GP43B-0850 1340h
Was the Regime of Geomagnetic Dipole Lows and Excursions Influenced by the Pleistocene Climate?
Paleomagnetic directions and relative paleointensities (NRM/ARM) as well as cosmogenic nuclide concentration (authigenic Be-10/Be-9 ratio, i.e. cosmogenic vs lithogenic isotopes adsorbed on the sedimentary particles) were measured along clayey-carbonate sequences deposited through the last 1300 kyr in high accumulation rate area of the North-East Atlantic (0-400 kyr BP) and West-Equatorial Pacific (600-1300 kyr BP). The Low Dipole Moment phases (LDM) generally match with large amplitude or even excursional deviations of paleomagnetic vector directions, at the time of excursions [Laschamp, Blake, Icelandic basin, Jamaica, Portuguese margin, Levantine .] or reversals [Brunhes/Matuyama, Jaramillo boundaries, Cobb Mountain]. Peaks of the Be-10/Be-9 ratio occur at the level of each LDM, or slighlty above (pDRM lock-in depth may reach ~15-20 cm, which has significant consequences for the use of LDM phases and/or excursions as global correlation markers at least at the submillennial scale). Over the last 400 kyr, paleomagnetic and geochemical evidences of the LDM very often coincide with the end of integlacial stages. Although such relationship generally cast doubts about the reliability and significance of sedimentary RPI records, this observation can be supported and extended to the last 800 kyr using other archives of the dipole moment variation: first the SINT 800 (Guyodo and Valet, 1999) but more importantly the S.E. Pacific near Sea Floor magnetization record (Gee et al. 2000). However this relationship (LDM - interglacials) can not be confirmed prior to 800 kyr BP. Complex Wavelet Analyses identify a modulation of RPI variations between periods of 80 and 120 kyr, and in few time windows, near a period of 40 kyr. A modulation between 40 and 100 kyr appears in the S.E. Pacific sea floor record and in the 600-1300 kyr window of the cosmogenic isotope records of theWest-Equatorial Pacific. Such coupling "geomagnetic field /climate" limited to the last 800 kyr, might be explained by a VDM regime triggered by variations of polar ice caps (dominated by the ~100 kyr eccentricity period), at the time of their maximum amplitude. Variations of the speed rotation due to strong polar load/unload alternation may have provoked variations of the geodynamo regime: Ice volume minimum = slower rotation = weakened geodynamo (low dipole moment) Ice volume maximum = faster rotation = enhanced geodynamo (high dipole moment).
GP43B-0851 1340h
Developing a Holocene Chronostratigraphic Template for the North Atlantic: Paleomagnetic, Radiocarbon and Tephra chronostratigraphies from Iceland (MD99-2269) and East Greenland (MD99-2322)
For paleomagnetism to reach its full chronostratigraphic potential, precise knowledge of the temporal variability of the full geomagnetic vector is required. Towards this rarely achieved goal, we present high-resolution Holocene sediment paleomagnetic direction and intensity records from the NW Iceland (MD99-2269: Lat 66.37.53 N, Long 23.51.16 W, water depth 365 m, length 2530 cm) and East Greenland margins (MD99-2322: Lat 67.08.18 N, Long 30.49.67W, water depth 714 m, length 2635 cm). The natural remanent magnetization (NRM) studied by progressive alternating field demagnetization of u-channel samples, preserves a strong, stable, single component magnetization. Other than coring induced inclination shallowing of 4 degrees and a linear trend in declination in MD99-2269, the directions of both records are consistent with each other and regional comparisons support a geomagnetic origin. Normalized intensity records are not clearly geomagnetic in the upper part of MD99-2269 possibly as a result of coring. Normalized intensity from MD99-2322 shows a more typical Holocene pattern and older (9-12 ka) highest accumulation rate part of both cores show a very similar pattern that correlates with the inverted cosmogenic isotope records supporting a geomagnetic interpretation. The chronostratigraphy is based on a multilevel approach. Twenty-five AMS radiocarbon dates from MD99-2269 are augmented by an additional 18 AMS radiocarbon dates from MD99-2322. Paleomagnetic synchronization of the two cores allows the radiocarbon dates to be combined into a single age to depth profile. The accuracy of the calibrated chronology using the standard ( 400 yr) marine reservoir correction was tested using eight terrestrially dated Icelandic tephras identified in MD99-2269. Temporal offsets are generally less than 200 yrs and are corrected for in our final age model. Comparison between paleointensity and independently dated cosmogenic isotope records further attest to the accuracy of the chronology. The use of these records as a North Atlantic (and beyond) chronostratigraphic template will be discussed.
