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doi:10.2204/iodp.pr.347.2014

Site summaries

Little Belt (Sites M0059 and M0067)

Site M0059 (proposed Site BSB-3B; 55°0.29′N, 10°6.49′E) is situated in the southern Little Belt at a water depth of 35 m, and Site M0067 (proposed Site BSB-4B; 55°08.14′N,09°48.03′E) is situated farther north in the Little Belt at a water depth of 23 m (Fig. F1). Seismic studies of the western BSB, east of the Island of Als, have revealed an exceptionally thick accumulation of Quaternary sediments in deep valleys and basins running parallel to the coast. It was proposed that the almost complete Eemian interglacial succession found exposed at Mommark on Als may have been originally related to the deposit found within a deep Quaternary channel that runs along the eastern coast of the Island of Als. Subsequent glacial activity may have transported this sequence to the west-northwest.

The kilometer-wide incised valley, which in places reaches 200 m below the seafloor, expresses an erosional unconformity. It was suggested that the valley infill included a thick MIS 5e (Eemian interglacial) succession. This interpretation was partly based on the occurrence of slices of dislocated MIS 5e sediments in the coastal areas around the western BSB and partly on direct evidence from wells penetrating a similar valley infill. One example of such a well is located close to the end of an incised valley in the Åbenrå Fjord, northwest of Als.

The aim of drilling in the central part of one of the deep incised valleys east of the Island of Als was to obtain an extremely high resolution MIS 5e record, providing a similar sequence to the Mommark section but thicker, more complete, and still in situ.

To ensure an undisturbed sediment sequence from the Eemian, two sites were proposed at this location. Both were identified as primary sites, as there was a risk that methane gas would occur in parts of the Quaternary sediment sequence, possibly preventing coring reaching the target depth at one of the sites. Of these two sites, M0059 (proposed Site BSB-3) was the primary objective (Figs. F8, F9).

Site M0067 (proposed Site BSB-4) (Figs. F10, F11), located north of Site M0059 in the same deep valley, was expected to basically contain a similar stratigraphy as a nearby terrestrial site studied by well drilling at Ensted, which indicated the presence of Eemian deposits (Eiríksson et al., 2006).

Anholt Loch (Site M0060)

Site M0060 (proposed Site BSB-1; 56°37.21′N, 11°40.24′E) is situated in the southern Kattegat at a water depth of 34 m (Figs. F12, F13). On the basis of a rather coarse grid of seismic lines (Fig. F14), an erosional valley was mapped southeast of Anholt Island in the Kattegat Sea (Fig. F15). The valley is ~25 km long and ~2 km wide, is oriented northwest to southeast, and was probably formed by subglacial meltwater erosion during repeated glaciations in the early and middle Quaternary. The distance from the seafloor to the valley bottom is 200–300 m. Based on the stratigraphy from a well boring on the island of Anholt in the vicinity of the proposed drill site, it was suggested that the valley was infilled by a sequence from MIS 6, MIS 5e, and MIS 3 (Lykke-Andersen et al., 1993). Based on later boomer profiles and shallow cores, the younger parts of the infill were referred to as late glacial and Holocene age (MIS 2–MIS 1; Jensen et al., 2002). The area was inundated by the SIS during the LGM, but according to the most recent seismic profiles, this inundation did not lead to disturbances of the sequence in the Anholt region. Therefore, because the valley acted as a sediment trap and later disturbance was minimal, it was expected to contain an unusually complete sequence of sediments from the last interglacial–glacial cycle (Fig. F16), as well as a deglacial–Holocene sequence that would be valuable in establishing linkage between the BSB and the open Atlantic oceanic systems.

Ångermanälven River estuary (Sites M0061 and M0062)

Site M0061 (proposed Site BSB-10; 62°46.70′N, 18°02.95′E) is situated near the mouth of the estuary at a water depth of 86 m, and Site M0062 (proposed Site BSB-11; 62°57.35′N, 17°47.70′E) is situated farther into the estuary at a water depth of 68 m. In the Ångermanälven River estuary, it has been proposed that varves have been deposited during the last 10,000 y and that they are still being deposited. Varve thicknesses in open sections along the river valley have been measured, and varve diagrams have been cross-correlated to a local varve chronology, which is in turn correlated to the Swedish late glacial varve chronology, together constituting the so-called Swedish Time Scale (STS). No scientific drilling has been performed previously in the estuary, but short piston cores indicated a sediment accumulation rate of 5–10 mm/y during the early Holocene.

