Lichens and Mosses Used as Biomonitors in Environmental Magnetic Studies
Plants are widely used in classical biomonitoring studies, due to the ability of different vegetation species to accumulate dust and toxic elements in their tissues or to bound them to surface structures. The aim of the present investigation is to study magnetic signature of foliose and fruticose lichens and mosses, gathered from several polluted and clean sites in Bulgaria and to evaluate their suitability as enviromagnetic indicators. Plant material was sampled from rocks and tree branches. Various species demonstrate different preferences in grain size distribution of accumulated dust particles. Measurements of magnetic susceptibility, acquisition of isothermal remanence, anhysteretic remanence and hysteresis loops reveal that lichens preferentially accumulate coarser anthropogenic grains as compared to the magnetic fraction in clean localities. This suggests that conclusions related to grain size distribution of industrial emissions should be carefully considered in respect to bioindicator used in each particular study. The main ferromagnetic phase in accumulated dust from various vegetation species is magnetite-like with possible Al-, Si- and Mn-substitutions, related to the specific chemistry of source emissions. This phase has varying grain size distribution, deduced by dP parameter from fitted Gausian functions to IRM-acquisition curves and shows inverse relationship to the remanent coercivity of the soft IRM component. It is probably related to sorting effect with distance and the presence of single/multiple pollution sources.
Geochemical and Rock Magnetic Properties of a Purple Sand Belt on Cedar Beach, Western Lake Erie, Ontario, Canada
Limited environmental magnetic studies have been carried out on beaches and shoreline processes. Therefore, we initiated a rock magnetic study at Cedar Beach on the north shore of western Lake Erie (42°N, 81°W) and noted the highest susceptibility values (9.6 x 10-2 SI) occurred in a purple sand belt that is 200m in length and 3.5m in width and located just outside the swash zone. To understand the rock magnetic properties of these purple sands, we systematically performed frequency-dependence of low-field susceptibility (Kfd%), partial anhysteretic remanent magnetization (pARM) spectra, saturation isothermal remanent magnetization (SIRM) acquisition, S-ratio, hysteresis measurements and Curie-point determinations for a set of representative purple sand samples. Most Kfd% values are lower than 1%, indicating that the dominant magnetic minerals fall in the large pseudo-single domain (PSD) to multidomain (MD) magnetic size, while single domain/super paramagnetic (SD/SP) grains are rare. pARM curves show only one group with a peak in the pARM spectra at 10 mT, indicating a low coercivity and a PSD to MD grain size (around 4 um). However, in a few samples, the pARM spectra at high coercivities (> 90mT) is not zero, suggesting the presence of either fine magnetic grains (SD magnetites) or high coercivity minerals (hematite or maghemite). Most IRM acquisition spectra show 95% of the magnetization gained below 200 mT, and S-300 ratios close to one, indicating the presence of fully saturated, coarse and fine magnetite. Some samples did not saturate until after 800 mT and showed two saturation levels, suggesting the presence of high coercivity minerals. High temperature susceptibility curves (K-T) and magnetization curves (Js-T) show Curie points (Tc) around 585°C, suggesting magnetite is the main magnetic mineral. The Js-T cycles are almost reversible, indicating that magnetic minerals have not been created or destroyed. Hysteresis curves are normally wasp- waisted, indicative of mixtures of ferrimagnetic and antiferromagnetic minerals and of different grain sizes, as suggested by the other experiments such as IRM acquisition curves. On a Day diagram, all samples fall into the PSD to MD field. For the geochemical observations, the magnetic grains were separated using a hand magnet. The ESEM observations reveal large (90-300 um) sub-rounded to sub-angular magnetite grains with etching pits, mechanical cracks and some fine grains on the surface. The combination of magnetic and ESEM data suggests that these large magnetites have experienced partial oxidation during erosion and weathering processes, resulting in an effective subdivision of the larger magnetite grains to smaller grains and the creation of finer maghemite or hematite grains. However, we cannot eliminate the possibility that there are two separate populations of magnetite and maghemite/hematite. X-ray diffraction (XRD) spectra of the separated magnetic grains show a mixture of oxidized and unoxidized grains, matching the spectrum expected for magnetite and maghemite with minor hematite. This study confirmed the presence of magnetite as a heavy mineral in the purple sand belt. Evaluation of the grain size distribution may allow estimation of the degree of erosion and/or oxidation of these detrital beach sediments.
Holocene History of the East Antarctic Ice Sheet: Environmental Magnetic Record from Mac.Robertson Land
A 24-m jumbo-piston core containing a Holocene sedimentary record was collected from Mac.Robertson Land during United States Antarctic Program cruise NBP01-01. This study uses environmental magnetism to trace the Holocene history of the East Antarctic Ice Sheet (EAIS), which is generally thought of as stable since it is grounded above sea level. Magnetic analyses are used to identify periods of terrigenous sedimentation from the EAIS, for example ice rafted debris layers or meltwater pulses. Terrigenous material contains magnetic minerals, which we expect to stand out from the dominantly biosiliceous sediment deposited along the East Antarctic Margin. Magnetic susceptibility and Anhysteretic Remanent Magnetization (ARM) have been analyzed, while Saturation Isothermal Remanent Magnetization (SIRM) and core x-ray analysis are in progress. Magnetic susceptibility is weak down core, -2 to 5 *10-8 m3/kg, until approximately 1650 cm where there is a gradual increase in susceptibility with depth, most likely due to increased silt and sand content. There is a steep increase in susceptibility due to diamict at the base of the core. This change in lithology represents deglaciation following the last glacial maximum and the transition to an open marine environment. ARM exhibits a similar profile to magnetic susceptibility from 0 to 1650 cm. Below 1650 cm ARM remains at a low intensity, possibly representing a low concentration of fine-grained magnetic particles in the lower part of the core. Periodic, small-scale oscillations are also evident in the ARM (50-110 cm wavelengths) and magnetic susceptibility (40-70 cm wavelengths) profiles above 1650 cm. Stepwise IRM acquisition was used to determine the intensity of the applied field needed to saturate the samples. Saturation was achieved between 0.2 - 0.4 T treatment. A -300 mT backfield was applied to the samples after saturation to calculate the S-ratio. The S-ratios for the samples observed are generally above 0.9 for the upper 1700 cm, and drop to 0.80-0.85 at the base of the core. SIRM will be used in conjunction with magnetic susceptibility and ARM to assess downcore variations in magnetic mineralogy and magnetic grain size. In addition, core x-rays are being analyzed to identify horizons containing gravel grains and to assess laminae thickness down core.
Holocene Environmental Reconstruction of Sediment-Source Linkages at Crummock Water, English Lake District, Based on Magnetic Measurements
We present the results of an attempt to reconstruct Holocene sediment-source linkages from Crummock Water, a large lake in the English Lake District, based on magnetic measurements. The lake sediment magnetic properties indicate a series of changes in sediment composition during the Holocene, which correlate well with sediment lithology, water content and weight-loss-on-ignition. The first change corresponds to the Lateglacial/Postglacial climatic shift at around 11 400 years ago; the subsequent suite of changes corresponds to a probable regional onset of human activity at 2000 BC, and particularly to the intensification of human activity at around AD 900. A comparison of the lake sediment magnetic properties and those of the catchment soils shows a clear linkage for the Late-glacial period, and for the period after AD 900. In contrast, detailed magnetic measurements of the early- through mid-Holocene sediments suggest that their magnetic properties are dominated by bacterial magnetosomes.