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Cooper, A. K., P. J. Barrett, H. Stagg, B. Storey, E. Stump, W. Wise, and the 10th ISAES editorial team, eds. (2008). Antarctica: A Keystone in a Changing World. Proceedings of the 10th International Symposium on Antarctic Earth Sciences. Washington, DC: The National Academies Press. Late Cenozoic Climate History of the Ross Embayment from the AND-1B Drill Hole: Culmination of Three Decades of Antarctic Margin Drilling T. R. Naish,1,2 R. D. Powell,3 P. J. Barrett,1 R. H. Levy,4 S. Henrys,1 G. S. Wilson,5 L. A. Krissek,6 F. Niessen,7 M. Pompilio,8 J. Ross,9 R. Scherer,3 F. Talarico,10 A. Pyne,1 and the ANDRILL-MIS Science team11 ABSTRACT Cenozoic is poorly known and dated. The latest Antarctic geological drilling program, ANDRILL, successfully cored Because of the paucity of exposed rock, the direct physical a 1285-m-long record of climate history spanning the last 13 record of Antarctic Cenozoic glacial history has become m.y. from subsea-ï¬oor sediment beneath the McMurdo Ice known only recently and then largely from offshore shelf Shelf (MIS), using drilling systems specially developed for basins through seismic surveys and drilling. The number operating through ice shelves. The cores provide the most of holes on the continental shelf has been small and largely complete Antarctic record to date of ice-sheet and climate conï¬ned to three areas (McMurdo Sound, Prydz Bay, and ï¬uctuations for this period of Earthâs history. The >60 cycles Antarctic Peninsula), but even in McMurdo Sound, where of advance and retreat of the grounded ice margin preserved Oligocene and early Miocene strata are well cored, the late in the AND-1B record the evolution of the Antarctic ice sheet since a profound global cooling step in deep-sea oxygen isotope records ~14 m.y.a. A feature of particular interest is a ~90-m-thick interval of diatomite deposited during the warm Pliocene and representing an extended period (~200,000 1 Antarctic Research Centre, Victoria University of Wellington, Wel- years) of locally open water, high phytoplankton productiv- lington, New Zealand (email@example.com, firstname.lastname@example.org, alex. email@example.com). ity, and retreat of the glaciers on land. 2 Geological and Nuclear Sciences, Lower Hutt, New Zealand (t.naish@ gns.cri.nz, firstname.lastname@example.org). 3 Department of Geology and Environmental Geosciences, Northern HISTORICAL OVERVIEW Illinois University, DeKalb, IL, USA (email@example.com). 4 The remarkable late Cenozoic record of glacial history in the ANDRILL Science Management Office, University of Nebraska- Lincoln, 126 Bessey Hall, Lincoln, NE 68588-0341, USA (rlevy2@unl. Ross Embayment recovered in late 2006 by the ANDRILL- edu). MIS Project is the culmination of work begun over three 5 Department of Geology, University of Otago, PO Box 56, Dunedin, New decades ago to document and understand the more recent Zealand (firstname.lastname@example.org). glacial history of Antarctica by drilling close to the margin. 6 Department of Geosciences, The Ohio State University, Columbus, OH, Ironically, although the last 3 million years of Earthâs climate USA (email@example.com). 7 Department of Marine Geophysics, Alfred Wegener Institute, Postfach is often said to be the best studied interval of the Cenozoic, 12 01 61, Columbusstrasse, D-27515, Bremerhaven, Germany (fniessen@ the contribution of Antarctic ice volume changes is the most awi-bremerhaven.de). poorly understood. Until this most recent hole was drilled, 8 Istituto Nazionale di Geoï¬sica e Vulcanologia, Via della Faggiola, 32, the middle Cenozoic record was better known from several 9 I-56126 Pisa, Italy (firstname.lastname@example.org). holes in both the McMurdo region and Prydz Bay (Table New Mexico Geochronology Research Laboratory, Socorro, NM 87801, USA (email@example.com). 1). This is in general because the older strata were exposed 10 UniversitÃ di Siena, Dipartimento di Scienze delle Terra,Via Laterina closer to the coast through basin uplift, where younger strata 8, I-53100 Siena, Italy (firstname.lastname@example.org). had been removed by Neogene erosion, but also because in 11 See http://www.andrill.org/support/references/appendixc.html. offshore basins glacial debris from the last glacial advance 71
