Sedimentary records of relative paleointensity of the geomagnetic field: Theory and practice

Abstract

the record prior to about 300 ka; thus they are probably not an inherent feature in the geomagnetic field, and the correspondence of the period of oscillation to that of obliquity is probably coincidence. 1.clides as •4C and •øBe, and the radiocarbon timescale is built upon the assumption that the rate of production is constant. Detailed calibration of the radiocarbon dates using tree rings and varved sediments of known age resulted in the discovery of rather large discrepancies [e.g., Stuiver et al., 1986], and these deviations can be reasonably well modeled as resulting from changes in Earth's magnetic field (see Figure 1). This calibration is available only for the last 10 ka or so, and direct calibration prior to that time is based on a very few spot calibration points comparing radiocarbon ages with those derived from thorium decay [see Bard Observatory measurements effects in order to obtain the contribution of the magnetic field. Under the ideal conditions shown in Figure 3, DRM normalized by magnetic activity yields estimates of the paleofield intensity B* that is linearly related to the ancient magnetic field. Of course, the slope relating B* and B may never be known. The experimental and theoretical basis for estimating [ma] is relatively undeveloped. I will highlight results from landmark papers that serve as the foundations for all sedimentary paleointensity studies. These papers provide hope that there may be sediments that retain a recoverable record of paleofield intensities if properly normalized. I will describe the conditions under which such records are created and the available techniques for analyzing them. Finally, I have collected some representative records of relative paleointensity from sediments and will draw a few general conclusions from them. The paleomagnetic literature suffers from confusion about units and terms and is therefore at times difficult to read and to interpret. I have redrafted all figures in order to standardize the units according to the Syst•rne International (SI). Table 1 lists some useful conversions among the various units in the original literature. I will describe all magnetizations in terms of moment rn or magnetization M. Magnetic fields are denoted either as H or as B. B is usually referred to as the magnetic induction, but often in the paleomagnetic context B is also called the magnetic field; the two will •20 be distinguished by using the appropriate units and associated letter. I note in passing that the change from using cgs units to SI units in the early 1980s has led to an enormous amount of confusion because many measurements are still made in cgs units and conversion is ubiquitous and frequently done incorrectly. Nearly every table in the paleomagnetic literature listing unit conversion contains confusing, misleading, or erroneous statements [see Payne, 1981; King et al., 1983;Butler, 1992]. I will adhere to the usage in Table 1 throughout the paper. Magnetic remanence is the magnetization measured in the absence of an applied field. I will call, by...

Paleointensity record from Pleistocene sediments (1.4–0 Ma) off the California Margin

“…The downcore changes in magnetic carrier concentration were investigated by using the concentration dependent records of ARM and magnetic susceptibility (Figures 5a, and 5b). The downcore changes of these two parameters do not exceed a factor of 3, a value that is well within the desirable range for records of relative paleointensity [Tauxe, 1993] and confirms the previous observations obtained with single samples. Except for some specific intervals, the variations depicted by the two parameters are globally identical.…”

The Blake geomagnetic excursion recorded in a radiometrically dated speleothem

“…After demagnetisation of these samples, an anhysteretic remanent magnetisation (ARM) was imparted at 200 mT followed by very detailed AF demagnetisation and finally acquisition of isothermal remanent magnetisation (IRM) at 200 mT. These experiments were conducted to study the magnetic properties variations (Supplementary Table Sl) and to perform relative palaeointensity (RPI) analyses (Tauxe, 1993). Stepwise acquisition of isothermal remanent magneti sation (IRM) up to 1800 mT followed by isothermal remagne tisation in three orthogonal directions and subsequent progressive thermal demagnetisation were carried out on selected samples in order to identify the magnetic mineralogy of the samples (Lowrie, 1990).…”

How does Chinese loess become magnetized?

“…All of the evidence from our experiments indicates that the postdepositional presence of water is crucial, above a threshold water content of ~40% (which effectively corresponds to water saturation), for producing an efficient PDRM in eolian sediments. If a sedimentary sequence is magnetically homogeneous, and if the remanence was acquired under uniform conditions, one would expect the NRM/ARM ratio to provide a proxy estimate of the relative geomagnetic field strength (e.g., Tauxe, 1993). In our experiments, the sediment is magnetically homogeneous and the field strength was constant, therefore NRM/ARM variations provide an indication of variations in the conditions under which the remanence was acquired.…”

“…In such a case, we can normalize the NRM by a parameter that provides an indication of magnetic mineral concentration such as ARM, IRM, or χ. Alternatively, one can use more complicated proxies that involve selective demagnetization of the NRM or of the respective normalizer to estimate the relative paleointensity. Tauxe (1993Tauxe ( , 2006 summarized several criteria for relative paleointensity studies using sediments.…”