Jürgen Mienert

THE NORWEGIAN–GREENLAND SEA CONTINENTAL MARGINS: MORPHOLOGY AND LATE QUATERNARY SEDIMENTARY PROCESSES AND ENVIRONMENT

The continental margins surrounding the Norwegian–Greenland Sea are to a large degree shaped by processes during the late Quaternary. The paper gives an overview of the morphology and the processes responsible for the formation of three main groups of morphological features: slides, trough mouth fans and channels. Several large late Quaternary slides have been identified on the eastern Norwegian–Greenland Sea continental margin. The origin of the slides may be due to high sedimentation rates leading to a build-up of excess pore water pressure, perhaps with additional pressure caused by gas bubbles. Triggering might have been prompted by earthquakes or by decomposition of gas hydrates. Trough mouth fans (TMF) are fans at the mouths of transverse troughs on presently or formerly glaciated continental shelves. In the Norwegian–Greenland Sea, seven TMFs have been identified varying in area from 2700 km2 to 215 000 km2. The Trough Mouth Fans are depocentres of sediments which have accumulated in front of ice streams draining the large Northwest European ice sheets. The sediments deposited at the shelf break/upper slope by the ice stream were remobilized and transported downslope, mostly as debris flows. The Trough Mouth Fans hold the potential for giving information about the various ice streams feeding them with regard to velocity and ice discharge. Two large deep-sea channel systems have been observed along the Norwegian continental margin, the Lofoten Basin Channel and the Inbis Channel. Along the East Greenland margin, several channel systems have been identified. The deep-sea channels may have been formed by dense water originating from cooling, sea-ice formation and brine rejection close to the glacier margin or they may originate from small slides on the upper slope transforming into debris flows and turbidity currents.

Geological controls on the Storegga gas-hydrate system of the mid-Norwegian continental margin

The geologic setting of the formerly glaciated mid-Norwegian continental margin exerts specific controls on the formation of a bottom-simulating reflector (BSR) and the inferred distribution of gas hydrates. On the continental slope the lithology of glacigenic debris flow deposits and pre-glacial basin deposits of the Kai Formation prevent gas-hydrate formation, because of reduced pore size, reduced water content and fine-grained sediment composition. Towards the continental shelf, the shoaling and pinch-out of the gas-hydrate stability zone terminates the area of gas-hydrate growth. These geological controls confine the occurrence of gas hydrates and ensuing formation of a BSR to a small zone along the northern flank of the Storegga submarine slide and the slide area itself. A BSR inside the slide area indicates a dynamically adjusting gas-hydrate system to post-slide pressure–temperature equilibrium conditions. These observations, together with widespread evidence for fluid flow and deep-seated hydrocarbon reservoirs, suggest that the formation of BSR and gas hydrates on the mid-Norwegian continental margin is dominated by an advection of gas from the strata distinctly beneath the gas-hydrate stability zone. Fluids migrate upward within the Naust Formation and are deflected laterally by hydrated sediments and less permeable layers. Gases continually accumulate at the top of the slope, where overpressure eventually results in the formation of blow-out pipes and consequent pockmark development on the seabed.

Large-scale sedimentation on the glacier-influenced polar North Atlantic Margins: Long-range side-scan sonar evidence

Long-range side-scan sonar (GLORIA) imagery of over 600,000 km² of the Polar North Atlantic provides a large-scale view of sedimentation patterns on this glacier-influenced continental margin. High-latitude margins are influenced strongly by glacial history and ice dynamics and, linked to this, the rate of sediment supply. Extensive glacial fans (up to 350,000 km³) were built up from stacked series of large debris flows transferring sediment down the continental slope. The fans were linked with high debris inputs from Quaternary glaciers at the mouths of cross-shelf troughs and deep fjords. Where ice was slower-moving, but still extended to the shelf break, large-scale slide deposits are observed. Where ice failed to cross the continental shelf during full glacials, the continental slope was sediment starved and submarine channels and smaller slides developed. A simple model for large-scale sedimentation on the glaciated continental margins of the Polar North Atlantic is presented.