72 ANTARCTICA: A KEYSTONE IN A CHANGING WORLD TABLE 1 Antarctic Coastal and Continental Shelf Rock-Drilling Sites, 1973 to 2006 Ross Sea DSDP 28 1973 270 77Â°26'S 178Â°30'W -634 m 423 m 62% gneiss - E Paleozoic Hays, Frakes et al., 1975 271 77Â°26'S 178Â°30'W -562 m 233 m 7% diamict clasts - E Pliocene 272 77Â°26'S 178Â°30'W -629 m 439 m 37% diamict - E Miocene 273 77Â°26'S 178Â°30'W -491 m 333 m 25% diamict - E Miocene McMurdo Sound area - onshore DVDP 1973 1 77Â°50'S 166Â°40'E 67 m 201 m 98% basalt - L Quat Kyle, 1981 1973 2 77Â°51'S 166Â°40'E 47 m 179 m 96% basalt - L Quat 1973 3 77Â°51'S 166Â°40'E 48 m 381 m 90% basalt - L Quat 1974 10 77Â°35'S 163Â°31'E 3m 182 m 83% diamict - L Miocene Powell, 1981 1974 11 77Â°35'S 163Â°25'E 80.2 m 328 m 94% diamict - L Miocene 1974 12 77Â°38'S 162Â°51'E 75.1 m 185 m 98% migmatite - E Paleozoic McMurdo Sound area - offshore DVDP 15 1975 15 77Â°26'S 164Â°23'E -122 m 62 m 52% black sand - E Pleist Barrett and Treves, 1981 MSSTS 1979 1 77Â°34'S 163Â°23'E -195 m 230 m 62% mudstone - L Oligocene Barrett, 1986 CIROS 1986 1 77Â°05'S 164Â°30'E -197 m 702 m 98% boulder congl - L Eocene Barrett, 1989 CIROS 1984 2 77Â°41'S 163Â°32'E -211 m 168 m 67% gneiss - E Paleozoic Barrett and Hambrey, 1992 CRP 1997 1 77Â°00'S 163Â°45'E -154 m 148 m 86% diamict - E Miocene CRST, 1998 1998 2 77Â°00'S 163Â°43'E -178 m 624 m 95% mudstone - Oligocene CRST, 1999 1999 3 77Â°00'S 163Â°43'E -295 m 939 m 97% sandstone - Devonian CRST, 2000 ANDRILL 2006 1 77Â°55'S 167Â°01'E -840 m 1285 m 98% basalt - E Miocene Naish et al., 2006 Prydz Bay ODP 119 1988 739 67Â°17'S 75Â°05'E -412 m 487 m 34% diamict - L Eo-E Oligocene Barron, Larsen et al., 1988 1988 740 68Â°41'S 76Â°43'E -808 m 226 m 32% red beds - ?Triassic 1988 741 68Â°23'S 76Â°23'E -551 m 128 m 26% sandst, siltst -?E Cretaceous 1988 742 67Â°33S 75Â°24'E -416 m 316 m 53% mudst, diamict - ?Eo-Olig 1988 743 66Â°55'S 74Â°42'E -989 m 97 m 22% diamict - Pleistocene ODP 188 2000 1166 67Â°42'S 74Â°47'E -475 m 381 m 19% claystone - L Cretaceous O'Brien, Cooper, Richter et al., 2001 Antarctic Peninsula Barker, Camerlenghi, Acton et ODP 178 1998 1097 66Â°24'S 70Â°45'W -563 m 437 m 14% diamict - E Pliocene al., 1999 1998 1098 64Â°52'S 64Â°12'W -1010 m 47 m 99% mud - Holocene 1998 1099 64Â°57'S 64Â°19'W -1400 m 108 m 102% mud - Holocene 1998 1100 66Â°53'S 65Â°42'W -459 m 111 m 5% diamict - Pleistocene 1998 1102 66Â°48'S 65Â°51'W -431 m 15 m 6% diamict - Pleistocene? 1998 1103 64Â°00'S 65Â°28'W -494 m 363 m 12% diamict - L Miocene SHALDRIL 2005 1 62Â°17'S 58Â°45'W -488 m 108 m 87% mud - L Pleistocene http://shaldril.rice.edu/ SHALDRIL 2006 3 63Â°51'S 54Â°39'W -340 m 20 m 32% mudst - L Eo/E Oligocene Anderson et al., 2007 2006 5 63Â°15'S 52Â°22'W -506 m 23 m 40% muddy sand - mid Miocene 2006 6 63Â°20'S 52Â°22'W -532 m 21 m n/a muddy sand - E Pliocene 2006 12 63Â°16'S 52Â°50'W -442 m 4m 64% mudst - Oligocene
NAISH ET AL. 73 prevented penetration and recovery of the older record from much more frequent cycles of climate, ice volume, and sea gravity coring (Anderson, 1999). Plainly deep-drilling tech- level in the late Quaternary (Hays et al., 1976) in response nology was required from an appropriate location. Here we to variations in Earthâs orbital parameters calculated by summarize the history and rationale behind this successful Milankovitch. How far back in time these should be evident outcome. was not clear, but there seemed no reason why they should By the early 1970s the International Geophysical Year not have inï¬uenced the Antarctic ice sheet from the time of (1956-1958) had already resulted in a vast increase in knowl- its inception. edge of rock types, ages, and history of the continent itself While deep ocean sediments were useful for their (Bushnell and Craddock, 1970), but the history of its ice continuous record of past ocean chemistry, providing an sheet was still a mystery. This changed in early 1973 with the ice volume-temperature signal, they could not provide voyages of the Glomar Challenger to the South Indian Ocean information on the extent of ice or regional climate in the and Ross Sea (Leg 28) (Hayes et al., 1975) and the Tasman high latitudes. This could only come out of sediment cores Sea and Southwest Paciï¬c Ocean (Leg 29) (Kennett et al., from the Antarctic margin, where the direct inï¬uence of ice 1974). The ï¬rst of these cruises showed that the Antarctic advance and retreat (and perhaps also sea-level fall and rise) ice sheet, which had previously been seen as a Quaternary could be obtained. feature, had been in existence since at least the Oligocene. In the early 1970s two drilling platforms were available. The second yielded the ï¬rst oxygen isotope measurements The Glomar Challenger operated by the Deep Sea Drilling on deep-sea calcareous microfossils, providing the ï¬rst Project and a land-based system put together for the Dry evidence of dramatic ocean cooling and global ice volume Valley Drilling Project, an initiative of the United States, increase at the Eocene-Oligocene boundary (Shackleton and Japan, and New Zealand to explore the late Cenozoic history Kennett, 1974). of the McMurdo Dry Valleys (Smith, 1981). Over the next The 1970s was also a period in which detailed paleonto- three decades both of these platforms (and their successors) logical and chronological studies of cores from continuously ran in parallel (Table 1, Figure 1) with varying degrees of deposited deep-sea sediments displaced the long-held view success. of four major Quaternary glaciations (Flint, 1971), showing FIGURE 1 Locations of geological drill sites on land and on the Antarctic continen- tal shelf. Details and references are given in Table 1. McMurdo Sound drill sites are shown in Figure 2A.
