Clathrate Destabilization Research Articles

Questioning the role of methane in the wake of a snowball Earth: Insights from isotopically anomalous cap dolostone cements with a complex diagenetic history

Methane has been widely regarded as an important source of greenhouse gas that modulated Earth’s paleoclimate. Notably, it has been proposed that a massive release of methane via clathrate destabilization may have played a critical role during and/or in the immediate aftermath of the Marinoan deglaciation. One key piece of supporting evidence for this hypothesis comes from the isotopically anomalous (δ13Ccarb < –30 ‰) methane-derived authigenic calcite (MDAC) cements within post-Marinoan cap dolostones in South China. However, the origin of these MDAC cements remains hotly contested, including a clumped isotope study that reinterpreted them as resulting from late hydrothermal fluids. To further evaluate these critical yet controversial cements, a detailed investigation by secondary ion mass spectrometry (SIMS) was conducted. Here we report the largest range of δ13Ccarb values (–53.1 ‰ to +6.3 ‰) yet measured in Precambrian carbonates. Our study shows that the MDAC cements are post-depositional, void-filling, and rich in Mn. Importantly, integrated petrographic and SIMS results consistently show that MDAC cements post-date disrupted dolomite laminae that bear surprisingly positive δ13Ccarbvalues up to +6.3 ‰. This is the first report that authigenic carbonates with positive δ13Ccarb signals are found within post-Marinoan cap dolostone. Although the precise age of these authigenic carbonates remains ambiguous, integrated chemostratigraphic and SEM-SIMS results suggest that the dolomite laminae formed after the deposition of the uppermost cap dolostone interval, when the seawater δ13C had already evolved to positive values; and the MDAC formed even later. The dolomite laminae and MDAC cements, therefore, represent distinct post-depositional, exogenous, diagenetic carbon signals that are irrelevant to the deglaciation. Our new results challenge the hypothesis that methane played a central role at the end of or in the immediate aftermath of the Marinoan glaciation. Rather, the infiltration of methane-bearing fluids within cap dolostones could have occurred much later.

Evolution of the reservoirs of volatiles in the protosolar nebula

The supersolar abundances of volatiles observed in giant planets suggest that a compositional gradient was present at the time of their formation in the protosolar nebula. To explain this gradient, several studies have investigated the radial transport of trace species and the effect of icelines on the abundance profiles of solids and vapors formed in the disk. However, these models only consider the presence of solids in the forms of pure condensates or amorphous ice during the evolution of the protosolar nebula. They usually neglect the possible crystallization and destabilization of clathrates, along with the resulting interplay between the abundance of water and those of these crystalline forms. This study is aimed at pushing this kind of investigation further by considering all possible solid phases together in the protosolar nebula: pure condensates, amorphous ice, and clathrates. To this end, we used a one-dimensional (1D) protoplanetary disk model coupled with modules describing the evolution of trace species in the vapor phase, as well as the dynamics of dust and pebbles. Eleven key species are considered here, including H2O, CO, CO2, CH4, H2S, N2, NH3, Ar, Kr, Xe, and PH3. Two sets of initial conditions are explored for the protosolar nebula. In a first scenario, the disk is initially filled with icy grains in the forms of pure condensates. In this case, we show that clathrates can crystallize and form enrichment peaks up to about ten times the initial abundances at their crystallization lines. In a second scenario, the volatiles were delivered to the protosolar nebula in the forms of amorphous grains. In this case, the presence of clathrates is not possible because there is no available crystalline water ice in their formation region. Enrichment peaks of pure condensates also form beyond the snowline up to about seven times the initial abundances. Our model can then be used to compare the compositions of its different volatile reservoirs with those of comet C/2016 R2 PanSTARRS, Jupiter, Uranus, and Neptune. We find that the two investigated scenarios provide compositions of solids and vapors consistent with those observed in the bodies considered.

