High-resolution chronostratigraphy from downhole susceptibility logging tuned by palaeoclimatic orbital frequencies

High-resolution magnetic records have been obtained by downhole measurement with the Geological High Resolution Magnetic Logging Tool (GHMT) in a well drilled in the North Sea for hydrocarbon exploration. This tool enables a continuous measurement of the polarity of the magnetic remanence and of the magnetic susceptibility throughout the drilled formations. A high-resolution magnetic chronostratigraphy between 2.6 and 1.6 Ma is deduced from the tie points given by the downhole magnetostratigraphy and from a detailed spectral analysis of climatically influenced variations of susceptibility. The orbital frequencies identified by spectral analysis of the magnetic susceptibility in the depth domain allow a refined estimation of the sedimentation rate and determination of a high-resolution chronology of the sequence. Characteristic features of the onset of Northern Hemisphere glaciation and its fluctuations during the Late Pliocene are identified through this analysis of the susceptibility record. Our analysis indicates that massive discharge events of non pelagic deposits — similar, but with a higher amplitude, to Heinrich events that are observed during the last glaciation — occurred between 2.5 and 2.0 Ma and were probably related to instabilities of the Laurentide ice sheet. The relative power of the orbital forcing factors is compared with that documented at other North Atlantic Ocean sites. Comparison with an insolation model does not indicate direct correlation but suggests a strong influence of the Northern Hemisphere ice cover and its local variations.