Recording and dating geomagnetic reversals, events and excusrions in palaeokarst and pseudokarstic deposits from southern Africa and Australia

English Abstract:

This paper outlines the use of caves and palaeokarstic deposits for the recording and dating of high resolution Plio-Pleistocene geomagnetic field data from the southern hemisphere; including magnetic reversals, events and excursions. Examples are given of recently discovered high resolution magnetic reversals in palaeocave sediments from South Africa, including the base of the Olduvai SubChron and an event at ~2.047-2.005 Ma based on the direct uranium-lead dating of the speleothem. This is somewhat younger than recent estimates for the Huckleberry Ridge excursion based on other methods, but older than estimates for the Pre-Oluvai excursion at ~1.977 Ma. Two short geomagentic field events are dated to slightly later than 2.069 and 1.959 Ma at a second cave and could incorporate both the Huckleberry Ridge and Pre-Olduvai excursions. However, in all cases full normal polarity directions are recorded, suggesting these are perhaps events, not excursions. In an older flowstone, another event is recorded and dated to between 2.38 and 2.24 Ma, suggesting that it relates to the older Réunion event at ~2.20 Ma. X-ray fluorescence microscopy and X-ray absorption spectroscopy at the Australian Synchrotron has been used to map iron within the speleothems, identify areas of potential recrystalisation and identify the main magnetic minerals present. This work, combined with palaeomagnetic and mineral magnetic analysis indicates that while the remanence is carried by detrital magnetite, this constitutes only a few percent of the overall iron mineralogy within the speleothems. This work indicates that recystalisation can have an effect on the palaeomagnetic signal in spelothem and so it is important to evaluate this through thin section analysis. Micromorphology has also been used to evaluate the potential effects of sedimentary structures on the geomagentic field data recorded from cave sediments. This work shows the great potential of Pliocene and Pleistocene cave deposits for recording geomagnetic field data in high resolution as well as the direct dating of reversals, events and excursions, but also highlights the difficulties and new challenges faced in this work. The potential of younger pseudokarstic sea caves are also highlighted, including the direct dating of the Punaruu Event (1.11 and 1.09).