A High-Resolution Record of Early Miocene Antarctic Glacial History From Downhole Logs, Site 1165, ODP Leg 188

ODP Site 1165, located 400 km NW of Prydz Bay, contains the most detailed record yet obtained of Early Miocene Antarctic glacial history. The 520-m-thick Early Miocene consists of dark grey claystone with silt laminae (contourites) alternating with decimeter-scale layers of greenish-grey bioturbated claystone that contain common ice rafted debris (IRD). These sediments are interpreted to record pulses of IRD from the continent, and alternations in the position or strength of the currents at the site. Downhole logs of electrical resistivity, natural gamma radiation, and density also record the alternations between the two facies. Here we assess the lithological significance of the logs by examining the opal, IRD, and susceptibility of two short intervals at high resolution, with samples taken every 10 cm, or about 1250 yr. The IRD-bearing greenish-grey claystone corresponds to higher resistivity and density because of increased cementation by silica, and corresponds to lower natural gamma radiation because of decreased clay content. Thus, using the logs, lithology can be inferred for intervals where no core was recovered. Based mainly on magnetostratigraphy, the age of the base of the hole is 22 Ma, and the sedimentation rate for the Early Miocene about 9 cm/kyr. The regular spacing of the facies alternations in the logs suggests they are astronomically paced. Between 18-20 Ma, the main periodicity is in the precessional range, and between 20-22 Ma, the logs have a saw-toothed form, with a major peak in IRD at the beginning of each, repeating about every 100 kyr. We think that the IRD pulses represent deglaciations. Hence our data suggest that the Antarctic continent, at least around the Prydz Bay region, was glaciated in the Early Miocene, and that the size of the ice sheet oscillated on Milankovitch time scales.