New approaches for integrating palaeomagnetic and mineral magnetic methods to answer archaeological and geological questions on Stone Age sites
Year: 2009
Type: book-chapter
Abstract: Mineral magnetismSediment sourcing, input, alteration and palaeoclimatic modelling Mineral magnetic studies examine the magnetic nature (minerals, grain sizes and concentration) of the sedimentary input into a depositional system and the various processes that have acted on that material both before and after its deposition in that system.Soil formed on volcanic soils will have a very different magnetic character to soils formed on karstic landscapes.A study of the magnetic mineralogy of cave sediments can therefore show changes in sedimentary input, such as a change from locally derived material to material derived from further afield.This particularly occurs for sites where sediments are deposited by aeolian activity due to changes in wind speed and direction, which can change between glacial and interglacial periods (Begét 2001).In fluvially derived deposits, changes generally tend to reflect more locally changing conditions, such as changes in pedogenic activity, soil cover, weathering and erosion.A study of the magnetic mineralogy of sediment sequences can often be used as a palaeoclimatic proxy record because these processes are governed by climatic factors such as rainfall and temperature variations.These are in turn driven by glacial/interglacial cycles.Such palaeoclimatic proxy records have primarily been undertaken on lake and marine sequences (for example, Peck et al. 2004;Williamson et al. 1998), but work has shown that cave sediments are also suitable, as they are relatively protected from external climatic processes after deposition (Herries 2006).Mineral magnetic analysis of sediment sequences can reveal a climatically-driven signal for a number of reasons.Primary and secondary iron phases in rocks and sediments are transformed by weathering and various other climatically driven processes into secondary iron oxides (such as magnetite, maghaemite and haematite), hydroxides (such as goethite) and sulphides (such as pyrrhotite).Most often, these pedogenic processes convert primary and secondary iron into magnetite, with later oxidisation to maghaemite through oxidisation/reduction cycles (Maher 1998).This is known as pedogenic enhancement and it produces a dominance of these ferrimagnetic minerals.With prolonged weathering, these processes may also cause the formation of anti-ferromagnetic minerals, haematite and goethite.The degree of pedogenic enhancement is not only determined by climatic and environmental processes but also by the local lithology, which provides the initial magnetic input into the pedogenic system through weathering of underlying rock strata.In certain rocks, iron oxides, hydroxides and sulphides may already coexist with amorphous iron.In igneous landscapes magnetite is the dominant iron oxide, whereas sedimentary rocks may contain significant haematite, especially in sandstones.As large amounts of highly magnetic phases are weathered directly from the local rock strata in volcanic landscapes, the soils tend to show little pedogenic enhancement.The greatest enhancement is seen in rock types that do not contain large amounts of primary iron oxides, such as limestone (Maher 1998).Pedogenic formation of ferrimagnets is driven by temperature and moisture.It appears to be favoured in well-drained, poorly-acidic soils on weatherable, Fe-bearing (but often not Fe-rich) substrates in a climate that produces wetting/drying cycles (Maher 1998).Excessively arid, waterlogged or acidic soils display little pedogenic enhancement.Maher and Thompson (1995) suggest that in many cases maximum values of strong ferrimagnetic mineral phases, and so greatest pedogenic enhancement, correlate well with absolute rainfall.However, in cases where a certain rainfall threshold is exceeded, water-logging occurs and can result in the depletion of magnetic minerals by processes such as gleying (Liu et al. 1999).Detailed in situ magnetic susceptibility (MS) measurements of multiple sections can quickly identify the potential of a site for palaeoclimatic analysis by identifying bulk changes in ironbearing minerals.However, to understand fully the magnetic mineralogical changes, a suite of mineral magnetic measurements needs to be taken.Other analytical techniques for characterising mineralogy (e.g.XRF, XRD, FTIR) can most often not be used due to the small amounts of magnetic material within the samples.
Source: ANU Press eBooks
Author: Andy I. R. Herries
Institutions: University of Liverpool, UNSW Sydney
Language: English
Subfield: Molecular Biology
Domain: Life Sciences
SDG: Climate action
Open Access status: bronze
OpenAlex ID: W2911898940