GP43B-0852 1340h
Geomagnetic Reversals and Apparent Excursions in Icelandic Lava Sequences
Detailed paleomagnetic results from composite stratigraphic sections containing 2600 Icelandic lava flows of 1 to 15 Ma age have been published in 1985-2004: a bibliography of these and other papers on paleomagnetism in Iceland is available from the author. Some 11 per cent of the lavas have yielded "transitional" virtual geomagnetic poles, i.e. below 40 degrees N or S latitude. If we define a polarity zone in a sampled profile as consisting of one or more (commonly 10-20) lavas with VGP latitudes above 40 deg., then only about one-third of the transitional poles are found at boundaries between polarity zones. The remaining two-thirds of the transitional-pole lavas were emplaced during apparent excursions. The deviations of the field from the axial central dipole direction are difficult to classify into two distinct categories of "secular variation" and "transitions", as they most probably had a continuous range of amplitudes. The above results are similar to those found by the present author and Ian McDougall from older surveys comprising 2400 sampled lavas (Geophys. J. Royal Astr. Soc. vols. 68, 1982 and 80, 1985). In these papers it was concluded that the geomagnetic field reversed its polarity on average about 8 times per Ma in the period 1-15 Ma ago; as few radiometric dates have been obtained from Icelandic lavas subsequently, this estimate has not been revised. Excursions and polarity zones containing a few lava flows sometimes aid in stratigraphic correlation between lava profiles separated by several km or more. However, large spatial and temporal variations occurring in the lava production tend to limit the usefulness of such features.
http://www.raunvis.hi.is
GP43B-0853 1340h
On the Discovery of Cryptochron C2r.2r-l (ca. 2.42-2.44 Ma) Recorded on Koolau Volcano at Halawa: Evidence from Paleomagnetic and $^{40}$Ar/$^{39}$Ar studies
New paleomagnetic measurements, coupled with precise $^{40}$Ar/$^{39}$Ar radioisotopic dating, are revolutionizing our understanding of the geodynamo by providing detailed terrestrial lava records of the short-term behavior of the paleomagnetic field. As part of an investigation of the Koolau Volcano, Oahu, and the short-term behavior of the geomagnetic field, we have sampled a long volcanic section located on the volcano\'s buttressed flank within Halawa Valley. Prior paleomagnetic and K-Ar investigations of the Koolau (Volcano) Series revealed excursional directions (Site F of Doell and Dalrymple, 1973). The alkaline composition of lava flows, easy access, and close geographical proximity to K-Ar dated lava flows made this newly studied 120 m thick sequence of flows an excellent candidate for detailed paleomagnetic analysis. At least eight samples, collected from each of 28 successive flow-sites, were stepwise demagnetized by both alternating field (5mT to 100mT) and thermal (from 28$^{o}$C to 575-650$^{o}$ C) methods. Mean directions were obtained by principal component analysis. All samples yielded a strong and stable ChRM trending towards the origin based on no less than seven to nine steps, with thermal and AF results agreeing to a very high degree. Low field susceptibility vs. temperature (k-T) analyses were conducted for individual lava flows, with approximately half showing reversible curves. Curie point determinations revealed a temperature close to or equal to 580$^{o}$ C, indicative of almost pure magnetite for most of the flows. The mean directions of magnetization of the entire section sampled indicate that about 10 m of the section are characterized by excursional directions (5 lava flows). The corresponding VGPs are located off the southeast part of Africa, close to Madagascar. Initial $^{40}$Ar/$^{39}$Ar incremental heating experiments on groundmass from four flow-sites located at different stratigraphic levels yielded isochron ages ranging from 2.68+/-0.25 to 2.40+/-0.46 Ma indicating that the excursion may correlate with the C2r.2r-l Cryptochron of Cande and Kent [1995]. Pending further $^{40}$Ar/$^{39}$Ar dating underway, this is potentially the first terrestrial record of the ca. 2.4 Ma Cryptochron, a finding that will place important constraints on the evolution of the entire Koolau shield edifice.
