Weddell Sea Y Kristoffersen and W Jokat

The principal features in the Weddell Sea sector relevant to resolving the Antarctic paleoclimate and paleoceanographic history are prograding wedges of glacigenic sediments along the entire margin, a major trough-mouth fan (Crary Fan), and numerous sediment drifts on the slope and in the deep basin, particularly along the western and northwestern side of the Antarctic Peninsula. Ice sheet flow-line patterns suggest that the continental margin of the eastern and southern Weddell Sea east of 45°W receives drainage from the EAIS, whereas the continental margin west of 45°W receives drainage from the WAIS (Fig. WS-1).

Figure WS-1: Track lines of multichannel seismic data in the Weddell Sea and locations of ODP drill sites. Bathymetry after Schenke et al. (1998). Areal extent of proximal and distal deposits of the Crary Trough Mouth Fan are outlined by light brown shaded area. True extent of sediment drifts in the southwestern and western Weddell Sea is poorly defined due to lack of data coverage. CTMF, Crary Trough Mouth Fan; EE, Explora Escarpment.

5.5.1. The Regional Seismic and Geologic Database

Modern geophysical data in the Weddell Sea comprise about 45,000 km of MCS lines from surveys principally by German, Norwegian and Russian research institutions since 1976 (Fig. WS-1). ODP drilled four sites in the Weddell Sea during ODP Leg 113, and ODP Site 693 on the Dronning Maud Land continental slope has been the most useful for stratigraphic calibration (Figs. WS-1 and WS-2). Prior to the deep drilling, stratigraphic studies in the region were conducted by Elverhoi and Maisey (1983), Hinz and Krause (1982), Hinz and Block (1984), Haugland et al. (1985) and Hinz and Kristoffersen (1987). Correlations with ODP Site 693 were made by Miller et al. (1990), Kuvaas and Kristoffersen (1991), Moons et al. (1992), Michels et al. (2002) and most extensively by Rogenhagen et al. (2004) (Fig. WS-2).

5.5.2. Acoustic Stratigraphy of the Shelf/Slope/Rise Environment-Spatial and Temporal Characteristics

The continental shelf of the Weddell Sea is characterized by a prograding wedge of glacigenic sediments more than 1 km thick below the shelf edge (Fig. WS-3). The wedge downlaps onto older units, which are characterized by rather uniform thickness in the down-slope direction (Fig. WS-3a). Wedge deposition is a first order result of massive transport of unsorted texturally immature sediments by advance of a grounded ice sheet to the shelf edge (Barker et al., 1998). The acoustic response of coarse sediment in proximal positions below the shelf and uppermost slope is one of discontinuous reflection events. Continuity and definition of acoustic stratification improve in the down-slope direction as a result of progressive sorting and increased relative abundance of finer material. The shelf edge may appear rectilinear, but the three-dimensional wedge architecture in the eastern Weddell Sea reveals an amalgation of adjacent small discrete cones of glacial sediments sourced by smaller ice streams (Kristoffersen et al., 2000). The spectrum of cones reflects broad scale expansion of the EAIS, but adjacent cones may or may not be coeval. Topsets of the prograding wedge are generally truncated at the seabed. Shelf aggradation is indicated in the southern Weddell Sea west of the Crary Trough, but the vast shelf area west of 45°W has not been accessible for seismic surveys (Fig. WS-1).

The maximum thickness of prograding units below the mouth of the Crary Trough and also below the shelf north of Lyddan Ice Rise is more than 3 km (Rogenhagen et al., 2004). ODP Site 693 (Fig. WS-2) provides local

Figure WS-2: Stratigraphic summary column. Modified from Rogenhagen et al. (2004).

1800 2000 2200 2400 2600 2800 3000 3200 3400 3600 3800 4100

10km

1800 2000 2200 2400 2600 2800 3000 3200 3400 3600 3800 4100

10km

West Larsen Shelf East

West Larsen Shelf East

12900 11500 10000 8500 7000 5500 4000 2500 1000

Figure WS-3: (a) Seismic line AWI-90110 across the Dronning Maud Land margin showing the prograding wedge (modified from Michels et al., 2002). (b) Seismic line AWI-97051 across the Larsen Shelf and Slope, showing the prograding shelf and sediment drift on the lower continental slope (modified from Michels et al., 2001). Profile locations are in Fig. WS-1.

NARE-8517B2

NARE-8517B2

SHOT 4500 4000 3500

Figure WS-4: Seismic line NARE-8517 across the prograding Crary Trough Mouth Fan (modified from Kuvaas and Kristoffersen, 1991). Profile location is in Fig. WS-1.

calibration of the acoustic section (Miller et al., 1990), but regional extrapolations are inhibited along-slope by numerous canyons, and are inhibited down-slope by the steep Explora Escarpment (Fig. WS-1). The Crary Fan, a regional feature at the mouth of the Crary Trough, is associated with large channel/levee complexes, which extend up to 1,000 km to the north into the basin (Figs. WS-1, WS-4 and Foldout WS-1). Initial fan evolution is correlated with the resumption of sediment deposition above an Albian-early Oligocene hiatus at ODP Site 693 (Reflector W4).

