Late Neogene evolution of the Antarctic cryosphere as derived from paleo- and environmental- magnetic studies of New Harbour drill cores

Abstract

The Pliocene period is an attractive geological analogue from which to study the future behaviour of the cryosphere because global temperatures and atmospheric CO2 were inline with what is projected in the coming decades. A Pliocene succession recovered beneath the McMurdo Ice Shelf revealed repeated collapses of the West Antarctic Ice Sheet during the Pliocene period. This conflicts with geomorphic records from the Transantarctic Mountains, which have been used to argue for continued polar conditions since the middle Miocene. The New Harbour drill cores were examined because they contain Plio- Pleistocene sediments that were deposited in deep fiords located within the Transantarctic Mountains only a few kilometres from geomorphic records that were used to argue for stable polar conditions. New paleomagnetic age models were constructed for the core successions from AF and thermal demagnetisation of 559 specimens. Environmental magnetic records, which are sensitive indicators of the terrestrial climate evolution, were constructed for the drill core successions. In a parallel study, a database of the magnetic mineralogy of southern Victoria Land basement and cover rocks was compiled. The database was used to construct a model of the source to sink evolution of magnetic minerals and to identify how the modern climate is expressed in the offshore environmental magnetic records. The new age models indicate that the Taylor Glacier was thick and dynamic during latest Miocene and earliest Pliocene and that it retreated after 4.69 Ma. Maghemite and pure magnetite cycles between ∼4.9 Ma and ∼4.6 Ma in sediments are coherent with advance and retreat history of the Taylor Glacier. The presence of maghemite may indicate paedogenesis during interglacial periods under warmer and wetter conditions than today. Glaciers in the Taylor and Ferrar fiords retreated after ∼4.4 Ma resulting in the deposition of fine-grained sediments with very low magnetic concentrations. Magnetic mineralogy cycles are in phase with the benthic δ18O record and an up-core increase of coercivity may indicate greater input of terrigenous sediment by rivers. Evidence of terrestrial soil formation and ice-free fiords conflicts with the nearby contemporaneous geomorphic records used to argue for persistent hyper arid, modern style polar conditions. Significant unconformities after ∼4.2 Ma indicate a dynamic environment which agrees with the terrestrial geomorphic records of expansion of EAIS glaciers. Grounded eastern sourced ice entered New Harbour at 2.6 Ma and the appearance of paramagnetism in sediments may indicate a shift to a modern style climate and the onset of strong katabatic winds. A shift to sand dominated lithologies after ∼1.2 Ma indicates deposition in ice-dammed lakes during glacial maxima. Environmental magnetic behaviour of these sediments is identical to the behaviour of modern sediments from southern Victoria Land. The new chronologies of New Harbour successions demonstrate coherence between the emplacement age of the geomorphic features and unconformities in the drill cores, which were both caused by expansion of EAIS glaciers during the mid Pliocene. However, indications from New Harbour successions of much warmer than present conditions and deep, ice-free fiords conflict with contemporaneous evidence from the Dry Valleys of hyper polar conditions. It is possible that the geomorphic records are not sensitive indicators of climate evolution.