GP43B-0854 1340h
Late Miocene-Recent Magnetic Polarity Stratigraphy and Astro-Chronology From ODP Sites 1207, 1208, 1209, 1210, 1211 and 1212- Shatsky Rise
ODP Leg 198 to Shatsky Rise recovered a total of ~768 meters of Late Miocene to Recent sediments from 6 Sites. Neogene sediments at the sites consisted mostly of light gray to pale orange nannofossil oozes with varying amounts of clay, radiolarians and diatoms. Site 1208 was drilled on the Central High and provided a Late Miocene to Recent sequence with sedimentation rates ~4-5 cm/kyr. Sites 1209, 1210, 1211 and 1212 were drilled on the Southern High and yielded shorter sequences of Late Miocene to Recent sediments with sedimentation rates ~1-2 cm/kyr. Clearly interpretable magnetic stratigraphies have been obtained from all six sites based on shipboard data. These results have been augmented using discrete sample cubes (7cc) collected shipboard and measured post-cruise. Astrochronologies for Sites 1207 to 1211 were based on cycles seen in reflectance data from shipboard measurements. The reflectance data for these cores were measured using a digital imaging track system equipped with a line-scan camera. ODP Leg 198 was the first cruise after installation of the track, and these measurements provided a high-resolution stratigraphic record of color variations for visible wavelengths. These data were placed on an initial age model by pattern fit of the polarity zones to the geomagnetic polarity timescale (GPTS) of Cande and Kent (1995). Power spectra calculated using this initial age model often show concentration of power at orbital frequencies, particularly around the 41 kyr obliquity cycle, although the orbital power is more pronounced in some parts of the section than in others. The reflectance record was tuned to the astronomical solution for obliquity from Laskar et al. (1993). Astronomically tuned ages were estimated for polarity reversals in the 1-9 Ma interval at Site 1207. At Sites 1208 and 1209 tuning was performed in the 1-7 Ma interval and at Sites 1210 and 1211 in the 1-5 Ma interval. Cross spectral analysis on the tuned age model indicated significant coherence between the astronomic solution and the reflectance data. The tuned timescales are compared with other published ages for this part of the GPTS and indicate that the Shackleton et al., (1995) ages from ODP Leg 138 in the 8-9 Ma interval are too young by ~100 kyrs. The astronomical timescales are in better agreement with the timescale of Hilgen et al., (1995) in this interval.
GP43B-0855 1340h
30 Myr of polarity stratigraphy and relative paleointensity from Equatorial Pacific sediments (ODP Sites 1218 and 1219, Leg 199)
ODP Sites 1218 and 1219 yielded an exceptional record of the direction and relative intensity of the geomagnetic field during most of the Miocene and Oligocene. The sedimentation rate in the Oligocene and the lower Miocene carbonate-rich pelagic sediments is > 10 m/Myr, and measurements on u-channel samples allowed a high-resolution magnetostratigraphy to be resolved. Moreover, the sediment has uniform magnetic properties that made it suitable for relative paleointensity estimates. Relative paleointensity was computed using the slope of thenatural remanent magnetization (NRM) versus anhysteretic (ARM) and isothermal remanent magnetizations (IRM) during AF demagnetization. ARM and IRM were AF demagnetized using the same alternating fields used for NRM demagnetization This high-resolution record of magnetic polarity and relative paleointensity has been used to determine whether small-scale magnetic anomalies on the ocean floor (cryptochrons) represent short polarity subchrons, intensity fluctuations of the geomagnetic field, or both.
