GP43B-1215
An Indian Monsoon Story As Told By The Bengal Fan Sediments
Various climatic proxies indicate that significant change in the Indian monsoon system occurred at ~ 7-8 Ma. The exact nature of this change is debated and involves either the intensification of the monsoon or increased aridity in the region. We conducted a rock magnetic study of the Bengal Fan sediments to estimate the variation in the monsoon intensity, linked to precipitation in the source area of the fan – the Himalayan Foreland. Different magnetic minerals in the Bengal Fan sediments were identified based on their coercivity and low temperature properties; the contents of these minerals were estimated based on their saturation IRM values. Our results indicate the presence of several magnetic phases in the sediments, including goethite, hematite, a detrital component (likely a mixture of titano-magnetite and/or maghemite) as well as a very distinctive component that we interpret as biogenic magnetite. Variations in relative abundance of goethite and hematite in the studied section suggest that the Himalayan foreland had experienced an arid period that had started at approximate 7 Ma and lasted for about 600 kyr. The onset of aridity on the continent coincides with the disappearance of "biogenic magnetite" from the marine sediments, indicating a change in the oxidation state of bottom waters.
GP43B-1216
Rock magnetic and geochemical analyses of surface sediment characteristics in deep ocean environments: A case study across the Ryukyu Trench
Magnetic properties in marine sediments are one of the most powerful tools for marine sedimentological studies. Magnetic minerals, which are magnetic carriers of natural remanent magnetization and paleoclimate signals marine sediments are commonly dissolved and formed with burial depth, and mask the primary magnetic properties. In order to clarify the progressive changes in the magnetic properties, we studies seven cores collected from the Ryukyu Trench, southwest Japan. Magnetic properties, organic geochemistry, and interstitial water chemistry of seven cores are described. The magnetic carriers in the sediments are predominantly magnetite and maghemized magnetite with minor amount of hematite. Results of the topmost sediments from the landward slope showed that magnetic minerals were diluted by terrigenous materials and microfossils. The downcore variations in magnetic properties and geochemical data from three cores provided evidence for the dissolution of fine-grained magnetite with burial, and that the fine-grained magnetite is well preserved in oxic condition. http://staff.aist.go.jp/kawamura- noriko/@
GP43B-1217
Direct dating of remagnetizations in carbonate rocks: insights into the timing and mechanism from clay mineralogy and 40Ar/39Ar age analysis.
A significant problem for paleomagnetic study of carbonates is their propensity for remagnetization, especially during tectonic events. This phenomenon, recognized since the late 1980's, remains unexplained. We examined carbonate rocks of Devonian age from the Cantabrian arc of NW Spain, which are recognized to have become remagnetized during the Permo-Carboniferous amalgamation of Pangaea. Both whole rock samples and magnetic extracts have been investigated previously using conventional techniques of rock magnetism, demonstrating the presence of both single domain and superparamagnetic magnetite. X-ray diffraction of the insoluble, non-carbonate material in these rocks reveals abundant illite, with quartz and magnetite also identified. Smectite, in the form of interlayered illite-smectite or discrete smectite, is notably absent. Quantitative modeling of different grain size fractions (<0.05 μm, 0.05-2 μm, >2 μm, and ≫2 μm) indicates that the proportion of the detrital illite (polytype 2M1) is commensurate with grain size, with authigenic illite (polytype 1Md) predominating in the finest grain fractions. 40Ar/39Ar age dating of these different grain size fractions confirms the "younging" trend for the authigenic clay population. Extrapolating from the mixing line produced by the four size fractions to a 100% authigenic component yields a radiogenic age that overlaps with the known timing of remagnetization. We propose a mechanism for carbonate remagnetization in which Fe- rich smectite is transformed to Fe-poor illite + magnetite. The age of remagnetization (i.e., authigenic clay and the remanence-bearing magnetite) can be dated by 40Ar/39Ar analysis of the clay products.
