Calcium carbonate hexahydrate (ikaite) is a rare mineral that forms as metastable species in the organic-carbon-rich sediments of the King George Basin, Bransfield Strait, Antarctica, as a consequence of early diagenetic decomposition of organic matter under cold water (−1.4 °C) and high pressure (200 bar) conditions. Large crystals grow in the sediment immediately below the diagenetic transition between microbial sulfate reduction and methanogenesis at ~320 cm below sea floor (bsf). This process is reflected in the dissolved sulfate, total carbon dioxide, and methane concentrations, as well as in the carbon, hydrogen, and oxygen isotope chemistries of the interstitial fluids and dissolved gases of the host sediment. The ikaite crystal faithfully records in its zonal structure the changing carbon isotope ratio of the total dissolved carbon dioxide pool as it gradually diminishes during methanogenesis (δ13Cikaite = −17.5 to −21.4‰). These changes in the crystal’s host environment follow general Rayleigh carbon isotope fractionation. The oxygen isotopes of the ikaite carbonate (δ18Oikaite = 1.46 to 4.45‰) also show a strong zonal distribution, unrelated to temperature of formation, but perhaps controlled by the degree of recrystallization of ikaite to calcite. The crystal water of the ikaite is depleted 11‰ in 2H/1H (VSMOW) relative to the coexisting interstitial water, which is in excellent agreement with the isotope fractionation of other hydrated minerals. In addition to the in situ temperature and pressure, nucleation of the ikaite crystals in the Bransfield Basin sediments may be induced by the high alkalinity, high phosphate concentrations, and dissolved organic compounds. Intense microbial metabolism generates such compounds; of these, aspartic acid and glutamic acid may play an important role, as they do in biological and extracellular carbonate mineral precipitation. All indications are that low temperatures (such as of polar environments), high calcium carbonate supersaturation caused by interstitial methanogenesis, and a sufficiently large supply of dissolved phosphate and amino acids favor metastable ikaite formation. These conditions, modified by recrystallization, may be preserved in calcite glendonites, thinolites, and other calcitic pseudomorphs derived from ikaite and found throughout the ancient sedimentary record.

High alkalinity values observed in coastal seas promote the uptake of CO2 from the atmosphere. However, the alkalinity budget of coastal areas and marginal seas is poorly understood, even though some of the recently observed alkalinity enhancement can be ascribed to riverine fluxes and anaerobic processes in shelf sediments. Here, we investigate the alkalinity budget of the Baltic Sea to identify previously unrecognized alkalinity sources. We quantify the generation of alkalinity and dissolved calcium (Ca) in this marginal sea applying simple mass balance calculations. Using this approach, we identify alkalinity and Ca sources of approximately 324 Gmol yr-1 and 122 Gmol yr-1, respectively, that cannot be ascribed to the riverine input. The magnitude of the Ca source suggests that a major fraction of the excess alkalinity (244 Gmol yr-1) is induced by the dissolution of calcium carbonate (CaCO3). A review of available field data shows that carbonate-bearing rocks at the coast and the seabed of the Baltic Sea are rapidly eroded and may provide sufficient CaCO3 to close the Ca budget. Hence, dissolution of eroded CaCO3 is the most likely source for the Ca enrichment observed in Baltic Sea water. This hypothesis is supported by mass accumulation rates of sediments derived from radioisotope data that are evaluated to derive a basin-wide rate of mud to muddy sand accumulation at the bottom of the Baltic Sea. The resulting value (139 Tg yr-1) exceeds current estimates of riverine particle fluxes into the Baltic Sea by more than one order of magnitude and confirms that rates of till erosion are sufficiently high to account for the Ca and most of the alkalinity excess in Baltic Sea water. Finally, we show that deliberate addition of CaCO3 to sediments deposited in the Baltic Sea could neutralize significant amounts of CO2 and help to achieve net-zero greenhouse gas emissions in the Baltic region.

Pristinely preserved mineral pseudomorphs called glendonites, up to 1.6 m long, from the Palaeogene strata of Denmark allow detailed crystallographic characterisation and add to the understanding of the transformation of the precursor mineral, ikaite (CaCO3·6H2O), to calcite, which constitutes the glendonite. We describe Danish pseudomorphs after ikaite from two localities and formations: the Early Eocene Fur Formation and the Late Oligocene Brejning Formation. This detailed study highlights that key aspects such as morphology and mode of occurrence of these ancient glendonites are identical to those of their parent mineral ikaite, when it grows in marine sediments. Systematic distortion of the angles in glendonite and marine sedimentary ikaite relative to the ideal ikaite symmetry may arise due to the incorporation of organic matter into the crystal structure, and we demonstrate the similarity between modern and ancient ikaite formation zones in the marine sedimentary realm with respect to organic matter.

