GP51A-01 INVITED
What do the HIRM and S-ratio really measure in environmental magnetism?
The hard isothermal remanent magnetization (HIRM) and the S-ratio are widely used in environmental magnetism to quantify the absolute and relative concentrations, respectively, of antiferromagnetic minerals (hematite and goethite) in mineral mixtures. New results from synthetic Al-substituted hematite and goethite indicate that these minerals can have a wide range of coercivities, which significantly influences the HIRM and S- ratio. These parameters are therefore not necessarily straightforward indicators of the absolute and relative concentrations of hematite/goethite. To circumvent this problem, we propose a new parameter (the L-ratio), which is the ratio of two remanences after alternating field (AF) demagnetization of an IRM imparted in a 1 T field with a peak AF of 100 mT and 300 mT (IRMAF300mT/IRMAF100mT). The newly proposed L-ratio is straightforward to measure on a wide range of instruments, including modern vibrating sample or alternating gradient magnetometers (equivalent parameters can also be measured on other instruments). Changes in HIRM only reflect changes in the absolute concentration of hematite and/or goethite if the L-ratio is relatively constant. Conversely, L-ratio fluctuations indicate variable coercivities that possibly reflect changes in the source of hematite/goethite. Corresponding HIRM and S-ratio variations should be interpreted with caution in such cases. Examples are shown where variable and constant L-ratios, respectively, provide significant new insights and reduce ambiguities associated with interpretation of the HIRM and S-ratio in environmental magnetism.
GP51A-02 INVITED
Reconstructing Past Climates From Thin Loess-Paleosol Sequences - Results From Modern Loessic Soils From the Midwestern United States
Magnetic susceptibility records of loess-paleosol sequences from the Chinese loess plateau and elsewhere have been used successfully to quantify variations in past rainfall by comparing the magnetic properties of magnetically enhanced soil horizons and their respective parent materials. Such an approach may work for thick loess profiles with well separated paleosols, but our experience from modern loessic soils in the Midwestern United States has shown that a straightforward comparison between the magnetic properties (magnetic susceptibility, IRM, ARM etc.) of modern soils and their (assumed) parent material yields unreliable paleoclimate proxies. Such a simple approach works well for well developed, simple soil profiles but breaks down for more complicated soil profiles that may contain recent loess additions, welded paleosols, or redoximorphic features relatively high up in the C-horizon. Our recent rock-magnetic analyses of modern soils from North America (and older studies of Chinese loess) show that the pedogenic magnetic component of these sites is characterized by a narrow range of magnetic properties. This makes it possible to directly estimate the abundance of pedogenically produced magnetite using, for example, magnetic coercivity distributions, ARM/IRM ratios or, in some cases, hysteresis loops. Analysis of over sixty modern loessic soil profiles shows that such estimates can yield reliable climate proxies even if no unaltered parent material is available for comparison.
GP51A-03
Dusty Gusts and Rusty Dust: The Role of Iron in the Contrast in Physical Properties of Surface Soil between Sahara and Sahel
In the treatment of dust lofting and its advection out of Africa to the Americas, many studies neglect important distinctions between the Sahara Desert and the Sahel on its southern fringe, as source regions for the dust. The Saharan sources are primarily dry processes sustained by synoptic-scale winds and occasional cold fronts from Europe. The Sahelian sources involve moist convection and ensuing dry dusty gust fronts (aka, hoboobs), and have recently been investigated with a variety of methods (Doppler radar, visibility, thermodynamic) during the African Monsoon and Multi-Disciplinary Analysis. Evidence for a latitudinal contrast in optical properties in West Africa comes from satellite measurements of surface albedo, showing typical factor-of-two differences between albedo maxima (~0.5) in the Sahara to the regions in the Sahel (0.2-0.35) where vegetation begins to influence albedo strongly. Subjective visual observations indicate that the Sahara is ‘white' and the Sahel is ‘red'. In the present more quantitative study, soil samples were collected at ~50 km intervals between Bamako, Mali in the Sahel and Timbuktu, Mali in the Sahara Desert, to explore the role of iron compounds in influencing the physical properties of the soil: measurements included optical reflectance (the laboratory equivalent of albedo) and magnetic susceptibility (MS). Diffuse reflectance spectroscopy in the red, green and blue regions of the spectrum (250-800 nm in 1nm increments) were undertaken on these samples. Percent red is generally a good indicator of (red) hematite and increases from the Saharan samples to the Sahelian samples. The overall increase in reflectance is a factor-of-three from Sahel to Sahara. The MS measurements by three groups all show consistent results and a strong increase from Sahara to Sahel following the gradient in rainfall as in other studies elsewhere. The maximum spread in values is nearly two decades (from 1 x 10-8 m3/kg to 100 x 10-8 m3/kg). MS is known to be strongly influenced by ferrimagnetic minerals (magnetite and maghemite), which appear to be substantially more abundant in the surface material of the Sahel than the Sahara, but the hematite concentrations in the Sahel may be 2-3 orders of magnitude larger. The use of MS as a tracer of the origin of dust from Africa will be critically evaluated.
