Late Miocene Early Pliocene Ice Sheet Fluctuations

Long-term variation in the marine oxygen isotopic data (e.g. Hodell and Venz, 1992; Kennett and Hodell, 1995; Zachos et al., 2001) are suggestive of a warming in the earliest Pliocene, culminating in the mid-Pliocene Climatic Optimum at ~ 3 Ma. However, the oxygen isotope ratios do not allow major changes in ice volume on Antarctica. Obliquity-driven oscillations (41 kyr) in 818O of up to 0.6m, which dominate the Pliocene period, while capable of producing ~ 20-30 m sea-level changes (depending on the temperature contribution to the 818O signal), are also considered insufficient to cause large-scale deglaciation of Antarctica without a significant increase in the temperature of the deep ocean.

Marine sedimentary deposits exposed on land and on the continental shelf, slope and rise show evidence of dynamic ice-sheet behaviour in the Late Miocene-Early Pliocene and Early Pliocene warming. At Marine Plain in the Vestvold Hills, East Antarctica, a ca. 8 m thick early Pliocene sequence of diatomaceous sands and silts is exposed (Pickard et al., 1988). During deposition of the diatomaceous sediments, the ice margin must have been ca. 50 km further inland and no floating ice was covering the site. The presence of Cetacean skeletons, including dolphins and a right whale, and preliminary isotope measurements led Quilty (1993) to infer warmer conditions during deposition than currently prevail. Water temperatures may have been as high as 5°C. A marked increase in sea-ice indicator diatoms and a coarsening upward of the sediments in the upper 3 m of the deposit indicate cooling conditions and glacial expansion.

The sediments at ODP Sites 739, 742 and 1166 in Prydz Bay record evidence of repeated advances and retreats of the Lambert ice stream across the shelf in the Late Miocene through Early Pliocene (Hambrey et al., 1991; Passchier et al., 2003; O'Brien et al., 2004). During repeated advances of the Lambert ice stream to the shelf break glacigenic debris flows build up a trough-mouth fan on the continental slope.

It is worth mentioning that the IRD signal inferred from the >250-mm size fraction at Site 1165, in a sediment drift on the continental rise off Prydz Bay, suggests warmer conditions in the Early Pliocene

(Griitzner et al., 2005). The low sea-ice concentrations derived from diatom assemblages (Whitehead et al., 2005) and the warm-water periods inferred from high abundances of silicoflagellates (Dictyocha) at Site 1165 (Whitehead and Bohaty, 2003) are consistent with isotopic estimates of warmer conditions than present during the Early Pliocene (Hodell and Venz, 1992). In addition, three periods of enhanced accumulations of sediment originating from East Antarctica are recognized in the Late Miocene-Pliocene record of the East Kerguelen Ridge sediment drift, in the Indian Ocean to the north of Prydz Bay (Joseph et al., 2002). The sediment pulses are interpreted as periodically warmer, less stable, ice-sheet conditions during this time.

DVDP-10, 11 and 15, MSSTS-1 and CIROS-2 drill-holes in the Ross Sea Region have recovered partial late Neogene records that suggest a dynamic Plio-Pleistocene history of glacial advance and retreat in the Taylor and Ferrar valleys. Of these, a 166 m thick Plio-Pleistocene sequence of diatom-bearing glaciomarine strata was cored in Ferrar Fiord by the CIROS-2 drilling campaign (163°31'E; 77°41'S) (Barrett et al., 1992; Barrett and Hambrey, 1992). The Pliocene sequence is ~62m thick and is composed of diamictite dominated by basement lithologies interbedded with thin mudstone intervals, indicating several early Pliocene advances and retreats of ice through the Transantarctic Mountains. Further evidence for Pliocene Antarctic ice volume fluctuation is recorded by glaciomarine strata from the DVDP-10 and 11 drill-holes (163°32'E; 77°35'S) (McGinnis, 1981; Ishman and Reick, 1992; Wilson, 1993). Combined foraminiferal and magnetostratigraphic analyses indicate highly fluctuating palaeoenvironmental conditions in the Late Miocene-Early Pliocene with less ice-influenced conditions in the early Pliocene (Ishman and Reick, 1992).

Late Neogene trough-mouth fans have been identified in seismic data on the Ross Sea and Weddell Sea continental slopes. The Crary Fan on the Weddell Sea margin was formed by glacial sediment supply from Dronning Maud Land and ice drainage from East Antarctica through the Weddell Sea (Bart et al., 1999), whereas the Ross Sea fan was fed by ice draining through the Northern basin (Bart et al., 2000). In contrast, the geometry of Late Neogene sequences on the Wilkes Land margin is characterized as steep prograding foresets and trough-mouth fans that are less developed (Escutia et al., 2005). However, the close proximity to the deep subglacial Wilkes Land basin results in ice grounding below sea level, which may have contributed to ice-sheet behaviour that is different from the other portions of the East Antarctic margin. Interpretations of the seismic data record a dynamic Late Neogene EAIS with large expansions and contractions for the Ross Sea and Weddell Sea portions of the East Antarctic margin (Bart et al., 1999, 2000).

0 0

Post a comment