Environmental Magnetism: Techniques and Applications
Presiding: C E Geiss, Trinity College; R Egli, Institute of Rock Magnetism
GP41B-01 INVITED 08:30h
A review of component analysis based on magnetization curves: state-of-the art and future developments.
Rocks and sediments inevitably contain mixtures of magnetic minerals, grain sizes, and weathering states. Most rock magnetic interpretation techniques rely on a set of value parameters, such as susceptibility and isothermal/anhysteretic remanent magnetization (ARM or IRM). These parameters are usually interpreted in terms of mineralogy and domain state of the magnetic particles. In some cases, such interpretation of natural samples can be misleading or inconclusive. A less constrained approach to magnetic mineralogy models is based on the analysis of magnetization curves, which are decomposed into a set of elementary contributions. Each contribution is called a magnetic component, and characterizes a specific set of magnetic grains with a unimodal distribution of physical and chemical properties. Magnetic components are related to specific biogeochemical signatures rather than representing traditional categories, such as SD magnetite. This unconventional approach gives a direct link to the interpretation of natural processes on a multidisciplinary level. Despite the aforementioned advantages, component analysis is not yet come into wide use for three reasons: 1) the lack of quantitative magnetic models for natural, non-ideal magnetic grains and/or the statistical distribution of their properties, 2) the intrinsic mathematical complexity of unmixing problems, and 3) the need of accurate measurements that are beyond the usual standards. Since magnetic components rarely occur alone in natural samples, unmixing techniques and rock magnetic models are interdependent. A big effort has been recently undertaken to verify the basic properties of magnetization curves and obtain useful and reliable solutions of the unmixing problem. The result of this experience is a collection of a few hundred magnetic components identified in various natural environments. The properties of these components are controlled by their biogeochemical history, regardless of the provenance of the hosting sediment. For example, the coercivity of all detrital magnetites is tuned by the transport mechanism (air/water), and the ARM of biogenic magnetites is controlled by the (paleo)redox conditions of the sediment. The consistency of these results supports the linear additivity principle upon which all current magnetic unmixing methods are based. Once the rock magnetic properties of individual components and their statistical distribution is known, the solution of unmixing problems provides important benefits including a great simplification which makes it accessible to non-specialized users. Simplified unmixing algorithms are robust and deliver reliable results based on relatively fast measurements. Two key examples will be presented. In the first example, the magnetic composition of lake sediments is used to develop a model that describes the nonlinear response of a lake to environmental changes. The response function can be used to deconvolute magnetic measurements for paleoclimatology reconstructions. The second example deals with an application of component analysis to obtain a low-cost and fast assessment of the air quality in urban areas. A community effort in setting up a database of magnetic components occurring in the most varied environments will provide us with a new, powerful tool for rock- paleo- and environmental magnetism research.
GP41B-02 08:50h
Paleoclimatic and Paleomagnetic Variability over the Past 40,000 Years as Recorded by Sediments at ODP Site 1002 in the Cariaco Basin
The Cariaco Basin, an anoxic basin off the coast of Venezuela, acts as a depocenter for sediments of marine and terrestrial origin. Sedimentation rates for the upper 170 m of sediment recovered during ODP Leg 165 at Site 1002 average 35 cm/k.y. and are fairly constant over the entire 600-k.y.-long interval cored. The high sedimentation rates and anoxic conditions have resulted in a very favorable setting for studying short-term changes in the paleoenvironment as recorded by geochemical and rock-magnetic variations. Paleomagnetic field variability is also recorded, although the sediments are less than ideal owing to their relatively weak magnetizations. Prior geochemical studies of Cariaco Basin sediments have shown that variations in Ti and Fe content correlate with changes in precipitation. These results are being augmented by XRF determinations of a wider range of elements, obtained using the Advanced Light Source at the Lawrence Berkeley National Laboratory. In the laminated intervals, the light and dark lamina that comprise a varve correspond with the drier winter-spring season and wetter summer-fall season, respectively. These changes are interpreted mainly as resulting from changes in the position of the Intertropical Convergence Zone (ITCZ), which currently passes over or near the Cariaco Basin during the summer-fall seasons and migrates well south of the basin in the winter-spring seasons. Several of the rock magnetic parameters that are sensitive to magnetic (Fe) concentration have much larger variations than do the geochemical data, with increases by more than an order of magnitude in several discrete intervals indicating large changes in the amount of Fe entering the basin. Although these changes are ideal for monitoring how the marine and terrigenous sources have abruptly changed over time, they are not favorable for constructing paleosecular variation or relative paleointensity records. Even so, the mean paleomagnetic inclination corresponds to the expected value, but the variation about the mean is much larger than expected, which we interpret as noise related to the variable and weak magnetizations.
