Lake Ohrid is located at the border between North Macedonia and Albania (40°70′ N, 20°42′ E) and is assumed as the oldest lake in Europe. The lake with a surface area of 360 km² has trapped sediments and volcanic ashes over more than 1 million year and hence, contains essential information of major climatic and environmental change of the central northern Mediterranean region (Wagner et al., 2008). Seismic investigations indicate a sediment fill of the lake basin up to a thickness of 700 m. Several pre-site studies (e.g. multichannel seismic and shallow coring) have demonstrated the potential of Lake Ohrid to yield a complete climate record with continuous sedimentation and several tephra layers are promising to provide age control (Lindhorst et al., 2014; Vogel et al., 2010a; Wagner et al., 2014). In the frame of the ICDP project SCOPSCO (Scientific Collaboration on Past Speciation Conditions in Lake Ohrid), several scientific questions are addressed: age and origin of the lake, paleoclimatic change during the Quaternary, tephrostratigraphy, and driving forces for the outstanding biodiversity.
A deep drilling campaign was performed and four sites were multiple cored in spring 2013. The “Deep site” is located in the central deep basin of Lake Ohrid and was targeted for drilling operation to a depth of 569 m below lake floor and an overall core recovery of 95 % was achieved. Within two logging campaigns high-quality continuous downhole logging data were acquired by the use of the following tools: spectral gamma ray (SGR), magnetic susceptibility (MS), resistivity, dipmeter, borehole televiewer and sonic (vp).
Strong cyclicity is evident in Lake Ohrid’s pelagic sediment facies, whereas the signal is most pronounced in the total gamma ray and in the contents of potassium (and thorium). The data was compared with the widely used global climate reference record (LRO4-stack from benthic δ18O) (Lisiecki and Raymo, 2005, 2007). The time span which was tested for correlation, was framed based on current age estimates from tephrochronology on core material. These tephra layers could be identified clearly in the downhole logging data either by high values in potassium or magnetic susceptibility. After the anchor points from these tephra layers were set, the curves were correlated and a very similar cyclicity is evident. The data shows high correlation (R² = 0.75), whereas low gamma ray, potassium (K) and thorium (Th) correlate with interglacial periods and therefore glacial-interglacial dynamics can be read from these data easily (Fig. 3a).
To further investigate the cyclic characteristics of the data, spectral analysis was applied (sliding window method) and several emphasized wavelengths were observed. The distribution of the cycles is non-uniform over the dataset and a break in the spectral characteristics occurs at about 110 mblf (Fig. 3b). The high amplitudes were linked with orbital cycles and thereafter, a strong 100 ka cycle was observed which is in agreement with cyclostratigraphic studies of several sedimentary records for the past c. 900 ka (Berger and Loutre, 2010). Based on the distribution of the signals, sedimentation rates which range from 30 to 45 cm/ka were calculated. According to our calculations these rates (mean sedimentation rates) show a jump at 110 mblf and are apart from this constant over large parts of the sedimentary succession.