Non-uniform occurrence of smallterm fluctuations in the geomagnetic field? New results from Middle to Late Miocene sediments from the North Atlantic (DSDP Site 608)

Abstract

New magnetostratigraphic results from DSDP Site 608 in the North Atlantic reveal the presence of nine short-term polarity fluctuations that do not correspond to subchrons in the most recent GPTS. Comparison with published results from ODP Sites 845 and 1092, and from the continental Orera Composite Section, shows that all these polarity fluctuations are observed in more than one record. At DSDP Site 608, five of the intervals are ascertained by more than two samples and have an estimated duration that is larger than 10 kyr. We regard these as polarity subchrons, which define five new short polarity intervals in the Miocene: C4r.2r-1n, C4Ar.1r-1n, C5r.2r-1n, C5r.2r-2n, and C5r.3r-1n. Three polarity fluctuations within C5n and one in C5An.2n are each only determined by one sample and are hence not unambiguous. They have an estimated duration < 10 kyr and an inconsistent expression. Consequently, we believe they qualify better as directional excursions and that they are most likely associated with decreases in paleointensity (DIPs) of the geomagnetic field. The identification of these nine new magnetic events suggests that the occurrence of short-term polarity fluctuations in the geomagnetic field is indeed non-uniformly distributed through time. For example, they are relatively more common in the early Late Miocene and the Pleistocene but virtually absent in the latest Miocene and Pliocene. It remains uncertain, however, if this is related to real behavior of the geodynamo. The detection of several new short polarity subchrons in the Middle to Late Miocene time interval may be explained by earlier registration problems in the magnetic anomaly patterns, while the absence of such features in the latest Miocene-early Pliocene could be related to paleomagnetic records of inferior data quality.