MAGNETIC-SUSCEPTIBILITY CYCLICITY AND MAGNETIC DISSOLUTION IN CRETACEOUS LIMESTONES OF THE SOUTHERN ALPS (ITALY)

Sampling of a reversed polarity interval previously reported from limestones of the "Albian" Valle del Mis section (Southern Alps, Italy) indicates that it represents a recording of chron M3 of Barremian age rather than a brief reversed polarity 'event' in the Cretaceous Normal Polarity Superchron (K-N). Hysteresis measurements suggest that magnetic susceptibility in these limestones is dominated by the paramagnetic clay fraction. Bulk magnetic susceptibility values exhibit a cyclicity which is correlated with the 100-kyr Milankovitch eccentricity cycle. The stable record of reversed polarity is interrupted at six stratigraphically narrow intervals (< 1 m) by the apparent lack of any mineral having a stable remanent magnetization. I suggest these "paleomagnetic barren" intervals mark selective dissolution of magnetite related to increases in organic carbon. Such dissolution may explain some patterns of magnetization and remagnetization observed in world-wide carbonates.