Glendonites are pseudomorphs of syndepositional or early authigenic ikaite (CaCO3 center dot 6H(2)O) that often forms at near-freezing temperatures. Silicified glendonite has been reported from inner-shelf deposits of the lower Ediacaran Doushantuo Formation at a single stratigraphic section in South China, where they are stratigraphically associated with the positive delta C-13 excursion EP1 but predate the negative delta C-13 excursion EN3 (=Shuram excursion), indicating a period of cool climate somewhere between similar to 609 Ma and similar to 551 Ma. This interpretation predicts a wider geographic distribution of Ediacaran glendonites in equivalent strata in the Yangtze Block of South China. To test this prediction, we conducted a regional survey of Doushantuo Formation and found that, although glendonites are not universally present, they occur in expected stratigraphic intervals and in association with EP1 at two new sections representing inner-shelf and intrashelf basin facies. The wide but not ubiquitous distribution of Doushantuo glendonites indicates that ikaite precipitation and glendonite formation was controlled by both regional climatic and local geochemical factors. Glendonites at the new localities are stellate clusters pseudomorphed by calcite spar and sometimes are rimmed with silica. The calcite spar is characterized by highly variable and mostly negative delta C-13 values as low as -37 parts per thousand, indicating that diagenetic transformation of precursor ikaite to calcitic glendonite may be related to anaerobic oxidation of organic matter or methane in sediment. The new data suggest an early Ediacaran cold period prior to EN3 or the Shuram excursion and facilitation of glendonite/ikaite formation by both climatic and local geochemical conditions such as redox conditions and phosphate concentrations.

The Ediacaran Period is punctuated by the ca. 580 Ma Gaskiers glaciation in Newfoundland. However, paleoclimatic data are scarce in Ediacaran successions in South China, where abundant geochemical and paleobiological data are shaping current understanding of Ediacaran evolutionary and environmental history. Here, we report the occurrence of silicified glendonites in the Ediacaran Doushantuo Formation deposited in an inner-shelf environment on the South China block. Petrographic evidence suggests that these silicified glendonites are pseudomorphs after syndepositional or early authigenic ikaites formed at near-freezing temperatures. The glendonite- bearing stratigraphic interval is characterized by positive delta C-13 values. It predates both the negative delta C-13 excursion EN3 (widely believed to be an equivalent of the Shuram negative excursion) and excursion EN2. Although alternative interpretations may be possible, these glendonites may be related to and correlated with the Gaskiers glaciation. If confirmed, this correlation suggests that the Shuram event postdates the Gaskiers glaciation, thus having important implications for Ediacaran climate changes, carbon cycles, and biological evolution.

Hydrate Ridge has the distinction of hosting the first documented subduction-driven cold seep system that supports chemosynthetic life by Anaerobic Oxidation of Methane as well as the most widely researched methane hydrate setting at any active continental margin. Today this site is a vital node of Northeast Pacific regional long-term studies that constitute the most advanced cabled ocean network, the NSF’s Ocean Observatory Initiative. The illustrated time-line available as a poster at the 8 th International Conference of Gas Hydrate in Beijing documents highlights of field studies, persons involved, themes addressed and key results published from the beginning to the present. It chronicles submersible and ROV-deployments, deep drilling operations and surface ship expeditions and reviews selected results that for the first time addressed fundamental objectives of convergent margin dewatering and gas hydrate research that still persist today.

Authigenic carbonates and seep biota are archives of seepage history and record paleo-environmental conditions at seep sites. We obtained the timing of past methane release events at the northeastern slope of the South China Sea based on U/Th dating of seep carbonates and seep bivalve fragments from three sites located at 22°02′–22°09′N, 118°43′–118°52′E (water depths from 473 to 785 m). Also, we were able to reconstruct the paleo-bottom water temperatures by calculating the equilibrium temperature using the ages, the corresponding past δ18O of seawater (δ18Osw) and the δ18O of the selected samples formed in contact with bottom seawater with negligible deep fluid influence. A criterion consists of mineralogy, redox-sensitive trace elements and U/Th-isotope systematics is proposed to identify whether the samples were formed from pore water or have been influenced by deep fluid. Our results show that all methane release events occurred between 11.5 ± 0.2 and 144.5 ± 12.7 ka, when sea level was about 62–104 m lower than today. Enhanced methane release during low sea-level stands seems to be modulated by reduced hydrostatic pressure, increased incision of canyons and increased sediment loads. The calculated past bottom water temperature at one site (Site 3; water depth: 767–771 m) during low sea-level stands 11.5 and 65 ka ago ranges from 3.3 to 4.0 °C, i.e., 1.3 to 2.2 °C colder than at present. The reliability of δ18O of seep carbonates and bivalve shells as a proxy for bottom water temperatures is critically assessed in light of 18O-enriched fluids that might be emitted from gas hydrate and/or clay dehydration. Our approach provides for the first time an independent estimate of past bottom water temperatures of the upper continental slope of the South China Sea.

