In situ produced branched glycerol dialkyl glycerol tetraethers in suspended particulate matter from the Yenisei River, Eastern Siberia

Soil-derived branched glycerol dialkyl glycerol tetraethers (brGDGTs) in marine river fan sediments have a potential use for determining changes in the mean annual temperature (MAT) and pH of the river watershed soils. Prior to their incorporation in marine sediments, the compounds are transported to the marine system by rivers. However, emerging evidence suggests that the brGDGTs in freshwater systems can be derived from both soil run-off and in situ production. The production of brGDGTs in the river system can complicate the interpretation of the brGDGT signal delivered to the marine system. Therefore, we studied the distribution of brGDGT lipids in suspended particulate matter (SPM) of the Yenisei River. Chromatographic improvements allowed quantification of the recently described hexamethylated brGDGT isomer, characterized by having two methyl groups at the 6/6 ' instead of the 5/5 ' positions, in an environmental dataset for the first time. This novel compound was the most abundant brGDGT in SPM from the Yenisei. Its fractional abundance correlated well with that of the 6-methyl isomer of the hexamethylated brGDGT that contains one cyclopentane moiety. The Yenisei River watershed is characterized by large differences in MAT (>11 degrees C) as it spans a large latitudinal range (46-73 degrees N), which would be expected to be reflected in brGDGT distributions of its soils. However, the brGDGT distributions in its SPM show little variation. Furthermore, the reconstructed pH values are high compared to the watershed soil pH. We, therefore, hypothesize that the brGDGTs in the Yenisei River SPM are predominantly produced in situ and not primarily derived from erosion of soil. This accounts for the absence of a change in the temperature signal, as the river water temperature is more stable. Using a lake calibration, the reconstructed temperature values agree with the mean summer temperatures (MST) recorded. The brGDGTs delivered to the sea by the Yenisei River during this season are thus not soil-derived, possibly complicating the use of brGDGTs in marine sediments for palaeoclimate reconstructions

Stratified community responses to methane and sulfate supplies in mud volcano deposits: insights from an <i>in vitro</i> experiment

Numerous studies on marine prokaryotic communities have postulated that a process of anaerobic oxidation of methane (AOM) coupled with sulfate reduction (SR) is the main methane sink in the world's oceans. AOM has also been reported in the deep biosphere. But the responses of the primary microbial players in eliciting changes in geochemical environments, specifically in methane and sulfate supplies, have yet to be fully elucidated. Marine mud volcanoes (MVs) expel a complex fluid mixture of which methane is the primary component, forming an environment in which AOM is a common phenomenon. In this context, we attempted to identify how the prokaryotic community would respond to changes in methane and sulfate intensities, which often occur in MV environments in the form of eruptions, diffusions or seepage. We applied an integrated approach, including (i) biochemical surveys of pore water originated from MV, (ii) in vitro incubation of mud breccia, and (iii) prokaryotic community structure analysis. Two distinct AOM regions were clearly detected. One is related to the sulfate methane transition zone (SMTZ) at depth of 30-55 cm below the sea floor (bsf); the second is at 165-205 cm bsf with ten times higher rates of AOM and SR. This finding contrasts with the sulfide concentrations in pore waters and supports the suggestion that potential AOM activity below the SMTZ might be an important methane sink that is largely ignored or underestimated in oceanic methane budget calculations. Moreover, the incubation conditions below the SMTZ favor the growth of methanotrophic archaeal group ANME-2 compared to ANME-1, and promote the rapid growth and high diversity of bacterial communities. These incubation conditions also promote the increase of richness in bacterial communities. Our results provide direct evidence of the mechanisms by which deep AOM processes can affect carbon cycling in the deep biosphere and global methane biochemistry

Drastic changes in the distribution of branched tetraether lipids in suspended matter and sediments from the Yenisei River and Kara Sea (Siberia): Implications for the use of brGDGT-based proxies in coastal marine sediment

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 (d13Corg, C/N), confirming the use of the BIT index to trace the delivery of river-transported and coastal cliff-derived terrigenous organic matter

Origin of lipid biomarkers in mud volcanoes from the Alboran Sea, western Mediterranean

