TRANSIENT WARM EVENTS DURING THE EARLY PLIOCENE IN THE SOUTHERN OCEAN

Percent biogenic carbonate in surface sediments of the Southern Ocean marks the location of the overlying Polar Front Zone (PFZ). The transition across the PFZ is striking with high percent biogenic carbonate north of the PFZ and low values to the south. In sediments of the Last Glacial Maximum (LGM) the boundary between high and low biogenic carbonate is displaced some 6 degrees of latitude to the north in the Atlantic sector and lesser distances in the Indian and Pacific sectors. I suggest that this represents displacement of surface water isotherms and argue that percent biogenic carbonate can be used as an index of relative paleotemperature in the Southern Ocean, particularly during warm intervals when isotherms are expected to be displaced to the south. The utility of this approach is demonstrated by the presence (in the early Jaramillo magnetic subchron) of transient carbonate spikes in sediment cores to the south of the present PFZ. This carbonate spike is considered to be isochronous with the Oxygen Isotope Stage 31 extreme warm event (Froelich et al., 1991). Using this approach, four distinct warm events (e.g. carbonate spikes) are evident in the reversed section between the Thvera and Sidujfall magnetic subchron (4.40-4.47 myBP) of the Gilbert chron (early Pliocene). A lesser warming is seen in the Cochiti subchron (3.88-3.97 myBP). I refer to these as transient climate excursions and suggest that the discrete warm events between 4.40 and 4.47 myBP may be isochronous with early Pliocene sediments recorded in Marine Plain, East Antarctica.