74 ANTARCTICA: A KEYSTONE IN A CHANGING WORLD The ship-based system had the advantage that it could 1. Provide new knowledge on the late Neogene behav- be deployed at a number of places around the Antarctic ior and variability of the Ross Ice Shelf and Ross Ice Sheet margin, but was limited by ice conditions and poor recovery and the West Antarctic ice sheet, and their inï¬uence on global of glacimarine sediments from near-shore shelf basins. Nev- climate, sea-level, and ocean circulation. ertheless, the cores taken have provided useful constraints on 2. Provide new knowledge on the Neogene tectonic the Cenozoic history of the Antarctic ice margin and climate evolution of the West Antarctic Rift System, Transantarctic from the Ross Sea, Prydz Bay, and Antarctic Peninsula sec- Mountains, and associated volcanism. tors (Table 1). The land-based system, once adapted for sea ice, had the advantage that it could yield long and continuous A key outcome of the project will be to provide age core with near-complete recovery. However, it was rather control for, and determine the environmental signiï¬cance of, cumbersome and required an ice platform that was ï¬rmly seismic reï¬ectors that have been mapped regionally within tied to land. Although the system has been improved through the Victoria Land Basin (Fielding et al., 2007; Henrys et the MSSTS-1, CIROS, and Cape Roberts Projects (Table al., 2007) in order to assess the regional impact of global 1), continuing to exploit the fast-ice rim around McMurdo climatic and local tectonic events. A second key outcome of Sound as a drilling platform, attractive drilling targets the project will be to use paleoclimatic proxies and boundary beneath fast sea ice elsewhere on the Antarctic margin have conditions to help constrain numerical climate and dynami- yet to be identiï¬ed. cal ice-sheet models. This paper presents an overview of the The results from both ship-based and sea-ice-based MIS Project, and the details are reported in the Initial Results drilling have provided a framework for the Cenozoic history volume (Naish et al., 2007). of the Antarctic ice sheet (Kennett and Warnke, 1992, 1993; Barrett, 1999), but more detail has become known for Oli- TECTONIC AND STRATIGRAPHIC SETTING gocene and early Miocene times, especially from the 1500 m of strata cored off Cape Roberts (Naish et al., 2001; Barrett, Ross Island lies at the southern end of the Victoria Land 2007), than in subsequent times. The ï¬rst results of the shift Basin (VLB), a ~350-km-long, half-graben hinged on its from sea-ice- to shelf-ice-based drilling reported below add western side at the TAM front (Figure 2). Major rifting a great deal to the late Cenozoic story. in the VLB has occurred since the latest Eocene, perhaps having been initiated in the Cretaceous (Cooper and Davey, 1985; Brancolini et al., 1995), and has accommodated up THE ANDRILL MCMURDO ICE-SHELF PROJECT to 10 km of sediment. A new rift history, based on the CRP The aim of the MIS Project was to obtain a continuous sedi- drill cores linked to a new regional seismic stratigraphic ment core through approximately 1200 m of Neogene (~0-10 framework (Fielding et al., 2007; Henrys et al., 2007), Ma) glacimarine and volcanic sediment that had accumulated indicates that crustal stretching during the Oligocene syn- in the Windless Bight region of the MIS (Figure 2A). The rift phase produced rapid subsidence, followed by ther- present-day MIS forms the northwest part of Ross Ice Shelf mally controlled slower subsidence in the early Miocene. where it has been pinned by Ross Island for the last ~10 ka Renewed rifting within the center of the VLB beginning in (McKay et al., 2007), and is nourished by ice sourced locally the late Miocene has continued through to present day. This and from the East Antarctic ice sheet (EAIS) outlet glaciers forms the Terror Rift (Cooper et al., 1987) and is associated in the southern Transantarctic Mountains (TAM). The drill with alkalic igneous intrusions and extrusive volcanism site was situated above a ï¬exural moat basin formed in (e.g., Beaufort Island and Ross Island). Quaternary loading response to Quaternary volcanic loading of the crust by Ross of the crust by the Ross Island volcanoes has added signiï¬- Island, superimposed on regional subsidence associated with cantly to subsidence near Ross Island, and the development Neogene extension of the Terror Rift (Horgan et al., 2005; of an enclosing moat (Stern et al., 1991). The Terror Rift has Naish et al., 2006) (Figure 2B). accommodated up to 3 km of Neogene sediment beneath Between October 29 and December 26, 2006, a single Windless Bight. Here the load-induced subsidence caused 1284.87-m-deep drill core (AND-1B) was recovered from by Ross Island has contributed signiï¬cantly to the genera- the bathymetric and depocentral axis of the moat in 943 m tion of accommodation space, especially during the last 2 of water from an ice-shelf platform. The drilling technology m.y. (Horgan et al., 2005). employed