GP11A-0684
On the Reliability of Paleomagnetic Poles Obtained From Permo-Carboniferous Rocks From Oaxaca Mexico, Belize and Guatemala: Insights From Rock Magnetic Studies
In the reconstruction of past movements of tectonic plates, the determination of reliable paleomagnetic poles is of utmost importance. To achieve accurate results, a full knowledge of the rock magnetic properties of the samples, is required particularly for Curie points and for grain-size analyses of the minerals carriers of the magnetization in addition to thermal and af demagnetization experiments. In this study we present the comparative results of 20 sites drilled at three different Paleozoic areas: The Permian rocks of the Juchatengo area in Oaxaca, Mexico; the Carboniferous (336 Ma) Mountain Pine Ridge Granite, the Triassic (237 Ma) Hummingbird Granite in Belize, and the Permian Chochal Limestone in Guatemala. A minimum of 8 cores were drilled and sun-oriented at each site. The samples of all 20 sites were subjected to AF demagnetization in sixteen steps from NRM to 1,000 Oe. Next, the samples were thermally cleaned in fifteen increasing temperature steps from NRM up to 675 C. Principal component analysis was applied to each one of the samples in order to obtain their respective mean directions. SIRM, hysteresis loops, and coercivity experiments performed on samples from the sites in question indicate that about 90 percent of the samples were characterized by Multi-Domain (MD) grain sizes and the rest were PSD. Curie point determinations results ranged from relatively low temperatures of 190 up to 660 C, indicating the presence of Ti-rich, Ti- poor magnetites as well as hematite. The Juchatengo complex of southern Mexico, a green rock tholeiitic sequence emplaced beetwen the Oaxaquia and Xolapa terranes is made up from bottom to top, of gabbroic and plagiogranitic dikes and stocks (289 +/- 6 Ma, K-Ar hornblende); basalts and pyroclastic rocks and a series of deep water marine sediments. The deposition and deformation of the complex occurred in the Early Permian followed by the emplacement of Permian-Triassic calc-alkaline plutons (K-Ar hornblende ages from 286 +/- 6 Ma to 238 +/- 5 Ma). At this area we have sampled nine different sites. Thus far, reliable poles were obtained from 3 sites and the results obtained are: Juchatengo sites JU: PLON= 128.2; PLAT= 82.4; A95= 5.7. LS: PLON= 182.5; PLAT= 61.2; A95= 13.8 and ES: PLON= 171.9; PLAT= 74.6; A95=17.5. This inclination corresponds to a latitude of (220) slightly higher than expected for the sampling site at 160 north. However it falls within the analytical error. At Belize, eight sites were sampled in the Maya Mountains region: 3 sites in the Carboniferous (336 Ma) Mountain Pine Ridge Granite; 3 other sites in the Triassic (237 Ma) Hummingbird Granite, and 2 other sites in the sandy facies of the Santa Rosa Formation of Pennsylvanian age. Only three sites gave some reliable results as follows: Belize site 1 at Mountain Pine Ridge Granite: PLON= 171.9 PLAT= -22.2; A95= 12.8 and Belize site 2 also at Mountain Pine Ridge Granite: PLON= 279.5; PLAT= -75.3; A95= 11.3. Belize site 3 at Hummingbird Granite yielded: PLON= 106.8; PLAT= 32.1; A95= 7.7. At Guatemala, out of three sampling sites in the Permian Chochal Formation north of the Motagua Fault Zone, two sites yielded useful results with a calculated pole at: Guatemala site 9: PLON= 162.5; PLAT= -66.2; A95= 11.6. Guatemala site 11: PLONG= 130.1; PLAT= 57.8; A95= 13.7. The 8 reliable sites that yielded tectonically coherent results are characterized by samples that showed reversible Curie curves that contained almost pure magnetite and are PSD in size.
GP11A-0685
Three-Component Magnetic Logging in the Outokumpu Borehole
Magnetic measurements in boreholes are normally used for the interpretation of total field anomalies or for tool orientation. Some more sophisticated tools may use inclinometers to obtain the vertical and the horizontal field components and estimate magnetic field inclination. However, in particular in the ore exploration, the declination of the magnetic field is of interest to reduce the ambiguity of the interpretation. Another potential application is the reorientation of borehole cores. With the knowledge of the declination one can easily combine the results of laboratory measurements with other logging results. To determine the declination of the rock magnetisation, it is critical to obtain the inclination as well as the azimuthal position with a high accuracy. In our tool, the "Göttinger Bohrloch Magnetometer" (GBM), the rotation along the x-, y- and z-axis is recorded by three fibre-optic gyros. These instruments have the big advantage of a very small drift per hour (approx. 1 °/h) in combination with a high resolution (86 μ° per increment). The small drift per hour is critical due to the measuring time of several hours in deep boreholes, and can be further reduced by different reorientation techniques. The GBM has been successfully tested in different boreholes, like KTB or HSDP. In HSDP-2 the GBM was used to determine the declination of the rock magnetization with an accuracy of 6 °. To further improve the accuracy, we carried out several calibration and test measurements, using the Braunbek Coil System of the magnetic laboratory "Magnetsrode" in Braunschweig. The tool will be applied in September 2008 in the Outokumpu drillhole/Finland. The aim is to obtain an understanding of the deep structure of the Cu-Co-Zn ore deposits as well as their formation process. The ore is hosted in the so-called Outokumpu assemblage, consisting of black shales, serpentinite and skarn rock. The integrated interpretation of our results with data from laboratory investigations will enable us to test various hypotheses on the tectonic evolution of the Outokumpu assemblage.