GP51A-04 INVITED
Rock Magnetic Detection of Two Coercivity Components of Magnetosomes in Lake Ely Sediments
The modeling of IRM acquisition curves has become an important tool used by environmental magnetists to characterize a sample's magnetic mineralogy. We have applied IRM modeling to lake sediments from Lake Ely, a small, post-glacial lake located in northeastern Pennsylvania. The lake is unique in having a strong sedimentary magnetization dominated by the magnetosomes produced by magnetotactic bacteria. Measurements of Fe, S, and O2 at different depths in the lake's water column, as well as the magnetization of material filtered from the water, indicate that magnetotactic bacteria live in the water column at the oxic-anoxic interface. Magnetic measurements and TEM observations of lake sediments and material caught in a sediment trap indicate that magnetosomes are the major constituent of the magnetic minerals in the lake sediments. IRM modeling shows two separate coercivity components at about 30 mT and 60 mT that are consistent with Egli's (2004) observations of BS (biogenic soft) and BH (biogenic hard) coercivity components for magnetosomes in lake sediments. However, the peaks are poorly determined because only 6-7 data points constrain the 0-100 mT part of the IRM coercivity spectrum. We used pARMs (97 μT DC field) applied in 5 mT alternating field steps between 0 and 100 mT to better resolve the BH and BS coercivity peaks. pARM spectra collected from lake sediment samples reveal one strong coercivity peak at approximately 30 mT in the 0-100 mT range. A second higher coercivity peak at about 80 mT is weakly evident in some samples. Alternating field demagnetization in 104 steps of an ARM applied to one sample also suggests the presence of two coercivity peaks but at 20 and 40 mT. First order reversal curves (FORCs) measured at the Institute for Rock Magnetism show that the magnetic grains behave as non-interacting single domain grains, implying that the magnetosome chains are relatively far apart and probably still intact. The FORC diagrams also show a large coercivity peak at 30-40 mT and a weak peak at about 70 mT consistent with the pARM spectra and the IRM modeling. Although subsequent independent coercivity measurements support the IRM acquisition modeling, direct measurement of coercivity spectra by application of pARMs appears to be the best way to resolve the two different coercivity components of magnetosomes present in lake sediments and to determine their relative size.
GP51A-05
Magnetic Properties of Surface Sediments in Small Temperate Lakes: Modern Analogues for Paleolimnologic Research
Magnetic properties of lake sediments are routinely measured as part of paleolimnological and paleoclimatic research. Basic parameters such as magnetic susceptibility (MS), anhysteretic remanent magnetization (ARM), and isothermal remanent magnetization (IRM) are used for correlating cores from different sites in the same basin, tracking erosion history and lake level changes, or investigating eutrophication and microbial processes. However, a detailed investigation of the syn-depositional processes that control the distribution of magnetic minerals across lake basins is lacking for most types of lake systems. In order to understand the main controls on environmental magnetic records, we systematically investigated the magnetic properties of surface sediments collected along transects in nine Minnesota lakes. The lakes are small (<1 sq. km), have simple morphologies, are hydrologically closed, and are distributed across an east-west moisture gradient, as well as a north-south temperature gradient. The structure of lake water columns was investigated by measuring temperature, specific conductivity, dissolved oxygen, and pH. Sediment composition was determined via loss on ignition (LOI). The magnetic properties of the sediments reflect the change in depositional environments from shallow to deep water, as defined in sedimentological context by LOI and sediment granulometry. All lake basins exhibit a characteristic pattern in terms of concentration (MS and IRM) and grain size (ARM/IRM) of magnetic minerals. Sediments above wave base (0.5 m) have high concentrations of coarse grained magnetic minerals. Below wave base, but in the thermally mixed layer, magnetic particles are finer-grained and present in lower concentrations. Profundal slope sediments are characterized by variable magnetic and compositional parameters, indicative of a dynamic sedimentological and geochemical environment. In the deep, anoxic regions, magnetic concentration increases again, associated with a coarsening in grain size, most likely an effect of sediment focusing. The generalized model of lacustrine magnetic response to both external and internal forcing factors provided by our modern analogue approach will provide an improved basis for paleolimnologists to interpret records of lacustrine environmental magnetism.
