The potential of Lake Ohrid for long-term palaeoenvironmental reconstructions

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Palaeogeography, Palaeoclimatology, Palaeoecology 259 (2008) 341 – 356 www.elsevier.com/locate/palaeo

The potential of Lake Ohrid for long-term palaeoenvironmental reconstructions Bernd Wagner a,⁎, Klaus Reicherter a , Gerhard Daut b , Martin Wessels c , Andreas Matzinger d , Antje Schwalb e , Zoran Spirkovski f , Mitat Sanxhaku g a

c

Institut für Geophysik und Geologie, Universität Leipzig, Talstrasse 35, D-04103 Leipzig, Germany b Institut für Geographie, Universität Jena, Löbdergraben 32, D-07743 Jena, Germany Institut für Seenforschung, Landesamt für Umweltschutz Baden-Württemberg, Argenweg 50/1, D-88045 Langenargen, Germany d Swiss Federal Institute of Aquatic Science and Technology (Eawag), Seestrasse 79, CH-6047 Kastanienbaum, Switzerland e Institut für Umweltgeologie, TU Braunschweig, Pockelsstrasse 3, D-38106 Braunschweig, Germany f Hydrobiological Institute, Naum Ohridski 50, MK-9600 Ohrid, Macedonia g Institute of Hydrometeorology, Durresi Street, 219, Tirana, Albania Received 28 September 2005; accepted 28 February 2007

Abstract Lake Ohrid, at the Macedonian/Albanian border, was likely tectonically formed during the Tertiary and therefore is one of the oldest lakes in Europe. However, only a few studies exist concerning the potential of Lake Ohrid sediments for long-term palaeoenvironmental reconstructions within the scope of future potential deep-drilling campaigns. Therefore, as a first step, a transect of short surface sediment cores was investigated for chronology, physical properties, grain size, and biogeochemistry. The results were compared with information derived from a shallow hydro-acoustic seismic survey. The investigations indicate a rather uniform and bioturbated sedimentation in the central part of the lake basin with mean sedimentation rates of ca. 0.5–1 mm/year. The sediment composition is dominated by authigenetic carbonates. Diatom frustules or fragments form the major part of biogenic matter deposits, as indicated by the relatively high contents of biogenic opal and low contents of total organic carbon and total nitrogen. The shallow hydro-acoustic seismic survey indicates that horizons of sediment redeposition occur sporadically. Towards the shore of the lake, the sedimentation rate increases and sedimentation is increasingly influenced by local inflows or massmovement processes triggered by tectonic activities. Thus Lake Ohrid has a high potential for palaeoenvironmental reconstructions on a multi-decadal scale and provides additional information concerning tectonic activity in the region. © 2007 Elsevier B.V. All rights reserved. Keywords: Lake Ohrid; Surface sediments; Shallow-seismic survey; Tectonic activity

1. Introduction

⁎ Corresponding author. Present address: Institute of Geology and Mineralogy, University of Cologne, Zülpicher Strasse 49a, D-50674 Köln, Germany. Fax: +49 221 470 5149. E-mail address: [email protected] (B. Wagner). 0031-0182/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.palaeo.2007.10.015

Lake Ohrid has been the subject of numerous limnological, biological, and biogeographical studies. The lake accommodates more than 200 endemic species (e.g., Jerkovic, 1972; Kenk, 1978; Decraemer and Coomans, 1994; Michel, 1994). This high number of

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endemic species, mainly invertebrates and algae, but also with some fishes, indicates that the overall environmental conditions have been relatively stable over a long period. The biological and biogeographical studies, particularly the high degree of endemism, for the first time compiled extensively in Stankovic (1960), suggest that the origin of Lake Ohrid dates back to Pliocene times, about 3 to 5 million years ago. Taking this age estimation as correct, Lake Ohrid is certainly one of the oldest lakes in Europe and one of the few lakes in the world that date back to the Tertiary (e.g., Meybeck, 1995). Hence Lake Ohrid is an interesting target for palaeoclimatic and palaeoenvironmental research, because lakes act as continental sedimentary basins and thus normally contain continuous sedimentary records of high temporal resolution. However, investigations of the sedimentary record of Lake Ohrid in order to reconstruct the regional climatic and environmental history are sparse. Gravity cores containing near-surface sediments revealed weak eutrophication in recent times (Matzinger et al., 2007). The only data available from longer sediment records originate from an 8.85 m long sediment sequence recovered in 1973 (Roelofs and Kilham, 1983). This core is supposed to represent the past ca. 30,000 years, however, uncertainties in the age-depth correlation of the core complicated the interpretation of the proxies investigated, mainly the contents of water, organic matter, CaCO3, and diatom stratigraphy. The uncertainties in the age-depth correlation can probably be explained by disturbed sedimentation in the core as a result of mass-movement processes due to tectonic activities, which are known to be common in the region (e.g., Aliaj et al., 2004). Because of its presumed great age, Lake Ohrid could present a valuable link between climatic and environmental records from the Mediterranean Sea and the adjacent terrestrial areas. In the eastern Mediterranean Sea, most records focus on the Late Pleistocene and Holocene history (e.g., Geraga et al., 2005) and only a few cover several glacial–interglacial cycles (e.g., Schmiedl et al., 1998; Howell et al., 1998). Similarly, most records from the northern terrestrial region are restricted to the Late Pleistocene or Holocene (e.g., Denèfle et al., 2000; Ramrath et al., 2000; Schmidt et al., 2000; Sadori and Narcisi, 2001); longer continuous records are relatively sparse (e.g., Wijmstra, 1969; Ryves et al., 1996; Brauer et al., 2000). In order to evaluate the potential of Lake Ohrid for palaeoenvironmental reconstructions, surface-sediment cores were taken from different locations in the lake basin and a shallow-seismic survey was carried out

