GP44A-01 INVITED
Calibration and Validation of Relative Geomagnetic Paleontensity Records
Over the past few decades a burgeoning number of relative paleointensity records derived from sediments has contributed greatly to knowledge of geomagnetic paleosecular variation. It is now broadly accepted that there have been repeated large changes in field strength during the Brunhes polarity interval and there are a significant number of geomagnetic excursions corresponding to lows in the geomagnetic dipole moment. However, the calibration of relative paleointensity data to provide absolute field strengths remains a thorny problem, and it can also be difficult to evaluate the reliability of paleointensity data. For the Holocene relative paleointensity data have been calibrated using the CALS7K.2 time varying paleomagnetic field model. We compare strategies for calibrating relative data, including local or global averaging of VADM records and the use of time varying paleomagnetic field models. Results are used in an assessment of the internal consistency of relative intensity data compared with absolute intensity observations.
GP44A-02 INVITED
Full Vector Analyses of the Short-Term Behavior Recorded in Long Volcanic Sequences in Hawaii, USA.
The Hawaiian volcanoes, in principle, offer the opportunity of observing the geomagnetic field behavior from present back to 5.72 Ma (from the Big Island of Hawaii to the island of Kauai). Thus, new paleomagnetic measurements coupled with radioisotopic dating are revolutionizing our understanding of the geodynamo by providing terrestrial lava records of the short-term behavior of the paleomagnetic field. As part of our investigations of some of these Hawaiian volcanoes, we have sampled long volcanic sequences of the Waianae, Koolau (island of O'ahu) and Mauna Loa (Big Island of Hawaii) volcanoes. These volcanic edifices have collapsed in the past leaving highly dissected lava sequences ideal for paleomagnetic sampling. We have sampled and studied in detail the directional characteristics and their respective absolute paleointensities of three successive reversals, namely, Gilbert-Gauss, Lower and Upper Mammoth polarity transitions recorded on Waianae lavas, Cryptochron C2r.2r-1 (ca. 2.514 +/- 0.030 Ma) as well as the Kaena Subchron recorded in the Koolau Volcano and the Laschamp and Pringle Falls excursions recorded on lavas from the Mauna Loa volcano (Big Island of Hawaii). The records of the three successive Gilbert-Gauss, Lower and Upper Mammoth reversals confirm that large oscillations of directions precede or follow the reversals, which reminds waveforms typical of paleosecular variation with their amplitude being considerably amplified by the decrease of the dipole. Determinations of absolute paleointensity were attempted on more than 750 samples. Special care was taken at selecting data obtained from segments covering more than 50% of the remanent magnetization of the samples. This procedure limited the success rate to 13% for the Waianae lavas, 70% for the Cryptochron flows and 25% for the Kaena samples but provided consistent and reliable paleointensities. In addition to other time intervals, the results document the field variations surrounding the five excursions and polarity transitions. For the 0.4 Ma record of the Waianae sections a period of weak field dominated before the reversal, then the transition was initiated by a transit from normal to reverse polarity and followed by a short restoration of field intensity in reverse polarity. A second episode of very weak field was accompanied by a return to positive inclinations before reaching the reverse polarity. The very strong and apparent rapid recovery of the dipole following completion of the reversal culminated at a value of 16 x 1022 Am2 similarly to field intensities reported for the other detailed volcanic records of reversals studied so far. The asymmetry between the pre- and the post-reversal phases appears as a dominant characteristic and indicates the importance of field regeneration to initiate a new stable polarity interval. All transitional VGP's lie within the longitudinal bands of America, central Africa, western Europe and eastern Asia. This distribution does not support the hypothesis of a direct link with heterogeneities of the lower mantle underneath Americas and eastern Asia. Clusters of VGP's are observed in most records at various geographical locations without preference for specific longitudes, which in the present case most likely seems to result from intense volcanism during short time periods rather than from specific transitional states.
