GP53B-1212
On detection of small changes in frequency-dependent magnetic susceptibility of weakly magnetic sediments: implications for magnetic granulometry
The frequency-dependent magnetic susceptibility is one of the useful tools for magnetic granulometry diagnostics. However, the instrumentation common in environmental research does not enable measuring the frequency-dependent susceptibility with sufficient precision, in magneticaly weak sedimentary rocks. Recently, the MFK1-FA Kappabridge was developed that enables such measurements much more precisely. The instrument measures the susceptibility in variable fields and at three operating frequencies, viz. 976 Hz, 3,904 Hz and 15,616 Hz, the field ranges being 2 to 700 A/m at 976 Hz, 2 to 350 A/m at 3,904 Hz and 2 to 200 A/m at 15,616 Hz. A collection of Late and Middle Pleistocene loess and paleosol samples and Late Holocene cave deposits was collected, with bulk susceptibility being about 1 x 10-4 [SI], and the frequency-dependence (defined as difference between susceptibilities measured at the lowest and highest frequencies normed by the measured value) ranging from 1% to about 10%. The collection was measured 5 times, each collection measurement being executed in different day; the instrument was newly calibrated in each measuring day. The differences between individual specimens were evaluated statistically. It was revealed that the variation in the frequency-dependence in the order of 1% is well reproducible and the measurements can be interpreted in terms of magnetic granulometry even in the weakly magnetic sedimentary horizons. The trends obtained are verified through the investigation using the method of time-dependent IRM.
GP53B-1213
Dynamics of Anthropogenic Ferrimagnetics (fly-Ashes) in Soils - Experiments on Test Fields
Magnetic properties of recent soils are often studied with the aim to asses the level of contamination due to atmospheric deposition of pollutants. In order to interpret properly magnetic data, several factors have to be taken into account. Lithogenic effect on top soil magnetic properties, pedogenic processes in soils, magnetic stability of anthropogenic ferrimagnetics in real soil conditions have been discussed in earlier studies. The other limiting effect, quite omitted in the past, is possible migration of anthropogenic farrimagnetics in vertical soil profiles. In our contribution we report on experimental investigation of temporal variation of power-plant fly ashes under different soil properties (soil, grass, sand), as well as different land morphology. In experiments on test fields we used fly ashes from two coal-burning power plants, showing relatively narrow grain-size distribution, with the mean of 10.5 Ým. Magnetite and/or maghemite was identified as dominant ferromagnetic phase. Temporal variations of vertical distribution of magnetic susceptibility along soil columns, with surface artificially contaminated by the power-plant fly ashes, were investigated using MAGPROX SM400 susceptibility meter. Our preliminary results, covering a period of more than one year, indicate that vertical distribution of particles in topsoil layers is in the investigated cases relatively stable. Vertical migration of deposited particles is practically buffered in the top 1-2 cm, and surface magnetic susceptibility was not distinctly modified. On the other hand, surface dynamics of anthropogenic ferrimagnetics may be radically affected by water regime (precipitation) especially during the winter period. This resaerch was carried out within GA AVCR Project No IAA300120701.
GP53B-1214
A magnetic quantification of anthropogenic pollution for Asian dust
It was previously found that significant amounts of iron-oxides had been incorporated into the Asian dust during its transportation. Intensive electron microscopic observations identified that the added materials were carbon- bearing anthropogenic iron-oxides (resulting from fossil fuel combustion). To quantify magnetic concentration, values of saturation magnetization (Ms) were determined from the hysteresis measurements for 33 Asian dust samples and 22 dust source region samples. Then, observed values of Ms were divided by the Ms of magnetite (92.36 Am2 kg-1), yielding an apparent magnetic concentration, AMC (= observed Ms / Ms of magnetite). The AMC conceptually represents the lower limit of the net concentration of magnetic materials, because thermomagnetic analyses found magnetite and other less magnetic iron-oxides as a dominant magnetic phase in samples. Apparent magnetic concentrations were 0.06~1.22 × 103 ppm and 0.77~2.65 × 104 ppm for the source region and Asian dust samples, respectively. This result suggests that more than 90 % of magnetic materials were added in the Asian dust during its transportation. This study reveals that the AMC can be used as a powerful magnetic concentration parameter in environmental magnetism.
