Geomagnetism and Paleomagnetism [GP]

 CC:718A  Wednesday  0800h

Environmental Magnetism: Established Methods and New Approaches II

Presiding:  M T Cioppa, University of Windsor; K L Verosub, University of California, Davis


Grand Challenges for Environmental Magnetism

* Verosub, K L (, Geology Department, University of California - Davis, Davis, CA 95616, United States

The development of new, inexpensive, and rapid geochemical methods for determining the ages of geologic materials, their elemental composition, and their isotopic ratios over a broad array of elements puts into sharp focus the question: What information can environmental magnetic methods provide that can't be obtained using these other methods? Because iron is ubiquitous in the Earth's crust and because it exists in so many different forms, a discipline that looks in detail at iron-bearing minerals does have the potential to make significant contributions to the study of surficial processes. However, to reach that potential requires the development of new environmental magnetic methods. I would like to put forward three Grand Challenges for environmental magnetism that have the potential to move the field forward to a new level of scientific sophistication and that will allow environmental magnetists to compete successfully in a world increasingly dominated by geochemists. The first Grand Challenge is the development of new techniques that lead to the direct and unambiguous identification of the full suite of magnetic minerals. For many environmental magnetic applications, the key magnetic minerals are not just magnetite and hematite but also iron oxy-hydroxides (goethite, lepidocrocite, akaganeite, ferrihydrite), carbonates (siderite) and sulfides (pyrrhotite and greigite) as well as compounds involving iron and other transition metals (cobalt and nickel). The second Grand Challenge is the development of new analytical methods that provide specific quantitative values for the amount of each magnetic mineral present in a sample. One promising approach to this problem is the application of two- or three-component multivariate analysis to arrays of downcore environmental magnetic parameters. The third Grand Challenge is the development of new ways of determining, not just the average values, but the actual distributions of grain sizes and coercivities of each mineral species. Some progress has been made in this area in recent years but considerably more can and should be done. Fortunately environmental magnetists have several new techniques at their disposal, and the capabilities of these techniques have not yet been fully explored, especially at high and low temperatures and using relatively small steps for the acquisition and demagnetization of laboratory magnetizations.


Magnetoclimatology From Quaternary Loess And Palaeosol Sequences.

* Maher, B (, Lancaster Environment Centre, University of Lancaster, Lancaster, LA1 4YQ, United Kingdom

The loess and palaeosols of the famous Chinese Loess Plateau span more than the last 2 million years of Earth's history, preserving the longest, most detailed record of Quaternary climatic changes yet found on land. More loess/palaeosol sequences occur in both hemispheres, also acting as potential climate archives albeit for shorter time spans. Given the prospect of future changes in monsoonal rainfall in the densely populated East Asian region, retrieval of quantitative palaeoclimatic data from these natural archives is an important and timely task. Here are reviewed the climate-recording mechanisms of these terrestrial sediments, and the current and prospective strengths of magnetic measurements as quantitative proxies of palaeorainfall and monsoon variations.


Reexamining the long-term astronomical periodicities in the Lake Baikal deep drilling sediments

* Kravchinsky, V A (, Department of Physics, 11322 - 89 Avenue, University of Alberta, Edmonton, AB T6G 2G7, Canada

Records of climatic change, such as lacustrine and eolian sediments agree well with the marine records for the last couple million years. Continuous terrestrial records for longer time intervals were unavailable until last decade when deep drilling Lake Baikal cores were recovered. Lake Baikal's middle-high latitude position within the Asian interior ensures highest sensitivity of the climate to the Earth orbital parameter variations. Our earlier publications examined Milankovitch cyclicity in a number of the lake cores with ages spanning from the present day to 6.7 Ma. It was possible to demonstrate the high signal power in the narrow obliquity band confirming the strong orbital forcing on the climate. However, a 100 kyr period found in these records did not align with the orbital eccentricity in a way predicted by theory. Available Be10 chronology enabled me to adjust the magnetostratigraphy age model for certain time intervals and to revise the magnetic susceptibility and biogenic silica datasets. I compare both untuned and tuned datasets with the theoretical insolation and oceanic oxygen isotope records and discuss the findings of the precession, obliquity, and eccentricity cycles as well as "non-Milankovitch" cycles of ~29 kyr and ~70 kyr. A notable absence of considerable eccentricity amplitude at 400 kyr in the oceanic records during the last few million years is well known in spite of this being one of the largest component of eccentricity forcing. At the same time, reported earlier 400 kyr and larger periodicities for the Lake Baikal sedimentary records were derived from the erroneous age model. I reexamine the long-term periodicities (400 kyr and longer) which are confidently represented in the Lake Baikal records.


Paleo-monsoon Route Reconstruction in Chinese Loess Plateau Using the Anisotropy of Magnetic Susceptibility Measurements

* Zhang, R (, Physics Department, 11322 - 89 Avenue University of Alberta, Edmonton, AB T6G 2G7, Canada
Kravchinsky, V A (, Physics Department, 11322 - 89 Avenue University of Alberta, Edmonton, AB T6G 2G7, Canada
Zhu, R, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, 100029, China
Yue, L, Department of Geology, Northwest University, Xi'an, 710069, China

The anisotropy of magnetic susceptibility (AMS) was investigated in three Chinese Loess Plateau sedimentary sections along the W-E transect (400 km). The loess-paleosol sequences in study represent a 130 kyr time interval. The measured AMS in the loess was compared to the expected theoretically derived magnetic fabrics occurred in the current or air flow conditions. The major and minor AMS axes orientation was interpreted in order to evaluate the paleowind direction in the studied loess. Previously published models assumed that the cold and dry winter monsoons from the northwestern deserted areas were responsible for the main magnetic fabrics in the loess. While in our new interpretation, the summer monsoons from the southeast played the major role in the magnetic fabrics orientation. Although the material was brought by the winter monsoon, the AMS was formed during the rainy summer monsoon when the sedimentary particles including magnetite were rearranged and settled. The paleo-monsoon route reconstructed for the last 130 kyr appeared to be affected by the regional topographic factors. For example, in the west section, the corridor between the north and south of Liupan Mountains disturbed the summer monsoon route shifting it from the SE direction to the SEE.


