Research paper Luminescence properties and optically (post-IR blue-light) stimulated luminescence dating of limnic sediments from northern Lake Malawi e Chances and limitations Annette Kadereit a, * , Regina DeWitt b , Thomas C. Johnson c a Geographisches Institut, Universität Heidelberg, Im Neuenheimer Feld 348, D-69120 Heidelberg, Germany b Department of Physics, East Carolina University, Howell Science Complex, Rm C-209, 1000 E. 5th Street, Greenville, NC 27858, USA c Large Lakes Observatory and Department of Geological Sciences, University of Minnesota Duluth, MN 55812, USA article info Article history: Received 12 October 2011 Received in revised form 17 February 2012 Accepted 22 February 2012 Available online 28 February 2012 Keywords: Lake Malawi Lake sediments Luminescence dating Dose rate uncertainties abstract Affected by the shifting Intertropical Convergence Zone, Lake Malawi holds an outstanding limnic sediment archive storing palaeo-environmental information of a climatically highly responsive landscape in the East African Rift Valley. Reliable chronological data are essential to interpret the lake-bottom sediments and optically stimulated luminescence (OSL) dating is an important method to build up the chronology. We report on material-property studies and OSL-dating results for polymineral fine grains extracted from piston cores obtained from the northern basin of Lake Malawi during the 1998 campaign of the International Decade of the East African Lakes (IDEAL). OSL-dating is based on a post-IR blue (pIRB) light stimulation single aliquot regeneration (SAR) protocol. Our results support earlier publications demonstrating the general applicability of OSL-dating on Lake Malawi sediments of Holocene and late Pleistocene age. However, the study also shows that dose rate estimations imply major challenges which need to be investigated in more detail to improve future OSL-dating results. Ó 2012 Elsevier B.V. All rights reserved. 1. Introduction Lake Malawi provides a tropical sedimentary archive of outstanding quality over 1 Ma of duration. The archive records the palaeo-environmental history of part of the East African Rift Valley, which is situated in the activity area of the Intertropical Convergence Zone (ITCZ) (Fig. 1). It is highly sensitive to climate variations controlled by orbital forcing and the southward expansion of the northern inland glaciers, leading to respective shifts of the ITCZ and therewith connected aridity or monsoonal moisture import over Lake Malawi (Scholz et al., 2006, 2007). Therefore, Lake Malawi became scientific object of the International Decade of the East African Lakes (IDEAL), the International Continental Drilling Project (ICDP) and the Lake Malawi Drilling Project (e.g. Scholz et al., 2006). To interpret the sediment archive, a reliable chronology is needed. Published age models are based mainly on radiocarbon ages for the period up to ca. 50 ka, for which a correction is suggested by sub- tracting 450 a prior to the calibration to calendar years (Johnson et al., 2002). A few isolated ages were obtained by 10 Be-, U/Th-, palaeomagnetic and luminescence dating for the period up to 130 ka (Scholz et al., 2007; Johnson et al., 2011). Luminescence dating is an ideal method to cover the relevant period, from recent decades (e.g. Ballarini et al., 2003) up to several hundred ka to 1 Mio a (e.g. Watanuki et al., 2005; Rhodes et al., 2006). Here we report on material-property studies and optically stimulated luminescence (OSL-) dating of sediments from northern Lake Malawi. Due to the small sample volumes of less than w3 cm 3 special attention was paid to (1) the grain sizes and dating properties of the available minerals; (2) the effective dose rates which are challenging to gain from small drilling-core samples; and (3) a changing water content usually decreasing with time as sediment compaction progresses. 2. Optically stimulated luminescence analyses Sample material, in cubes of w3 cm 3 for each sample, was provided from piston cores M98-1P and M98-3P of the 1998 IDEAL expedition (Table 1 , figure i Supplementary material). Due to the small sample sizes, dose rate determinations using low level gamma-spectrometry were performed in lightproof containers prior to any sample preparation for equivalent-dose (D E ) determi- nation. To obtain good counting statistics the small samples were measured for 8e17 days each. * Corresponding author. Tel.: þ49 0 6221 54 4385; fax: þ49 0 6221 54 4997. E-mail addresses: annette.kadereit@geog.uni-heidelberg.de (A. Kadereit), dewittr@ecu.edu (R. DeWitt), tcj@d.umn.edu (T.C. Johnson). 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. doi:10.1016/j.quageo.2012.02.021 Quaternary Geochronology 10 (2012) 160e166