Palaeomagnetism of Palaeogene strata from southern Zealandia: Implications for ice in the greenhouse

Abstract

When did the first ice form on Antarctica? Large, stable ice sheets started to appear in the Oligocene, but there may have been earlier, transient Palaeocene glaciations which left only brief traces in the sedimentary record. Correlation of such traces across the New Zealand region requires the accuracy provided by magnetostratigraphic dating. However, the sediments that may contain these traces have extremely weak magnetizations, high glaucony concentrations, and other characteristics which complicate magnetic measurement.

To address these problems, I developed measurement techniques for weakly magnetized samples and wrote a software package called PuffinPlot to process the measurements efficiently. (PuffinPlot is a fully-featured palaeomagnetic data plotting and analysis program also intended for use outside the scope of the thesis.) I also conducted an in-depth rock magnetic study to identify the remanence-bearing minerals and determine the effects of glaucony on magnetic behaviour. The rock magnetic results indicated that the remanence in the glauconitic sediments was carried by single-domain magnetite at extremely low concentrations, and the model I developed for remanence acquisition showed that this magnetite was capable of carrying a stable primary remanence.

Using PuffinPlot and the results from the rock magnetic experiments, I conducted palaeomagnetic studies of early Palaeogene sections at the mid-Waipara River in Canterbury, Fairfield Quarry in eastern Otago, and Campbell Island, 700 km south of New Zealand. At each site I also measured the anisotropy of magnetic susceptibility (AMS) to determine variations in palaeocurrent. The wide spacing of the sites allows regional effects to be distinguished from local ones. The sections had several features in common: very weak magnetization, necessitating special measurement and analysis protocols; poor response to alternating-field demagnetization, necessitating thermal demagnetization; and thermal alteration at relatively low temperatures, necessitating great-circle remagnetization analysis to infer primary remanences.

At Fairfield Quarry, I sampled a 25-metre composite section; 31 of the 58 sites sampled yielded usable data, all of them with reversed polarity. In conjunction with the known location of the K-Pg boundary within the section, this constrained the entire section to the C29r chron. At the mid-Waipara River, 9 sites (of 21 originally sampled) gave reliable directions, all reversed, constraining a 16-metre continuous section to the C26r chron. At Campbell Island, I sampled two sections in different parts of the island and constructed an integrated stratigraphy from a total of 38 site directions, which expanded the known duration of a major unconformity from around 9.5 Ma to 13.4 Ma.

The improved age constraints on the Campbell Island section allowed the unconformity there to be correlated with a change in palaeocurrent at the mid-Waipara River, with a previously reported Palaeocene horizon of ice-rafted debris from eastern New Zealand, and with known fluctuations in oxygen isotopes during the Palaeocene, implying an extensive glaciation. Some of the glauconitic horizons at Fairfield Quarry may also be linked to earlier transient glaciations. Since Antarctica was still attached to Australia and South America during the Palaeocene, these results imply that circum-Antarctic ocean gateways are unnecessary for Antarctic glaciation.