GP43A-0921
Paleosecular variation during the PCRS based on a new database of sedimentary and volcanic records
We present a paleosecular variation study using a generalised global paleomagnetic sedimentary and volcanic database. We made use of all available (and suitable) - published and some new- sedimentary and volcanic paleomagnetic records corresponding to the Permo-Carboniferous Reversed Superchron (PCRS) interval to reanalyse all data. We focused on records with a sufficient number of samples, and acquired – whenever possible – the original data, or – as a second choice - parametrised published site means. Analysis of these paleomagnetic data in terms of latitude variation of the scatter of the virtual geomagnetic poles (VGPs) suggests that careful data selection is required and that some of the older studies may need to be redone using more modern methods, both in terms of sampling and laboratory treatment. In addition, high (southern and especially northern hemisphere) latitudes are notably lacking in published records. The transitional data is removed using a variable VGP cut-off angle which varies with latitude. We use also our extended sedimentary records from Permian red beds from the Lodève and Dôme de Barrot basins (S. France), a new detailed paleomagnetic study of the Permian volcanics in the Oslo graben (Norway), as well as new data from Carboniferous-Permian sediments from the Donbas basin (Ukraine). We compare our results with those from published paleosecular variation models and with recent (re)analyses of VGP scatter during different periods of the geological archive.
GP43A-0922
Geomagnetic paleosecular variation during the Creteacous Normal Superchron as recorded by new and published data from lavas
The secular variation of the ancient geomagnetic field can be quantified by measuring the angular dispersion of directions of magnetization recorded in sequences of lavas of roughly the same age. The latitudinal dependence of this dispersion can provide important information about the long-term symmetry and stability of the field. More than 16 years have passed since the last detailed examination of paleosecular variation of lavas (PSVL) prior to 5 Myr ago and a significant amount of reliable data has been made available since then. Here, we examine PSVL in the Cretaceous Normal Superchron using more than 100 new site mean data from Mongolia together with an updated database of published studies. We also compare this analysis with recent studies of PSVL for the last 5 Myr allowing us an insight into the relationship between patterns of PSV and reversal frequency and the role of mantle convection in forcing geodynamo behavior.
GP43A-0923
The 2.7 Ga Pilbara Drilling Project, Western Australia: Paleomagnetic Results
The Pilbara Drilling Project diamond drillhole intersects subaerial basaltic flows (Maddina Formation, 2715 Ma), stromatolitic carbonates, interbedded black shale (Meentheena Member, Tumbiana Formation) and underlying volcaniclastic sandstones (2719 Ma). Three to four 15 mm diameter and 13 mm long tiny cylindrical specimens were sampled every metre downcore and subjected to thermal and alternating field demagnetization. The demagnetization results of the upper Maddina basalts (0-40 m depth) revealed one or two distinct magnetization components. The high temperature component was observed for more than 50% of the samples and could not be isolated before 550°C. It shows normal polarity and is always almost antiparallel to the low temperature component of reverse polarity. Alternating field demagnetization displayed similar characteristics with opposite directions for the low reverse and high normal coercivity components. We thus consider that the high unblocking temperature and high coercivity grains carry the same normal characteristic component. In contrast to the basaltic flows, the Tuffaceous materials (98-104m depth) show a unique component of reverse polarity which could either reflect thermo-chemico-viscous overprint or an actual reverse polarity before emplacement of the basaltic flows. Paleomagnetic studies on interbedded sedimentary materials and complementary rock magnetic experiments are under way to discriminate between the two hypotheses. If the second scenario had to be retained we would have identified the second oldest known reversal of the Earth's magnetic field (first one was presented in Strik et al., doi:10.1029\slash2003JB002475, 2003). Other implications of the present study would concern the characteristics of the paleosecular variation during Precambrian time but also lateral drift of continental plates. This would also constrain the apparent polar wander path of the Pilbara Craton and the origin of plate velocity which is suggested to be 10 times faster than now.
