GP51A-01 INVITED
Possibilities for Extending Time-Varying Geomagnetic Field Models to 100~ka
Knowledge of millennial scale variations in the past geomagnetic field has been greatly enhanced over the past decade by extensive data gathering efforts combined with a systematic development of global time-varying spherical harmonic models like CALS7K.2 which covers the past 7 thousand years. I explore prospects for extending such modeling to 100~ka, using marine sediment and lava flow records of the paleosecular variation. Such modeling must take account of a variety of limitations in the paleomagnetic data available, including restricted geographic and temporal coverage. Only secular variation on the largest spatial scales can be recovered. Another major consideration is evaluating the uncertainty in age associated with individual observations of either direction or field strength, and treating such uncertainties in an appropriate way during the modeling process. These age uncertainties directly limit the temporal resolution attainable in global field modeling. Relative paleointensity records derived from sediments require calibration during the modeling process, and all paleomagnetic data require a careful assessment of their accuracy so that the best quality model can be produced. Current modeling strategies based on temporally and spatially regularized inversion will be evaluated and simple modifications proposed to accommodate the limited information content in paleomagnetic data.
GP51A-02
An Improved Hawaiian Secular Variation Curve for the Last 25 ka
The Big Island of Hawaii has been the subject of numerous palaeomagnetic studies. The surface lava flows from the Kilauea, Mauna Loa and Hualalai volcanoes are abundant and accessible, resulting in 77 published absolute palaeointensity determinations for the time period 0--25 kyrs. These palaeointensity estimates are well constrained in time due to an extensive 14C dating program carried out by the US Geological Survey. 15 of the surface flows have been re-dated using accelerator mass spectrometry (AMS) and the implications of the revised 14C ages will be presented. In addition to the outcrop data, borehole data from the Hawaii Scientific Drilling Project (HSDP) core, Scientific Observation Hole 1 (SOH1) drill core and Scientific Observation Hole 4 (SOH4) drill core add a further 252 palaeointensity values to the database. Lake sediment data is also available for this period from Lake Waiau, located near the summit of Mauna Kea. Combining all the Hawaiian data and using the cubic-B spline method of curve fitting, a smooth, weighted curve has been obtained for each of the palaeomagnetic components. The trends seen in the improved secular variation curve for Hawaii will be discussed and compared to the trends seen in the global datasets PINT2003, SINT-800 and NAPIS-75, as well as global 10Be isotope data and archaeomagnetic data. The fit of the CALS7K.2 global model to the data will also be discussed.
GP51A-03 INVITED
Paleosecular Type Curves for South America Based on Holocene-Pleistocene Lake Sediments Studies
Most of the high-resolution paleomagnetic secular variation (PSV) results were obtained from records on sediments from the Northern Hemisphere. Experimental results from South America are scarce. The first results were obtained by Creer et al. (1983) and have been continued since few years ago by the author and collaborators. This review deals with studies of PSV records from bottom sediments from three lakes: Escondido, Moreno and El Trébol (south-western Argentina, 41° S, 71° 30'W). Measurements of directions (declination D and inclination I) and intensity of natural remanent magnetization (NRM), magnetic susceptibility at low and high frequency (specific, X and volumetric, k), isothermal remanent magnetization (IRM), saturation isothermal remanent magnetization (SIRM), and back field were carried out. Stability of the NRM was investigated by alternating-field demagnetization. Rock magnetic studies suggest that the main carriers of magnetization are ferrimagnetic minerals, predominantly pseudo single domain magnetite. The correlation between cores was based on magnetic parameters as X and NRM. The tephra layers were identified from the lithologic profiles and also from the magnetic susceptibility logs. Due to their different chronological meaning and their rather bad behavior as magnetic recorder, these layers were removed from the sequence and the gaps that were produced along the profiles by the removal were closed, obtaining a "shortened depth". Radiocarbon age estimates from these cores and from earlier studies allow us to construct paleosecular variation records for the past 22,000 years. Inclination and declination curves (Gogorza et al., 2000a; Gogorza et al., 2002; Irurzun et al., 2006) show trends that are similar to a paleomagnetic secular variation curve for SW of Argentina (Gogorza et al., 2000b). References Creer, K.M., Tucholka, P. and Barton, C.E. 1983. Paleomagnetism of lake sediments, in Geomagnetism of Baked Clays and Recent Sediments, edited by K. M. Creer, P. Tucholka and C. E. Barton, pp 172-197, Elsevier, Amsterdam. Gogorza C.S.G., Sinito A.M., Di Tomasso I., Vilas J.F., Creer K.M., Nuñez, H., 2000a. Geomagnetic secular variations 0-12000 year as recorded by sediments from Moreno Lake (South Argentina). J. South Am. Earth Sci., 13(7), 627-645. Gogorza, C., Sinito, A. M., Vilas, J. F., Creer, K. M., Nuñez, H., 2000b. Geomagnetic Secular Variations 0-6500 Yr. As Recorded By Sediments from Lakes of South Argentina. Geophys. J. Int., 143(3), 787-798. Gogorza, C.S.G., Sinito, A. M., Lirio, J.M., Nuñez, H., Chaparro, M.,Vilas, J. F., 2002. Paleosecular variations 0-19,000 years recorded by sediments from Escondido Lake (Argentina). Phys. Earth Planet. Inter., 133, 35-55. Irurzun M.A. Gogorza C.S.G, Chaparro M.A.E., Lirio J.M., Nuñez H., Vilas J.F., Sinito A.M., 2006. Paleosecular variations recorded by Holocene-Pleistocene sediments from Lake El Trébol (Patagonia, Argentina). Phys. Earth and Planet. Inter., 154(1), 1-17.