The vertical transport of methane from different potential emission types on Mars

AbstractThe contrasting evolutionary behavior of the vertical profile of methane from three potential release scenarios is analyzed using a global circulation model with assimilated temperature profiles. Understanding the evolving methane distribution is essential for interpretation of future retrievals of the methane vertical profile taken by instruments on the ExoMars Trace Gas Orbiter spacecraft. We show that at methane release rates constrained by previous observations and modeling studies, discriminating whether the methane source is a sustained or instantaneous surface emission requires at least 10 sols of tracking the emission. A methane source must also be observed within 5 to 10 sols of the initial emission to distinguish whether the emission occurs directly at the surface or within the atmosphere via destabilization of metastable clathrates. Assimilation of thermal data is shown to be critical for the most accurate backtracking of an observed methane plume to its origin.

How Large Igneous Provinces affect global climate, sometimes cause mass extinctions, and represent natural markers in the geological record

Large Igneous Provinces (LIPs) can have a significant global climatic effect as monitored by sedimentary trace and isotopic compositions that record paleo-seawater/atmosphere variations. Improved U-Pb dating (with better than 0.1Myr resolution) for several LIPs is confirming a long-proposed mass extinction-LIP link. The most dramatic climatic effect is global warming due to greenhouse-gases from LIPs. Subsequent cooling (and even global glaciations) can be caused by CO2 drawdown through weathering of LIP-related basalts, and/or by sulphate aerosols. Additional kill mechanisms that can be associated with LIPs include oceanic anoxia, ocean acidification, sea level changes, toxic metal input, essential nutrient decrease, producing a complex web of catastrophic environmental effects. Notably, the size of a LIP is not the only important factor in contributuing to environmental impact. Of particular significance are the rate of effusion, and the abundance of LIP-produced pyroclastic material and volatile fluxes that reach the stratosphere. While flood basalt degassing (CO2, SO2, halogens) is important (and is also from associated silicic volcanism), a significant amount of these gases are released from volatile-rich sedimentary rocks (e.g. evaporites and coal horizons) heated by the intrusive component of LIPs. Feedbacks are important, such as global warming leading to destabilization of clathrates, consequent release of further greenhouse gases, and greater global warming. In the broadest sense LIPs can affect (or even induce) shifts between Icehouse, Greenhouse and Hothouse climatic states. However, the specific effects, their severity, and their time sequencing is specific to each LIP. Based on the robust array of environmental effects due to LIPs, as demonstrated in the Phanerozoic record, it is suggested that LIP events represent useful time markers in the Precambrian Era as proxies for some significant global environmental changes that are preserved in the sedimentary record.

High-resolution carbon isotope records of the Toarcian Oceanic Anoxic Event (Early Jurassic) from North America and implications for the global drivers of the Toarcian carbon cycle

The Mesozoic Era experienced several instances of abrupt environmental change that are associated with instabilities in the climate, reorganizations of the global carbon cycle, and elevated extinction rates. Often during these perturbations, oxygen-deficient conditions developed in the oceans resulting in the widespread deposition of organic-rich sediments — these events are referred to as Oceanic Anoxic Events or OAEs. Such events have been linked to massive injections of greenhouse gases into the ocean–atmosphere system by transient episodes of voluminous volcanism and the destabilization of methane clathrates within marine environments. Nevertheless, uncertainty surrounds the specific environmental drivers and feedbacks that occurred during the OAEs that caused perturbations in the carbon cycle; this is particularly true of the Early Jurassic Toarcian OAE (∼183.1 Ma). Here, we present biostratigraphically constrained carbon isotope data from western North America (Alberta and British Columbia, Canada) to better assess the global extent of the carbon cycle perturbations. We identify the large negative carbon isotope excursion associated with the OAE along with high-frequency oscillations and steps within the onset of this excursion. We propose that these high-frequency carbon isotope excursions reflect changes to the global carbon cycle and also that they are related to the production and release of greenhouse gases from terrestrial environments on astronomical timescales. Furthermore, increased terrestrial methanogenesis should be considered an important climatic feedback during Ocean Anoxic Events and other similar events in Earth history after the proliferation of land plants.

