Expedition 318 summary

Abstract

Understanding the evolution and dynamics of the Antarctic cryosphere, from its inception during the Eocene–Oligocene transition (~34 Ma) through the significant subsequent periods of likely coupled climate and atmospheric greenhouse gas changes, is not only of major scientific interest but also is of great importance for society. Drilling the Antarctic Wilkes Land margin along an inshore to offshore transect was designed to provide a long-term record of the sedimentary archives of Cenozoic Antarctic glaciation and its intimate relationships with global climatic and oceanographic change. The principal goals were (1) To establish the timing and nature of the first arrival of ice at the Wilkes Land margin inferred to have occurred during the earliest Oligocene (reflecting Oligocene isotope Event 1 around ~34 Ma), (2) To reconstruct the nature and age of the changes in the geometry of the progradational wedge interpreted to correspond with large fluctuations in the extent of the East Antarctic Ice Sheet and possibly coinciding with the transition from a wet-based to a cold-based glacial regime, (3) To obtain a high-resolution record of Antarctic climate variability during the late Neogene and Quaternary, and (4) To obtain an unprecedented ultrahigh resolution (i.e., annual to decadal) Holocene record of climate variability. The Wilkes Land drilling program was developed to constrain the age, nature, and paleoenvironment of deposition of the previously only seismically inferred glacial sequences. Drilling the Wilkes Land margin has a unique advantage in that seismic unconformity WL-U3, inferred to separate preglacial strata below from glacial strata above in the continental shelf, can be traced to the continental rise deposits, allowing sequences to be linked from shelf to rise. Integrated Ocean Drilling Program Expedition 318 recovered ~2000 m of high-quality middle Eocene–Holocene sediments from Sites U1355, U1356, U1359, and U1361 on the Wilkes Land rise and Sites U1357, U1358, and U1360 on the Wilkes Land shelf at water depths between ~400 and 4000 meters below sea level. Together, the cores represent ~53 m.y. of Antarctic history. Recovered cores successfully date the inferred glacial seismic units (WL-S4–WL-S9). The cores reveal the history of the Wilkes Land Antarctic margin from an ice-free “greenhouse Antarctica,” to the first cooling, to the onset and erosional consequences of the first glaciation and the subsequent dynamics of the waxing and waning ice sheets, all the way to thick, unprecedented “tree ring style” records with seasonal resolution of the last deglaciation that began ~10,000 y ago. The cores also reveal details of the tectonic history of the so called Australo-Antarctic Gulf from 53 Ma portraying the onset of the second phase of rifting between Australia and Antarctica, to ever subsiding margins and deepening, all the way to the present continental and ever widening ocean/continent configuration. Tectonic and climatic change turned the initially shallow broad subtropical Antarctic Wilkes Land shelf into a deeply subsided basin with a narrower, ice-infested margin. Thick Oligocene and notably Neogene deposits, including turbidites, contourites, and larger and smaller scaled debris mass flows, witness the erosional power of the waxing and waning ice sheets and deep ocean currents. The recovered clays, silts, and sands and their microfossils also reveal the transition of subtropical ecosystems and a vegetated Antarctica into sea ice–dominated ecosystems bordered by calving glaciers, separated by seismic unconformity WL-U3.