GP43A-0924
New Paleomagnetic and Rock Magnetic Results from the Late-Cretaceous Kap Kane Volcanics, North Greenland Large Igneous Province
We present new paleomagnetic and rock magnetic data from our ongoing study of the 60-90 Ma volcanics from Kap Kane, North Greenland Large Igneous Province (NGLIP). During our July 2006 fieldwork in North Greenland, we obtained more than 100 samples of various lithology, including thick, massive ignimbrites, volcaniclastic sediments, mafic lavas (basalt to latite), crater breccia, and rhyolites. The samples have been subjected to detailed alternating field and thermal demagnetization experiments to understand their magnetization history and screen out those significantly contaminated by lightning or other secondary magnetizations for paleointensity work (on going). Generally, the Kap Kane volcanics display multicomponent behavior with a fairly large amount of scatter. After removal of a low-temperature component that resembles recent field directions, a well-defined characteristic component remains in most samples. Both normal and reversed magnetizations are observed, which is expected as the rocks presumably formed within 60-90 Ma. Selected samples have also been subjected to a series of rock magnetic measurements that focused on determining the Curie temperature and hysteresis parameters of the magnetic carriers in the rocks. Measurements of the weak-field susceptibility vs. temperature suggest that magnetite is the dominant magnetic mineral. However, resistance to alternating field demagnetization and the magnetic hysteresis behavior indicate that a significant amount of hematite is present in several samples as well. Our interpretation of the data from this investigation is ongoing, but the preliminary results have bearings on the paleogeography of North Greenland, the tectonic history of NGLIP, as well as the variation in intensity of the magnetic field during the Cretaceous Normal Superchron.
GP43A-0925
Paleomagnetic Evidence From Volcanic Units of Valsequillo Basin for the Laschamp Geomagnetic Excursion, and Implications for Early Human Occupation in Central Mexico
Alleged human and animal footprints were found within the upper bedding surfaces of the Xalnene volcanic ash layer that outcrops in Valsequillo basin, south of Puebla, Mexico (Gonzalez et al., Quaternary Science Reviews doi: 10.1016/j.quascirev, 2005). The ash has been dated to 40 ka by means of optically stimulated luminescence analysis. This was held as new evidence that America was colonized earlier. We carried out paleomagnetic and rock magnetic analysis of 18 Xalnene ash block and core samples collected at two distinct localities, and nineteen standard paleomagnetic cores belonging to nearby monogenetic volcanoes. Our data yield evidence that both volcanic lava flow and Xalnene ash were emplaced at during the Laschamp geomagnetic event spanning from about 45 to 39 ka. This interpretation indicates that Valsequillo probably remains one of the sites of early human occupation in the Americas, producing evidence of early arrival.
GP43A-0926
Laschamp and Mono Lake Geomagnetic Excursions Recorded in New Zealand
Eight basaltic lavas from the Auckland Volcanic Field, New Zealand, record three distinct sets of excursional geomagnetic field directions with low paleointensities, however the timing and therefore paleomagnetic significance of these records have been poorly understood. Radiocarbon, K-Ar, and thermoluminescence dating constrain these lavas to have erupted during the last 75 ka, a period during which at least three excursions are recorded in the northern hemisphere. Thirty-four 40Ar/39Ar incremental heating experiments conducted on groundmass from seven of these excursional lavas indicate that they erupted during at least two periods, at 39.1 ± 4.1 and 30.1 ± 4.4 ka, coincident with 40Ar/39Ar and astrochronologic ages determined for the Laschamp and Mono Lake excursions, respectively*. Experiments on lavas associated with a third cluster of virtual geomagnetic poles (VGPs) are complicated by a low concentration of radiogenic argon in the presence of excess argon, yielding discordant age spectra and an imprecise age of 27.7 ± 7.9 ka. VGP clusters from the lavas that we correlate with the Laschamp and Mono Lake excursions fall close to the broad looping VGP pathways obtained from high resolution sediment records of these two excursions. Our findings imply that these excursions, previously identified unequivocally only in the northern hemisphere, were globally synchronized. This supports the hypothesis that during short-lived excursions the geomagnetic field, although weakened, remains largely dipolar. *ages relative to 1.194 Ma ACs and 28.34 Ma TCs standards, reported with 2 σ analytical uncertainties.
GP43A-0927
Stratigraphic and Paleomagnetic Study of Glacial Lake Missoula Lacustrine and Flood Sediments
During the late Wisconsinan, the Purcell Trench lobe of the Cordilleran Ice Sheet advanced south into Idaho where it dammed the Clark Fork River and formed glacial Lake Missoula in western Montana. The lake repeatedly filled and emptied through its dam between about 16,000 and 12,500 14C yr ago. The floodwaters entered glacial Lake Columbia, Washington, and spilled across the plateaux of eastern Washington. Repeated floods excavated the Channeled Scablands and deposited slackwater sediments in several valleys in Washington and in the Willamette Valley in Oregon. Stratigraphic study of the flood and slackwater sediments in Washington and Oregon, and of glacial Lake Missoula sediments near Missoula, Montana, provide evidence for at least 46 glacial Lake Missoula floods. Paleomagnetic analyses were done on oriented samples of (1) glacial Lake Missoula sediments at the Ninemile section and at a section in Missoula (348 samples); (2) interstratified flood and lacustrine sediments at the Manila Creek site in the Sanpoil Valley, Washington (615 samples); and (3) backwater sediments in the Yakima, Walla Walla, and Willamette valleys (813 samples). The sediments record strong and stable magnetic remanence carried by either magnetite or hematite. Magnetic remanence directions of flood beds and of interbedded lacustrine units show secular variation over a time span of hundreds to a few thousands of years, consistent with deposition by many tens of floods separated on average by several decades. Rock magnetic properties of the flood deposits give insight into depositional processes involved and the source of the floodwaters. We are now attempting to date the floods using radiocarbon and optically stimulated luminescence techniques, to correlate the paleomagnetic records between our study sites, which are up to 750 km apart, and to correlate them with well dated paleomagnetic records from Fish Lake, Oregon, and Mono Lake, California.