GP43B-0856 1340h
Deep-tow Study of Magnetic Anomalies in the Pacific Jurassic Quiet Zone
The Jurassic Quiet Zone (JQZ) is a region of low-amplitude, short-wavelength, difficult-to-correlate magnetic anomalies located on Jurassic seafloor and thought to represent a time of decreased field strength and rapid reversals. We collected new deep-tow magnetic data over the Pacific JQZ that complement 2 deep-tow profiles reported in Sager et al. (J. Geophys. Res., vol.103, p. 5269, 1998). Our primary goals were to extend the correlation of deep-tow magnetic anomalies farther back in time, crossing ODP Site 801 (where Jurassic ocean crust has been drilled and cored), to evaluate the correlation of anomalies, and to refine the Jurassic geomagnetic polarity reversal time scale developed by Sager et al. (1998). These new data include: (1) closely spaced lines around M34 and Site 801, (2) two long lines extending from the previous survey, across Site 801 to the southeast, and (3) one line between the previous lines in the area of difficult-to-correlate anomalies. Systematic changes in anomaly amplitudes occur along the deep-tow lines, perhaps indicating changes in field strength. From northwest to southeast (i.e., increasing in age) anomaly amplitudes and wavelengths decrease, become nearly constant, and then increase slightly. The zone of smallest, shortest wavelength anomalies corresponds to a period of ~4 m.y. that appears to have an abrupt end. Comparing anomalies between lines, correlations were excellent on the closely-spaced profiles over M34 and around Hole 801C. Correlation over supposedly older seafloor to the south of Site 801 was also good. However, anomaly correlation in the region between M34 and Site 801 was difficult. As with other studies of magnetic profiles, it is impossible to uniquely determine which anomalies are caused by reversals and which are not. Many of the larger anomalies are likely caused by changes in polarity, whereas smaller anomalies may be intensity fluctuations. The new deep-tow data, being closer to the source than the previous lines, show more short-wavelength anomalies in some areas, particularly the area where anomaly amplitudes are least. This observation suggests that many of these short-wavelength anomalies may result from intensity fluctuations. To construct a reversal time scale, we limit short wavelengths by modeling magnetic profiles upward continued to mid-water depth.
GP43B-0857 1340h
Short wavelength magnetic anomalies in the Indian and Pacific Oceans after the Cretaceous Normal Superchron (40-83 Ma).
Temporal variations of the magnetic field of the Earth span a large spectrum of timescales : from year to hundred of millions of years (from secular variation to variations of the magnetic reversals frequency). Many questions remain unsolved, particularly about the long term evolution of the intensity of the magnetic field and about a possible link between secular variation and the magnetic reversals frequency. Study of marine magnetic measurements may complement measurements of the remanent magnetisation acquired on volcanic rocks or sediments in order to recover the long term evolution of the intensity of the magnetic field. Indeed, the magmatic oceanic crust is a good recorder of the magnetic reversals but also of the variations of the intensity of the magnetic field. The goal of this study is to use marine magnetic anomaly profiles to estimate fluctuations of the magnetic field for a period when the reversal rate was low (40-83 Ma) and to compare it with the last millions of years which are characterised by a high reversals frequency. Many sea-surface magnetic profiles are available. Magnetic anomalies are of two origins : the variations of the magnetic field during the cooling of the oceanic crust and local effects due to the structure of the lithosphere. In order to discriminate between the paleovariations of the magnetic field (the signal) and the local effects (the noise), the study deals with different regions of the Earth. The Indian and Pacific Oceans exhibit fast spreading rates during the period investigated and insure better resolution. In each study area, selected profiles are reduced to the pole, stretched with the help of the main magnetic anomalies (due to reversals) and stacked in order to increase the signal to noise ratio. Preliminary results reveals systematic micro-anomalies which correspond to variations of the magnetic dipole intensity or short polarity intervals. Over the 40 Ma period investigated, we observed the same micro-anomalies than Cande and Kent(1992) (between 53 and 61 Ma) but also many additional micro-anomalies in other chrons. The number of observed micro-anomalies observed is heavily dependent on the spreading rate. Cande, S.C., and D.V. Kent, Ultrahigh resolution marine magnetic anomaly profiles: A record of continuous paleointensity variations?, J. Geophys. Res., 97, 15075-15083, 1992.
