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doi:10.2204/iodp.sp.347.2012

Proposed drill sites

Proposed site locations

For a list of proposed site locations, see Table T1.

Anholt Loch (proposed Sites BSB-1B and BSB-2B)

On the basis of a rather coarse grid of seismic lines, an erosional valley was mapped southeast of Anholt Island in the southern Kattekat Sea. The valley is ~25 km long and ~2 km wide and is oriented northwest to southeast. The distance from the sea bottom to the valley bottom is 200–300 m. The valley was probably formed by subglacial meltwater erosion during repeated glaciations in the early and middle Quaternary, and based on the stratigraphy in the Anholt boring, it is suggested that the valley is 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 being of late glacial and Holocene age (MIS 2-1; Jensen et al., 2002). The area was inundated by the SIS during the LGM, but according to our new seismic profiles, this did not lead to disturbances of the sequence in the Anholt region. Because the valley acted as a sediment trap, it may contain an unusually complete sequence of sediments from the last interglacial–glacial cycle (Theme 1) (Figs. F5, F6), as well as a deglacial–Holocene sequence, which will be valuable for our linkage between the BSB and the open Atlantic oceanic systems (Theme 3).

Little Belt (proposed Sites BSB-3 and BSB-4)

Seismic studies of the western BSB, east of the Island of Als, reveal an exceptionally thick accumulation of Quaternary sediments in deep valleys and basins running parallel to the coast. It is proposed that the almost complete interglacial succession found exposed at Mommark may be related to original deposition within a deep Quaternary channel along the east coast of the Island of Als, and subsequent transportation by glaciers to the west-northwest.

The kilometer-wide incised valley, which in places reaches 200 m below the seafloor, expresses an erosional unconformity, and it is suggested that the valley infill includes a thick marine MIS 5e (Eemian interglacial) succession. This interpretation is partly based on the occurrence of slices of dislocated marine MIS 5e sediments in the coastal areas around the western BSB and partly by direct evidence from wells penetrating similar valley infill, for instance 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 is to obtain material from an in situ extremely high resolution marine MIS 5e record, providing a similar sequence to the Mommark section, but thicker and more complete (Theme 1).

To ensure an undisturbed sediment sequence from the Eemian, two sites are proposed at this location, both primary sites (Figs. F7, F8), as there is a risk that methane gas can occur in parts of the Quaternary sediment sequence, possibly preventing acquiring core to target depth at one of the sites.

Hanö Bay and Bornholm Basin (proposed Sites BSB-5B, BSB-6B, BSB-7B, and BSB-8)

Based on information from a short piston core and the radiocarbon age of its lowermost part, 25,930 ± 250 uncorrected radiocarbon years (T. Andrén, unpubl. data), this location was included in the R/V Heincke seismic cruise although it was not a part of original IODP pre-Proposal 672. The new seismic profiles indicate 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 (Figs. F9, F10, F11, F12). The lower unit is therefore of Eemian or early or mid-Weichselian age. The entire area is covered by ~10 m of Holocene mud and clay. Furthermore, during this 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 have also included these sites in the proposal as they will give the unique opportunity to study the development of this lake both in littoral phases in the Hanö Bay area and in deep lake phases in the Bornholm Basin.

Landsort Deep (proposed Site BSB-9)

The Landsort Deep is the deepest basin in the BSB, and its geometry makes it an excellent “sediment trap,” preventing subsequent glacial erosion. Its location just south of the postulated margins of the early Weichselian glacial advances (MIS 5d and MIS 5b) makes it particularly promising for registering the early Weichselian development of the BSB, and 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 68162141, which both run along 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 (Fig. F13).

From approximately shot point (SP) 1000 in Profile 68170648 and SP 1700 in Profile 68162141, the uppermost horizontally stratified sediment layer is thickening. Assuming a sediment sound speed of 1500 m/s, the sediment section is ~122 m thick at the thickest part near SP 1900 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.

Ångermanälven River estuary (proposed Sites BSB-10 and BSB-11)

In the Ångermanälven River estuary, varves have been deposited during the last >10,000 y and are still being deposited. Varve thicknesses in open sections along the river valley have been measured and varve diagrams 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 drilling in the estuary has been performed, but short piston cores have been collected, and they indicate a sediment accumulation rate of 5–10 mm/y in the early Holocene.

Two target areas were surveyed north of Härnösand in the easternmost part of Ångermanälven (Figs. F14, F15) using an EdgeTech chirp sonar subbottom profiler with the SB-216s tow fish. A chirp frequency modulation pulse between 3 and 9 kHz was used.

The surveyed area, located at about 62°47′N, contains a suitable area for drilling where an apparently undisturbed sediment section fills a trough and reaches a thickness of >40 ms two-way traveltime (TWT) (30 m assuming a sound speed of 1500 m/s).

Sediment thickness appears to exceed the limit of the used chirp sonar. The proposed site is located on Profile 71021235, and Profile 71021316 ends 34 m southwest of the proposed site and thus nearly provides a crossing.

Profile 71021706 crosses this site and shows no difference in the acoustic stratigraphy. The site could be moved to another location where two chirp sonar profiles cross, but the selected location, albeit not on a crossing, represents the most promising and longest undisturbed record.

The second proposed site (BSB-11) is located close to Kramfors for the purpose of connecting the clay varve series with modern varves in Ångermanälven. Cross-correlation between this site and proposed Site BSB-10 will further strengthen the varve chronology, as some varves may be missing when analyzing only one site. It will also strengthen the correlation of the varve records to the STS, something that is needed in order to achieve true calendar-year accuracy in the dating. This site has been 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 71021604.

Site prioritization

For site prioritization by theme, see Table T2. Further details regarding site prioritization and proposed coring order will be made available on the ESO Expedition 347 web page (www.eso.ecord.org/​expeditions/​347/​347.php).

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