PITS FORMATION FROM VOLATILE OUTGASSING ON 67P/CHURYUMOV–GERASIMENKO

We investigate the thermal evolution of comet 67P/Churyumov-Gerasimenko's subsurface in the Seth_01 region, where active pits have been observed by the ESA/Rosetta mission. Our simulations show that clathrate destabilization and amorphous ice crystallization can occur at depths corresponding to those of the observed pits in a timescale shorter than 67P/Churyumov-Gerasimenko's lifetime in the comet's activity zone in the inner solar system. Sublimation of crystalline ice down to such depths is possible only in the absence of a dust mantle, which requires the presence of dust grains in the matrix small enough to be dragged out by gas from the pores. Our results are consistent with both pits formation via sinkholes or subsequent to outbursts, the dominant process depending on the status of the subsurface porosity. A sealed dust mantle would favor episodic and disruptive outgassing as a result of increasing gas pressure in the pores, while high porosity should allow the formation of large voids in the subsurface due to the continuous escape of volatiles. We finally conclude that the subsurface of 67P/Churyumov-Gerasimenko is not uniform at a spatial scale of similar to 100-200 m.

Constraining the cause of the end-Guadalupian extinction with coupled records of carbon and calcium isotopes

A negative δC13 excursion in carbonate sediments near the Guadalupian/Lopingian (Middle/Late Permian) boundary has been interpreted to have resulted from a large carbon cycle disturbance during the end-Guadalupian extinction event (ca. 260 Ma). However, the carbon isotope data alone are insufficient to uniquely determine the type and magnitude of perturbation to the global carbon cycle. Calcium isotopes can be used to further constrain the cause of a carbon isotope excursion because the carbon and calcium cycles are coupled via CaCO3 burial. In this study, we present coupled carbon and calcium isotope records from three Guadalupian–Lopingian (G/L) sections in China (Penglaitan and Chaotian) and Turkey (Köserelik Tepe). The δC13 and δCa44/40 records differ among our studied sections and do not co-vary in the same manner. No section shows δC13 and δCa44/40 changes consistent with massive, rapid volcanic CO2 emissions or methane clathrate destabilization. Additionally, many sections with large (>3‰) changes in δC13 exhibit δO18 evidence for diagenetic alteration. Only one section exhibits a large excursion in the δCa44/40 of limestone but the absence of a similar excursion in the δCa44/40 of conodont apatite suggests that the limestone excursion reflects a mineralogical control rather than a perturbation to the global calcium cycle. Hence, we interpret the large isotopic changes observed in some sections to have resulted from local burial conditions or diagenetic effects, rather than from a large carbon and calcium cycle disturbance. Perturbations to the global carbon and calcium cycles across the G/L transition were much less intense than the disturbances that occurred across the subsequent Permian–Triassic boundary. This finding is consistent with the much smaller magnitude of the end-Guadalupian extinction relative to the end-Permian.

Signs of instability

The finding that pools of gas hydrates — compounds that trap natural gas emissions — in ocean sediments are deeper than expected implies that the hydrates are destabilizing, and might release gigatonnes of methane. See Letter p.527 Clathrates store enormous amounts of methane, much of it in shallow ocean shelf environments. It has been proposed that sudden methane release from clathrates could cause abrupt climate changes, and this has been invoked to explain past warming events such as the Paleocene–Eocene thermal maximum. Clathrate destabilization in the modern climate is possible, especially in cases of pronounced ocean warming. Here, Benjamin Phrampus and Matthew Hornbach use seismic data and modelling to reveal clathrate destabilization along the United States Eastern margin. The destabilization is thought to be linked to warming of the Gulf Stream, or to a slight shift in its position. The climatic impact of the methane released by clathrate destabilization remains uncertain, however, because it is not clear how much of the methane would actually enter the atmosphere.