along several profiles. The surface sediments were intended to be used to obtain information about the recent sedimentation and to distinguish between local and regional effects in the lake basin. The shallow-seismic survey provides information about the long-term sediment accumulation and its dependence on sediment supply and basin morphology. In addition, the survey can be used to indicate areas of tectonic activity and sediment redeposition, which would hamper palaeoenvironmental reconstructions. 2. Setting Lake Ohrid (40°54′–41°10′ N, 20°38′–20°48′ E, Fig. 1) is located at an altitude of 693 m a.s.l., and is shared between the Republics of Macedonia and Albania. The lake has a length of ca. 30 km, a width of ca. 15 km, and covers an area of 360 km2. The basin morphology is relatively simple and maximum water depth is 286 m (e.g., Stankovic, 1960). Lakes with comparable volumes and water depths, located in temperate and Mediterranean regions, are in general monomictic with a mixing of the water column during winter. Complete overturn of the water column in Lake Ohrid occurs very irregularly, roughly every 7 years (Stankovic and Hadzisce, 1953; Hadzisce, 1966). Recent investigations have shown that in years without complete overturn the water column is mixed down to depths of 125 to 175 m in winter, creating a partly isolated water body below the mixing depth. Slow exchange with the overlying water by weak turbulence leads to upward transport of dissolved salts and retains oxic conditions in the irregularly mixed bottom layer (Matzinger et al., 2006b). Today, Lake Ohrid has an oligotrophic status with a Secchi disc transparency of more than 14 m (Stankovic, 1960; Ocevski and Allen, 1977; Naumoski, 2000). The oligotrophic conditions in Lake Ohrid are supported by inflows, which are depleted in mineral suspensions and nutrients. Most of the inflow to the lake is supplied by groundwater from more or less abundant karstic sources in the relatively small natural catchment area of 1042 km2, which was artificially enlarged to 1487 km2 in 1962 (Matzinger et al., 2006b). However, the effective size of the catchment is substantially larger, because several surface springs and likely also some sub-aquatic inflows into Lake Ohrid are supplied from Lake Prespa, located 20 km to the east, 150 m above Lake Ohrid and separated by a mountain range (Stankovic, 1960; Anovski et al., 1980; Matzinger et al., 2006a; Fig. 1). The only surface outflow of Lake Ohrid is the river Crni Drim in the northern part of the

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Fig. 1. Map of the northern Mediterranean region with location of Lakes Ohrid and Prespa at the Macedonian/Albanian border.

lake, which accounts for 63% of the water loss, with the remaining 37% accounted for by evaporation (Watzin et al., 2002). Groundwater outflow cannot be completely excluded, but has not been observed so far. The theoretical water residence time is estimated to be ca. 70–85 years, which is long in comparison with other European lakes. The climate in the Lake Ohrid watershed is influenced by the proximity of the Adriatic Sea, by the surrounding mountains, and by the thermal capacity of Lake Ohrid itself (Watzin et al., 2002). Average monthly temperatures range from ca. 26 °C during summer to −1 °C during winter. Precipitation is lowest during summer, but varies significantly depending on the geographical position. Rainfall ranges from ca. 600 to 1450 mm/year in the watershed, and averages around 750 mm/year at shoreline stations along Lake Ohrid. Main wind directions are controlled by the shape of the lake valley and, thus, are dominantly northerly or southerly. Both Hercynian and Alpine orogenies left their imprint on the terrain in the Lake Ohrid watershed. The bedrock constitution is made up of various lithologies from Palaeozoic to Cenozoic age (Watzin et al., 2002). Palaeozoic metamorphic and magmatic

rocks form the country rock of the entire western Macedonian Zone. Triassic carbonates and clastics are widely exposed to the southeast and northwest of the lake. The carbonates are intensely rugged, broken, and karstified. The Ohrid basin, including the lake in its center, was formed during a later phase of the Alpine orogeny. Cenozoic sediments include Pliocene and Quaternary deposits and are particularly exposed to the southwest of the lake. The Pliocene–Quaternary period is characterized by enhanced and progressive uplift in Albania. Most data in the world stress map (Reinecker et al., 2004) point to SW–NE directed extension and normal faulting. Lake Ohrid is situated in a graben, which extends from the Korça plain (after the city of Korça) to north of Lake Ohrid (Aliaj et al., 2001; Fig. 2). The graben formed during extension, which has affected the entire interior of Albania since Pliocene times. The Ohrid– Korca seismic zone comprises the Pliocene–Quaternary normal-fault-controlled Ohrid graben, and the Korça and Erseka half-grabens, which are generally northtrending (Aliaj, 2000; Aliaj et al., 2004). Active normal faulting with horst and graben structures is seen in the geomorphology and also determined from earthquake

