Evidence for Relative Climatic Stability of Antarctica During the Early Pliocene: A Marine Perspective

ABSTRACT

Before ∼3 Ma, during the early Pliocene, polar ice sheets were restricted to Antarctica, and much evidence suggests global warmth greater than at any time during the last seven million years, including today. Did the East Antarctic Ice Sheet withstand this early Pliocene warmth or did it experience major instability and deglaciation? This problem is of central importance in considering the consequences of global warming that might result from anthropogenic increases in atmospheric CO₂ and other greenhouse gases.

Marine stratigraphic data from the southern high latitudes is incompatible with earlier suggestions that invoke major Pliocene warming of the Antarctic continent, increase in temperature of the surrounding Antarctic water mass to 5–10°C, and associated major deglaciation (>50% ice volume reduction) of the East Antarctic Ice Sheet. An oxygen isotopic record of high stratigraphic resolution from the Subantarctic region indicates an increase in average sea surface temperatures of no more than ∼3°C during the warmest interval of the Pliocene (4.8 to 3.2 Ma). These data suggest that the Antarctic ice sheets did not experience major deglaciation during this warmer interval. Instead the oxygen isotopic data indicate relative stability of the Antarctic climate/cryosphere system during the early Pliocene compared with the large-scale fluctuations that mark the latest Miocene prior to ∼4.8 Ma. We suggest that the well known early Pliocene marine transgression was marked by sea levels no higher than 25 m above the present, and probably significantly less than this for most of this interval.

Relative stability of the Antarctic cryosphere during the early Pliocene is supported by other evidence. The deep sea carbon isotopic record lacks large changes in δ¹³C values that would have resulted from major glacioeustatic changes. Significant ice rafting of sedimentary detritus to the Subantarctic far to the north of the Antarctic continent continued throughout the entire Pliocene, indicating a continued presence of major ice sheets on Antarctica even during the warmest Pliocene intervals. An almost complete lack of biocalcareous sediments in Antarctic waters during the Pliocene and continued dominance of biosiliceous sediments suggests oceanographic conditions broadly similar to those of the present day. This would not have been the case if the surface waters in the Antarctic had increased by more than 5°C, reflecting values of present day southern Subantarctic waters. Limited stratigraphic evidence suggests that marine sedimentary basins adjacent to West Antarctica have been relatively starved of terrigenous sediments throughout the Pliocene and Quaternary, probably as a result of the near-permanency of the East and perhaps the West Antarctic Ice Sheets since the Miocene, thereby reducing continental erosion.

Early Pliocene planktonic microfossils suggest warmer average surface water temperatures in the Antarctic compared with the present day. However, conditions were not sufficiently warm to affect increasing endemism of Pliocene siliceous planktonic assemblages. Biosiliceous faunas and floras dominate throughout the Pliocene and there is no return of calcareous nannofossil deposition which had ceased during the late Miocene. During the Pliocene, conditions were never sufficiently warm to cause significant displacement of Antarctic by Subantarctic planktonic assemblages.

The marine sedimentary record of oceanographic, climatic and cryospheric stability in the Antarctic during the early Pliocene agrees with predictions based on coupled ocean-atmospheric models of relatively stable Antarctic sea surface temperatures in response to global warming.