GP31B-0833 0800h
Magnetic Evidence for Metastable Ferrihydrite Particles in a Vertisol
Poorly crystalline iron oxides in soils are of ecological importance since their chemical properties (e.g. solubility, surface area and reactivity) affect the bioavailability of iron and promote processes such as adsorption and transport of metals, pesticides, and organic nutrients. In natural environments these highly reactive phases are difficult to detect because they form generally a minor component with a nano-sized nature. Ferrihydrite is the best known phase among these materials with ambiguous identity. For the magnetic and soil-chemical investigation a Vertisol in southern Mali was chosen. Southern Mali has a subtropical climate with a wet and a dry season, an annual rainfall of 1300mm, and an average temperature of $27\deg$C. The Vertisol mainly consists of montmorillonite and quartz, and no iron oxides were detected by X-ray diffractometry. It has a relatively high amount of oxalate extractable Fe and reveals a strong increase of IRM acquisition at 77K compared to room temperature. In order to characterize the low temperature magnetic phase, AC susceptibility frequency experiments on bulk samples were conducted between 2 and 300K with 10 and 1000Hz. The in-phase susceptibility exhibited an increase at 120K followed by two peaks at 65 and 35K and a steep increase at 25K. In the temperature range of the peaks a clear frequency dependency was observed. The out-of-phase component revealed a similar behavior with a more pronounced frequency dependency below 65K. The beginning of magnetic ordering in the out-of-phase component at about 120K and the two peaks are assigned to two ferrihydrites with different magnetic properties. After oven-drying the bulk samples at $60\deg$C, the two peaks had vanished. This behavior suggests that ferrihydrites are metastable and convert into a thermodynamically more stable phase.
GP31B-0834 0800h
Holocene Centennial to Millennial-Scale Climatic Variability: Insights From High-Resolution Magnetic Analyses of the last 10 cal. kyr off North Iceland (IMAGES core MD99-2275).
High resolution magnetic analysis has been performed on the top 25 m of core MD99-2275 (66°33.06'N, 17°41.59'W; 440 m water depth) taken with the R. V. Marion Dufresne from the mid- outer shelf off North Iceland. This is a key boundary region for climate changes related to atmospheric and oceanographic variations, influenced by the moving limit between the cold and warm water masses at the Polar Front and the low depression track with its associated westerlies situated across Iceland. A precise age model based on tephrochronology was previously reported for this core and has allowed to determine precise timing of significant variations in the magnetic concentration and grain-size over the last 10000 cal. yr BP. Our record shows a clear increase in oceanographic instability from about 6 cal. kyr BP to the present compared with the previous period. The Holocene `climatic optimum', identified in North Iceland between about 9 and 6 cal. kyr BP, is characterized by minor variations in the magnetic record. A tendency to a decrease of the magnetic content toward the present will be discussed in terms of variations in the strength of the Irminger Current. Shorter oscillations are also identified, the youngest one (AD 1020 to AD 1330) corresponding in time to the Medieval Warm Period. The older ones (around 3.5 and 5 cal. kyr BP) might be associated with increased activity of the warm saline Irminger Current related to stronger input of North Atlantic waters into the Nordic seas. Spectral analysis of selected magnetic parameters also demonstrates centennial scale periodicities, which are more clearly expressed in the last 6 cal. kyr BP than in the previous period. This record therefore shows that the Holocene climate was more unstable than generally assumed hitherto, and it demonstrates the importance of high-resolution climatic studies of this recent time period to improve climate model predictions.
