Palaeoclimatic Significance of Hematite/Goethite Ratio in Bulgarian Loess-Palaeosol Sediments Deduced by DRS and Rock Magnetic Measurements

Abstract

The role of hematite and goethite as palaeoenvironmental indicators in loess-palaeosol sediments is studied by diffuse reflectance spectroscopy (DRS) and rock magnetic methods. Forty five selected samples from four loess-palaeosol profiles in Bulgaria were used to deduce the behaviour of the hematite/goethite ratio in loess and palaeosol units, which were formed under contrasting palaeoclimate conditions -- cold (glacial) and warm (interglacial). According to DRS, the pedogenesis is accompanied by preferential formation of hematite over goethite in all sites. At the same time, rock magnetic data prove that this process is concomitant with in-situ formation of a strongly magnetic ferrimagnetic fraction, responsible for the observed magnetic enhancement of palaeosol units. Systematically higher amount of hematite in the loess-palaeosol profile Orsoja, situated at the fifth Danube river terrace and the Durankulak profile at the Black sea coast is supposed to be due to additional coarse-grained hematite in the aeolian dust blown from local dust sources during glacial periods. Component analysis of the curves of stepwise acquisition of isothermal remanence reveals the presence of two components -- pedogenic (P) with median coercivity at half-width of the Gaussian function of 31 {\textpm} 3.2 mT; detrital unweathered component D1 in loesses from Lubenovo and Koriten profiles with B1/2 of 58 {\textpm} 4 mT, and weathred detrital component D2 (B1/2 of 123 {\textpm} 46 mT) in all palaeosols and weathered loess units from Orsoja and Durankulak profiles. Mineral phases, carrying these coercivity components are supposed to be pedogenic maghemite (P), aeolian (titano)magnetites (D1) and hematite (D2).