GP51A-06 INVITED
Magnetochemical Features in Recent Marine Sediments Across the Galician Atlantic Margin and their Stratigraphic Significance
A significant part of the sedimentary magnetochemical signal in the marine realm is controlled by the variability of detrital inputs; especially ice-rafted debris (IRD), fall-outs from nepheloid layers (NL) and wind blow-outs; which have been widely reported in the scientific literature. Their magnetic properties have been commonly and quite successfully used to establish the stratigraphic correlation of cores from a given location, or more intently, as proxies that document climatically forced events. Soon after deposition, the balance between oxygen diffusion from oxic bottom waters into the sediment; sea bed remobilization by benthic organisms and currents; primary productivity; and sediment accumulation rates will determine the degree of alteration of these detrital signals during redoxomorphic diagenesis and subsequently, the degree and conditions of preservation of their detritally-based environmental information (mostly held by magnetite) and the addition of new information during the authigenesis of magnetically-interesting minerals (chiefly goethite, greigite and pyrite). The correct interpretation of the magnetochemical properties and their environmental significance depends on our ability to asses the intensity and nature of magnetic-sensitive early-diagenetic processes resulting from changes in the sediment accumulation rates and on the palaeoceanographic conditions affecting productivity through time. Much of the changes that took place over the last 30 ky in the different depositional and diagenetic realms of the Galician Continental Margin -spanning from high productivity low depth Rías, storm swept continent shelf, and down slope turbiditic aprons, and abyssal plain- can be assessed estimating the concentration and half life of magnetite; which is respectivelly controlled by detrital forcements and the balance between C/S and the site sedimentation rate. This paper will deal with the magnetochemical expression of these changes in the depositional and/or diagenetic environment from the point of view of the enviromagnetism, solid-phase geochemistry, with the additional support of radiometric dating and scanning electron microscopy texture analysis; and of how these information have been used to reconstruct the Late Pleistocene to Holocene sedimentation history across the Galician Atlantic Margin.
GP51A-07
Rock magnetic and geochemical process modelling of overpressured sediments from the northwestern Gulf of Mexico (IODP Exp. 308)
Four late Quaternary sediment series were recovered from the Brazos-Trinity Minibasin IV and the Ursa Region in the northwestern Gulf of Mexico, offshore Texas and Louisiana (USA) to study in detail the sedimentation processes, slope stability, overpressure and fluid flow mechanisms of rapid sedimentation areas on this passive continental slope. Within the framework of this project, it is specifically important to understand the time frame of the studied processes. Due to a very high sedimentation rate under unconformable conditions (several m/kyr) the classical paleomagnetic approaches are not suitable for time constraints here. Magneto-mineralogic studies as well as electron microscopic analyses yield results on the highly complex magnetic assemblage, which includes various Fe-Ti mineral phases such as (titano-) magnetite, hemoilmenite, and hematite next to Fe-sulphides (mainly greigite and pyrite). The rock magnetic records also show a distinct pattern of either iron oxide or iron sulphide dominated sediment sections, thus it is possible to distinguish between the detrital/continental and postdepositional magnetic signal of the sediments. A conceptual model has been developed combining magnetic and geochemical methods. This enables us to identify various successive non-steady state sedimentation events and to understand the prevailing past ad present environmental conditions. Subsequently we develop a novel approach to further interpret this conceptual model and to constrain a detailed stratigraphic and environmental model for these low-permeability sediments by using multi-parameter correlations as well as non-steady state quantitative geochemical modelling. High resolution rock magnetic records are calibrated with geochemical data sets of acid volatile sulphur (AVS) and chromium reducible sulphur (CRS) estimations. Like that we aim to calculate ages for the distinct sediment packages. The model for the tectonically less complicated Brazos-Trinity drill sites will be linked to the overpressured and tectonically complex Ursa drill sites, where conventional dating methods are very limited.
GP51A-08 INVITED
Magnetic and Modelling Approaches to Ocean Circulation at the Last Glacial Maximum
The N. Atlantic plays a key role in global climate through formation of N. Atlantic Deep Water, which is a major driver of the thermohaline circulation. To understand past and future climate change, it is essential to understand the processes occurring within the N. Atlantic both at the present day and in past glacial stages. Debate exists over the ocean circulation state during glacial stages, some geological and modelling studies suggesting similar, or higher, rates of NADW advection during the Last Glacial Maximum (LGM), others indicating decreased NADW and increased formation of Southern Ocean deep water. Here, we use a suite of susceptibility and remanence measurements of ocean-wide N Atlantic sediments to test two different potential LGM ocean general circulation model results, by comparing the modelled iceberg trajectories each produces with magnetically- mapped patterns and sources of LGM ice-rafted debris (IRD). The two LGM states are characterised by: vigorous NADW formation; and deep water production in the Southern Ocean. We use cluster analysis of sediment magnetic properties to characterise N. Atlantic IRD patterns and sources at the present day and at the LGM. The LGM IRD patterns match most closely iceberg trajectories arising from the 'southern-sinking' ocean circulation state. Two major IRD sources at the LGM are indicated by the magnetic data, Fennoscandia and Greenland/Iceland, and one minor source, the St Lawrence region. The model and magnetic data suggest that the LGM N. Atlantic circulation was dominated by a cyclonic central N. Atlantic gyre, separated from the N. Atlantic Current which was displaced south of ~ 42°N. http://geography.lancs.ac.uk/cemp/publications/07_Watkins%20etal.pdf