Leg 188 Synthesis: Transitions in the Glacial History of the Prydz Bay Region, East Antarctica, from ODP Drilling

Abstract

Drilling during Leg 119 (1988) and Leg 188 (2000; Sites 1165–1167) of the Ocean Drilling Program (ODP) provides direct evidence for longand short-term changes in Cenozoic paleoenvironments in the Prydz Bay region. Cores from across the continental margin reveal that in preglacial times the present shelf was an alluvial plain system with austral conifer woodland in the Late Cretaceous that changed to cooler Nothofagus rainforest scrub by the middle to late Eocene (Site 1166). Earliest recovered evidence of nearby mountain glaciation is seen in late Eocene–age grain textures in fluvial sands. In the late Eocene to early Oligocene, Prydz Bay permanently shifted from being a fluviodeltaic complex to an exclusively marine continental shelf environment. This transition is marked by a marine flooding surface later covered by overcompacted glacial sediments that denote the first advance of the ice sheet onto the shelf. Cores do not exist for the early Oligocene to early Miocene, and seismic data are used to infer the transition from a shallow to normal depth prograding continental shelf with submarine canyons on the slope and channel/levees on the rise. Cores from the continental rise at Site 1165 show long-term (millions of years) early Miocene and younger decreases in sedimentation rates as well as short-term (Milankovitch periods) cyclicity between principally biogenic and terrigenous sediment supply—resulting from the cyclic presence of onshore glaciers and changes in ocean circulation. Middle Miocene transitions include rapid decreases in sedimenta1Cooper, A.K., and O’Brien, P.E., 2004. Leg 188 synthesis: transitions in the glacial history of the Prydz Bay region, East Antarctica, from ODP drilling. In Cooper, A.K., O’Brien, P.E., and Richter, C. (Eds.), Proc. ODP, Sci. Results, 188, 1–42 [Online]. Available from World Wide Web: . [Cited YYYY-MM-DD] 2Department of Geological and Environmental Sciences, Stanford University, 450 Serra Mall, Building 320, Room 118, Stanford CA 94305, USA. akcooper@pangea.stanford.edu 3Geoscience Australia, GPO Box 378, Canberra ACT 2601, Australia. Initial receipt: 3 July 2003 Acceptance: 28 April 2004 Web publication: 20 July 2004 Ms 188SR-001 A.K. COOPER AND P. O’BRIEN LEG 188 SYNTHESIS: GLACIAL HISTORY OF THE PRYDZ BAY REGION 2 tion rates, increased ice-rafted debris, shifts in clays and other minerals, and regional erosion of the slope and rise. These transitions may reflect enhanced glacial erosion and reduced glacial meltwater from progressively colder ice. At this time, seismic data show that depocenters began to shift from the outer continental rise to the base of the continental slope coincident with the initial stages of the glacial erosion and overdeepening of the continental shelf. During the late Miocene to early Pliocene there was a transition to greater subglacial activity on the shelf and more pronounced cyclic facies variations on the continental rise. At this time, severe glacial morphologies initiated on the shelf with the erosion of Prydz Channel and other troughs by fast-moving ice and the deposition of overcompacted glacial diamictons by slow-moving ice on adjacent banks. The Prydz trough-mouth fan also began to form with alternating deposition of debris flows (ice at shelf edge) and muddy units (reduced ice) (Site 1167). The fan also records a transition during the late Pleistocene for times younger than 780 k.y. when short-term glacial variations continued but ice reached the shelf edge only a few times. Both short-term and long-term transitions characterize the Cenozoic evolution of the Prydz Bay region from the Cretaceous nonglacial to late Neogene full-glacial paleoenvironments. These transitions are known only from ODP cores, and further insights will require additional drilling. INTRODUCTION Three sites (1165, 1166, and 1167) were drilled during Ocean Drilling Program (ODP) Leg 188 in the Prydz Bay region of Antarctica (Fig. F1) to achieve three principal objectives: 1. Date the earliest evidence of glacial activity in the region and decipher the Paleogene environment of Antarctica by drilling on the continental shelf; 2. Link Oligocene and younger events in the East Antarctic Ice Sheet with changes in the Southern Ocean by drilling a transect of holes across the continental shelf, slope, and rise; and 3. Acquire a record of late Miocene and younger ice advances to the shelf edge and interglacial periods by drilling into the troughmouth fan built by advances of the Lambert Glacier–Amery Ice Shelf. Leg 188 is also notable for the many technical advances made in successfully acquiring the desired cores under unusually harsh operating conditions. This was the first Antarctic drilling leg to work without the aid of a support ship and conduct logging-while-drilling operations in polar-glacial sediments. Site 1165 is the deepest drill hole in Antarctica to date, and Site 1167 is the first drill hole to sample deeply into a polar trough-mouth fan. These accomplishments and others lay the path for future Antarctic one-ship drilling operations to more fully decipher south polar paleoenvironmental history. The East Antarctic Ice Sheet has a long-lived record of growth and decay since the Paleogene (Barron et al., 1991) and is now the largest ice mass on Earth. The ice sheet has been a significant driving force in global climates and sea level changes and in ocean and atmospheric circulation. The long-term cooling of the world’s oceans and climates over F1. Prydz Bay region, p. 28.