Antarctic Late Cenozoic Glaciation: Evidence for Initiation of Ice Rafting and Inferred Increased Bottom-Water Activity

摘要

Micropaleontological and sedimentological studies have been conducted on three sub-antarctic-northern Antarctic piston cores ranging in age from the middle Miocene to earliest Pliocene. Middle to early late Miocene intervals in two of these cores (E 34-6; E 36-37) contain no ice-rafted debris. The other core (E 34-5) is of latest Miocene to earliest Pliocene age (approximately 4.0 to 5.0 m.y. B.P.) and is the oldest known late Cenozoic core containing ice-rafted debris. Quartz sand first appears in this core within the Gilbert c paleomagnetic event and increases in abundance toward the Nunivak paleomagnetic event (Gilbert c) near the top of the core. Examination of these quartz grains with a scanning electron microscope reveals that only a small number are of primary glacial origin; the remainder show varying degrees of reworking by subaqueous processes. The occurrence of quartz grains of primary glacial origin appears to reflect the first major late Cenozoic development of Antarctic ice with accompanying initiation of ice rafting to subantarctic regions. The simultaneous appearance of significant numbers of subaqueously transported grains with ice-rafted glacial grains may reflect a synchronous increase in Antarctic bottom-water activity associated with the development of Antarctic ice. A concomitant increase in micromanganese nodules in sediments of this age may also be related to increased bottom-current activity. This evidence perhaps indicates that during the early part of the late Cenozoic, there was an increase in the degree of Antarctic glaciation, as reflected in the amount of ice rafting as well as an increase in bottom-current activity.