Influence of meteorite impacts on the geomagnetic field from the Manicouagan and Rochechouart craters

The Manicouagan (Canada) and Rochechouart (France) impact structures are similar in age and could be the product of a fragmented meteorite that struck Earth nearly simultaneously at different places on the globe. We pose the question whether either, or both, of these events influenced geomagnetic activity. Our paleomagnetic investigations show that suevite and impact melt from these craters possess distinctly different magnetic characteristics. For Manicouagan, stepwise demagnetization reveals a single normal polarity magnetization direction carried by magnetite with minor hematite - both in surface outcrops and in drill cores to 1400 m depth. The minor hematite fraction decreases with depth in the cores and is likely related to surface weathering. For Rochechouart, stepwise thermal demagnetization reveals three kinds of stable magnetization directions: solely normal, solely reversed and others with multiple, often antipodal directions. The magnetic carriers in the Rochechouart lithologies consist of ilmeno-hematite and magnetite; the multiple carriers could contribute to antiferromagnetic coupling that leads to self-reversal behavior. If so, and if the antipodal directions are not manifest of a geomagnetic reversal, then the question remains as to which of the polarities is representative of the Late Triassic field at the time of impact. We will present rock magnetic, stepwise demagnetization, and paleointensity results from both craters.