GP23A-01 INVITED
A Multi-Parameter Approach to Assessing the Origin of Sediment-Magnetic Signatures in Small, Temperate-Climate Lakes
To assess the controls on sedimentary magnetic signatures in lakes, we analyzed the characteristics of core, surface, sediment trap, and catchment samples collected from small, temperate-climate lakes, which are commonly used in paleoenvironmental reconstructions. Magnetic properties of sediments were evaluated using room temperature measurements (susceptibility, IRM, ARM), hysteresis parameters, low temperature magnetization curves, and modeling of bulk coercivity spectra, obtained from IRM and ARM acquisition and demagnetization curves. The results highlight several characteristics of lacustrine sediments: 1) Magnetic- mineral assemblages are dominated by low-coercivity minerals (e.g. magnetite); 2) Saturation magnetization, rather than remanent magnetization, is a better overall indicator of concentration, due to differences in grain size; 3) Shoreline samples contain almost exclusively detrital multi-domain (MD) magnetite grains, while littoral and profundal sediments contain a mixture of MD and in-lake produced single-domain (SD) grains; 4) Profundal sediments, deposited in seasonally-anoxic waters, exhibit dissolution of finer-grained particles; 5) Calculated contributions of a biogenic source differ depending on method utilized. ARM/IRM ratios and delta/delta ratios are more conservative in this aspect, indicating a maximum of 50% contribution of biogenic magnetite. Hysteresis and coercivity analyses yield up to 75% contribution of biogenic particles; 6) There is a direct linear relation between percent mass organic matter (OM) and ARM/IRM, showing a dependence of magnetic grain size on organic carbon concentration. This correlation highlights the role OM has in the process of production and preservation of magnetosomes. Organic matter decomposition depletes the dissolved oxygen and leads to temporary or permanent anoxic conditions in the sediment. If conditions are suboxic (e.g. in the littoral zone, where oxygen can diffuse into the sediment from the overlying oxic water column), magnetite has a better chance of being preserved. If bottom waters and sediments are anoxic, dissolution of magnetic grains will occur, with smaller particles being preferentially dissolved.
GP23A-02 INVITED
Detecting Magnetosomes in Freshwater Lakes and Lake Sediments: A comparison of techniques
We will present a summary of the work done to date on detecting magnetosomes in the lake sediments and water column of Lake Ely, a small post-glacial lake in northeastern Pennsylvania. To establish that magnetosomes dominate the magnetic mineralogy of the Lake Ely sediments we sampled the water column every meter down to its maximum depth of 23 m and measured the dissolved oxygen, sulfide, and iron, as well as the ARM of the material filtered from the water. We examined the water samples for magnetotactic bacteria. These results established an increase in the ARM of the filtered material at the oxic-anoxic transition. They also showed that the ARM was carried by magnetosomes produced by magnetotactic bacteria living in the water column at depths from 15-19 m. TEM of magnetic separates collected from the lake sediments show that magnetosomes are transferred to the sediments from the water column and are a significant fraction of the magnetic minerals in the sediments. We used a variety of mineral magnetic techniques to magnetically characterize the magnetosomes in the lake sediments. The delta-delta ratio test of low temperature behavior at the Verwey transition (Moskowitz et al., 1993) gave values of 1.2 to 1.5, lower than the theoretically predicted level of 2 for magnetosomes, but a numeric unmixing technique could resolve higher delta-delta ratios in the dark organic-rich layers in the sediments where magnetosomes were more prevalent. ARM/SIRM ratios of 0.15 to 0.35 with Raf values (the crossover of an IRM acquisition curve versus its alternating field demagnetization curve) of 0.45 to 0.5 are consistent with the presence of magnetosomes in the sediments, the water column, and in a sediment trap located at the bottom of the lake. IRM and ARM acquisition modeling of samples collected from a 160 cm piston core revealed two components of magnetization with coercivities of about 25 mT and 65 mT that are identified as Egli's (2004) biogenic soft (BS) and biogenic hard (BH) magnetosome components. High-resolution FORC measurements support the coercivities observed by the IRM and ARM acquisition modeling. The FORC data also indicate that the magnetic grains are non-interacting SD grains, as predicted for magnetosomes. Finally, ferromagnetic magnetic resonance (FMR) measurements of piston core samples show that component fits support the presence of bacterial magnetite throughout the core. The FMR data also suggest that a detrital iron oxide component undergoes significant biological recycling within the top 15 cm. A broad ferromagnetic resonance spectrum with positive anisotropy below this depth may include a contribution from magnetotactic bacteria producing elongate particles. Modeling of detailed IRM or ARM acquisition, ARM/SIRM vs. Raf plots, and FMR measurements all provide evidence of magnetosomes in Lake Ely's sediments.
