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South China Sea (SCS) is the largest Western Pacific marginal sea. However, microbial studies have never been performed in the cold seep sediments in the SCS. In 2004, "SONNE" 177 cruise found two cold seep areas with different water depth in the northern SCS. Haiyang 4 area, where the water depth is around 3000 m, has already been confirmed for active seeping on the seafloor, such as microbial mats, authigenic carbonate crusts and bivalves. We investigated microbial abundance and diversity in a 5.55-m sediment core collected from this cold seep area. An integrated approach was employed including geochemistry and 16S rRNA gene phylogenetic analyses. Here, we show that microbial abundance and diversity along with geochemistry profiles of the sediment core revealed a coupled reaction between sulphate reduction and methane oxidation. Acridine orange direct count results showed that microbial abundance ranges from 10 5 to 10 6 cells/g sediment (wet weight). The depth-related variation of the abundance showed the same trend as the methane concentration profile. Phylogenetic analysis Production and hosting by Elsevieravailable at www.sciencedirect.com China University of Geosciences (Beijing) GEOSCIENCE FRONTIERS journal homepage: www.elsevier.com/locate/gsf GEOSCIENCE FRONTIERS 3(3) (2012) 301e316indicated the presence of sulphate-reducing bacteria and anaerobic methane-oxidizing archaea. The diversity was much higher at the surface, but decreased sharply with depth in response to changes in the geochemical conditions of the sediments, such as methane, sulphate concentration and total organic carbon. Marine Benthic Group B, Chloroflexi and JS1 were predominant phylotypes of the archaeal and bacterial libraries, respectively. ª 2011, China University of Geosciences (Beijing) and Peking University. Production and hosting by Elsevier B.V. All rights reserved.

Gas hydrates represent a huge reservoir of methane in marine sediments, prone to dissociation in response to environmental changes. There is consensus that past events of gas hydrate dissociation in the marine environment mainly occurred during periods of low sea level. Here, we report geochemical data for 2-m-thick layers of seep carbonate collected from a hydrate-bearing drill core from~800-m water depth in the northern South China Sea. The aragonite-rich carbonates reveal positive δ 18 O values, confirming a genetic link with gas hydrate dissociation. Uranium-thorium dating of seep carbonates indicates that gas hydrates at the study site dissociated between 133,300 and 112,700 years BP, hence coinciding with the Last Interglacial (MIS 5e) sea-level highstand. We put forward the concept that a climate-driven increase in temperature was responsible for a period of pronounced gas hydrate dissociation.Plain Language Summary The gas hydrate reservoir is a dynamically changing system extremely susceptible to variations of seafloor temperature and pressure. Therefore, gas hydrate dissociation and subsequent methane seepage frequently occur during times of global climate change, especially during sea-level lowstands with reduced seabed pressure. However, this conclusion was mainly based on dating of seep carbonates sampled from the seabed. As a consequence, one cannot exclude that previous results have been compromised by a sampling bias since seafloor samples are easier to collect. Authigenic seep carbonates from drill cores represent a continuous record of gas hydrate dynamics. Our uranium-thorium dating of seep carbonate from drill cores provides a unique example of the effects of temperature and pressure on the stability of the hydrate system in the Dongsha area, northern South China Sea (SCS), during the last interglacial stage (MIS 5e, about 130,000 years BP). Representing the most similar and most contemporary analog to the current interglacial, the study of a methane release event in the SCS during MIS 5e will shed light on the expected trend of methane release events in the future, while providing insight into the response of low-latitude oceans to climate change.Supporting Information:• Data Set S1