Timescales for the development of methanogenesis and free gas layers in recently-deposited sediments of Arkona Basin (Baltic Sea)

Arkona Basin (southwestern Baltic Sea) is a seasonally-hypoxic basin characterized by the presence of free methane gas in its youngest organic-rich muddy stratum. Through the use of reactive transport models, this study tracks the development of the methane geochemistry in Arkona Basin as this muddy sediment became deposited during the last 8 kyr. Four cores are modeled each pertaining to a unique geochemical scenario according to their respective contemporary geochemical profiles. Ultimately the thickness of the muddy sediment and the flux of particulate organic carbon are crucial in determining the advent of both methanogenesis and free methane gas, the timescales over which methanogenesis takes over as a dominant reaction pathway for organic matter degradation, and the timescales required for free methane gas to form

Sulphur and Carbon Isotopes as Tracers of Past Sub-seafloor Microbial Activity

Microbial life below the seafloor has changed over geological time, but these changes are often not obvious, as they are not recorded in the sediment. Sulphur (S) isotope values in pyrite extracted from a Plio- to Holocene sequence of the Peru Margin (Ocean Drilling Program, ODP, Site 1229) show a down-core pattern that correlates with the pattern of carbon (C) isotopes in diagenetic dolomite. Early formation of the pyrite is indicated by the mineralogical composition of iron, showing a high degree of pyritization throughout the sedimentary sequence. Hence, the S-record could not have been substantially overprinted by later pyrite formation. The S- and C-isotope profiles show, thus, evidence for two episodes of enhanced microbial methane production with a very shallow sulphate-methane transition zone. The events of high activity are correlated with zones of elevated organic C content in the stratigraphic sequence. Our results demonstrate how isotopic signatures preserved in diagenetic mineral phases provide information on changes of past biogeochemical activity in a dynamic sub-seafloor biosphere

Energy Gradients Structure Microbial Communities Across Sediment Horizons in Deep Marine Sediments of the South China Sea

The deep marine subsurface is a heterogeneous environment in which the assembly of microbial communities is thought to be controlled by a combination of organic matter deposition, electron acceptor availability, and sedimentology. However, the relative importance of these factors in structuring microbial communities in marine sediments remains unclear. The South China Sea (SCS) experiences significant variability in sedimentation across the basin and features discrete changes in sedimentology as a result of episodic deposition of turbidites and volcanic ashes within lithogenic clays and siliceous or calcareous ooze deposits throughout the basin\u27s history. Deep subsurface microbial communities were recently sampled by the International Ocean Discovery Program (IODP) at three locations in the SCS with sedimentation rates of 5, 12, and 20 cm per thousand years. Here, we used Illumina sequencing of the 16S ribosomal RNA gene to characterize deep subsurface microbial communities from distinct sediment types at these sites. Communities across all sites were dominated by several poorly characterized taxa implicated in organic matter degradation, including Atribacteria, Dehalococcoidia, and Aerophobetes. Sulfate-reducing bacteria comprised only 4% of the community across sulfate-bearing sediments from multiple cores and did not change in abundance in sediments from the methanogenic zone at the site with the lowest sedimentation rate. Microbial communities were significantly structured by sediment age and the availability of sulfate as an electron acceptor in pore waters. However, microbial communities demonstrated no partitioning based on the sediment type they inhabited. These results indicate that microbial communities in the SCS are structured by the availability of electron donors and acceptors rather than sedimentological characteristics

Anaerobic oxidation of methane in the Concepcion Methane Seep Area, Chilean continental margin

