Recent progress on rock and paleomagnetism by means of deepsea drilling

Abstract

This review considers the progress in rock and paleomagnetism since the start of the Integrated Ocean Drilling Program (IODP) in 2003. Here, we focus on geomagnetic paleointensity and related topics, although many other studies have applied rock and paleomagnetic techniques to achieve various aims on IODP expeditions. High-quality, relative paleointensity records derived from North Atlantic cores have led to the establishment of a high-resolution paleointensity stack for the last 1.5 m.y. Several relative paleointensity records of the Eocene to Miocene have been obtained from equatorial Pacific cores; previously, few records were available before 3 Ma. These records show large-amplitude paleointensity variations on timescales of 10⁴ year during a stable polarity, including paleointensity minima similar to those at polarity transitions. These are similar to the variations known for the last 800 kyr, and are considered to be characteristic of geomagnetic field variations. Inter-core correlation and age estimation using relative paleointensity variations can now be applied back to the Eocene. It has been proved that variations in the magnetic properties of sediments, which are induced by paleoenvironmental changes, influence estimates of relative paleointensity. In particular, fluctuations in the relative abundances of biogenic and terrigenous magnetic minerals and sedimentation rates contaminate relative paleointensity records. To overcome this problem, we propose two possible strategies: (1) extraction of a real geomagnetic component using principal component analysis from a global set of relative paleointensity records, obtained from various oceanographic settings, and (2) independent estimation of paleointensity using beryllium isotope ratios. It is also important to gain a better understanding of the biogeochemical remanent magnetization carried by biogenic magnetite. The successful recovery of uncontaminated paleointensity records will enable us to explore controversial issues in paleomagnetism, such as correlations between geomagnetic field intensity and polarity length, and possible linkages among geomagnetic field variations, the Earth's orbit, and paleoclimatic change.