GP31B-0835 0800h
Volcanic Influence on the Susceptibility Signal: a Case Study in Indian Ocean
Magnetic susceptibility of sediments is now widely used as a climate proxy in paleoclimatic and paleoceanographic studies. The correlation between the susceptibility and the oxygen isotopic variations is either positive or negative and thus cannot be only caused by carbonate dilution. So far no convincing model has been proposed to entirely explain the dependency of the two signals. Many studies used the fact that the weathering products, which are transported by rivers or by winds to the sea, keep the signature of their source. In order to evaluate the contribution of magnetic particles from different origins to the budget of susceptibility signal, we measured mass normalized susceptibilities for sands and suspended load from the world major rivers and from rivers draining volcanic lithologies. We found that weathering products transported by rivers which drain volcanic terrains have a susceptibility signal which is 10 to 100 fold higher than for granites. Taking into account the size of river basins, their lithology and the fact that the weathering rates of basalts are much higher than that of continental silicates, the contribution of volcanic particles to the susceptibility of oceanic sediments is estimated to be 100 to 1000 fold stronger than the contribution of particles eroded from granites. Thus we infer that the susceptibility signal of oceanic sediments is mostly dominated by basaltic inputs to the sea. With this assumption in mind, we can compare the carbonate-free susceptibility signals of several cores collected in the Arabian Sea [1,2], the Somali Basin [3] and from the Ninety East Ridge (ODP Leg 121). The time-depth calibration was based on the oxygen isotopic variations measured in the same cores. In addition we measured the susceptibility of individual samples to improve and validate the calibration between different cores with different material. Comparison of absolute values from carbonate-free sediments to our river sands data should allow us to estimate the contribution of the volcanic input for each area. Ultimately this approach should lead us to retrace the amount of alteration of volcanic terrains through time. 1 P. DeMenocal, J. Bloemendal and J. King, 1991, Proc. Ocean Drilling Program Sci. Results 117, 389-401. 2 M.W. Hounslow and B.A. Maher, 1999, J. Geophys. Res. 104, 5047-5061. 3 L. Meynadier, J.P. Valet and F.E. Grousset, 1995, Paleoceanography 10(3), 459-472.
GP31B-0836 0800h
Environmental magnetic record of paleoclimate change from the Eocene-Oligocene stratotype section, Massignano, Italy
A high-resolution environmental magnetic study of the Massignano section, Italy (Global Stratotype Section and Point for the Eocene-Oligocene boundary), has been performed to test whether a clear magnetic signature associated with climatic change is recognizable in this record. Our results reveal the existence of alternating intervals with high and low magnetic mineral concentrations similar to the pattern of rock magnetic property variations observed from an environmental magnetic study of the CIROS-1 sediment core from Antarctica. These results suggest that an external forcing mechanism drove the sedimentary response to global climate change prior to the major Oi-1 cooling event at the Eocene-Oligocene boundary.
GP31B-0837 0800h
Magnetic Characterisation and Mapping of Urban Dust in Madrid, Spain.
Air quality and particulate loadings are important contemporary issues, and their characterisation is of great interest. A study of the magnetic properties of dust deposited on horizontal surfaces and tree leaves across the main urban area of Madrid, Spain, has been carried out. In both cases the magnetic signal is dominated by a similar, low coercivity component, which temperature-dependent susceptibility and remanence measurements identify as oxidised magnetite or maghaemite. Subtle coercivity variations are seen in the samples, showing both tree species and time dependencies. Evergreen (pine) needles exhibit higher coercivities than deciduous leaves, whilst the coercivity of deciduous leaves increases with time. This can be ascribed to the progressive oxidation of the magnetic phase. Isothermal remanence intensity and susceptibility decrease as the distance from traffic-bearing roads increases, indicating traffic as the main source of the magnetic signal. City-wide maps highlight magnetic "hot-spots" where the accumulation of magnetic material is more pronounced. Sample collection, measurement and map production are quick and easy and provide a useful tool for delineating time-averaged pollution loadings.
GP31B-0838 0800h
Estimating Magnetic Susceptibility and Frequency Dependent Susceptibility in Soils to aid Landmine Clearance
Mine and UXO clearance using metal detectors is often hampered by the presence of strongly magnetic minerals in soils overlying targets that are known to significantly affect detector sensitivity and performance. Specifically, the concentration of frequency dependent superparamagnetic magnetic minerals, which have a time-dependent magnetic response, is shown to be correlated with impeded detector performance. Nanoscale iron oxides with frequency-dependent properties that form during soil development in topsoils are widespread and result from the interaction of soil forming processes at different scales. The development of a modelling tool was based on exploiting the known links between soil forming processes and the development of frequency dependent magnetic minerals in soils. The model was further refined using analogue information to determine likely magnetic contrasts between different soil types. The production of a national map of magnetic categories for soil types indicates areas where soils may be problematic for mine detection using metal detectors. Estimates of soil magnetic properties provide essential information for planning and deployment of mine clearance operations.
