MAGNETOBIOSTRATIGRAPHIC CHRONOLOGY OF THE EOCENE-OLIGOCENE TRANSITIONIN THE CIROS-1 CORE, VICTORIA LAND MARGIN, ANTARCTICA - IMPLICATIONS FOR ANTARCTIC GLACIAL HISTORY

In 1986, cores were obtained to a depth of 702 m (with 98 % recovery) from the CIROS-1 drill hole beneath the Ross Sea on the Victoria Land margin. Glaciogene sediments identified near the base of the hole mark the earliest known record of Antarctic glaciation, Initial biostratig raphic analysis indicated that the lower 336 m of the core is early Ol igocene in age, and that the upper 366 m is of late Oligocene-early Mi ocene age, Recently, the chronology of the CIROS-1 core has been quest ioned, We developed a magnetostratigraphy for the lower 400 m of the C IROS-1 core to clarify the chronology, Our magnetobiostratigraphic res ults indicate that the base of the CIROS-1 core is early-late Eocene i n age (corresponding to Chron C16r; ca, 36.5 Ma), We identify the Eoce ne-Oligocene boundary at about 410-420 m, within a 20-m-thick, poorly stratified, bioturbated sandy mudstone, This makes the CIROS-1 core th e highest latitude site (77.1 degrees S) from which this datum event h as been recognized, At 366 m, a 4 m,y, hiatus, which lies immediately beneath fluvial sediments, accounts for most of Chrons C11 and C12. We recognize three major climatic episodes in the CIROS-1 core: (1) the late Eocene (34.5-36.5 Ma, 430-702 m), when relatively warm conditions dominated and there were high sedimentation rates and some glacial ac tivity; (2)the late Eocene-early Oligocene boundary interval (28.5-34. 5 Ma, 340-430 m), which was a transition from relatively warm to coole r conditions that coincided with glacial intensification, sea-level fa ll, and subaerial erosion of the shelf; and; (3) the late Oligocene-ea rly Miocene (22-28.5 Ma, 50-340 m), when large-scale glaciation domina ted the region and glaciers grounded across the continental shelf. Fro m correlation with global oxygen isotope and sea-level records, we inf er that the Antarctic climate and surrounding oceans cooled after sepa ration of Australia and Antarctica and development of deep-water circu lation between them, This marked the onset of the Eocene-Oligocene tra nsition at ca, 34.5 Ma. A major East Antarctic ice sheet did not devel op until the early-late Oligocene boundary, toward the end of the Eoce ne-Oligocene transition (ca, 28.5 Ma), Outlet glaciers did not breach the Transantarctic Mountains and ground across the Ross Sea Shelf unti l 0.5 m.y., later (ca, 28 Ma).