The Glacial Interglacial Environment

The change from a glacial to an interglacial environment was associated with major changes in sediment flux. Average sediment deposition on the eastern Weddell Sea margin (10°W) during the last two climatic cycles (300 k.y.) varies from 5 cm/k.y. on the upper slope to over 1 cm/k.y. on the lower slope (Grobe and Mackensen, 1992). Sedimentation was most rapid during the beginning of interglacials, with rates on the middle slope four to five times higher than during glacials. We note, however, that the grounded EAIS only reached the mid-shelf in this area during the LGM (Kristoffersen et al., 2000). Sediment input in the southern Weddell Sea was focused in the Crary Trough Mouth Fan (Figs. WS-1, WS-4 and Foldout WS-1). The fan comprises large channel-levees on the flanks of deep-water channels, such as the Cold Water Channel and the Deutschland Channel (Foldout WS-1). Grounded ice reached the shelf edge at the trough mouth during the last glaciation (Bentley and Anderson, 1998), and deposition on the levees (in water depths of 2,000-3,000 m) ranged from 100-200 cm/k.y. during the LGM to a few cm/k.y. during the present interglacial (Weber et al., 1994). Episodic sediment transport into the basin also occurred by mass flows during interglacials, probably as partial collapse of the deposits on the upper continental slope. A 90-m-thick sandy turbidite unit was deposited within 0.5 m.y. during the early Gilbert Chron (4.8 Ma) at ODP Site 694 (Fig. WS-1), and may be the distal expression of mass wasting events on the continental slope in the southwestern Weddell Sea (Shipboard Scientific Party, 1988). Also, major early Pliocene drawdown of East Antarctic ice is postulated to have triggered extensive mass flows that originated from the Crary Trough Mouth Fan (Bart et al., 1999).

In the western Weddell Sea, upper Miocene and younger sediments (above Reflector W5) are mostly drift deposits that reach a thickness of more than 1 km below the middle slope, seaward of the Larsen Shelf (Rogenhagen and Jokat, 2000; Michels et al., 2001; Maldonado et al., 2005). Present and past bottom currents circulated in nearly the opposite direction to channel transport, and cross-channel flow was in the same direction as the Coriolis force acting on down-slope turbidity currents in the southern Weddell Sea. Sediments scavenged from turbid channel flow by cross-channel bottom currents sourced the benthic boundary layer and enhanced formation of sediment drifts along the western and northern Weddell Basin. The actual drift distribution was mainly controlled by the physiography of the basin and bottom current flow directions (Maldonado et al., 2005). These drifts represent a storehouse of paleoceanographic and climatic proxies not yet sampled by scientific drilling.

0 0

Post a comment