GP21B-0778
Diagenetic Alteration on Magnetic Properties of Core MD06-3040 from East China Sea Inner Continental Shelf
Reductive diagenesis on magnetic minerals has been increasingly documented in coastal and continental shelf sediments, which is mostly characterized by a sharp decrease of all magnetic parameters at shallow depth and sometimes by two steplike decreases depending on the stability of magnetic minerals and chemical environment of the sediments. Magnetic results of Core MD06-3040, taken from East China Sea (ECS) inner continental shelf during IMAGESXIV Marco Polo 2 cruise on the R. V. Marion Dufresne (IPEV), presents two steplike decreases due to post depositional reductive diagenesis. The first occurs at a depth of 1.5-3.5 m and is characterized by reduction of both ferrimagnetic (magnetite) and antiferromagnetic (goethite/hematite) mineral components. The second occurs at 8.45-9 m and is characterized by a loss of fine-grained ferrimagnetic material. The steplike decline of magnetic parameters provides a mechanism for the relative stability of the magnetic minerals, suggesting that fine-grained ferrimagnetic mineral may be more resistant to the process of reductive diagenesis than coarse magnetic grains and antiferromagnetic material in this area, which is different from previous studies.
GP21B-0779
Diagenetic Control Of The Magnetic Susceptibility Variations In Core MD98-2172 In The Eastern Timor Sea
Detailed mineral magnetic measurements, integrated with grain-size distribution and X-ray diffraction (XRD) analyses, were made on the marine sediments of Core MD98-2172, retrieved from the Eastern Timor Sea. Values of magnetic susceptibility of this core drop sharply down-core since the depth of ~3.85 m and get to very low at ~5.35 m. But both results of XRD and grain-size distribution show no sudden change of the terrigenous input through the whole core. Mineral magnetic results indicate that the depth of ~3.85 m below the sediment/water interface may be an oxic/anoxic boundary and the sediments below ~3.85 m have been greatly subjected to the reductive diagenesis, while the sediments of the top ~3.85 m are seldom affected. The magnetic properties of the top 3.85-m sediments are dominated by pseudo-single domain (PSD) magnetite, and its content and grain size show little down-core variations. While the magnetic mineral assemblages that have survived in the sediments below ~3.85 m may record different stages of the reductive diagenesis: (1) the sediments from the 3.85-5.35 m interval are at the stage of iron oxide reduction; PSD magnetite is the major magnetic contributor, but it becomes less abundant and coarser down-core; (2) the sediments below ~5.35 m are at the stage of sulphate reduction; ferrimagnetic minerals almost vanish and paramagnetic minerals contribute to the susceptibility down-core variations, including pyrite as evidenced by high-temperature magnetic susceptibility measurements. However, the susceptibility variations below ~5.35 m of Core MD98-2172 show obvious periodicity, despite the intense effect of reductive diagenesis. Furthermore, the susceptibility down-core variations are coincident with the fluctuations of the amounts of fine detrital particles (<8 μm), which may mainly come from the advection of the Indonesia Throughflow and/or the river input from the Timor Island. Therefore, for Core MD98-2172, the susceptibility variations below ~5.35 m, implying the changes of the amounts of fine particles, may record the history of the development of the ITF and the precipitation of the Timor Island.
GP21B-0780
Multi-Polarity Magnetisations Acquired During Progressive Cooling of Young, Lower Oceanic Crust
IODP Expedition 304/305 sampled lower oceanic crust exhumed by detachment faulting in the Atlantis Massif oceanic core complex (Mid-Atlantic Ridge at 30°N). Palaeomagnetic analyses on gabbros from Hole U1309D reveal a complex record of magnetic remanences. Essentially all samples record a high temperature reversed component of magnetisation, R1. An intermediate temperature normal polarity overprint, N1, and a lower temperature reversed overprint, R2, are also observed in some samples. The transition between these distinct components occurs over very narrow and consistent temperature intervals, providing strong evidence for a thermal origin of magnetic remanence. However, the variations in remanence are unrelated to either: (i) igneous stratigraphy, suggesting that the overprints are not produced by partial thermal resetting of earlier remanences by later intrusions during subsequent geomagnetic chrons; or (ii) the distribution of alteration in the core, arguing against overprinting by partial chemical re-magnetisation. Instead the pattern of multi- component magnetisations appears to be controlled by subtle variations in the grain size and related blocking temperature characteristics of the remanence carriers. We conclude that the magnetisation signature was acquired during protracted cooling of the section over a period encompassing chrons C1r.2r, C1r.1n (Jaramillo) and C1r.1r. The R1 magnetisation direction provides constraints on the tectonic rotation of the section. However, mean inclinations of the N1/R2 components are steeper/shallower than that of the R1 component respectively. These differences cannot be explained by any plausible tectonic mechanism. Instead, they appear to relate to the biasing effects of a residual vertically-oriented drilling overprint that persists to intermediate unblocking temperatures despite an initial low temperature demagnetisation treatment. This highlights the need to carefully consider the potential effects of drilling overprints prior to tectonic interpretation of IODP inclination data, especially in geological settings where rotation angles inferred from palaeomagnetic analyses are sensitive to small changes in inclination.