Characteristics of cold seeps at different geologic settings are the subject of this review primarily based on results of the Research Consortium SFB 574. Criteria are drawn from examples on the erosive convergent margin off Costa Rica, the accretionary margin off Chile supplemented by examples from the transform margin of the Golf of Cadiz and the convergent Hikurangi margin off New Zealand. Others are from well-studied passive margins of the Black Sea, the Golf of Mexico, the eastern Mediterranean Sea and the South China Sea. Seeps at all settings transport water and dissolved compounds to the ocean through the seafloor by different forcing mechanism and from different depths of the submerged geosphere (10s of meters to 10s of km). The compounds sustain oasis-type ecosystems by providing bioactive reductants sulfide, methane and hydrogen. Hereby, the interaction between fluid composition, flux rates and biota results in a diagnostic hydrocarbon–metazoan–microbe–carbonate association; currently, well over 100 active sites are known. The single most important reaction is microbially mediated anaerobic oxidation of methane with secondary reactions involving S-biogeochemistry and carbonate mineral precipitation. Seep fluids and their seafloor manifestations provide clues as to source depth, fluid–sediment/rock interaction during ascent, lifetime and cyclicity of seepage events but less so on the magnitude of return flow. At erosive margins, Cl-depleted and B-enriched fluids from clay dehydration provide criteria for source depth and temperature. The upward material flow generates mud volcanoes at the seafloor above the projected location of dehydration at depth. At accretionary margins, fluids are derived from more shallow depths by compaction of sediments as they ride on the incoming oceanic plate; they are emitted through thrust faults. At highly sedimented margins, organic-rich and evaporite-containing strata (when present) determine the final fluid composition, by emitting characteristically gas hydrate-derived methane, brine-associated non-methane hydrocarbons or leached elements and their isotopes (Li, δ7Li, B, Ba) from host sediments. Smectite–illite transformation and associated Cl-depletion from release of interlayer water is a pervasive process at these margins. Rare earth element pattern in conjunction with redox-sensitive metals retained in seep carbonates indicate whether or not they precipitated in contact with oxic bottom water or suboxic fluids; clear environmental characterization, though, currently remains inconclusive. More deeply sourced fluids as in transform margins may be characterized by their 87Sr/86Sr ratios from interaction with oceanic crustal rocks below. Quantification of flow and reliable estimates of total volatile output from fore-arcs remain a challenge to seep research, as does understanding the role of geologically derived methane in the global methane cycle.

combine field work on land with marine cruises probing the seafloor. During the course of the SFB 574, many different aspects of subduction zone processes were investigated. Geophysical investigations identified and quantified the input of water through hydration of the bend-faulted subducting plate as far as mantle depths. This process and water release from the subducting slab deeper in the subduction zone was investigated by numerical modeling. These studies highlighted the important role of hydration and dehydration of mantle rocks for the global water cycle. Subducted oceanic fracture zones are another conduit for water transported deep into the subduction zone. Compactional and thermal dehydration of the subducted sediment layer affects the strength of interplate coupling and the depth and lateral extension of the seismogenic zone where disastrous earthquakes are generated. The fluids, largely generated by clay mineral transformation, are expelled through the forearc by splay faults. At the seafloor, the sites of cold seeps, often associated with mud volcanoes, are populated by biota that control the carbon transfer to the ocean. The most prominent manifestations of cold seeps are authigenic carbonates that form from anaerobic oxidation of methane and which serve as archives of forearc processes. The largest cold seep emissions occur at faults generated by the subduction of volcanic seamounts. Petrological analyses of ancient, exhumed subduction zone metamorphic rocks revealed the important role of pervasive, typically channelized fluids in high-pressure metamorphic reactions of dewatering subducted igneous crust and mantle. Slab-derived fluids hydrate the mantle wedge along the slab–wedge interface and form a subduction channel in which mixing of different fluids and fluid–rock interaction causes metasomatic overprinting. The element redistributions associated with the liberation of fluids from Sonderforschungsbereiche (SFBs) are a successful funding model in use by the German Science Foundation (DFG) for over 30 years to strengthen basic research first locally at universities and later also supra-regionally by including academic institutions at different cities and states. Literally translated, SFB means “special research area” that comprises research that complements but does not duplicate research at participating institutions and departments. The English terminology used by the DFG is “Collaborative Research Centre,” which better describes the expected approach by emphasizing collaboration and interdisciplinary efforts in such a way that the overall result is better than the sum of individual results. The SFB 574 had united more than 70 scientists with expertise in structural geology, geophysics, sedimentology, geochemistry, empirical and experimental petrology, volcanology, and biology for 11 years (2001–2012). The overarching theme addressed the role of volatiles in subduction zone tectonic, hydrological, metamorphic and magmatic processes, and resulting hazards. The main areas of research were the subduction zones of Central America and southern Chile. Both extend across shorelines from deepsea trenches to arc-volcano summits and thus required to

