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doi:10.2204/iodp.pr.318.2010

Background

Physiographic and geologic setting

The Adélie and George V Coasts of the eastern Wilkes Land margin drain the EAIS with a mostly divergent flow pattern (Figs. F4, F5). Ice cliffs and two prominent outlet glaciers, the Mertz and the Ninnis, characterize the present coastline. These outlet glaciers extend seaward as ice tongues and have an important role in ice drainage and sediment delivery to the ocean (Anderson et al., 1980; Drewry and Cooper, 1981). Drainage velocities in outlet glaciers range from >0.5 to ~3.7 km/y (Fig. F4) (Lindstrom and Tyler, 1984; MacDonald et al., 1989), whereas drainage in the areas between outlet glaciers, occupied by ice cliffs and tidewater glaciers, may range from a few meters to tens of meters every year (Anderson, 1999). In the eastern Wilkes Land margin the continental shelf has an average width of 125 km and an average water depth of 450–500 meters below sea level (mbsl) (Figs. F6, F7). As commonly observed around Antarctica, the shelf exhibits an overdeepened and landward-sloping bathymetric profile that is caused principally by glacial erosion and flexural loading by grounded ice (Ten Brink and Cooper, 1992). The topography is irregular, with depths from 200 mbsl on outer shelf banks to 1000 mbsl in shelf troughs. The troughs, which shoal as they traverse from inner shelf (>1000 mbsl) to outer shelf (500 mbsl), were formed by ice streams extending across the shelf during times of glacial maxima. The adjacent outer shelf banks are areas that have been bypassed by the most recent ice streams and where grounded ice has been relatively immobile or stagnant. The continental slope is steep (1:9–1:30) and narrow (15 km average, but as wide as 25 km) and extends from the shelf break to ~2000–2500 mbsl. The continental rise is also steep, with average gradients between 1:100 and 1:150.

The eastern Wilkes Land Continental Margin, conjugate to the southern Australian margin, formed during the Late Cretaceous separation of Australia and Antarctica (Cande and Mutter, 1982; Sayers et al., 2001; Veevers, 1987; Colwell et al., 2006; O'Brien and Stagg, 2007; Leitchenkov et al., 2007; Close et al., 2009). Slow seafloor spreading began between Australia and Antarctica in the Late Cretaceous, with the precise timing of onset still under debate (~83 Ma, according to Sayers et al., 2001). At ~50 Ma (middle Eocene), rapid seafloor spreading commenced in the Australian-Antarctic Basin (AAB) as Australia continued to move northwards from Antarctica (Cande and Mutter, 1982; Close et al., 2009), with the final clearance of the Australian and Antarctic plates southwest of the South Tasman Rise taking place by the middle late Eocene (~35.5 Ma) (Stickley et al., 2004). This resulted in the establishment of more open marine conditions and eventually the development of the Antarctic Circumpolar Current (ACC) in the Miocene. The continental crust underlying the thick postrift and synrift sediments in the eastern Wilkes Land/Terre Adélie margin extends more than 400 km seaward of the shelf break (Close et al., 2009). This seaward salient, named the Adélie Rift Block (ARB), is associated with anomalously shallow bathymetry, an atypical continental margin free-air gravity edge-effect anomaly, and an absence of magnetic anomalies related to seafloor spreading. Close et al. (2009) interpret this salient, which is underlain by stretched continental crust and pre- and synrift sediment, as a deeply subsided marginal plateau.

The stratigraphy of the eastern Wilkes Land margin is known mainly from the seismic stratigraphic analyses of numerous multichannel seismic (MCS) reflection surveys in the eastern Wilkes Land margin (Sato et al., 1984; Wannesson et al., 1985; Tsumuraya et al., 1985; Eittreim and Hampton, 1987; Ishihara et al., 1996; Tanahashi et al., 1997; Brancolini et al., 2000; Stagg et al., 2004) complemented by gravity and piston sediment cores (Payne and Conolly, 1972; Hampton et al., 1987; Domack et al., 1980; Domack, 1982; Tsumuraya et al., 1985; Ishihara et al., 1996; Tanahashi et al., 1997; Brancolini et al., 2000; Escutia et al., 2003; Michel et al., 2006; Leventer et al., 2006), dredging (Mawson 1940, 1942; Domack et al., 1980; Sato et al., 1984; Leventer et al., 2001), and limited deep geological sampling recovery at DSDP Site 269 (Hayes, Frakes, et al., 1975).