GP43B-0858 1340h
Relative paleointensity variation during the last 300 kyrs in the Northwestern Pacific region
We have obtained 8 relative paleointensity records from gravity and piston cores collected from the Northwestern Pacific Ocean (33N-39N/146E-159E). The gravity cores consist of NGC108 (6.5m, 3390m), NGC109 (6.5m, 4530m), CGC12 (4.4m, 5600m), and CGC16 (4.2m, 5670m), whereas the piston cores comprised of KR0215-PC2 (11.7m, 5936m), KR0215-PC3 (18.8m, 5970m), KR0215-PC7 (17.6m, 5750m), and MR01K02-PC1 (11.5m, 5758m). Although most of the water depths of these cores are below the Calcium-carbonate Compensation Depth (CCD), oxygen isotope records are available for two cores, NGC102 (last 300 kyr, Kawahata et al., 1999) and NGC108 (last 180 kyr, Maeda et al., 2002). NGC102 (3.3m, 2612m) was obtained in the vicinity of the studied cores. The present cores showed large decrease in S-ratio (-0.3T) at several horizons. This suggests that magnetic minerals were dissolved at these depths. Hence, we excluded such intervals from the normalized intensity records. Then the depth of each core was converted to a common depth scale of KR0215-PC3 using magnetic susceptibility. They were further transformed to ages based on the oxygen isotope stratigraphy. The age estimation is supported by two ash layers identified as Aso-4 (86-90ka) and Ata-Th (240 ka) in MR01K02-PC1. The resultant relative paleointensity curves commonly showed lows and highs at similar ages, though their amplitude did not necessarily agree among the curves. There are not only well-known lows around 40 ka (Laschamp), 120 ka (Blake) and 190 ka (Icelandic basin) but also other minima around 60 ka, 80ka, 160ka and 240 ka. Although further tuning may slightly shift these ages, they are possibly associated with recently reported or unknown geomagnetic excursions.
GP43B-0859 1340h
Authigenic Be-10/Be-9 Recorded in Western Equatorial Pacific Sediments over the Laschamp and Mono Lake Excursions. Implication for Cosmonuclide Production Rates.
The timing and the amplitude of geomagnetic moment variations and their direct impact on atmospheric cosmonuclides production rate during the late pleistocene are still debated. The effects of the Laschamp and Mono Lake excursions that occurred during Marine Isotope Stage 3 are particularly important for the radiocarbon chronology. Core MD97-2134 (9.9°S, 144.6°E, 760 m water depth) has been collected on the South New Guinean margin in the vicinity of the Fly river. The chronostratigraphy was established on 13 accelerator mass spectrometer (AMS) C-14 ages on planktonic foraminifera and on the age of the MIS4/3 boundary identified in the oxygen isotopes record. The average sedimentation rate of 25 cm/kyr allows a high resolution record of paleoclimate and paleoenvironmental variations through the 0-60 kyr BP time interval. Paleomagnetic measurements performed on Uchannels led to construct a quasi-continuous record of inclination and relative paleointensity (RPI) of the geomagnetic field. Therefore we have concentrated our sampling strategy for Be isotopes measurements around identified paleomagnetic features : one major directional deviation at 41 kyr BP and three major dipole lows at 32, 37 and 41 kyr BP. After extraction, authigenic Be-10 concentration was measured by AMS facility and normalized by the authigenic Be-9 concentration in order to reconstruct the authigenic Be-10/Be-9 ratio, proxy of the atmospheric Be-10 production rate during the studied interval. Three significant increases of the authigenic Be-10/Be-9 ratio coincide with the 3 RPI decreases between 30 and 45 kyr BP, including both the Laschamp and the Mono Lake events, usually dated at 41 and 32 kyr respectively. However, the strongest Be-10/Be-9 peak is centered at 37 kyr BP. The occurrence of enhanced deposits of atmospheric cosmonuclides between 30 and 40 kyr BP is consistent with the Be-10 flux reconstructed from the Vostok and Greenland (GRIP and GISP2) cores. Although these results may suggest that the VDM minimum associated with the Laschamp event is younger than previously admitted, our interpretation rather introduces a specific response of the cosmonuclide production rate during a significant, though until now undetected, dipole drop between the Laschamp and Mono excursions. If confirmed by other studies, these results should be considered in the reconstruction of cosmonuclides production rate variation, namely for C-14 dating.
