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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 δO of seawater (δO) and the δO 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 δO of seep carbonates and bivalve shells as a proxy for bottom water temperatures is critically assessed in light of O-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. [ABSTRACT FROM AUTHOR]

The surface sediments of two mud mounds ('Mound 11' and 'Mound 12') offshore southwest Costa Rica contain abundant authigenic carbonate concretions dominated by high-Mg calcite (14-20 mol-% MgCO). Pore fluid geochemical profiles (sulfate, sulfide, methane, alkalinity, Ca and Mg) indicate recent carbonate precipitation within the zone of anaerobic oxidation of methane (AOM) at variable depths. The current location of the authigenic carbonate concretions is, however, not related to the present location of the AOM zone, suggesting mineral precipitation under past geochemical conditions as well as changes in the flow rates of upward migrating fluids. Stable oxygen and carbon isotope analysis of authigenic carbonate concretions yielded δO values ranging between 34.0 and 37.7 ‰ Vienna standard mean ocean water (VSMOW) and δC values from −52.2 to −14.2 ‰ Vienna Pee Dee belemnite (VPDB). Assuming that no temperature changes occurred during mineral formation, the authigenic carbonate concretions have been formed at in situ temperature of 4-5 °C. The δO values suggest mineral formation from seawater-derived pore fluid (δO = 0 ‰ VSMOW) for Mound 12 carbonate concretions but also the presence of an emanating diagenetic fluid (δO ≈5 ‰) in Mound 11. A positive correlation between δC and δO is observed, indicating the admixing of two different sources of dissolved carbon and oxygen in the sediments of the two mounds. The carbon of these sources are (1) marine bicarbonate (δC ≈0 ‰) and (2) bicarbonate which formed during the AOM (δC ≈−70 ‰). Furthermore, the δO composition, with values up to +4.7 ‰ Vienna standard mean ocean water (VSMOW), is interpreted to be affected by the presence of emanating, freshened and boron-enriched fluids. Earlier, it has been shown that the origin of O-enriched fluids are deep diagenetic processes as it was indicated by the presence of methane with thermogenic signature (δC = −38 ‰). A combination of present geochemical data with geophysical observations indicates that Mounds 11 and 12 represent a single fluid system interconnected by deep-seated fault(s). [ABSTRACT FROM AUTHOR]

Abstract: The aim of this study is to provide combined new insights into the geochronological framework, isotope geochemical signatures and structural observations of methane related authigenic carbonate settings and associated cold-water corals from offshore New Zealand. The analysed samples are obtained from calcified sediments of three different cold seep areas at the Hikurangi Margin: Opouawe Bank, Uruti and Omakere Ridge. We focused the sub-sampling on aragonitic precipitates in vein like structures, partly still open fluid channel systems and related chemoherm structures in order to identify the timing and signature of focused marine methane emanation. The presented initial U/Th age data set indicates different generations of intensified seep activity and related carbonate precipitation between 12,400±160 and 2090±850years BP. The youngest stage so far, was identified as contemporaneous cold seep activity at the southernmost (North Tower, Opouawe Bank) and northernmost (Bear''s Paw, Omakere Ridge) sampling sites around 2300years BP. Sharing the same water depth (1050 to 1100m) these sites imply regional margin-wide tectonic or hydrological changes as controlling process. An intermediate phase of vein and channel structures within the sediment was detected for a time interval between approximately 5000 and 4000years BP with contemporaneous settings of focused seep activity around 4300years BP at Uruti Ridge (LM-10) and Opouawe Bank. δ 13CPDB data reflect site and carbonate type specific signatures, clustering around −52‰ (Uruti and Omakere Ridge) and −47‰ for the fluid pathway system and the uppermost surface at North Tower site (Opouawe Bank). Late stage precipitates in chemoherm cavities of the latter reflect significantly heavier values of about −38‰. Porous precipitates within open fluid channel systems are characterized by decreased δ 234U( T ) values, exceptional high Th and U concentrations and slightly lighter δ 13CPDB signatures when compared to adjacent rim-like and dense cements. This specific kind of precipitate is interpreted as indicator for phases of less vigorous fluid seepage. The observed occurrence of cold-water corals seems to be mostly depending on the abundance of authigenic carbonates as a substrate exposed to erosive bottom water currents. But, seafloor observations combined with preliminary age data indicate a significant time gap between the inferred end of cold seep activity and coral colonization. U–Th analyses of recent reef-forming coral provided an initial δ 234U(0) value of 146.3±3.9‰ and 0.0013±0.0002 as starting 230Th/234U activity ratio for coral growth in the bottom water. [Copyright &y& Elsevier]

Abstract: Four seep sites located within an ∼20km2 area offshore Georgia (Batumi seep area, Pechori Mound, Iberia Mound, and Colkheti Seep) show characteristic differences with respect to element concentrations, and oxygen, hydrogen, strontium, and chlorine isotope signatures in pore waters, as well as impregnation of sediments with petroleum and hydrocarbon potential. All seep sites have active gas seepage, near surface authigenic carbonates and gas hydrates. Cokheti Seep, Iberia Mound, and Pechori Mound are characterized by oil-stained sediments and gas seepage decoupled from deep fluid advection and bottom water intrusion induced by gas bubble release. Pechori Mound is further characterized by deep fluid advection of lower salinity pore fluids. The Pechori Mound pore fluids are altered by mineral/water reactions at elevated temperatures (between 60 and 110°C) indicated by heavier oxygen and lighter chlorine isotope values, distinct Li and B enrichment, and K depletion. Strontium isotope ratios indicate that fluids originate from late Oligocene strata. This finding is supported by the occurrence of hydrocarbon impregnations within the sediments. Furthermore, light hydrocarbons and high molecular weight impregnates indicate a predominant thermogenic origin for the gas and oil at Pechori Mound, Iberia Mound, and Colkheti Seep. C15+ hydrocarbons at the oil seeps are allochtonous, whereas those at the Batumi seep area are autochthonous. The presence of oleanane, an angiosperm biomarker, suggests that the hydrocarbon source rocks belong to the Maikopian Formation. In summary, all investigated seep sites show a high hydrocarbon potential and hydrocarbons of Iberia Mound, Colkheti Seep, and Pechori Mound are predominantly of thermogenic origin. However, only at the latter seep site advection of deep pore fluids is indicated. [Copyright &y& Elsevier]