GP53B-1215
Magnetic Fingerprint for Holocene Climate Change in a High Alpine Lake
High alpine lakes provide high-resolution archives for environmental change since steep climatic gradients can enhance the effect of small climatic shifts, and human influence is minimal. The magnetic properties of lacustrine sediments from Schwarzsee ob Soelden (SOS), a high alpine lake located in western Austria, were evaluated and compared with other climate proxies. The depth of this oligotrophic lake is currently 17 m and the lake covers 3.05 ha. It lies above the present tree line and the catchment area is characterized by fine-grained crystalline rock and scree with a sparse soil cover. A 159 cm core from the deepest anoxic part of the lake covers the past 10,000 years. The magnetic parameters; i.e., magnetic susceptibility, isothermal remanent magnetization and anhysteretic remanent magnetization, show a good correlation with Fe content. The mineralogy of the lacustrine sediments indicates four units: (1) the bottommost 6 cm, represent the weathered country rock and contain pyrite; (2) the following 14 cm of bottom sediments, consist of clays, feldspars, and quartz; (3) the next 120 cm, contain siderite in addition to the clay, feldspar and quartz; and in the uppermost 19 cm of sediment siderite is again absent. The lowest unit has a low concentration of low coercivity, ferromagnetic phases. The second unit has the highest concentration of ferromagnetic phases, with slightly higher coercivity. There is an abrupt change in the magnetic properties around 9000 yr BP. The next 140 cm of sediments are dominated by a low coercivity, ferromagnetic phase, and the constant values for the S-ratio and ARM/IRM suggest that its grain size spectrum does not vary greatly. The only exception is in a small zone around 5000 yr BP, which is associated with a rapid cooling event.
GP53B-1216
A Rockmagnetic and Palaeomagnetic Record of two Glacial Lakes in the Wind River Range, Wyoming, U.S.A.
We analyzed four cores from Louis Lake (42.596 ° N,108.846 ° W, 2610 m elev.) and nearby Fiddlers Lake (42.6312 ° N, 108.8786 ° W, 2868 m elev.), two glacial lakes in the Wind River Range of Wyoming. The lakes were dammed by lateral moraines and hold the possibility of long records of mid-continental climate change. Due to the lack of dateable material towards the base of the cores it is impossible to establish a precise maximum age for both records. We use comparisons of magnetic secular variation with other sites from the Western and Midwestern United States to constrain the age of these lakes as well as rock magnetic analyses to obtain a paleoenvironmental record. Glacial sediment in both lakes is characterized by inorganic, sometimes varved deposits containing high concentrations of coarse-grained (MD) ferrimagnetic minerals (high values of magnetic susceptibility, ARM and IRM, low ARM/IRM ratios). The post-glacial sediments at Fiddlers Lake contain mostly organic material with low concentrations of fine-grained (SD and finer) ferrimagnetic minerals (low values of magnetic susceptibility, ARM and IRM, high ARM/IRM ratios). In contrast, post glacial sediments at Louis Lake are more strongly magnetized containing a higher proportion of detrital material leading. Bulk susceptibility and IRM are still controlled by the presence of abundant organic material, bulk ARM and ARM/IRM ratios, however remain high, indicating the abundance of fine (SD) magnetic minerals. Paleomagnetic analyses show that all samples are demagnetized in fields below 80 mT and that all glacial samples as well as the post-glacial samples from Louis lake yield stable paleomagnetic directions. Holocene samples from Fiddlers Lake were too weakly magnetized to have their NRM analyzed. Both inclination and declination records from both lakes are consistent and paleosecular variations can be used to correlate the records of both sites. Both lakes show several pronounced dips in inclination and a shift in declination in the glacial sediments that can be correlated with other paleomagnetic records and constrains the maximum age of both lake records to approximately 15 - 20 ka B.P.
GP53B-1217
Rock-magnetic Analyses of two Lake Sediment Records Yield a Late-Holocene Climate Record for Eastern South Dakota, U.S.A.