a sea-riser system in a similar fashion to the Cape Neogene strata have now been extensively mapped in Roberts Project (CRP), but utilized a combination of soft southern McMurdo Sound from the Drygalski Ice Tongue sediment coring in upper soft sediments and continuous wire south to Ross Island. These Neogene strata show a thicken- line diamond-bit coring. Innovative new technology, in the ing and eastward-dipping succession extending under Ross form of a hot-water drill and over-reamer, was used to make Island in the vicinity of the MIS Project drill site. Analysis an access hole through 85 m of ice and to keep the riser free of these strata, which have now been sampled by MIS project during drilling operations. drilling, will contribute signiï¬cantly to the young tectonic The MIS project has two key scientiï¬c objectives: history of the West Antarctic Rift System.
NAISH ET AL. 75 FIGURE 2 (A) Location of key geographi- cal, geological, and tectonic features in southern McMurdo Sound. Volcanic centers of the Erebus Volcanic Province include Mt. Erebus (E), Mt. Terror (T), Mt. Bird (B), White Island (W), Black Island (B), Mt. Discovery (D), Mt. Morning (M), and Minna Bluff (MB). Boundary faults of the southern extension of Terror Rift are also shown. Location of ANDRILL Program drill sites and previous programs (DVDP, CIROS, MSST) are shown. (B) Schematic structural-stratigraphic cross-section across the VLB (located in [A] as âA-Aâ) shows the stratigraphic context of the MIS and SMS drill sites with respect to previous drilling in Southern McMurdo Sound. The cross-section is compiled from interpreted seismic reï¬ection data, previous drill core data from MSSTS-1 and CIROS-1 (Barrett, 1986, 1989), and models for the evolution of the VLB. CHRONOSTRATIGRAPHY OF âAND-1Bâ representing more than half of the last 7 Ma. Thus the AND- 1B record provides several highly resolved âwindowsâ into A preliminary age model for the upper 700 m of drill core the development of the Antarctic ice sheets during the late constructed from diatom biostratigraphy (Scherer et al., Cenozoic. Strata below ~620 mbsf are late Miocene in age 2007) and radiometric ages on volcanic material (Ross et (5-13 m.y.). At the time of writing, the chronostratigraphic al., 2007) allows a unique correlation of ~70 percent of the data available for this interval include three radiometric ages magnetic polarity stratigraphy with the Geomagnetic Polar- on volcanic clasts from near 1280 mbsf constraining the age ity Time Scale (Wilson et al., 2007). The age model provides for the base of the AND-1B drill core to <13.5 m.y. Work several well-constrained intervals displaying relatively continues to improve the age control on the lower part of the rapid (<1m/k.y.) and continuous accumulation of sediment cored interval. punctuated by several 0.5 m.y. to 1.0 m.y. stratal hiatuses
76 ANTARCTICA: A KEYSTONE IN A CHANGING WORLD RELATIONSHIP TO REGIONAL SEISMIC 1. Rg (Surface C, bilious green reï¬ector): This STRATIGRAPHY regionally extensive discontinuity is correlated with top of a ~60-m-thick interval late Miocene volcanic sandstone (LSU Prior to drilling the MIS target, ï¬ve distinctive reï¬ectors 7) and the base of a 150-m-thick, high-velocity (3000 msâ1) marking regional stratal discontinuities had been mapped interval of diamictite cycles (LSU 6.4). 40Ar/39Ar dates on through a grid of seismic data in the vicinity of the drill ashes beneath Rg indicate that this discontinuity is <13.8 site (HPP and MIS lines) (Naish et al., 2006) (Figure 2), Ma. and linked to marine seismic reï¬ection data and reï¬ector 2. Rh (Surface B, dark green): This regionally nomenclature in McMurdo Sound (Fielding et al., 2007). We extensive discontinuity is correlated with the base of a have used whole-core velocity measurements and VSP ï¬rst ~180-m-thick interval of late Miocene-early Pliocene, pyrite- arrival travel-time picks (see Hansaraj et al., 2007; Naish et cemented, high-velocity volcanic sandstone and mudstone al., 2007) to derive a time-depth conversion curve to convert (LSU 5). Regionally the green reï¬ector correlates with the the seismic reï¬ection section to depth. We have summarized base of volcanic bodies in the VLB north of Ross Island. It in Figure 3 and below our correlation of the regional seismic is also correlated with the base of White Island volcano (Fig- stratigraphy with the AND-1B drill core. ure 2) dated at ~7.6 Ma (Alan Cooper, University of Otago, unpublished data). FIGURE 3 Integrated plot correlating seismic reï¬ectors with the lithologic log. We have used the whole-core velocity (c) to derive a time-depth conversion curve (b) together with VSP arrival times to map the seismic reï¬ection proï¬le from MIS-1 (a) to drill hole depth and correlate with core lithologies and lithostratigraphic units (e). Seismic stratigraphic units identiï¬ed are from the MIS Science Logistics and Implementation Plan (SLIP) (Naish et al., 2006) and have been mapped regionally (Fielding et al., 2007).
