Rock-magnetic and geochemical characteristics of relict Vertisols—signs of past climate and recent pedogenic development. Issue 3 (24th February 2016)

• Record Type: Journal Article
• Authors: Jordanova, Neli; Jordanova, Diana
• Is Part Of: Geophysical journal international. Volume 205:Issue 3(2016:Jun.)
• Page Start: 1437
• Page End: 1454
• Publication Date: 2016-02-24
• Subjects: Environmental magnetism -- Rock and mineral magnetism; Geophysics -- Periodicals; 550
• DOI: 10.1093/gji/ggw067
• Languages: English
• ISSNs: 0956-540X

Abstract

Abstract: Rock-magnetic and geochemical characteristics of three Vertisol profiles with different degree of textural differentiation have been studied. Thermomagnetic analyses, thermal demagnetization of laboratory remanences and acquisition of isothermal remanence curves are applied for identification of iron oxide mineralogy. The main magnetic minerals in Vertisols are ferrihydrite, single-domain magnetite, maghemite and hematite. Variations in magnetic susceptibility, anhysteretic remanent magnetization, isothermal remanent magnetization, as well as different ratios (Xarm/X, ARM/SIRM, S-ratio) along depth are studied. Concentration of magnetic minerals in Vertisols is low, influenced by the intense reductomorphic processes. The lowest magnetic susceptibility is found in the most texturally differentiated soil. However, rock-magnetic data suggest the presence of small, but well defined fraction of single domain-like magnetite with relatively wide grain-size distribution found in those parts of the profiles, which are subjected to most intense and frequent seasonal changes in oxidation-reduction conditions. It is suggested that this fraction is formed as a result of transformations of ferrihydrite under repeated cycles of anaerobic/aerobic conditions. Based on geochemical data, CALMAG weathering index was calculated for the three Vertisols. Using the established relation between CALMAG and mean annual precipitation (MAP), palaeo-MAP was evaluated for the studied profiles. The obtained MAP estimations fall in the range 1000–1200 mm and are much higher compared to contemporary precipitation in the area (MAP in the interval 540–770 mm). This finding confirms the relict character of Vertisols on Bulgarian territory and gives more information about the palaeoclimate during the initial stages of Vertisol formation.