New statistical analysis on diatom proxy data for the estimation of past Southern Ocean properties

The annual sea-ice cycle of formation and retreat influences atmospheric and oceanic processes on a global scale. An understanding of the paleo environmental occurrence of sea ice is therefore necessary when studying past and future climatic change. Currently, diatom abundances provide the most robust proxy for Southern Ocean paleo sea-ice cover and extent. Therefore, this thesis evaluated the performance of four statistical models on a southern hemisphere diatom relative abundance and winter sea-ice concentration (wSIC) training dataset. A Generalised Additive Model (GAM) provided the most robust estimates of wSIC when compared with the Modern Analog Technique, the Imbrie and Kipp transfer function, and Weighted Averaging Partial Least Squares. The GAM derived wSIC estimates were comparable with previously published paleo sea-ice data. The application of GAM was then focused on estimating a new wSIC record throughout the southwest Pacific sector of the Southern Ocean during the Last Glacial Maximum, the ensuing deglaciation, and the Holocene. A diatom proxy for February sea-surface temperature and the sea-surface height signature of the Polar Front’s southern branch were developed to complement the paleo wSIC estimates. The GAM derived estimates provided the first indication that wSIC increased during the Antarctic Cold Reversal and early Holocene. Paleo wSIC and February sea-surface temperature data suggested that the Last Glacial Maximum lasted for ~11 kyr BP throughout the southwest Pacific, similar to the well documented extended Last Glacial Maximum of New Zealand. Notably, the Last Glacial Maximum was distinguished by two cold phases separated by a period of climatic amelioration, known as Antarctic Isotopic Maxima 2. The proposed diatom proxy for the sea-surface height signature of the Polar Front’s southern branch indicated a northern migration of this Polar Front jet, prior to the expansion of wSIC, during the Last Glacial Maximum and the Antarctic Cold Reversal. A northern migration of the Polar Front’s southern jet, and resulting expansion of wSIC, resulted from a northern migration of the westerly winds and the subsequent northern migration of the Antarctic Circumpolar Current.