Luminescence chronology of the northeastern Bulgarian loess-paleosol sequences (Viatovo and Kaolinovo)

摘要

The key reference loess sequence of Viatovo in NE Bulgaria, lying within the lower Danube basin, records climatic and environmental changes over the last 800 ka. This 20 m thick loess sequence consists of seven loess units (L1 to L7) intercalated by six paleosols (S1 to S6) below the modern soil (SO). The Matuyama-Brunhes palaeomagnetic boundary has been identified in the lowest loess unit (L7). This reference loess sequence remains so far undated by radiometric techniques. Its chronostratigraphy relies only on pedostratigraphic and magnetostratigraphic evidence. This contrasts with the adjacent reference loess sequences of SE Romania (Tuzla, Mostistea, Mircea Voda) whose luminescence chronology is now securely established up to MIS 8.The aim of this study is to validate the chronostratigraphic framework of the Viatovo loess sequence using the luminescence dating method. For this purpose, the luminescence method is applied to the three upper loess units (L1, L2, L3) from two adjacent sites: the reference site of Viatovo and the nearby site of Kaolinovo, both located in the Ludogorie region (NE Bulgaria) and showing the same superposition of loess (L1 to L7) and interbedded paleosols. Luminescence dating (Infrared stimulated luminescence; IRSL) was carried out using the feldspar fine sand-size grain fraction (60-80 mu m), applying the IRSL50 and the pIRIR(290) dating techniques. For the equivalent dose (De) determination we used both the SAR post-infrared infrared (pIRIR(290)) protocol on single aliquots and the multiple-aliquot additive dose (MAAD IRSL50) protocol. The pIRIR(290) ages and the fading corrected MAAD-IRSL50, ages are self-consistent. It demonstrates for the first time, that the three upper loess layers (L1, L2, L3) at Viatovo and Kaolinovo were deposited during the marine isotope stages (MIS) 2-4, 6 and 8, thus constraining the youngest well-developed paleosol (S1, a chernozem) to the Last Interglacial. These IRSL50, and pIRIR(290) ages are in good agreement with the luminescence ages obtained for (1) the two upper loess layers (L1, L2) from the Harletz sequence in NW Bulgaria and (2) the three upper loess units (L1, L2, L3) from the adjacent SE Romanian loess sequences.The luminescence chronology of the loess sequences is presented along with the pedogenic and magnetic property data. The pedostratigraphic records of the NE Bulgarian and SE Romanian loess sequences show the same palaeoclimatic trends, with progressive aridification during interglacial periods (from S6 to S0).