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International audience; Seven sediment cores were taken in the Sea of Okhotsk in a south-north transect along the slope of Sakhalin Island. The retrieved anoxic sediments and pore fluids were analyzed for particulate organic carbon (POC), total nitrogen, total sulfur, dissolved sulfate, sulfide, methane, ammonium, iodide, bromide, calcium, and total alkalinity. A novel method was developed to derive sedimentation rates from a steady-state nitrogen mass balance. Rates of organic matter degradation, sulfate reduction, methane turnover, and carbonate precipitation were derived from the data applying a steady-state transport-reaction model. A good fit to the data set was obtained using the following new rate law for organic matter degradation in anoxic sediments: RPOC = _______________Kc____________ . Kx.POC C (DIC) + C (CH4) + Kc The rate of particulate organic carbon degradation (RPOC) was found to depend on the POC concentration, an age-dependent kinetic constant (kx) and the concentration of dissolved metabolites. Rates are inhibited at high dissolved inorganic carbon (DIC) and dissolved methane (CH4) concentrations. The best fit to the data was obtained applying an inhibition constant KC of 35 ±5 mM. The modeling further showed that bromide and iodide are preferentially released during organic matter degradation in anoxic sediments. Carbonate precipitation is driven by the anaerobic oxidation of methane (AOM) and removes one third of the carbonate alkalinity generated via AOM. The new model of organic matter degradation was further tested and extended to simulate the accumulation of gas hydrates at Blake Ridge. A good fit to the available POC, total nitrogen, dissolved ammonium, bromide, iodide and sulfate data was obtained confirming that the new model can be used to simulate organic matter degradation and methane production over the entire hydrate stability zone (HSZ). The modeling revealed that most of the gas hydrates accumulating in Blake Ridge sediments are neither formed by organic matter degradation within the HSZ nor by dissolved methane transported to the surface by upward fluid flow but rather through the ascent of gas bubbles from deeper sediment layers. The model was further applied to predict rates of hydrate accumulation in Sakhalin slope sediments. It showed that only up to 0.3% of the pore space is occupied by gas hydrates formed via organic matter degradation within the HSZ. Gas bubble ascent may, however, significantly increase the total amount of hydrate in these deposits.

Measurements of the calcium isotopic composition (δ44/40Ca) of planktonic foraminifera from the western equatorial Pacific and the Indian sector of the Southern Ocean show variations of about 0.6‰ over the past 24 Myr. The stacked δ44/40Ca record of Globigerinoides trilobus and Globigerina bulloides indicates a minimum in δ44/40Casw (seawater calcium) at 15 to 16 Ma and a subsequent general increase toward the present, interrupted by a second minimum at 3 to 5 Ma. Applying a coupled calcium/carbon cycle model, we find two scenarios that can explain a large portion of the observed δ44/40Casw variations. In both cases, variations in the Ca input flux to the ocean without proportional changes in the carbonate flux are invoked. The first scenario increases the riverine calcium input to the ocean without a proportional increase of the carbonate flux. The second scenario generates an additional calcium flux from the exchange of Ca by Mg during dolomitization. In both cases the calcium flux variations lead to drastic changes in the seawater Ca concentrations on million year timescales. Our δ44/40Casw record therefore indicates that the global calcium cycle may be much more dynamic than previously assumed.

Along the erosive convergent margin off Costa Rica a large number of mound-shaped structures exist built by mud diapirism or mud volcanism. One of these, Mound 12, an intermittently active mud volcano, currently emits large amounts of aqueous dissolved species and water. Chemosynthetic vent communities, authigenic carbonates, and methane plumes in the water column are manifestations of that activity. Benthic flux measurements were obtained by a video-guided Benthic Chamber Lander (BCL) deployed at a vent site located in the most active part of Mound 12. The lander was equipped with 4 independent chambers covering adjacent areas of the seafloor. Benthic fluxes were recorded by repeated sampling of the enclosed bottom waters while the underlying surface sediments were recovered with the lander after a deployment time of one day. One of the chambers was placed directly in the centre of an active vent marked by the occurrence of a bacterial mat while the other chambers were located at the fringe of the same vent system at a lateral distance of only 40 cm. A transport-reaction model was developed and applied to describe the concentration profiles in the pore water of the recovered surface sediments and the temporal evolution of the enclosed bottom water. Repeated model runs revealed that the best fit to the pore water and benthic chamber data is obtained with a flow velocity of 10 cm yr− 1 at the centre of the vent. The flux rates to the bottom water are strongly modified by the benthic turnover (benthic filter). The methane flux from below at the bacterial mat site is as high as 1032 μmol cm− 2 yr− 1, out of which 588 μmol cm− 2 yr− 1 is oxidised in the surface sediments by microbial consortia using sulphate as terminal electron acceptor and 440 μmol cm− 2 yr− 1 are seeping into the overlaying bottom water. Sulphide is transported to the surface by ascending fluids (238 μmol cm− 2 yr− 1) and is formed within the surface sediment by the anaerobic oxidation of methane (AOM, 588 μmol cm− 2 yr− 1). However, sulphide is not released into the bottom water but completely oxidized by oxygen and nitrate at the sediment/water interface. The oxygen and nitrate fluxes into the sediment are high (781 and 700 μmol cm− 2 yr− 1, respectively) and are mainly driven by the microbial oxidation of sulphide. Benthic fluxes were much lower in the other chambers placed in the fringe of the vent system. Thus, methane and oxygen fluxes of only 28 and 89 μmol cm− 2 yr− 1, respectively were recorded in one of these chambers. Our study shows that the aerobic oxidation of methane is much less efficient than the anaerobic oxidation of methane so that methane which is not oxidized within the sediment by AOM is almost completely released into the bottom water. Hence, anaerobic rather than aerobic methane oxidation plays the major role in the regulation of benthic methane fluxes. Moreover, we demonstrate that methane and oxygen fluxes at cold vent sites may vary up to 3 orders of magnitude over a lateral distance of only 40 cm indicating an extreme focussing of fluid flow and methane release at the seafloor.

