Timing Between a Large Impact and a Geomagnetic Reversal and the Depth of NRM Acquisition in Deep-Sea Sediments

Abstract

The hypothesis that large meteorite impacts on earth may trigger geomagnetic reversals is partly based on the close stratigraphic occurrence of the Australasian microtektite layer to the Brunhes/Matuyama reversal boundary in deep-sea sediments. However, the precise timing between these two events is unclear because the original stratigraphic position of the microtektites has been altered by bioturbation and the position of the reversal boundary has been changed by post-depositional remanent magnetization. The palaeomagnetic stratigraphy and microtektite distributions from 20 deep-sea cores have been utilized to solve the problem. After correction for bioturbation, a model to account for the distance (Q) separating the microtektites and the reversal boundary includes the time difference (Δt) between the two events, the sediment accumulation rate (w), and an exponential term (n e^-w) representing the depth of NRM acquisition in an expression:

Q = Δtw + ne^-w

The fitted value for Δt is -11.8 ± 5.6 kyr, which means the microtektites were deposited 11,760 yr before the midpoint of the Brunhes/Matuyama reversal. Taking into account the uncertainty, and a reversal transition time of 5000 yrs, the impact either slightly predates or is essentially simultaneous to the reversal. However, this information does not prove causality between the two events. The second term in the model shows that depth of NRM acquisition decreases exponentially with increasing sedimentation rates. Although this result was unexpected, it probably reflects the relationship between sediment grain size and water content of the sediment.