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Fig. 3. Seismic profiles from 2004 and coring locations of surface sediments taken between 2002 and 2005. Bold lines (A–C) indicate seismic profiles shown in Figs. 8–10. The stars indicate the locations of the Saint Naum (N) and Tushemisht (T) springs, suppliers of ground water from Lake Prespa to Lake Ohrid.

focal mechanisms (Aliaj et al., 2001; Goldsworthy et al., 2002). Other sets of morphological lineations, directed NW–SE, ENE–WSW, and NNE–SSW, are also very prominent (Fig. 2A). Lake Ohrid is framed in the west by N–S trending lineations, and in the east by sets of lineations with N–S and NNE–SSW trending directions. The lake is surrounded by mountains reaching approx. 1500 m to the west (“Mokra Mountain” Chain) and more than 1750 m to the east (“Galiçica Mountain” Chain, Fig. 2B). This relief, accompanied by several escarpments, together with the simple and relatively straight shorelines of the lake, suggests active faulting accompanied by significant uplift.

The central mountain chain, i.e., the internal zone of Albania or the Peshkopia–Korça zone, especially the intramontane basins of Late Neogene age, forms one of the most active seismic zones in Albania/Macedonia with several moderate earthquakes reported during the last few centuries (Muço, 1994, 1998; Sulstarova et al., 2000; Muço et al., 2002; NEIC database, USGS). Major earthquakes occurred during historical times (Ambraseys and Jackson, 1990; see also Goldsworthy et al., 2002). The last prominent earthquake took place on 18 February 1911 close to Lake Ohrid basin (M = 6.7, corresponding to European Macroseismic Scale X; 15 km depth, 40.9° N, 20.8° E). Hypocenter depths

Fig. 2. (A) Shaded relief of the Lake Ohrid area (based on Shuttle Radar Topographic Mission data, light from east). Morphological lineations (white lines) frame the lake. Note locality of topographic section (X–X′). (B) Topographic section in E–W direction (X–X′ in Fig. 2A). Note active normal faulting on both sides of Lake Ohrid, responsible for the very steep morphology along the N–S trending coastlines. (C) Simplified geological map of the Lake Ohrid and Lake Prespa region (modified from the geological maps of Albania 1:200,000 and Yugoslavia 1:500,000, sheet Sarajevo).

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Table 1 Sediment cores from Lake Ohrid with year of recovery, water depths at the coring locations, lengths, analyses performed, and related sample intervals Core

Lz1120

Lz1083

OHR02-1

Lz1084

Lz1085

OHR03-2

OHR03-1

Lz1086

Recovery in Water depth (m) Length (cm)

2005 105 108

2004 270 73

2002 282 77

2004 250 120

2004 232 48

2003 202 52

2003 122 53

2004 85 62

Analyses Magnetic susceptibility Grain size Water content Total carbon (TC) Total nitrogen (TN) Total sulphur (TS) Total organic carbon (TOC) Total inorganic carbon (TIC) C/N ratio Biogenic opal 14 C dating 137 Cs dating 210 Pb dating

2 mm 2 cm 2 cm 2 cm 2 cm 2 cm 2 cm 2 cm 2 cm – – – –

1 mm – 2 cm 2 cm 2 cm 2 cm 2 cm 2 cm 2 cm – – – –

– – 1–3 cm 1–3 cm 1–3 cm – 1–3 cm 1–3 cm 1–3 cm 7 samples – 13 samples 13 samples