We obtained rock magnetic and geochemical records from Pickerel Lake (45.5114 ° N, 97.2796 ° W) and Enemy Swim Lake (45.4409 ° N, 97.2650 ° W), two lakes in eastern South Dakota. Susceptibility data from Pickerel Lake show cyclical fluctuation which are thought to reflect short term climate change. Similar susceptibility variations, however, are absent in nearby Enemy Swim Lake, which casts doubt on the value of magnetic susceptibility as a climate proxy. At Pickerel Lake magnetic susceptibility and remanence parameters (ARM, IRM) covary and are caused by a dilution of the ferrimagnetic component by diamagnetic material, with reductive dissolution playing a secondary role. At Enemy Swim Lake the susceptibility record is consistently high throughout the top 5 m of sediment and drops off below that depth. Remanence parameters, however, do show cyclical variations similar to the ones observed at Pickerel Lake, and the two lakes can be correlated using variations in ARM. At Enemy Swim Lake variations in magnetic remanence are not due to the dilution of a ferrimagnetic component as they are poorly correlated with the abundance of dia- or paramagnetic magnetic material. Variations in magnetic grain size as expressed by ARM/IRM ratios are more pronounced and regular at Enemy Swim Lake than at Pickerel Lake, where ARM/IRM ratios and hysteresis data show little systematic change throughout the record. The different magnetic responses to a regional climate signal are likely due to the different hydrologic settings of the two lakes. Enemy Swim Lake is a closed basin lake, while Pickerel Lake is a throughflow lake with presumably more stable hydrologic conditions. Under these circumstances dryer climatic conditions are likely reflected in the increased deposition of (windblown?) detrital material, resulting in higher values for concentration dependent magnetic parameters. Variable hydrologic conditions at Enemy Swim may have caused changes in sediment redox conditions leading to varying degrees of reductive dissolution of ferrimagnetic grains and an abundance of ultrafine superparamagnetic (SP) minerals. These changes lead to a susceptibility record controlled by the SP component.
GP53B-1218
Environmental Magnetism of mid-Pleistocene Lacustrine Sediments of the Valles Caldera, New Mexico
Rock magnetic measurements are applied to almost 80 m of lacustrine sediment (VC-3) to augment independent means of interpreting the environmental conditions of a mid-Pleistocene lake in the Valle Grande of the Valles Caldera, northern New Mexico. An age model for the core is pinned to an Ar/Ar age determination of 552 ± 3 ka for a sanidine bearing ash layer at 78 m depth and major transitions in the organic carbon record at 53 m, 40 m, 27 m, and 17 m; these data suggest that deposition of VC-3 spans MIS 14 to MIS 10, including glacial terminations VI (531 ka) and V (426 ka). AF demagnetization resolves positive inclination magnetizations from most of the core, consistent with Brunhes normal polarity. Three thin (< 22 cm) intervals of shallow to negative inclination magnetizations may indicate poorly-recorded geomagnetic polarity events at ~406 ka (11α), ~536 ka (14α) and the Big Lost excursion (~580 ± 8 ka). Data from an array of rock magnetic investigations indicate magnetite, titanomagnetite, and pyrhotite characterize VC-3 sediments. Susceptibility and frequency dependent susceptibility experiments at low temperature reveal an abundance of paramagnetic and super-paramagnetic material. Scanning electron microscopy of magnetic separates show an array of Fe-oxide and Fe-sulfide grains, including titanomagnetites with trellis ilmenite intergrowths. NRM intensities of sediment deposited during glacial periods typically range from 0.04 mA/m to values as high as 1.6 mA/m; interglacial sediment NRM intensities range from 0.05 mA/m to 0.2 mA/m. NRM values increase to 3.3 mA/m, between 48 m to 43 m, where the sediment exhibits shallow mud crack, bioturbation, and oxidation. Overall, trends in susceptibility, ARM, and SIRM are similar to those in NRM intensity. Bivariate plot of susceptibility/ARM shows little variation in concentration of low coercivity minerals (i.e. magnetite) in VC-3 sediments. Also, the concentration of high coercivity minerals (i.e. hematite) indicated by the IRM/susceptibility curve show similar variation between sediments deposited during glacial and interglacial times as the remanence data sets but the relative intensities and exact shape of the curve indicate these materials are strongly influenced by local oxidation and reduction conditions within the lake environment.
