Paleointensity of the geomagnetic field during the last 80,000 years

Abstract

High-resolution records of the relative paleointensity of the geomagnetic field have been obtained from five marine cores. Three duplicate records were used to estimate the regional coherency of the data within a single area (Tyrrhrenian Sea) while the two others document the field variations in the eastern Mediterranean and the southern Indian Ocean. Careful investigations of distinct rock magnetic parameters have established the downcore unifornfity of the sediments in terms of magnetic nfineralogy and grain sizes. The time-depth control was provided by oxygen isotopes, and small-scale variations in the deposition rates were constrained by means of tephrachronology. The synthetic curve calculated from the Mediterranean records provides a continuous record of the intensity variations during the last 80,000 years (80 kyr), which correlates well with the sparse volcanic data available for the period 0-40 kyr. The fact that identical behavior is seen in both data sets and that they also compare quite well with results from a core collected in the Pacific Ocean establishes the truly dipolar character of these variations. The dipole field moment is characterized by large-scale changes as shown by the existence of pronounced drops (at 39 and 60 kyr) alternating with periods of higher intensity. The record suggests a periodic nature for these intensity variations; however, the period studied is not sufficiently long to state this conclusively. These results demonstrate the potential of sediments for such studies and constitute a first step towards obtaining a global paleointensity record over a long period of time.The aim of the present study is to extend the global record of the geomagnetic field intensity over the last 80 kyr. The approach consists, first, of obtaining duplicate records from the same area in order to determine the reproducibility of the results. Records from different regions can then be compared and evaluated as a group. As a first step this paper summarizes the results obtained from four cores in the Mediterranean Sea. MEASUREMENTS AND ROCK MAGNETIC PROPERTIFfS Natural Reinanent Magnetization

Changes in solar activity and Holocene climatic shifts derived from 14C wiggle-match dated peat deposits

“…Most of the long-term variability in 14 C production is due to changes in the Earth's magnetic eld (Mazaud et al, 1991;Tric et al, 1992), while short-term variability is driven by changes in the solar wind (Stuiver and Quay, 1980;Stuiver, 1994). Because of these changes in production, wiggles ('kinks', 'windings' or 'warps', Taylor et al, 1996) are readily visible when the 14 C ages of tree rings are plotted against calendar age (see for example the INTCAL98 radiocarbon age calibration curve within the OxCal v3.5 program, http://www.rlaha.ox.ac.uk/orau/index.htm).…”

Deconvolution of Paleomagnetic Directions from Marine Sediments in the Boso Peninsula, Japan, and Its Implications for the Matuyama-Brunhes Transition Field.

“…Relative field intensity has been estimated from NRM (natural remanent magnetization) intensity normalized to ARM (anhysteretic remanent magnetization) intensity (King et al, 1983). Although this technique appears successful (e.g., Tric et al, 1992), its physical basis has been unclear. The second problem arises from the effects of the post-depositional DRM, which is common in marine sediments (Kent, 1973): the sediment magnetization is gradually acquired after deposition and locked in at some depth below the depositional surface, consequently convolving the geomagnetic field.…”

Orbital-scale variation in the magnetic content as a result of sea level changes in Papua New Guinea over the past 400 ka

“…In general, ARM, SIRM, and χ were adopted to normalize NRM to verify a best way in removing the effects from magnetic lithology. Tric et al (1992) suggested that identical demagnetization treatment on NRM and ARM would offer the best information about magnetization carried by similar RM carriers. Based on the RPI evaluation, several stacks describing virtual axial dipole moment (VADM) have been reported (e.g., Guyodo and Valet 1999;Yamazaki and Oda 2005;Valet et al 2005;Channell et al 2009).…”

“…These properties indicate that all these ratios could offer similar information for the RPI evaluation. Following the suggestion of Tric et al (1992), we adopted the ratio NRM 20mT /ARM 20mT as the RPI proxy and then compared our RPI curve with the PISO-1500 stack (Channell et al 2009) to get the RPI age control.…”