Ines Dumke

Temporal Constraints on Hydrate-Controlled Methane Seepage off Svalbard

What Does It All Mean? Strong emissions of methane have recently been observed from shallow sediments in Arctic seas. Berndt et al. (p. 284, published online 2 January) present a record of methane seepage from marine sediments off the coast of Svalbard showing that such emissions have been present for at least 3000 years, the result of normal seasonal fluctuations of bottom waters. Thus, contemporary observations of strong methane venting do not necessarily mean that the clathrates that are the source of the methane are decomposing at a faster rate than in the past. Seasonal gas hydrate destabilization has been releasing methane from marine sediments near Svalbard for at least 3000 years. Methane hydrate is an icelike substance that is stable at high pressure and low temperature in continental margin sediments. Since the discovery of a large number of gas flares at the landward termination of the gas hydrate stability zone off Svalbard, there has been concern that warming bottom waters have started to dissociate large amounts of gas hydrate and that the resulting methane release may possibly accelerate global warming. Here, we corroborate that hydrates play a role in the observed seepage of gas, but we present evidence that seepage off Svalbard has been ongoing for at least 3000 years and that seasonal fluctuations of 1° to 2°C in the bottom-water temperature cause periodic gas hydrate formation and dissociation, which focus seepage at the observed sites.

Sidescan backscatter variations of cold seeps on the Hikurangi Margin (New Zealand): indications for different stages in seep development

Cold seeps on the Hikurangi Margin off New Zealand exhibit various seabed morphologies producing different intensity patterns in sidescan backscatter images. Acoustic backscatter characteristics of 25 investigated seep sites fall into four distinct types characterised by variations in backscatter intensity, distribution and inferred structural heights. The types reflect different carbonate morphologies including up to 20-m-high structures (type 1), low-relief crusts (type 2), scattered blocks (type 3) and carbonate-free sites (type 4). Each seep corresponds to a single type; intermediates were not observed. This correlates well with published data on seep fauna at each site, with the four types representing three different faunal habitats of successive stages of seep development. Backscatter signatures in sidescan sonar images of cold seeps may therefore serve as a convenient proxy for variations in faunal habitats.

Gas hydrate distribution and hydrocarbon maturation north of the Knipovich Ridge, western Svalbard margin

A bottom-simulating reflector (BSR) occurs west of Svalbard in water depths exceeding 600 m, indicating that gas hydrate occurrence in marine sediments is more widespread in this region than anywhere else on the eastern North Atlantic margin. Regional BSR mapping shows the presence of hydrate and free gas in several areas, with the largest area located north of the Knipovich Ridge, a slow-spreading ridge segment of the Mid Atlantic Ridge system. Here, heat flow is high (up to 330 mW m-2), increasing towards the ridge axis. The coinciding maxima in across-margin BSR width and heat flow suggest that the Knipovich Ridge influenced methane generation in this area. This is supported by recent finds of thermogenic methane at cold seeps north of the ridge termination. To evaluate the source rock potential on the western Svalbard margin, we applied 1D petroleum system modeling at three sites. The modeling shows that temperature and burial conditions near the ridge were sufficient to produce hydrocarbons. The bulk petroleum mass produced since the Eocene is at least 5 kt and could be as high as ~0.2 Mt. Most likely, source rocks are Miocene organic-rich sediments and a potential Eocene source rock that may exist in the area if early rifting created sufficiently deep depocenters. Thermogenic methane production could thus explain the more widespread presence of gas hydrates north of the Knipovich Ridge. The presence of microbial methane on the upper continental slope and shelf indicates that the origin of methane on the Svalbard margin varies spatially.

Underwater hyperspectral imaging as an in situ taxonomic tool for deep-sea megafauna

Identification of benthic megafauna is commonly based on analysis of physical samples or imagery acquired by cameras mounted on underwater platforms. Physical collection of samples is difficult, particularly from the deep sea, and identification of taxonomic morphotypes from imagery depends on resolution and investigator experience. Here, we show how an Underwater Hyperspectral Imager (UHI) can be used as an alternative in situ taxonomic tool for benthic megafauna. A UHI provides a much higher spectral resolution than standard RGB imagery, allowing marine organisms to be identified based on specific optical fingerprints. A set of reference spectra from identified organisms is established and supervised classification performed to identify benthic megafauna semi-autonomously. The UHI data provide an increased detection rate for small megafauna difficult to resolve in standard RGB imagery. In addition, seafloor anomalies with distinct spectral signatures are also detectable. In the region investigated, sediment anomalies (spectral reflectance minimum at ~675 nm) unclear in RGB imagery were indicative of chlorophyll a on the seafloor. Underwater hyperspectral imaging therefore has a great potential in seafloor habitat mapping and monitoring, with areas of application ranging from shallow coastal areas to the deep sea.