No abstract

The multispecimen protocol (MSP) is a method to estimate the Earth's magnetic field's past strength from volcanic rocks or archeological materials. By reducing the amount of heating steps and aligning the specimens parallel to the applied field, thermochemical alteration and multi-domain effects are minimized. We present a new software tool, written for Microsoft Excel 2010 in Visual Basic for Applications (VBA), that evaluates paleointensity data acquired using this protocol. In addition to the three ratios (standard, fraction-corrected, and domain-state-corrected) calculated following Dekkers and Böhnel (2006) and Fabian and Leonhardt (2010) and a number of other parameters proposed by Fabian and Leonhardt (2010), it also provides several reliability criteria. These include an alteration criterion, whether or not the linear regression intersects the y axis within the theoretically prescribed range, and two directional checks. Overprints and misalignment are detected by isolating the remaining natural remanent magnetization (NRM) and the partial thermoremanent magnetization (pTRM) gained and comparing their declinations and inclinations. The NRM remaining and pTRM gained are then used to calculate alignment-corrected multispecimen plots. Data are analyzed using bootstrap statistics. The program was tested on lava samples that were given a full TRM and that acquired their pTRMs at angles of 0, 15, 30, and 90 • with respect to their NRMs. MSP-Tool adequately detected and largely corrected these artificial alignment errors.

A B S T R A C TTo create meaningful models of the geomagnetic field, high-quality directional and intensity input data are needed. However, while it is fairly straightforward to obtain directional data, intensity data are much scarcer, especially for periods before the Holocene. Here, we present data from twelve flows (age range ∼ 200 to ∼ 470 ka) in the East Eifel volcanic field (Germany). These sites had been previously studied and are resampled to further test the recently proposed multi-method palaeointensity approach. Samples are first subjected to classic palaeomagnetic and rock magnetic analyses to optimise the subsequent palaeointensity experiments. Four different palaeointensity methods -IZZI-Thellier, the multispecimen method, calibrated pseudo-Thellier, and microwave-Thellier -are being used in the present study. The latter should be considered as supportive because only one or two specimens per site could be processed. Palaeointensities obtained for ten sites pass our selection criteria: two sites are successful with a single approach, four sites with two approaches, three more sites work with three approaches, and one site with all four approaches. Site-averaged intensity values typically range between 30 and 35 μT. No typically low palaeointensity values are found, in line with paleodirectional results which are compatible with regular palaeosecular variation of the Earth's magnetic field. Results from different methods are remarkably consistent and generally agree well with the values previously reported. They appear to be below the average for the Brunhes chron; there are no indications for relatively higher palaeointensities for units younger than 300 ka. However, our young sites could be close in age, and therefore may not represent the average intensity of the paleofield. Three of our sites are even considered coeval; encouragingly, these do yield the same palaeointensity within uncertainty bounds.

Regional full-vector palaeosecular variation (PSV) curves are essential to further our understanding of geodynamo operation. Such curves typically lack palaeointensity information (with palaeodirectional being much more common), particularly for periods before the Holocene. Notably low palaeointensity values have been reported for significant portions of the Late Quaternary. Here, we present full-vector palaeomagnetic from the El Golfo section on El Hierro (Canary Islands, Spain), spanning the age range between ∼120 and ∼450 ka, to substantiate these reported low palaeointensity values for the Canary Islands. We sampled 28 flows (mostly alkaline oceanic basalts of the shield building stage) along the Camino de Jinama, a few kilometres to the south of a section studied by Széréméta et al. for PSV. In line with their results, we observe an easterly declination deviation of ∼14 • for the middle and lower part of our section. We relate this to undersampling of the complete PSV spectrum; rotation of El Hierro since ∼500 ka is deemed unlikely. Attempts to date the flows with the 40 Ar/ 39 Ar method on a multicollector instrument were unsuccessful due to the presence of copious amounts of methane in the extracted gases. Stratigraphic correlation to Széréméta et al.'s section lends support to ages (technically of very low quality) obtained for the sites reported on here. Importantly, we add palaeointensity information in the present study, for the first time on El Hierro. Three different palaeointensity protocols were utilized, IZZI-Thellier, multispecimen, and pseudo-Thellier, to provide an additional consistency check and to increase the success rate. After applying strict selection criteria, reliable palaeointensities could be obtained for 18 flows, a success rate of ∼64 per cent. If more than one protocol yielded results for the same flow, the obtained intensities were often within error of each other, testifying to their robustness. Flows with a common true mean direction tend to produce similar intensity values as well; this testifies to the presumed under-sampling of PSV within the section. Obtained palaeointensity values typically range between ∼20 and ∼35 μT, distinctly lower than the present-day field value of ∼39 μT and lower than the average intensity during the Brunhes Chron. The determinations are in line with other rather low values obtained elsewhere for this particular time span.