Geohazard studies offshore the Faroe Islands: slope instability, bottom currents and sub-seabed sediment mobilisation

Prior to the 1990s only few geological investigations of the seabed and the shallow geology around the Faroe Islands had been undertaken (Waagstein & Rasmussen 1975; Nielsen et al. 1981). However, in the 1990s marine geological and in particular seismic investigations were markedly intensified. Since 1993 several studies on the structure of the Faroe Islands margin and seafloor processes have been funded by the European Union, namely the ENAM (European North Atlantic Margin) project I and II (1993–1999) and the STRATAGEM (Stratigraphy of the Glaciated European Margin) project (2000–2003), and these have provided significant new information on the mechanisms shaping the Faroe Islands margin (e.g. Boldreel et al. 1998; Kuijpers et al. 1998a; Nielsen & van Weering 1998; van Weering et al. 1998). Due to the expertise and regional geological knowledge obtained during these projects the Geological Survey of Denmark and Greenland (GEUS) became involved in socalled ‘geohazard’ seabed studies of the Faroe–Shetland Channel in 1997. These investigations were financed by the petroleum industry that had begun to show significant interest in exploration of the Faroe–Shetland Channel area. The studies focused on possible natural risks that would affect submarine structures, such as slope instability and strong bottom currents, and included both shallow seismic data acquisition and sediment core analyses. Most of the work at sea was undertaken with the Russian research vessel Prof. Logachev, and carried out within the framework of the international, UNESCO-supported ‘Training-Through-Research’ (TTR) programme co-ordinated by Moscow State University, Russia. Since 1997, more than three million DKK have been granted for various projects and this work has been documented in 14 classified reports. This paper presents some of the main results from these ‘geohazard’ studies, in particular with respect to the sediment instability affecting the western flank of the Faroe–Shetland Channel, the occurrence of very strong bottom currents in the channel, and the newly discovered mud diapirs at the northern entrance of the channel (Fig. 1)

Climatic warming: a trigger for glacial iceberg surges (‘Heinrich events’) in the North Atlantic?

In the present-day western North Atlantic, icebergs can be observed off north-east Canada, drifting south along the coast in the cold Labrador Current. Normally they melt in the area off Newfoundland where they reach warmer waters. Most of these icebergs originate from calving glaciers in West Greenland or in the Canadian Arctic. Jakobshavn Isbræ in West Greenland (Fig. 1) deserves particular mention as it is the fastest known ice stream in the world draining 6–7% of the entire Greenland ice sheet (Joughin et al. 2004). Southward drifting icebergs also occur along the east coast of Greenland (Fig. 2), but most of these melt when they approach the southernmost tip of Greenland. The iceberg limit in the north-western Atlantic varies from year to year, but isolated icebergs may reach far south of Newfoundland (Fig. 1). Many icebergs carry a load of rock debris and soil incorporated by their parent glacier that leads to deposition of ice rafted debris on the deep ocean sea floor. In the past decade the Geological Survey of Denmark and Greenland (GEUS) has initiated marine geological investigations in the North Atlantic on the late Quaternary variability of North Atlantic thermohaline circulation, with special focus on the possible link between climate change and variations in deep-water flow intensity (Kuijpers et al. 1998, 2002, 2003). Moreover, glaciological projects in Greenland undertaken by GEUS have significantly contributed to the current debate of present-day climatic warming. Notably work carried out in East Greenland fjords has provided crucial information relevant for the study of glacial iceberg surges in the North Atlantic (Reeh et al. 1999). These surges are suggested to have been triggered under the influence of extreme cold climate conditions, but the actual trigger mechanism involved has been a matter of much debate. Evidence from modern glacier process studies referred to above, combined with results of recent studies in the North Atlantic carried out by GEUS and partner institutions, has provided new insights into the possible trigger mechanism of these massive glacial iceberg surges. These new findings have great significance for the current climate debate, since they strongly suggest that ongoing ocean warming can trigger a sudden, massive break-up of ice shelves. Such processes may already be in progress in the Arctic (e.g. Vincent et al. 2004), where rapid ice-shelf disruption on the margin of the Canadian Arctic Ocean has been reported to be the result of significant warming over the past few decades. During this period intensified inflow of Atlantic water to the Eurasian sector of the Arctic has been noted. It is evident that for Antarctic ice shelves large-scale disruption and break-up may lead to significant destabilisation of the Antarctic ice sheet with the serious risk of a sudden, drastic sea-level rise