GP11A-0686
Paleomagnetism and Rock Magnetic Properties from Quaternary Lavas and Tuffs of the Yellowstone Plateau Volcanic Field
We report paleomagnetic and rock magnetic from rhyolite lava flows, ignimbrites, and basalt flows associated with the Yellowstone Caldera, within and surrounding Yellowstone National Park. These data were collected in order to understand sources of magnetic variations observed in high resolution aeromagnetic data reported by Finn and Morgan (2002), and to better understand the evolution of the Yellowstone magmatic system. Most paleomagnetic samples are from volcanic rocks of the third eruptive cycle (1.2 Ma to 0.070 Ma), including the ca. 0.640 Ma Lava Creek Tuff, postcaldera rhyolite flows, and contemporaneous marginal or post-caldera basalt flows. Magnetic intensities for samples ranged from 0.12 A/m to 5.9 A/m, with volume susceptibilities of 2.14x10-4 to 1.45x10-3 SI; Q ratios range from 0.67 to 23.8. As expected, most sites yield well-defined paleomagnetic directions of north declination and moderate positive inclination consistent with remanence acquisition during the Brunhes polarity chron. However, a few sites from older units such as the rhyolites of the Harlequin Lake (0.839 ± 0.007 Ma) and Lewis Canyon (0.853 ± 0.008 Ma) flows, and the basalts from the Junction Butte flow (at Tower Falls, 2.16 ± 0.04 Ma) and Hepburn Mesa (2.2 Ma) yield reverse polarity magnetizations (40Ar/39Ar dates from Obradovich, 1992, and Harlan, unpublished (Hepburn Mesa flow)). Rock magnetic behavior, including high coercivities during AF demagnetization, high laboratory unblocking temperatures, and susceptibility vs. temperature determinations indicate that remanence in the rhyolitic samples is carried by a combination of single or pseudo-single domain magnetite and/or hematite; in the basalt flows magnetite and high-Ti titanomagnetite carrys the remanence. Paleomagnetic results from 46 sites in 27 separate flows yields a grand mean direction with a declination of 356.9° and inclination of 61.9° (k = 35.2, α95 = 4.8°). VGPs calculated from the site-mean directions yield a paleomagnetic pole at 128.4°E, 89.1°N (K = 18.9, A95 = 6.6°), which is 0.9° ± 5.0° from Earth's spin axis. The results suggests that our sample population averages paleosecular variation, despite the episodic and discontinuous record of volcanic activity associated with the Yellowstone Caldera.
GP11A-0687
A Comparison of Spherical Harmonics and Slepian Basis Functions in Magnetic Field Analysis
Planetary probes often have equatorial, low inclination, orbits around the planet in question, leaving polar data gaps. For example, the orbital path of the Cassini probe around Saturn is essentially limited to a narrow latitudinal band around the equator. For potential field studies this can lead to problems as such studies traditionally employ spherical harmonics, which require global support, to describe the potential field (Simons 2006, Simons 2006a, Wieczorek 2005). Here we consider the use of different basis functions that have been specifically designed to deal with such situations. In contrast to using spherical harmonics for a spectral representation of such 'equatorial' data, an approach which can be prone to errors because global support is required to achieve orthogonality over the whole sphere, Slepian functions, which are orthogonal over both the whole sphere and the region of data coverage, and which have their energy optimally concentrated in the spatial and spectral domain, should yield better results. We have set out to compare the use of these new basis functions in magnetic field analysis with the canonical spherical harmonics. Are the Slepian basis functions better for resolving the magnetic field? Such comparisons have been done in the past, (Simons 2006, Simons 2006a) but took a more theoretical stance, evaluating only a bandlimited white noise source field. We are more interested in the performance under practical circumstances. We employ a number of criteria to evaluate the performance of both basis functions such as: errors, variances, spectral leakage. We have used an Earth magnetic field model, POMME, to generate realistic magnetic field values and Cassini's trajectory scaled to Earth to generate typical equatorial coverage. Simons, F., Dahlen, F., 2006. Spherical Slepian functions and the polar gap in geodesy. Geophysical Journal International 166(3). Simons, F., Dahlen, F., Wieczorek, M. A., 2006. Spatiospectral concentration on a sphere. SIAM Review 48(3). Wieczorek, M., Simons, F., 2005. Localized spectral analysis on the sphere. Geophysical Journal International 162.
GP11A-0688
A relationship between giant earthquakes and core flow instabilities?
Giant earthquakes of exceptional energy release induce large coseismic deformation field, with measured offsets of the order of millimeter recorded by GPS stations at distances of thousand kilometers from the epicenter. The recorded deformation field is just the surface expression of a volume deformation affecting the whole Earth. According to theoretical models of global coseismic deformation, the 2004 Sumatra-Andaman earthquake, which was the second-greatest occurred in the instrumental age, is expected to have affected the core-mantle boundary with static deformations of the order of millimeter over a large area. At the same time, high-resolution core magnetic-field models based on satellite observations evidenced a short-timescale geostrophic fluid acceleration at the top of the core. These anomalies are located in a region corresponding to the Indian ocean and occur in a time window containing the earthquake occurrence. More interestingly, the acceleration pattern is close to the modeled displacement due to earthquake coseismic effects. These evidences suggest us the possible existence of a relationship between giant earthquakes and rapid perturbations in the Earth core dynamics.