1 mm 2 cm 2 cm 2 cm 2 cm 2 cm 2 cm 2 cm 2 cm 2 cm 3 samples – –

1 mm – 2 cm 2 cm 2 cm 2 cm 2 cm 2 cm 2 cm – – – –

– – 0.5–2 cm 0.5–2 cm 0.5–2 cm – 0.5–2 cm 0.5–2 cm 0.5–2 cm – – – –

– – 0.5–3 cm 0.5–3 cm 0.5–3 cm – 0.5–3 cm 0.5–3 cm 0.5–3 cm – – – –

1 mm – 2 cm 2 cm 2 cm 2 cm 2 cm 2 cm 2 cm – – – –

generally scatter between 10 and 25 km in the seismogenic zone (Muço, 1994, 1998; Sulstarova et al., 2000; Muço et al., 2002; NEIC database, USGS). The Ohrid– Korça zone is considered to be the region of the highest seismic hazard in the Albanian–Macedonian corridor based on present-day seismicity (Sulstarova et al., 2003; Aliaj et al., 2004). 3. Materials and methods 3.1. Core recovery Between the years 2002 and 2005 near-surface sediment cores were recovered at different locations in the lake basin (Fig. 3) using a MARKASUB and an UWITEC gravity corer (Table 1). Both corer systems are equipped with PVC liners of 6 cm in diameter. The penetration depth is controlled by both the weight and the level of release above the sediment surface and is limited by the length of the liner to a maximum of 120 cm. After recovery, the sediment surface was stabilized with a sponge and the liners were closed at the upper and lower ends with plastic caps and tape. The cores were stored in the dark and cold until further processing in the laboratory. 3.2. Analytical work Core description and photographic documentation were carried out immediately after core opening in the laboratory. One of the core halves was used to measure magnetic susceptibility in 1 or 2 mm steps using a

Bartington system with a high-resolution MS2E sensor, before the core half was archived for future work. Subsamples were taken continuously at 0.5–3 cm intervals (Table 1) from the other core half. Aliquots of these subsamples were freeze-dried and their water contents determined by the loss of weight. For grain-size analyses, 10 ml 15% H2O2 was added to 2 g of fresh sediment in a beaker. After the reaction stopped, 0.3 g of Na4P2O7 was added to the suspension, which was then boiled. Subsequent to settling of the suspension the supernatant water was removed and the sediment homogenized using a spatula. 30 s of ultrasonic treatment avoided flocculation prior to detection. Sample material was then transferred into a Micromeritics DigiSizer 5200 laser-diffractometer until an obscuration of 10% was reached. The DigiSizer 5200 has a 1280 × 1024 pixel CCD for the detection of light scattered by the sample. The signal was deconvoluted using the Mie theory (Loizeau et al., 1994; Wen et al., 2002) and transformed into 160 size classes between 0.1 and 1000 μm. Two runs were made with each sample and an average calculated. Calculations of grain-size parameters and statistics were made using the program gradistat (Blott and Pye, 2001). Fresh sediment was also used for separating ostracode valves. After pre-treating the sample with NaHCO3 and water, the ostracode valves were separated by sieving with a 63 μm mesh size. Aliquots of the freeze-dried samples were ground to b 63 μm, homogenized, and used for biogeochemical analyses. Contents of total carbon (TC), total nitrogen (TN), and total sulphur (TS) were measured with

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combustion CNS elemental analyzers (VARIO Co. and EuroVector Co.). In cores recovered in 2002 and 2003 (Table 1), total inorganic carbon (TIC) was measured by infrared absorption of CO2 after acidifying the samples with 3M HCl (Skoog et al., 1996). Total organic carbon (TOC) was calculated from the difference in TC and TIC. In cores recovered in 2004 and 2005, total organic carbon (TOC) contents were measured with a Metalyt CS 1000S (ELTRA Co.) analyzer, after pre-treating the sediment with 10% HCl at 80 °C in order to remove carbonate. Total inorganic carbon (TIC) was calculated from the difference in TC and TOC. Biogenic silica (opal) was measured according to the wet chemical method described by Müller and Schneider (1993).

thus exponential decrease with depth to background activity (Appleby and Oldfield, 1978). Radiocarbon dating of core Lz1084 was conducted by accelerator mass spectrometry (AMS) at the Leibniz Laboratory for Radiometric Dating and Isotope Research in Kiel, Germany. Since macrofossil remains were not present in sufficient amount throughout the core, three sediment samples were selected for bulk TOC dating. However, the dating of bulk TOC in general is a relatively imprecise method to establish a robust chronology. Therefore, reservoir correction and calibration into calendar years of the obtained 14C ages were not carried out.

3.3. Dating

In order to observe and compute bathymetric data together with information concerning the sedimentary architecture at the lake floor, hydro-acoustic methods were used in a shallow seismic survey. At Lake Ohrid, a parametric sediment echosounder (SES-96 light, INNOMAR Co.; www.innomar.com) was used, with the transducer mounted on the sides of local Albanian and

For the dating of the surface sediments in core OHR02-1, 210 Pb and 137 Cs activities were established from γ-counting in Ge–Li borehole detectors (Håkanson and Jansson, 1983). Sedimentation rates were calculated from 210Pb assuming constant rate of supply and

3.4. Shallow seismic survey

Fig. 4. Magnetic susceptibility, water content, grain-size distribution, biogeochemical proxies, and ostracod abundances of core Lz1084 from the central lake basin. Black dots to the right indicate horizons of bulk TOC radiocarbon dating and related ages.

