Neli Jordanova

Continental sedimentary sequences of alternating loess and palaeosol horizons preserve detailed records of past global climate changes during the Pleistocene. Obtaining deeper and genuine knowledge on the history of past climates using proxy data depends on interdisciplinary approaches, novel techniques and thinking “out-of-the-box”. The LOEs-CLIMBE team members gather around this concept and present here the first pilot magnetic data from the Kolobar loess-palaeosol section in NE Bulgaria. The 25 m thick section is exposed in an active quarry and was sampled at 2-cm-resolution, covering the Holocene soil, seven palaeosol units and loess horizons L1 to L7 of varying thicknesses. New high resolution magnetic susceptibility data, delineates palaeosol horizons with high values of mass specific magnetic susceptibility except the special case of fourth palaeosol S4, showing no magnetic enhancement as compared to the underlying thin loess. Such depletion of pedogenic magnetic enhancement in paleosol units from the Lower Danube area is rarely reported. This phenomenon will be further examined by detailed magnetic and colorimetric methods. The strongest pedogenic magnetic signal is observed in the three youngest palaeosol units S1, S2 and S3, tentatively related to the interglacial stages MIS 5, MIS 7 and MIS 9. The weakest magnetic susceptibility is typical for the younger part of the loess unit L2, punctuated by the signal of a tephra layer, which is a widespread chronostratigraphic marker in the region. This research is carried out and financed within the framework of the second Swiss Contribution MAPS, LOEs-CLIMBE project № IZ11Z0_230102.

Acknowledgements: funded by the Ghent University Special Research fund and Scientific Research Fund (FWO; K208225N).

Bu çalışma, kayaçların mıknatıslanma özelliklerinden yararlanarak Troya ve çevresinin paleoçevre koşullarının belirlenmesine odaklanmıştır. Troya ören yerinin kuzey yamacında açılan sondaja ait örneklere, Bulgar Bilimler Akademisi Paleomanyetizma Laboratuvarı’nda kaya manyetizması analizleri uygulanmıştır. Sonuçlar, delta çökel ortamında biriken ve arkeolojik materyaller içeren taşkın ovası sedimanlarının ~340 cm derinliğe kadar sondaj örneklerinden ve sert izotermal kalıntı mıknatıslanma (HIRM) parametresindeki kademeli düşüş göstergesinden oksik bir ortamı temsil ettiğini göstermiştir. 450 cm derinliğin altındaki örnekler, manyetit açısından zengin uzak kaynaklı bir taşkının etkisiyle yoğun demir indirgenmesinin ve olası demir sülfitlerinin varlığını göstermektedir. Bu durum aynı zamanda doygun izotermal kalıntı mıknatıslanma (SIRM) değerlerinin yüksek çıkmasına yol açmıştır.

Magnetic susceptibility variations in loess–paleosol successions are widely utilized proxy records for reconstructions of global climate change during the Pleistocene. Analysis of the role of local factors in the establishment of magnetic signatures is rarely addressed. This study compares magnetic records along several adjacent profiles exposed in three open quarries near Kaolinovo (NE Bulgaria). The effect of the position of the sampled locations in the local landscape on the magnetic enhancement is revealed by differences in the thickness and degree of pedogenic magnetic enhancement. The profile, situated in a local paleo-depression, revealed disturbed sedimentation and depletion in the magnetic susceptibility. At lateral distances of 2–3 km (between quarries) the magnetic records show firmly repeatable patterns. Magnetic, geochemical, and diffuse reflectance data demonstrate a trend of increasing content of pedogenic hematites towards older paleosols, while goethite has major contribution to dithionite extractable iron phases. A representative stacked record of magnetic susceptibility for the Kaolinovo site is established using the results from mineralogical analyses. Comparison of the stacked susceptibility record from Kaolinovo with other sites from Bulgaria reveal that loess–paleosol sequences preserve reliable and repeatable magnetic records of global climate change for the last three glacial–interglacial cycles.

Loess paleosol sequences carry valuable information on global climate change during the Quaternary. The main geochemical paleoenvironmental proxies rely on the well established sequence of mineral weathering and its products under earth surface conditions. On the other hand, mineral magnetic signal of the loess – paleosol sequences represent easily measurable and sensitive tool for identification and semi-quantification of the degree of secondary alterations of the initial loess substrate. We have analyzed geochemical and magnetic properties of a collection of loess – paleosol sediments from North Bulgaria from 8 profiles, spanning the time interval of the last 800 kyrs. Weathering indices calculated from the bulk geochemical data on major oxides indicate that loess samples are characterized by wide range of Chemical Index of Alteration (CIA) values – from 57.8 to 83.5 with the highest values typically obtained in the oldest deposits, formed during marine oxygen isotope stages (MIS) 16 and 18. On the other hand, CIA values for paleosols are restricted in the range 73.4 – 82, indicating an intermediate weathering degree. Classic ternary A-CN-K diagram implies dominant role of plagioclase weathering. Magnetic signature of the studied collection shows typical widely observed magnetic enhancement in paleosols as compared to weakly magnetic non weathered loess material. The content of ultra fine grained superparamagnetic magnetite/maghemite give rise to the percent frequency dependent magnetic susceptibility, which shows linear relationship with CIA values, proving the genetic linkage between weathering and pedogenic magnetic fraction. On the other hand, calculated background magnetic susceptibilities χbg for the loess-paleosol couplets included in the study show reverse linear relationship with CIA values for the loess samples. Searching for possible effect of changing dust source areas, we observe decreasing χbg for sites located progressively eastward (longitudes varying from 23oE to 29oE) along with linear increase of χbg with increasing Cr/V ratio, indicative for increasing contribution of mafic component. At the same time, samples from loess horizons with the lowest χbg are characterized by the highest Al2O3/SiO2 values, e.g clay content. Thus, decreasing χbg along W – E transect probably reflects combined effects of dust source change and grain size fining during aeolian dust transportation. The relationships between CIA, χbg and pedogenic magnetic signatures suggest that aeolian dust material was already weathered before its deposition. As a result, the following periods of paleosol formation during interglacial epochs occurred under supply – limited weathering regimes.This contribution is supported by project No KP-06-H34/2 of the Bulgarian National Science Fund

Identification of magnetic minerals, which determine the magnetic properties of natural rocks, sediments and soils, is of crucial importance for any further interpretation of their magnetic signature in environmental or paleogeographical context. One of the major and widely used methods for determination of kind of magnetic phases in natural materials is by obtaining the Curie/Neel temperature of the respective magnetic mineral. In this contribution, we report a set of thermomagnetic measurements of high-temperature behavior of magnetic susceptibility for topsoils from the territory of Bulgaria, aiming to reveal the pedogenic and lithogenic signature. The data are considered and reported in respect to the parent rock type. Our results suggest that pedogenic magnetic minerals are represented mainly by fine-grained maghemite and/or hematite with possible Al-substitutions in the crystal lattice. This phase is expressed on the heating run of the thermomagnetic curve as a “hump” with a maximum at 250–300 °C. Coarse-grained magnetite is identified as a dominant lithogenic magnetic mineral. Hematite’s presence in a sub-set of red-colored soils is confirmed by additional analyses of isothermal remanence acquisition curves. Two hematite coercivity components were identified, related to pedogenic and lithogenic origin, respectively.