GP43B-0860 1340h
Different magnetostratigraphic approaches: Lake Baikal sediments and the J/K boundary strata in the Tethyan realm
Rock magnetic and paleomagnetic parameters were studied on two cores drilled in the Academician Ridge, Lake Baikal, Russia. The rock magnetic parameters were used to identify variations in the concentration, grain size and mineralogy of the magnetic material. Three intervals of deviating declinations and inclinations with steep totally reversed inclinations are clearly present in the cores. The ChRM directions were clearly dominated by normal polarity indicating the Brunhes Chron age of the sediments. The observed excursions were interpreted as the Blake excursion, the Iceland Basin excursion and the Biwa II excursion. The reversal excursions fall within the intensity minima. On the basis of the identification of excursions we correlated the magnetostratigraphic results (relative paleointensity and polarity) from the Lake Baikal sediments to comparable data sets from ODP site 984. Data obtained from the cores indicate the age of deposits up to 300 ka. Comparing the variations of the paleointensity records the mean sedimentation rate in the range of 3 - 4 cm/ka can be estimated. The result of magnetostratigraphic and micropalaeontological investigations of the Jurassic/Cretaceous boundary strata in the Tethyan realm (Brodno - Slovakia, the Bosso Valley - Italy, and Puerto Escano - Spain) can be reasonably intercorrelated. Reverse subzones proposed to be named "Kysuca Subzone" in M20n and "Brodno Subzone" in M19n were precisely localized in all studied profiles. All the magnetozones and subzones can be related to the M-sequence of marine magnetic anomalies. At the locality of Brodno, the interpreted duration of the transition between N - R (R - N) polarity falls into the range of 5 - 10 ka. Stratigraphically significant calpionellid events occupy an identical position in relation to magnetozones and subzones derived in all the three sections. The base of the calpionellid zone Crassicolaria coincides with the base of the "Kysuca Subzone". Appearance of the species Calpionella grandalpina represents a significant horizon; it lies immediately below the base of the magnetozone M19r in all sections. The base of the standard zone Calpionella was used for the definition of the J/K boundary in calpionellid stratigraphy.
