Reconstruction of superparamagnetic particle grain size distribution from Romanian loess using frequency dependent magnetic susceptibility and temperature dependent Mössbauer spectroscopy
Introduction
The magnetic properties of loess deposits are of particular importance in reconstructing the paleoclimatic changes over the last 2.5 Ma (Maher and Thompson, 1999, Liu et al., 2007). Magnetic susceptibility variations across a typical loess deposit from the Northern Hemisphere mid-latitudes show higher values in the modern soils and paleosols than in the loess reflecting interglacial/glacial climatic changes [e.g., Heller and Liu, 1984, Heller and Liu, 1986, Kukla et al., 1988, Maher and Thompson, 1991, Maher and Thompson, 1992, Maher and Thompson, 1995, Ding et al., 2002, Deng et al., 2005, Deng et al., 2006]. The magnetic enhancement in paleosols is generally attributed to the formation of new fine-grained single domain (SD) and superparamagnetic (SP) particles of ferrimagnetic minerals such as magnetite and/or maghemite during pedogenesis (Zhou et al., 1990, Maher and Thompson, 1991, Maher and Thompson, 1992, Maher and Thompson, 1995, Verosub et al., 1993, Heller and Evans, 1995, Deng et al., 2000, Deng et al., 2001, Liu et al., 2003, Liu et al., 2004a, Liu et al., 2004b, Liu et al., 2005, Chen et al., 2005). The grain size distribution of superparamagnetic particles performed on Chinese loess showed a wide distribution with a maximum located around 20–25 nm (Liu et al., 2004b, Liu et al., 2005, Nie et al., 2008) or around 16–19 nm (Kodama, 2013, Nie et al., 2013, Kodama et al., 2014), depending on the methods used. In addition, this distribution was found to be independent of pedogenesis degree (Liu et al., 2004b, Liu et al., 2005, Nie et al., 2008).
The loess–paleosol deposits from the lower Danube basin are among the best preserved European loess deposits (Frechen et al., 2003, Haase et al., 2007, Fitzsimmons et al., 2012), providing detailed paleoclimatic information over the last 800 kyr (Jordanova and Petersen, 1999, Panaiotu et al., 2001, Jordanova et al., 2007, Buggle et al., 2009, Buggle et al., 2013, Balescu et al., 2010, Marković et al., 2011, Necula et al., 2013). The source material of these deposits is thought to be mainly derived from Danube sediments possibly with some minor input from the glaciofluvial source area in Ukraine and from local sand dune fields or from Black Sea aerosols (Buggle et al., 2008). In Romania, the most developed loess–paleosol deposits outcrop mainly in the Danubian Plain and the Dobrogea Plateau. Several studies over the last 15 years showed that the magnetic behavior of the Romanian loess follows the typical Chinese loess pattern: significant higher values of magnetic susceptibility in paleosols compared to the loess layers (Panaiotu et al., 2001, Buggle et al., 2008, Buggle et al., 2009, Buggle et al., 2014, Balescu et al., 2010, Timar et al., 2010, Timar-Gabor et al., 2011, Vasiliniuc et al., 2011, Necula and Panaiotu, 2012, Necula et al., 2013, Constantin et al., 2014). Detailed rock magnetic investigations performed on several Romanian loess sections revealed the magnetite close to superparamagnetic/stable single domain boundary as being the major contributor to the magnetic susceptibility signal in paleosol units (Panaiotu et al., 2001, Necula and Panaiotu, 2012, Necula et al., 2013, Buggle et al., 2014). Thus the magnetic enhancement in paleosols from Romanian loess deposits can be explained by the pedogenic model of in situ production of new ultrafine magnetic minerals during soil formation (e.g. Maher and Thompson, 1991, Evans and Heller, 1994, Hunt et al., 1995, Maher, 2011).
In this study we reconstruct, for the first time, the grain size distribution of the magnetite nanoparticles in two Romanian loess deposits using two independent methods. The first one is based on wide-band frequency spectrum of magnetic susceptibility (FSMS) developed by Kodama (2013), which provide also a quantitative estimate of the SP particle concentrations (Appendix). The second is based on temperature dependent Mössbauer spectroscopy (Kuncser et al., 2007) (Appendix). Our results provide further insight about the link between paleoclimatic changes and grain size distributions of pedogenic SP magnetite in Romanian loess deposits.
Section snippets
Geological settings and measurements
The Mircea Vodă section (44.322084°N, 28.189189°E) is situated in the Dobrogea (Dobrudja) Plateau (SE Romania), close to Mircea Vodă village, at about 15 km from the Danube River (Fig. 1, see also the Supplementary KMZ file). This loess section is approximately 26 m thick and consists of six paleosols (S0–S5, with S0 representing the Holocene soil) and intercalated loess layers (L1–L5), with no apparent evidence of remarkable hiatuses (Buggle et al., 2008, Buggle et al., 2009, Buggle et al., 2014
GSD determined by FSMS
The results of FSMS measurements are presented in Fig. 3. The magnetic susceptibility of all the samples decreases monotonically with frequency until about 32 kHz. Above this frequency the decrease becomes smoother with a slight tendency to saturation at 500 kHz. According to the FSMS typology pointed out by Kodama (2013), all the samples studied seem to belong to the FSMS II group. FSMS II type displays a monotonically decreasing magnetic susceptibility until about 100 kHz, followed by a
Comparison of GSDs from FSMS method and Mössbauer spectroscopy
The two methods indicate that both loess and paleosol samples have almost the same dominant grain size. However the temperature dependent Mössbauer spectroscopy shows peak values smaller than the FSMS method. In addition dispersion parameters provided by Mössbauer spectroscopy differ from sample to sample with no link to pedogenesis (Table 3). The differences in the estimates of the peak values and diameter ranges mainly depend on the used methods and underlying assumptions.
Firstly, both
Conclusions
We have reconstructed, for the first time, the superparamagnetic grain size distribution for Romanian loess using two independent methods resulting in the following conclusions.
The FSMS method shows that the SP particles are present both in loess and paleosols. The GSDs in loess are shifted to slightly higher diameters with respect to paleosols. The concentration of SP particles has an opposite trend. The largest value is reached in the forest paleosol and the lowest in the loess samples.
Acknowledgments
C.N. and C.P. were supported by CNCSIS grant ID-31/2010. G.S., P.P., and V.K. were supported by PNII-PCE 75/2011 grant. Useful comments from four anonymous reviewers help us to significantly improve our manuscript.
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