Large methane releases lead to strong aerosol forcing and reduced cloudiness

Abstract. The release of vast quantities of methane into the atmosphere as a result of clathrate destabilization is a potential mechanism for rapid amplification of global warming. Previous studies have calculated the enhanced warming based mainly on the radiative effect of the methane itself, with smaller contributions from the associated carbon dioxide or ozone increases. Here, we study the effect of strongly elevated methane (CH4) levels on oxidant and aerosol particle concentrations using a combination of chemistry-transport and general circulation models. A 10-fold increase in methane concentrations is predicted to significantly decrease hydroxyl radical (OH) concentrations, while moderately increasing ozone (O3). These changes lead to a 70 % increase in the atmospheric lifetime of methane, and an 18 % decrease in global mean cloud droplet number concentrations (CDNC). The CDNC change causes a radiative forcing that is comparable in magnitude to the longwave radiative forcing ("enhanced greenhouse effect") of the added methane. Together, the indirect CH4-O3 and CH4-OH-aerosol forcings could more than double the warming effect of large methane increases. Our findings may help explain the anomalously large temperature changes associated with historic methane releases.

A hydrothermal origin for isotopically anomalous cap dolostone cements from south China

The release of methane into the atmosphere through destabilization of clathrates is a positive feedback mechanism capable of amplifying global warming trends that may have operated several times in the geological past. Such methane release is a hypothesized cause or amplifier for one of the most drastic global warming events in Earth history, the end of the Marinoan 'snowball Earth' ice age, ∼635 Myr ago. A key piece of evidence supporting this hypothesis is the occurrence of exceptionally depleted carbon isotope signatures (δ(13)C(PDB) down to -48‰; ref. 8) in post-glacial cap dolostones (that is, dolostone overlying glacial deposits) from south China; these signatures have been interpreted as products of methane oxidation at the time of deposition. Here we show, on the basis of carbonate clumped isotope thermometry, (87)Sr/(86)Sr isotope ratios, trace element content and clay mineral evidence, that carbonates bearing the (13)C-depleted signatures crystallized more than 1.6 Myr after deposition of the cap dolostone. Our results indicate that highly (13)C-depleted carbonate cements grew from hydrothermal fluids and suggest that their carbon isotope signatures are a consequence of thermogenic methane oxidation at depth. This finding not only negates carbon isotope evidence for methane release during Marinoan deglaciation in south China, but also eliminates the only known occurrence of a Precambrian sedimentary carbonate with highly (13)C-depleted signatures related to methane oxidation in a seep environment. We propose that the capacity to form highly (13)C-depleted seep carbonates, through biogenic anaeorobic oxidation of methane using sulphate, was limited in the Precambrian period by low sulphate concentrations in sea water. As a consequence, although clathrate destabilization may or may not have had a role in the exit from the 'snowball' state, it would not have left extreme carbon isotope signals in cap dolostones.

Evidence for pervasive mud volcanism in Acidalia Planitia, Mars

We have mapped 18,000+ circular mounds in a portion of southern Acidalia Planitia using their sizes, shapes, and responses in Nighttime IR. We estimate that 40,000+ of these features could occur in the area, with a distribution generally corresponding to the southern half of the proposed Acidalia impact basin. The mounds have average diameters of about 1 km and relief up to 180 m and most overlie units mapped as Early Amazonian. High resolution images of mound surfaces show relatively smooth veneers, apron-like extensions onto the plains, moats, and concentric circular crestal structures. Some images show lobate and flow-like features associated with the mounds. Albedo of the mounds is generally higher than that of the surrounding plains. Visible and near-infrared spectra suggest that the mounds and plains have subtle mineralogical differences, with the mounds having enhanced coatings or possibly greater quantities of crystalline ferric oxides. Multiple analogs for these structures were assessed in light of new orbital data and regional mapping. Mud volcanism is the closest terrestrial analogy, though the process in Acidalia would have had distinctly martian attributes. This interpretation is supported by the geologic setting of the Acidalia which sits at the distal end of the Chryse–Acidalia embayment into which large quantities of sediments were deposited through the Hesperian outflow channels. In its distal position, Acidalia would have been a depocenter for accumulation of mud and fluids from outflow sedimentation. Thus, the profusion of mounds in Acidalia is likely to be a consequence of this basin’s unique geologic setting. Basinwide mud eruption may be attributable to overpressure (developed in response to rapid outflow deposition) perhaps aided by regional triggers for fluid expulsion related to events such as tectonic or hydrothermal pulses, destabilization of clathrates, or sublimation of a frozen body of water. Significant release of gas may have been involved, and the extensive mud volcanism could have created long-lived conduits for upwelling groundwaters, providing potential habitats for an in situ microbiota. Mud volcanism transports minimally-altered materials from depth to the surface, and mud volcanoes in Acidalia, therefore, could provide access to samples from deep zones that would otherwise be inaccessible. Since the distal setting of Acidalia also would favor concentration and preservation of potentially-present organic materials, samples brought to the surface by mud volcanism could include biosignatures of possible past or even present life. Accordingly, the mounds of Acidalia may offer a new class of exploration target.