Million years of Greenland Ice Sheet history recorded in ocean sediments

Geological records from Tertiary and Quaternary terrestrial and oceanic sections have documented the presence of ice caps and sea ice covers both in the Southern and the Northern hemispheres since Eocene times, approximately since 45 Ma. In this paper focussing on Greenland we mainly use the occurrences of coarse ice-rafted debris (IRD) in Quaternary and Tertiary ocean sediment cores to conclude on age and origin of the glaciers/ice sheets, which once produced the icebergs transporting this material into the adjacent ocean. Deep-sea sediment cores with their records of ice-rafting from off NE Greenland, Fram Strait and to the south of Greenland suggest the more or less continuous existence of the Greenland ice sheet since 18 Ma, maybe much longer, and hence far beyond the stratigraphic extent of the Greenland ice cores. The timing of onset of glaciation on Greenland and whether it has been glaciated continuously since, are wide open questions of its long-term history. We also urgently need new scientific drilling programs in the waters around Greenland, in particular in the segment of the Arctic Ocean to the north of Greenland

The Storebælt gateway to the Baltic

The present-day Storebælt (Great Belt), the waterway between the islands of Fyn and Sjælland (Fig. 1), contains deeply incised valleys, locally more than 50 m deep, and is of crucial importance to the water exchange between the fully marine Kattegat and the brackish Baltic Sea. The role of this important gateway changed significantly during the late and post-glacial period (since 15000 B.P.), when the Baltic Basin experienced alternating freshwater, brackish and marine conditions as a result of changes in relative sea level (Figs 2, 3). The importance of the Storebælt in understanding the dynamics of the Baltic Basin is reflected in the large number of studies carried out (see Bennike et al. 2004). The first detailed sedimentological and stratigraphic studies in the Storebælt area that demonstrated the presence of early Holocene freshwater deposits below the seabed were those of Krog (1960, 1965, 1971), who also presented the first shore-displacement curve for the area (Krog 1979)

The Darss Sill, hydrographic threshold in southwestern Baltic: Late quaternary geology and recent sediment dynamics

About 73% of the water exchange between the Baltic and the North Seas is via the Darss Sill. Joint Danish and German marine geological investigations, including shallow seismic surveys and sediment sampling, have been carried out in the Darss Sill area between 1989 and 1991. Results from these studies and unpublished archive data from the Institute of Baltic Sea Research, Warnemünde, are presented. These show that the entire Quaternary sedimentary sequence in most of the area is at least 40 m thick. A zone of glacial till outcropping between Denmark and Germany belongs to the Late Weichselian ice marginal line G (“Velgaster Staffel”). The formation of this line caused the damming of a pre-existing (sub)glacial meltwater discharge system. At the beginning of the Baltic Ice Lake formation, a deep and presently buried channel incised in this ice marginal line was part of a (glacio)fluvial drainage system with discharge towards the northeast, i.e. into the Baltic Ice Lake. During the Baltic Ice Lake transgression damming of the Darss Sill occurred due to accumulation of sandy sediments within this channel. On the other hand, the recent Kadet Channel was only occasionally overflown during the Baltic Ice Lake highstand maximum, which was 18 m BSL (below present sea level) in the Darss Sill area. The main erosion and deepening of the Kadet Channel began during the Ancylus Lake highstand maximum. At present, sediment transport on the Darss Sill is governed by northeasterly inflow of saline bottom waters in the entire area from the Kadet Channel to the German coast. Baltic outflow affects the seabed at shallower depth in the Danish sector. Large-scale current-induced bedforms in the area include, amongst others, sandwaves with a height of 5 m. Changes of the bedforms observed indicate a maximum bottom flow speed of 70–100 cm s−1, both for inflow and for outflow

Methane and possible gas hydrates in the Disko Bugt region, central West Greenland

Current climate models predict an annual temperature increase in the Arctic between 4° and 6°C by the end of the 21st century with widespread impact on the Arctic environment. Warming will lead to thawing of the widespread, permanently frozen, high-latitude peat-lands and to degradation of marine gas hydrates, both of which may increase the rate of methane release to the atmosphere. This will influence global climate as methane is a potent greenhouse gas with a large global warming potential. Marine gas hydrates are found worldwide on continental margins and frequently occur in the Arctic. Interpretation of seismic profiles has also indicated their presence in the Disko Bugt region in western Greenland

Subarctic Front migration at the Reykjanes Ridge during the mid- to late Holocene:Evidence from planktic foraminifera