GP53B-1219
Magnetic Susceptibility and Geochemistry of Soils and Alluvial Sediments in the Area of Vistula River Delta (N Poland).
Magnetic iron minerals, whose presence in soils can easily be detected by magnetic susceptibility measurements, are components of numerous industrial dusts. There is a linear dependence between magnetic particles content and content of heavy metals like: Cu, Fe, Pb and Zn (in urban dusts). A similar dependence was observed in atmospheric dusts (for Pb, Cu, Zn, and Cd). If the origin of magnetic particles and a considerable part of heavy metals permeating into soils as a result of industrial emission or imission is the same, the application of magnetic susceptibility measurements for the detection of potential high risk-areas is possible. Studies of magnetic susceptibility of alluvial sediments, beach sands, soils and subsoils were conducted in the area of Vistula river delta (N Poland). Maps of magnetic susceptibility were prepared basing on laboratory measurements, using Bartington MS2 device. The magnetic susceptibility pattern was compared and correlated with available geochemical determinations that were elaborated for the same sample set. Positive correlation is observed between magnetic susceptibility of topsoils and such elements as Ba, Co, Mg, Mn and Ni. Differential map, constructed as a difference between maps of topsoils and subsoils, might be a useful tool for preliminary detection of heavy metal pollution in soils. Negative anomaly observed at this kind of map suggests natural origin of increased susceptibility. A direct link between lithology of surrounding bedrocks and susceptibility values of soils and alluvial sediments is evident. A very high values of magnetic susceptibility reflect anthropogenic pollution of soils in the areas of big cities and industrial centrums. There is no such relationship in the case of magnetic susceptibility of alluvial sediments. The anthropogenic pressure is cumulated in the coastal zone of the Baltic Sea, where alluvial and beach sediments display the highest values of magnetic susceptibility.
GP53B-1220
Modern Sediments as Enviromagnetic Archives. A Case Study: Danube Delta and Northwestern Black Sea
The Danube Delta and the Razim - Sinoie Lagoonal Complex are two special units of the Danube - Black Sea hydrosedimentary geosystem, acting as a buffer interface between the Danube River water and sediment supplies collected from a large catchment area, and the Black Sea semi-enclosed marine basin. A vast enviromagnetic archive of modern sediments from this aquatic system was sampled in the 1976-2007 time span. The magnetic signatures identified in the sediments are able to characterize and differentiate the depositional environments inside of an extended, diversified, and at the same time, unitary complex of fluvial, lacustrine, lagoonal, deltaic and marine ecosystems. Confined versus dynamic deltaic environments were compared and defined by specific magnetic fingerprints. The sediments of the lakes that are successively located along a main water flowing way modify their lithological, mineralogical and chemical characteristics, as a consequence of differential transport and sedimentation, and by passing from a dynamic aquatic environment towards a more or less stagnant environment. This situation is well reflected by the magnetic signatures and it is investigated at a regional scale, when two lacustrine areas - situated far, respectively close of the main fluvial influx points - are discussed, and at a local scale, when several interconnected `couples of lakes` are analyzed. Besides, as in certain cases the bottom sediments were sampled in the same zones, in successive phases, at time intervals of 5-10 years, the modified magnetic fingerprints clearly demonstrated the presence or the absence of the anthropogenic impact on the ecosystems. Special cases are concerned with the harmful influence exerted by (artificial) canals crossing lacustrine areas or cutting Danube Delta Branch meanders. Much higher intensities were detetermined for the magnetic fingerprints of the fluvial-deltaic environments of the Danube Branches as compared with those detected in the lake sediments. With regard to the Razim - Sinoie Lagoonal Complex, the magnetic susceptibility (MS) maps pointed out a good correlation with the morphostructural details of the emerged adjacent areas, showing connections between the (higher intensity) magnetic signatures and the structure of the surrounding sand ridges. Besides, an increasing trend of finer sediments occurrence southwards is accompanied by a simultaneous decreasing trend of the MS values. This remark is supported by the results obtained within three periods spanning 31 years. As regards the fluvial-marine interaction zone, the MS maps of the sediments sampled in the northwestern Black Sea showed good connections with the main sedimentary environments in this area. The magnetic signatures with a higer intensity reflect the area directly influenced by the Danube River sediment discharge, particularly the Danube Delta front area and the Danube prodelta area. The area showing the southward drift of the Danube originated sediment flux, as well as the sediment starved area - both located within the northwestern Black Sea shelf area - are reflected by higher, and respectively lower intensities of the magnetic fingerprints. To calibrate the modern sediments and to compare different magnetic signatures recovered from the various aquatic environments, an original Magnetic Susceptibility Scale was conceived by the authors. The MS classes characterizing the recent sediments are used to define the sediment quality, which is also evaluated by geochemical and ecological scales.