GP11A-0689
Dynamo Region Geometry Controlling Stars and Planets Magnetic Behavior
We address the issues raised by comparing the geodynamo with some planetary and stellar dynamos. The recently discovered field structure of a fully convective star happens to be more reminiscent of the Earth magnetic field than the Solar magnetic field, despite the fact that the physical and chemical properties of these objects clearly differ. We therefore want to test a simple assumption: the internal geometry of these objects being very different, could the aspect ratio be one of the key parameters controlling some aspects of the magnetic behavior? We report the results of fully three dimensional dynamo simulations which show that varying the aspect ratio of the active dynamo region can yield sharp transition from Earth like steady dynamos to Sun like dynamo waves (Goudard and Dormy, EPL, 83, 2008). Moreover recent study of large longitude libration have shown that the core of Mercury is partially molten. Relying on the above geometrical approach, we offer a tentative explanation for the low intensity of Mercury's magnetic field.
GP11A-0690
Establishing a new Archaeomagnetic Record for the SW Pacific
The goal of this study is to establish the first archaeomagnetic record for the SW Pacific for the last few millennia using an outstanding collection of well-dated archaeological ceramics from a wide region, namely: 1. Bismarks-Northern Solomons 3500-0 BP, 2. Western Solomons 2000-0 BP, 3. Southeast Solomons 3100- 2000 BP, 4. Vanuatu 3100-700 BP, 5. New Caledonia 3000-0 BP, 5. Fiji -Tonga 3000-0 BP. Ultimately, we aim to investigate the potential for archaeomagnetic dating, especially of un-orientated inter-tidal samples that have proved impossible to date using C14 methods because of their disturbed contexts. Because the currently available samples are un-orientated, we are using palaeointensity measurements to establish the archaeomagnetic record. Although much archaeomagnetic intensity data (especially of the last three millennia) are already available, the geographic distribution of sites is very poor with a particular lack of data for the southern hemisphere. The new data will be the first such from the SW Pacific and should therefore contribute significantly to improving global field models. Preliminary measurements of palaeointensity from samples from the W-Solomon (site New Georgia, 3100 BP) and SE-Solomon (site Santa Cruz, 600 BP) Islands show a mean value of 33.4 ± 5.4 μT and 31.6 ± 4.2 μT, respectively. In addition we will present further results, primarily from Lakeba in Fiji (18.2° S, 178.8° E) and the Duke of York Islands in the Bismarck Archipelago (4.2° S, 152.5° E). Palaeointensity measurements are being carried out using the latest state of the art microwave demagnetisation methods in order to minimise alteration. We use pTRM and tail-checks in order to detect any thermo-chemical alteration and multi domain (MD) effects. Anisotropy and cooling rate are also determined and corrected for.
GP11A-0691
Time scales separation and the Geodynamo
The study of dynamo action classically involves two timescales: the diffusive and the advective ones. We have recently demonstrated the possibility of magnetic field amplification on an intermediate timescale associated with time dependent modulations of the flow (Dormy & Gérard-Varet, EPL, 81, 2008). This mechanism relies on an ordering of three relevant timescales, even vanishing low frequency perturbations can yield exponential growth of the magnetic field on the typical time scale of oscillation. In this approach, the dynamo mechanism relies on a parametric instability associated with transient amplification by shear flows (sometimes called the Ω--effect). The timescale ordering is here crucial. We investigate the potential importance of this mechanism on the Geodynamo. Arguing that the rapidly drifting thermal-Rossby waves in the core provides the necessary timescale ordering for this mechanism, we show how it can account for the observed decrease of the dynamo threshold with the Ekman number.
GP11A-0692
Geomagnetic Reversals Caused by Breaking Mirror Symmetry of Core Dynamics
The Earth's magnetic field can be geometrically described by a strong axial dipole and higher degree terms, which belong to the dipolar (even) or quadrupolar (odd) family depending on their symmetry with respect to the equatorial plane. It is established that the field has frequently (and maybe always) reversed its polarity. It has been suggested by Merrill and Mc Fadden (1988) that reversals occur because the fluid flow in the outer core breaks the equatorial symmetry. This results in a coupling between the dipolar and quadrupolar families. Field reversals have now been reported in several numerical simulations of dynamos and very recently for the first time in a laboratory experiment involving a Von Karman swirling flow of liquid sodium (VKS) (Berhanu et al, 2007). In this experiment, reversals are observed when the velocities of the two counter rotating disks driving the flow are different, thus when a symmetry is broken. Here, we show how the interaction between the dipolar and quadrupolar modes, that results from such lack of symmetry, can lead to polarity reversals or excursions. We identify the mechanism at work and discuss its consequences on the shape of reversals and excursions, and the existence of long periods of time without reversals. We then test the predictions of the model with the characteristics of reversals obtained from the paleomagnetic records as well as from the laboratory experiment.