Estimating the potential for methane clathrate instability in the 1%‐CO2 IPCC AR‐4 simulations

The recent work of Reagan and Moridis (2007) has shown that even a limited warming of 1 K over 100 years can lead to clathrate destabilization, leading to a significant flux of methane into the ocean water, at least for shallow deposits. Here we study the potential for methane clathrate destabilization by identifying the 100‐year temperature increase in the available IPCC (Intergovernmental Panel on Climate Change) AR‐4 1%‐CO2 increase per year (up to doubling over pre‐industrial conditions, which occurs after 70 years) simulations. Depending on assumptions made on the possible locations (in this case, only depth) of methane clathrates and on temperature dependence, our calculation leads to an estimated model‐mean release of methane at the bottom of the ocean of approximately 560‐2140 Tg(CH4)/year; as no actual geographical distribution of methane clathrates is considered here, these flux estimates must be viewed as upper bound estimates. Using an observed 1% ratio to estimate the amount of methane reaching the atmosphere, our analysis leads to a relatively small methane flux of approximately 5–21 Tg(CH4)/year, with an estimated inter‐model standard deviation of approximately 30%. The role of sea‐level rise by 2100 will be to further stabilize methane clathrates, albeit to a small amount as the sea‐level rise is expected to be less than a few meters.

Role of hydrogen sulfide in a Permian‐Triassic boundary ozone collapse

Using a three‐dimensional chemistry‐climate model of the troposphere and stratosphere, we find that hydrogen sulfide alone is unlikely to directly affect stratospheric ozone, even for hydrogen sulfide emission rates as large as 5000 Tg(S) per year. However, we also find that large quantities of hydrogen sulfide create a significant decrease in tropospheric hydroxyl radical, leading to a commensurate increase in atmospheric methane. Therefore a large methane flux (possibly from methane clathrate destabilization, Siberian traps or hydrothermal vent complexes) combined with a large hydrogen sulfide oceanic flux is much more likely to lead to an ozone collapse than methane or hydrogen sulfide alone with implications to the Permian‐Triassic boundary extinction 250 million years ago.

Interglacial clathrate destabilization on Mars: Possible contributing source of its atmospheric methane

The presence of methane has been recently detected in the martian atmosphere, suggesting a contemporary source such as volcanism or microbial activity. Here we show that methane may be released by the destabilization of methane clathrate hydrates, triggered by the interglacial climate change starting 0.4 Ma. Clathrate hydrates are nonstoichiometric crystalline compounds in which a water ice lattice forms cages that contain apolar gas molecules, such as methane [CH4· n H2O] and carbon dioxide [CO2· n H2O]. The loss of shallow ground ice eliminates confining pressure, initiating the destabilization of clathrate hydrates and the release of methane to the atmosphere. This alternative process does not restrict the methane's age to 430 yr (maximum residence time of methane gas in martian atmosphere), because clathrate hydrates can preserve (encage) methane of ancient origin for long time periods.