Carbonate precipitates on mounds and along tectonic scarps off the Costa Rica margin are manifestations of subduction-induced dewatering. The long-term dewatering history is recorded in mineralogical, petrological and isotope signals of carbonates recovered from these sites. The carbonates are strongly depleted in δ13C (−11 to −53‰ PDB) and enriched in δ18O (+4 to +8‰ PDB). Thermogenic methane and biogenic methane were identified as sources of the carbon. Chemoherm carbonates and seepage-associated carbonates formed in a focused flow regime have lighter δ13C values, while others formed in a more diffusive flow regime have slightly enriched C isotope values. Three fluid components were inferred based on the calculation of equilibrium δ18O: clay dehydration water, gas hydrate water and seawater. Calculated equilibrium δ18O values of carbonates from different down-core depths as well as from different precipitation stages show that the δ18O of the precipitating fluid is progressively depleted with time. Dolostones showing a methane-C source and a well constrained O-isotope signature are thought to have formed at depth in the sediment and subsequently became exhumed. Glauconitic sandstones cemented by methane-derived carbonate provide evidence that fluid and solid material have been expelled by the mud volcano.

[1] A new model is developed and applied to simulate the Phanerozoic evolution of seawater composition (dissolved Ca, Sr, dissolved inorganic carbon, alkalinity, pH, δ18O), marine carbonates (Sr/Ca, 87Sr/86Sr, δ13C, δ18O), atmospheric CO2 and surface temperature. The marine carbonate records (Sr/Ca, 87Sr/86Sr, δ13C) are used to reconstruct changes in volcanic/tectonic activity and organic carbon burial over the Phanerozoic. Seawater pH is calculated assuming saturation with respect to calcite and considering the changing concentration of dissolved Ca documented by brine inclusion data. The depth of calcite saturation is allowed to vary through time and the effects of changing temperature and pressure on the stability constants of the carbonate system are considered. Surface temperatures are calculated using the GEOCARB III approach considering also the changing flux of galactic cosmic radiation (GCR). It is assumed that GCR cools the surface of the Earth via enhanced cloud formation at low altitudes. The δ18O of marine carbonates is calculated considering the changing isotopic composition of seawater, the prevailing surface temperatures and seawater pH. Repeated model runs showed that the trends observed in the marine δ18O record can only be reproduced by the model if GCR is allowed to have a strong effect on surface temperature. The climate evolution predicted by the model is consistent with the geological record. Warm periods (Cambrian, Devonian, Triassic, Cretaceous) are characterized by low GCR levels. Cold periods during the late Carboniferous to early Permian and the late Cenozoic are marked by high GCR fluxes and low pCO2 values. The major glaciations occurring during these periods are the result of carbon cycling processes causing a draw-down of atmospheric CO2 and a coevally prevailing dense cloud cover at low-altitudes induced by strong GCR fluxes. The two moderately cool periods during the Ordovician - Silurian and Jurassic - early Cretaceous are characterized by both high pCO2 and GCR levels so that greenhouse warming compensated for the cooling effect of low-altitude clouds. The very high Jurassic δ18O values observed in the geological record are caused by low pH values in surface waters rather than cold surface conditions.

[1] The role of sediment diagenesis in the marine cycles of Li and B is poorly understood. Because Li and B are easily mobilized during burial and are consumed in authigenic clay mineral formation, their abundance in marine pore waters varies considerably. Exchange with the overlying ocean through diffusive fluxes should thus be common. Nevertheless, only a minor Li sink associated with the low-temperature alteration of volcanic ash has been observed. We describe a low-temperature diagenetic environment in the Black Sea dominated by the alteration of detrital plagioclase feldspars. Fluids expelled from the Odessa mud volcano in the Sorokin Trough originate from shallow (≈100–400 m deep) sediments which are poor in volcanic materials but rich in anorthite. These fluids are depleted in Na+, K+, Li+, B, and 18O and enriched in Ca2+ and Sr2+, indicating that anorthite is dissolving and authigenic clays are forming. Using a simple chemical model, we calculate the pH and the partial pressure of CO2 (PCO2) in fluids associated with this alteration process. Our results show that the pH of these fluids is up to 1.5 pH units lower than in most deep marine sediments and that PCO2 levels are up to several hundred times higher than in the atmosphere. These conditions are similar to those which favor the weathering of silicate minerals in subaerial soil environments. We propose that in Black Sea sediments enhanced organic matter preservation favors CO2 production through methanogenesis and results in a low pore water pH, compared to most deep sea sediments. As a result, silicate mineral weathering, which is a sluggish process in most marine diagenetic environments, proceeds rapidly in Black Sea sediments. There is a potential for organic matter-rich continental shelf environments to host this type of diagenesis. Should such environments be widespread, this new Li and B sink could help balance the marine Li and Li isotope budgets but would imply an apparent imbalance in the B cycle.