GP11A-0693
Rock Magnetic Characterization of Lavas Erupted From Haleakala Volcano, Maui, Hawaii
We have studied rock magnetic properties of 103 lava flows erupted from Haleakala Volcano on the Island of Maui in Hawai'i. Ages of the flows range between 365±40 years to 52,900±1800 years BP, as determined by 14C dating and comparisons to paleomagnetic directions between flows. Sherrod et al. [JGR, 111, 2006], and Herrero-Bervera and Valet [PEPI, 167, 2007] have previously undertaken paleomagnetic investigations of these lava flows. Few rock magnetic experiments, however, have been done on samples from these flows other than alternating field (a.f.) and thermal demagnetization experiments. Thus, we have performed saturation isothermal remanent magnetization (SIRM), hysteresis loop, coercivity, and Curie-point determinations for the lava flows in question, using a variable field translation balance (VFTB) instrument. The results indicate that 90 percent of the magnetic grain sizes are within the pseudo- single domain (PSD) range, with the remainder being of either single domain (SD) or multi-domain (MD) grain size. Curie point determinations have identified diverse magnetic mineral phases ranging from Ti-rich, pure magnetite to Ti-poor magnetite grains within the lava flows. After the performance of these rock magnetic experiments, in addition to a.f. and thermal demagnetization procedures, we conclude that only about 10 percent of the flows are suitable for absolute paleointensity determinations using the Thellier method.
GP11A-0694
Constraints on Pacific Plate Paleo-equator from Detailed Skewness Analysis of Magnetic Isochrons 32-25 near the Molokai, Clarion and Clipperton Fracture Zones
Plate tectonics study based on total field marine magnetic anomalies measured by proton precession magnetometers has always been challenging in the equatorial area due to the relatively smaller amplitude of the anomaly and larger diurnal variations of the magnetic field resulting from the varying current of the ionosphere including the effect of the electrojet. Previous investigation of the magnetic anomalies in the equatorial Pacific has been used to study the paleomagnetic pole of the Pacific plate. However, the skewness of the magnetic data was low in coherence and accuracy, and the anomaly pattern is unable to be determined between the Clipperton fracture zone and the modern equator. Here we corrected a set of shipboard magnetic profiles using Comprehensive Model : Phase 4 ( CM4 ) which is designed to estimate the Near- Earth magnetic field by sources in the Earth's core, ionosphere and magnetosphere. We use the CM4 model to remove the diurnal variation of the ionosphere magnetic field which reduces the long-wavelength component of noise in magnetic data. In this study we deskewed more than 40 ship magnetic data sets in the central Pacific, including the Nathaniel B Palmer cruise NBP0207, by comparing the data profile with zero- skewness synthetics. The magnetic profiles are then mapped on a satellite gravity map along with their skewness values. Theoretical models show that for a ridge trending 170 degrees from north, a sudden ~90 degree change in skewness occurs when the magnetized crust moves across the paleo-equator. On the map we were able to identify that the skewness generally decreases systematically as the latitude increases. Despite the noisy data, we located positions where the ridge crossed the paleo-equator at the time of creation of the individual magnetic isochrons. The track of the paleo-equator moves southward relative to the Pacific plate during chron 32-25 time. The track of the paleo-equatorial crossing also has implications for constraints on the paleomagnetic pole.
GP11A-0695
Astronomical calibration of the middle Eocene Contessa Highway section (Gubbio, Italy)
The Earth's Eocene to early Oligocene climatic system experienced an important transition with a long-term cooling trend from warm greenhouse to icehouse conditions. Today, it is a priority to understand the causes and consequences that drove this major climatic change. In this context, a multidisciplinary study has been carried out on the middle Eocene sedimentary succession of the Contessa Highway (Gubbio, Italy). Spectral analysis and CWT technique of seven multidisciplinary high-resolution records demonstrate that climatic changes, in the western Neo-Tethys (Umbria-Marche basin) during the middle Eocene, are sensitive to eccentricity, obliquity and precession astronomical variations. In the Contessa Highway section, the lithology shows high-frequency cyclicity, which is strongly modulated by insolation. The lithologic cyclostratigraphy combined with the ~7 My-long astronomically driven climate proxy records, provide a first astronomical calibration of the middle Eocene. Here, we present astronomical age for the bio-magnetostratigraphic events along the middle Eocene Contessa Highway section. These astronomically calibrated ages mark significant improvements for the dating of biostratigraphic events and minimal correction to chronostratigraphy. Based on the available high-resolution bio-, isotope- and magnetostratigraphy and the precise multi-proxy astronomical tuning of the sedimentary record we retain that the Contessa Highway section represents an excellent candidate as GSSP for the Lutetian/Bartonian boundary.
GP11A-0696
Ultra-fine scale magnetostratigraphy of Mn crust by SQUID microscopy with spacial domain inversion
We have conducted ultra-fine scale magnetostratigraphy on a Mn crust sample D96-m4 from northwest Pacific (38 degrees 48.7'N, 138 degrees 19.14'E, water depth 1940 m) with a high-resolution SQUID microscope at Vanderbilt University. Various rock-magnetic techniques delineate the presence of well dispersed single domain magnetic mineral with coercivity of about 30 mT and Curie temperature of about 550-570 degrees C. Two thin sections of 5 mm width x 35 mm length x 0.2 mm thickness were taken from a Mn crust, which are perpendicular to each other. The thin sections were subjected to scanning by a SQUID microscope on 85 microns grids with a spatial resolution of about 100 microns for NRM and ARM. The SQUID microscopy revealed fine scale magnetic anomaly parallel to the apparent laminations. The preliminary interpretation of polarity pattern was successfully conducted and we could correlate with the standard paleomagnetic timescale. The identification of polarity boundaries made it possible to estimate growth rate of the Mn crust as 4.5 mm/Myr, which is consistent with the estimate by 10Be/9Be (5.9 mm/Myr). Furthermore, we conducted spacial domain inversion descibed by Weiss et al. (2007) with unidirectional constraint on the magnetization. The inversion on ARM was successfully conducted and gave optimum results directing to the axis of DC field of ARM imparted. The inversion of NRM on restricted surface area corresponding to Brunhes gave optimum direction similar to the direction obtained from independent thin sliced samples next to the thin sections. The results of inversion with bidirectional constraint for the whole sample surface of NRM will also be investigated and presented.
