The dominant dipolar component of the Earth's magnetic field has been steadily weakening for at least the last 170 years. Prior to these direct measurements, archaeomagnetic records show short periods of significantly elevated geomagnetic intensity. These striking phenomena are not captured by current field models and their relationship to the recent dipole decay is highly unclear. Here we apply a novel multi-method archaeomagnetic approach to produce a new high-quality record of geomagnetic intensity variations for Hawaii, a crucial locality in the central Pacific. It reveals a short period of high intensity occurring similar to 1,000 years ago, qualitatively similar to behaviour observed 200 years earlier in Europe and 500 years later in Mesoamerica. We combine these records with one from Japan to produce a coherent picture that includes the dipole decaying steadily over the last millennium. Strong, regional, short-term intensity perturbations are superimposed on this global trend; their asynchronicity necessitates a highly non-dipolar nature.

In environmental magnetism, the properties of magnetic minerals are used as proxy parameters for many purposes. Examples are paleoclimate analysis, paleoceanographic studies, provenance studies of sediments, studies of anthropogenically-induced pollution, and archeological investigations. Mineral-magnetic techniques are sensitive, require little sample preparation, are rapid, often grain-size indicative, and usually non-destructive. These techniques involve 'bulk' properties which makes them complementary to geochemical micro-analytical techniques. Measurements include the field- and temperature-dependence of various types of induced and remanent magnetizations. Mineral-magnetic methods are continuously being improved. The underlying causal relations between observed mineral-magnetic properties and the processes that led to those properties, are becoming increasingly better understood, and the extended use of such properties as proxy parameters for many processes is foreseen. The following environmental magnetic applications are reviewed: the analysis of paleoclimatic variations in loess and other sediment types, the untangling of sedimentary features in piston cores, and the interpretation of the anthropogenic impact on the environment, in archeological studies and in studies of present-day pollution. The pathway between the provenance area and depositional site is shown to have a crucial impact on the magnetic properties.

Northern Iberia exposes a series of Mesozoic sedimentary basins, whose final formation history is tied to a rotation episode of Iberia during the Aptian. related to the opening of the Bay of Biscay. Many of these basins experienced widespread remagnetization, previously loosely tied to a 'Mid-Cretaceous thermal event'. Here we make use of the improved apparent polar wander path (APWP) of Iberia to narrowly constrain the age of their remagnetization to show that several basins experienced a regional diachronous remagnetization. We apply the small circle intersection (SCI) method to reconstruct the paleomagnetic field direction during remagnetization in the Organya Basin in the Southern Pyrenees, Spain, where the age of remagnetization is stratigraphically well-dated to around the Barremian-Aptian boundary, and where the paleomagnetic direction during remagnetization has been well-established. The resulting direction of D/I = 316.8 +/- 5.7 degrees/54.8 degrees +/- 3.3 degrees (where the uncertainty in declination is approximated by alpha(95)/cos I and the error in inclination by alpha(95)) corresponds closely to a recently improved APWP of Iberia. providing support for the applicability of this method. Application of the SCI method allows us to show that the older portion of the remagnetized beds was already tilted by similar to 15-20 degrees to the South during the remagnetization as a result of half-graben formation in the Organya Basin. Moreover, the positive outcome of the SCI technique enables us to argue that previously published results from three other basins-the Cabuerniga and Cameros Basins and the Iberian Range-can be straightforwardly compared to the APWP. Hence, we show that the four pervasively remagnetized basins under consideration all acquired their remagnetization at different times. The remagnetization events were thus confined to these sedimentary basins on an individual 'per basin' scale-albeit occurring on a regional scale over a period of at least similar to 10-15 Myr. Therefore, we propose that the Cretaceous remagnetization events in northern Iberia are related to the extensional tectonic rifting. The pervasive remagnetization is most likely basin confined because there is no apparent overall regional or temporal trend among the basins. Reasonable explanations for remagnetization are those that can occur on the scale and within the context of individual sedimentary basins. These include the interplay of burial depth and an elevated geothermal gradient to reach the diagenetic temperatures required for magnetite-producing reactions. In the Iberian situation. there is no need to invoke more speculative regional Iberia-wide mechanisms. (C) 2009 Elsevier B.V. All rights reserved.

Widespread Cretaceous remagnetization is documented in several Mesozoic basins in North Central Spain. Organya Basin (South Central Pyrenean foreland) is atypical in the sense that the lower part of the rock sequence (Berriasian-Barremian limestones) is remagnetized while the upper portion (Aptian-Albian marls) is not (Dinares-Turell and Garcia-Senz, 2000). Here, this view is confirmed by the analysis of 41 new paleomagnetic sites over the entire basin, so that a 3D view is obtained. Thermoviscous resetting of the natural remanent magnetization can be ruled out, hence the remagnetization is chemical in origin. A positive breccia-test on remagnetized strata constrains the remagnetization age to older than the Paleocene-Eocene, when the backthrust system was active. The remagnetization is argued to have occurred early in the geological history of the Organya Basin either in the elevated geothermal gradient regime during the syn-rift extension or at the earliest phase of the later compression. Burial is considered the most important cause combined with the lithological effect that limestones are more prone to express remagnetization than marls. The observed pressure solution in the remagnetized limestone is likely associated with the remagnetization, whereas it is unlikely that externally derived fluids have played an important role.