GP41B-03 09:05h
Paleomagnetic and Rock Magnetic Results from Sediments from the Adriatic Sea
In the framework of two EC funded project (Eurodelta and Promess-1) we present paleomagnetic and rock-magnetic results from six cores collected in the Adriatic Basin. Four of the cores were collected by using a gravity corer within sequences representing an expanded section of the HST above the maximum flooding surface (KS02-246; CSS00-23 and CSS00-07) or the entire HST extending to the late Glacial-Interglacial transition (AMC99-01). Two long cores were collected by drilling on the Central Adriatic slope and aimed at obtaining 1) a continuous record of sedimentary sequences representing the last four glacial-interglacial cycles extending back approximately 100 kyr (PRAD1-2) and 2) a muddy succession that formed during the last sea-level rise and highstand (PRAD2-4). Paleomagnetic measurements on U-channel samples were carried out at the laboratory of the University of California at Davis using an automated 2G Enterprises cryogenic magnetometer. The NRM measurements from the gravity cores mostly exhibit a declination and inclination pattern characterized by a single paleomagnetic direction whereas a significant magnetic overprint, probably induced by the drilling, has been observed in the long cores. The upward pointing secondary direction associated with the overprint was removed by 10-20 mT of AF demagnetization which allowed us to retrieve the characteristic and primary magnetization of these sediments. The mean, cleaned inclination agrees with the geocentric axial dipole direction for this site. Variations in declination and inclination reveal comparable features during the Holocene and can be compared with secular variation records from other marine and lacustrine cores as well as with lava flows of historical age. These results allow us to develop a detailed age-depth model that will be helpful to reconstruct the history of sedimentation in the time interval under study. Variations in both concentration and grain-size related magnetic parameters reflect changes in sediment supply during the glacial-interglacial stages, as well as changes in environmental conditions, including the formation of sapropel layers.
GP41B-04 09:20h
Environmental Magnetism Study in Zirahuen Lake, Michoacan, Central Mexico
In Michoacan area major civilizations developed in a time of climatic shifts and volcanic activity, as the historically emplaced Jorullo (1759 AD) and Paricutin (1942 AD) volcanoes. Research in the central-western Mexico, has pointed several uncertainties and difficulties in paleoclimatic reconstructions for periods between the late glacial and the mid Holocene. Rock-magnetism data and non magnetic parameters are combined in order to provide insights in the environmental history for the last ca. 14 kyr in Zirahuen basin, Michoacan, Mexico (19.4° N, 101.7° W). A core 6.5 m long was collected in the shallow NE part of Zirahuen lake. Three main zones reflect major changes in environmental conditions. Sediments older than 10.3 kyr, have low concentration of ferrimagnetic minerals. Between 10.3 to 4 kyr, a hiatus occurred. For the mid and late Holocene sediments, the data suggests higher catchment erosion. A period of relative catchment stability is recorded in 630-1200 AD, followed by a return to previous conditions.
GP41B-05 09:35h
Multi-proxy Analysis of Lake Sediments from Western Connecticut Reveals Changes in Past Environments and Land-use
Mudge Pond (lat. 41.90 deg. N, long 73.48 deg. W) is a small glacial lake in the highlands of western Connecticut. Our analyses of a transect of eight piston cores aim at reconstructing the combined effects of past climatic changes and human land use practices on the chemical, biological and physical properties of lake sediments. Magnetic analyses (magnetic susceptibility, hysteresis loops, remanence parameters and low-T analyses) characterize the terrigenous components of all cores and allow for correlation between sites. Sediments deposited prior to 10 ka B.P. are poor in organic carbon and its magnetic properties are dominated by paramagnetic minerals, leading to low remanence values and high values of magnetic susceptibility. Low lake levels during part of the Holocene, as reconstructed from sedimentary facies changes along our transect of cores, led to erosion and partial redeposition of these sediments in the deepest part of the lake. Late Holocene sediments are composed of Marl, which is only weakly magnetic and displays a magnetic remanence carried by large MD and PSD-sized ferrimagnets. The onset of Euroamerican settlement in New England led to increased erosion rates, which are reflected in higher concentrations of magnetic minerals. Analyses of macroscopic charcoal show that, despite extensive logging activities in the watershed, the severity of forest fires did not change noticeably compared to pre-settlement times. Our magnetic and sedimentological studies are supplemented by a palynological analyses, which confirm our reconstruction of past environmental change and help to clarify the onset of Euroamerican settlement and it's influence on the watershed of Mudge Pond.
GP41B-06 09:50h
Chemical, morphological, and magnetic properties of loessic soils and the development of a multiproxy model to reconstruct paleorainfall
In recent publications, we have been able to show that changes in the relative abundance of ferrimagnets along a rainfall gradient from less than 500 mm/year in southwestern Nebraska to almost 1000 mm/year in western Illinois correlates well with the modern precipitation gradient. This suggests that we can use a grain-size dependent magnetic parameter as a potential paleorainfall proxy when analyzing paleosols in central North America. Because prior attempts of others to develop a paleoclimate proxy based on magnetics alone have proved challenging, we are proposing that a multiproxy approach using other soil properties in addition to ferrimagnets will be more fruitful. Soil carbon, nitrogen, color, and depths to the top of the B horizon are examples of soil properties that have been shown to correlate to climate. We have sampled loessic soils with a truck-mounted soil probe from more than 70 stable undisturbed upland positions from Nebraska to Illinois. We described and subsampled the cores in the field. Laboratory measurements of soil carbon and nitrogen show a strong correlation to the rainfall gradient. Soil color and other morphological properties also show promise as climatic indicators. When combined with our measured magnetic parameters, such as magnetic susceptibility, anhysteretic remanent magnetization, hysteresis loops, and viscous remanent magnetization, these soil parameters will result in a robust model that tracks modern precipitation and in a promising tool that can be applied to paleosols for reconstructing paleoprecipitation.