At Hydrate Ridge (HR), Cascadia convergent margin, surface sediments contain massive gas hydrates formed from methane that ascends together with fluids along faults from deeper reservoirs. Anaerobic oxidation of methane (AOM), mediated by a microbial consortium of archaea and sulfate-reducing bacteria, generates high concentrations of hydrogen sulfide in the surface sediments. The production of sulfide supports chemosynthetic communities that gain energy from sulfide oxidation. Depending on fluid flow, the surface communities are dominated either by the filamentous sulfur bacteria Beggiatoa (high advective flow), the clam Calyptogena (low advective flow), or the bivalve Acharax (diffusive flow). We analyzed surface sediments (0 to 10 cm) populated by chemosynthetic communities for AOM, sulfate reduction (SR) and the distribution of the microbial consortium mediating AOM. Highest AOM rates were found at the Beggiatoa field with an average rate of 99 mmol m(-2) d(-1) integrated over 0 to 10 cm. These rates are among the highest AOM rates ever observed in methane-bearing marine sediments. At the Calyptogena field, AOM rates were lower (56 mmol m(-2) d(-1)). At the Acharax field, methane oxidation was extremely low (2.1 mmol m(-2) d(-1)) and was probably due to aerobic oxidation of methane. SR was fueled largely by methane at flow-impacted sites, but exceeded AOM in some cases, most likely due to sediment heterogeneity. At the Acharax field, SR was decoupled from methane oxidation and showed low activity. Aggregates of the AOM consortium were abundant at the fluid-impacted sites (between 5.1 x 10(12) and 7.9 x 10(12) aggregates m(-2)) but showed low numbers at the Acharax field (0.4 x 10(12) aggregates m(-2)). A transportreaction model was applied to estimate AOM at Beggiatoa fields. The model agreed with the measured depth-integrated AOM rates and the vertical distribution. AOM represents an important methane sink in the surface sediments of HR, consuming between 50 and 100 % of the methane transported by advection.

A new model for the marine cycles of particulate organic carbon (POC), oxygen, nitrate, and phosphorus has been developed and applied to explore the controls and constraints on marine productivity and nutrient inventories. The coupled benthic-pelagic model uses a new approach for the simulation of the reactive phosphorus turnover (Preac corresponding to the sum of organic P, authigenic P, and adsorbed phosphate) in marine sediments. The simulated POC/Preac burial ratio in shelf, slope and rise, and deep-sea sediments increases under strongly reducing conditions in agreement with field observation. The model runs revealed that the spread of anoxia in bottom waters may enhance the productivity of the global ocean by one order of magnitude if sufficient nitrate is provided by N2-fixation. Thus anoxic bottom waters promote eutrophic conditions and vice versa. Additional model runs showed that the productivity and nutrient inventory of the glacial ocean were probably enhanced due to the falling sea level. Marine regression induced a narrowing of the depositional areas on the continental shelves and thereby an increase in the fraction of POC exported to the deep ocean. The accelerated POC delivery, in turn, decreased the oxygen contents of the deep water and thus favored the release of phosphate from deep-sea and rise sediments. Enhanced recycling of phosphate at the seafloor promoted further POC export in a positive feedback loop.

International audience; During the expeditions ANT-XV/2 with R/V Polarstern in 1997/98 and NBP 99-04 with R/V IB N.B. Palmer in 1999, the first samples of hydrothermally influenced sediments of Bransfield Strait were obtained at Hook Ridge, a volcanic edifice in the Central Basin of the Strait. The vent sites are characterized by white siliceous crusts on top of the sediment layer and temperatures measured immediately on deck are up to 48.5°C. The shallow depth of these vent sites (1050 m) particularly controls the chemistry of the pore fluids that are enriched in silica and sulfide and show low pH values. Chloride is depleted up to 20% and the calculated hydrothermal endmember concentration is in the range of 1–84 mM. Since other mechanisms for Cl depletion can be ruled out clearly, the composition of this fluid is attributed to phase separation. While the Cl-depleted fluid is emanating at Hook Ridge, a Cl-enriched fluid can be identified in the adjacent King George Basin. Using a p,x diagram the two corresponding endmember concentrations reveal that the phase separation takes place at subcritical conditions (total depth: ∼2500 m), probably along the whole volcanic edifice.

The relationship between carbon dioxide and climate over millions of years has been a source of controversy. Fossilized liverwort leaves can help illuminate both temperature and atmospheric carbon dioxide levels from 200 to 60 million years ago.

The weathering of silicate minerals exposed on the continents is the largest sink of atmospheric CO2 on time scales of millions of years. The rate of this process is positively correlated with global mean temperature and atmospheric CO2 concentration, resulting in a negative feedback that stabilizes Earths’ climate (Berner, 2004). Detrital silicates derived from the physical denudation of the continents are a major component of marine sediments (Li and Schoonmaker, 2003). However, their geochemical behaviour is poorly understood and they are considered to be unimportant to the long-term carbon cycle. We show that in organic matter-rich sediments of the Sea of Okhotsk detrital silicates undergo intense weathering. This process is likely favoured by microbial activity, which lowers pore water pH and releases dissolved humic substances, and by the freshness of detrital silicates which originate from the cold, poorly weathered Amur River basin. Numerical simulations of early diagenesis show that submarine weathering rates in our study area are comparable to average continental weathering rates (Gaillardet et al., 1999). Furthermore, silicate weathering seems to be widespread in organic matter-rich sediments of continental margins, suggesting the existence of a significant CO2 sink there. These findings imply a greater efficiency of the silicate weathering engine also at low surface temperatures, resulting in a weakening of the negative feedback between pCO2, climate evolution and silicate weathering.

