Bipolarity, the presence of a species in the high latitudes separated by a gap in distribution across the tropics, is a well-known pattern of global species distribution. But the question of whether bipolar species have evolved independently at the poles since the establishment of the cold-water provinces 16-8 million years ago, or if genes have been transferred across the tropics since that time, has not been addressed. Here we examine genetic variation in the small subunit ribosomal RNA gene of three bipolar planktonic foraminiferal morphospecies. We identify at least one identical genotype in all three morphospecies in both the Arctic and Antarctic subpolar provinces, indicating that trans-tropical gene flow must have occurred. Our genetic analysis also reveals that foraminiferal morphospecies can consist of a complex of genetic types. Such occurrences of genetically distinct populations within one morphospecies may affect the use of planktonic foraminifers as a palaeoceanographic proxy for climate change and necessitate a reassessment of the species concept for the group.

Strontium isotope stratigraphy provides a chronology for Cenozoic cryogenic strata in the northern Antarctic Peninsula and allows an assessment of diachronism in onset of glacial conditions between East and West Antarctica. The earliest observed event on the Antarctic Peninsula was late Early Oligocene (29.8 0.6 Ma) (at least c. 4 Ma later than in East Antarctica), with a second in the early Early Miocene (22.6 0.4 Ma). Both glacials and intervening interglacial were continent-wide phenomena. Two late Neogene glaciations are distinguished: early Late Miocene (9.9 0.97 Ma) (Hobbs Glacier/Jones Mountains) and latest Miocene (Alexander Island). A further late Neogene glacial deposit (Weddell Formation) can be constrained only as

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Strontium isotope stratigraphy is used to date two interglacial-marine deposits in the Antarctic Peninsula region. On King George Island, interglacial pectinid-rich sediments in the Low Head Member of the Polonez Cove Formation give a strontium isotope stratigraphy age of 29.0 +0.7 0.6 to 29.8 +0.8 0.7 Ma (mid-Oligocene), which, in conjunction with previous K-Ar dating of volcanic rocks, indicates a glacial episode in the Antarctic Peninsula between middle Eocene (42.0 1.0 Ma) and mid-Oligocene time. In addition, an inter-glacial deposit (Pecten Conglomerate) from tectonically-elevated exposures on Cockburn Island is dated as Pliocene (3.5-5.3 Ma). Published data suggest these latter sediments were deposited under shallow marine conditions, which were warmer than those of present-day Antarctica.