Pedogenic transformation of magnetic minerals in Pliocene-Pleistocene palaeosols of the Siwalik group, NW Himalaya, India

We present preliminary rock magnetic and geochemical analyses of thirty-one pedogenic intervals from six palaeosol profiles in a Pliocene-Pleistocene fluvial sequence of the Siwalik Group in NW Himalaya. The palaeosols are characterised by low magnetic susceptibility (X-1f (mean) = 9 x 10(-8) m(3)/kg), low frequency dependent susceptibility (X-fd (mean) = 0.93%), high coercivity of remanence ((B-0)(cr) (mean) = 447 mT) and high mean hysteresis ratios (M-rs/M-s (mean) = 0.6) as a result of the predominance of imperfect antiferromagnetic minerals, hematite and goethite. Stepwise IRM acquisition to 7 T and backfield demagnetisation to 1 T suggest the predominance of goethite in several intervals. The relative contributions of goethite and hematite are derived from differential rates of IRM acquisition, which are further confirmed using stepwise thermal demagnetisation of orthogonal IRM components acquired at 4, 1 and 0.3 T. The absence of a significant superparamagnetic fraction, and the apparent depth control of X-fd%, are attributed to post-depositional processes of compaction and burial diagenesis. Anomalously high coercivities within the goethite-enriched horizons may have resulted from TRM acquisition due to the high crustal heat flows due to cumulative burial to depths of over 1 km. Pedogenically induced magnetic mineral production and transformation within the palaeosol profiles are inferred using a range of rock magnetic parameters, geochemical ratios (e.g. Rb/Sr and At/Si), and percentage organic/inorganic carbon content. We use magnetic ratios based on the relative contributions of goethite, hematite and soft ferrimagnetic minerals to infer climate-sensitive processes of oxidation-reduction, hydration-dehydration, hydroxylation-dehydroxylation and humification. In the earliest Pliocene palaeosol, we infer the pedogenic depletion of magnetic susceptibility due to the transformation of parental ferrimagnetic minerals mainly into anti ferromagnetic minerals. In contrast, the youngest (middle Pleistocene) profile shows significant modification of parental antiferromagnetic minerals into ferrimagnetic forms. We suggest that further detailed rock magnetic and geochemical studies using a higher sampling density and at a regional scale would help reconstruct the pedogenic transformation of iron oxides in the tropical-subtropical Siwalik soils and the effect of post-depositional and burial processes upon them. (C) 2004 Elsevier B.V. All rights reserved.