NAISH ET AL. 77 3. Ri (Surface A2, b-clino, red reflector): This regionally extensive reï¬ector marks the base of a ~100-m- thick seismically opaque interval that separates high-ampli- tude reï¬ections of the underlying unit. It corresponds with the base of prograding clinoforms north of Ross Island, and locally marks the base of ï¬exure associated with Ross Island volcanic loading (Horgan et al., 2005). In AND-1B, Surface A2 correlates with the boundary between the ~90-m- thick, low-density, low-velocity (1700 msâ1), early Pliocene diatomite interval (LSU 4.1), and the higher-velocity (<2500 msâ1) diamictites of LSU 4.2 beneath. Diatom assemblages indicate the age of this surface lies between 5.0 Ma and 4.0 Ma. Regionally this reï¬ector has been traced into western VLB, where it is correlated biostratigraphically in MSSTS-1 to core at about 20 mbsf that yields a Pliocene age of 4.6-4.0 Ma based on diatom microfossils (Naish et al., 2006). 4. Rj (Surface A1, turquoise reï¬ector): This region- ally extensive reï¬ector marks the base of a ~150-m-thick unit of strongly alternating high- and low-amplitude reï¬ections. These dramatic cycles in density and velocity reï¬ect regular alternations between diatomite and diamictite in late Plio- cene (LSU 3). The turquoise reï¬ector separates strata above that are younger than ~3.0 Ma and below greater than ~3.5 Ma. STRATIGRAPHIC ARCHITECTURE The 1285-m-long AND-1B drill core provides the ï¬rst high- resolution, late Neogene record from the Antarctic margin (Figure 4) as well as the ï¬rst long geological record from under a major ice shelf. Details of the lithostratigraphic sub- division, facies analysis, and sequence stratigraphy are pre- sented in an Initial Results volume (Krissek et al., 2007). Glacial-Interglacial Cyclostratigraphy At the time of writing, 60 unconformity-bounded glacima- rine sedimentary cycles, of probable Milankovitch duration have been identiï¬ed, representing repeated advances and retreats of an ice sheet across the drill site during the late Neogene. Bounding unconformities, glacial surfaces of ero- sion (GSEs), are typically sharp and planar and mark disloca- tions between enclosing facies. Facies immediately beneath display a range of intraformational deformation features, including physical mixing of lithologies, clastic intrusions, faulting, and soft-sediment deformation. Facies above these surfaces are typically diamictites and conglomerates, and are interpreted as subglacial tillites or near grounding-line FIGURE 4 Lithostratigraphy, chronostratigraphy, and sequence stratigraphy of the AND-1B drillcore. Cyclic variations in litholo- gies reï¬ect periodic ï¬uctuations of the ice margin in western Ross Embayment during the last 13 Ma. Sources: * = After Ross et al. (2007); ^ = After Wilson et al. (2007).