GP31B-0839 0800h
Environmental Magnetism as an Instrument for Characterizing Paleoclimatic Variations in the Sediment Record of Lake Tanganyika, East Africa.
Due to their age and their continuous record of sedimentation, the lacustrine sediments of Lake Tanganyika, East Africa, provide an excellent resource for paleoclimatic research. During an eight-day cruise in July of 2004, participants in the Nyanza Project collected four Kullenburg piston cores in the vicinity of the Kalya horst, a mid-lake topographic high located south of the Mahale Mountains. Thirty meters of core were recovered. Initial lithologic analysis of the cores revealed that they consist of massive silty clay beds alternating with laminated diatomaceous oozes. U-channel samples were collected from the cores in order to obtain a continuous record of paleomagnetic directions recorded by the sediments as well as an environmental record of changes in the composition and concentration of magnetic minerals. In conjunction with other techniques, the directional record will help to provide a chronology for the cores, which are thought to extend well into Marine Isotope Stage 3. This chronology will be used to place the evolution of the lake system and its sedimentary processes within the context of global climate variability. The environmental magnetic record will provide information about both large-scale and small-scale climatic variations. The paleomagnetic and environmental magnetic information obtained from these cores will make it possible to draw definitive conclusions about past climate variations, current atmospheric composition, and the present-day quality of the lake.
GP31B-0840 0800h
Rock Magnetic Properties Across Paleocene-Eocene Boundary Sediments from the North Atlantic, South Atlantic, and Eastern Pacific
We present new rock magnetic data from Wilson Lake, NJ (N. Atlantic continental shelf), ODP Site 1262, (Walvis Ridge, S. Atlantic), and Lodo, CA (continental shelf, coastal California) in an effort to evaluate the comet-impact trigger hypothesis (Kent et al., 2003) for the carbon isotope excursion (CEI) associated with the Paleocene-Eocene Thermal Maximum (PETM). The comet-impact trigger hypothesis is based primarily on anomalous magnetic properties in outer shelf Paleocene-Eocene boundary sediments from the New Jersey continental margin; proponents of this hypothesis suggest these magnetic properties indicate the presence of extraterrestrial nanoparticles of magnetite. Our results suggest that these nanoparticles of magnetite may be terrestrial, not cosmic, in origin. Changes in magnetic hysteresis properties (e.g., Mr/Ms and Hc/Hcr) across the Wilson Lake section are similar to those from other regional stratigraphic sections (re: Kent et al., 2003), but similar changes are not observed at S. Atlantic or coastal California sites; thus, there may be a regional rather than global source of magnetic material. Squareness plots (Tauxe et al., 2003) and Day plots (Day, 1977) of hysteresis data suggest that Wilson Lake magnetite is single domain (SD flower structure to SD cubic), but the grain size of magnetite from the other two sites is mixed, possibly with a significant component of pseudo-single domain grains. Weak-field high temperature susceptibility experiments suggest that the primary magnetic mineral in these sections is magnetite. Low-temperature zero- and strong-field saturation isothermal remanent magnetization (SIRM) experiments indicate that the fine-grained magnetite is partially oxidized, probably to maghemite. More significant, however, the low-temperature experiments suggest that the fine-grained magnetite from the CIE portion of the Wilson Lake section may have a biogenic origin. Although it is clear that intact chains of magnetosomes are not preserved, the single-domain magnetite in the Wilson Lake section may have a significant population of isolated magnetosomes or other biogenic magnetite. We present additional comparative studies of low-temperature data to better assess the component of biogenic magnetite in the Wilson Lake section. We also present geochemical data to better understand the redox conditions that could favor or inhibit the production of biogenic magnetite at the three sampling sites.