Methane-induced early diagenesis of foraminiferal tests in the southwestern Greenland Sea

Planktic and epibenthic foraminiferal δ 13C records at Site PS62/015-3 (southwestern Greenland Sea) reveal a series of transient events of extreme 13C depletion down to − 6‰ during the period 90–40 ka. Scanning electron microscope studies of the ultrastructures of foraminiferal tests suggest that 13C depleted specimens are affected by some 10–20% overgrowth by authigenic calcite contributing to the light δ 13C signal. Incremental-leaching experiments and census counts of pristine versus overgrowth-affected specimens show that the 13C depleted foraminiferal tests incorporate a primary δ 13C signature most likely ranging from + 1‰ to − 1.7‰ and a post-depositional δ 13C signature around − 17‰ to − 19.5‰. Extremely low values of productivity and organic carbon in Late Quaternary sediments along the east Greenland margin preclude organic matter as potential source of the isotopically light carbon. In contrast, thermal instability of clathrates and subsequent aerobic oxidation of (highly 12C enriched) methane in pore and ocean water provide a compelling mechanism to account for the negative δ 13C excursions of both primary and post-depositional carbonates. Here, pore water methane may have led to a supersaturation of 13C depleted bicarbonate and precipitation of isotopically light authigenic calcite on and in foraminiferal tests, a feature that may serve as a tracer to former sites of clathrate destabilization.

Methane-driven late Pleistocene δ13C minima and overflow reversals in the southwestern Greenland Sea

A core transect across the southwestern Greenland Sea reveals coeval events of extremely negative planktic and benthic δ 1 3 C excursions between 40 and 87 ka. The most pronounced event, event 1, began at peak Dansgaard-Oeschger stadial 22 (85 ka) with a duration of 18 k.y. During this episode, incursions of Atlantic Intermediate Water caused a bottom-water warming of up to 8 °C. The amplitude, timing, and geographic pattern of the δ 1 3 C events suggest that this bottom-water warming triggered clathrate instability along the East Greenland slope and a methane-induced depletion of δ 1 3 C D I C (DIC-dissolved inorganic carbon). Since δ 1 3 C event 1 matches a major peak in atmospheric CH 4 concentration, this clathrate destabilization may have contributed to the rise in atmospheric CH 4 and thus to climate warming over marine isotope stage 5.1.

Does the oxidation of methane leave an isotopic fingerprint in the geologic record?

Pore water ΣCO2 and δ13C data are presented for the dysoxic Santa Barbara and Santa Monica Basins in an attempt to constrain the influence of methane oxidation on the pore water gradients and benthic foraminiferal δ13C. Although these two basins are adjacent to one another Santa Barbara Basin has steeper pore water ΣCO2 and δ13C gradients. Methane oxidation could be contributing to the higher gradients in this basin. The isotope mass balance for pore water ΣCO2 indicates, however, that the pore water gradients in the upper 2 cm of both basins are controlled only by the rate of photosynthate carbon oxidation and the rate of diffusion. Methane derived CO2 has no discernable influence on pore water or carbonate δ13C values at these sediment depths in either basin. Benthic foraminiferal species inhabiting different depth horizons in the top centimeter incorporate these steep isotopic gradients in their calcite tests and can be used therefore to reconstruct the history of carbon oxidation within the basins. On the basis of these modern geochemical constraints, a more likely explanation for the apparent δ13C excursions recorded in the late Pleistocene sediments from Santa Barbara Basin involves enhanced carbon rain and carbon oxidation in the basin sediments rather than clathrate destabilization.

Effect of water content on the solid-state stability in two isomorphic clathrates of cephalosporin: cefazolin sodium pentahydrate (alpha form) and FK041 hydrate.

This study clearly demonstrates that clathrated water molecules can contribute to both chemical stabilization and destabilization of clathrates. The solid-state stabilities for two isomorphic clathrates of cephalosporin, cefazolin sodium and FK041, were investigated in terms of the effects of water content. The isomorphic ranges of water content were estimated to be 3.5-5 mol/mol for alpha-form cefazolin sodium and 2-4 mol/mol for FK041 hydrate. Upon the isomorphic dehydration, alpha-form cefazolin sodium was destabilized as the water content decreased below 4.25 mol/mol owing to the disruption of hydrogen bonding network in lattice channels. In this case, the hydration of clathrated water up to 4.25 mol/mol contributed to the physical and chemical stability of the crystals. On the contrary, the isomorphic hydration in FK041 hydrate contributed to the chemical destabilization owing to the high water activity. The difference in water activity between alpha-form cefazolin sodium and FK041 hydrate could be attributed to the size of water channels.