Pre–ice sheet stratigraphy

Presumed pre-Oligocene synrift strata are as thick as ~3 km and are highly variable in seismic character, with discontinuous, faulted, and tilted strata onlapping the flanks of the acoustic basement (Eittreim and Smith, 1987; Eittreim, 1994; De Santis et al., 2003; Stagg et al., 2004; Leitchenkov et al., 2007; Close et al., 2009). Postrift strata across the eastern Wilkes Land margin may be as thick as 5 km, well layered on the continental rise, and less stratified and discontinuous landward (Eittreim and Smith, 1987; Wannesson, 1990; Tanahashi et al., 1994; Eittreim, 1994; De Santis et al., 2003). On the continental shelf, a prominent regional unconformity (WL-U3) within the Cenozoic postrift section (Fig. F8) has been interpreted to record erosional processes related to the first advance of grounded ice sheets onto the continental shelf (Eittreim and Smith, 1987; Tanahashi et al., 1994; Eittreim et al., 1995; Escutia et al., 1997).

Prior to Expedition 318, the only pre–ice sheet strata sampled from this margin were a series of dredges from the inner continental shelf and slope. Erosion by late Cenozoic glaciers near the Mertz ice tongue exposed Mesozoic sediments at the seafloor, which allowed recovery of lignite (Mawson, 1940, 1942) and Lower Cretaceous brecciated carbonaceous siltstone (Domack et al., 1980). Dredges collected in the area by Leventer et al. (2001) also recovered Paleogene lignites with reworked Early Cretaceous flora. Dredging of the upper continental slope off Terre Adélie, sampled Oligocene and Miocene limestones and undated sedimentary, metamorphic, and igneous rocks of mostly ice-rafted origin (Sato et al., 1984).

Continental margin glacial stratigraphy

Glacial sequences on the shelf thicken seaward in prograding wedges (Fig. F8). The sequences are deeply eroded by broad troughs that cross the shelf. Foreset strata are commonly truncated at or near the seafloor beneath the troughs. Topset strata form the banks adjacent to the troughs.

Two main unconformities of regional character, Unconformities WL-U3 and WL-U8, are identified as truncating the glacial seismic sequences on the shelf (Wannesson et al., 1985; Eittreim and Smith, 1987; Hampton et al., 1987; Escutia et al., 1997; De Santis et al., 2003) (Fig. F8). The erosional events represented by these unconformities are interpreted to result from grounded ice sheets moving across the continental shelf (Tanahashi et al., 1994; Eittreim et al., 1995, Escutia et al., 1997). Eittreim et al. (1995) calculated an erosion of 300–600 m of strata below Unconformity WL-U3 and of 350–700 m of strata below Unconformity WL-U8. Unconformity WL-U3 marks the start of progradation in this sector of the Wilkes Land margin. Across Unconformity WL-U8, a change in the geometry of the outer shelf progradational wedge, from shallower dips below Unconformity WL-U8 to steeper dips above (foreset slopes as much as ~10°), can be recognized.

Seismic units have been correlated from shelf to rise and abyssal plain by tracing and projecting unconformities and seismic units. Seismic units above Unconformity WL-U8 downlap and pinch out at the base of the continental slope, but deeper units (i.e., between Unconformities WL-U8 and WL-U3) continue across the margin (Hampton et al., 1987; Eittreim et al., 1995; Escutia et al., 1997; De Santis et al., 2003) (Fig. F8). The principal marker is Unconformity WL-U3, which in Wilkes Land can be traced from the shelf, where it marks the onset of progradation on the Wilkes Land margin (Eittreim and Smith, 1987), to the rise, where it correlates with an upsection increase in turbidite and contourite deposition (Escutia et al., 1997, 2000; Donda et al., 2003) (Fig. F9).

On the eastern Wilkes Land continental rise, strata above Unconformity WL-U3 include six glacial-related seismic units (WL-S4–WL-S9) (De Santis et al., 2003; Donda et al., 2003) (Fig. F9). The two deepest units, WL-S4 and WL-S5, consist of stratified and continuous reflectors that onlap at the base of the slope (Escutia et al., 1997; Donda et al., 2003). Acoustic signatures of isolated channel-levee complexes that characterize turbidite deposition are first observed during deposition of Unit WL-S5 (Escutia et al., 1997; Donda et al., 2003). Channel-levee complexes became widespread during deposition of Units WL-S6 and WL-S7, and turbidity flows were likely the dominant process building the large sedimentary ridges on the rise. Wavy reflectors that are characteristic of bottom contour-current deposition occur on the lower rise in Unit WL-S6 and on the upper rise in Unit WL-S7. Unit WL-S8 mostly fills previous depressions, although there is evidence for bottom contour-current and turbidite flows (Escutia et al., 2002; Donda et al., 2003). Unit WL-S9 is a discontinuous unit on the rise and, where present, is composed of channel and levee complexes and layered reflectors (Donda et al., 2003).