78 ANTARCTICA: A KEYSTONE IN A CHANGING WORLD glacimarine deposits. In many cycles the facies succession itoid and metamorphic rocks from the Byrd Glacier region reï¬ects retreat of the grounding line through ice shelf into imply the long-term existence of a grounded Ross Ice Sheet open-ocean environments at the interglacial minimum, fol- with short ice-shelf phases, and ice streaming from the south- lowed by ice readvance characterized by progressively more ern Transantarctic Mountains. Once a magnetostratigraphy glacially inï¬uenced facies in the upper parts, culminating in is developed for this section of the core together with new 40 a GSE at the glacial maximum. Ar/39Ar ages, further sedimentological and petrographic work should provide an important history of Antarctic ice sheet behavior during the âbigâ late Miocene, Mi-glaciations Major Chronostratigraphic Intervals and of Miller et al. (1991). Their Cyclic Character Here we summarize the sedimentary succession cored in Late Miocene, Diamictite/Mudstone-Sandstone Sedimentary AND-1B by subdividing it into nine chronostratigraphic Cycles of LSU 6.1-6.3 (1069.2-759.32 mbsf) intervals largely on the basis of characteristic facies cycles, with their glacial and climatic implications (Figure 4). The This section is characterized by cycles of subglacial and latter are addressed in more detail by Powell et al. (2007). grounding-line diamictites that pass upward into a gla- Three of the intervals lack cyclicity: interval 1 (a volcanic cimarine retreat succession of redeposited conglomerate, sandstone), interval 4 (largely volcanic ash), and interval 6 sandstone and mudstone. These units are overlain by a (the ~90-m-thick diatomite). more distal hemipelagic terrigenous mudstone with out- size clasts and lonestones. The retreat facies succession is 1. Late Miocene volcanic sandstone (1275.24-1220.15 then followed by a proglacial advance facies assemblage in mbsf), LSU 7. which clast abundance increases together with the occur- 2. Late Miocene diamictite-dominated sedimentary rence of submarine outwash facies, which is truncated by cycles (1220.15-1069.2 mbsf), LSU 6.4. a glacial surface of erosion. In contrast with the underly- 3. Late Miocene, diamictite/mudstone and sandstone ing diamictite-dominated interval, over 60 percent of this sedimentary cycles (1069.2-759.32 mbsf), LSU 6.1-6.3. unit comprises strata representing periods of both ice-shelf 4. Late Miocene-early Pliocene? lapilli tuff, lava ï¬ow, cover and open ocean, though still with signiï¬cant ice raft- and volcanic sandstone and mudstone (759.32-586.45 mbsf), ing, over the drill site. These observations imply a warmer LSU 5. climate for this whole period, and the association increased 5. Early Pliocene diamictite/diatomite sedimentary subglacial meltwater. cycles (586.45-459.24 mbsf), LSU 4.2-4.4. 6. Early Pliocene diatomite (459.24-382.98mbsf), Late Miocene-early Pliocene Volcanic Sandstone and LSU 4.1. Mudstone of LSU 5 (759.32-586.45mbsf) 7. Late Pliocene diamictite and diatomite sedimentary cycles (459.25-146.79 mbsf), LSU 3. This interval is dominated by subaqueously redeposited 8. Late Pliocene-early Pleistocene diamictite/volcanic volcanic sediments, many with near-primary volcanic char- mudstone and sandstone cycles (146.79-82.72 mbsf), LSU 2. acteristics. The volcaniclastic sediments are organized into 9. Middle-late Pleistocene, diamictite-dominated sedi- sediment gravity ï¬ow deposits (mostly proximal turbidites), mentary cycles (82.72-0mbsf), LSU 1. indicating a nearby active volcanic center delivering primary volcanic material into a deep basin (several hundred meters water depth). A series of ï¬ning upward, altered and degraded Late-Miocene Diamictite-Dominated Cycles of LSU 6.4 pumice lapilli tuffs occur at 623, 603, and 590 mbsf, and a (1220.15-1069.2 mbsf) pure ï¬ne volcanic glass sand occurs at 577 mbsf. All of these This interval is dominated by thick massive diamictite with have been targeted for argon geochronology. A plagioclase thin stratiï¬ed mudstone interbeds, representing alterna- Hawaiite lava with chilled margins and âbaked sedimentsâ tions between basal glacial and glacimarine deposition. under its lower contact interrupts the volcanic sediment The diamictites, which form over 90 percent of the interval, gravity ï¬ows at 646.49-649.30 mbsf, and has been dated contain medium- to high-grade metamorphic basement clasts by 40Ar/39Ar at 6.38 Ma. The source of this lava and the known from the Byrd Glacier region (Talarico et al., 2007). redeposited volcanic units is thought to be nearby because At this stage we have been conservative in our recognition of the freshness of the glass and angular nature of clasts in of grounding-line features in the core that might be used to the breccias. The unit is virtually devoid of out-size clasts identify boundaries of the sedimentary cycles. More detailed or other features indicative of iceberg rafting or grounding- study is likely to ï¬nd features such as sets of alternating line proximity, suggesting that much of the 120-m-thick sharp-based massive and stratiï¬ed diamictites, representing interval was deposited rapidly in open water. At this stage we subglacial and pro-grounding-line environments. The domi- consider this volcanic interval to represent a single eruptive nance of subglacial deposition and the occurrence of gran- sequence with very high rates of accumulation.