This chapter contains sections titled: Introduction Geological Setting and Geochemical Environment of Carbonate Formation Carbonate Complexes and Sampling Sites Analytical Methods Results and Discussion Conclusions

The microbial community inhabiting anaerobic sediments of Hydrate Ridge, an area characterized by extensive methane-venting and gas hydrate occurrence on the Cascadia accretionary prism, is dominated by a complex consortium of archaea and bacteria which performs anaerobic methane oxidation (AMO). The process is indicated by the dominance of archaeal-derived biomarkers such as crocetane, pentamethylicosane, and archaeol in anaerobic sediments and authigenic carbonates. The biomarkers are highly depleted in 1 3C relative to their methane carbon source, which results in carbon isotopic compositions as low as -128%o PDB. The essentially limitless reservoir of methane available to the archaea in this gas hydrate environment allows the kinetic carbon isotope effect (ϵCH4-biomarker) associated with AMO to be fully developed (ϵmax = 72%o). No substrate depletion effects were detected. Varying isotope effects and concentrations of the biomarkers on and off Hydrate Ridge indicate that the abundance of these compounds is essentially dependent on the supply of methane from this special environmental setting.

This chapter contains sections titled: Introduction Methane Hydrate Manifestations Chloride-Anomaly Pattern Floating Hydrates: Evidence from Hydrate Fabric and Low Bulk Densities Hydrate Water Composition Conclusions

Focused gas and fluid flow of cold seep ecosystems is often characterized by carbonate precipitation processes fueled by hydrocarbon-rich fluids and microbial activity. In various geological settings the seabed leakage is methane dominated and accompanied by the formation of long-lasting hard substrates and open channels. These fluid pathways are connecting deeper levels of the sediment column with the bottom water, bridging the diffusive processes at the sediment/water-interface. Understanding and quantifying feedback mechanisms between hydrocarbon-sources, ocean chemistry, and climate requires detailed data about the dynamics of seafloor methane emanation throughout geological time. Authigenic carbonates from these ecosystems represent in many cases unique archives of marine methane emanation by their geobiological, geochemical, mineralogical, and structural inventory. Precise and high resolution geochronology of these archives provides new insights into the rate and duration of precipitation processes, the related microbial activity and a base for the reconstruction of paleoactivity of natural seepage. The actual data set of our compilatory study is spanning a wide range in space and time. It covers different geological circum-Pacific settings (South China Sea, Costa Rica & Nicaragua, Chile, New Zealand), including more than 200 thousand years old archives (Hydrate Ridge, off Oregon) and recent methane-related carbonates from Black Sea and Mediterranean Sea. Special emphasis is actually given to new insights into growth structures, emplacement processes, mineralogy and high resolution geochemistry of mud mound and escarpment related carbonates from the Central American Forearc as well as to new findings on seep systems off Chile (cruise Sonne 210) applying a ROV-operated diamond chain saw sampling system. In a rather complex case study carbonate drill cores decipher the late stage evolution of mound growth and related methane enriched fluid emanation during the last 70 000 years off Costa Rica. A broad range in &13C from -22 to -36‰ (mounds) and -43 to -56‰ (escarpment) is covered, reflecting different hydrocarbon sources and/or varying fluid/seawater-ratios. Whereas the &18O signatures indicate a systematic variation between 3.8 - 5.3‰ (mounds) and 4.2 - 5.1‰ (escarpment) in close correlation with their ages (U-Th geochronology [1]) and the record of seawater evolution. Combining high resolution observations of growth structures (fluorescence microscopy) and analyses of Cl-S-C distribution pattern (electron microprobe) decipher multiple phases of carbonate precipitation separated by micrometer scaled layers of residual organic matter (e.g. 50 alternations on 3 mm). The latter are interpreted to be attached onto crystal surfaces during phases of rather stagnant or low fluid flow, respectively. On long time scales, the circum-Pacific data set indicates sea level decrease as an important enhancement factor for focused fluid flow via increasing pore water buoyancy, destabilization of gas hydrates and related fluxes from underlying free gas deposits upon hydraulic pressure release. Data from tectonically highly active settings imply structural changes as major control on initiation and position of cold seeps and their activation on short time scales [2, 3]. References: [1] Hammerich et al. (2007) Terra Nostra. [2] Kutterolf et al. (2008) Geology, doi: 10.1130/G24806A [3] Liebetrau et al. (2010) MG, doi:10.1016/j.margeo.2010.01.003.