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Macedonian research vessels. The SES-96 light system is a very-high-resolution sediment sub-bottom profiler that takes advantage of the parametric effect (Grant and Schreiber, 1990; Spieß, 1993; Wunderlich and Müller, 2003). Sinusoidal pulses are emitted simultaneously at two different frequencies. Due to the parametric effect a narrow beam signal is produced at the difference frequency. The portable SES-96 light system operates with effective frequencies between 4 and 12 kHz, which are generated by two primary frequencies near 100 kHz. Because of the narrow emitting cone the generation of diffraction hyperbolae is generally avoided. However, slopes inclined more than 2° do not reveal any interpretable information due to reflection signal loss. The data were digitally stored, not compensated for heave and roll. Post-processing was carried out with the INNOMAR software tool ISE 2.5. The sound velocity in the water was taken as 1440 m/s. During the shallow seismic survey in spring 2004 approx. 280 km of sub-bottom profiles were obtained.

Table 2 Radiocarbon ages determined on bulk TOC from core Lz1084, Lake Ohrid. The surface sediment age most likely represents the recent hard water effect Sample KIA25877

Depth (cm)

Material

0–2

C weight δ13C (mg) (‰)

Bulk (humic 1.5 acid free) KIA25878 66–68 Bulk (humic 2.5 acid free) KIA25879 116–118 Bulk (humic 5.8 acid free)

14

C age (year BP)

− 29.35 1560 ± 30 − 31.06 3030 ± 30 − 30.68 3270 ± 40

The profiles are oriented mainly north–south and east– west with a grid distance of maximum 4 km (Fig. 3). Navigation along the profiles was managed using GPS mapping software OziExplorer. The position was recorded by two GPS (GARMIN Co.), one for navigation and one linked to the seismic system. 4. Results and discussion 4.1. Surface sediments

Fig. 5. SEM-photos of bulk sediment at a depth of 112–114 cm in core Lz1084. (A) indicates that large authigenetic carbonates are deposited in Lake Ohrid sediments. A valve of Cyclotella ocellata is shown in the upper left corner. (B) shows that diatoms, i.e., Stephanodiscus alpinus (in front) and Cyclotella ocellata (in the background and fragments), contribute significantly to the sediment composition. Diatoms were determined by Holger Cremer.

Core Lz1084 from the central, deep basin of Lake Ohrid (Fig. 3) is 120 cm long and is mainly composed of fine-grained minerogenic matter of light brownish to greyish color. A few (not very significant) color changes most likely document minor changes in sedimentary or post-sedimentary conditions. The lack of lamination can probably be explained by bioturbation by organisms, which live at or in the uppermost sediments (Stankovic, 1960) and have been observed during the field campaigns even in the deepest part of the lake. Measurements of physical properties, including grain-size and biogeochemical composition, revealed an overall rather homogeneous sediment sequence with only few significant changes. Magnetic susceptibility varies between ca. 150 and 1200 × 10− 5 SI and shows a minor peak at 78 cm depth and a more distinct peak at 102 cm depth (Fig. 4). These peaks correspond with minima in the water content, which is relatively low throughout the core (50–67%), and also with minima in the mean grain sizes. The grain-size data support the macroscopic observation that fine sediments dominate. Minima with mean grain sizes of less than 15 μm are recorded below 92 cm and above 18 cm depth, whereas coarser sediments with values of more than 15 μm occur in between. TOC values of less than 2.5% and TN values of less than 0.32% indicate that the organic matter content in the sediment is low. TOC/TN ratios of less than 8 suggest that autochthonous organic matter

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dominates (Meyers and Ishiwatari, 1995), which can be explained by the distal coring location in the large oligotrophic lake and low input of allochthonous TOC, respectively. However, TOC/TN minima of less than 5 imply that the organic matter is affected by bacterial decomposition and loss of TOC. The contents of biogenic opal, ranging between about 4 and 12%, indicate that diatoms contribute significantly to the amount of organic matter preserved in the sediment (Fig. 5). Calcareous fossil remains, such as ostracode valves, have been observed only sporadically in a very few levels (Scharf, pers. comm.; Fig. 4). TS does not correlate with TOC and TN and this suggests that formation by sulphides rather than by organic matter. TIC contents vary between ca. 2 and 7%. Assuming that TIC mainly represents CaCO3, the proportion of CaCO3 from the total sediment composition can be calculated to ca. 17 to 58%. Given the low amount of calcareous fossils, most of the carbonates originate either from allochthonous sources or, more likely, are formed authigenetically by precipitation (Fig. 5). Radiocarbon dating of bulk TOC in core Lz1084 revealed a surface sediment age of 1560 14C year BP (Table 2; Fig. 4). This age is most likely due to a significant hard-water effect primarily caused by the input of old dissolved carbonate from the karstic environment. At 66–68 cm sediment depth an age of 3030 14 C year BP was determined, and at 116–118 cm an age of 3270 14C year BP (Fig. 4). Assuming that all radio-

