Early Pliocene paleoenvironment of the Sorsdal Formation, Vestfold Hills, based on diatom data

Comparison of diatom data from modern surface sediments in Prydz Bay and th e Kerguelen Plateau with diatom assemblages from the Sorsdal Formation, Ves tfold Hills, indicates that the climate was warmer than present during the early Pliocene (4.5-4.1 Ma). Extant, sea-ice associated diatoms are signifi cantly less abundant throughout the Sorsdal Formation than in the modem Ant arctic coastal zone. Extant diatoms in the Sorsdal Formation, including Ste llarima stellaris, Thalassiosira oliverana, Fragilariopsis sublinearis, Pse udo-nitzschia turgiduloides and Eucampia antarctica var. recta, are consist ent with annual sea-surface temperatures (SST) of between -1.8 and 5.0 degr eesC. The presence of S. stellaris indicates that the summer SSTs were >3 d egreesC during some intervals. The absence of calcareous coccoliths and the silicoflagellate Dictyocha suggests that the upper limit for summer SST wa s <5<degrees>C. These data indicate that early Pliocene summer SST were bet ween 1.6 and 3 degreesC warmer than today. Abundant Chaetoceros cysts infer that stratified, open-water conditions were present during summer/spring. Ice sheet models suggest that warming of the magnitude evident in the Sorsd al Formation (less than or equal to3 degreesC) should have resulted initial ly in increased snow accumulation and ice sheet growth. However, ice sheet growth was probably short-lived, as the long-term response to this warming in the early Pliocene resulted in a significant decrease in ice volume and deposition of the Sorsdal Formation. Other factors, such as increased basal -ice sliding and higher discharge (icebergs and melt-water), probably led t o significantly elevated ablation rates from the Pliocene ice sheet, result ing in ice sheet retreat. (C) 2001 Elsevier Science B.V. All rights reserve d.