GP43B-1218
A scoring scheme for evaluating magnetofossil identifications
In many Quaternary lacustrine and marine settings, fossil magnetotactic bacteria are a major contributor to sedimentary magnetization [1]. Magnetite particles produced by magnetotactic bacteria have traits, shaped by natural selection, that increase the efficiency with which the bacteria utilize iron and also facilitate the recognition of the particles' biological origin. In particular, magnetotactic bacteria generally produce particles with characteristic shapes and narrow size and shape distributions that lie within the single domain stability field. The particles have effective positive magnetic anisotropy, produced by alignment in chains and frequently by particle elongation. In addition, the crystals are often nearly stochiometric and have few crystallographic defects. Yet, despite these distinctive traits, there are few identified magnetofossils that predate the Quaternary, and many putative identifications are highly controversial. We propose a six-criteria scoring scheme for evaluating identifications based on the quality of the geological, magnetic, and electron microscopic evidence. Our criteria are: (1) whether the geological context is well-constrained stratigraphically, and whether paleomagnetic evidence suggests a primary magnetization; (2) whether magnetic or microscopic evidence support the presence of significant single-domain magnetite; (3) whether magnetic or ferromagnetic resonance evidence indicates narrow size and shape distributions, and whether microscopic evidence reveals single-domain particles with truncated edges, elongate single-domain particles, and/or narrow size and shape distributions; (4) whether ferromagnetic resonance, low-temperature magnetic, or electron microscopic evidence reveals the presence of chains; (5) whether low-temperature magnetometry, energy dispersive X-ray spectroscopy, or other techniques demonstrate the near-stochiometry of the particles; and (6) whether high-resolution TEM indicates the near- absence of crystallographic defects. We use criterion 1 to set the threshold for determining whether a magnetofossil identification is robust. Criteria 3 and 4 are assigned numerical scores that range from 0 to 4, while criteria 2, 5, and 6 are evaluated based on presence or absence. Based on this scheme, the oldest robust magnetofossils yet found come from the Cretaceous chalk beds of southern England [2], though Lower Cambrian limestones of the Pestrotsvet Formation, Siberia Platform, only marginally fail to meet our robust criteria [3]. Although magnetofossils have also been reported from Proterozoic, Archean, and Martian rocks, none of these identifications are robust. References: [1] R. E. Kopp and J. L. Kirschvink (2007). Earth Sci. Rev. doi:10.1016/j.earscirev.2007.08.001. [2] P. Montgomery et al. (1998). Earth Planet. Sci. Lett. 156: 209-224. [3] S. B. R. Chang et al. (1987). Phys. Earth Planet. Int. 46: 289-303.
GP43B-1219
Detrital Controls on Magnetosusceptibility and Cyclostratigraphy Records
Magnetic susceptibility (MS) is a rapid, simple, and nondestructive measurement frequently made on marine sediments and sedimentary rocks. The practical utility of MS is for correlation purposes, however many records demonstrate Milankovitch climatic cycles and are also used for the astronomical tuning of cyclostratigraphic records. To correctly interpret long-term cyclostratigraphic MS records, it is important to understand the factors influencing MS values. It is often argued (and occasionally documented) that MS values exhibit an inverse relationship with calcium carbonate concentrations in marine sediment samples. As a result, MS records are often interpreted as being controlled by calcium carbonate concentrations, and therefore, that MS may be a reliable indication of past carbonate productivity. However, other studies demonstrate strong detrital controls on MS values and conclude that MS variations are a function of changes in terrigenous input. To test these two models, we present new experimental results and extensive data sets that demonstrate that changes in mineralogy and/or detrital input are the main controls on MS variability, not carbonate productivity. To simulate the effect of variations in biological productivity on marine sediments, we prepared three separate samples of iron-rich minerals found in marine sediments and then incrementally diluted them with powdered diamagnetic calcium carbonate (MS ~ -3x10-9 m3/kg) and measured the new MS value. Prepared samples were designed to cover the common range of MS observed for lithified marine sediments. In order of decreasing initial MS, the samples are: 1) ~1 g of powdered Iceland basalt (MS = 7.58x10-6 m3/kg); 2) a ~1 g paramagnetic illite standard (MS = 1.20x10-7 m3/kg); and 3) a ~2 g 50/50 mixture of illite and quartz sand (MS = 4.53x10-8 m3/kg). The calcium carbonate dilution required to reduce initial sample susceptibilities by one order of magnitude is over 700% for all samples, which is far beyond natural variations in carbonate productivity. Even the mixed illite-quartz sample required ~880% dilution by calcite to reduce the susceptibility one order of magnitude. A nonlinear regression line (R=0.99) fits all experimental data. To further test whether MS is controlled by calcium carbonate concentrations in marine sediments, we present MS and calcium carbonate data for a 210 m section of the Upper Cretaceous Niobrara Chalk, Kansas. Overall, MS and calcium carbonate are not significantly correlated (R=0.14, n=1840).
GP43B-1220
Enhancement Mechanisms of Magnetic Susceptibility in the Chinese Red-Clay Sequence
Little is known about the mechanisms of magnetic susceptibility (÷) enhancement for the Red-Clay sequence on the Chinese Loess Plateau (CLP), in comparison to the overlying loess-paleosol sequence. Here we present a rock magnetic study of the Red-Clay sediments from the central CLP. Our results show that frequency dependence of ÷ (÷fd = ÷lf – ÷hf , where ÷lf and ÷hf are ÷ measured at 470 Hz and 4700 Hz, respectively), ÷lf, and susceptibility of anhysteretic-remanent-magnetization (÷ARM) are linearly correlated within the Red-Clay sequence. This linear correlation indicates that the pedogenic magnetic minerals of the Red-Clay have a rather uniform grain size distribution as in the loess-paleosol sequence, and the grain size is independent of the degree of pedogenesis. Nevertheless, Red-Clay sediments are slightly more enriched in superparamagnetic magnetic particles than the overlying loess-paleosol sediments as indicated by the higher slope of the regression line between ÷fd and ÷lf.