http://www.gli.cas.cz/Departments/paleomagnet/projects.htm
GP43B-0861 1340h
Magnetic Excursion Recorded in Basalt at Newberry Volcano, Central Oregon
Paleomagnetic study of basalt flows on the north flank of Newberry Volcano has identified a major eruptive episode that occurred during a magnetic excursion. The measured direction of the basalt flows erupted during the excursion shallows from $81\deg$ to $76\deg$ inclination along a declination of $\sim$ $155\deg$. The Virtual Geomagnetic Pole also shallows from $29\deg$ to $19\deg$ paleolatitude, along a paleolongitude of $\sim$ $250\deg$, and is located off the west coast of Mexico. Geologic evidence combined with limited argon dating indicate that the basalt erupted from multiple sites about 80,000 years ago, probably during the time of anomalous magnetic directions recorded by ($\sim$80 ka) ocean sediments in the Norwegian Sea and the North Atlantic. The westernmost flows erupted from spatter vents located a few km south of the city of Bend, and flowed north through lava tube(s) which form Stevens Cave, Horse Cave, and Redmond Cave among others. This western lobe flowed more than 50 km to the north, over NW-trending faults of the Tumalo Fault Zone that cut the adjacent and underlying basalt of Bend (40Ar/39Ar plateau age of 78$\pm$9 ka; isochron age of 77$\pm$19 ka); it is overlain by the basaltic andesite of Klawhop Butte (40Ar/39Ar plateau age of 39$\pm$6 ka). One sample of the transitional magnetic direction basalt has a K-Ar age of 77$\pm$40 ka; another sample has a 40Ar/39Ar plateau age of 92$\pm$25 ka and an isochron age of 73$\pm$24 ka. The eastern lobe erupted from vents at and near Lava Top Butte, located approximately 15 km SE of the western vents. These eastern lavas flowed through Arnold Cave and formed a broad ~10-12 km rootless shield known as the Badlands, the NE extent of which is about 30 km from Lava Top Butte. The west and east lobes each cover about 150 km$^{2}$, and comprise an estimated volume of 3-5 km$^{3}$. Newly acquired 10-meter DEM's and compilation of the mapping in ArcGIS will allow more precise calculation of the total area covered and the volume erupted. Chemical analyses of multiple samples from both lobes, together with the paleomagnetic directional variations, suggest that most of the western and eastern lobes erupted at approximately the same time. Basalt erupted early has 49-50 wt.% SiO2, but the stratigraphically youngest eastern lavas are more silicic (50.5-51.5 wt.% SiO2). We propose to name these combined basalt flows as the basalt of the Badlands, and the transitional paleomagnetic behavior as the Badlands Excursion.
GP43B-0862 1340h
The Potential of Magnetostratigraphy for a Global Correlation of the Germanic Triassic - Case Study Buntsandstein
The Buntsandstein represents the lower group of the tripartite classic Germanic Triassic supergroup. In its type area of Central Germany, the thickness of the Buntsandstein is about 1000 m. The predominantly clastic sediments were deposited during latest Permian to earliest Middle Triassic times in mainly fluvio-lacustrine environments of a large intracratonic basin. Traditionally, the Buntsandstein is subdivided by lithological criteria, showing a distinct cyclicity, pragmatically subdivided in 5 to 25 m thick small-scale fining-upward cycles. Above all, these cycles are correlation cycles, that are obvious in wireline logs, cores and outcrops. With wireline logs (e.g., gamma-ray logs) they can be correlated over almost the entire epicontinental Central European Basin. The thickness of these fining-upward cycles corresponds to that of genetic cycles, which are considered to reflect climatic fluctuation of alternating drier and wetter periods due to solar-induced 100 ka eccentricity cycles. On the basis of this robust high-resolution lithostratigraphic framework a very precise positioning and verification of paleomagnetic results has been realized. In Central Germany eighteen sections (twelve outcrops, six wells) were collected at 1-2 m intervals, yielding a total of nearly 2100 oriented standard samples. From about 81 % of them a characteristic remanence was obtained, being carried by magnetite in the gray lithologies and by hematite in the red-brown lithologies, respectively. The inter-section correlation of all investigated profiles allows the creation of a well-defined composite magnetic polarity record for Central Germany, being in good agreement with polarity scales from the Boreal and Tethyan realms. The magnetozones of the Buntsandstein last 0.1-0.9 Ma, with an average duration of approx 0.3 Ma. According to magnetostratigraphic and biostratigraphic data, the position of the Hindeodus parvus calibrated Permian-Triassic boundary is located within the so-called "Graubankbereich" (= gray bed interval), about 20 m above the lithostratigraphic Zechstein-Buntsandstein boundary. In terms of magnetostratigraphy, it is situated within the lowermost part of a remarkable thick normal polarity zone, which is a distinctive feature occuring in virtually all magnetic records across the Permian-Triassic boundary. The boundary between the Calvörde and Bernburg formations of the Lower Buntsandstein is close to the Griesbachian-Dienerian boundary.