Nucleation of calcium carbonate on microbial cell material may have been the dominant mode of microbial carbonate formation during most of Earth's history. Current knowledge predicts that nucleation takes place on the cell surface or on extracellular polymeric substances. However, the initial nucleation steps have not been described in detail and the process remains elusive. Here we describe the bacterial nucleation of calcium carbonate at the nanometer scale. In our precipitation experiment with sulfate reducing bacteria (SRB), the bulk of calcium carbonate precipitates on hundreds of individual globules 60– 200 nm in diameter. Globules originate from the SRB cell surface but calcify significantly only when released to the culture medium. Similar globules have been observed, albeit at a much larger scale, in other bacterial precipitation experiments and in many natural microbial carbonates, suggesting that the process we describe could be an important step in microbial calcification.

A large number of mound-shaped structures that originated from mud extrusions is present along the convergent continental margin off Costa Rica and Nicaragua. Active fluid venting is indicated by the existence of CH4- and H2S-rich pore fluids as well as associated benthic fauna and authigenic carbonates. End-member fluid samples from all mounds are significantly depleted in dissolved Cl and other major elements, suggesting a general process of freshwater addition and thus a common source of the fluids. Our data clearly rule out dilution by gas hydrate dissociation as a dominant source of the freshwater. Enrichments of the fluids in B (up to 2 mmol/L) and inversely correlated δ18O vs. δD values point to clay-mineral dehydration as the cause for these anomalies. Calculations assuming a δ18O vs. δD equilibrium between the pore fluid and clay minerals at depth of formation indicate temperatures of dehydration between 85 and 130 °C. This temperature range is in agreement with the B enrichments and the presence of thermogenically formed CH4. Because temperatures above 50 °C are not reached within the sediment cover of the upper plate, the fluids most likely form within the subducted sediments and flow upward along deep-seated faults from ≥12 km depth. Mound-related fluid expulsion may contribute significantly to the recycling of mineral-bound water.

Hydrothermal activity in the Central Bransfield Basin revealed an active low-temperature vent field on top of a submarine volcanic structure. A temperature anomaly was detected and the sea floor showed various patches of white silica (opal-A) precipitate exposures and some yellow–brown Fe-oxyhydroxide crusts. Enriched dissolved methane concentrations were encountered. Sediment was near 24°C just after the grab came on deck. No dense population of chemosynthetically based macrofauna known from other hydrothermal venting areas was present, except for pogonophora. The observations suggest that the sedimented hydrothermal field at Hook Ridge is a low-temperature end-member branch from a deeper hydrothermal source.

Fluid venting has been observed along 800 km of the Alaska convergent margin. The fluid venting sites are located near the deformation front, are controlled by subsurface structures, and exhibit the characteristics of cold seeps seen in other convergent margins. The more important characteristics include (1) methane plumes in the lower water column with maxima above the seafloor which are traceable to the initial deformation ridges; (2) prolific colonies of vent biota aligned and distributed in patches controlled by fault scarps, over- steepened folds or outcrops of bedding planes; (3) calcium carbonate and barite precipitates at the surface and subsurface of vents; and (4) carbon isotope evidence from tissue and skeletal hard parts of biota, as well as from carbonate precipitates, that vents expel either methane- or sulfide-dominated fluids. A biogeochemical approach toward estimating fluid flow rates from individual vents based on oxygen flux measurements and vent fluid analysis indicates a mean value of 5.5 + 0.7 L m -2 d -1 for tectonics-induced water flow ( Wallmann et al., 1997b). A geophysical estimate of dewatering from the same area (von Huene et al., 1997) based on sediment porosity reduction shows a fluid loss of 0.02 L m -2 d-1 for a 5.5 km wide converged segment near the deformation front. Our video-guided surveys have documented vent biota across a minimum of 0.1% of the area of the convergent segment off Kodiak Island; hence an average rate of 0.006 L m -2 d -1 is estimated from the biogeochemical approach. The two estimates for tectonics-induced water flow from the accretionary prism are in surprisingly good agreement.

Extraordinary helium and argon signatures were observed in a recently discovered brine lake district in the eastern Mediterranean Sea at water depths of ∼ 3500 m. In the Urania Basin 4He concentrations are enriched by a factor of up to 32000 compared to normal seawater. Low 3He/4He ratios of 1 × 10−7 and high 40Ar/36Ar ratios of up to 470 indicate a radiogenic source. These data represent by far the highest radiogenic supersaturations ever observed in an oceanic environment. We propose a new conceptional model assuming that the Urania Basin is fed by advective transport from a deep fluid reservoir below the Messinian evaporite sequence. The evaporite seal provides the unique condition for accumulating the extraordinary high amounts of radiogenic helium from deep sedimentary strata. Injection of radiogenic helium from brine pools like the Urania Basin may contribute significantly to the inventory of terrigenic helium in the deep waters of the eastern Mediterranean.