The target area for Site M0061 was surveyed north of Härnösand in the easternmost part of Ångermanälven (Figs. F17, F18). The surveyed area contains a suitable location for drilling where an apparently undisturbed sediment section fills a trough and reaches a thickness of >40 ms two-way traveltime (>30 m assuming a sound velocity of 1500 m/s) (Fig. F19). The sediment thickness appears to exceed the limit of the CHIRP sonar used in the survey. The proposed site is located on Profile 71021235, which ends 34 m southwest of the proposed site and provides a proximal perpendicular view of the proposed site geometry (Fig. F17).

Site M0062 is located close to the town Kramfors and has the purpose of connecting the clay varve series from Site M0061 with the modern varves in Ångermanälven (Figs. F20, F21). Cross correlation between this site and Site M0061 should further strengthen the varve chronology, as some varves may be missing when analyzing only one site. It should also strengthen the correlation of the varve records to the STS, which is needed in order to achieve true calendar-year accuracy in the dating. This site was moved ~50 m along Profile 71021706 in order to avoid some apparent undulations in the acoustic stratigraphy at the exact location of the crossing between this profile and Profile 71021606 (Figs. F20, F22).

Landsort Deep (Site M0063)

Site M0063 (proposed Site BSB-9; 58°37.34′N, 18°15.25′E) is situated in the central part of the Landsort Deep at a water depth of 451 m. The Landsort Deep is the deepest basin in the BSB, and its geometry makes it an excellent “sediment trap,” protecting the sediments from subsequent glacial erosion. Furthermore, its location just south of the postulated margins of the early Weichselian glacial advances (MIS 5d and MIS 5b) made it particularly promising for registering the early Weichselian development of the BSB. It also displays an expanded late Weichselian and Holocene sediment sequence.

A 20 inch³ par air gun was used together with a 50 m long single-channel streamer to collect Profiles 68170648 and 68161201, which run along the axis of (Fig. F23) or cross (Fig. F24) the Landsort Deep. The seismic reflection data acquired clearly show the most suitable drilling target selected for the purpose of recovering the longest undisturbed postglacial (possibly partly deglacial) sequence. From Profile 68161201 (Fig. F25), it is evident that the uppermost horizontally stratified sediment layer thickens toward the center of the depression. Assuming a sediment sound speed of 1500 m/s, the sediment section is ~122 m thick at the thickest part in Profile 68170648. This profile is intersected by Profile 68161201. This intersection provides an acoustically well constrained drilling target at a location in the Landsort Deep where the sediment section appears to reach the maximum thickness in the area and where there are no signs of erosion or other disturbances in the acoustic data.

Hanö Bay and Bornholm Basin (Sites M0064, M0065, and M0066)

Site M0064 (proposed Site BSB-5; 55°43.27′N, 15°13.59′E) is situated in the Hanö Bay off the south coast of Sweden at a water depth of 61 m (Figs. F26, F27). Based on information from a short piston core and the radiocarbon age of its lowermost part (26 k.y. uncorrected radiocarbon years; Andrén, unpubl. data), Site M0064 was included in the presite survey. The seismic profiles around Site M0064 indicated the possible occurrence of a thick, relatively transparent sequence of sediments with some internal reflectors, below a relatively thin till cover of late Weichselian age (Fig. F28). Previous offshore studies in the southern Baltic have documented the presence of marine brackish sediments, dated to MIS 3 or older, that were overridden by a glacier (Klingberg, 1998) at Kriegers Flak, along with two varved clay sequences—the upper one dating from the last deglaciation—separated by an organic-rich layer dated to older than 35 k.y. ¹⁴C BP (bulk date) in Hanö Bay (Björck et al., 1990). The lower unit could therefore be of Eemian or early or mid-Weichselian age.

In the Bornholm Basin, Site M0065 (proposed Site BSB-7C; 55°28.09′N, 15°28.63′E) is situated northeast of the island Bornholm at a water depth of 87 m, and nearby Site M0066 (proposed Site BSB-7D; 55°27.77′N, 15°29.56′E) is situated at a water depth of 87 m (Fig. F29). The area of Site M0065 is covered by ~10 m of Holocene mud and clay (Fig. F30), whereas the area of Site M0066 has no distinct Holocene sequence. During the seismic survey, pockets of sediments below late Weichselian till and Holocene mud were discovered in the deeper part of the Bornholm Basin. These sediments were probably deposited in the lake that occupied the southern Baltic Basin from the Eemian interglacial up to the last Weichselian ice advance over the area. We included the two sites (Figs. F29, F30, F31) in the proposal because they could give a unique opportunity to study the development of this lake in two different types of settings, the shallow coastal setting in the Hanö Bay area (Site M0064) at a water depth of 61 m and the deep lake setting at a water depth of 87 m in the Bornholm Basin at Site M0065.

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