GP23A-03
Comparison of Geochemical, Grain-Size, and Magnetic Proxies for Rock Flour and Ice- Rafted Debris in the Late Pleistocene Mono Basin, CA
Advance and retreat of mountain glaciers are important indicators of climate variability, but the most direct proxy record, mapping and dating of moraines, is by nature discontinous. The Sierra Nevada form the western boundary of the Mono Lake basin, and the proximity of the large Pleistocene lake to the glacial canyons of the Sierra presents a rare opportunity to examine glacial variability in a continuous, well-dated lacustrine sequence. We have applied a geochemical proxy for rock flour to the glacial silts of the late Pleistocene Wilson Creek Formation, but because it is time- and sample-intensive, another method is required for a high-resolution record. Previous microscopic examination, thermomagnetic measurements, XRD analysis, and new isothermal remnant magnetization (IRM) acquisition curves show that the magnetic mineralogy is dominated by fine-grained, unaltered magnetite. Bulk measurements show strong susceptibility (mean ~ 16 x 10- 6 m3/kg) and remanent magnetization (mean IRM ~ 10-2 Am2/kg) compared to diluting components (carbonate, smectite, rhyolitic ash). The Wilson Creek type section sediments also contain a coarse lithic fraction, quantified by counting the >2cm clasts in outcrop and the >425 μm fraction in the bulk sediment. Susceptibility, IRM, and ARM (anhysteretic remnant magnetization) are quite similar throughout the type section, with the abundance of coarse lithic fraction correlative to the ratio k/IRM. Because the magnetic fraction of the rock flour is fine-grained magnetite, IRM should capture the changes in concentration of flour through time, and the major features of the (low-resolution) geochemical flour proxy record are identifiable in the IRM record. Flux-correction of the IRM results in a rock flour proxy record with major peaks between 36 and 48 ka, similar to a rock flour record from neighboring Owens Lake. This regional glacial signal contrasts with peaks in coarse lithics between 58 and 68 ka in the Wilson Creek record; coupled with coeval high lake levels and a lack of geomorphic evidence of glacier-lake interaction, this is taken to indicate that the rafting was due to shore ice, rather than glacial icebergs.
GP23A-04
The magnetic properties of Rio Tinto Basin sediments (Spain): a step towards Martian enviromagnetics
Exploration by the NASA rover Opportunity revealed sulphate-and haematite-rich sedimentary rocks exposed in surface features of Meridiani Planum, Mars. Sediments of the Rio Tinto Basin, Spain, provide at least a partial analog of Meridiani rocks, thereby facilitating an understanding of mineral precipitation and diagenesis on the Martian surface. Here we present the results of a mineral magnetic study of river terrace deposits of the Rio Tinto system, the deposits ranging from Modern to Plio-Pleistocene in age. A broad range of temperature- and field-dependent measurements have been made which have identified the presence of goethite, haematite and sulphides. Seasonally driven processes are thought to lead to the precipitation of nano-phase goethite which increases in crystallinity with age, eventually being replaced by haematite. Diverse micro-organism populations inhabit the acidic and seasonally dry environments, although organic matter does not persist in the sediments. However, biosignatures are left as casts and molds in the iron oxide minerals. The potential for recognizing their characteristic magnetic signatures, and their extrapolation to Martian environments, will be discussed.