In situ oxygen fluxes were measured at vent sites in the Aleutian trench at a water depth of almost 5000 m using a TV-guided benthic flux chamber. The flux was 2 orders of magnitude greater than benthic oxygen fluxes in areas unaffected by venting on the continental margin off Alaska. Porewater profiles taken from the surface sediment below a vent site showed high concentrations of sulfide, methane, and ammonia. The reduced carbon and nitrogen compounds are transported to the vent site by fluids expelled from deeper anoxic sediment layers by the forces of plate convergence. The tectonically driven fluid flow was determined from the biochemical turnover in vent communities and was found to be 3.4 ± 0.5 m yr−1. A model was used to quantify the transport of silica, Ca2+, and sulfate via diffusion, advection, and bioirrigation through the surface sediments of a vent site. A nonlocal mixing coefficient of 20–30 yr−1 was determined by fitting the model curves to the measured porewater profiles showing that the transport of solutes within the near-surface sediments and across the sediment-water interface is dominated by the activity of the vent fauna. Sulfate-containing oceanic bottom water and methane-rich vent fluids were mixed below the clam colony to produce sulfide and a CaCO3 precipitate. The vent biota shape their immediate environment and control the sediment-water exchange and the benthic fluxes at vent sites. The oxygen consumption at vent sites is a major sink for oxygen at the study area.

Highlights • A biogeochemical baseline of sediment geochemistry at potential offshore CCS sites. • Diagnostic indicators of CO2 leakage based on stoichiometry of porewater chemistry. • Porewater chemistry is modified by reverse weathering processes at Goldeneye site. Abstract Injection of carbon dioxide (CO2) into subseafloor reservoirs is gaining traction as a strategy for mitigating anthropogenic CO2 emissions to the atmosphere. Yet, potential leakage, migration and dissolution of externally-supplied CO2 from such reservoirs are a cause for concern. The potential impact of CO2 leakage on the biogeochemistry of sediments and overlying waters in the North Sea was studied during a controlled subsurface CO2 release experiment in 2019 at a potential carbon capture and storage site (Goldeneye). This study describes the natural (unperturbed) biogeochemistry of sediments. They are classified as muddy sand to sandy mud with low organic carbon content (∼0.6 %). Distributions of dissolved inorganic carbon (DIC) and total alkalinity (TA) in sediment porewaters are reported in addition to in situ benthic fluxes of dissolved nutrients and oxygen between the sediments and the overlying water. Oxygen fluxes into the sediment, measured using benthic chambers and eddy covariance, were 6.18 ± 0.58 and 5.73 ± 2.03 mmol m−2 d-1, respectively. Diagnostic indicators are discussed that could be used to detect CO2 enrichment of sediments due to reservoir leakage at CCS sites. These include the ratio TA and ammonium to sulfate in sediment porewaters, benthic fluxes and chloride-normalized cation distributions. These indicators currently suggest that the organic carbon at Goldeneye has an oxidation state below zero and is mainly degraded via sulfate reduction. Carbonate precipitation is apparently negligible, whereas decreases in Mg2+ and K+ point toward ongoing alteration of lithogenic sediments by reverse weathering processes.

According to many prognostic scenarios by the Intergovernmental Panel on Climate Change (IPCC), a scaling-up of carbon dioxide (CO 2) capture and storage (CCS) by several orders-of-magnitude is necessary to meet the target of ≤2 °C global warming by 2100 relative to preindustrial levels. Since a large fraction of the predicted CO 2 storage capacity lies offshore, there is a pressing need to develop field-tested methods to detect and quantify potential leaks in the marine environment. Here, we combine field measurements with numerical models to determine the flow rate of a controlled release of CO 2 in a shallow marine setting at about 119 m water depth in the North Sea. In this experiment, CO 2 was injected into the sediment at 3 m depth at 143 kg d -1. The new leakage monitoring tool predicts that 91 kg d -1 of CO 2 escaped across the seafloor, and that 51 kg d -1 of CO 2 were retained in the sediment, in agreement with independent field estimates. The new approach relies mostly on field data collected from ship-deployed technology (towed sensors, Acoustic Doppler current profiler—ADCP), which makes it a promising tool to monitor existing and upcoming offshore CO 2 storage sites and to detect and quantify potential CO 2 leakage.

The Cretaceous with extremely high atmospheric pCO2 is one of the warmest periods in the Phanerozoic, providing an endmember to test climate models. Repeated periods of widespread dysoxic / anoxic conditions (Oceanic Anoxic Events; OAEs) were characterized by enhanced burial of organic matter and different degrees of marine faunal turnovers, that can be seen as extreme equivalents for the already observed trend of expanding Oxygen Minimum Zones (OMZ) in modern oceans caused by anthropogenic climate warming. One of the largest OAEs occurring at the Cenomanian / Turonian boundary is OAE2. This thesis presents results from Core SN°4 drilled in the Tarfaya Basin (SW Morocco). This continuous record spanning the time from the late Albian to early Turonian allowed 1) to reconstruct climatic and paleoceanographic variability during this period in the Tarfaya Basin, 2) to unravel climate processes and biospheric changes during the onset of OAE2 and 3) to study phosphorus dynamics during the Cenomanian and early Turonian. Die Kreide zeichnet sich als eine der wärmsten Perioden des Phanerozoikums durch signifikant höhere atmosphärische CO2-Konzentrationen aus, was sie zu einem idealen Vergleichsobjekt für Klimamodellierungen macht. Wiederholt auftretende Ozeanische Anoxische Events (OAEs) sind gekennzeichnet durch weiträumige dysoxische / anoxische Bedingungen, welche sich in einer starken Anreicherung organischen Kohlenstoffes in marinen Sedimenten, häufig begleitet von Aussterbeereignissen, wiederspiegeln. Diese Ereignisse können als extreme Äquivalente des sich bereits heute zu beobachtenden Trends der Expansion der Sauerstoff-Minimum-Zonen (OMZ) infolge der globalen Klimaerwärmung angesehen werden. Das OAE2 an der Cenomanium-Turonium-Grenze ist eines der bedeutendsten dieser Events. Im Rahmen dieser Dissertation wurde der Bohrkern SN°4 aus dem Tarfaya Becken (SW Marokko) untersucht. Dieser kontinuierliche Sedimentkern besteht aus offen marinen Ablagerungen der mittleren Kreise vom oberen Albium bis zum unteren Turonium und gestattete: 1) die Rekonstruktion von klimatischen und ozeanographischen Änderungen dieser Zeit, 2) eine hoch aufgelöste Studie klimatischer und sedimentärer Variabilität während der Anfangsphase des OAE2 und begleitender Umwälzungen in der Biosphäre und 3) die Rekonstruktion von Änderungen im Phosphorhaushalt während des Cenomaniums und unteren Turoniums.