GP44A-03 INVITED
The Microwave Palaeointensity Technique
Reducing laboratory induced alteration during palaeointensity experiments was the motivation for the group at Liverpool in the 1990s (led by John Shaw and Derek Walton) to start developing a new variant of palaeointensity methodology. Using high frequency microwaves, as opposed to heating using conventional ovens, it is possible to demagnetise and remagnetise samples. The microwaves directly excite the magnetic system so that it is not necessary to heat the whole bulk sample and excite the magnetic system via lattice vibrations. Thus, using microwaves can reduce sample alteration and the success rate of palaeointensity experiments can be increased. As with conventional methods, it is still necessary that the remanence is an original thermal remanence, which is held by non interacting single domain behaving grains. The development of the microwave palaeointensity technique from the early experiments using a domestic microwave oven, to the current systems operating at 14 GHz will be described. Results from studies that compare conventional thermal (Thellier) and microwave techniques, using both artificial and natural remanence will be described as well as summarising some of the palaeointensity results that have been obtained using the microwave palaeointensity method from archaeological, geological and even extra terrestrial material. Finally, future developments and applications of the method will be discussed.
GP44A-04 INVITED
Long-term trends in paleointensity examined using Thellier analyses of single silicate crystals
Available data suggest a remarkable feature of the Permian to Recent time-averaged geomagnetic field: its strength apparently varied by only a factor of ~3. Thus, the task of investigating more subtle trends becomes challenging, and has given rise to continued constructive debate. For example, several data sets (specifically Thellier analyses of single silicate crystals and submarine basaltic glass, and natural remanent magnetization intensities of submarine basalt) suggest an inverse relationship between reversal frequency and field strength. This relationship has not, to date, been seen in Thellier results from whole rocks, perhaps due to geologic and laboratory-induced alteration. In addition, it is likely that the presently available paleointensity database from whole rocks is underrepresented in samples that have seen high-temperature oxidation (and that might otherwise yield more accurate values due to favorable grain size/domain state characteristics). Long-term paleointensity trends are largely based on comparisons of values from the Cretaceous Normal Polarity Superchron and preceding and succeeding mixed polarity intervals. Here we discuss such comparisons using new paleointensity data from single silicate crystals. For Archean times, prior paleointensity estimates based on whole rocks suggest much weaker fields. On the basis of data from single silicate crystals, we argue that some prior results from whole rock samples underestimate field strength due to the ubiquitous thermochemical alteration seen in Archean terrains. Field strengths at 3.2 Ga appear to be comparable to those of the more recent field.
GP44A-05
Relative Paleointensity of the Geomagnetic Field 12-20 kyr. From Sediment Cores, Lake Moreno (Patagonia, Argentina)
Four cores labeled Lmor1, Lmor2, Lmor3, Lmor98-1, Lmor98-2 from the bottom sediments of Lake Moreno (south-western Argentina) have been used to estimate regional geomagnetic paleointensity. Lake Moreno is on the east side of the Andean Cordillera Patagónica; it is located in the Llao Llao area, San Carlos de Bariloche, Argentina (41° S, 71° 30'W). The following measurements were performed: Natural Remanent Magnetization (NRM), magnetic susceptibility at low and high frequency (specific, X and volumetric, k), Isothermal Remanent Magnetization (IRM) reaching the Saturation Isothermal Remanent Magnetization (SIRM), Back Field, Anhysteric Remanent Magnetization with a direct field of 0.1mT and an alternating field between 2.5 and 100mT (ARM100mT). Associated parameters were calculated: S-ratio, Remanent Coercitive Field (BCR, anhysteric volumetric susceptibility (kanh), SIRM/k, ARM100mT/k, and SIRM/ ARM100mT. The rock magnetic studies indicate that the magnetic mineralogy of the clay-rich sediments is dominated by pseudo- single domain magnetite in a narrow range of grain size (between 1 and 4μm) and concentration (between 0.05 and 0.1%), thereby meeting established criteria for relative paleointensity studies. The remanent magnetization at 20mT (NRM20mT) has been normalized using the anhysteric remanent magnetization at 20mT (ARM20mT), the saturation of the isothermal remanent magnetization at 20mT (SIRM20mT) and k. A comparison of these results with relative paleointensity records obtained in previous works, Lake Escondido (Gogorza et al., 2004) and Lake El Trébol (Gogorza et al., 2006) allows obtaining detailed information about the disagreement observed in the period 12-20 kyr between both records. References Gogorza, C.S.G., J.M. Lirio, H. Nunez, M.A.E. Chaparro, H.R. Bertorello, A.M. Sinito. Paleointensity studies on Holocene-Pleistocene sediments from Lake Escondido, Argentina, Phys. Earth and Planet. Inter. 145: 219-238, 2004. Gogorza, C.S.G., M.A. Irurzun, M.A.E. Chaparro, J.M. Lirio, H. Nunez, P.G. Bercoff, A.M. Sinito. Relative paleointensity of the geomagnetic field over the last 21,000 years BP from sediment cores, Lake El Trébol (Patagonia, Argentina). Earth Planet Space. 58: 1323-1332, 2006.