GP43B-0863 1340h
Does Lithology Influence Relative Paleointensity Records? A Statistical Analysis on South Atlantic Pelagic Sediments
The relative paleointensity (RPI) method assumes that the intensity of Post Depositional Remanent Magnetization (PDRM) depends exclusively on the magnetic field strength and the concentration of the magnetic carriers. Sedimentary remanence is regarded as an equilibrium state between aligning geomagnetic and randomizing interparticle forces. Just how strong these mechanical and electrostatic forces are, depends on many petrophysical factors related to mineralogy, particle size and shape of the matrix constituents. We therefore test the hypothesis that variations in sediment lithology modulate RPI records. For ninety selected Late Quaternary sediment samples from the subtropical and subantarctic South Atlantic Ocean a combined paleomagnetic and sedimentological dataset was established. Misleading alterations of the magnetic mineral fraction were detected by a routine Fe/kappa test (Funk et al., 2004). Samples with any indication of suboxic magnetite dissolution were excluded from the dataset. The parameters under study include carbonate, opal and terrigenous content, grain size distribution and clay mineral composition. Their bi- and multivariate correlations with the RPI signal were statistically investigated using standard techniques and criteria. While several of the parameters did not yield significant results, clay grain size and chlorite correlate weakly and opal, illite and kaolinite correlate moderately to the NRM/ARM signal used here as a RPI measure. The most influential single sedimentological factor is the kaolinite/illite ratio with a Pearson's coefficient of 0.51 and 99.9$%$ significance. We find that kaolinite has a positive and illite a negative effect on magnetic alignment, while smectite is more indifferent. This is certainly related to the contrasting unit-layer charges of the three clay minerals, eventually also to their crystalline versus flaky structure and low versus medium to high plasticity Our regionally restricted results also indicate an inhibition of magnetic particle alignment by the presence of siliceous microfossils. This could be simply due to the porous structure of diatoms, radiolaria and sponge spicules which favor a high initial porosity followed by extreme postdepositional compaction. A three-member regression model suggests that all considered matrix effects can make up over 50$%$ of the observed RPI dynamics.
GP43B-0864 1340h
Holocene relative paleointensity and paleosecular variation from the Southern Okinawa Trough (ODP Hole 1202B)
We investigated u-channels from the top 36 meters of Hole 1202B collected in the Southern Okinawa Trough during Ocean Drilling Program Leg 195. Detailed rock magnetic data demonstrate that the sediments preserve a high resolution record of paleosecular variation and a paleointensity signal spanning almost the entire Holocene. The sediments consist of homogenous, slightly calcareous, bioturbated clayey silt with isolated sandy intervals and fine sand laminae. An age-depth model was established through C-14 accelerator mass spectrometry dating of planktonic foraminfera. The studied section spans almost the entire Holocene (0-9.4 kyr) and exhibits sedimentation rates close to 400 cm/kyr. The magnetic properties are dominated by stable, pseudo-single domain low-titanium magnetite with a Curie temperature of about 540 deg C. High-field hysteresis data and the grain-size sensitive ratio of anhysteretic remanent magnetization (ARM) to low field magnetic susceptibility indicate a narrow range of grain sizes and concentrations. Key magnetic parameters vary by less than a factor of four, thereby fulfilling the criteria for relative paleointensity determinations. The relative paleointensity was extracted by normalizing the intensity of the natural remanent magnetization (NRM) by the ARM and by the low field magnetic susceptibility. Both normalizations yield very similar results. Spectral analysis indicates that the record is not significantly affected by local environmental conditions. Comparison of this Western Pacific paleointensity curve with other curves suggests a geomagnetic origin for the observed variations. Millennial-scale features in our record correlate with variations of the archeomagnetic dipole moment, which implies that the sediment from Hole 1202B recorded changes of the geomagnetic field over the investigated time interval.