Expansion of fresh and sea-ice loaded surface waters from the Arctic Ocean into the sub-polar North Atlantic is suggested to modulate the northward heat transport within the North Atlantic Current (NAC). The Reykjanes Ridge south of Iceland is a suitable area to reconstruct changes in the mid- to late Holocene fresh and sea-ice loaded surface water expansion, which is marked by the Subarctic Front (SAF). Here, shifts in the location of the SAF result from the interaction of freshwater expansion and inflow of warmer and saline (NAC) waters to the Ridge. Using planktic foraminiferal assemblage and concentration data from a marine sediment core on the eastern Reykjanes Ridge elucidates SAF location changes and thus, changes in the water-mass composition (upper ˜200 m) during the last c. 5.8 ka BP. Our foraminifer data highlight a late Holocene shift (at c. 3.0 ka BP) in water-mass composition at the Reykjanes Ridge, which reflects the occurrence of cooler and fresher surface waters when compared to the mid-Holocene. We document two phases of SAF presence at the study site: from (i) c. 5.5 to 5.0 ka BP and (ii) c. 2.7 to 1.5 ka BP. Both phases are characterized by marked increases in the planktic foraminiferal concentration, which coincides with freshwater expansions and warm subsurface water conditions within the sub-polar North Atlantic. We link the SAF changes, from c. 2.7 to 1.5 ka BP, to a strengthening of the East Greenland Current and a warming in the NAC, as identified by various studies underlying these two currents. From c. 1.5 ka BP onwards, we record a prominent subsurface cooling and continued occurrence of fresh and sea-ice loaded surface waters at the study site. This implies that the SAF migrated to the southeast of our core site during the last millennium

Late-Holocene Atlantic bottom-water variability in Igaliku Fjord, South Greenland, reconstructed from foraminiferal faunas

A high-resolution record of late-Holocene subsurface water-mass characteristics in outer Igaliku Fjord, South Greenland, is presented based on benthic foraminifera faunas from core PO 243–451 collected from a water depth of 304 m. Strati” cation with Atlantic water masses present in the lower part of the water-column is suggested to have prevailed during the last 3200 cal. years, except for a period referred to as the‘Mediaeval Warm Period’ (MWP). During the MWP (c. ad 885–1235) the outer part of Igaliku Fjord experienced enhanced vertical mixing and a high hydrodynamic energy level which we ascribe to increasing wind stress through this period, corresponding to the period of the Norse settlement. The transition from the MWP to the‘Little Ice Age’ (LIA) shows a two-step pattern with a short climatic amelioration around AD 1520 before maximum cooling occurred. The intensified wind stress and the overall environmental change are suggested to have contributed to the loss of the Norse settlement in Greenland. Periods with strong stratification and marked in uence of Atlantic subsurface water masses around 2.6, 1.3 ka BP and during the LIA are correlated to North Atlantic Holocene ice-rafting events reported by Bond et al. (1997)

Size differences of Arctic marine protists between two climate periods - using the paleoecological record to assess the importance of within-species trait variation

Mean body size decreases with increasing temperature in a variety of organisms. This size–temperature relationship has generally been tested through space but rarely through time. We analyzed the sedimentary archive of dinoflagellate cysts in a sediment record taken from the West Greenland shelf and show that mean cell size decreased at both intra‐ and interspecific scales in a period of relatively warm temperatures, compared with a period of relatively cold temperatures. We further show that intraspecific changes accounted for more than 70% of the change in community mean size, whereas shifts in species composition only accounted for about 30% of the observed change. Literature values on size ranges and midpoints for individual taxa were in several cases not representative for the measured sizes, although changes in community mean size, calculated from literature values, did capture the direction of change. While the results show that intraspecific variation is necessary to accurately estimate the magnitude of change in protist community mean size, it may be possible to investigate general patterns, that is relative size differences, using interspecific‐level estimates

Evidence for influx of Atlantic water masses to the Labrador Sea during the Last Glacial Maximum

The Last Glacial Maximum (LGM, 23–19,000 year BP) designates a period of extensive glacial extent and very cold conditions on the Northern Hemisphere. The strength of ocean circulation during this period has been highly debated. Based on investigations of two marine sediment cores from the Davis Strait (1033 m water depth) and the northern Labrador Sea (2381 m), we demonstrate a significant influx of Atlantic-sourced water at both subsurface and intermediate depths during the LGM. Although surface-water conditions were cold and sea-ice loaded, the lower strata of the (proto) West Greenland Current carried a significant Atlantic (Irminger Sea-derived) Water signal, while at the deeper site the sea floor was swept by a water mass comparable with present Northeast Atlantic Deep Water. The persistent influx of these Atlantic-sourced waters entrained by boundary currents off SW Greenland demonstrates an active Atlantic Meridional Overturning Circulation during the LGM. Immediately after the LGM, deglaciation was characterized by a prominent deep-water ventilation event and potentially Labrador Sea Water formation, presumably related to brine formation and/or hyperpycnal meltwater flows. This was followed by a major re-arrangement of deep-water masses most likely linked to increased overflow at the Greenland-Scotland Ridge after ca 15 kyr BP