GP11A-0697
Evidence for a 3.45 Billion-Year-Old Geodynamo
The behavior of the early geomagnetic field is key for investigations of Earth's thermal history, evolution of its atmosphere and the development of life. A major obstacle to obtaining primary remanences from the oldest terrestrial rocks is ubiquitous low-grade metamorphism which has resulted in magnetic mineral alteration and magnetic overprints of varying age. Measurements on pristine single silicate crystals provide the oldest unambiguous paleomagnetic evidence of strong thermoremament magnetization from 3.2 Ga rocks (Tarduno et al., 2007). Specifically, these data indicate that Earth's magnetic field at 3.2 Ga was within 50% of the strength of the modern field. Here, we report rock magnetic and paleomagnetic analysis on ~3.45 Ga rocks from the Barberton Greenstone Belt. The data provides the first unambiguous evidence for a geodynamo at ~3.45 Ga. We focus on an exceptionally well-preserved dacitic intrusion of the Hooggenoeg Formation. This lithology is preserved in situ and as clasts comprising stratigraphically overlying conglomerates. Detailed progressive thermal demagnetization of the conglomerate clasts reveals a common overprint component of low unblocking temperature and a high unblocking temperature component which is directionally distinct for each clast. These data comprise a positive conglomerate test, providing paleomagnetic evidence that the high unblocking temperature component was acquired before the formation of the conglomerate. Theory and empirical data indicate that a thermoviscous overprint induced by low-grade metamorphism and carried by multi-domain grains cannot be completely thermally demagnetized, prohibiting the further use of the bulk rock to constrain the paleointensity of the ancient field. The positive conglomerate test also indicates that the underlying dacitic intrusion could hold a primary magnetization. Single-crystal paleointensity experiments on this lithology are underway.
GP11A-0698
The Amaranth Formation of the Williston Basin: Paleomagnetic, Petrologic and Geochemical studies
Major debate continues to exist concerning the time of deposition of the Amaranth Formation in the Williston Basin of North America, with postulated ages of Pennsylvanian, Permian, Triassic, Lower and Middle Jurassic. A multidisciplinary study of the lower member of the Amaranth Formation was conducted in six wells in Manitoba. The lower Amaranth red beds are composed of red carbonate-rich and carbonate-poor interbedded sandstones/siltstones/shales containing dolomite and anhydrite and lacking diagnostic fossils. Preliminary analysis of the oxygen and carbon isotope values measured for replacive and cement dolomite show variations related to particular lithologies that can be correlated to the types of dolomite present in the rocks. The siliciclastic sections are dominated by detrital, zoned dolomite that has recrystallized rims, whereas in the more carbonate-rich and evaporitic samples with little to no clastic content, replacive matrix dolomite is the dominant phase. Dolomite samples from the siliciclastic sections are characterized by relatively depleted carbon and oxygen isotope values, the dolomite matrix samples have relatively enriched oxygen and carbon isotope values and a few samples containing replacement matrix dolomite with minor clastic input have intermediate isotope values. These variations reflect primary and diagenetic overprints. Hematite is the major magnetization carrier, with occasional softer magnetic minerals such as magnetite. Optical microscopy revealed the existence of two types of hematite: detrital specular hematite and very fine red pigment hematite. The paleomagnetic data reveals at least three episodes of magnetization. The most pervasive magnetization, B, was formed during the Permian-Carboniferous Kiaman Reverse Superchron. An isolated magnetization in a couple of wells, C, suggests a remagnetization event that happened sometime between mid-Jurassic and Neogene, possibly resulting from a localized oxidizing fluid flow event. The oldest magnetization, D, was acquired sometime between mid-Devonian and Pennsylvanian, but it is usually poorly defined and is not common within the studied samples. B and D are both carried in both specular and pigmentary hematite and are candidates for a primary magnetization preserved in the lower Amaranth samples. The paleomagnetic data presented in this study indicate that the lower Amaranth member red beds are certainly older than early- to mid-Jurassic, and probably even older than Triassic. Our results suggest that these sediments were deposited either in Pennsylvanian or during the Permian-Carboniferous Kiaman Reverse Superchron.
GP11A-0699
magnetostrophic MRI in the Earth's outer core : linear and non-linear developements.
We have shown that a simple, modified version of the Magnetorotational Instability (MRI) can develop in the outer liquid core of the Earth, in the presence of a background shear (Petitdemange, Dormy, Balbus, GRL,35, 2008). It requires either a thermal wind, or a primary instability, such as convection, to drive a weak differential rotation within the core. The force balance in the Earth's core is unlike classical astrophysical applications of the MRI (such as gaseous disks around stars). Here, the weak differential rotation in the Earth core yields an instability by its constructive interaction with the planet's much larger rotation rate. The resulting destabilising mechanism is just strong enough to counteract stabilizing resistive effects, and produce growth on geophysically interesting timescales. We refer to this instability as the magnetostrophic MRI (MS-MRI). We investigate linear and nonlinear developments of the MS-MRI. We present global numerical simulations both axisymmetric and weakly-three- dimensional (i.e. involving a limited number of azimuthal modes). We discuss the possible signature of MS- MRI in geomagnetic observations and address the connection between the MS-MRI and the Geodynamo.
