RELATIVE VARIATIONS IN GEOMAGNETIC INTENSITY FROM SEDIMENTARY RECORDS: THE PAST 200,000 YEARS

Abstract

Eighteen records of relative paleointensity from marine sediments have been compiled in order to retrace the global changes in geomagnetic intensity during the past 200 kyr. Before dealing with any other source of noise we focused on problems inherent to the depth versus time correlations. The accuracy and resolution of dating (mostly deduced from isotopic stratigraphy) were identified as significant factors of distortion; we found also that different time scales have been used by authors. After readjustment with respect to the same time scale, the records were stacked together and the statistical significance of the results tested by a bootstrap technique. Seventeen records shown to be coherent were combined into a synthetic curve defined as Sint-200. Other stacks, combining various subsets of data, were found to be identical to Sint-200, except for changes in dispersion. In fact, the dispersion around Sint-200 could be reduced further by improving the correlations without violating the stratigraphy, a procedure which, at least for some records, emphasizes the need for more detailed depth-time correlations. Another characteristic emerging from the results is that paleointensity features as short as 10 kyr are observed between 120-200 ka, and can thus, in principle, be resolved. However, subsequent refinement is still necessary for this period which is documented by fewer records than the following 120 kyr. Finally, we compared Sint-200 with the records from the Labrador Sea and the Lac du Bouchet (LdB), both of which rely mostly on 14C chronology. The two records show satisfactory agreement with Sint-200 when transferred with respect to the age model established by correlating their δ18O (Labrador) and susceptibility features (LdB) with the δ18O reference record. The offset of 7 kyr at 40 ka measured in both cases between the 14C and δ18O ages is higher than the value deduced from a recent model of a geomagnetic field control of 14C production [1] but is not inconsistent with recent U-Th dates.