GP44A-06
Towards the establishment of an archeointensity master curve for Mesoamerica
We present new rock-magnetic and archaeointensity results of some pre-Columbian potteries found at archaeological caves in Chiapas, Southeast Mexico. The ceramics studied consist mostly of tiles of different archaeological artifacts. Their archeological ages cover mainly two ranges: from 550-250 BC and from 100-500 AD. ‘Rock-magnetic' experiments which included susceptibility vs. temperature and hysteresis measurements were used to select magnetically stable and suitable material for Thellier absolute intensity determinations. Six out of thirteen archaeological tiles, which showed no evidence of strong secondary components and presenting almost reversible susceptibility curves, were selected for archeointensity studies. Each of these tiles were further divided into at least seven fragments and then set in salt (NaCl) pellets in order to treat them as standard paleomagnetic cores. Magnetization per unit volume of ‘blank' pellets is negligible with respect to that of ceramic samples. Archeointensity experiments were carried out using the Coe version of the Thellier method. Cooling rate and anisotropy effects were investigated on each sample. Thellier experiments yielded unusually high success rate and data obtained are of excellent technical quality, attested by reasonably high Coe's factors. This pioneering study proves the usefulness of Mesoamerican ceramics for archeointensity studies and opens new perspectives towards the establishment of a reference intensity variation curve for the region.
GP44A-07
Paleomagnetic behavior of volcanic rocks from Isla Socorro, Mexico
Isla Socorro, Mexico is located in the eastern Pacific Ocean on the Mathematician Ridge, an abandoned spreading center that ceased activity ~3.5 Ma when activity shifted to the East Pacific Rise. Silicic volcanism continues to the present day. A lack of vegetation in the southeastern portion of the island has resulted in extensive erosion, exposing in steep-sided canyons a sequence of volcanic units the oldest of which are ~540ka. We carried out a joint expedition between the Scripps Institution of Oceanography and the Universidad Nacional Autonoma Mexico to Isla Socorro in January of 2005. We collected oriented paleomagnetic samples from 21 sites from as many as 10 different volcanic units. We subjected over 100 specimens to an "IZZI" type (Tauxe and Staudigel, 2004) paleointensity experiment in which in-field and zero-field first steps were alternated to detect high-temperature tails. We also included the standard pTRM checks. Although some specimens worked extremely well, we observed non-ideal behavior in many specimens. We categorized this ‘misbehavior' into four general groups reflecting contamination by non-single-domain magnetic carriers and alteration during the experiment. In an earlier study, Bohrson et al. (1996) proposed a series of widespread eruptive events, based on similarities of argon/argon dates. Paleointensity from specimens that conform to the strictest acceptance criteria are available from both the (unoriented) original sample collection and our fully oriented (but as yet undated) new collection. Comparison of the two sample sets allows us to tie dates unambiguously from the dated collection to the new fully oriented data set. Bohrson, W. A., M. R. Reid, et al. (1996). "Prolonged History of Silicic Peralkaline Volcanism in the Eastern Pacific Ocean." Journal of Geophysical Research-Solid Earth 101(B5): 11457-11474. Tauxe, L. and H. Staudigel (2004). "Strength of the geomagnetic field in the Cretaceous Normal Superchron: New data from submarine basaltic glass of the Troodos Ophiolite." Geochem. Geophys. Geosyst. 5(2): Q02H06, doi:10.1029/2003GC000635.
GP44A-08
Absolute geomagnetic intensities between 900 and 1521 d.c., case study of some pre- Columbian ceramics from Quiahuiztlan (Veracruz State, Mexico)
The Quiahuiztlan, known as a city-cemetery-fortress belongs to coastal Totonacapan culture at the Gulf of Mexico. The site was active from about 900 d.c. and until arrival of Spaniards. In total, 26 samples from three different localities were analyzed. Although hysteresis measurements combined with isothermal remanent magnetization experiments were performed on all samples, pre-selection of samples for Thellier archeointensity experiments was mainly based on the demagnetization of natural remanent magnetization and temperature dependence of initial magnetic susceptibility. In absence of local archeointensity reference curve, we tried to use recently reported global intensity variation curves for the last 3 millennia for the dating purposes.