Mud volcanoes (MVs) are the most prominent indicators of active methane/hydrocarbon venting at the seafloor on both passive and active continental margins. Their occurrence in the western Mediterranean is patent at the West Alboran Basin, where numerous MVs develop overlaying a major sedimentary depocentre containing overpressured shales. Although some of these MVs have been studied, the detailed biogeochemistry of expelled mud so far has not been examined in detail. This work provides the first results on the composition and origin of organic matter, anaerobic oxidation of methane (AOM) processes and general characteristics on MV dynamics using lipid biomarkers as the main tool. Lipid biomarker analysis was performed on MV expelled material (mud breccias) and interbedded hemipelagic sediments from Perejil, Kalinin and Schneider's Heart MVs located in the northwest margin of the Alboran Sea. The n alkane distributions and n alkane-derived indices (CPI and ACL), in combination with the epimerization degree of hopanes (22S/(22S+22R)) indicate that all studied mud breccia have a similar biomarker composition consisting of mainly thermally immature organic matter with an admixture of petroleum-derived compounds. This concordant composition indicates that common source strata must feed all three studied MVs. The past or present AOM activity was established using lipid biomarkers specific for anaerobic methanotrophic archaea (irregular isoprenoids and dialkyl glycerol diethers) and the depleted carbon isotope composition (δ13C) of crocetane/phytane. The presence of these lipid biomarkers, together with the low amounts of detected glycerol dialkyl glycerol tetraethers, is consistent with the dominance of anaerobic methanotrophs of the ANME-2 over ANME-1, at least in mud breccia from Perejil MVs. In contrast, the scarce presence or lack of these AOM-related lipid biomarkers in sediments from Kalinin and Schneider's Heart MVs, suggests that no recent active methane seepage has occurred at these sites. Moreover, the observed methane concentrations support the current activity of Perejil MV, and the very low methane seepage activity in Kalinin and Schneider's Heart MVs.This study was supported by Project CTM2009-07715, CGL2011-1441 and CGL2012-32659 (MINECO, Spain), by the VENI grant from the Netherlands Organization for Scientific Research (NWO) and the Royal Netherland Institute for Sea Research (NIOZ via ZKOprogramme) (Texel, The Netherlands). C. F. López-Rodríguez was funded by a JAE-PhD fellowship from the CSIC (Spain)

Bacterial GDGTs in Holocene sediments and catchment soils of a high Alpine lake: application of the MBT/CBT-paleothermometer

A novel proxy for continental mean annual air temperature (MAAT) and soil pH, the MBT/CBT-paleothermometer, is based on the temperature (T) and pH-dependent distribution of specific bacterial membrane lipids (branched glycerol dialkyl glycerol tetraethers – GDGTs) in soil organic matter. Here, we tested the applicability of the MBT/CBT-paleothermometer to sediments from Lake Cadagno, a high Alpine lake in southern Switzerland with a small catchment of 2.4 km2. We analysed the distribution of bacterial GDGTs in catchment soils and in a radiocarbon-dated sediment core from the centre of the lake, covering the past 11 000 yr. The distribution of bacterial GDGTs in the catchment soils is very similar to that in the lake's surface sediments, indicating a common origin of the lipids. Consequently, their transfer from the soils into the sediment record seems undisturbed, probably without any significant alteration of their distribution through in situ production in the lake itself or early diagenesis of branched GDGTs. The MBT/CBT-inferred MAAT estimates from soils and surface sediments are in good agreement with instrumental values for the Lake Cadagno region (~0.5 °C). Moreover, downcore MBT/CBT-derived MAAT estimates match in timing and magnitude other proxy-based T reconstructions from nearby locations for the last two millennia. Major climate anomalies recorded by the MBT/CBT-paleothermometer are, for instance, the Little Ice Age (~14th to 19th century) and the Medieval Warm Period (MWP, ~9th to 14th century). Together, our observations indicate the quantitative applicability of the MBT/CBT-paleothermometer to Lake Cadagno sediments. In addition to the MWP, our lacustrine paleo T record indicates Holocene warm phases at about 3, 5, 7 and 11 kyr before present, which agrees in timing with other records from both the Alps and the sub-polar North-East Atlantic Ocean. The good temporal match of the warm periods determined for the central Alpine region with north-west European winter precipitation strength implies a strong and far-reaching influence of the North Atlantic Oscillation on continental European T variations during the Holocene

The enigmatic ichnofossil Tisoa siphonalis and widespread authigenic seep carbonate formation during the Late Pliensbachian in southern France

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

Impact of riverine suspended particulate matter on the branched glycerol dialkyl glycerol tetraether composition of lakes: The outflow of the Selenga River in Lake Baikal (Russia)

Branched glycerol dialkyl glycerol tetraethers (brGDGTs) are bacterial membrane lipids found in several environments, including soils, rivers and lakes, whose distribution varies with temperature and pH, although this dependence is apparently not the same for the different environments. Mixing of brGDGT sources may thus complicate palaeoenvironmental reconstruction. The extent to which brGDGTs in a lake outflow reflect the distribution delivered by upstream rivers was studied for Lake Baikal (Russia), one of the largest freshwater lakes worldwide. Fifteen brGDGTs in suspended particulate matter (SPM) of the Selenga River and its outflow from the lake were quantified. The river and lake SPM had rather different distributions. The riverine distribution was still apparent in the SPM of the lake surface water 5 km from the river mouth, but shifts in the distribution were already apparent in the SPM of the surface water after 1 km. Based on the brGDGT distributions of the SPM of the Selenga outflow and that of the lake, conservative mixing between the river and the lake brGDGT distributions could not fully explain the observed shifts in distributions. Both preferential degradation and in situ production of brGDGTs in the surface and, especially, bottom water of the river outflow were potentially responsible. This implies that a riverine lipid distribution delivered to a lake can be modified prior to being transported downstream. The lacustrine brGDGT distribution, that possibly could have reflected a mixture of mountainous and Selenga River SPM, was not recognized in downstream Yenisei River SPM. The watershed of Lake Baikal thus does not seem to contribute to the brGDGTs transported to the marine system. As many large rivers have major lakes in their watershed, this has implications for palaeoclimate reconstruction from river fan sediments globally

Molecular and carbon isotopic variability of hydrocarbon gases from mud volcanos in the Gulf of Cadiz, NE Atlantic.

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