GP43A-0928
Magnetostratigraphy of Plio-Pleistocene Lake Sediments from Armenia
The Pleistocene geology of Armenia is dominated by the widespread occurrence of lake sediments which testify rapid changes of the environmental conditions during the last 2 million years. These sediments are perfectly suited for paleoenvironmental studies and yield various fossil remains, which allow to reconstruct flora and fauna as well as climatic conditions during dispersal of early man like homo georgicus (1.8 Ma) into Eurasia. Unfortunately, however, precise dating of these sequences is still a matter of debate and absolute ages for basalts directly overlying the lacustrine diatomites range from 1.2 to 0.935 Ma. Addressing the age debate, a detailed magnetostratigraphic study has been carried out in the Vorotan river area near the town of Sisian. A total number of 250 oriented drill cores have been obtained from 5 sections covering 75 meters in stratigraphic thickness. First results demonstrate the high quality of the sampled material and, after removal of a present day overprint at demagnetization temperatures of around 150° C, a stable component of magnetization can be identified in the majority of the samples. Magnetizations of both normal and reverse polarities have been determined. In cases where the magnetic spectrum could not be resolved due to overlapping blocking spectra, great circle analysis was applied successfully to identify the characteristic direction of the NRM. The resulting mean direction for both polarity components plots slightly to the west of the direction of the present day magnetic field at the sampling site and has an inclination value of 55° . A positive reversal test suggests the primary character of these magnetizations. Plotting the characteristic directions in stratigraphic order reveals that the lowermost 50m of the composite profile are of inverse polarity, followed by 20m of normal and 5m of inverse polarity. Based on these preliminary results, and in combination with existing radiometric data, we interpret the zone of normal polarity as the Olduvai normal subchron (1.778 - 1.945 Ma according to the Astronomically Tuned Neogene Time Scale, ATNTS04) within the Matuyama chron. These results provide an important correlation tool for the region and furthermore improve the dating accuracy of the sedimentary sequences in the Vorotan river area.
GP43A-0929
Late Pleistocene and Holocene Paleomagnetic Record From Lago Cardiel, Argentina: Correlation of Geomagnetic Paleointensity Records and Geomagnetic Modulation of the Radionuclide Production
Paleomagnetic analyses of sedimentary cores provide a powerful tool for high-resolution stratigraphic correlation, however, the database of high-resolution sedimentary paleomagnetic records, including paleointensity data, for the last 10 kyr is sparse, particularly for the southern hemisphere. We have generated high-resolution records of paleomagnetic directional secular variation (PSV) and relative paleointensity (RPI) from three long piston cores from Lago Cardiel, southern Argentina (CAR 99-7P, CAR 99-9P, and CAR 99-10P) covering the last ~16 kyr. A single-component low-coercivity magnetization carried by magnetite, and homogeneity in magnetic concentration, allows the reconstruction of well-defined PSV and RPI records. Comparison with two other Argentinean lacustrine PSV records from Lake Escondido and El Trébol revealed comparable features. The Holocene RPI records from Lago Cardiel include a broad low around 7-5 ka and a high around 2 ka, followed by a decrease comparable to published archeomagnetic records from the northern hemisphere, implying that the RPI records from Lago Cardiel are strongly influenced by the main axial dipole field. Holocene geomagnetic field intensity has been implicated as a potential driver for climate (and regime) change in the Middle East and elsewhere, through the links between field intensity and cosmic ray flux, and cosmic ray flux and low cloud formation. In order to test the former linkage, the RPI records from Lago Cardiel were band-pass filtered and subsequently compared with the identically treated radiocarbon production record. The remarkable similarity between both records points towards a link between cosmogenic radionuclide production rate and geomagnetic paleointensity on millennial or even shorter timescales. With a sampling resolution of less than two years, the late Holocene RPI record of core CAR 99-10P can be directly compared with the radiocarbon production rate. Both records show similar characteristics with a dominant periodicity around the ~ 205 yr de Vries cycle. As the de Vries cycle is considered of solar origin, this implies a climatic contamination of the RPI record from this core in spite of its similarity with RPI and archaeomagnetic records from the northern hemisphere.
