GP42A-01
The Matuyama-Brunhes Reversal: a Global and Temporal Perspective
We present new palaeointensity and palaeodirectional results from the Matuyama-Brunhes geomagnetic field reversal. Dated volcanic sequences from four global locations: La Palma, Chile, Iceland and Guadeloupe, are investigated. These sections sample the Matuyama-Brunhes boundary, possible precursors, and fluctuations in the magnetic field prior to the main polarity reversal. Absolute palaeointensity values were determined using the 14 GHz microwave systems at the Geomagnetism Laboratory, University of Liverpool. New directional results (from thermal demagnetisation) and published directional data are combined with new microwave intensity data to define the full vector of the field during the reversal. We compare our data with 1) other published data for the Matuyama-Brunhes reversal and other reversal records that have both intensity and directional data; 2) a simple mathematical reversal model based upon CALS7K.2 (Brown et al., Geophys. J. Int, 168, 541-551, 2007); 3) the iterative Bayesian model of the Matuyama-Brunhes reversal, IMMAB4 (Leonhardt and Fabian, Earth Planet. Sci. Lett., 253, 172-195, 2007).
GP42A-02
Paleointensity study of the Brunhes-Matuyama reversal recorded in lavas on Tahiti Island by the LTD-DHT Shaw method
We have been investigating the Brunhes-Matuyama (B-M) polarity reversal recorded in lavas of the northern side of the Punaruu Valley, Tahiti Island. The lavas were sampled successively as possible as we can and in total 34 sites were located. From the bottom to the top of the sampled lavas, the directional data show a reversed (R) polarity, intermediate-normal-reversed (I-N-R) change and subsequently normal (N) period. These directional changes suggest that these lavas are considered to record the paleomagnetic field variation from the Matuyama chron to the whole stage of the B-M reversal. Paleointensity determinations were made by the LTD-DHT Shaw method (Tsunakawa and Shaw, 1994; Yamamoto et al., 2003; Mochizuki et al., 2004). For the directionally unstable period (I-N-R), the paleomagnetic field intensity was weak (4.7 μT). For the Matuyama reversed period prior to the directional reversal, the paleointensity results suggest that the field intensity varied in oscillation-like manner between 2 and 43 μT. It is noted that the virtual dipole moments (VDMs) in the oscillation-like change show a clear correlation with the VGP latitudes change, suggesting one of the important processes in the geodynamo for the polarity reversal.
GP42A-03
Transitional Evolution of the Geomagnetic Field Recorded in Long Volcanic Sequences: Insights From Icelandic and Hawaiian Lavas
We have studied the short-term evolution of the geomagnetic field recorded by long volcanic sequences in Iceland and Hawaii. The eastern Icelandic sequences correspond to 45 and 49 individual lava flows sampled and range in age from 12.9 to 10.21 Ma as reported by Watkins and Walker [Watkins and Walker, 1977. Am J. Sci. 277, 513-584] and are labeled as profiles C and D. Also sampled were long volcanic sequences in the Waianae, Koolau (island of O"ahu) and Mauna Loa (Big Island of Hawaii) volcanoes. We have studied in detail the directional characteristics of three successive reversals, the Gilbert-Gauss, the Lower and the Upper Mammoth reversals recorded by the Waianae lavas. These results confirm that large oscillations of directions precede or follow the reversals, which shows waveforms typical of paleosecular variation with their amplitude being considerably amplified by the decrease of the dipole. There is no apparent preferred location for the virtual geomagnetic poles (VGP). In addition to the directional analyses, determinations of absolute paleointensity were attempted on more than 540 samples, which document the field variations surrounding the Lower Mammoth transition. A period of a weak field dominated before the reversal; then the transition was initiated by a transit from normal to reverse polarity followed by a short restoration of field intensity in reverse polarity. A second episode of a 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 similar 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. In addition we have obtained reversal records from the Koolau Volcano corresponding to Cryptochron C2r.2r-1 (ca. 2.514 ±0.030 Ma) and of the Laschamp and Pringle Falls excursions recorded by the Mauna Loa volcano (Big Island of Hawaii). The Icelandic as well as the Hawaiian transitional VGPs 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 the Americas and eastern Asia. Clusters of VGPs 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.
GP42A-04 INVITED
Can Transitional Paleomagnetic Data Track Changes in the Pattern of Mantle-held Flux?