Methane (CH4) and nitrous oxide (N2O) are greenhouse gases, which atmospheric concentrations increased since preindustrial times by ~150 and ~20%, respectively, mainly due to the increase in anthropogenic emissions. The atmospheric increase of greenhouse gases (incl. carbon dioxide (CO2), CH4 and N2O) led to various effects on the Earth’s surface and atmosphere, summarized as global climate change. In the marine environment, temperature rise, sea level rise, ocean acidification, and decreased oxygen concentrations are the most significant effects of climate change. To predict possible changes through climate change in the future, natural and anthropogenic sources and production/consumption pathways of greenhouse gases need to be determined carefully. To date, research is sparse on the sources for oceanic CH4 and N2O, thus leading to uncertainties in global ocean emission estimates. Marine, organic-rich sediments (e.g. in coastal areas) are known to be major benthic sources for CH4 and N2O, which eventually could end up in the atmosphere. Both greenhouse gases are produced by microbial processes during the degradation of organic matter in marine sediments, namely methanogenesis and denitrification (next to nitrification), respectively. However, knowledge about magnitude and environmental controls of these microbial processes is still limited. In the present study, benthic CH4 and N2O production was investigated in three different marine areas with the focus on the surface sediment (0-30 cmbsf=centimeter below surface): the upwelling region off Peru, the Eckernförde Bay in the southwestern Baltic Sea and the Gulf of Mexico. In sediments from Peru and Eckernförde Bay, the focus was set on surface methanogenesis within the sulfate-reducing zone, which has been thought to be negligible due to the successful competition of sulfate reducers for the mutual substrates hydrogen (H2) and acetate. In oil-influenced sediments from the Gulf of Mexico, the focus was set on benthic denitrification and sulfate reduction, with benthic N2O production as a side effect. The investigations showed the following: 1) In sediments off Peru, methanogenesis and sulfate reduction co-occurred within the upper 0-30 cmbsf, explained by usage of non-competitive substrates (such as methanol or methylated compounds) by methanogens. In the deeper sediment horizons (>30 cmbsf), usage of the competitive substrates H2 and acetate was confirmed, probably due to a relief of the competitive situation by sulfate depletion. Surface methanogenesis activity varied spatially along the Peruvian margin (70-1024 m water depth), with the major driving factor being the availability and variety of organic matter, followed by oxygen. Thus, highest surface methanogenesis activity was observed on the shelf, where organic carbon load was highest together with hypoxic or even anoxic conditions in the bottom water. At the same time, dissolved CH4 concentrations were also highest on the shelf, indicating a previously underestimated contribution of surface methanogenesis to benthic methane emissions. If not escaping to the water column, the methane produced by surface methanogenesis could act as important methane supplier for anaerobic oxidation of methane in surface sediments. 2) Surface methanogenesis was detected on a seasonal basis within the sulfate-reducing zone in sediments (0-30 cmbsf) from the Time Series Station Boknis Eck in Eckernförde Bay (SW Baltic Sea). Methanogenesis activity changed seasonally with highest rates in September/November after the summer/autumn phytoplankton blooms, and lowest rates in March after the period of low production during winter. The main controlling factor for surface methanogenesis was suggested to be the organic matter quantity and quality, followed by temperature and oxygen. The major part of surface methanogenesis was probably facilitated from usage of non-competitive substrates (e.g. methanol and methylated amines) to avoid competition with sulfate reducers, indicated by increased activity after methanol addition and concomitant detection of the family Methanosarcinaceae, whose members are known for usage of non-competitive substrates. Accordingly, usage of competitive substrates such as H2 increased in deeper sediment horizons (>30 cmbsf) when sulfate was depleted. The results revealed that surface methanogenesis could potentially fuel surface anaerobic oxidation of methane with up to 13%. In addition, not only surface methanogenesis activity but also benthic methane emissions showed seasonal variation, indicated by dissolved methane concentrations in the bottom water. 