NAISH ET AL. 79 Early Pliocene Diamictite and Diatomite Sedimentary Cycles tite (glacial and glacimarine) to diatomite (open ocean) are of LSU 4.2-4.4 (586.45-459.24 mbsf) dramatic, in many cases occurring in less than a meter of core. Powell et al. (2007) discuss these abrupt facies transi- These cycles can be summarized as comprising, in ascending tions in terms of rapid retreat of an ice sheet or collapse of an stratigraphic order: (1) a sharp-based massive diamictite with ice shelf. Our preliminary age model suggests that the cycles variable amounts of volcanic glass (subglacial to grounding in this interval may have formed in response to orbital forc- zone) passing up into (2) stratiï¬ed diamictite, sandstone, ing of the ice sheet. This interval will be the focus of further and mudstone with dispersed clasts (grounding-line to dis- work as we evaluate its signiï¬cance in terms of the global tal glacimarine) followed by (3) stratiï¬ed diatomite (open climatic deterioration coincident with the development of ocean), which in turn commonly passes up into glacimarine large ice sheets on the northern hemisphere continents (e.g., mudstone and sandstone with dispersed clasts (perhaps an Shackleton et al., 1984; Maslin et al., 1999). indication of an approaching grounding line). The interval that includes the lower part of the diamictite and the under- lying glacimarine lithologies is often physically intermixed Late Pliocene-Early Pleistocene Diamictite/Volcanic and displays deformation associated with glacial overriding Mudstone-Sandstone Cycles of LSU2 (146.79-82.72 mbsf) and shearing but, interestingly, appears to represent little This interval displays lithologic alternations between ice- signiï¬cant erosion by the advance. These cycles show a dra- proximal facies and open-water volcaniclastic facies. A matic change from hemipelagic open-water sedimentation GSE at 150.90 mbsf is correlated with the Rk-pink reï¬ector to open-water biogenic sediments (diatomite) at interglacial and spans as much as 0.7 m.y. between 2.4 Ma and 1.7 Ma. ice minima. Above this the interval from 146.79 mbsf to 134.48 mbsf represents another glacial-interglacial cycle; that above the Early Pliocene Diatomite of LSU 4.1 (459.24-382.98 mbsf) diamictite is almost entirely made up of redeposited volcanic (basaltic) sandstone. The sandstone is organized into a series This interval is almost solely diatomite, indicating an of normally graded turbidites. The interval from 134.48 mbsf extended period of high-productivity open water over the to 120 mbsf is characterized by an open-water assemblage site. It is of early Pliocene age (~4.2 Ma) and is likely to span of interstratiï¬ed mudstone and volcanic sandstone that lies a period of about 4.2-4.0 m.y., involving a number of glacial- stratigraphically above weakly stratiï¬ed diamictite alternat- interglacial cycles. The onset of an inferred warmer period is ing with sparsely fossiliferous claystone and mudstone, captured in the 10 m transition beginning at 460 mbsf from a more typical of grounding line and iceberg zone systems. A clast-rich to clast-poor muddy diamictite through terrigenous muddy-sandy volcanic breccia with near-primary volcanic mud to diatomite, with a decline in mud component, out- material at 117 mbsf has yielded a preliminary 40Ar/39Ar age size clasts, and frequency of cm- to dm-thick gravity ï¬ow of ~1.6 Ma on basaltic volcanic glass. deposits. Further research is planned in order to quantify A dramatic change upward into massive diamictite the contemporaneous climate and will involve examination from 109.7 mbsf to 97.2 mbsf indicates the return of the ice of the terrestrial microfossils and estimation of sea-surface sheet to the proximity of the drill site. Our preliminary age temperatures from geochemical and biological proxies. This model suggests that up to ~0.4 m.y., between 1.5 Ma and diatomite appears to coincide with the widely recognized 1.1 Ma is missing between GSE at 109.7 mbsf and at least early Pliocene global warming and accompanying higher sea 2 GSEs in the overlying interval of amalgamated diamictite levels (Dowsett et al., 1999; Ravelo et al., 2004), for which above. From 97.2 mbsf to 92.5 mbsf diamictite and volcanic there is evidence elsewhere on the Antarctic margin (e.g., sandstone passes downward into interstratiï¬ed bioturbated Whitehead et al., 2005). However, well-dated deposits and volcanic sandstone and mudstone. The interval from 92.5 geomorphological evidence from the adjacent McMurdo Dry mbsf to 86.6 mbsf contains the youngest biosiliceous-bearing Valleys indicate a polar climate persisted there throughout sediments in the AND-1B core. The presence of numerous this period (Marchant et al., 1996). volcaniclastic units and biosiliceous sediments in this interval indicate an extended period of open-water conditions with no Middle to Late Pliocene Diamictite and Diatomite sea ice beyond the calving line. An immediately overlying, Sedimentary Cycles of LSU 3 (382.98-146.79 mbsf) ï¬ning upward lapilli tuff between 85.86 mbsf and 85.27 mbsf has yielded a high-precision 40Ar/39Ar age on sanidine phe- Within this interval the core is strongly cyclic in nature, and nocrysts of 1.01 Ma within a short normal polarity interval is characterized by 12 glacial-interglacial cycles that is typi- interpreted as the Jaramillo Subchron. Thus the underlying cally composed of a sharp-based lower interval of diamictite biosiliceous interglacial sediments are tentatively correlated in the upper few meters that passes upward into a ~5- to with warming during the âsuper-interglacialâ associated with 10-m-thick unit of biosiliceous ooze or biosiliceous-bearing Marine Isotope Stage 31. This interval will be the focus of mudstone (diatomite). The lower part of diamictites and the a future concentrated effort to better characterize the impact upper few meters of diatomites are sheared and deformed, of this warm period on ice-sheet behavior. presumably from grounded ice. The transitions from diamic-