GP11A-0700
Are Apparent Polar wandering paths smooth?
Recent advances in Earth Sciences, such as provided by global tomography or climate modeling, have emphasized the essential need for accurate reconstructions of the Earth's surface, in order for instance to compare the initial positions of down going slabs or emerging hot spots with deeper mantle features, or to understand why and how time-varying climates and biomes may be related. However, in a large number of paleomagnetic studies in the literature, uncertainty related to age (or the exact time window) is very often larger than 10 Myr. The result is that small amplitude (<10°) or rapid (<10 Myr) APW features are often irresolvable. Based on two examples, one extending in the Jurassic and Cretaceous, the other during the Paleocene, we discuss various aspects of the construction of APWPs and reconstruction of past plate positions using combined magnetostratigraphies. Improved APWP segments document 2 loops respectively during Paleocene and Mid Cretaceous, and a fast and abrupt change in plate motion close to the Late Jurassic/Early Cretaceous boundary. We will discuss for these two periods of times some paleo- environmental and geodynamic aspects.
GP11A-0701
Paleomagnetic Field Properties at High Southern Latitude, Erebus Volcanic Province, Antarctica
We present new paleointensity and paleodirectional data from 100 volcanic sites in the Erebus Volcanic Province (EVP), Antarctica and twenty-one new age determinations by the 40Ar/39Ar incremental heating method. The new EVP data are combined with previously published paleomagnetic and geochronological results, providing a total of 141 sites. 91 sites have been dated and span 0.03 to 13.42 Ma. The 125 high-quality EVP directional data selected from the merged dataset have a non-Fisherian distribution and a mean direction with an inclination anomaly of ~3°, but the 95% confidence limit includes the prediction from a geocentric axial dipole. Virtual geomagnetic pole (VGP) dispersion is high compared with values from mid to low-latitude regions regardless of the criterion used to determine transitional fields. With VGP latitude cut-off at 45° the dispersion of 24.0 ± 2.2° for the combined 0-5 Ma EVP dataset is consistent with earlier high latitude data and paleosecular variation (PSV) in Model G but not with some more recent statistical PSV models. The mean EVP paleointensity of 31.5 ± 2.4° μT, derived from modified Thellier-Thellier experiments is about half the current value at McMurdo. This result is independent of our data selection criteria. High VGP dispersion and low intensity values support the global observation of anti-correlation between directional variability and field strength. Simulations of time-varying dipole strength show that uneven temporal sampling may bias the mean EVP intensity estimate, but the possibility of persistently anomalous field behavior at high latitude cannot be excluded.
GP11A-0702
Magnetic Carriers In Archean Cherts from the Barberton Greenstone Belt, South Africa
The ~3.4 billion-year-old Buck Reef Chert of the Barberton Greenstone Belt, South Africa has been the focus of continued study, especially in regard to the presence of potential biogenic microstructures. Here, we address the nature of magnetic carriers in the Buck Reef Chert and other cherts from the Barberton area. Magnetic properties of bulk samples and magnetic separates have been measured. Scanning Electron Microscope observations of magnetic separates in Scanning Transmission Electron Microscope (STEM) mode reveal cubo-octahedral to quasi-rectangular shaped grains that fall within a stable single domain range. Energy Dispersive Spectrometer (EDS) analysis shows that iron, sulfur, carbon, and oxygen are contained within the magnetic separates. Measurements of magnetic hysteresis properties using a Princeton Measurements Corporation Alternating Gradient Force Magnetometer show that bulk samples have a variety of hysteresis behaviors (including multi-domain, pseudo-single domain and wasp-waisted curves). Preliminary analyses of bulk magnetic susceptibility versus temperature (using a KLY-4S Kappabridge) highlight the tendency for alteration on heating (100-400 °C). Our results to date should be interpreted in terms of the original depositional environment, and the subsequent geologic history. While it is possible that some of the magnetic minerals we have separated and observed using STEM analysis are primary, it is also important to recognize that the host rocks have undergone low grade metamorphism at peak temperatures of 250 to 320 °C. Therefore any depositional sulfides (e.g. greigite) in the originally sulfur rich environment may have been converted to other iron oxides. This may be a common process in Archean cherts. We will discuss attempts to better distinguish primary from secondary magnetic minerals in the Barberton rocks.
GP11A-0703
Search for Vertical Redox Gradients in the Late Archean Ocean: No Bacterial Magnetofossils
The timing and rate of Earth's Great Oxygenation Event (sometime between 2.7 and 2.2 billion years ago) and the origin of vertical redox gradients in sedimentary and water columns are currently in dispute. Magnetotactic bacteria today most commonly inhabit oxic-anoxic transition zones in freshwater and marine systems. Hence, the presence or absence of their fossilized magnetite precipitates may be used as an indicator of paleo-redox conditions. Using conventional rock magnetic techniques, ferromagnetic resonance (FMR) spectroscopy, Raman spectroscopy, and electron microscopy, we screened late Archean material from a ~1.33 km Agouron-Griqualand Paleoproterozoic Drilling Project (AGPDP) core for the presence of microbial magnetofossils. Of all facies, only the magnetic and FMR properties of sideritic cherts from the Kuruman and Nagua formations somewhat resemble typical magnetofossil signatures. However, most evidence for magnetite in these samples is also associated with trace or dominant siderite. We suspect that thermal decomposition of the carbonate phase may be responsible for genesis of magnetite from siderite during burial diagenesis and/or Paleoproterozoic crustal reheating. The absence of magnetofossils in the AGPDP core could indicate that environmental conditions in the late Archean were not conducive to magnetotactic bacteria, that low- temperature diagenetic or metamorphic events destroyed preexisting magnetofossils, and/or that magnetotactic bacteria evolved following the Archean epoch.
