Fast geomagnetic field variations recorded in glacial Black Sea sediments

Over the last 12 years, from magnetostratigraphic investigation of 16 sediment cores from the Southeastern Black Sea, a high-resolution paleo-secular variation record spanning from 64 to 14 ka could be compiled. Due to glacial erosion in the catchment sedimentation rates are in the range of 25 to 50 cm/kyr. Obtained relative paleointensity data reflect a highly dynamic geomagnetic field during that time. By comparison to absolute paleointensity data it could be estimated that the field intensity varied between about 3 µT and 60 µT (present day field intensity 48 µT). The lowest value is associated with the Laschamps polarity excursion centred at 41.0 ka. Its N-R and R-N transitions lasted just 200 and 270 years, respectively, with a 440 year long stable reversed phase (intensity max. 12 µT, VGP latitudes higher than 65°S) in between. The second deepest intensity minimum at 64.5 ka with about 8 µT is associated with fast directional changes but still in the range of normal secular variation (VGP latitudes above 45 °N). This is likely the mid-latitude expression of the postulated Norwegian-Greenland Sea excursion, as derived from Arctic marine sediments. In contrast to this, the Mono Lake excursion at 34.5 ka was characterized by intermediate field intensities of 20 µT, but clearly non-dipolar directional variations, indicating a multi-polar field configuration during the excursion. Another moderate field minimum of 25 µT at 18.5 ka is coeval with the postulated Hilina Pali excursion. But, despite high sedimentation rates (50 cm/kyr), only normal secular variation amplitudes were recorded in Black Sea sediments with VGP latitudes higher than 60°N. Ongoing re-sedimentation experiments with Black Sea mud indicate a very rapid lock-in of the paleomagnetic signal and a fairly linear relationship between field strength (1.8 to 110 µT) and acquired magnetization. Thus it can be concluded that data from Black Sea sediments reflect true geomagnetic field behaviour.