The development of antarctic glaciation and the Neogene paleoenvironment of the Maurice Ewing Bank

Abstract

A micropaleontologic, magnetostratigraphic, and sedimentologic analysis of 56 piston cores provides the basis of a geologic study of the late Miocene to Holocene depositional and erosional history of the intermediate-depth Maurice Ewing Bank (MEB) located at the eastern extremity of the Falkland (Malvinas) Plateau, southwest Atlantic Ocean. Sedimentation on the MEB is controlled presently by the position of the Polar Front and strong bottom currents beneath the axis of the Antarctic Circumpolar Current (ACC). Fluctuations in the position of the Polar Front and in the intensity of the ACC are inferred to have largely controlled deposition on the MEB since the initiation of ACC flow over the bank in the Miocene. The sedimentary record of the MEB is used to infer the late Miocene to Holocene oceanographic and climatic conditions in the South Atlantic sector of the Southern Ocean. The MEB suffered major erosion by intense ACC flow during late Miocene time which exposed Cretaceous—Miocene sediment near the surface and shaped the present configuration of the bank. This erosional event is inferred to have occurred during late Miocene to early Pliocene time (∼7.2–4.7 m.y.B.P.) with the major phase of erosion occurring between middle magnetostratigraphic Chron 7 and late Chron 6 (∼7.2–6.2 m.y.B.P.). We suggest that the Miocene sedimentary record of the MEB and other paleoenvironmental evidence from the circum-Antarctic region precludes the existence of a West Antarctic ice sheet or extensive ice shelves along the Antarctic margin prior to the late Miocene. During the late Miocene a West Antarctic ice shelf formed in the former West Antarctic Sea and rapidly thickened until it grounded below sea level to form the West Antarctic ice sheet. Formation of this ice sheet, with floating and partially grounded extensions (ice shelves) in the Ross and Weddell embayments, led to the first major production of true Antarctic Bottom Water (formed in a manner similar to today) which permanently altered global abyssal circulation. Late Miocene changes in oceanic CCD levels and a permanent shift in the oceanic 13 C 12 C composition may be attributed to this major change in abyssal circulation. A decrease in ACC velocity during an early Pliocene amelioration of climate led to resumption of deposition on the bank from ∼4.7 to 3.9 m.y.B.P. Intensification of the ACC during the late Gilbert and early Gauss Chrons again resulted in limited deposition and widespread erosion and/or non-deposition over most of the MEB from ∼4.0 to 3.2 m.y.B.P. ACC velocity apparently began to wane during the middle of the Gauss Chron and allowed widespread deposition on the bank during late Gauss time (2.9–2.48 m.y.B.P.) and over more limited areas during earliest Matuyama time (2.48 to about 2.2 m.y.B.P.). During middle to late Matuyama time bottom current intensity reached its greatest post-Miocene level forming a regional disconformity between sediments of about 2.0 and 1.0 m.y. in age. This increased circumpolar circulation and associated erosion is inferred to have peaked during the late Matuyama Chron (1.2–1.0 m.y.B.P.) and is approximately the same age as the tentatively dated greatest Patagonian Glaciation in nearby southern Argentina. Between ∼1.0 and 0.7 m.y.B.P., the bank was blanketed with a coarse, erosion-resistant layer of ice-rafted detritus which armored the older sediment thereby protecting it from major subsequent erosion. Sedimentation on the MEB during the Bruhnes Chron (720,000 yr B.P.–Present) was intermittent and finally culminated with the deposition of a veneer of carbonate ooze during or since latest Pleistocene time.