GP53B-1221
Magnetomineralogical Characterization of Heinrich Events Preserved on Turbiditic Sediments From the Galicia Bank (NW Iberian Margin)
Abyssal marine environments combine the ideal conditions for recording the climatic instability occurred during the last glacial period. However, on a more dynamic zone, such as the Galicia Bank slope, the turbiditic activity may compromise the fidelity of a given record. In this context, we will show how very detailed magnetic measurements make possible the detection and characterization of distinct climatic events in these environments, such as the Heinrich events H1 to H3 described in this paper. The study area is located on the south-western flank of the Galicia Bank, at depths ranging from 3363 to 4171 meters. It is influenced by the Lower North Atlantic Deep Water (LNADW) and Antarctica Bottom Water (AABW) deep currents that flow northwards as a result of thermohaline equilibrium. Three gravity cores of around 3 m long and representative of the main sedimentary environments in the area (fault scarp; turbidite lobe and interlobe depression) were selected for detail magnetic analysis. Measurements at room (magnetic susceptibility, ARM, hysteresis) and low temperature (susceptibility and SIRM) revealed the occurrence of magnetic-enriched laminae within the magnitude range documented for Heinrich events recorded in sediment of the abyssal plain bordering the Iberian continental margin. The formation of loops in the ARM-100 vs. susceptibility plots highlighted the typical increase in magnetic grain size that characterizes Heinrich sediments at latitudes inside the Rudimann Belt. Recognition of IRD layers in very detrital-diluted horizons was possible after the evaluation of magnetic domain state. Low temperature measurements were also useful to clarify on the mineralogy and grain size of this ice rafted particles, which proved to be stoichiometric magnetite. This was supported by the occurrence of a clear Verwey transition at 120K. The significant loss of total remanence after a complete SIRM-300K cycle in these horizons indicated the presence of larger grain sizes, when compared to other samples studied along the cores.
GP53B-1222
Tracing Late Holocene Warm Periods in the Galician Continental Margin (NW Spain): Detrital Control vs. Early Diagenetic Modulation.
The sediments of the Galician continental margin (NW Spain) exhibit great but variable degrees of early diagenetic dissolution of magnetic minerals. This process completely erases any detrital magnetic signal at time-scales that range from about 1,000 years to less than 50 years in the highly productive estuarine-like environment of the Galician Rias Baixas. The more open marine conditions encountered in the adjoining continental shelf exhibit however a significantly different behavior. The singular balance between early diagenetic dissolution and lower sedimentation rates allows partial and variable preservation of the also variable detritally controlled magnetic inputs. In this context it is possible to reconstruct the recent environmental history of the area attending to the changes in the concentration of magnetically-interesting iron oxides. High-resolution magnetic and geochemical measurements carried out in 6 cores from the continental self evidenced the occurrence of correlatable peaks of magnetic mineral concentration that were interpreted as periods of enhanced detrital input. Magnetically depleted sediments were related to lower detrital input and/or to enhanced productivity that intensified the reductive conditions. The magnetic concentration peaks occurred within the Medieval (MWP) and Roman Warm Periods. Similar concentrations of hematite in the MWP and the RWP suggest that the lower concentration-dependent magnetic properties in the RWP are most likely caused by a greater degree of dissolution of magnetic detrital oxides due to a longer exposure to reducing conditions. These features could be traced over all the studied area, despite the great heterogeneity of the shelf sediments. This highlights the potential of magnetic properties as proxies of paleoenvironmental conditions in areas of similar complexity. This approach can be used as a rapid and cost-effective tool to screen large areas in the search for suitable settings for more precise, but time-consuming analysis, as well to extrapolate results through correlation of the appropriate magnetic features. We thank Prof. T. von Dobeneck and Dr. M. Dekkers for their kind support and advice during the stays of K. Mohamed in Bremen and Utrecht. The research of K. Mohamed in Bremen was funded by the EU Paleostudies program.
