Brad Pillans

The last major polarity reversal of the Earth's magnetic field, known as the Matuyama-Brunhes polarity transition or reversal, has long been used as a chronostratigraphic marker in Pleistocene studies. The Matuyama-Brunhes boundary (MBB), dated at 0.78 Ma, has been identified globally in both marine and continental sequences and is also a key time marker for the chronology of human evolution and migration. It has gained wide acceptance as the boundary between the Lower and Middle Pleistocene, although it has never been formally defined as such. A working group of the international Subcommission on Quaternary Stratigraphy is currently considering the status of the boundary.

In New Zealand, the MBB is precisely located in shallow marine sediments of Wanganui Basin, where it corresponds with the base of the New Zealand Putikian Substage. A combination of marine biostratigraphy, sequence stratigraphy and tephrostratigraphy permit correlation from Wanganui Basin to other on-land sections and deep-sea cores (Fig. 1). Major loess/paleosol sequences, in both islands of New Zealand, post-date the MBB.

In Australia, the MBB is identified in many continental sequences, particularly saline lake basins. A major arid shift in paleoclimate is interpreted to have succeeded the MBB at sites including Lakes Lewis (NT), Lefroy (WA) and Bungunnia (NSW), as well as in southern South Australia (Adelaide and Kangaroo Is.), and is also evidenced by increased aeolian dust concentrations in Tasman Sea cores (Fig. 1). At Lake Amadeus the arid shift predates the MBB, perhaps as a result of differing responses to regional and local hydrologic thresholds.

During the period from about 1 to 0.6 Ma there is a marked change from 40 ka to 100 ka cyclicity in oxygen isotope records from deep-sea cores, often called the Mid-Pleistocene Transition (MPT) ­ Fig. 1. Placement of the Lower-Middle Pleistocene boundary at the MBB, which occurs in the middle of the MPT, would constitute a widely recognisable chronostratigraphic marker in both marine and continental deposits.