapropel formation, surface-water salinity was reduced and the deep eastern Mediterranean Sea, below 1,800 m depth, was devoid of oxygen. This resulted in the preferential basin-wide preservation of sapropel S1 with different characteristics above and below 1,800 m depth as a result of different redox conditions. We conclude that climate-induced stratification of the ocean may therefore contribute to enhanced preservation of organic matter in sapropels and potentially also in black shales. © 2008 Macmillan Publishers Limited.

[1] The residual dark unit of the most recent eastern Mediterranean sapropel (S1) is usually overlain by sediments with enhanced concentrations of MnOx in two separated layers. The variability and magnitude of the Mn enrichment at different locations and water depths indicate that Mn must have been added preferentially to sediments at intermediate (1–2 km) water depths. We propose a two-stage mechanism for the Mn enrichment that involves decreasing oxygenation with increasing water depth. This mechanism involves the loss of reduced Mn2+ from the deepest sediments (>2 km water depth) into overlying anoxic waters and a variable gain of MnOx in sediments in contact with oxygenated waters at shallower depth. In the S1 unit that receives the extra MnOx input, an upper higher Mn-enriched zone (>3 wt %) is maintained continuously at the top of the accumulating S1 unit because the pore waters are anoxic at shallow sediment depth while bottom waters are oxic to some degree. In a reactive-transport model, the Mn enrichment in the upper zone could not be supported by normal sediment diagenesis. Thus the MnOx in the upper Mn horizon must have formed mainly in the water column. The MnOx in the upper Mn-enriched zone adsorbed Mo and Li from seawater in a similar manner as other Mn-enriched oxic sediments, nodules, and crusts, with a Mn:Mo ratio of ∼600:1, a Mn:Li ratio of ∼750:1, and a δ98/95MoMOMO of −2.5 ‰.

The explosive eruption on the island of Santorini in ~1630 B.C. in Minoan times had a large environmental impact over the eastern Mediterranean region. It has even been suggested that the Mn-enriched layer (the bMarker BedQ) above the most recent sapropel (S1) in sediments of a crestal area of the Mediterranean Ridge gained Mn from a hydrothermal source related to the Santorini eruption. Radiocarbon dating of two cores from this area sampled at high resolution demonstrate that this large Mn peak in fact pre-dates the Santorini event by ~2.8 ky and forms part of a pattern seen in Mn profiles from all over the eastern Mediterranean. This same Mn profile shape is altered in areas that experienced substantial deposits of either the tephra layer emitted by the Santorini eruption or the turbidites that were triggered by it. Evidence of both of these perturbations is readily identified from compositional element/Al and Sr/Ca profiles that are distinct from those of the enclosing sediments. In one core with a 37 cm thick Santorini ash layer an oxidation front succeeded in penetrating the whole ash layer after emplacement to form a Mn peak but is now retreating. In cores where thin (b15 cm) Santorini turbidites or ash layers lie above S1, oxidation fronts initially form additional Mn peaks on top of the turbidites and subsequently alter the characteristic double peaked Mn profile shape usually observed above sapropel S1.

Biogenic barium concentrations obtained from direct determination by a three-step sequential extraction procedure are compared to those obtained from the widely used indirect normative calculation based on total digestion. A comparison of the biogenic barium from the direct/sequential extraction and the indirect/normative calculation clearly shows that the detrital Ba/Al ratio is the critical factor in the normative approach, and that erroneous assumptions based on this ratio may introduce significant errors to the calculated biogenic barium. Overall, the crustal average Ba/Al ratio of 0.005–0.01 is much higher than our directly determined global average of 0.0037. This would result in an underestimation of the biogenic barium and thus of the primary productivity calculated using the Ba-flux obtained from the normatively calculated Ba record. Using our (Ba/Al)det ratio of ∼0.0037 leads to normatively calculated biogenic barium results that are in reasonable agreement with the biogenic barium from sequential extraction for samples of the Atlantic, Pacific and Indian Ocean. Our directly determined ‘regional’ (Ba/Al)det ratios deviate from those calculated or assumed from hinterland ratios and our global average (Ba/Al)det is lower than the one commonly reported. Therefore, in sediments with a significant terrigenous fraction, the sequential extraction technique is always required.

Corrigendum to ‘‘Impact of hot fluid advection on hydrocarbon gas production and seepage in mud volcano sediments of thick Cenozoic deltas’’ [Earth Planet. Sci. Lett. 341–344 (2012) 139–157] Marianne Nuzzo , Marcus Elvert , Mark Schmidt , Florian Scholz , Anja Reitz , Kai-Uwe Hinrichs , Christian Hensen a a GEOMAR Helmholtz Centre for Ocean Research Kiel, Wischhofstrabe 1-3, D-24148 Kiel, Germany b Organic Geochemistry Group, Department of Geosciences and MARUM—Centre for Marine Environmental Sciences, University of Bremen, Leobener Str., D-28359 Bremen, Germany