GP53B-1223
Sedimentology, geochemistry and rock magnetic properties of beach sands in Galapagos Islands - implications for nesting marine turtles
Marine turtles are well known for their navigation ability in the open ocean and fidelity to nesting beaches. Green turtle adult females migrate from foraging areas to island nesting beaches, traveling hundreds or thousands of kilometers each way. The marine turtle breeding in the Galapagos Islands is the Green Sea Turtle (Chelonia mydas agassisi); fairly common throughout the islands but with nesting sites located at Las Bachas (Santa Cruz), Barahona and Quinta Playa (Isabela), Salinas (Baltra), Gardner Bay (Espaņola) and Bartolomé Islet. In order to characterize and to identify the geochemical signature of nesting marine turtle beaches in Galapagos Islands, sedimentological, geochemical and rock magnetic parameters are used. A total of one hundred and twenty sand samples were collected in four beaches to relate compositional characteristics between equivalent areas, these are: Las Bachas, Salinas, Barahona and Quinta Playa. Grain size is evaluated using laser particle analysis (Model Coulter LS 230). Bulk ICP-MS geochemical analysis is performed, following trace elements are analyzed: Al, V, Cr, Co, Ni, Cu, Zn, Cd, Ba, Pb, Fe, Mn, K, Na, Mg, Sr, Ca and Hg; and low-field magnetic susceptibility is measured in all samples at low and high frequencies. Granulometric analysis showed that Barahona and Quinta Playa are characterized for fine grained sands. In contrast, Salinas and Las Bachas exhibit medium to coarse sands. Trace metals concentrations and magnetic susceptibility show different distribution patterns in the beach sands. Calcium is the most abundant element in the samples. In particular, Co, K, and Na show similar concentrations in the four beaches. Las Bachas beach shows highest concentrations of Pb and Hg (maximum values 101.1 and 118.5 mg/kg, respectively), we suggest that the enrichment corresponds to an anthropogenic signal. Salinas beach samples show high concentrations of Fe, V, Cr, Zn, Mn and the highest values of magnetic susceptibility (maximum 932 10-6 SI), we propose, a human activity influence less evident than in Bachas beach that could overlap the contribution of continent source. Quinta Playa sands show the maximum concentration of calcium and also high concentration of Fe and Mg, and relatively high values of magnetic susceptibility. Ca results from marine biogenic carbonates (mainly coral reefs). Barahona also show high concentrations in calcium that could be correlated with the presence of biogenous source around the beach.
GP53B-1224
Environmental magnetic and physical grain size data of surface sediments across a variety of depositional environments in the Gulf of Alaska region
Modern depositional environments vary significantly across the Gulf of Alaska (GoA) region depending on sediment type, proximity to glacial sediment sources, and relative balance of terrestrial input and biological productivity. Multi-core samples were collected from fjord, continental shelf and deep sea fan sites along the southern GoA margin in 2004 (cruise EW0408). Depositional environments range from ice-proximal glacimarine to biologically productive ice-free bays. As expected, magnetization intensities (NRM, ARM, IRM) and magnetic susceptibility (MS) are a strong function of biogenic dilution as measured by organic carbon (TOC) and biogenic silica content. In organic rich sediments, located in the south of the study area, the magnetic grain-size parameter kARM/k (anhysteretic susceptibility divided by susceptibility) indicates fine-grained magnetic material (kARM/k = ~14) throughout the radioisotopically defined (210Pb, 234Th) biological surface mixed layer (typically 5-20 cm thick). Just below the base of this mixed layer, magnetic grain size coarsening (to kARM/k values of ~2) occurs over several tens of cm. This coarsening can be attributed to dissolution of fine-grained (possibly biogenic) magnetite, accompanied by the formation of authigenic iron sulfides. Sedigraph particle-size analyses show no down-core coarsening corresponding with the kARM/k profile, implying that the kARM/k parameter is sensitive to magnetic grain sizes largely below the ~0.5 μm lower limit of the Sedigraph. Sediments with low biogenic content, from the northern part of the study area, are characterized by low TOC concentrations and stronger magnetization intensities and susceptibilities. Here, in shelf environments, kARM/k values are not reflected in physical grain size data, implying that the two parameters are again sensitive to different grain size (and mineralogical) populations. For near-shore environments influenced by river plumes, down-core kARM/k changes (dominated by changes in k) tend to parallel physical grain size changes in the sortable silt population. The apparent magnetic coarsening driven by the higher susceptibility is most likely associated with an observed strengthening in river discharge velocity, which would increase the total amount of sortable silt and magnetite transported within the plume in the coastal ocean.