Arctic methane sources: Isotopic evidence for atmospheric inputs

By comparison of the methane mixing ratio and the carbon isotope ratio (δ13CCH4) in Arctic air with regional background, the incremental input of CH4 in an air parcel and the source δ13CCH4 signature can be determined. Using this technique the bulk Arctic CH4 source signature of air arriving at Spitsbergen in late summer 2008 and 2009 was found to be −68‰, indicative of the dominance of a biogenic CH4 source. This is close to the source signature of CH4 emissions from boreal wetlands. In spring, when wetland was frozen, the CH4 source signature was more enriched in 13C at −53 ± 6‰ with air mass back trajectories indicating a large influence from gas field emissions in the Ob River region. Emissions of CH4 to the water column from the seabed on the Spitsbergen continental slope are occurring but none has yet been detected reaching the atmosphere. The measurements illustrate the significance of wetland emissions. Potentially, these may respond quickly and powerfully to meteorological variations and to sustained climate warming.

Gas hydrates along the northeastern Atlantic margin: possible hydrate-bound margin instabilities and possible release of methane

Abstract The presence of gas hydrates and free gas in oceanic sediments along the northeastern European Margin is documented in high-frequency near-vertical and wide-angle seismic reflection data. Shallow-water and deep-water gas hydrate instabilities can cause free gas to escape from oceanic sediments. Particularly, methane from shallow-water gas hydrate destabilization may then get transferred from the sediments into the water column, and eventually into the atmosphere. Deep-water gas hydrates are coincident with areas and depths of slope failures in continental margin sediments. Comparisons between seismicity and the potential hydrate distributions suggest a correlation between hydrate instability and margin instabilities along the north-eastern Atlantic Margin.

Polygonal fault systems on the mid-Norwegian margin: A long term source for fluid flow

Abstract 2D and 3D seismic data from the mid-Norwegian margin show that polygonal fault systems are widespread within the fine-grained, Micene sediments of the Kai Formation that overlie the Mesozoic/Early Cenozoic rift basins. Outcropping polygonal faults show that de-watering and development of polygonal faults commenced shortly after burial. On the other hand, the polygonal fault system’s stratigraphic setting, upward decreasing fault throw and the association with fluid flow features that are attributed to de-watering of the polygonal fault systems shows that polygonal faulting and fluid expulsion is an ongoing process since Miocene times.

Late Quaternary sedimentation on the Portuguese continental margin: climate-related processes and products

The late Quaternary sedimentary history of the continental margin off Portugal was reconstructed from sediment gravity cores. Hemipelagic sedimentation (lithofacies A) was dominant during glacial times. It was interrupted periodically by deposition of shelf- and upper-slope-derived silty and sandy terrigenous material by dilute turbidity currents (lithofacies B and C), ice-rafted debris during distinct periods of breakdown of North Atlantic ice sheets (Heinrich events, lithofacies D) and large amounts of pteropods (lithofacies F). Settling of biogenic particulate material (lithofacies E) prevailed during the Holocene, when sea level and sea surface temperatures were high and terrigenous shelf-input was low. Downslope transport was dominant on the northern part of the Portuguese margin, culminating in frequent turbidity current transport between 35 and 70 ka. This may be due to a humid climate and a high fluvial input. Pteropod muds are confined to cores south of 41°N. Prominent peaks in pteropod concentration were radiocarbon dated at 17.8 and 24.6 ka. Layers rich in ice-rafted debris (IRD) were found along the entire margin. The base of these layers have been dated at 13.6–15.9 14C ka, 21.0–22.0 14C ka, 33.8 14C ka and ±64.5 ka, which correspond well with the ages of Heinrich events 1, 2, 4 and 6 in the central North Atlantic. Heinrich events 0 (10.5 ka), 3 (27 ka) and 5 (50 ka) rarely influenced sedimentation on the Portuguese slope. A mineralogical study of the IRD within Heinrich layers suggests that most icebergs were derived from the Laurentide Ice Sheet in the Hudson Strait and Hudson Bay area through the Labrador Current and the Canary Current and flowed in a southward direction along the margin. IRD from European ice sheets may have been mixed in during Heinrich event 6. On their way along the margin the icebergs lost much of their sediment load due to melting of the ice in a progressively warmer climate. The southernmost latitude studied (37°N) may be close to the southeastern extension of iceberg transport during Heinrich events.

