Late Pliocene climate and south-west Atlantic abyssal circulation

Abstract

THE late Pliocene is characterised by a global climatic cooling^1–4 and by widespread erosion of abyssal sediments by Antarctic Bottom Water (AABW)^5–6. The global cooling is dated between 2.5 and 3.0 Myr based on the age of the oldest tillite in Iceland⁷, the initiation of glaciation in the Sierra Nevada of the USA⁸, faunal changes in Antarctic and sub-Antarctic deep-sea cores^9–11, and a change in carbonate preservation in the equatorial Pacific¹². This cooling may be synchronous with the development of the Northern Hemisphere ice sheet which has been dated by the first appearance of ice-rafted debris in the North Atlantic at 3 Myr followed by a major influx at 2.6 Myr (ref. 13), and by a large change in the oxygen isotope record of benthic foraminifera indicating an increase in ice volume at ∼ 2.6 Myr (refs 2, 3). The global cooling and deep-sea erosion events have been identified in widely separated studies and the ages assigned to each are based on biostratigraphic zonations with long durations. The apparent synchroneity of the two events (within the limits of the methods used) has led to the suggestion of a cause and effect relationship with the global cooling driving the increase in abyssal circulation^{14, 15}. In an attempt to find the cause of the relative timing of both the climatic and circulation events, which has not been determined with sufficient stratigraphic resolution, we have examined two late Pliocene sections in the south-west Atlantic.