CLIMATIC EVOLUTION OF THE SOUTHERN-OCEAN DURING THE PLIOCENE EPOCH FROM 4.8 MILLION TO 2.6 MILLION YEARS AGO

We studied the paleoclimatic history of the Southern Ocean during the Pliocene, before the intensification of northern hemisphere glaciation at 2.4 Ma, from isotopic and sedimentologic data from ODP Leg 114 sites (subantarctic South Atlantic). Relatively warm climatic conditions prevailed in the Southern Ocean during the early early Pliocene (between approximately 4.6 and 4.2 Ma) as evidenced by benthic and planktonic delta O-18 values that were generally less than the Holocene. Despite this warmth, significant ice-rafting episodes are recorded at several drill sites and piston-core locations throughout the Southern Ocean. This interval correlates with a eustatic highstand and represents a climatic optimum for the Southern Ocean when low- and mid-latitude species migrated into the Antarctic and surface waters were warmer than at present. Although it is generally agreed that this interval was warm in the southern hemisphere, divergent opinions exist as to whether this warming led to the partial decay or growth of the Antarctic ice-sheet. A distinct mid-Pliocene cooling occurred between approximately 3.5 and 3.2 Ma, as evidenced by an increase in delta O-18 values of planktonic and benthic foraminifers in Hole 704A, and by faunal changes indicative of progressive cooling of Antarctic surface waters and development of biotic provinciality in the Southern Ocean. During the late Pliocene (3.25-2.6 Ma), planktonic and benthic delta O-18 values commonly exceeded those of the Holocene and several distinct cooling and/or ice volume events occurred during the late Gauss Chron (most notably at 2.9 and 2.63 Ma). Faunal studies suggest several northward advances of the Polar Front Zone and expansion of sea ice in the Antarctic during this period. These events preceded the major climatic transition at approximately 2.4 Ma when the PFZ moved to the north, sea ice expanded in the Antarctic, and glaciation intensified in both hemispheres.