GP11A-0704
Reverse Paleomagnetic Polarity in Exposed Lacustrine Sediment in Searles Valley, California, Dated 34,000-46,000 Calendar Years B.P.
The late Pleistocene history of Searles Lake in the western Great Basin of the U.S. is known in detail from field work and cores recovered during industrial exploration of Searles Valley, in which the lake formed (Smith et al., 1979; Liddicoat et al., 1980). Exposed siltstone assigned the age about 34,000 to 46,000 calendar years B.P. (eight AMS 14-Carbon dates on gastropods and mollusks from fine- to medium-grain sand units that bracket the siltstone) records reverse paleomagnetic polarity following thermal demagnetization to 600°C at two localities 3 km apart. For 12 samples (six pairs from six horizons, three pairs from each locality), the mean paleomagnetic directions are I = -37.5°, D = 180.2°, alpha-95 = 19.5° and the mean Virtual Geomagnetic Pole (VGP) is 73.6° S, 231.8°E, Alpha-95 = 20.6°. The reverse polarity in the 12 samples is not part of the Mono Lake Excursion (Denham and Cox, 1971) that never has a VGP in the Southern Hemisphere (Liddicoat and Coe, 1979). Other samples from the two Searles Valley localities do not reach a definite reverse direction but contain a component of magnetization that approaches reverse polarity above 400°C. The remanence in the Searles Lake siltstone is very low, and when examined in polished thin section, the siltstone contains detrital opaque grains that have a diameter of about 0.2 microns.
GP11A-0705
Correlation of Glaciations: a Comparison of Northwestern North America and the South American Cordillera
Northwest Canada is one of the relatively few regions of the world where ice sheets from mountains (valley glaciers) and plains (Laurentide Ice Sheet) coalesced. This also occurred to a minor degree in N.W. Siberia (Arctic Ice Sheet and northern Ural Mountain valley glaciers) while in southern Argentina and Chile large coalescing Piedmont glaciers (Patagonian Ice Sheet) radiated out to the east and west, reaching both the Atlantic and Pacific Oceans. For the most part, build up of continental and Cordilleran ice was coincident in northwestern North America, clearly reflecting the 100 ka Milankovitch cycles. The notable absence of ice from MIS 56 to 38 in both N.A. and S.A. coincides with the low amplitude del 18O values, and bridges the time span during which the transition from 40 ka to 100 ka cycles is seen in the marine isotope record. The Argentinean record, which is the most complete thus far, appears to indicate a record of glaciation which considerably precedes that of N.A. Records from 5-6 Ma in N.A. and S.A have some similarities but the Argentinean record indicates a much earlier initiation of glaciation, dating back as far as 7.5 Ma. Small local glacier occurrences are rare in the earlier N.A. record (e.g. Yakataga Formation, Alaska) but much more common in the Argentinean record, suggesting that Argentinean valley glaciers may have developed earlier in response to cooling associated with the presence of the Antarctic Ice Sheet and the associated northward outflow of cold water along S.A. coasts. In N.A. the major Cordilleran/Montane glacial events commenced immediately preceding the Gauss/Matuyama boundary (2.8-2.6 Ma) and increased in frequency and magnitude with the advance of time. At the time of the Gauss-Matuyama boundary the N.A. and S.A. records are nearly synchronous. Proceeding back in time from the mid-Pleistocene transition (MIS 38), periods of ice free conditions appear to have been more extensive. In southern Argentina the Cordilleran glacial record appears to be somewhat independent from the global ice volume record. The extensive nonglacial periods that are noted in both continents correspond to major periods of tectonic stability and regional denudation. In northwestern North America there are at least 5 and possibly as many as 7 levels of pedimentation documented. In S.A. this record of pedimentation is considerably more extensive.
GP11A-0706
Magnetic Parameter Analysis in Studies of Holiday Beach, Western Lake Erie, Ontario, Canada
Magnetic measurements offer advantages such as low costs, speed, sensitivity and flexibility over other techniques in studying a wide range of environmental processes. Many beaches of Lake Erie (42¨¬N, 81¨¬W) in Canada contain highly magnetic sands. In this work, we utilize magnetic susceptibility (¥öLF), isothermal remanent magnetization (IRM) acquisition curves, partial anhysteretic remanent magnetization (pARM) spectra and hysteresis measurements to investigate five profiles at Holiday Beach on the northern shoreline of western Lake Erie. A contour plot of the ¥öLF values for the 5 profiles shows a high concentration of magnetic minerals near to, but not right at the water's edge, and lower concentrations at the water's edge and in the inland area, close to vegetation. This occurs in three profiles (3, 4 and 5, with the highest ¥öLF value (4.3¡¿10-3 SI) in profile 5. Profiles 1 and 2 are relatively flat with low ¥öLF (< 5¡¿10-4 SI) values throughout, similar to the background values in the other three profiles. A distinctive magnetite component,with low coercivity and of PSD-MD size, is present in the beach sand, and produces the aomalies mentioned previously. pARM spectra for profiles 3, 4, and 5 show a peak at 8¢¦15 mT, indicating lower coercivity and a PSD to MD magnetic size. In contrast, such features are not distinct in the latter two profiles. The variation of pARM is consistent with the ¥öLF distribution, suggesting coarser magnetic minerals contribute to the strong magnetic susceptibility ¥öLF anomalies. The average Bloemendal S-300 ratio of all samples is close to 1 (¢¦0.96), indicating the dominant magnetic mineral of the beach sand has low coercivity. The difference between S-300 and S-100 ratio is lower than 0.1, suggesting that little fine- grained material is present in these samples. However, this difference is >0.1 at the water's edge in profile 1 and close to the inland area in profile 4, suggesting that some finer grains probably occur in the sand. Hysteresis coercivity (Bc) is low and generally decreases from water edge to inland area, and remanent coercivity Bcr is generally consistent, so the ratio Bcr/Bc increases. The results suggest that magnetic parameters may be used to understand shoreline processes.
