Early Pliocene paleoenvironment of the Sùrsdal Formation, Vestfold Hills, based on diatom data J.M. Whitehead a, * , P.G. Quilty b,1 , D.M. Harwood c,2 , A. McMinn a,3 a Antarctic CRC/Institute of Antarctic and Southern Ocean Studies, University of Tasmania, G.P.O. Box 252-80, Hobart, Tasmania 7001, Australia b School of Earth Sciences, University of Tasmania, G.P.O. Box 252-79, Hobart, Tasmania 7001, Australia c Department of Geosciences, University of Nebraska Ð Lincoln, Lincoln, NE 68588-0340, USA Received 22 December 1999; revised 26 October 2000; accepted 8 November 2000 Abstract ComparisonofdiatomdatafrommodernsurfacesedimentsinPrydzBayandtheKerguelenPlateauwithdiatomassemblages from the Sùrsdal Formation, Vestfold Hills, indicates that the climate was warmer than present during the early Pliocene 4.5±4.1 Ma). Extant, sea-ice associated diatoms are signi®cantly less abundant throughout the Sùrsdal Formation than in the modern Antarctic coastal zone. Extant diatoms in the Sùrsdal Formation, including Stellarima stellaris, Thalassiosira oliver- ana, Fragilariopsis sublinearis, Pseudo-nitzschia turgiduloides and Eucampia antarctica var. recta,areconsistentwithannual sea-surface temperatures SST) of between 21.8and5.08C.Thepresenceof S. stellaris indicates that the summer SSTs were .38C during some intervals. The absence of calcareous coccoliths and the silico¯agellate Dictyocha suggests that the upper limitforsummerSSTwas ,58C.ThesedataindicatethatearlyPliocenesummerSSTwerebetween1.6and38Cwarmerthan today. Abundant Chaetoceros cysts infer that strati®ed, open-water conditions were present during summer/spring. Ice sheet models suggest that warming of the magnitude evident in the Sùrsdal Formation #38C) should have resulted initially 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 signi®cant decrease in ice volume and deposition of the Sùrsdal Formation. Other factors, such as increased basal-ice sliding and higher discharge icebergs and melt-water), probably led to signi®cantlyelevatedablationratesfromthePlioceneicesheet,resultinginicesheetretreat. q 2001ElsevierScienceB.V.All rights reserved. Keywords: Antarctica; Ice sheet; Pliocene; diatom; Sùrsdal Formation 1. Introduction The aim of this study is to reconstruct the paleo- environmental conditions of the Sùrsdal Formation, marine sediments deposited in the Vestfold Hills, Antarctica, during Pliocene 4.5±4.1 Ma) climate warming and glacial retreat Pickard et al., 1988; Harwood et al., 2000; Quilty et al., 2000) Fig. 1). This study will reconstruct the summer sea-surface temperature and sea-ice conditions through the use Marine Micropaleontology 41 2001) 125±152 0377-8398/01/$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII:S0377-839800)00060-8 www.elsevier.nl/locate/marmicro * Corresponding author. Tel.: 161-3-6226-7888; fax: 161-3- 6226-2973. E-mail addresses: jm_whitehead@hotmail.comJ.M.Whitehead), P.Quilty@utas.edu.au P.G. Quilty), dharwood@unlserve.unl.edu D.M. Harwood). 1 Tel.: 161-3-6226-2814; fax: 161-3-6223-2547. 2 Tel.: 11-402-472-2648; fax: 11-402-472-4917. 3 Corresponding author.