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carbon ages obtained are correct, they imply either a distinct shift in the hard-water effect or significant changes in sedimentation rates over time. A plausible reason for a shift in the hard-water effect could be a significant change in sediment supply from the catchment, however, such lacks further evidence. Assuming that the hard-water effect has remained relatively constant during the period recovered, a much higher sedimentation rate can be inferred for the lower part of core Lz1084. The relatively high sedimentation rate in the lower part could be due to redeposited material from mass movement processes or, less likely, dispersed admixture of volcanic ashes originating from Italy or Greece. Volcanic ashes from these regions are common in other marine and terrestrial sediment cores from around the northern Mediterranean region (e.g., Wulf et al., 2004), but have not been observed macroscopically in core Lz1084. Nevertheless, the admixture of volcanic ashes could explain the significant peak in magnetic susceptibility in the lower part of the core (Fig. 4). The low sedimentation rate in the upper part of core Lz1084 is apparently confirmed by 137Cs and 210Pb dating on core OHR02-1 (Fig. 6). Assuming that the maximum of 137Cs activity at ca. 2.5 cm depth corresponds with the maximum of nuclear weapons tests in 1962/1963, a sedimentation rate of 0.5 mm/year can be calculated for the surface sediments. Assuming that this maximum corresponds with the Chernobyl peak from 1986 and the 1962/1963 peak is too weak to be detected,

Fig. 6. 137Cs and 210Pb activities in the surface sediments of core OHR02-1. The dashed line indicates a linear sedimentation rate of 1.0 ± 0.2 mm/year for the whole dataset; the full line indicates a linear sedimentation rate of 0.9 ± 0.2 mm/year if one neglects the uppermost 1.5 cm of recently bioturbated surface sediments.

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Fig. 7. Magnetic susceptibilities (MS, light line) and total inorganic carbon (TIC, heavy line) contents of surface sediment cores from Lake Ohrid. The cores are located along a S–N transect; for locations see Fig. 3. The grey dashed lines indicate core correlations based on TIC contents, the black lines those based on magnetic susceptibilities. Cores correlate relatively well in the central basin.

sedimentation rate can be calculated to ca. 1.3 mm/year. However, because 137Cs can migrate in the sediment column and bioturbation likely has occurred in core OHR02-1, an unambiguous sedimentation rate cannot be calculated on basis of the 137Cs dating. Sedimentation rates calculated on the basis of the 210Pb activities are ca. 1.0 ± 0.2 mm/year taking all samples measured, and 0.9 ± 0.2 mm/year if one neglects the uppermost 1.5 cm of recently most bioturbated sediments, where 210 Pb values were found to be constant (Fig. 6). However, bioturbation can reach much deeper (Håkanson and Jansson, 1983), which precludes more reliable interpretation of the 210Pb dating. Overall, a sedimentation rate of between ca. 0.5 and 1 mm/year seems to prevail in the central part of Lake Ohrid. These results are consistent with the pre-Holocene sedimentation rate that has been calculated for the lower part of a piston core studied by Roelofs and

Kilham (1983). Unfortunately, the top of this core, containing the Holocene sediments, apparently was not completely archived and thus cannot be used for comparison. The intra-basin comparison of surface sediments, based on the TIC contents and magnetic susceptibilities of eight gravity cores, reveals that sedimentation rates and sediment composition are relatively uniform in the central basin (Fig. 7). Towards the northern shore, the sediment composition is similar to the central basin, but the sedimentation rates seem to increase. In contrast, coring location Lz1120 close to the southern shore has a differing sediment composition, indicated by the relatively low TIC contents and slightly higher magnetic susceptibilities. Such complicates a comparison of the sedimentation rates with those from the other sites (Fig. 7). The differences in sediment composition in core Lz1120 can be explained by the influence of inlets

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close to the coring site (Fig. 3). The Saint Naum springs are fed for ca. 40% with water originating from Lake Prespa (Anovski et al., 1980; Eftimi and Zoto, 1997; Matzinger et al., 2006a), and they form one of the major tributaries to Lake Ohrid. Although a nutrient supply from the eutrophic Lake Prespa via this hydraulic conection can be widely neglected today (Matzinger et al., 2006a), its changes may have affected sedimentation at site Lz1120 in the past. For example, changes in nutrient supply would affect primary productivity and, thus, formation of authigenetic carbonates. Additional influence comes from the nearby inlet of Cerava River (Fig. 3) that may have contributed changing amounts of clastic and organic matter. The proximity of the Saint

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Naum and Tushemisht springs and the Cerava River may also have affected the mixing conditions of the water column or other hydrologic conditions at location Lz1120, which also control, for example, carbonate precipitation and organic matter preservation. As a result, coring site Lz1120 seems to be most affected by local influences amongst all investigated sites. 4.2. Shallow seismic survey A main focus of the very-high-resolution seismic profiling was to characterize the relationship between morphology of the lake floor, tectonic activity, and sedimentation. The results were needed to find adequate

Fig. 8. Hydro-acoustic seismic line (profile A in Fig. 3) and interpretation (VE = vertical exaggeration). The steep eastern coastline of Lake Ohrid is due to intense normal faulting with multiple sets of faults. A small roll-over anticline developed, with secondary normal faults in the crestal region. A thick relatively young debris flow forms a homogenite layer basinward (shaded), limited by an antithetic normal fault.