GP11A-0707
Precessionally-Driven Dynamo in a Spheroid
It has been shown by Tilgner [Physics Fluids 17 (3): Art. No. 034104, 2005] that the luni-solar precession is capable of powering a geodynamo model. Tilgner's model assumes a spherical core-mantle boundary and therefore relies on viscous coupling to transmit the precessional motion of the mantle to the fluid in the outer core. It is probable that a more potent coupling of mantle and core is topographic, i.e., is due to the oblateness of the core-mantle boundary. We have studied this mechanism through numerical simulations of incompressible fluid flow in a precessing oblate spheroidal container. The computer code advances the flow in time using finite differences on overlapping grids; in this way the numerical difficulty known as the pole problem is completely avoided. In this study, as in the study of Kerswell [Geophys. Astrophys. Fluid Dynamics, 72, 107-144, 1993], Poincaré's basic solution in a precessing spheroid is found to be linearly unstable. Furthermore, it is shown that the resulting finite amplitude precessional flow can maintain a magnetic field by dynamo action.
GP11A-0708
Managing Rock and Paleomagnetic Data Flow with the MagIC Database: from Measurement and Analysis to Comprehensive Archive and Visualization
The Magnetics Information Consortium (MagIC) is commissioned to implement and maintain an online portal
to a relational database populated by rock and paleomagnetic data. The goal of MagIC is to archive all
measurements and derived properties for studies of paleomagnetic directions (inclination, declination) and
intensities, and for rock magnetic experiments (hysteresis, remanence, susceptibility, anisotropy). Organizing
data for presentation in peer-reviewed publications or for ingestion into databases is a time-consuming task,
and to facilitate these activities, three tightly integrated tools have been developed: MagIC-PY, the MagIC
Console Software, and the MagIC Online Database.
A suite of Python scripts is available to help users port their data into the MagIC data format. They allow the
user to add important metadata, perform basic interpretations, and average results at the specimen, sample
and site levels. These scripts have been validated for use as Open Source software under the UNIX, Linux,
PC and Macintosh© operating systems.
We have also developed the MagIC Console Software program to assist in collating rock and paleomagnetic
data for upload to the MagIC database. The program runs in Microsoft Excel© on both Macintosh©
computers and PCs. It performs routine consistency checks on data entries, and assists users in preparing
data for uploading into the online MagIC database.
The MagIC website is hosted under EarthRef.org at http://earthref.org/MAGIC/ and has two search nodes,
one for paleomagnetism and one for rock magnetism. Both nodes provide query building based on location,
reference, methods applied, material type and geological age, as well as a visual FlashMap interface to
browse and select locations. Users can also browse the database by data type (inclination, intensity, VGP,
hysteresis, susceptibility) or by data compilation to view all contributions associated with previous databases,
such as PINT, GMPDB or TAFI or other user-defined compilations. Query results are displayed in a digestible
tabular format allowing the user to descend from locations to sites, samples, specimens and measurements.
At each stage, the result set can be saved and, when supported by the data, can be visualized by plotting
global location maps, equal area, XY, age, and depth plots, or typical Zijderveld, hysteresis, magnetization
and remanence diagrams.
http://earthref.org/MAGIC/
GP11A-0709
REVISTING MAGNETIC INTENSITY AND CLIMATE CHANGE: A STRONG CORRELATION
Relations between Earth's magnetic intensity and climatic temperatures were suggested and investigated during the 1970's and early 1980's. The strong statistical correlation was dismissed owing to no explanation for the process. However, research shows that the intensity of a material's magnetic field changes as the material's temperature changes, thus suggesting that the Earth's core temperature varies. Additional and more complete global-scale datasets and advanced analytical techniques indicate that global and, to a lesser degree, continental average annual temperatures respond significantly to secular variations of core- generated magnetic intensity. Simple polynomial-regression techniques show that globally-averaged secular variations predict and explain 79-percent of the variability in global average-annual temperatures 7-years in the future; thus suggesting another or additional process contributing to climate change.
GP11A-0710
Reduction of Aeromagnetic Surveys to a Common Base
Two reduction methods of aeromagnetic surveys are compared and a simple correction scheme between systems is presented, provided that a geomagnetic observatory record is available. Both methods are based on a base station for correcting the daily variation. Longer period variations are reduced either by average field at one or more geomagnetic observatories or at the base station. The former is commonly used in northern Eurasia and the latter by commercial contractors worldwide. By the correction these two sets can be made more compatible by a ground based scheme. A measurement and reduction scheme is presented to reduce long tie lines by a moving ground station network jointly with geomagnetic observatories. This method is useful to tie aeromagnetic surveys during a long time span, provided that reduction of survey levels fail by other methods. Examples from NW-Europe are presented.