Research paper Multiple dating of varved sediments from Lake q azduny, northern Poland: Toward an improved chronology for the last 150 years Wojciech Tylmann a, * , Dirk Enters b , Ma1gorzata Kinder a , Piotr Moska c , Christian Ohlendorf b , Grzegorz Pore ˛ ba c , Bernd Zolitschka b a Department of Geomorphology and Quaternary Geology, Institute of Geography, University of Gda nsk, Ba _ zynskiego 4, 80952 Gda nsk, Poland b GEOPOLAR, Institute of Geography, University of Bremen, Germany c Department of Radioisotopes, Institute of Physics, Silesian University of Technology, Gliwice, Poland article info Article history: Received 5 April 2012 Received in revised form 12 September 2012 Accepted 5 October 2012 Available online 23 October 2012 Keywords: Lake sediments Varve chronology Radionuclides Age-depth model Masurian Lake District OSL dating Sediment focusing abstract On two short sediment cores from Lake qazduny (northeastern Poland) different methods for age esti- mation were applied including varve counting, 210 Pb, 137 Cs and OSL dating. The investigated sediment consists of finely laminated, organic-rich calcareous gyttja interrupted by a sand layer. For 210 Pb we tested the CFCS, CIC, CRS and SIT models and compared the results with the established varve chronology and the 137 Cs distribution. The total 210 Pb activity distribution is consistent between cores and shows a regular decrease with depth. However, the total inventory of unsupported 210 Pb was significantly different and indicates varying rates of sediment focusing. Our results show that all 210 Pb dating models used in this study produced different results, while the SIT model performed best in comparison with varve counts and 137 Cs peaks. The CFCS model produced a realistic age-depth trend but obtained ages were too old compared to the varve time scale. The age estimates provided by the CRS model were too old as well with an increasing offset downcore and the CIC model failed completely by producing an unrealistic chronology with age inversions. This confirms that only systematic testing of different 210 Pb dating models and routine validation using independent time markers produces reliable 210 Pb chro- nologies. OSL dating of two samples from the sand layer provided much too old results which indicate incomplete bleaching during deposition of the analyzed quartz grains. This multiple dating approach demonstrates the considerable potential but also potential pitfalls of dating such young sediments. Ó 2012 Elsevier B.V. All rights reserved. 1. Introduction Modern lake sediments preserve a wide range of information about natural and man-made environmental change. Under favorable circumstances, lake sediments provide the potential to obtain a temporal resolution of one year or less resulting from sedimentation rates of 0.5 mm to several mm per year (Brauer, 2004). However, high-resolution records of environmental proxies are valuable only if a reliable time scale is established. Although precise dating of sediments is of crucial importance in all types of sedimentological studies, it is especially critical for high- resolution studies which require a considerably reduced uncer- tainty of age estimation. Only a precise age control allows the comparison and correlation of cores from different sites or even different types of archives. Dating of the most recent past presents significant challenges (Gale, 2009), which is also the case for lacustrine sedimentary archives. The best time control is available when annually lami- nated (varved) sediments provide an accurate chronology in calendar years. However, verification using independent dating methods is necessary to confirm the annual nature of lamination and the stratigraphic continuity of the record (Lamoureux, 2001; Zolitschka, 2003). The most widely used method for estimating the age of organic remains in lake sediments is radiocarbon dating. The time range covered by this method is theoretically up to about 50,000 years before present (BP), but there are substantial prob- lems in dating “modern” samples. This is related to anthropogenic changes of the radiocarbon concentration in the atmosphere and modern biosphere in the last centuries due to Suess effect and nuclear weapon tests effect (Reimer et al., 2004). As the accuracy of radiocarbon dating is very limited for sediments younger than 300 years, methods that employ short-lived radionuclides are best for modern sediments, for instance 210 Pb (T 1/2 ¼ 22.3 yr) and 137 Cs (T 1/2 ¼ 30.2 yr). The 210 Pb analysis may provide a continuous * Corresponding author. E-mail address: geowt@ug.edu.pl (W. Tylmann). Contents lists available at SciVerse ScienceDirect Quaternary Geochronology journal homepage: www.elsevier.com/locate/quageo 1871-1014/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.quageo.2012.10.001 Quaternary Geochronology 15 (2013) 98e107