Within subduction zones of active continental margins, large amounts of methane can be mobilized by dewatering processes and transported to the seafloor along migration pathways. A recently discovered seep area located off Concepción (Chile) at water depth between 600 to 1100 mbsl is characterized by active methane vent sites as well as massive carbonates boulders and plates which probably are related to methane seepage in the past. During the SO210 research expedition “Chiflux” (Sept-Oct 2010), sediment from the Concepción Methane Seep Area (CSMA) at the fore arc of the Chilean margin was sampled to study microbial activity related to methane seepage. We sampled surface sediments (0-30cm) from sulfur bacteria mats, as well as clam, pogonophoran, and tubeworm fields with push cores and a TV-guided multicorer system. Anaerobic oxidation of methane (AOM) and sulfate reduction rates were determined using ex-situ radioisotope tracer techniques. Additionally, porewater chemistry of retrieved cores as well as isotopic composition and age record of surrounding authigenic carbonates were analyzed. The shallowest sulfate-methane-transition zone (SMTZ) was identified at 4 cm sediment depth hinting to locally strong fluid fluxes. However, a lack of Cl- anomalies in porewater profiles indicates a shallow source of these fluids, which is supported by the biogenic origin of the methane (�13C -70h PDB). Sulfide and alkalinity was relatively high (up to 20 mM and 40 mEq, respectively). Rates of AOM and sulfate reduction within this area reached magnitudes typical for seeps with variation between different habitat types, indicating a diverse methane supply, which is affecting the depths of the SMTZ. Rates were highest at sulfur a bacteria mats (20 mmol m-2 d-1) followed by a large field of dead clams, a pogonophoran field, a black sediment spot, and a carbonate rich clam field. Lowest rates (0.2 mmol m-2 d-1) were measured in close vicinity to these hot spots. Abundant massive carbonate blocks and plates hint to a very old seep system with a probably much higher activity in the past. The U-Th age record of these authigenic carbonates reach back to periods of venting activity with more than 150 ka ago. Carbon isotopic signatures of authigenic carbonates (�13C -50 to -40hPDB) suggest a biogenic carbon source (i.e. methane), also in the past. We found several indications for the impact of recent earthquakes within the seep area (cracks, shifted seafloor), which could be an important mechanism for the triggering of new seepage activity, change in fluid expulsion rates and colonization patterns of the cold seep fauna

TRACKING SULFUR DIAGENESIS IN METHANE RICH MARINE SEDIMENTS ON THE CASCADIA MARGIN: COMPARING SULFUR ISOTOPES OF BULK SEDIMENT AND CHROMIUM REDUCIBLE SULFUR

Methane gas is produced in anoxic marine sediments by methanogenic bacteria and can be ephemerally stored in gas hydrate deposits, escape to the seafloor at methane seeps, and/or be consumed at depth by the anaerobic oxidation of methane (AOM). One way to examine changes in methane flux in cold seep environments through time is to identify past positions of the sulfate methane transition zone (SMTZ) where AOM results in sulfate and methane consumption and bicarbonate and hydrogen sulfide production, often resulting in the precipitation of authigenic carbonates and iron sulfides. One method to identify paleo-positons of the SMTZ is through the sulfur isotopic composition of sulfides produced by AOM, which are typically enriched in 34S relative to sediments not influenced by AOM. Traditionally, a chemical extraction technique called chromium reduction has been used to extract reduced forms of sulfur, mainly pyrite and other iron sulfides, from the sediment. This separates only reduced sulfur from sediments leaving behind sulfur bound to organic matter and from oxidized sulfur species such as barite. In this study, bulk sediment δ34S values are compared to measured δ34S from chromium reducible sulfur (CRS) to assess the utility of using bulk sediment δ34S measured alone to investigate paleo-diagenetic conditions in methane rich marine sediments. The upper ~25 meters of sediment at three ODP core sites (1252A, 1247B, and 1244C) from Hydrate Ridge on the central Cascadia margin are examined. In addition to bulk sediment and CRS δ34S, total sulfur and total organic carbon were also measured. The results reveal the bulk sediment is generally more enriched in 34S, compared to the CRS, except for a few samples at Site 1247B and some larger intervals at Site 1244C. At Site 1247B the greatest weight percent of total sulfur occurred at the modern SMTZ, and at Sites 1252A and 1244C the highest weight percent of sulfur occurred above the modern SMTZ, coincident with the most enriched (heaviest) bulk sediment sulfur isotope values at these two sites. Peaks in the δ34S value of the bulk sediment and total sulfur weight percent could be due to the presence of barite, making the bulk sediment in these locations more enriched in 34S than the CRS. In the majority of the sedimentary records examined here, the intervals where the chromium reducible sulfur has a heavier δ34S composition than the bulk sediment could indicate that the sediment has experienced intense AOM, leaving the iron sulfides to form from the heaviest hydrogen sulfide. This would make the δ34S value of the bulk sediment lighter than that of the highly enriched iron sulfides. There is a positive, linear relationship between the δ34S values of the CRS and the bulk sediment, which shows that the δ34S value of the bulk sediment is strongly influenced by the sulfur isotope composition of the CRS portion of the sediment. While the bulk sediment did not have the same δ34S values as the CRS it often showed the same trends and may be helpful in assessing the extent of AOM in methane rich marine sediments. Further comparisons of the sulfur isotope composition of sedimentary iron sulfides and bulk sediment, as well as other sulfur containing species, at other locations could indicate if the relationship observed at Hydrate Ridge exists in other methane rich seafloor environments

