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In this study, we report lipid biomarker patterns and phylogenetic identities of key microbes mediating anaerobic oxidation of methane (AOM) communities in active mud volcanos (MVs) on the continental slope of the Canadian Beaufort Sea. The enriched δ13C values of total organic carbon (TOC) as well as lipid biomarkers such as archaeol and biphytanes (BPs) relative to δ13CCH4 values suggested that the contribution of AOM-related biomass to sedimentary TOC was in general negligible in the Beaufort Sea MVs investigated. However, the δ13C values of sn-2- and sn-3-hydroxyarchaeol were more negative than CH4, indicating the presence of AOM communities, albeit in a small amount. The ratio of sn-2-hydroxyarchaeol to archaeol and the 16S rRNA results indeed indicated that archaea of the ANME-2c and ANME-3 clades were involved in AOM. Further studies are needed to investigate the diversity and distribution of AOM communities and to characterize their habitats in the uppermost surface sediments of Beaufort Sea MV systems.

The distribution of branched glycerol dialkyl glycerol tetraethers (brGDGTs) in soils has been shown to correlate with pH and mean annual air temperature. Because of this dependence brGDGTs have found an application as palaeoclimate proxies in coastal marine sediments, based on the assumption that their distribution is not altered during the transport from soils to marine systems by rivers. To study the processes acting on the brGDGT distributions, we analysed the full suite of brGDGTs, including the recently described 6-Me brGDGTs, in both the suspended particulate matter (SPM) of the Siberian Yenisei River and the SPM and sediments of its outflow in the Kara Sea. The brGDGT distribution in the SPM of the Yenisei River was fairly constant and characterized by high abundances of the 6-Me brGDGTs, reflecting their production at the neutral pH of the river water. However, the brGDGT distribution showed marked shifts in the marine system. Firstly, in the Yenisei River Mouth, the fractional abundance of the 6-Me brGDGTs decreases sharply. The brGDGT signature in the Yenisei River Mouth possibly reflects brGDGTs delivered during the spring floods that may carry a different distribution. Also, coastal cliffs were shown to contain brGDGTs and to influence especially those sites without major river inputs (e.g. Khalmyer Bay). Further removed from the river mouth, in-situ production of brGDGTs in the marine system influences the distribution. However, also the fractional abundance of the tetramethylated brGDGT Ia increases, resulting in a distribution that is distinct from in-situ produced signals at similar latitudes (Svalbard). We suggest that this shift may be caused by preferential degradation of labile (riverine in-situ produced) brGDGTs and the subsequent enrichment in less labile (soil) material. The offshore distribution indeed agrees with the brGDGT distribution encountered in a lowland peat. This implies that the offshore Kara Sea sediments possibly carry a soil-dominated signal, indicating potential for palaeoclimate reconstructions at this site. Both in the river system and coastal cliffs, brGDGTs were much more abundant than crenarchaeol, an archaeal isoprenoid GDGT, resulting in high (>0.93) Branched and Isoprenoid Tetraether (BIT) index values. Moving downstream in the marine sediments, a decrease in brGDGT concentrations, coeval with an increase in crenarchaeol, resulted in decreasing BIT index values. This decrease correlates with changes in bulk proxies for terrigenous input (δ13Corg, C/N), confirming the use of the BIT index to trace the delivery of river-transported and coastal cliff-derived terrigenous organic matter.

Bacteriohopanepolyols (BHPs) are ubiquitous bacterial membrane lipids, encountered in soils, river and marine suspended particulate matter (SPM) and sediments. Their abundance and distribution provides a direct means to identify bacterial inputs and can be used to trace soil-derived bacterial organic matter (OM) and in some cases the presence of bacterial groups and their activities in aquatic systems. We have studied the BHP distribution in the SPM of a major Siberian River (Yenisei River) that crosses a large latitudinal gradient, draining a large part of Mongolia and Siberian Russia. The Yenisei River is the main river to flow into the Kara Sea, a shelf sea of the Arctic Ocean. We show that the BHP distribution and concentration of SPM and surface sediments of the Yenisei Outflow in the Kara Sea allow to trace soil-marker BHPs and evaluate the performance of the R′soil index, a proxy developed to trace bacterial soil-derived OM. Soil-marker BHPs are present in the Yenisei River, and their concentration decreases from the Yenisei River Outflow into the offshore marine sediments. The R′soil correlates well with an independent proxy for bacterial OM, the BIT-index (r2 = 0.82) and has a moderate correlation with the δ13Corg values, a bulk OM proxy for terrigenous input (r2 = 0.44). Consequently, the R′soil index performs well in the Kara Sea, strengthening its application for tracing bacterial OM in the Arctic Ocean, both in modern and downcore sediments. Furthermore, a suite of BHPs that are characteristic for methanotrophic bacteria, i.e. 35-aminobacteriohopane-30,31,32,33,34-pentol (aminopentol) and 35-aminobacteriohopane-31,32,33,34-tetrol (aminotetrol), is encountered in the Yenisei Outflow sediments. These components are partly sourced from terrigenous sources, but are likely also produced in-situ in the marine sediments. The distribution of the pentafunctionalized cyclitol ether BHP in the marine systems is noteworthy, and indicates that it can possibly be applied as a marker for cyanobacterial biomass in marine sediments.