GP23A-05
The Latitudinal Gradient of Rainfall, Mineralogy, Albedo and Magnetic Susceptibility in West Africa
In order to investigate the effect of climate on soil and surface sediment properties we examined four transects around the Sahara Desert. The transects were located in Mali, Niger, Benin, Togo, Egypt and Morocco and, with the exception of Egypt, each crossed a significant climatological rainfall gradient. The Egyptian transect was designed to characterize one of the driest portions of the Sahara Desert. Our study included laboratory measurements of mineralogy (XRD), elemental composition (XRF), grain size, optical reflectance (lab), magnetic susceptibility (MS)and remanences. In addition, albedo was determined from the MODIS satellite imagery from space. Many of our laboratory measurements exhibited variations with the rainfall gradient. Iron oxides (hematite and goethite), kaolinite, Al2O3, and TiO2 increased with increasing rainfall whereas SiO2, illite, and grain size decreased with increasing rainfall. Both laboratory-determined reflectivity and satellite-determine albedo decreased as rainfall increased. In part, this decrease in reflectivity/albedo with increasing rainfall appears to be the result of hematite, the dominant coloring agent for the soil in this region and the origin of the 'red' Sahel. The physical interpretation of these results centers on rainfall as a long-term leaching agent of surface material, and the control of physical properties by specific mineralogy. SiO2 is highly reflective and iron oxides are strongly absorptive in the visible range. The solubility of SiO2 in rainwater is orders of magnitude larger than all the iron oxides, with hematite the least soluble. It has long been recognized that leaching by rainfall produces dark red laterite in the near-surface oxidizing environment, a prominent geological feature throughout the high rainfall belt of West Africa. Laterite beds represent simultaneous enrichments of all iron oxides and a reduction in SiO2 by leaching. In the Sahara desert where rainfall is minimal (<10 mm/yr), SiO2 is concentrated, and in conjunction with eolian surface abrasion, produces the highest reflectivity/albedo values in Africa. MS and ferrimagnetic mineral concentrations also increase with increasing rainfall and, of all the laboratory analyses, exhibits the highest correlation to rainfall. The alternating wetting and drying conditions - tropical wet dry climate of the Sahel and the Mediterranean climate of Morocco – enhance the production of secondary ferrimagnetic oxides through the alternating reducing (wet) and oxidizing (dry) conditions necessary to form them. The findings in this study are consistent with a number of published results.
GP23A-06
Pedogenic Magnetic Minerals in Soils: Some Tests of Current Models
The magnetic enhancement of soils is increasingly used as a proxy for continental climate, since it is related to the formation of pedogenic iron minerals under warm, humid conditions. Ultrafine magnetite is believed to be the major responsible of the magnetic enhancement, however, very little is known on the detailed formation mechanism, ant its relation to the soil iron cycle. Furthermore, the 'textbook' case of the Chinese Loess Plateau is not well replicated around the World: Loessic soils from the Midwestern US are systematically less enhanced than their Chinese counterpart under similar climatic conditions, and many loessic soils in Argentina are not enhanced at all. In trying to find a rationale behind these differences, I will address three main questions that need to be answered in a bottom-up approach to the problem. The first question is whether susceptibility is indeed controlled by fine magnetite, excluding any significant role of other minerals such as ferrihydrite, goethite, and hematite. This is a rock magnetic problem addressing the interpretation of magnetic measurements: is susceptibility an adequate proxy for the concentration of magnetic minerals in soils? Answering this question allows us to think directly in terms of abundance specific magnetic minerals, which is fundamental for any subsequent interpretation. The second question is directed to understanding the role of magnetic minerals in the soil iron cycle and how they are formed. This brings us to a discussion of the transfer function linking magnetic enhancement with climate. Is indeed rainfall the only parameter controlling pedogenesis? Why is rainfall apparently related with the logarithm of susceptibility in enhanced soils? Can we test current pedogenetic models against this empirical transfer function? The third question points to the role of parent material and later dust inputs. Midwestern US and Argentinian loesses are different from Chinese loess. Is this a reason for the differences observed in the magnetic enhancement of the respective soils? Enough material is now available to test current models and hypotheses with respect to the first two questions.