Methane dynamics in Santa Barbara Basin (USA) sediments as examined with a reaction-transport model

Here we describe a new reaction-transport model that quantitatively examines δ13C profiles of pore-water methane and dissolved inorganic carbon (DIC) (δ13CCH4 and δ13CDIC) in the anoxic sediments of the Santa Barbara Basin (California Borderland region). Best-fit solutions of the model to these data suggest that CO2 reduction is the predominant form of methanogenesis in these sediments. These solutions also accurately reproduce the isotope depth profiles, including a broad minimum in the δ13CDIC profile and a much sharper (angular) minimum in the δ13CCH4 profile, both of which appear near the base of the transition zone in the sediments between sulfate reduction and methanogenesis (referred to here as the sulfate-methane transition zone, or SMTZ). Such minima in pore-water profiles of δ13CCH4 near the base of the SMTZ have been seen in a number of other marine sediments across a range of depth and timescales. We show here that this minimum in the δ13CCH4 profile in Santa Barbara Basin sediments results from the balance between (1) anaerobic oxidation of methane (AOM), which leads to an increase in δ13CCH4 with decreasing depth in the sediment column through and above the SMTZ; (2) methanogenesis, which produces 13C-depleted methane, both in and below the SMTZ; and (3) an upward flux of CH4 from depth that is relatively enriched in 13C as compared with the methane in these pore waters. Possible sources of this deep methane include the following: geologic hydrocarbon reservoirs derived from ancient source rocks; decomposition of buried gas hydrates; and biogenic (or perhaps thermogenic) methane produced hundreds of meters below the seafloor stimulated by increasing temperatures associated with the sediment geothermal gradient. Although we are unable to resolve these possible sources of deep methane, we believe that the significance of an upward methane flux as an explanation for minima in δ13CCH4 pore-water profiles may not be limited to Santa Barbara Basin sediments but may be common in many continental margin sediments

The existing states and evolution of P and Fe in shallow sediments from the continental slope of northern South China Sea and their indication for the decomposition of gas hydrate

天然气水合物作为当前极具开发潜力的新型清洁能源，受到科学界的广泛关注。海平面下降、洋壳的构造运动、海底沉积物滑塌及洋底水温变化等都可能造成天然气水合物的失稳分解，分解释放的CH4在垂向运移过程中与上层电子受体发生AOM，使SMTZ等作用带磷铁的赋存形态发生变化。磷作为海洋生物的重要营养元素，在海洋生物地球化学循环中有着重要意义，与铁、硫元素及沉积有机质关系紧密，在早期成岩过程中扮演了重要角色；而铁作为地壳中丰度最高的氧化还原敏感性金属元素，在海洋沉积物中的氧化还原过程对磷、硫、碳等生物地球化学重要元素的成岩循环都有重要影响。因此对南海北部陆坡沉积物中磷铁的赋存形态及演化规律的研究，不仅有助我们...Gas hydrates have been receiving extensive attention of the scientific community for their considerable exploration potential. Geological events like eustasy, tectonic movement of ocean crust, seabed sediment collapse and variation of bottom water temperature may cause the decomposition of gas hydrates, the released CH4 in this process reacts with upper electron acceptors(AOM) during upward migrat...学位：理学硕士院系专业：海洋与地球学院_海洋地质学号：2232013115143

Multi-elemental composition of authigenic carbonates in benthic foraminifera from the eastern Bering Sea continental margin (International Ocean Discovery Program Site U1343)

