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Magnetic susceptibility measurements have played a significant role in understanding the Earth’s climate. The main source of information has come from the study of wind blown dusts that accumulate as sediments at the bottom of the oceans or on raised plateau areas of the world.

The first magnetic study of deep-sea sediments (Bloemendal and deMenocal 1989) used the Bartington Instrument’s whole core sensor to measure magnetic susceptibility over the past 3.5 million years.

Their data from the Arabian Sea and eastern Atlantic Ocean showed how tropical climates are strongly controlled by cyclical variations in solar radiation. The stronger the tropical monsoons, the more wind blown dust reached the oceans. The interesting finding was that from 2.4 million years ago the tropical climate

was strongly influenced by the northern hemisphere glaciations. Since this study, whole core scanning of magnetic susceptibility has become a standard tool in climate change studies of deep-sea sediments.

On land, long-term dust accumulation produces deposits known as loess. In China, the loess sequences are over 100m thick and show alternating bands of light and dark soil, corresponding to past cool, dry and wet, warm conditions respectively (Figure 2a). Magnetic measurements of these sequences provide a way of interpreting the bands into climate data. Maher and Thompson (1995) measured soil magnetism in different climate zones across China and showed that there was a positive relationship between magenetic susceptibility and annual rainfall. When they applied that same relationship to the loess sequences (Figure 2b) it was possible to reconstruct rainfall back to over one million years ago.

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8Figure 1. Magnetic susceptibility curves for two deep-sea cores covering last 3.5 million years with spectral analyses showing the periodicity of cycles for each 400,000 year period.

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Figure 2. a) Chinese loess plateau showing horizontal banding of light and dark soils (photo Qinzheng Hu); b) magnetic susceptibility (upper axis) for the Xifeng loess sequence over 1.1 million years (left axis) showing alternating values corresponding to variations in rainfall (lower axis).

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But why do the soil magnetic values relate to climate? To explain this, scientists have sought to understand the effects of climate on the production of secondary ferrimagnetite minerals. The most recent study (Boyle et al 2010) uses a computer model based on known chemical reactions (Figure 3a) to simulate how climate affects the magnetic minerals (Figure 3b). The results show that that soil magnetism is affected primarily by parent material and climate. Where parent material is constant, like the loess, the magnetism acts as a proxy for annual precipitation and to a lesser extent annual temperature.

In some situations, magnetic susceptibility can be interpreted in terms of much more recent climate change or events. On the Peace River the lack of instrument records of flooding has led to the search for flood proxy records. Sediments from oxbow lakes were analysed for magnetic susceptibility in order to provide proxy records of flood history spanning the past 200–300 years (Wolfe et al 2006). Whole core magnetic measurements using the MSE2 sensor show a strong relationship with the sediment stratigraphy (Figure 4a). The magnetic fluctuations can be used as a high resolution time series of flood events (Figure 4b).

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Figure 3. a) Simplified scheme for production of magnetite (MGT), maghemite (MHT), and goethite (Goet.) and/or hematite (Hem.) from hydrous ferric oxides (HFO, e.g. ferrihydrite) through cycles of oxidizing and reducing conditions in the presence of dissimilatory iron reducing bacteria (DIRB). T– temperature, Min – minerals; b) Model simulations of the effects of Fe (chlorite), mean annual temperature (MAT) and mean annual temperature (MAT) on production of magnetite (solid line), goethite (long dash), and hematite (short dash) after 10,000 years of weathering (Boyle et al 2010).

Figure 4a. Peaks in magnetic susceptibility are associated with dark-coloured flood event laminations (Wolfe et al 2006).

Figure 4b. Inferred flood record for the past ~300 years from magnetic susceptibility. Vertical dashed lines show estimates of the sill threshold (when the river was able to flow into the lake) and the flood threshold for the last major flood in 1974. Years along the right-hand side of the graphs are estimated dates of flood events that are equivalent to or exceed the1974 flood threshold (Wolfe et al 2006).

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The results show that the 1974 flood magnitude was more common in the past, especially between the 1780s and 1900, when it was exceeded ~9 times. The recent declining trend in the data suggests that major floods have become less frequent since the late 19th century as a result of a drier climate. This means that plans for dams and hydroelectric power need to accommodate the possibility of more frequent floods associated with any shift towards a wetter climate.

References

Bloemendal, J. and deMenocal, P. 1989. Evidence for a change in the periodicity of tropical climate cycles at 2.4 Myr from whole-core magnetic susceptibility measurements. Nature 342, 897- 900.

Boyle, J.F., Dearing, J.A., Blundell, A. and Hannam, J.A. 2010. Testing competing hypotheses for soil magnetic susceptibility using a new chemical kinetic model. Geology 38, 1059-1062.

Maher, B.A. and Thompson, R. 1995. Paleorainfall Reconstructions from Pedogenic Magnetic Susceptibility Variations in the Chinese Loess and Paleosols. Quaternary Research 44, 383-391.

Wolfe, B.B., Roland I. Hall, R.I., Last, W.M., Edwards, T.W.D., English, M.C., Karst-Riddoch, T.L., Paterson, A. and Palmini, R. 2006. Reconstruction of multi-century flood histories from oxbow lake sediments, Peace-Athabasca Delta, Canada. Hydrological Processes 20, 4131–4153.