THE AGE OF THE AUSTRALASIAN IMPACT EVENT AND ITS RELATION TO EVENTS AT THE BRUNHES/MATUYAMA BOUNDARY

Tektites are natural glasses formed by melting of upper crustal materials during the hypervelocity impact of an extraterrestrial object. The Australasian strewnfield, covering at least one-tenth of the Earth's surface, is the largest and the youngest of four known tektite strewnfields. The stratigraphic proximity of the Australasian microtektite layers with the Brunhes/Matuyama boundary has prompted the hypothesis that large impacts may trigger geomagnetic reversals. As the Australasian microtektite preceded the Brunhes/Matuyama reversal by estimates of ~12 to 16 kyr, these results are inconsistent with simple geophysical models linking geomagnetic reversals with impact events. Recently, detailed paleomagnetic records from the Pacific and Atlantic Oceans consistently found a decrease in paleointensity approximately 15 kyr prior to the Brunhes/Matuyama transition. In some of these records, a directional excursion to nearly full normal polarity with a low paleointensity remains after demagnetization. The close temporal proximity of the pre-transition low (the so-called precursor) to the B/M transition raised questions about its relationship to the reversal itself as well as to an apparently unrelated event - the Australasian impact, that just occurred before the reversal. A through examination of the question hinges on how accurate one can define the temporal relationships among these events. In this study, we (1) present detailed paleomagnetic records from deep-sea cores through the Brunhes/Matuyama reversal interval in association with well-defined Australasian microtektite layers, (2) reassess the lock-in depth of post-depositional remanence acquisition based upon a compilation of eight deep-sea cores, and (3) test the impact-reversal hypothesis by examining the stratigraphic and temporal relation between the Australasian impact event and the precursor of the B/M reversal.