This study evaluates the glycerol dialkyl glycerol tetraether (GDGT) distribution and provenance in sediments (spanning a minimum of 13.3 ka) from the St. Anna Trough (Northern Kara Sea). The site has experienced extensive fluctuation in the delivery of river-derived organic matter (OM), caused by a eustatic change in sea level. This is in line with the record of the concentration of the isoprenoid GDGT, crenarchaeol, produced by marine Thaumarchaota, which was low at the bottom of the core, increasing gradually in the most shallow unit. The concentration of branched (br)GDGTs showed an opposite trend and a marked shift in distribution. The deepest sediments (> 10 ka), with a distribution currently encountered in surface sediments in front of the Yenisei River, are characterized by terrigenous brGDGTs, whereas the distribution in the shallowest unit (< 10 ka) is strongly influenced by marine, in-situ brGDGTs. During the shift from terrigenous to marine-sourced brGDGTs, there was one horizon where a pronounced shift in the brGDGT distribution was observed and the brGDGT concentration significantly decreased. As the brGDGTs delivered to the current Kara Sea system are derived from several sources, we postulate that a temporary change in the relative importance of the brGDGT sources happened during this interval. Both in-situ production and changing brGDGT provenance have implications for paleoclimate reconstruction using brGDGTs. In-situ production of marine brGDGTs results in a higher reconstructed pH. However, these in-situ produced brGDGTs did not influence the reconstructed mean annual air temperature (MAT), when the MATmrs calibration was used. Changes in the relative contribution of brGDGT sub-pools were shown, however, to influence both soil pH reconstruction and MAT reconstruction.

This study evaluates the glycerol dialkyl glycerol tetraether (GDGT) distribution and provenance in sediments (spanning a minimum of 13.3 ka) from the St. Anna Trough (Northern Kara Sea). The site has experienced extensive fluctuation in the delivery of river-derived organic matter (OM), caused by a eustatic change in sea level. This is in line with the record of the concentration of the isoprenoid GDGT, crenarchaeol, produced by marine Thaumarchaota, which was low at the bottom of the core, increasing gradually in the most shallow unit. The concentration of branched (br)GDGTs showed an opposite trend and a marked shift in distribution. The deepest sediments (> 10 ka), with a distribution currently encountered in surface sediments in front of the Yenisei River, are characterized by terrigenous brGDGTs, whereas the distribution in the shallowest unit (< 10 ka) is strongly influenced by marine, in-situ brGDGTs. During the shift from terrigenous to marine-sourced brGDGTs, there was one horizon where a pronounced shift in the brGDGT distribution was observed and the brGDGT concentration significantly decreased. As the brGDGTs delivered to the current Kara Sea system are derived from several sources, we postulate that a temporary change in the relative importance of the brGDGT sources happened during this interval. Both in-situ production and changing brGDGT provenance have implications for paleoclimate reconstruction using brGDGTs. In-situ production of marine brGDGTs results in a higher reconstructed pH. However, these in-situ produced brGDGTs did not influence the reconstructed mean annual air temperature (MAT), when the MATmrs calibration was used. Changes in the relative contribution of brGDGT sub-pools were shown, however, to influence both soil pH reconstruction and MAT reconstruction.

