Variability of the climatic antarctic system during the Plio-Pleistocene: Paleomagnetism contribution

Abstract

In the framework of a gradual global warming, which is one of the topic of major interest in the recent years and which importance is resumed in the Intergovernmental Panel on Climate Change (IPCC), it is important the study of the variability of the Earth’s system at the high latitudes i.e., in Artic and Antarctic areas, because these are the regions more sensitive to climatic changes. The possibility to study marine sedimentary sequences from Antarctica thus represented an important opportunity to investigate such climatic variability. Cold water mass formation in the Southern Ocean is involved in the global thermohaline circulation (THC) through the convection and inter-ocean exchanges of surface, intermediate and bottom waters. This work focus on the study of marine sequences cores from mid-high latitudes from strategic locations far and near the continental margin of Antarctica: (1) ANTA 95-157 (62°05.95'S) and ANTA 96-16 (66°20.09'S) cores are a long transect from New Zealand and Ross Sea slightly south of the present day Polar Front and the Pacific Antarctic Ridge, respectively; (2) MD97-2114 (42°22.32'S) core is on the northern side of Chatham Rise, east of New Zealand; (3) ODP Site 1166 (67°41.77'S) and 1167 (66°24.01'S) are in Prydz Bay continental shelf and slope, respectively. It has been provided a detailed magnetostratigraphy of the sequences and have been investigated magnetic proxies, which reflect variations in mineralogy, grain-size, and concentration of the magnetic fraction. Environmental magnetic data, along with the chronology derived by the magnetostratigraphy, provided an important contribution to our understanding of environmental and climate changes during the time intervals here examined. The cores ANTA and MD97-2114 span the shift from predominant 41-ka to predominant 100-ka glaciation cycles at around 1.0–0.9 Ma, (the Mid-Pleistocene Climate Transition (MPT)) that was centered at 922±12 ka and lasted about 40±9 ka. The amplitude of the 100-ka cycle abruptly increased much later on, at 641±9 ka. The transition was accompanied by an increase in 18O and decrease in 13C but the causes may be many and further information about changes in global ice volume during this period needed. In this core it has been found evidence of orbital influence on sedimentary processes. The core MD97-2114 records long-term variation of the upper Circumpolar Deep Water component of the Deep Western Boundary Current and manifests a stepwise modification of the THC during the transition. ODP Site 1166 and 1167 provide a record of the process involved in the story of the glacial expansion and retreat of the ice at the margin of the continent in Prydz Bay during the Plio-Pleistocene. Preliminary results from SEDANO cores shows downcore variations in concentration, mineralogy, and grain-size of magnetic minerals. In particular, there is a relative increase of moderate coercivity respect to low-coercivity minerals (magnetite) during glacials and a millennial scale variability of the magnetic grain size characterizes the last glacial (core SED 12 and 13) and it may be related to changes in the bottom current velocity.