During the interglacial open-marine Holocene, thick laminated diatom mud and oozes were deposited in deep (>1000 mbsl) inner shelf basins, including the Adélie Drift (Costa et al., 2007) (Figs. F7, F10, F11, F12). Based on accelerator mass spectrometry radiocarbon dates on a 50 m long sediment core, this drift has unusually high accumulation rates, as much as 20–21 m/k.y. Opal, Ti, and Ba time series show decadal to century variance suggestive of solar forcing and El Niño Southern Oscillation (ENSO) forcing (Crosta et al., 2005; Denis et al. 2006; Leventer et al., 2006; Maddison et al., 2006; Costa et al., 2007).

Previous drilling on the Wilkes Land margin

Site 269 was drilled during Leg 28 on the eastern Wilkes Land abyssal plain to determine the geologic and climate history of Antarctica and the Southern Ocean (Hayes, Frakes, et al., 1975). Site 269 is north of the Expedition 318 drill area and was intermittently cored to 958 meters below seafloor (mbsf) in a water depth of 4285 mbsl, with 42% recovery of (?)Eocene–Holocene rocks (Hayes, Frakes, et al., 1975). The cored sections consist predominantly of silts and clays with variable amounts of microfossils. Diatom oozes and diatom mud dominate the upper half of the section, which is dated as Quaternary to late Miocene in age (Hayes, Frakes, et al., 1975). In the lower half, which is late(?) Eocene to late Miocene in age, diatoms are absent but calcareous nannofossils are found in trace amounts, with abundant palynomorphs including dinoflagellate cysts and sporomorphs (Kemp et al., 1975). There is a transition in facies from more distal facies in the lower part of the hole to more proximal facies near the surface. Piper and Brisco (1975) interpret this facies change as a result of substantially increased supply of sand, coarse silt, and clay from the Antarctic continent, possibly in response to progradation of the continental margin. The cores document extensive Antarctic glaciation beginning at least by Oligocene to early Miocene times and indicate that water temperatures were cool to temperate in the late Oligocene and early Miocene then cooled during the Neogene, presumably as glaciation intensified.

Inferred long-term record of glaciation

Unconformity WL-U3 is interpreted to mark the first preserved grounding of an ice sheet across Wilkes Land, eroding the continental shelf (Tanahashi et al., 1994; Eittreim et al., 1995; Escutia et al., 1997, 2005), ~40 m.y. ago (Eittreim et al., 1995) to 33.5 to 30 Ma (Escutia et al., 2005) (Fig. F13). Early glacial strata (e.g., likely glacial outwash deposits) above Unconformity WL-U3 were delivered by fluctuating temperate glaciers and deposited as low dip–angle prograding foresets. The increase in stratal dips across Unconformity WL-U8 in the prograding wedge at the shelf edge is interpreted to record a change in the glacial regime inferred to correspond with the transition from intermittent fluctuating glaciers to persistent oscillatory ice sheets during the late Miocene–early Pliocene (Escutia et al., 2005), or ~3 Ma (Rebesco et al., 2006) (Fig. F13). The steep foresets above Unconformity WL-U8 likely consist of ice proximal (i.e., water-lain till and debris flows) and open-water sediments deposited as grounded ice sheets extended intermittently onto the outer shelf, similar to sediments recovered from ODP Site 1167 on the Prydz Trough fan (O'Brien, Cooper, Richter, et al., 2001; Passchier et al., 2003).

On the continental rise, the upsection response to shelf progradation (i.e., seismic facies indicative of distal turbidites to large channel-levee systems modified by bottom contour-current deposition) likely resulted from enhanced shelf progradation. Maximum rates of sediment delivery to the rise are reported during the development of seismic Units WL-S6 and WL-S7, interpreted to be of Miocene age (Escutia et al., 1997, 2000, 2005; De Santis et al., 2003) (Fig. F13). During deposition of seismic Units WL-S8 and WL-S9, sediment supply to the lower continental rise decreased and depocenters shifted landward to the base of the slope and outer shelf (Escutia et al., 2002, 2005; De Santis et al., 2003; Donda et al., 2003). Inferred age for Units WL-S8 and WL-S9 is Pliocene to Holocene (De Santis et al., 2003). Unit WL-S9 is inferred to be deposited under a polar regime with a persistent ice sheet during the Pliocene–Pleistocene. At that time, most sediment delivered to the margin was trapped on the outer shelf and slope, forming steep prograding wedges, with some sediment bypassing the slope in channelized turbidity currents (Escutia et al., 2002, 2005; De Santis, 2003) (Fig. F13).

During the Holocene, thick open-water interglacial sections of diatom mud and oozes are deposited in deep inner shelf basins (Domack, 1982; Dunbar et al., 1985; Crosta et al., 2005; Denis et al., 2006; Leventer et al., 2006; Maddison et al., 2006; Costa et al., 2007). These sediments hold an ultrahigh resolution record of climate variability and provide a means of tracking interannual- to centennial-scale variability in the response of the ocean to forcing by solar processes, ENSO, and the Southern Annular Mode (SAM).

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