Evaluation of Magnetic Proxies from Matrixes of Coarse Paleosols in the Atacama Desert: Environmental or Climatic Signal?

Magnetic proxies obtained from fine-grain sediments have been used to unravel climatic and environmental settings (Evans and Heller, 2003), yet similar studies from coarse-grain sediments lack. We present magnetic and mineralogical measurements of 115 samples collected on four coarse-grained Neogene paleosol profiles (gyspi-, gypsi/verti-, calci-sol and a wetland deposit) in the Atacama Desert, aiming to establish if the magnetic signal results from climate and/or environment processes. Three grain-size fractions were recovered from each sample: <0.5 (A), 0.5-1 (B), 1-2 (C) mm, from which magnetic susceptibility and frequency-dependent susceptibility were measured under 200A/m at two frequencies. To assess the contribution of each fraction to the total signal we defined: (1) kw=sum(Ci*kf1(i)) where i=A,B or C fractions and Ci their concentrations (Hrouda, 2011), and (2) kfdw%=sum(Ci*kfd%(i)) where kfd(i) is the frequency-dependent susceptibility of each fraction (Dearing et al., 1996). Kw is controlled by the A fraction, suggesting that ferromagnetic grains are concentrated in the matrix. Kw tends to be higher in coarse-grain horizons and lower in fine-grain ones; an opposite relationship when considering kfdw%. Kw depends on the concentration/composition of magnetic grains, whereas kfdw% depends on the concentration of superparamagnetic/single-domain (SP/SD) magnetic grains (Evans and Heller, 2003). From microscopic observations, magnetite is always present whereas hematite and goethite are scarce. Compositional variations may result from heterogeneities in the sediment source, whereas variations in the SP/SD content have been commonly attributed to generation or destruction of larger pseudo-single-domain/multi-domain (PSD/MD) magnetic grains due to diagenesis (Dearing et al., 1996). Weathering of large (PSD/MD) magnetite grains will result in a grain-size reduction and in a release of Fe-ions to form new (SD/SP) magnetic grains; hence, magnetite destruction will decrease kw whereas formation of new grains will increase kfdw%. Interestingly, kw seems to be related to a humidity trend, from wet-like (calcisol) to arid-like (gypsisol) environments, and kfdw% related to destruction/generation of ferromagnetic grains. In summary, erosional-derived horizons yield relatively high kw and low Xfdw% values, whereas those affected by diagenesis yield and opposite relation. Identification of erosional- and diagenetic-horizons can be achieved by using kw in combination with kfdw%. Moreover, humidity trends from paleosols may be detected by using kw. Evans, M., Heller, F. 2003. Environmental Magnetism, principles and applications of enviromagnetics. Acad. Press, 311p Hrouda, F. 2011. Models of frequency-dependent susceptibility of soils and rocks revisited and broadened. Geoph. J. Inter. 187, 1259-1269. Dearing, J., Hay, K., Baban, S., Huddleston, A., Wellington, E., Loveland, P. 1996. Magnetic susceptibility of soil: an evaluation of conflict theories using national data set. Geoph. J. Inter. 127, 728-734.