GP43B-1221
Anisotropy of Magnetic Susceptibility (ASM) as a powerful tool to study sedimentary process in continental basin
Syn-orogenic sedimentary accumulations provide long, continuous and detailed records of both tectonic and climate changes ongoing during mountain building. Deciphering these two end-member processes from sedimentary records remains challenging but of primary importance to understand how tectonics and climate interact. This is of particular relevance in continental area where detailed age control and environmental data are often lacking due to the absence of abundant good biostratigraphic records. Here, we present high resolution measurements of Anisotropy of Magnetic Susceptibility (AMS) combined with sedimentological analyses in three Neogene sections located in the Junggar and Tarim basins at edges of Tianshan ranges. In these sections we observed that AMS magnetic parameter changes are strongly correlated to important sedimentary environmental variation or sedimentation rate changes. We found that the AMS shape parameter T can provide information related to the hydrologic regime and transport conditions experienced by sediments. Indeed, we believe that T is sensitive to a preferred orientation of the particles acquiring during deposition or to the particle shapes themselves. The magnetic susceptibility is mainly related to the concentration of magnetite in sediment and may reflect potential changes in source or indicate environmental and climatic changes. This study shows that AMS analyses of continental sediments can be a powerful tool to document environmental changes as it provide high resolution records that are complementary to classic sedimentology.
GP43B-1222
Application of the Analytic Signal Technique to Calculate Magnetization ¡V Density Ratio of a structural boundary
The Poisson theorem provides suitable relationships between the gravity and magnetic anomalies. Based on Poisson theorem, magnetization-density ratio can be estimated. An accurate estimate of magnetization-density ratio (MDR) can provide better constraints for further interpretation such as modeling. When combined with existing geologic and rock property data, it can serve as a supplement to geologic interpretation. In this study, we present simple analytical solutions by applying analytic signal to the Poisson's theorem. The major advantage of using the analytic signal is that we can determine the magnetic sources locations and magnetization density contrast. Therefore, using this method, we can not only estimate the MDR distribution of the subsurface sources, but also determine the sources boundaries.
GP43B-1223
Extending the Sensitivity of Paleomagnetic Techniques: Magnetostratigraphy of Weakly- Magnetized, Organic-Rich Black Limestone from the Permian of Japan
Despite their importance for biostratigraphy, bituminous, organic-rich limestone has rarely yielded reliable paleomagnetic results, principally due to the weak magnetization levels encountered; NRMs for such samples often start with moments below 10-10Am2 (10-7 emu), and they become unmeasurable or unstable rapidly upon demagnetization. Recently, we have developed a fairly simple and inexpensive automation system for standard paleomagnetic and rock magnetic measurements that has the additional benefit of using an extremely low-noise sample holder. A vacuum pick-and-put system holds samples at the end a thin-walled quartz-glass tube, moving them between a measurement queue and the sense region of a vertically-aligned superconducting rock magnetometer. The system minimizes the amount of extraneous material introduced into the magnetometer's sense region. With frequent cleaning in strong acid, the glass holder system is capable of reliably measuring samples with moments below 1 pAm2 (10-9 emu), on a DC-SQuID magnetometer system with stable instrument noise of a few hundred fAm2. Signal averaging and clean-lab handling techniques are often needed to achieve this sensitivity, however. To test this system, we collected a series of densely-spaced samples for magnetostratigraphy from two sections in the Middle-Late Permian limestone at Kamura in Kyushu, Japan, which is the remnant of lagoonal sediments formed on an oceanic island atoll (Isozaki et al., 2007). According to fusulinids, the Saraito section spans Wordian time, which should be near the end of the Kiaman Reversed superchron. Similarly, strata at the Kamura section cover late Capitanian to early Wuchiapingian time, which should display a shift from predominantly Normal to Reversed magnetization. Raman spectroscopy suggests no appreciable heating. NRM intensities of the samples range generally between 10-10 and 10-11 Am2, with little change in the vectors either upon low-temperature cycling or alternating-field demagnetization up to 7 mT. However, thermal demagnetization to temperatures as low as 75 ° C in an N2 atmosphere often removes a present-field component, revealing a two-polarity NW/SE shallow magnetization; further thermal cleaning at 10 to 15 ° C intervals up to ~ 250 to 300 ° C allows proper isolation of this characteristic component in about 2/3rds of the specimens analyzed. When viewed stratigraphically, these data agree remarkably well with the magnetic reversal time scales, and imply formation at about 12° South latitude. MAD values from the principal component analysis, plotted against the intensity of the recovered component, indicate that the system is able to recover reliable data for characteristic components that are initially as weak as 10 pAm2 (10-8 emu). Continued advances in the sensitivity of rock magnetometers could open many additional types of weakly- magnetized sediments to paleomagnetic and magnetostratigraphic analysis. Isozaki, Y., Kawahata, H., and Minoshima, K. (2007). Palaeoworld 16, pp16-30.