GP21B-0781
Direct Comparison of the Geomagnetic Paleointensity Record and the Atmospheric Radiocarbon Content Based on Fossil Corals
The atmospheric radiocarbon content (denoted as Δ14C) is thought to be controlled by the geomagnetic shielding effect, the solar variability and the Earth's carbon cycle (e.g. the changes of deep water formation and the exchange rate between carbon reservoirs). The geomagnetic shielding effect is believed to dictate the large-scale trend in the atmospheric Δ14C record, and the other two factors are believed to control the smaller but rapid fluctuations. One puzzling feature of the long-term atmospheric Δ14C record is the anomalously high Δ14C values for the period older than 30,000 years BP. Some investigators attributed these anomalously high values to hypothetical carbon cycle conditions very different from those in the modern times. Even in the extreme carbon cycle scenarios, modeled Δ14C values and measured values during the last glacial period still differ largely. Furthermore, there is little paleoceanographic evidence which supports a prolonged stagnant ocean during that time. The atmospheric Δ14C values are usually being compared to the modeled Δ14C values based on the calculated 14C production and an assumed carbon cycle condition. However, the coral-based atmospheric Δ14C record discussed in this study seems to capture some finer structures and rapid changes observed in the paleointensity records obtained from the deep-sea sediments, despite the potential discrepancies due to differences in chronologies. This implies a more direct atmospheric response to the geomagnetism-induced 14C production change than we previously thought, and the carbon cycle change might actually play a much lesser role in controlling the atmospheric 14C variations during the last glacial period. The paleointensity may be the only dominant factor determining both the overall shape and the secondary structures of the atmospheric Δ14C record for the past 50,000 years BP. Since the coral-based atmospheric Δ14C record can be dated by 14C, 230Th/234U/238U and 231Pa/235U methods with high precision and accuracy, it could potentially serve as a check for deep-sea sediment-based paleointensity reconstruction.
GP21B-0782
Modeling Holocene Paleomagnetic Secular Variation: Is a Dipole all we Need?
Valet et al. (2008) have suggested that the quality of late Holocene paleomagnetic data, which have century- scale resolution at best, are not sufficient to resolve higher terms of the geomagnetic field (equal or greater than g2). Essentially, the tilt of a simple dipole can explain the data equally well. We will present new Holocene paleomagnetic data from the western Greenland shelf. The sediments are rich in ferrimagnetic minerals, most likely magnetite, and contain a stable natural remanent magnetization. The most prominent paleomagnetic feature of the last 4000 years, observed in the data, is a large westwards swing in declination associated with very steep, almost vertical, inclination. The declination swing can be associated with the northern European "f-event" first described by Turner and Thompson (1981). Comparison of the data with previous paleomagnetic studies of eastern Greenland shelf (MD99-2322) and north Iceland shelf (MD99- 2269) sediments (Stoner et al. 2007) reveal similar paleomagnetic development, which imply that the data are describing real behavior of the geomagnetic field. Following the work of Valet et al. (2008), a simple model of a time varying geocentric dipole has been constructed based on multiple paleomagnetic data records from localities widespread over the northern hemisphere. While the study by Valet et al. (2008) was based exclusively on archeomagnetic and volcanic data covering the last 2000 years BP, we extend the model back to 9000 Cal yrs BP by using sedimentary paleomagnetic data. Comparison with other sedimentary paleomagnetic data (not used to constrain the model) show that it is possible to reproduce the shape, amplitude and timing of the "f-event", as recorded in different places over the globe, with a relatively simple dipole model. The coherency of VGP paths constructed from these paleomagnetic records is difficult to explain with, for example, waveform theory. We do not suggest, however, that the geomagnetic field has been strictly dipolar during the last 9000 years, but rather that the non-dipolar influence may have been overestimated in some paleomagnetic reconstructions. Known dating errors are potentially large enough to produce large displacements of VGP's during geomagnetically active periods, such as the "f-event". These dating errors force models of geomagnetic secular variation to invoke higher terms to explain the paleomagnetic data. References: Stoner, J. S., Jennings, A., Kristjánsdóttir, G. B., Dunhill, G., Andrews, J. T. and Hardardóttir, J. 2007. A paleomagnetic approach toward refining Holocene radiocarbon-based chronologies: Paleoceanographic records from the north Iceland (MD99-2269) and east Greenland (MD99- 2322) margins. Paleoceanography 22(PA1209), doi:10.1029/2006PA001285. Turner, G. M. and Thompson, R. 1981. Lake sediment record of the geomagnetic secular variation in Britain during Holocene times. Geophysical Journal International 65(3), 703-725. Valet, J.-P., Herrero-Bervera, E., Le Mouel, J.-L. and Plenier, G. 2008. Secular variation of the geomagnetic dipole during the past 2000 years. Geochemistry Geophysics Geosystems 9(Q01008), doi:10.1029/2007GC001728.
