MAGNETIC PROPERTIES OF THE AND-2A CORE, ANDRILL SOUTHERN McMurdo SOUND (SMS) PROJECT, ANTARCTICA

During the austral summer of 2007, the ANDRILL Southern McMurdo Sound (SMS) Project cored Site AND-2A (77°45.488’S, 165°16.613’E, ~383 m water depth) in order to recover sediment from the middle Miocene, in an ice-proximal setting, which has long been held as one of the fundamental time intervals in development of the modern Antarctic ice sheets. The stratigraphic section recovered was divided into 14 distinct Litho-Stratigraphical Units (LSU). We present here the magnetic properties (hysteresis loops, saturation and backfield curves) of 849 samples collected from the AND-2A core. Mr ranges from 146.927 to 0.016 Am2/kg with an arithmetic mean of 12.7 and Ms ranges from 4169.677 to 0.668 with an Am2/kg arithmetic mean of 71.25. Those parameters have a similar trend, which shows wide long-wavelength variations below 436.18 mbsf. Hcr, with an average of 50.57 mT and standard deviation of 18.45 (36.5%), has a similar trend of Hc, which has an average of 20.2 mT and standard deviation of 7.03 (34.83 %). Hcr and Hc trends show fluctuations below 436.18 mbsf but with lower amplitude than Mr and Ms. Hcr and Hc have a trend opposite to the one of Mr and Ms. The stratigraphic trend of Mr and Ms mirrors that of the magnetic susceptibility measured both on the whole core and on discrete samples. Overall, the main magnetic carrier is magnetite but, at this stage, occurrence of other magnetic minerals is not excluded. When plotted on a Day Plot, the data lie within the Pseudo Single Domain (PSD) field for most of the samples and are compatible with mixtures of Single Domain (SD), PSD, and Multi Domain (MD) magnetites. Even if the data appears to define two distinct clusters, one tightly grouped between 0.16-0.2 Mr/Ms and 2-2.3 Hcr/Hc and another one, more disperse, around 0.2-0.3 Mr/Ms and 2-3 Hcr/Hc, these two clusters are not directly related to distinct stratigraphic intervals or lithofacies. Magnetic properties vary along the core in relation to lithology and LSU in a complex mode. We recognize that there is a tendency for finer magnetic grain sizes and higher Mr and Ms to occur within periods when Site AND-2A was in a marine setting (warmer and more distal from the ice sheets), and for intervals with coarser magnetic grain sizes and lower Mr and Ms to occur in diamictite layers and hence to correlate to periods when the site was proximal to or underlying an ice sheet.