3) Denitrification was significantly elevated in oil-influenced (oiled=sedimented oil layer on top) sediments compared to control sediments with no oil influence, identifying this process as a major degrading process. Sulfate reduction was not elevated, hinting towards the restricted availability of sedimented oil to only specific microbial groups. Sulfide-induced N2O production, resulting from elevated sulfate reduction, was not observed. However, all investigated sediment samples revealed the potential for being a N2O source, probably resulting from elevated denitrification rates. In summary, the successful detection of surface methanogenesis in two, organic-rich, coastal systems shows its previously underestimated role in benthic methane budgeting, e.g. as a methane supplier for anaerobic oxidation of methane or as a contributor to benthic methane emissions to the water column. In addition to spatial variation, also seasonal variation was identified to play an important role in benthic methane production and emission, which should be included in ocean emission estimates. Production of methane in surface sediments was found to be mainly dependent on organic matter input, temperature and oxygen, and thus could be affected by predicted climate change (e.g. temperature rise). The findings on benthic N2O production indicate the crucial importance for studying environmental controls on denitrification in organic-rich sediments as a source of N2O to the water column, including coastal areas and cold seeps. Both systems experience high sulfide concentrations, which would also increase the potential for sulfide-induced N2O production in theses environments. Methan (CH4) und Distickstoffmonoxid (N2O) sind Treibhausgase, deren atmosphärische Konzentration seit Beginn der Industrialisierung jeweils um 150% bzw. 20% angestiegen ist, hauptsächlich durch den Anstieg von anthropogenen Emissionen. Der Anstieg von Treibhausgasen (unter anderem Kohlenstoffdioxid (CO2), CH4 und N2O) in der Atmosphäre hat zu unterschiedlichen Auswirkungen in der Erdatmosphäre sowie an der Erdoberfläche geführt, die zusammenfassend als globaler Klimawandel bezeichnet werden. Im marinen Lebensraum gehören Temperaturanstieg, Anstieg des Meeresspiegels, Ozeanversauerung und die abnehmenden Sauerstoffkonzentrationen im Ozean zu den signifikantesten Auswirkungen des Klimawandels. Um zukünftige Veränderungen durch den Klimawandel vorherzusagen, ist es wichtig, die natürlichen und anthropogenen Quellen von Treibhausgasen sowie deren Produktions- und Abbauwege zu bestimmen. Die Forschung bzgl. der Quellen für gelöstes CH4 und N2O im Ozean ist immer noch gering, was wiederum zu Unsicherheiten in der Berechnung von globalen Ozean-Emissionen führt. Marine, organik-reiche Sedimente (z.B. in Küstengebieten) gelten als Hauptquelle für im Ozean-gelöstes CH4 und N2O, welches schlussendlich auch in die Atmosphäre gelangen kann. Sowohl CH4 als auch N2O werden in marinen Sedimenten von Mikroben hauptsächlich während der Methanogenese bzw. Denitrifikation (neben der mikrobiellen Nitrifikation) produziert. Beide Prozesse finden während der mikrobiellen Remineralisation von organischem Material statt. Das Wissen in Bezug auf die Größenordnung dieser Prozesse, sowie deren Umwelt-Kontrollen, ist noch sehr lückenhaft. In der vorliegenden Studie wurde die Produktion von CH4 und N2O in Sedimenten von drei verschiedenen, marinen Gebieten untersucht: dem Auftriebsgebiet vor Peru, der Eckernförder Bucht in der südwestlichen Ostsee und dem Golf von Mexiko. Hierbei lag der Fokus auf der Sediment-Oberfläche (0- 30 cm).In den Gebieten Peru und Eckernförder Bucht wurde die Oberflächen-Methanogenese in der Sulfatreduktionszone untersucht. Diese galt bis dato als vernachlässigbar aufgrund der erfolgreichen Konkurrenz der Sulfatreduzierer für die gemeinsamen Substrate Wasserstoff (H2) und Acetat. In den Öl-beeinflussten Sedimenten aus dem Golf von Mexiko wurden die benthischen Prozesse Denitrifikation und Sulfatreduktion untersucht, einschließlich der benthischen N2O Produktion als Nebeneffekt. Die Untersuchungen ergaben folgende Ergebnisse: 1) In den Sedimenten vom peruanischen Auftriebsgebiet kamen Methanogenese und Sulfatreduktion gleichzeitig in den oberen 0-30 cm auf vor. Dies war möglich durch Nutzung von nicht-kompetitiven Substraten von den methanogenen Mikroben (Methanogenen), wie z.B. Methanol oder methylierte Verbindungen. In den tieferen Sedimentschichten (> 30 cm) konnte eine verstärkte Nutzung der kompetitiven Substrate H2 und Acetat beobachtet werden. Möglicherweise wurde durch die starke Abnahme an Sulfat die Konkurrenz zwischen Methanogenen und Sulfatreduzierern in diesen Tiefen abgeschwächt. Oberflächen-Methanogenese zeigte räumliche Schwankungen entlang des Kontinentalhanges (70 bis 1024 m Wassertiefe), welche hauptsächlich durch die Verfügbarkeit und die Variabilität des organischen Materials, sowie der Sauerstoffkonzentrationen erklärt wurde. Folglich wurden die höchsten Raten von Oberflächen-Methanogenese auf dem Schelf gemessen, wo der Organik-Gehalt am höchsten und die Sauerstoffkonzentrationen im Tiefenwasser am geringsten waren (hypoxische bis zu anoxische Bedingungen). Gleichzeitig wurden auch die höchsten Methankonzentration im Tiefenwasser auf dem Schelf gemessen, was darauf hindeutet, dass Oberflächen-Methanogenese eine größere Rolle in benthischen Methan-Emissionen spielen kann, als vorher beschrieben. Das an der Oberfläche produzierte Methan kann, wenn es nicht direkt in die Wassersäule diffundiert, auch als wichtige Methanquelle für anaerobe Methanoxidation an der Oberfläche fungieren. 