Bacteriohopanepolyols (BHPs) are ubiquitous bacterial membrane lipids, encountered in soils, river and marine suspended particulate matter (SPM) and sediments. Their abundance and distribution provides a direct means to identify bacterial inputs and can be used to trace soil-derived bacterial organic matter (OM) and in some cases the presence of bacterial groups and their activities in aquatic systems. We have studied the BHP distribution in the SPM of a major Siberian River (Yenisei River) that crosses a large latitudinal gradient, draining a large part of Mongolia and Siberian Russia. The Yenisei River is the main river to flow into the Kara Sea, a shelf sea of the Arctic Ocean. We show that the BHP distribution and concentration of SPM and surface sediments of the Yenisei Outflow in the Kara Sea allow to trace soil-marker BHPs and evaluate the performance of the R′soil index, a proxy developed to trace bacterial soil-derived OM. Soil-marker BHPs are present in the Yenisei River, and their concentration decreases from the Yenisei River Outflow into the offshore marine sediments. The R′soil correlates well with an independent proxy for bacterial OM, the BIT-index (r2 = 0.82) and has a moderate correlation with the δ13Corg values, a bulk OM proxy for terrigenous input (r2 = 0.44). Consequently, the R′soil index performs well in the Kara Sea, strengthening its application for tracing bacterial OM in the Arctic Ocean, both in modern and downcore sediments. Furthermore, a suite of BHPs that are characteristic for methanotrophic bacteria, i.e. 35-aminobacteriohopane-30,31,32,33,34-pentol (aminopentol) and 35-aminobacteriohopane-31,32,33,34-tetrol (aminotetrol), is encountered in the Yenisei Outflow sediments. These components are partly sourced from terrigenous sources, but are likely also produced in-situ in the marine sediments. The distribution of the pentafunctionalized cyclitol ether BHP in the marine systems is noteworthy, and indicates that it can possibly be applied as a marker for cyanobacterial biomass in marine sediments.

Tubular carbonate concretions of up to 1 m in length and perpendicular to bedding, occur abundantly in the Upper Pliensbachian (upper Amaltheus margaritatus Zone, Gibbosus Subzone) in outcrops (Fontaneilles section) in the vicinity of Rivière-sûr-Tarn, southern France. Stable isotope analyses of these concretions show negative δ13C values that decrease from the rim to the center from −18.8‰ to −25.7‰ (V-PDB), but normal marine δ18O values (−1.8‰). Carbon isotope analyses of Late Pliensbachian bulk carbonate (matrix) samples from the Fontaneilles section show clearly decreasing C-isotope values across the A. margaritatus Zone, from +1‰ to −3‰ (V-PDB). Isotope analyses of coeval belemnite rostra do not document such a negative C-isotope trend with values remaining stable around +2‰ (V-PDB). Computer tomographic (CT) scanning of the tubular concretions show multiple canals that are lined or filled entirely with pyrite. Previously, the formation of these concretions with one, two, or more central tubes, has been ascribed to the activity of an enigmatic organism, possibly with annelid or arthropod affinities, known asTisoa siphonalis. Our results suggest tisoan structures are abiogenic. Based on our geochemical analyses and sedimentological observations we suggest that these concretions formed as a combination of the anaerobic oxidation of methane (AOM) and sulfate reduction within the sediment. Fluids rich in methane and/or hydrocarbons likely altered local bulk rock carbon isotope records, but did not affect the global carbon cycle. Interestingly, Tisoa siphonalis has been described from many locations in the Grands Causses Basin in southern France, and from northern France and Luxemburg, always occurring at the same stratigraphic level. Upper Pliensbachian authigenic carbonates thus possibly cover an area of many thousand square kilometers. Greatly reduced sedimentation rates are needed to explain the stabilization of the sulfate-methane transition zone in the sedimentary column in order for the tubular concretions to form. Late Pliensbachian cooling, reducing run-off, and/or the influx of colder water and more vigorous circulation could be responsible for a halt in sedimentation. At the same time (thermogenic) methane may have destabilized during a major phase of Late Pliensbachian sea level fall. As such Tisoa siphonalis is more than a geological curiosity, and its further study could prove pivotal in understanding Early Jurassic paleoenvironmental change.

Molecular and stable carbon isotope properties of hydrocarbon gases (methane through pentanes and sometimes hexanes) from seven sediment cores collected from five mud volcanoes (MVs) in the Sorokin Trough (NE Black Sea) suggest that these gases are initially derived from the comparable hydrocarbon pools and are likely initial products of non-microbial oil cracking processes. Our results state that dry characteristics of gas and 13 C-depleted signatures of methane are result of a high admixture of secondary microbial gas formed due to subsequent microbial anaerobic degradation of redeposited hydrocarbons in the shallow reservoirs. The wet gas components in all MVs and gas hydrates are related to each other. The compositional variations in C 2+ content appear to result from a complex of secondary processes such oil cracking in the deep subsurface, migration and mixing of resulted gaseous and liquid hydrocarbons, biodegradation of possibly redeposited hydrocarbons forming shallow reservoirs, and additional alterations of hydrocarbon gases in the surface sediments due to currently active microbial processes, such as AOM, C 2+ consumption, etc. Our data show that the most unaltered gas is in the mud breccia from the Kazakov MV. The gas mixture possibly represents the original properties of the hydrocarbons trapped in the deep subsurface of the Sorokin Trough. Analysis of the hydrocarbon gas data, complemented with published maturity characteristics of organic matter from Maycopian rock clasts and mud breccia matrix implies that the original source of gases is likely to be located below the Maycopian Shale Formation.