GP21B-0783
Paleosecular Variation of Late Pleistocene and Holocene Sediment, Gulf of Salerno, Western Mediterranean Sea
Sediment in a 6-m gravity core from the Gulf of Salerno in the western Mediterranean Sea records long-term change (secular variation) of Earth's magnetic field during much of the last approximately 40,000 years. The age of the sediment is based on tephrochronology and correlation to the relative intensity in the NAPIS-75 (Laj et al., 2000) and SAPIS (Stoner et al., 2002). The core (GS1202, 40º08.34'N, 14º43.57'E, 243 m) has a hiatus between about 14,000 and 24,000 yrs B.P. caused by slumping of shelf margin sediments (Trincardi et al., 2003). Where GS 1202 overlaps in time with a second core (GS1201, 40º28.92'N 14º42.24'E, 300 m) also from the Gulf of Salerno and that terminates at about 25,000 yrs B.P. (Iorio et al., 2006), there is good agreement of paleomagnetic directions and normalized intensity. Both cores confirm very well the paleomagnetic record for the last 8,000 years in a third core from the Gulf of Salerno (Iorio et al., 2004). An objective of our investigation is a search for the Laschamp (LE) and Mono Lake (MLE) excursions, believed to be either a single excursion (Kent et al., 2002) or two that differ in age by 6,000 to 8,000 years (Benson et al., 2003). Unstable field directions in GS1202 occur during relative field minimums at about 34,000 and 40,000 yrs B.P.; these portions of the record that may represent the LE and MLE are being further investigated using the archive half of the core.
GP21B-0784
Origin of Apparent Magnetic Excursions in Arctic Deep-sea Sediments
Natural remanent magnetization (NRM) of u-channel samples collected from cores, recovered by the Healy- Oden Trans-Arctic Expedition 2005 (HOTRAX), resolved using alternating field demagnetization, yields down- core inclination patterns of positive and negative intervals that mimic polarity zones. Component inclinations in these cores are generally much lower than the expected for a geocentric axial dipole field. NRM inclination variation of cores from the Lomonosov Ridge (20 JPC) and Yermak Plateau (22 JPC) can be correlated to previously studied records from the region. As the cores are likely to have been deposited during the Brunhes Chronozone, based on sparse stratigraphic information from a variety of sources, the negative NRM inclination zones may be associated with magnetic excursions. On the other hand, "excursion" zone thicknesses reach several tens of cm implying excursional durations that far exceed excursional durations determined outside the region. This abnormal duration of Brunhes-aged excursions has been commonly observed in the high latitude deep-sea sediments from the Arctic and Norwegian-Greenland Sea. Thermal demagnetization results of samples from the Mendeleev Ridge cores (6, 8, and 10 JPC) indicate that magnetization components with negative inclination have blocking temperatures below 300° C, whereas components with positive inclination have blocking temperatures up to 580° C. Thermomagnetic tests indicate that the two components are likely carried by an iron sulfide (greigite) and magnetite, and that samples from negative inclination intervals have a higher proportion of greigite. Low temperature magnetic experiments further suggest the existence of magnetite, and tend to rule out pyrrhotite as the dominant iron sulfide. Low temperature results also indicate that there is a lower proportion of magnetite in samples from negative inclination intervals. We suggest that negative NRM inclinations recorded in these Brunhes-aged high latitude deep-sea sediments are possibly caused by magnetic interactions (partial self-reversed chemical remanent magnetization) between a secondary authigenic greigite and a primary magnetite rather than any special behavior of the geomagnetic field at these high latitudes.
