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West Antarctic Ice Sheet Grounding Events on the Ross Sea Outer Continental Shelf During the Middle Miocene

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West Antarctic Ice Sheet Grounding Events on the Ross Sea Outer Continental Shelf During the Middle Miocene Palaeogeography, Palaeoclimatology, Palaeoecology 198 (2003) 169^186 www.elsevier.com/locate/palaeo West Antarctic Ice Sheet grounding events on the Ross Sea outer continental shelf during the middle Miocene Juan M. Chow Ã, Philip J. Bart Louisiana State University, Department of Geology and Geophysics, Howe-Russell Geoscience Complex E235, Baton Rouge, LA 70803, USA Received 14 May 2002; accepted 3 March 2003 Abstract New seismic^stratigraphic analysis of the Ross Sea continental shelf suggests that there were a at least five shelf- wide grounding events of the West Antarctic Ice Sheet (WAIS) during the middle Miocene. Although the number of WAIS grounding events generally matches the number of extreme N18O enrichments and eustatic lowstands, these results do not support the long-standing assumption that West Antarctica was substantially ice-free. Instead, seismic^ stratigraphic evidence from the Ross Sea shelf documents waxing and waning of a well-developed WAIS in the marine environment at least on the Pacific sector of the West Antarctic continental shelf. ß 2003 Elsevier B.V. All rights reserved. Keywords: middle Miocene; ice sheet; West Antarctica; glaciation; Ross Sea 1. Introduction eration of TISW in the Southern Ocean probably was a major agent in the meridional heat trans- During the early Miocene (V24 Ma to V16 port that helped maintain relatively warm cli- Ma), thermohaline circulation was much di¡erent mates in Antarctica during the early Miocene than today because low-latitude inter-oceanic pas- (Woodru¡ and Savin, 1989; Wright et al., 1992; sages (i.e. through the Isthmus of Panama and Flower and Kennett, 1995). Tethys) permitted well-developed equatorial circu- At the start of the middle Miocene (i.e. V16 lation. This equatorial surface £ow produced Ma), closure of the Tethys at the eastern portal of warm saline water masses (i.e. Tethyian Indian the Mediterranean Sea severely interrupted equa- Saline Water; TISW) that sank to intermediate torial circulation (Hsu and Bernoulli, 1978; Stei- depths and £owed southward (Woodru¡ and Sa- ninger et al., 1985) and resulted in reduced pro- vin, 1989). The upwelling and subsequent refrig- duction of TISW (Woodru¡ and Savin, 1989). According to conventional interpretations of deep-sea proxy data, the reduced meridional heat transport led to climatic cooling in the Southern Ocean which thereafter fostered rapid * Corresponding author. Fax: +1-225-578-2302. growth of the Antarctic Ice Sheet. Indeed, the E-mail address: [email protected] (J.M. Chow). large magnitudes of N18O enrichments (Shackleton 0031-0182 / 03 / $ ^ see front matter ß 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0031-0182(03)00400-0 PALAEO 3123 21-8-03 170 J.M. Chow, P.J. Bart / Palaeogeography, Palaeoclimatology, Palaeoecology 198 (2003) 169^186 and Kennett, 1975; Woodru¡ et al., 1981; Wood- full-bodied marine-based West Antarctic Ice Sheet ru¡ and Savin, 1989, 1991; Wright et al., 1992) (WAIS) existed during middle Miocene glacial pe- and eustatic falls (Haq et al., 1987) in the middle riods, because by analogy with the modern ice- Miocene (V16MatoV10 Ma) suggest signi¢- sheet drainage pattern (see Fig. 1 insert), West cant long-term cooling and several stepped expan- Antarctica constitutes a major part of drainage sions of ice volume on Antarctica. Primarily on basin for the Ross Sea continental shelf. If cor- the basis of these deep-sea proxy and eustatic re- rect, ice-volume £uctuations on West Antarctica cords, it is generally assumed that: (1) middle may have played a larger role on middle Miocene Miocene ice-volume increases were associated climatic and eustatic change than previously with the large land-based East Antarctic Ice Sheet thought. (EAIS) attaining physical dimensions similar to De Santis et al. (1995) also proposed that there that existing on East Antarctica today (Savin et were multiple Ross Sea grounding events, but al., 1975; Woodru¡ et al., 1981; Wright et al., they argued that the extent of grounded ice was 1992); and (2) West Antarctica, a much smaller relatively limited, covering only subaerial banks block of low-lying continental crust and volcanic on the outer continental shelf. They concluded highlands, remained substantially ice-free until the that submarine areas of West Antarctica were latest Miocene (Kennett and Barker, 1990). ice-free even during the peak of middle Miocene Although the view of an ice-free West Antarc- glacials and that the WAIS was not fully devel- tica during the middle Miocene is widely ac- oped until the end of the late Miocene, when ice cepted, eustatic and deep-sea proxy records do caps coalesced across the sediment-¢lled marine not uniquely de¢ne the speci¢c locations on Ant- basins between the subaerial banks (De Santis et arctica where the ice-volume £uctuations oc- al., 1995, 1999). This view of small West Antarctic curred. Moreover, direct geologic evidence sug- ice caps during the middle Miocene and a latest gests that ice cover on West Antarctica may Miocene transition to extensive ice cover generally have been more extensive than has traditionally is consistent with the theme of conventional inter- been deduced from proxy data. For example, ra- pretations of deep-sea proxy data, i.e. that West diometrically dated hyaloclastites (subglacially Antarctic glaciation was not signi¢cant until the erupted volcanics) require that thick ice-covered latest Miocene. subaerial highlands of West Antarctica at Marie These di¡erent views on the evolution and ex- Byrd Land (Fig. 1, inset) existed during this time tent of Antarctic ice volume during the middle frame (LeMasurier and Rex, 1982). In the marine Miocene are incompatible. Ultimately, before we environment, detailed seismic^stratigraphic stud- can fully resolve the global-scale question con- ies of the Ross Sea outer continental shelf dem- cerning the meaning and cause of the dramatic onstrate that grounded ice (ice in contact with the climatic and eustatic shifts deduced from middle sea£oor) existed during several intervals in the Miocene proxy records, it is necessary in the ¢rst middle Miocene (Anderson and Bartek, 1992; place to redress the local debates and precisely De Santis et al., 1995). The exact number of de¢ne ice-sheet extent on West Antarctica. In grounding events has not been determined and this study, we used single-channel seismic data debate still exists concerning the extent of that were not available at the time of the An- grounded ice on the Ross Sea continental shelf derson and Bartek (1992) and De Santis et al. during the middle Miocene. Anderson and Bartek (1995) studies to evaluate two speci¢c questions (1992) argued that the middle Miocene grounding with respect to the nature of middle Miocene events on the Ross Sea continental shelf (i.e. lat- ice-volume changes on the Ross Sea outer conti- eral advances of grounded ice into the marine nental shelf (i.e. West Antarctica). (1) Were Ross environment) were shelf-wide (i.e. the entire Sea grounding events shelf-wide or restricted to continental shelf was covered by ice in contact local topographic highs? and (2) How many with the sea£oor). This line of reasoning sug- grounding events occurred during the middle gested to Anderson and Bartek (1992) that a Miocene? PALAEO 3123 21-8-03 J.M. Chow, P.J. Bart / Palaeogeography, Palaeoclimatology, Palaeoecology 198 (2003) 169^186 171 Fig. 1. Ross Sea continental shelf, showing seismic traces used in this study. Thicker lines correspond to segments of pro¢les shown in Figs. 2, 3 and 5. Present-day banks are highlighted in grey. The inset shows the location of hyaloclastites on Marie Byrd Land, Roosevelt, Crary, and Berkner Islands as well as the modern ice-£ow patterns. CT = Crary Trough. 2. Materials and methods Miocene section was performed by hand on paper copies of seismic sections plotted at a vertical The middle Miocene section was identi¢ed from scale of 33:1. Three single-channel seismic surveys Deep Sea Drilling Project (DSDP) sites 273 and were used in this study: PD90, NBP94, and 272 (Hayes and Frakes, 1975; Savage and Ciesiel- NBP95 (Fig. 1). Data set PD90 consists of nearly ski, 1983). From these drill sites, the top and base 6000 km of seismic data acquired aboard the R/V of the middle Miocene were directly correlated to Polar Duke in the 1990 austral summer using a seismic data (Table 1; Figs. 2 and 3) using the 150-in3 generator^injector airgun. Data sets time^depth conversions from the ANTOSTRAT NBP94 and NBP95 were acquired aboard the R/ Project (1995). Regional correlation of the middle VIB Nathaniel B. Palmer in 1994 and 1995 ¢eld PALAEO 3123 21-8-03 172 J.M. Chow, P.J. Bart / Palaeogeography, Palaeoclimatology, Palaeoecology 198 (2003) 169^186 seasons, respectively, and include more than 7000 longitude in radians, B = degree latitude in radi- km of single-channel data acquired with a 50-in3 ans. Sediment volume within the grid cell was generator^injector airgun. determined with the following formula: volu- To evaluate whether grounding events were me = (AUf)T, where A = area of grid cell; f = frac- shelf-wide or were restricted to bank crests, we tion of grid cell area within which middle Mio- conducted three seismic-based experiments. If cene section is found; T = estimated average the extent of grounded ice was restricted to topo- thickness of middle Miocene section within the graphic highs on the Ross Sea shelf, then the mid- grid cell. The volume from each grid cell was dle Miocene section should have been primarily added to obtain the volume for the entire middle derived from the banks. In the ¢rst experiment, Miocene section. we divided the total volume of the middle Mio- In addition, we calculated the volumes of the cene strata by the area of the topographic highs to middle Miocene section using the RSS-5 isopach estimate a minimum reconstructed height of the (pl.
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