Environmental factors as complicating influences in the recovery of quantitative geomagnetic-field paleointensity estimates from sediments

We have recently recovered replicate records of the Earth's magnetic-field (relative) paleointensity for 12-71,000 years BP from marine sediments of the western North Atlantic Ocean. Our records are remarkably similar to two other recently published relative paleointensity records from marine and lacustrine sediments in Europe. This suggests to us that, even over this wide region, sediments may serve as reasonable, correlatable recorders of fluctuations in the strength of the Earth's magnetic field. If this is true, then similar records from around the globe could eventually provide valuable information about the deep-Earth processes which create the field. We note, however, that our intensity record is significantly correlated with the down-core ratio of magnetic susceptibility (chi) to anhysteretic remanent magnetism (ARM). This ratio is a measure of the relative grain size of magnetite (the primary magnetic mineral in these sediments), and as such is a sediment magnetic property which is controlled only by the local depositional environment (and indirectly by global climate). We are concerned that the European records may be similarly biased by climatic or other environmental factors, possibly synchronous with ours. We caution that extreme care must be taken to understand and remove any such magnetic influences from sediment relative paleointensity records before they are used as quantitative estimators of the past intensity of the Earth's magnetic field.