GP53B-1225
Environmental Magnetic Record of the past 220 kyr from Timor Sea
Environmental magnetic study was conducted on long piston core MD05-2970 from Timor Sea off northwest of Australia during IMAGES cruise. The site is located at Lat. 9°25f00S and Lon. 130°60f00E with a water depth of 437 m and the total length of the core is 29.82 m. Sediment consists of calcarious silt to clay with abundant foraminifera. 14C and oxygen isotope measurements revealed that the core preserved 220 kyr long record and that the sedimentation rates range from 18 cm/kyr at the top to 10 cm/kyr at the bottom. Intensity of natural remanent magnetization is reduced two orders of magnitude by diagenetic dissolution of magnetic minerals below 0.3 mbsf, which resulted in a poor paleomagnetic record. There are some horizons which show reversed or intermediate polarity directions with higher magnetization intensity and higher coercivity compared with the adjacent samples. This can be interpreted as CRM acquired in the direction different from the Earthfs magnetic field. Low temperature magnetometry and thermal demagnetization of composite IRM revealed that dominant magnetic mineral is magnetite with trace amount of hematite. Zone of dissolution is characterized by downward decrease in Sratio to 0.7 mbsf and subsequent increase to 2 mbsf. This can be interpreted as an initial dissolution of fine grained (possibly biogenic) magnetite and subsequent dissolution of hematite below 0.7 mbsf. Median destructive field of ARM is lower (~15 mT) during marine oxygen isotope stage (MIS) 7 compared with the values above. Grain size parameters also show relatively higher values (finer grained) and paleomagnetic directions are defined better (smaller MAD values) for MIS 7. These indicate that finer grained magnetic minerals deposited or remained during MIS 7, which might have been resulted from environmental change between MIS 7 and later stages.
GP53B-1226
Spectral Analysis of Environmental Magnetic Parameters at IMAGES Sites MD012380
We present both FFT and wavelet spectral analysis of the environmental magnetic parameters obtained at IMAGES Sites MD012380 covering the time 0-780 ka. These parameters, representing the abundance of magnetic minerals, include magnetic susceptibility (χ), anhysteretic remanent magnetization (ARM), and saturated isothermal remanent magnetization (SIRM).The FFT spectral results show clear peak of power at the frequency corresponding to the Earth eccentricity (100 ka). However there are several frequencies that we do not know their meanings yet in the spectral results. In order to distinguish the temporal discontinuities and the non- stationary signals in our data, we alternatively use the wavelet analysis. All these parameters present obvious periods corresponding to the eccentricity (100 ka) and precession (23 ka). Besides, as same as the FFT spectral result,and SIRM also show clear signal at ~160 ka period while ARM shows a period of ~66 ka. Furthermore we apply these parameters to cross spectral with Oxygen isotope (δ18O) and clear frequencies corresponding to the Earth orbital parameters (Eccentricity, obliqyity, precession) have been found. It indicates that magnetic parameters are good indexes to provide informations for paleo-environment study. One of the factors that affects the paleo-environment variation in the Banda Sea area may be contributed by the orbital forces.