Cold-seeps and authigenic carbonate formation in Monterey Bay, California

Authigenic carbonate associated with modern `cold seep' biological communities and their extinct analogues exhibit a broad range in stable isotope and mineral composition within the limited geographic area of Monterey Bay. Although such variations in ancient samples have been used to infer differing tectonic settings, these carbonates all formed within a faulted continental margin environment and chemical variations reflect local differences in the sources and flux of carbon to sediment pore fluids. The slow seepage of fluid, and with it dissolved carbon, along the transform-faulted continental margin results in discrete areas of enhanced microbial sulfate reduction, oxidation of methane from both biogenic and thermogenic origins, as well as the active precipitation of both high-Mg calcite (HMC) and dolomite. The authigenic carbonates include semicontinuous pavements of shallow cemented sediments surrounding benthic communities; circular or pipe-like `chimneys' interpreted as cemented conduits formed as a result of methane gas expulsion; centimeter- to meter-scale rings, doughnuts or slabs winnowed from variable depths within sediments; and carbonate veins (ankerite or calcite) or cements in faulted basement rocks draped with bacterial mat. Abundant pyrite framboids, preferentially filling the tests of the benthic foraminifer Uvigerina peregrina and characteristic of the HMC-bearing samples, are products of a zone of shallow microbial sulfate reduction, a process fundamental to the nourishment of the chemosynthetic cold seep communities. Sites on a sedimented ridge west of the San Gregorio Fault Zone have carbon isotopic values between −35 and −56‰ that are strongly influenced by carbon derived from methane. The higher values of δ 18 O (more enriched in 18 O ) are found closer to the active fault zone. Carbonates from sites within the San Gregorio Fault Zone or from Monterey Canyon floor exposures of Miocene sediments have carbon isotope values between −7 and −26‰ that are mixtures of sedimentary organic carbon and microbially oxidized hydrocarbons from organic-rich, petroleum source rocks. Dolomite-bearing samples are more enriched in both 18 O and 13 C . Bulk carbonate samples with heavier 18 O ( δ 18 O >3.5‰) result from the abundance of authigenic dolomite and low temperatures of formation. Local destabilization of clathrates might also create pore fluids that are more enriched in both 13 C and 18 O .

Low-sea-level emplacement of a very large Late Pleistocene ‘megaturbidite’ in the western Mediterranean Sea

Large-volume turbidites, termed 'megaturbidites' or 'megabeds', result from catastrophic slope failures and the associated downslope transport of enormous quantities of sediment from continental margins to the deep sea. Such large sediment failures can generate tsunamis and, in terrains underlain by gas hydrates (clathrates), may be associated with the release of substantial amounts of the greenhouse gas methane. It has been proposed that the megaturbidite events may be triggered by seismic activity, or may result from gas hydrate release itself, caused by a lowering of hydrostatic pressure on clathrates as a result of low sea level. Previous conclusions on the significance of sea-level change, however, have been conditional because of the lack of absolute times of turbidite emplacement. Here we use accelerator-mass-spectrometry radiocarbon dating in five widely spaced cores to constrain the date of emplacement of a large-volume (500 km3) bed in the Balearic Basin of the western Mediterranean. This turbidite is exceptional in its magnitude and represents the main sedimentation event in the Balearic Basin over the past 100 kyr. Our data provide an estimate of 22,000 calendar years before present for emplacement of the megabed, a time when sea level stood at its lowest level during the Last Glacial Maximum. The coincidence of these dates is consistent with emplacement due to clathrate destabilization caused by low sea level, although other triggering mechanisms, such as seismic shock, cannot be ruled out.