80 ANTARCTICA: A KEYSTONE IN A CHANGING WORLD Middle-Late Pleistocene, Diamictite-Dominated Sedimentary imply changes in ice-sheet volume that must have contrib- Cycles of LSU 1 (82-0 mbsf) uted signiï¬cantly to eustasy (e.g., 10-20 m). Cold early Miocene and Pleistocene till-dominated Between 83 mbsf and 27 mbsf there are at least 8 cycles of intervals with clasts originating in the TAM to the south diamictite and thinner bioturbated and stratiï¬ed intervals imply that grounded ice from the big outlet glaciers to the of mudstone, volcanic sandstone, and muddy conglomer- south was reaching McMurdo Sound during these times. ate. Like the late Miocene diamictite-dominated cycles, the Today Byrd Glacier-sourced ice in the shelf ï¬ows east of high proportion of subglacial and grounding-line proximal Ross Island. Glaciological reconstructions (e.g., Denton and deposits, together with the occurrence of granitoid and meta- Hughes, 2000) require signiï¬cant ice volume from WAIS to morphic rocks known from the Byrd Glacier region (Talarico direct the ï¬ow lines of the southern outlet glaciers into the et al., 2007), implies existence of a grounded Ross Ice Sheet McMurdo region during glacial periods, and also to main- in the middle and late Pleistocene. The implied presence of tain an ice shelf during ensuing interglacial retreats. Thus, a grounded ice sheet for much of this time is intriguing as it we view the sedimentary cycles representing primarily the also corresponds with a period of Earth history dominated expansion and contraction of WAIS in concert with ï¬uctua- by 80-120 ka ï¬uctuations in large Northern Hemisphere ice tions in the ï¬ow of TAM outlet glaciers. The TAM outlet sheets. Further work will focus on (1) how this ice sheet glaciers alone do not provide enough ice volume to maintain responded to the late Pleistocene interglacials, which in Ant- an ice sheet or ice shelf in the Ross Embayment. arctic ice-core records indicated polar temperatures warmer Work is planned in the near future to further understand than today (EPICA Community, 2004; Jouzel et al., 2007), and constrain ice-sheet dynamics, especially during periods and (2) the role of orbital forcing on Antarctic ice sheets of hypothesized global warmth that will provide useful ana- (e.g., Huybers and Wunsch, 2005; Raymo et al., 2006) in logues for constraining the behavior of Antarctic ice sheets regulating Pleistocene climate. in the context of future climate change. To achieve this we will integrate our core dataâsuch as clast provenance, bio- IMPLICATIONS FOR ANTARCTIC GLACIAL AND logical and geochemical proxiesâwith the new generation CLIMATE HISTORY of ice-sheet and climate models. The AND-1B core has the potential to contribute signiï¬cant new knowledge about the dynamics of the West Antarctic SUMMARY ice sheet (WAIS) and Ross Ice Shelf and Ice Sheet system, Repetitive vertical successions of facies imply at least 60 as well as contributing to understanding the behavior of ï¬uctuations of probable Milankovitch duration between the EAIS outlet glaciers during the late Cenozoic. New subglacial, ice-proximal, and ice-distal open marine envi- chronological data will allow certain intervals of the core to ronments. These have been grouped into three main facies be correlated with other proxy climate records (e.g., ice and associations that correspond to glacial-interglacial variability marine isotope records) and hence the sensitivity of the ice during climatically distinct periods of the late Neogene: sheet to a range of past global climate changes to be evalu- ated. The signiï¬cant results thus far are that the Ross Ice 1. Cold polar climate with MIS site dominated by Sheet, fed by WAIS and EAIS outlet glaciers, has undergone grounded ice but some retreat to ice-shelf conditions (late signiï¬cant cyclic variations in extent and timing during the Miocene, ~13-10 Ma and Pleistocene, ~1-0 Ma). late Neogene. A relatively colder and more stable ice sheet 2. Warmer climate with MIS site dominated by dominated the Ross Embayment in the early late Miocene ice-shelf and open-water conditions (hemipelagites), with between 13 Ma and 10 Ma, becoming more dynamic in the occasional periods of grounded ice (early-late Miocene, ~9-6 latest Miocene (9-7 Ma) with subglacial water discharge, Ma). still with periodic grounded ice in the Ross Embayment. 3. Warmer climate with extended periods of open- These conditions were followed by a period around 4 Ma ocean conditions (pelagic diatomites) with periods of sub-ice when the Ross Embayment was relatively ice-free, with shelf and grounded ice deposition (Pliocene, ~5-2 Ma). highly productive, warmer oceanic conditions, followed by a return to cycles of advance and retreat of grounded ice. From The ~90-m-thick early Pliocene (~4.2 Ma) interval of middle Pleistocene to Recent the ice sheet is characterized diatomite shows no apparent glacial cyclicity and represents by a change back to more stable, colder conditions. Our pre- an extended period of ice-free conditions indicative of a liminary analysis of the more than 25 Pliocene sedimentary reduced WAIS. Late Pliocene (~2.6-2.2 Ma) glacial-inter- cycles indicates signiï¬cant glacial-interglacial variability, glacial cycles characterized by abrupt alternations between with regular oscillations between subglacial/ice proximal subglacial/ice-proximal facies and open marine diatomites and open-ocean ice distal environments, including extended imply signiï¬cant WAIS volume ï¬uctuations around the time periods of interglacial warmth when the ice was not calving of the early Northern Hemisphere glaciations. A ~4-m-thick into the ocean. Our environmental reconstructions to date interval of diatomaceous mudstone in the middle Pleistocene
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