This is the final version. Available from Elsevier via the DOI in this record. Bering Sea sediments represent exceptional archives, offering the potential to study past climates and biogeochemistry at a high resolution. However, abundant hydrocarbons of microbial origin, especially along the eastern Bering Sea continental margin, can hinder the applicability of palaeoceanographic proxies based on calcareous foraminifera, due to the formation of authigenic carbonates. Nonetheless, authigenic carbonates may also bear unique opportunities to reconstruct changes in the sedimentary redox environment. Here we use a suite of visual and geochemical evidence from single-specimens of the shallow infaunal benthic foraminiferal species Elphidium batialis Saidova (1961), recovered from International Ocean Discovery Program (IODP) Site U1343 in the eastern Bering Sea, to investigate the influence of authigenic carbonates on the foraminiferal trace metal composition. Our results demonstrate that foraminiferal calcite tests act as a nucleation template for secondary carbonate precipitation, altering their geochemistry where organoclastic sulphate reduction and anaerobic oxidation of methane cause the formation of low- and high-Mg calcite, respectively. The authigenic carbonates can occur as encrusting on the outside and/or inside of foraminiferal tests, in the form of recrystallization of the test wall, or as banding along natural laminations within the foraminiferal test walls. In addition to Mg, authigenic carbonates are enriched in U/Ca, Mn/Ca, Fe/Ca, and Sr/Ca, depending on the redox environment that they were formed in. Our results demonstrate that site-specific U/Ca thresholds are a promising tool to distinguish between diagenetically altered and pristine foraminiferal samples, important for palaeoceanographic reconstructions utilising the primary foraminiferal geochemistry. Consistent with previous studies, U/Mn ratios of foraminifera at IODP Site U1343 increase according to their degree of diagenetic alteration, suggesting a potential response of authigenic U/Mn to the microbial activity in turn linked to the sedimentary redox environment.BGS University Funding Initiative Ph.D. studentshi

Paleo-methane emissions recorded in foraminifera near the landward limit of the gas hydrate stability zone offshore western Svalbard

We present stable isotope and geochemical data from four sediment cores from west of Prins Karls Forland (ca. 340 m water depth), offshore western Svalbard, recovered from close to sites of active methane seepage, as well as from shallower water depths where methane seepage is not presently observed. Our analyses provide insight into the record of methane seepage in an area where ongoing ocean warming may be fueling the destabilization of shallow methane hydrate. The ?13C values of benthic and planktonic foraminifera at the methane seep sites show distinct intervals with negative values (as low as ?27.8‰) that do not coincide with the present-day depth of the sulfate methane transition zone (SMTZ). These intervals are interpreted to record long-term fluctuations in methane release at the present-day landward limit of the gas hydrate stability zone (GHSZ). Shifts in the radiocarbon ages obtained from planktonic foraminifera toward older values are related to methane-derived authigenic carbonate overgrowths of the foraminiferal tests, and prevent us from establishing the chronology of seepage events. At shallower water depths, where seepage is not presently observed, no record of past methane seepage is recorded in foraminifera from sediments spanning the last 14 ka cal BP (14C-AMS dating). ?13C values of foraminiferal carbonate tests appear to be much more sensitive to methane seepage than other sediment parameters. By providing nucleation sites for authigenic carbonate precipitation, foraminifera thus record the position of even a transiently stable SMTZ, which is likely to be a characteristic of temporally variable methane fluxes

Biogeochemical investigations of methane seepage at the ultraslow-spreading Arctic mid-ocean ridge: Svyatogor ridge, Fram Strait

The Arctic continental shelves are important reservoirs of methane stored in gas hydrates and deeper geological formations. However, little is known about methane dynamics in deeper oceanic settings such as mid-ocean ridges, mainly due to operational challenges related to their remoteness. This study investigates a recently discovered methane seepage environment at Svyatogor Ridge, a sediment-covered transform fault on the western flank of the Arctic mid-ocean ridge west of Svalbard. Svyatogor Ridge was previously hypothesized to host deep gas hydrates and active fluid flow systems, which is a unique combination for this setting and requires geochemical evidence. Based on sediment and foraminiferal geochemistry, we demonstrate that Svyatogor Ridge hosts a shallow methane cycle with anaerobic oxidation of microbial methane, sustaining chemosynthetic communities at the seafloor. Our geochemical datasets also suggest that methane fluxes were higher in the past than today and long-lasting, with episodes of intense methane oxidation recorded in pre-Holocene sediments. Methane seepage is still ongoing at this location, although no evidence exists that methane is reaching the sea surface. The results provide the first evidence of a methane seepage environment ever reported from the ultra-slow spreading Arctic mid-ocean ridge, thus calling for a reevaluation of the role of this type of ridge in the ocean methane cycle