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Fig. 9. Hydro-acoustic seismic line (profile B in Fig. 3) and interpretation (VE = vertical exaggeration). The steep eastern coastline of Lake Ohrid is due to intense normal faulting with multiple sets of faults. Small grabens and halfgrabens developed with secondary normal faults, indicating partly synsedimentary tectonics. Note a thick relatively undisturbed sedimentary sequence basinward.

drilling sites for future palaeoclimatic studies with undisturbed and continuous sedimentary sequences, characterized by pelagic sedimentation and by flatlying and parallel reflectors. Future coring and dating of mass movement deposits and unconformities identified in seismic profiles can shed light on the neotectonic activity in the region. In the survey carried out in 2004, sub-surface information from depths reaching as much as 40 to 50 m of penetration was obtained. The parametric echosounding data are interpreted similarly to high-resolution seismic sections (e.g., Spieß, 1993). Penetration of the lake floor is strongly dependent on lithological and physical properties such as grain size and reflectivity. Folded, warped, or displaced reflectors indicate typical tectonic

structures in the profiles. Furthermore, sedimentary patterns such as pinching-out of strata, unconformities, stratal onlap patterns, and foresets can be observed. Homogenites formed by sub-lacustrine debris flows are characterized by chaotic to absent sediment layering and hummocky patterns, commonly with erosive bases. The hydro-acoustic/sedimentary data were analyzed with respect to structural elements that have been recently active and with respect to secondary evidence mass movements. Multiple sets of faults (Figs. 8 and 9) indicate that the steep eastern slopes of Lake Ohrid were formed by intense normal faulting. Small roll-over anticlines developed in the hanging wall with steep secondary normal faults near the crest of the scarp (Fig. 8) suggest a listric major fault. Calculating the true dip of

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Fig. 10. Hydro-acoustic seismic line (profile C in Fig. 3) and interpretation (VE = vertical exaggeration). The steep western coastline of Lake Ohrid is due to intense normal faulting along one major fault. A small halfgraben developed in the hanging wall, indicating partly synsedimentary tectonics. Note a thick relatively undisturbed sedimentary sequence basinward.

the faults, taking into account the vertical exaggeration, results in a fault dip of about 40–50° towards the west. This dip matches the observation of the topographic section (Fig. 2), where the active escarpments on both shores of Lake Ohrid are dipping 40–50°. A thick and relatively young debris flow forms a homogenite layer basinward, which pinches out towards the west and the

east. This layer suggests a flow in a northern or southern direction, with the eastern basin margin probably being the source area of the debris flow (Fig. 8). The western end of the homogenite is bound by a normal fault, antithetic to the eastern margin fault and may be of compactional origin due to loading by the sub-lacustrine debris flow.

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Some kilometers south of the section in Fig. 8, a parallel section exhibits a different picture (Fig. 9). Here, the steep eastern sub-lacustrine slope of Lake Ohrid is also cut by multiple sets of faults. However, small grabens and half-grabens developed between secondary normal faults, indicating partly syn-depositional tectonics. A thick, relatively undisturbed sedimentary sequence follows basinward with no indications of mass movements. The western shore of Lake Ohrid is defined by an active normal fault (Fig. 10). A small half-graben developed in the hanging wall, indicating partly syndepositional tectonics. The bathymetry indicates that the small half-graben is deeper than the adjacent thick undisturbed sedimentary sequence (i.e., the Ohrid basin), perhaps indicative of young tectonic processes. 5. Interpretation The central part of Lake Ohrid exhibits a relatively uniform sedimentation and is dominated by fine minerogenic matter. The minerogenic matter is mainly formed by carbonates, as indicated by a relatively high TIC content and relatively good correspondence with grain-size distribution (Fig. 4). Assuming that a significant proportion of the carbonates is of authigenetic origin, increased amounts of TIC in the central lake basin sediments could reflect periods of increased productivity in the lake. Diatoms are well preserved in the surface sediments and likely form the major part of primary production. The occurrence of Cyclotella is particularly characteristic for oligotrophic lakes with neutral to slightly alkaline conditions (Wetzel, 1983). Accordingly, the amount of biogenic silica could form a valuable proxy for past primary production. Calcareous fossils, such as ostracods, occur only sporadically in the surface sediments of the central basin, where the water column is probably depleted in nutrients due to the great distance from the shore. Additionally, the bottom-water conditions are probably unfavourable for the preservation or growth of benthic calcareous organisms and/or high abundances of ostracods. Despite the fact that low TOC and TN values suggest low primary productivity typical of an oligotrophic lake (Ocevski and Allen, 1977) and that the low TOC/TN ratio documents the dominance of autochthonous organic matter, TOC or TN values can hardly be used for interpretation of past primary production, because they are most likely affected by bacterial decomposition. Because the amounts of TOC and TN do not correlate with the amount of TS and because at present overturn of Lake Ohrid occurs only every few years, changes in the TS content could