Sedimentological, Magnetic and Geochemical Proxies for Holocene Climate Change and Paleoseismology from Marine Anoxic Inlet Sediments

* Enkin, R J (, Geological Survey of Canada, POB 6000, Sidney, BC V8L 4B2, Canada
Dallimore, A (, Royal Roads University, 2005 Sooke Road, Victoria, BC V9B 5Y2, Canada
Baker, J (, Geological Survey of Canada, POB 6000, Sidney, BC V8L 4B2, Canada
Ivanochko, T (, Earth and Ocean Sciences, 6339 Stores Road, Vancouver, BC V6T 1Z4, Canada
Chang, A S (, Earth and Ocean Sciences, U Victoria, POB 3065, STN CSC, Victoria, BC V8W 3V6, Canada

Sediments deposited in anoxic basins are not bioturbated and thus hold high temporal resolution proxy recordings of climate and other physical controls. This paper focuses on the inner basin of Effingham Inlet on the west coast of Vancouver Island, Canada, a 120m deep fiord basin restricted from the open ocean by a 46m deep sill. Fifteen years of oceanographic monitoring have helped establish the physical and sedimentary processes at play. Freeze cores, piston cores, and especially the 40 m long MD02-2494 core hold a15 ka record of Late Pleistocene deglaciation, relative sea level change, Holocene climate and paleoseismology, revealed by sedimentological, magnetic and geochemical analysis. The age model is established using terrestrial 14C dates complemented by varve counting and paleomagnetic secular variation correlations. Annual laminations are formed of spring/summer diatom deposits following algal blooms and dominantly- winter deposits of clastics. These sediments provide proxies of Holocene weather with annual resolution. There are several episodes of rapid regime change from high seasonality warm climate to low seasonality wet-cold climate. Interspersed with the the annual laminations, there are two types of massive deposits: "homogenites" formed by remixing of suspended sediments by bottom-hugging currents, and "seismites" formed by mass wasting events associated with ground shaking. Magnetically, both the laminations and homogenites feature similar single-domain magnetic grains, while the seismites feature larger magnetic grains with multi-domain signatures, thus providing a simple tool for distinguishing the two visually similar deposit types. Homogenites, which result from La Nina-like oceanic conditions, first appear in core MD02- 2494 approximately 8 ka and have been increasing in frequency ever since.


Sedimentology and Rock Magnetism of Bailey River Peat Cores, Sudbury Area: Preliminary Results

Yurtseven, A (, Department of Earth and Environmental Sciences University of Windsor, 401 Sunset Ave., Windsor, ON N9B3P4, Canada
* Cioppa, M T (, Department of Earth and Environmental Sciences University of Windsor, 401 Sunset Ave., Windsor, ON N9B3P4, Canada
Dean, K (, Department of Earth and Environmental Sciences University of Windsor, 401 Sunset Ave., Windsor, ON N9B3P4, Canada

Magnetic measurements on peat can reveal atmospheric anthropogenic contamination. Two cores were collected from a marsh surrounding the Bailey River, 10 km north of Sudbury, Ontario, using a Russian peat borer. The BR1 core (1.4 m) was collected right at the river's edge, whereas the BR2 core (2.5 m) was collected about 50 m away from the river's edge, close to the edge of the marsh and near the forest. Significant sedimentological variation between the two cores was observed: core BR1 had several centimeter to decimeter scale fine to coarse grey sand layers at 0.14 m, 0.46 m and 0.87 m between thicker organic-rich (peat) zones, whereas core BR2 had only one 5 cm sand-rich layer at 0.94 m within the organic-rich material. The cores were subsampled at 2.5 cm intervals for laboratory magnetic analysis. Volume susceptibility was measured using a Bartington MS2B meter, and mass-specific susceptibility was then calculated. In core BR1, the sand layers had relatively higher susceptibility (13 x 10-8 m3/kg) , while the organic rich layers had very low susceptibility (0 - 2 x 10-8 m3/kg). In core BR2, which had little sand, the susceptibility variation was dominated by higher values near-surface (10 x 10-8 m3/kg), and very low susceptibility (0.3 x 10-8 m3/kg) below 0.3 m depth. Since the lithology in this core did not vary substantially, susceptibility variations may be controlled by anthropogenic deposition in the near-surface during the peak mining and smelting decades. These preliminary results suggest that any anthropogenic signal in core BR1 appears to be masked by the sedimentological variation. On pilot results from eight samples in core BR1, saturation isothermal remanence acquisition showed 95% saturation by 200 mT, and the S-ratios (0.3T/0.9T) were above 0.93, suggesting that magnetite is the major magnetization carrier. In core BR2, six out of eight samples showed similar results; however, two samples had slightly more higher coercivity minerals (90% saturation by 200 mT, and S-ratios between 0.9 and 0.93). More detailed rock magnetic property measurements could reveal variations grain size and magnetic mineralogy that could reflect climatic or watershed changes.