There exists substantial evidence that the lowermost mantle has an influence over the pattern of magnetic flux leaving Earth's core. Although more controversial, there also exist both paleomagnetic and geomagnetic data compatible with the hypothesis that polarity reversal begins with the near total destruction of the axial dipole, leaving a complex field configuration whose source is the variation of physical properties about the core-mantle boundary. If so, it follows that quality, detailed paleomagnetic transition data spanning events over considerable geologic time may provide a unique tool to understanding spatial-temporal aspects of the mantle's effect on core fluid motion and related magnetic flux that reaches Earth's surface. In this talk we explore this possibility given published and newly-analyzed reversal data that span the latter half of the Cenozoic.
GP42A-05
Evidence for Two New Paleomagnetic Field Excursions ~2,500 and ~12,500 Years Ago from the South Pacific Ocean Region (Tahiti)
Paleomagnetic measurements have been made on 250 oriented samples from IODP Expedition 310 Sites 5, 7, 15-18 in the MARAA region of the Tahiti coral reef tract. The paleomagnetic inclinations (unoriented cores preclude declination measurements) average -31.1 degrees with an approximate 95 percent confidence interval of 2.8 degrees. This value is not significantly different from the axial-dipole expected inclination for this region of - 32.6 degrees. We believe that the paleomagnetic inclinations reflect the actual geomagnetic field values at this region over time. The directions are due to detrital magnetic grains (less than 1 μm to ~10 μm) from the Tahiti Island volcanics being washed out into the coral reef, oriented with the ambient field in quiet water conditions within the reef, and locked into place by primarily microbial precipitation of carbonate (95 percent of the samples come from the microbialite component of the reef). Relative paleointensity measurements have been made as well by normalizing the demagnetized natural remanence to magnetic susceptibility and demagnetized isothermal remanence. Both the inclination and relative paleointensity variability in each hole/site can be correlated with variability at the other independent holes/sites. Moreover, we can correlate this paleomagnetic variability with other sites from the Pacific Ocean Basin that surround Tahiti. This permits us to estimate a chronostratigraphy for the postglacial MARAA coral reef tract that extends from about 18,000 years ago to the present. This also lets us estimate the spatial pattern and timing of MARAA coral reef development over the last 18,000 years. We note, as well, the existence of 10 inclinations that average +30 degrees, almost antipodal to the rest of the inclinations. These anomalous inclinations are consistently located at two horizons in independent holes. Four anomalous inclinations from four different holes all occur at about 2,500 years ago and six anomalous inclinations from six different holes occur about 12,500 years ago. Our working hypothesis is that these are magnetic field excursions, which have never been recorded elsewhere. Yet, these two excursions occur at the two times of most anomalous paleomagnetic directions noted previously from sites that surround Tahiti: Indonesia, New Zealand, Hawaii, and Chile.
GP42A-06 INVITED
Proterozoic Geomagnetic Field: Intensity, Morphology and Stability
Information about the geometry, stability, and intensity of Earth's magnetic field during the Proterozoic is of great importance for understanding the early geodynamo and in the interpretation of paleomagnetic data with regards to paleogeographic reconstructions, true polar wander, global glaciations and other problems. What are the space-time characteristics of the Proterozoic field, such as the relative significance of the dipole and non-dipole components, and are these characteristics significantly different from their Phanerozoic counterparts? In the absence of strict theoretical constraints, paleomagnetic data are the principal source of information about the Precambrian field. Field geometry can be estimated by combining paleomagnetic data with independent latitudinal indicators such as evaporites and glacial deposits. A hotter Precambrian Earth coupled with absence of the inner core could have resulted in higher field variation, including more frequent reversals. The current magnetostratigraphic database is insufficient to test whether the early geodynamo reversed its polarity with the same range of frequencies as in the Mesozoic-Cenozoic interval. However, Precambrian rocks may preserve directional information useful for constraining paleosecular variation (PSV). The estimates of PSV based on the Precambrian paleomagnetic database will be discussed. Paleointensity data provide information on the energy state of geodynamo and may prove crucial for constraining models of the thermal evolution of the Earth (e.g., the timing of nucleation and growth of the solid inner core). However, many Precambrian rock sequences have been affected by alteration, which hinders the measurement of paleointensity using bulk rock samples. In addition, many paleointensity determinations for that time period come from intrusive rocks in which the paleointensity signal may be complicated by thermochemical remanent magnetization and other factors. Therefore, a caution should be exercised when concluding on the long-term behavior of paleointensity based on a very limited database for the Precambrian. Alternatively, single silicate crystals are less susceptible to alteration in nature and during experiments may be used as paleointensity recorders. Data from plagioclase crystals separated from mafic dikes, together with directional data from whole rocks, indicate a dipole-dominated field at 2.5–-2.7 Ga. The bulk of available data indicate that on a long-term scale the Proterozoic field was not grossly different from the present-day field.