Sediment drifts are common within the Neogene stratigraphic interval along the continental slope (Fig. WS-1) in the western Weddell Sea (Michels et al., 2001; Maldonado et al., 2005).

5.5.3. The Weddell Sea Pre-Ice-Sheet Depositional Environment

Acoustic stratigraphic information on the shelf is limited to subbottom depths comparable to the local water depth due to severe multiple reflections (e.g. Fig. WS-3). The pre-ice-sheet Cenozoic shelf edge was more than 10 km landward of its current position along the Dronning Maud Land continental margin (Kristoffersen et al., 2000), and 70 km to the south (Fig. WS-4) in the southern Weddell Sea (Kuvaas and Kristoffersen, 1991). The shoreward shift in the western Weddell Sea is unknown. The deeper strata below the continental slope (i.e. below W4) appear unstructured throughout. The older sediments are thickest (5-8 km) below the Larsen Shelf in the western Weddell Sea (Rogenhagen and Jokat, 2000), and may be up to 15 km thick along the front of the Ronne and Filchner Ice shelves (Leitchenkov and Kudryavtzev, 2000). In the central Weddell Sea Basin, the pre-Oligocene section of inferred turbidites is more than 1 km thick, and thins by basal onlap towards the margins to less than 0.5km (Rogenhagen et al., 2004). High seasonal variations in sea-surface temperatures and a well-developed seasonal thermocline characterized the early Paleogene Weddell Sea (Kennett and Barker, 1990). On Maud Rise, siliceous biogenic facies began to replace carbonate facies during the latest Eocene-earliest Oligocene (Kennett and Barker, 1990). A possible early Cenozoic seaway between East and West Antarctica could have been up to 700 m deep, and may have persisted into the Oligocene if no WAIS was present (Lawver and Gahagan,

2003). At ODP Site 693 on the middle continental slope, middle lower Oligocene and younger glacial sediments are separated by a hiatus from Albian radiolarian diatomite and claystones (i.e. Reflector W4). The unconformity may represent non-deposition and/or mild erosion (Kennett and Barker, 1990).

5.5.4. Change from Non-Glacial to Glacial Conditions

Sediment fluxes on high latitude continental margins are closely connected to climate extremes. In the Weddell Sea, environmental change is manifested by a basin-wide change in acoustic character within the sedimentary section (Reflector W4) at about 1 s TWT below the sea bed (Rogenhagen et al.,

2004). Younger deposits in the basin have finely laminated continuous acoustic stratification, and geometries on the slope are in the form of channel/levee complexes over a wide range of spatial scales. The change in depositional environment is interpreted to have originated from an increased sediment flux, caused by increased erosion of the continent and increased down-slope transport. At ODP Site 693 on the middle continental slope, the acoustic change correlates stratigraphically with resumed preservation of lower Oligocene sediments. The deposits include rounded dropstones in lower Oligocene (32-33 Ma) diatom muds, a signal of the first presence of glaciers on the adjacent parts of East Antarctica (Kennett and Barker, 1990). Subsequent early Miocene sedimentation rates at this site were low (7m/Ma). A more dramatic change in sediment flux to the margin is documented by a threefold increase in sedimentation rate (to 24m/m.y.), when sedimentation resumed following a hiatus that spanned the middle Miocene. Increased sediment input is related to expansion of ice on the East Antarctic continent. The hiatus at ODP Site 693 correlates with a regional acoustic reflection event (W5) identified below the continental slope and rise along the entire Weddell Sea margin (Rogenhagen et al., 2004). Shelf progradation accelerated dramatically along the eastern and western margins of the Weddell Sea (Fig. WS-3), with grounded ice extending to the shelf edge in the late Miocene (Michels et al., 2001; Michels et al., 2002). A range of contourite drifts formed on the slope and rise in the northwestern Weddell Sea (Michels et al., 2001; Maldonado et al., 2005). Sedimentation rates at ODP Site 693 reached 60m/m.y. in the early Pliocene, and subsequent Quaternary sedimentation rates were reduced to 16m/m.y. (Gersonde et al., 1990). Sediment input to the margin in the southeastern Weddell Sea was focused toward a trough mouth fan. The Crary Fan began to expand at the time of change to a glacial environment (above Reflector W4, Fig. WS-4 and Foldout WS-1), and major channel/levee complexes evolved in three phases. The last of these three phases (Reflector W5, Fig. WS-4 and Foldout WS-1) was from the late Miocene on (Kuvaas and Kristoffersen, 1991; Moons et al., 1992).

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