GP43A-0930
Reconstructing Holocene Paleomagnetic Secular Variation From High Arctic Sediments
Paleomagnetic studies of Holocene sediments from high Arctic lake and marine environments are beginning to document reproducible patterns of paleomagnetic secular variation (PSV), both in direction and intensity. Here we present new paleomagnetic data from Lakes Murray (81°34` N, 69°54` W) and C3 (82°50` N, 78°00` W) in Ellesmere Island, Nunavut Canada, as well as Lake Linne (78°05` N, 13°45` E) in Svalbard, Norway. The natural remanent magnetization (NRM) of all three lakes was studied by progressive alternating field (AF) demagnetization of u-channel samples. Though variable from lake to lake, the sediments preserve a generally strong, stable, well-defined (MAD <5) magnetization. Component inclinations are consistent with historical predictions and those expected for a geocentric axial dipole (GAD). Multiple records have been obtained from each lake, and varve and radiocarbon-based independent chronologies are being developed. Although these sediments represent a range of depositional environments, accumulation rates, and magnetic mineral assemblages, the records are largely consistent. Initial results from the laminated section of Murray Lake's sediments suggest that the record extends back at least 4500 years, providing one of the longest independently dated Holocene PSV records from the Canadian high Arctic. Comparisons will be made with previously obtained and independently dated records from the Beaufort Sea and Sawtooth Lake (Ellesmere Island), other non-independently dated PSV records from Arctic lakes, as well as both historic and prehistoric geomagnetic field models.
GP43A-0931
A Wavelet Approach of Determining the Origin of Orbital Periods in Relative Paleointensity Records
Power spectral analysis and continuous wavelet analysis with significance tests were applied to four relative paleointensity (RPI) records and their normalizer records from both North Atlantic (ODP sites 983 and 984) and North Pacific Ocean (NGC65 and NGC69). The results indicate the presence of significant power at 100 kyr and 41 kyr periods in these records during certain time intervals. Significant common power at orbital periods has been detected between the RPI records and their normalizer records for specific intervals in all these records using cross wavelet analysis. Squared wavelet coherence between the RPI records and their normalizer records further indicate that the two types of records are significantly coherent at orbital periods in specific time intervals. Squared wavelet coherence between RPI records from different climate regimes show out phase relationship on orbital periods during intervals where significant power of orbital periods were observed in the RPI records, and in phase relationship on orbital periods during other intervals. We conclude that orbital cycles detected in the paleointensity records are due to incomplete normalization of the natural remanent magnetization (NRM) records for changes in magnetic concentration and grain-size. Two lines of evidence indicate that the lithologic contamination, although present, does not debilitate the paleointensity records: (1) RPI records can be satisfactorily correlated across differing climatic regimes such as from the North Atlantic to Pacific oceans, and (2) filtering the orbital power from the time series does not appreciably alter the RPI records.
GP43A-0932
Robustness and Limitations of 0-1 Ma Absolute Paleointensity Data
Absolute paleomagnetic field intensity data derived from thermally magnetized lavas and archeological objects provide information about past geomagnetic field behavior, but the average field strength, its variability, and the expected distribution of these observations remain uncertain despite growing data sets. We investigate these issues for the 0-1 Ma field using data from igneous rocks compiled in the PINT03 database, 1124 samples of heterogeneous quality with restricted temporal and spatial coverage. Using virtual axial dipole moments (VADM) to accommodate variable spatial sampling and a bootstrap importance sampling technique to correct for uneven temporal sampling, we find an average VADM of 7.26 × 1022 Am2 ± 0.14. The associated statistical distribution appears bimodal with a subsidiary peak at approximately 5 × 1022 Am2. Tests indicate that this behavior is unlikely to arise from contamination by poor quality data, geographic bias in sampling, or over-representation of typically low intensity excursional data or to result from a mixture of two statistical distributions each corresponding to a distinct intensity state for the magnetic field. Simulations from a stochastic model based on the paleofield spectrum support the alternative possibility that a hypothetically smoother underlying distribution cannot be recovered from a 1 Myr time span. Possible influence of the restricted material type used in our analyses is assessed using additional data compilations and comparing Holocene data from lava flows, Submarine Basaltic Glass (SBG), and archeological objects. The SBG results are inconclusive because of dating issues, but average paleointensities from lavas are about 10% higher than for archeological materials and show greater dispersion. Uncorrected variable cooling rates during remanence acquisition provide a plausible explanation for these differences which may contribute to systematic bias in the 0- 1 Ma average field estimates.