Thermogenic methane injection via bubble transport into the upper Arctic Ocean from the hydrate‐charged Vestnesa Ridge, Svalbard

We use new gas-hydrate geochemistry analyses, echosounder data, and three-dimensional P-Cable seismic data to study a gas-hydrate and free-gas system in 1200 m water depth at the Vestnesa Ridge offshore NW Svalbard. Geochemical measurements of gas from hydrates collected at the ridge revealed a thermogenic source. The presence of thermogenic gas and temperatures of similar to 3.3 degrees C result in a shallow top of the hydrate stability zone (THSZ) at similar to 340 m below sea level (mbsl). Therefore, hydrate-skinned gas bubbles, which inhibit gas-dissolution processes, are thermodynamically stable to this shallow water depth. This was confirmed by hydroacoustic observations of flares in 2010 and 2012 reaching water depths between 210 and 480 mbsl. At the seafloor, bubbles are released from acoustically transparent zones in the seismic data, which we interpret as regions where free gas is migrating through the hydrate stability zone (HSZ). These intrusions result in vertical variations in the base of the HSZ (BHSZ) of up to similar to 150 m, possibly making the shallow hydrate reservoir more susceptible to warming. Such Arctic gas-hydrate and free-gas systems are important because of their potential role in climate change and in fueling marine life, but remain largely understudied due to limited data coverage in seasonally ice-covered Arctic environments.

Physical properties of Reykjanes Ridge sediments and their linkage to high-resolution Greenland Ice Sheet Project 2 ice core data

Five gravity cores taken from the Reykjanes Ridge have been used to establish a link between sediment physical properties and atmospheric records documented by δ18O variations in Greenland ice cores over the last 45,000 calendar years. Marine Gamma Ray Attenuation Porosity Evaluator density and magnetic susceptibility variations could be linked with the ice core Dansgaard-Oeschger and Bond cycles. This is supported by ice-rafted detritus (IRD), grain size, the quartz/feldspar ratio, and carbonate, isotopic, and foraminiferal records. The covariation of the sediment physical properties and δ18O in Greenland ice indicates a coupling of atmospheric temperature and paleocirculation variations. Gradual reduced bottom currents (Iceland-Scotland Overflow Water) and enhanced iceberg discharges have been reconstructed for cold atmospheric periods relative to interstadial times. In the study area the magnetic susceptibility signal is not related to the ice-rafted detritus input but most probably reflects the variations of the Iceland-Scotland Overflow Water intensity transporting titanomagnetite into the Reykjanes Ridge region.

Gas hydrates at the Storegga Slide: Constraints from an analysis of multicomponent, wide-angle seismic data

Geophysical evidence for gas hydrates is widespread along the northern flank of the Storegga Slide on the mid-Norwegian margin. Bottom-simulating reflectors (BSR) at the base of the gas hydrate stability zone cover an area of approximately 4000 km 2 , outside but also inside the Storegga Slide scar area. Traveltime inversion and forward modeling of multicomponent wide-angle seismic data result in detailed P- and S-wave velocities of hydrate- and gas-bearing sediment layers. The relationship between the velocities constrains the background velocity model for a hydrate-free, gas-free case. The seismic velocities indicate that hydrate concentrations in the pore space of sediments range between 3% and 6% in a zone that is as much as 50 m thick overlying the BSR. Hydrates are most likely disseminated, neither cementing the sediment matrix nor affecting the stiffness of the matrix noticeably. Average free-gas concentrations beneath the hydrate stability zone are approximately 0.4% to 0.8% of the pore volume, assuming a homogeneous gas distribution. The free-gas zone underneath the BSR is about 80 m thick. Amplitude and reflectivity analyses suggest a rather complex distribution of gas along specific sedimentary strata rather than along the base of the gas hydrate stability zone (BGHS). This gives rise to enhanced reflections that terminate at the BGHS. The stratigraphic control on gas distribution forces the gas concentration to increase slightly with depth at certain locations. Gas-bearing layers can be as thin as 2 m.