GP42A-07
The paleointensity of single silicate crystals from the Southern Volcanics of the Chatham Islands: What is the behavior of Earth's magnetic field leaving a superchron state?
While the strength of Earth's magnetic field during the Cretaceous Normal Polarity Superchron has been the subject of several recent (and on-going) studies, very little is known about the behavior of the magnetic field during the transition from a superchron to a mixed polarity state. To examine this issue, a new paleointensity study has been conducted using plagioclase crystals from the Late Cretaceous Southern Volcanics of the Chatham Islands, New Zealand. Plagioclase crystals are sought as paleointensity recorders because prior studies have demonstrated that they can contain minute (50 to 250 nm) magnetic inclusions with ideal magnetic properties. Such crystals have also been show to alter less than bulk lava samples during Thellier paleointensity experiments. Furthermore the silicate host can shelter the magnetic inclusions from in situ alteration that commonly occurs on geologic time scales, compromising the paleointensity fidelity of whole rock lava samples. Magnetic hysteresis data, collected using a Princeton Measurements Alternating Gradient Force Magnetometer and P1 probes, confirm that plagioclase crystals from lavas and dikes of the Southern Volcanics contain magnetic inclusions that are near single domain in character. Thus, the crystals are well-suited for paleointensity analysis. Magnetic hysteresis and first order reversal curves (FORCs) will be presented to examine potential differences in the magnetic inclusions between individual lavas and dikes. In addition, results derived from Thellier-Coe paleointensity analyses of single plagioclase crystals separated from select units of the Southern Volcanics will be presented and compared with values from other studies.
GP42A-08
Short Polarity Albian Events in the Cretaceous Normal Superchron - Magnetostratigraphy of the Morelos and Tlayua Formations (Mexico)
Interest in the long period of normal polarity in the Cretaceous has continued to increase since the early study of Helsley and Steiner (1969), who suspected that it corresponded to vast stretches of relatively low amplitude behaviour in the sea-floor magnetic anomaly profiles known informally as the "Cretaceous quiet zone". Since these first studies, paleomagnetists have continued to explore what is now known as the Cretaceous Normal Superchron, an interval of prolonged normal polarity from 118 to 84 Ma. Paleomagnetic data from sediment cores from the DSDP Holes 369, 386, and 364, were interpreted as brief reverse intervals (Keating and Helsley, 1978, 1979). Relatively fewer studies have been reported for land-based sections of pelagic and platform limestones; reports include studies in northern and central Italy (Vandenberg et al., 1978; Tarduno et al., 1992; Tarduno, 1990; Cronin et al., 2001) and Mexico (Urrutia-Fucugauchi, 1988; Benammi et al., 2006). We present here new magnetostratigraphic data for limestone sequences in central and southern Mexico from the Cretaceous Morelos and Tlayua Formations, which record short intervals of reversed polarity during the K normal superchron. The Morelos Formation has been revisited in Guerrero, southern Mexico, where the sequence of platform limestones with almost horizontal thick beds is well-exposed. The magnetostratigraphy shows dominant normal polarity for the sampled sections, with the occurrence of three short reverse polarity intervals corresponding to the Albian, which have been re-sampled and studied in additional detail for magnetic mineralogy and remanent acquisition. The Tlayua Formation is sampled in Tepeji de Rodriguez area, which is known for its rich and well-preserved fossil contents. Age control has mainly been provided from studies on ammonites, belemnites and benthic foraminifera (Cantu, 1987; Seibertz and Buitron, 1987; Kashiyama et al., 2003). Nine polarity intervals are documented in the Tlayua Middle Member, which correlate with middle part of chron C34n (C34n.1n-C34n.2n, 100-105 Ma). Results give further evidence for occurrence of short polarity events within the K normal superchron, and provide additional documentation on the polarity events.