2) Oberflächen-Methanogenese wurde saisonal in der Sulfatreduktionszone in Sedimenten der Zeitserienstation Boknis Eck in der Eckernförder Bucht (südwestliche Ostsee) nachgewiesen. Die Methanogenese-Aktivität zeigte saisonale Schwankungen mit den höchsten Raten im September/November nach den Sommer-/Herbst-Phytoplanktonblüten und den niedrigsten Raten im März nach dem weniger produktiven Winter. Die Hauptfaktoren, die die Oberflächen-Methanogenese kontrollierten, waren die Quantität und Qualität des organischen Materials, gefolgt von Temperatur und Sauerstoffgehalt. Der Hauptteil der Oberflächen-Methanogenese wurde wahrscheinlich durch die Nutzung von nicht-kompetitiven Substraten(z.B. Methanol oder methylierte Verbindungen) bestritten. Dies geht auch aus den erhöhten Methanogeneseraten nach Methanolzugabe und der gleichzeitigen Detektion der Familie Methanosarcinaceae hervor, deren Angehörige für die Nutzung von nicht-kompetitiven Substraten bekannt sind. Dementsprechend wurden kompetitive Substrate wie z.B. H2 erst in den tieferen Sedimentschichten (> 30 cm) verwendet, als Sulfat fast aufgebraucht war. Anhand der Ergebnisse konnte errechnet werden, dass Oberflächen-Methanogenese die anaerobe Methanoxidation an der Oberfläche mit bis zu 13% antreiben kann. Außerdem konnte festgestellt werden, dass nicht nur die Oberflächen-Methanogenese saisonale Schwankungen aufwies, sondern auch die benthischen Methan-Emissionen (erkennbar anhand der gelösten Methankonzentrationen im Tiefenwasser). 3) Denitrifikationsraten waren signifikant höher in Öl-beeinflussten (ölig=eine sedimentierte Ölschicht an der Oberfläche) Sedimenten im Vergleich zu Kontroll-Sedimenten ohne Öl-Einfluss. Dieses Ergebnis deutet darauf hin, dass Denitrifikation ein Haupt-Abbau-Prozess in öligen Sedimenten ist. Sulfatreduktionsraten waren dagegen nicht erhöht in den öligen Sedimenten, was dadurch zu erklären ist, dass die sedimentierte Ölschicht wahrscheinlich nur für spezielle mikrobielle Gruppen verfügbar war. Eine Sulfid-induzierte N2O Produktion, resultierend aus erhöhter Sulfatreduktion, konnte demnach nicht beobachtet werden. Allerdings zeigten alle untersuchten Sedimente erhöhte Sediment-N2O-Konzentrationen gegenüber dem Tiefenwasser, was sie als potentielle Quelle für N2O identifiziert. Zusammenfassend hat die erfolgreiche Detektion von Oberflächen-Methanogenese in zwei, organik-reichen Küstensystemen gezeigt, dass dessen Rolle im benthischen Methan-Zyklus (z.B. als Methan-Lieferant für anaerobe Methanoxidation) sowie dessen Beitrag zu benthischen Methan-Emission in die Wassersäule in der Vergangenheit unterschätzt wurde. Zusätzlich konnte gezeigt werden, dass nicht nur räumliche Schwankungen, sondern auch saisonale Schwankungen eine wichtige Rolle bei der benthischen Methan-Produktion und –Emission spielen, was mit in die Schätzungen von Ozeanemissionen einfließen sollte. Die Methanproduktion in Oberflächen-Sedimenten war hauptsächlich abhängig vom Eintrag des organischen Materials, von der Temperatur und von dem Sauerstoffgehalt. Dies macht deutlich, dass die vorausgesagten Änderungen durch den Klimawandel (wie z.B. der Temperaturanstieg) auch Auswirkungen auf Oberflächen-Methanogenese haben können. Die Resultate der benthischen N2O-Produktion weisen darauf hin, dass weitere Studien benötigt werden, die sich mit den Umwelt-Kontrollen von Denitrifikation in organik-reichen Sedimenten als eine Quelle für gelöstes N2O in der Wassersäule beschäftigen. Hierbei sollte der Fokus auf Küstengebieten und Kalten Quellen liegen, da beide Systeme hohe Sulfidkonzentrationen im Sediment aufweisen und somit auch eine Sulfid-induzierte N2O-Produkltion in diesen Sedimenten wahrscheinlich ist.

Gas hydrates are fascinating ice-like compounds made of water cages that retain various types of guest molecules. Natural gas hydrates on Earth form below the seafloor and permafrost and contain mainly methane (CH4). Methane from hydrate deposits could be considered as an energy resource. One possible production scenario of CH4 from hydrates is the injection of carbon dioxide (CO2) or carbon dioxide-nitrogen(CO2-N2) mixed gas into the reservoir. Depending on the thermodynamic constraints, the composition of the gas hydrate guest molecules changes: the energy source CH4 is released and the greenhouse gas CO2 is trapped. The aim of the present work is to study the mixed gas hydrates that form in gas hydrate reservoirs after injection of CO2 or CO2-N2 gas mixtures, using laboratory experiments and modeling.