GP51A-04
Mono Lake Excursion Reviewed
The Mono Lake Excursion as recorded in the Mono Basin, CA, has an older part that is about negative 30 degrees inclination and about 300 degrees declination during low relative field intensity. Those paleomagnetic directions are closely followed by greater than 80 degrees positive inclination and east declination of about 100 degrees during higher relative field intensity. A path of the Virtual Geomagnetic Poles (VGPs) for the older part followed from old to young forms a large clockwise loop that reaches 35 degrees N latitude and is centered at about 35 degrees E longitude. That loop is followed by a smaller one that is counterclockwise and centered at about 70 degrees N latitude and 270 degrees E longitude (Denham & Cox, 1971; Denham, 1974; Liddicoat & Coe, 1979). The Mono Lake Excursion outside the Mono Basin in western North America is recorded as nearly the full excursion at Summer Lake, OR (Negrini et al., 1984), and as the younger portion of steep positive inclination/east declination in the Lahontan Basin, NV. The overall relative field intensity during the Mono Lake Excursion in the Lahontan Basin mirrors very closely the relative field intensity in the Mono Basin (Liddicoat, 1992, 1996; Coe & Liddicoat, 1994). Using 14C and 40Ar/39Ar dates (Kent et al., 2002) and paleoclimate and relative paleointensity records (Zimmerman et al., 2006) for the Mono Lake Excursion in the Mono Basin, it has been proposed that the Mono Lake Excursion might be older than originally believed and instead be the Laschamp Excursion at about 40,000 yrs B.P. (Guillou et al., 2004). On the contrary, we favor a younger age for the Mono Lake Excursion, about 32,000 yrs B.P., using the relative paleointensity in the Mono Basin and Lahontan Basin and 14C dates from the Lahontan Basin (Benson et al., 2002). The age of about 32,000 yrs B.P. is also in accord with the age (32,000- 34,000 yrs B.P.) reported by Channell (2006) for the Mono Lake Excursion at ODP Site 919 in the Irminger Basin in the North Atlantic Ocean, which contains as well an excursion lower in the core at about 40,000 yrs B.P. that he identifies as the Laschamp Excursion. The paths of VGPs for the Irminger Basin and the younger half of the Mono Lake Excursion in the Mono Basin are similar in that they are counterclockwise loops and nearly adjacent to each other, and quite different from the VGP path for the Laschamp Excursion.
GP51A-05 INVITED
Secular Variation in Europe Derived From Archaeological Structures
The archaeomagnetic data base for Europe has prospered considerably during the EU funded network 'Archaeomagnetic Applications for the Rescue of Cultural Heritage' and because of other recent projects. While ten years ago archaeomagnetic secular variation calibration curves existed only for Bulgaria, England, France and Hungary, now Central, Southern and Western Europe are covered with data and the corresponding curves for at least the past 2000 years have been published. The present archaeomagnetic data base for Europe will be reviewed and analysed. While declination does not have a regular pattern, all archaeomagnetic inclination curves show during the past 2000 years a more or less sinusoidal shape with deep minima during Roman and Medieval times. The temporal correlation of such features is investigated and compared with results from global models of the archaeomagnetic field.
GP51A-06
Enhancing the European Archaeointensity Database: New Data From Italy
Global geomagnetic models using archaeomagnetic data can offer important insights in to geomagnetic field evolution and core mantle boundary processes. However, in order to provide a complete field description it is necessary to have many data giving good spatial and temporal coverage. Due mainly to experimental difficulties there are many more directional data than intensity data, and whilst data coverage is reasonable for some locations it is recognised that many more high quality data are still needed. Here we present results from two Italian sites: a Roman Amphorae workshop (dated to be between 200 BC and 100 AD), Albinia, Tuscany, and from a 7th Century BC site at Incoronata, Pisticii, Basilicata. The classical Thellier technique was used with anisotropy of thermal remanence and cooling rate corrections. At Albinia, samples were taken from five kilns and from the base of 39 amphorae. The mean archaeointensity per kiln / the amphorae range from 62 to 70 microT. This variation may reflect them being of differing ages however it could also reflect the limitations of the experimental protocol and/or samples. Thus it is desirable that a number of archaeointensity data per time interval are obtained in order to reliably determine the past variations of the geomagnetic field. At Incoronata samples were drilled from 39 bricks. The mean archaeointensity of 85 microT obtained for the older Incoronata bricks is consistent with the increase in field intensity seen going back through the first millennia BC in other regions such as Mesopotamia.
GP51A-07
New archeointensity results from Teotihuacan (Central Mexico)
We carried out systematic rock-magnetic and archeointensity invetigations on 84 pottery fragments (about 549 samples) of Teotihuacan ceramics. Three localities are sampled: Xalla, Teopancazco and Cueva de las Varillas. The principal aim of this study is to try to establish first archeointensity reference curve for Mesoamerica from 300 a.C. and 1500 d.C. The samples selected are related to about 50 C14 and AMS radiometric data available. Thus there are almost ideal conditions to know absolute geomagnetic intensity variation trough time. Rock-magnetic investigation included susceptibility vs temperature and hystresis measurements. In addition some X-Ray and microscopy studies were performed on selected specimens. The samples are characterized by stable remanent magnetization observed upon both thermal and alternating field demagnetizations. Both Ti-poor and Ti-rich titanomagnetites are responsible for the magnetization. 61 out 84 fragments yielded acceptable archeointensity results yielding an extremely high success rate. The cooling rate and anisotropy correction were applied to all samples. The mean intensities found range from 72.8 to 15.9 microT showing very low within fragment dispersion.