GP31B-0841 0800h
Archaeomagnetic Study of La Campana in Western Mesoamerica
Results of an archaeomagnetic and magnetometric prospecting study of sediments and archaeological remains from La Campana in western Mesoamerica are presented. La Campana is in the northern suburbs of the city of Colima over a terrain characterized by volcanic debris avalanche deposits. Our study area of about 12,600 m2 is divided into two sections corresponding to the main archaeological site and to potential extensions. We report measurements of soil magnetism on 6 vertical profiles in the archaeological excavations, and from a grid of surface sampling in the unexcavated northern area. In the main sector, excavations have uncovered three large structures made mainly of rounded volcanic boulders and built over large platforms. Southern structure consists of a pyramid (25x25 m at its base) and an adjacent structure (at least 30x15 m). Central structure is a pyramid (20x20 m) with stair-like side accesses. Northern structure is a large complex extending over an area at least 40x30 m. In the sector of the open plaza, a magnetic survey using the vertical gradient method reveals an elongated shallow linear feature, which corresponds to a channel network constructed with flat volcanic slabs. Inverted conical openings that ended in a small well are connected to the channel network, which was apparently designed to collect water from rain and distribute it to the surrounding. Rock-magnetic data provide information on landscape change and susceptibility enhancement in archaeological soils. Archaeological remains extend over an area, considerably larger than that excavated, which is manifested in susceptibility enhancement in the top soils. The characteristics and size of pyramidal structures, plazas and channel network and the apparent overall extension of the archaeological site indicates that La Campana constituted a major urban and ceremonial center in westernmost Mesoamerica.
GP31B-0842 0800h
Magnetic Measurements as Indicators of Soil Development in a Chronosequence in Southeast Spain
Soil profiles on a sequence of Quaternary river terraces in southeast Spain show age-related characteristics of colour and carbonate accumulation indicative of long-term soil development. The terraces exhibit similar relief and parent material that provide a unique soil sequence differentiated primarily by time and secondarily by climate. Magnetic measurements of relic soils show evidence for the pedogenic development of secondary magnetic minerals in association with the modification of the detrital signal from the fluvial gravels. Although time is a major contributor to soil development and magnetic characteristics, climate is likely to have affected pedogenic and magnetic mineral development in this region over the late Quaternary. Magnetic measurements highlight a threshold in the maximum concentration of neoformed superparamagnetic minerals, likely to be determined by climate regime and the longevity of soil development. The magnetic signature is deconvoluted into a dominant signal derived from primary minerals and their pedogenic alteration and a secondary neoformed viscous component. The investigation of complex magnetic signatures facilitates the proposal of a conceptual model of late Quaternary soil development in this region.
GP31B-0843 0800h
Magnetic and Sedimentological Records of Late Pleistocene - Holocene Climate Variability from a Western Canadian Coastal Inlet
Annually laminated marine sediments are potentially ideal paleoclimate recorders, particularly once proxy measurements for atmospheric, oceanographic and sedimentalogic conditions have been calibrated. In Effingham Inlet, a fiord on the southwest coast of Vancouver Island, the anoxic inner basin preserves annual varves consisting of dark terrigenous laminae related to winter precipitation, and light diatomaceous laminae related to algal blooms controlled by spring-summer sunlight and ocean productivity. The French ship Marion Dufresne, as part of the international IMAGES program, collected a continuous 40 meter long core which spans the Holocene and beyond. Since the core is continuous, paleomagnetic declination can be more accurately determined than in piecewise fitting of separate cores. The oldest dated twig is 12ka C14, and magnetic secular variation correlations extend the dating to the bottom of the core at 14ka C14. In the lower 8 metres (2000 years) of the core, magnetic properties show rapid jumps indicative of the variety of sediment sources perhaps transported by the glaciers feeding the inlet during their retreat. Throughout the rest of the core, the varve record is interupted by massive mud intervals of 1 to 50 cm thickness. Those caused by sediment slumps related to high rainfall or seismic events are identified by high magnetic susceptibility. Others, which magnetic properties indicate are reworkings of sediments already in place, are hypothesized to be caused by bottom currents related to coastal oceanographic conditions perhaps established by strong El Ni\~no periods. Variations in magnetic properties along with other paleoclimate proxies are being combined to establish annual through millennial cycles in Holocene climate on the Pacific coast.