Establishing fire incidence in temperate soils using magnetic measurements

Natural, unburnt topsoil samples from the Cotswolds region of England and samples from archaeological contexts in the UK and Hungary, some of which are known to have been affected by burning, have been used to identify a distinctive magnetic ‘signature’ linked to the effects of fire on magnetic properties. A combination of bulk, low field (χlf) and frequency dependent (χfd) magnetic susceptibility and normalized anhysteretic remanent magnetization (χARM) measurements have been used on both unburnt and experimentally burnt samples from the Cotswolds and on the archaeological samples. Complementary determinations were made for both sets of samples of the maximum potential susceptibility (χmax) and, hence, percentage fractional conversion (χconv), which provides independent evidence of burning. The results show that the ferrimagnetic mineral assemblages produced as a result of fire have a significantly finer grain size than those arising from weathering and soil formation alone. Burning thus gives rise to a distinctive envelope of values on a bilogarithmic plot of χARM/χfd versus χARM/χlf. Such a diagram provides a convenient basis for identifying magnetic signatures in soils, archaeological materials and palaeosols that have arisen mainly through burning.