be indicative of changes in the redox conditions of the bottom waters or sediments rather than of changes in organic matter accumulation. Significant changes in the grain-size composition or biogeochemistry apparently have not occurred in recent times. Such can be explained, at least partly, by bioturbation, which is common throughout all recovered sediment sequences. This uniformity might also indicate that the overall environmental conditions in Lake Ohrid have been relatively stable in the recent past, which is not surprising given the large volume of the lake. Indications of drastic hydrologic shifts or changes in clastic sediment supply are lacking. Bioturbation prevents the sediments of Lake Ohrid from being used for palaeoenvironmental reconstructions with annual resolution. With respect to the sedimentation rate, supposed to range between ca. 0.5 and 1 mm/year in the central basin, decadal-scale (20–40 years?) resolution of the information seems to be possible. The establishment of a reliable chronology, using different dating techniques, is indispensable for future work. Although significant changes of the hard water effect are rather unlikely over short periods, they likely have occurred in the long-term context. Therefore, in order to obtain a more precise chronology, radiocarbon dating of bulk TOC should be avoided and macrofossils should be used wherever possible. A control of the radiocarbon ages obtained can probably be given by tephrochronology, if significant tephra layers can be determined in longer sediment sequences. Towards the shore of the lake basin, the sedimentation is increasingly influenced by local phenomena, such as inflows that cause an increase of the sedimentation rates and/or affect sediment composition either directly via their suspension load or indirectly by changes of the hydrologic conditions in the lake. Additionally, sedimentation in these parts of the lake is particularly affected by the steep slopes, which facilitate mass movements. This effect is evidenced in the seismic profiles. The very-high-resolution hydro-acoustic profiles of Lake Ohrid demonstrate well the interplay between sedimentation and active tectonics. Due to relatively constant sedimentation rates the tectonic block movements can be reconstructed mainly along normal faults on the east and west coasts of the lake. Multiple sets of normal faults have their imprint on the structural style of the lake borders, partly with roll-over anticlines or back-tilted half-grabens. Multiple debris flows form homogenite layers of different age and extension mainly in the vicinity of the steep lake margins or the faults, but additionally also in the center of the lake (cf. Fig. 8). We infer that the sediments in lateral parts of Lake Ohrid are

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heavily influenced by frequent earthquakes, which have at least partly initiated mass wasting in the lake or seismite formation within the basin. One of the future tasks in the area should be a detailed mapping of the faults, their activity, and the accompanying effects of debris flows (e.g., are there coeval events?) to contribute to a hazard assessment in the Albanian/Macedonian corridor. In summary, Lake Ohrid has a high potential to provide evidence for reconstructing past climatic and environmental changes. High-resolution information in terms of annual climatic changes is likely masked by bioturbation and the large size and volume of the lake, thus forming a buffer for short-term events. However, long-term environmental changes are likely well recorded in the central part of the lake, making it a valuable link between marine records from the Mediterranean Sea and other long-term records onshore. Additionally noteworthy is that tectonic activities are recorded in the near-shore sediments, where faults and sediment redeposition are common in some areas. This tectonism provides new information concerning the geological history and setting of the region, but also has a distinct impact on the selection of suitable locations for future coring campaigns. Acknowledgements The project was funded by the Deutscher Akademischer Austausch Dienst (DAAD), the Deutsche Forschungsgemeinschaft (DFG; WA2109/1-1), the Swiss State Secretariat for economic affairs (seco), and the Swiss National Science Foundation (2000-067091.01). The SEM pictures were made by Viola Burkhardt, Institut für Seenforschung, Langenargen/Lake Constance. Gjeto Bushi, Zoran Brdarovski, and Boris Cakalovski are thanked for discussion and for their help during fieldwork. Martin Melles provided valuable comments on the manuscript. Ian Lerche kindly improved the English. Special thanks are due to the reviewers Steve Colman and Christian Beck. References Aliaj, S., 2000. Neotectonics and seismicity in Albania. In: Meco, S., Aliaj, S., Turku, I. (Eds.), Geology of Albania. Gebrüder Borntrager, Berlin. Beitrage zur regionalen Geologie der Erde, vol. 28, pp. 135–178. Aliaj, S., Baldassarre, G., Shkupi, D., 2001. Quaternary subsidence zones in Albania: some case studies. Bull. Eng. Geol. Env. 59, 313–318. Aliaj, S., Adams, J., Halchuk, S., Sulstarova, E., Peci, V., Muco, B., 2004. Probabilistic seismic hazard maps for Albania. 13th World

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