GP23A-07 INVITED
Testing the Potential of Integrated Field-Emission Electron Microscopy and Magnetic Analyses to Assess Airborne Particulate Matter Pollution in Rome (Italy)
Recent environmental magnetic studies on airborne particulate matter (PM) in Rome (Italy) proved that rock magnetic parameters may be useful proxies to delineate the degree and extent of air pollution. These studies also indicated that the main source of magnetic PM is represented by circulating vehicles and that the concentration of fine magnetic PM particles decrease rapidly away from high traffic roads. Rock magnetic data reveal that the main PM magnetic particles in Rome consist of magnetite-like grains with a uniform composition and grain size distribution. However, the rock magnetic data also show that the assumption that pure magnetite is the only magnetic phase on anthropogenic magnetic PM is overly simplistic and suggest that the chemical composition and structure of anthropogenic ferrimagnetic particles may be different from those of natural particles. To better constrain the nature and origin of these magnetic particles, we carried out coupled Field-Emission Electron Microscopy (FE-SEM) and magnetic hysteresis analyses on PM specimens from Quercus ilex leaves and from possible sources in motor vehicles. FE-SEM analysis reveals that Fe-rich particles are generally 0.1-5 microns in size and are mostly irregularly-shaped, occasionally rounded, aggregated, or flaky, and almost invariably with a rough, moss-like surface. These morphologies are remarkably different from the typical spherical shapes of magnetic fly ashes originated by industrial combustion of black and brown coal. The composition of these particles is similar to that of stoichiometric magnetite, with FeO accounting for more than 70 wt %, but also include a variety of other elements (mostly SiO22, SO3, CuO and ZnO). Overall, the data show that the magnetic PM in Rome is composed by a mixture of particles resulting from various vehicle-derived sources. Fe-rich particles coming from three studied vehicle-derived sources (i.e., powders collected around disk brakes, and from diesel and gasoline exhaust pipes) show different magnetic and compositional signatures, which may allow an approximate estimation of the contribution of each source to the airborne magnetic PM found in environmental magnetic studies.
GP23A-08
Testing Magnetic Resonance Spectroscopy for use in Microbial-Influenced Systems
Magnetic resonance spectroscopy is a technique that exploits the state of unpaired electrons associated with magnetic materials to provide important constraints on magnetic phases within a sample. Crystal and aggregate shape anisotropies of biogenic versus detrital magnetite, for example, can be exploited by ferromagnetic resonance (FMR) to provide a quick method for locating magnetofossils and estimating their abundance. Similarly, simultaneously acquired electron paramagnetic resonance (EPR) spectrum can distinguish and estimate abundances of para and ferri magnetic ions such as manganese and iron. The ability to rapidly identify and quantify trace amounts of these phases make magnetic resonance spectroscopy a potentially important technique for studying both biogenic and diagenetic processes in environmental systems but its efficacy must be tested. Previous geochemical and rock magnetic analyses of a series of drillcores through Holocene Blake Bahama Outer Ridge sediments, western North Atlantic Ocean, reveal highly structured biogeochemical zoning in the diagenetic window, with magnetofossils preserved in distinct zones throughout the oxic to sub-oxic and manganese-cycling zone, above a sulfidic zone. This study investigates the application of FMR and EPR spectroscopy to this structured system, and discusses the adaptation of parameter discrimination to assess different zones of microbially-influenced diagenesis in the shallow sediment column.