International audience; A new archaeal lipid biomarker, C 20,25 sn-2, C-7 hydroxy isoprenoid glycerol diether, has been identified in methane-related carbonates from six methane seep locations. The molecular structure resembles that of sn-2 hydroxyarchaeol but the hydroxy group-bearing moiety at the sn-2 position comprises a regular C 25 instead of a C 20 isoprenoid chain. Compound specific stable carbon isotope measurements revealed δ 13C values ranging from -96‰ to -129‰, which are similar to δ 13C values of sn-2 hydroxyarchaeol. This supports the origin of the novel compound from archaea involved in the anaerobic oxidation of methane. It also suggests a common source, i.e. archaea within the ANME-2 group, which is consistent with the previously reported 16S rDNA data for the same samples. The frequent appearance of the novel sn-2 isoprenoidal hydroxy glycerol diether in cold seepage sediments with high salinity environments, and its distinct structural similarities with membrane lipids of halophilic and methanogenic microbes, suggests that ANME-2 methanotrophic archaea thriving at elevated salinity are the possible biological source for this specific lipid.

Carbonate chimneys and other carbonate structures occur widespread in the Gulf of Cadiz and probably reflect the presence of cold seeps and associated release of methane in the geological past, possibly in the Early Pleistocene, but it is unclear under what conditions and by which processes these carbonates were formed. We studied a fossil methane-related carbonate crust collected from the Kidd mud volcano in the gulf. Concentrations of microbial lipids, their stable carbon isotope composition, sequences of fossil 16S rRNA genes of anaerobic methanotrophic archaea in combination with mineralogical and carbon and oxygen isotopic composition of carbonate were obtained for seven different horizons of the crust. This combination of organic and inorganic geochemical techniques with molecular ecological methods gave a consistent view on processes resulting in the formation of the crust and indicated that it took place in two phases and in a downward direction. Archaeal lipid biomarkers and fossil 16S rRNA gene sequence data revealed the dominance of archaeal ANME-2 group and elevated methane partial pressures during the formation of the top part of the crust. The lower part of the carbonate was likely formed in an environment with reduced methane fluxes as revealed by the dominance of fossil remains of ANME-1 archaea. The combination of these methods can be used as an effective tool to reconstruct in unprecedented detail the palaeo-biogeochemical processes resulting in the formation of carbonate fabrics. This interdisciplinary strategy may also be applied for other fossil methane-derived carbonates, generating new concepts and knowledge about past methane-related carbonate systems.

Mud volcanoes (MVs) are formed by expulsion due to lateral or vertical compression of overpressured fluids in the sub-surface. Their sediments are characterized by a specific lithology called mud breccia, which is composed of inorganic and organic matter sourced from different sedimentary units and transported from the sub-surface to the seafloor. Biomarker lipid distributions were determined for sediment cores collected from the Kazakov, NIOZ and Odessa MVs in the Sorokin Trough, northeastern Black Sea. This revealed different mud breccia sources that are directly linked to the location of the fracture zones or diapiric folds formed by tectonic compression. The Kazakov MV shows biomarker characteristics of relatively mature organic matter, likely related to the Maycopian Shale Formation. The NIOZ and Odessa MVs contain immature organic matter derived from much shallower sediments overpressed by rising Maycopian diapirs. In the mud breccia from the NIOZ MV, biomarkers clearly record two different mud eruption episodes reflected in distinct distributions from unrelated sedimentary facies. Hence, the study shows that, although the organic matter of a mud breccia is sourced from a complex mixture of lithofacies, the collective biomarker signal reveals specific signatures of the sedimentary sections that contributed most to the migrated fluid.

Investigations of molecular and carbon isotopic variability of hydrocarbon gases from methane through butanes (pentanes) have been performed on six mud volcanoes from two fluid venting provinces located in the Gulf of Cadiz, NE Atlantic. The main aims were to define the basic gas types, to describe their geochemical characteristics in relationship to their sources, and to determine the secondary effects due to migration/mixing and microbial alteration. Hydrocarbon gas data reveal two groups of gases. Despite the different maturation characteristics, both gas groups are allochthonous to the erupted mud breccia and represent a complex of redeposited, secondary migrated, mixed, and microbially altered hydrocarbons. It may possibly imply the presence of hydrocarbon accumulations in the deep subsurface of the Gulf of Cadiz. © 2005 Elsevier Ltd. All rights reserved.