RELATIVE CHANGES OF THE GEOMAGNETIC-FIELD INTENSITY DURING THE LAST 280 KYEAR FROM PISTON CORES IN THE ACORES AREA

Relative changes in geomagnetic field intensity over the last 280 kyea rs have been recovered from the study of three marine cores from the A cores area. One core was precisely dated by oxygen isotope study and t he other two records were linked to it using light reflectance analysi s which allowed precise correlation. Rock magnetic analysis shows that the main magnetic mineral is magnetite with a very homogeneous grain- size distribution in the pseudo-single domain range for the three core s. Changes in the amount of magnetite do not exceed a factor of 10. Th erefore, these cores appear to be suitable for relative palaeointensit y determinations. Two mineral magnetic components with periodicities o f 23 and 18 kyears are present in the records of bulk magnetic paramet ers and natural remanent magnetisation (NRM), but the power at these f requencies is not significant at the 95% level for the normalised rema nence records. For each core, normalisation of the NRM using different normalising parameters yields virtually identical results. Using satu ration isothermal remanent magnetisation (SIRM) as the normalising par ameter, the results of the three cores were then combined into a stack ed curve. Squared coherence analysis between this stacked curve and bu lk mineral-magnetic parameters reveals that the 18 kyear component is still present in the record, but that the power spectrum is barely abo ve noise level at this frequency. Some of the features of this North A tlantic record are consistent with the main characteristics already do cumented in other sedimentary or volcanic records. For instance, disti nct periods of low intensity are observed around 40, 120, and 190 kyea rs, and periods of high intensity at 50 and 80 kyears. Because of the uniformity of the mineral-magnetic characteristics of the three cores, we suggest that this record may be a suitable palaeointensity referen ce curve for the Central North Atlantic Ocean region.