A PALEOMAGNETIC AND GEOCHEMICAL RECORD OF THE UPPER COCHITI REVERSAL AND 2 SUBSEQUENT PRECESSIONAL CYCLES FROM SOUTHERN SICILY (ITALY)

A detailed paleomagnetic and geochemical study of the upper Cochiti (N-R) reversal recorded in marine marls on southern Sicily shows two consecutive and very rapid transitions (R-N and N-R) that coincide with distinct lithological boundaries. A 'Fe-migration model' is presented in which magnetite is formed under different diagenetic conditions. During sulphate reduction, originally formed ('primary') magnetite is preserved. This primary magnetite has recorded the pre-transitional normal polarity. After burial, migration of ferrous iron occurs and subsequent oxidation produces 'secondary magnetite', which records the post-transitional reversed polarity and produces the two apparent transitions. In addition, low temperature (LT) and high temperature (HT) components record the reversal in a slightly different way: the HT component being acquired with a delay. It is probable that both components reside in magnetite but have a different origin and grain size distribution, giving different blocking temperature spectra. The actual upper Cochiti reversal occurs at a level with an estimated astronomically calibrated age of 4.165 Ma.