GP21B-0785
A tale of TWO excursions (Mono Lake and Laschamp) recorded in sediments of Pyramid Lake, Nevada
We have recovered a 17-m lacustrine sediment core (PLC08-1) from Pyramid Lake, Nevada, near the site of core PLC92-2 (Benson et al., 1997; QR-47). PLC08-1 can be correlated with the well-dated PLC92-2 core on the basis of three volcanic ashes and lithostratigraphic variability. Initial natural remanent magnetization (NRM) and magnetic susceptibility measurements plus limited alternating field (AF) demagnetization of selected samples have produced an initial estimate of the paleomagnetic secular variation and relative paleointensity recorded in the PLC08-1 sediments. We see evidence for two magnetic field excursions associated with two broad relative paleointensity lows in the core. The first excursion, associated with the Timberlake ash (~32,400 cal ybp; Benson et al., 2003; QSR-22) contains anomalous inclinations as low as - 1.1 degrees; the second excursion, located about five meters lower in the core, contains anomalous inclinations as low as -40.1 degrees. The secular variation pattern above the younger excursion is consistent with and correlatable to secular variation above the Mono Lake Excursion at Mono Lake (Lund et al., 1988; GRL-15). The overall relative paleointensity variation is consistent with and correlatable to our previous studies at Mono Lake and the North Atlantic Ocean (Benson et al., 1998; QR-49). These correlations indicate that the younger excursion is the Mono Lake Excursion and the lower excursion is the Laschamp Excursion (~41,000 cal ybp), as documented by Lund et al. (2005; JGR-110). Fourteen radiocarbon dates from PLC92-2 can be correlated to the PLC08-1 core and provide independent age estimates consistent with our paleomagnetic correlations. All of these results corroborate our previous assessment that the Mono Lake Excursion at Mono Lake is a separate, younger excursion than the Laschamp Excursion.
GP21B-0786
Evidence for Two New Magnetic Field Excursions (11,000 and 13,000 Cal Yrs BP) from sediments of the Tahiti Coral Reef (Maraa tract)
IODP Expedition 310 drilled 37 boreholes (22 sites) into late Pleistocene/Holocene coral reef frameworks from three reef tracts surrounding the Island of Tahiti. The expedition recovered 632 m of reef rocks, mostly 9,000 to 18,000 years in age, in order to establish the timing and course of postglacial-max sea level rise and environmental variability in the Tahiti coral reef associated with the termination of the last global glaciation. Oriented paleomagnetic samples were collected from most boreholes with an average stratigraphic spacing of 50 cm. We report on paleomagnetic results on 316 samples recovered from the Maraa reef tract (12 boreholes, 6 sites). The samples span the time interval 9,500-13,600 Cal Yrs BP based on 44 radiocarbon dates. Paleomagnetic inclinations (characteristic remanences derived from detailed demagnetization) and relative paleointensity (cleaned natural remanence normalized to magnetic susceptibility and isothermal remanence) have been correlated consistently among all 12 boreholes. The inclinations of 304 samples average -30.6 degrees (alpha95 = 2.9 degrees) with an expected site axial dipole inclination of -32.7 degrees. The consistent pattern of inclination (and relative paleointensity) variability and the fact that average inclination is not significantly different from axial dipole expectation both indicate that the paleomagnetic data accurately record local geomagnetic field variability. Twelve samples from two narrow, but correlatable, intervals average +27.0 degrees. We interpret these anomalous inclinations to be evidence for two magnetic field excursions. The radiocarbon dates, which bracket the excursion intervals, limit the younger excursion age to between 10,600 and 11,100 Cal Yrs BP and the older excursion age to between 12,600 and 13,200 Cal Yrs BP. These excursion ages are not significantly different from the ages of the two largest global Delta14C cosmogenic isotope anomalies identified by Hughen et al. (2000;Science-290) from glacial termination sediments of Cariaco Basin. We hypothesize that the excursions (and associated modest paleointensity lows) are the cause of enhanced global cosmogenic isotope concentrations rather than changes in deep ocean ventilation as suggested by Hughen et al. (2000).