Lake Ohrid (Macedonia, Albania) is thought to be more than 1.2 million years old and host more than 300 endemic species. As a target of the International Continental scientific Drilling Program (ICDP), a successful deep drilling campaign was carried out within the scope of the Scientific Collaboration on Past Speciation Conditions in Lake Ohrid (SCOPSCO) project in 2013. Here, we present lithological, sedimentological, and (bio-)geochemical data from the upper 247.8 m composite depth of the overall 569 m long DEEP site sediment succession from the central part of the lake. According to an age model, which is based on 11 tephra layers (first-order tie points) and on tuning of bio-geochemical proxy data to orbital parameters (second-order tie points), the analyzed sediment sequence covers the last 637 kyr.
The DEEP site sediment succession consists of hemipelagic sediments, which are interspersed by several tephra layers and infrequent, thin (< 5 cm) mass wasting deposits. The hemipelagic sediments can be classified into three different lithotypes. Lithotype 1 and 2 deposits comprise calcareous and slightly calcareous silty clay and are predominantly attributed to interglacial periods with high primary productivity in the lake during summer and reduced mixing during winter. The data suggest that high ion and nutrient concentrations in the lake water promoted calcite precipitation and diatom growth in the epilimnion during MIS15, 13, and 5. Following a strong primary productivity, highest interglacial temperatures can be reported for marine isotope stages (MIS) 11 and 5, whereas MIS15, 13, 9, and 7 were comparably cooler. Lithotype 3 deposits consist of clastic, silty clayey material and predominantly represent glacial periods with low primary productivity during summer and longer and intensified mixing during winter. The data imply that the most severe glacial conditions at Lake Ohrid persisted during MIS16, 12, 10, and 6, whereas somewhat warmer temperatures can be inferred for MIS14, 8, 4, and 2. Interglacial-like conditions occurred during parts of MIS14 and 8.
In the light of recent climate warming, it has become fundamentally important to understand the characteristics and shaping of individual glacial and interglacial periods during the Quaternary, as these differences can reveal information about external forcing and internal feedback mechanisms in the global climatic system (Lang and Wolff, 2011). The global glacial–interglacial variability has widely been studied on ice cores (e.g., EPICA-members, 2004; NGRIP-members, 2004) and on marine sediment successions (e.g., Lisiecki and Raymo, 2005). In terrestrial realms, long continuous paleoclimatic records are sparse and mostly restricted to loess–paleosol sequences (e.g., Chen et al., 1999), to speleothem records (Bar-Matthews and Ayalon, 2004; Wang et al., 2008), and to lacustrine sediments (e.g., Prokopenko et al., 2006; Melles et al., 2012). In the eastern and southeastern Mediterranean region, long terrestrial paleo-records have become available from Lake Van (Stockhecke et al., 2014a), the Dead Sea (Stein et al., 2011), and from the Soreq Cave speleothem record (Bar-Matthews and Ayalon, 2004).
In the central Mediterranean region, the only terrestrial paleo-record that continuously covers more than 1 million years is the Tenaghi Philippon pollen record in northern Greece (cf. Fig. 1), which spans the last 1.3 million years and provides valuable insights into the vegetation history of the area (e.g., Tzedakis et al., 2006; Pross et al., 2015). The results from Tenaghi Philippon reveal that individual interglacial and glacial periods in the Mediterranean region can differ significantly in their duration and severity (e.g., Tzedakis et al., 2006; Fletcher et al., 2013). However, analytical methods for paleoclimate reconstructions at Tenaghi Philippon have so far been restricted to pollen analyses.
Lake Ohrid on the Balkan Peninsula is thought to be more than 1.2 million years old and has already demonstrated its high sensitivity to environmental change and the potential to provide high-resolution paleoenvironmental information for the last glacial–interglacial cycle (e.g., Wagner et al., 2008, 2009, 2014; Vogel et al., 2010a). Given that Lake Ohrid is also a hotspot for endemism with more than 300 endemic species in the lake (Föller et al., 2015), its sediments also have the potential to address evolutionary questions such as what the main triggers of speciation events are.
An ICDP deep drilling campaign took place at Lake Ohrid in spring 2013 using the Deep Lake Drilling System (DLDS) operated by the Drilling, Observation and Sampling of the Earths Continental Crust (DOSECC) consortium. More than 2100 m of sediments were recovered from four different drill sites (Fig. 1). The processing of the cores from the DEEP site from the central part of Lake Ohrid is still ongoing at the University of Cologne (Germany). Here, we present lithological, sedimentological, and (bio-)geochemical results from the upper part of the DEEP site sediment succession until 247.8 mcd (meter composite depth). According to an age model, which is based on 11 tephrochronological tie points and on tuning of bio-geochemical proxy data against orbital parameters, the analyzed sequence covers the period since 637 ka. Here, we aim to provide a chronological framework for the deposits, confirm the completeness of the record, provide first insights into the sedimentological, paleoenvironmental and paleoclimatological history of Lake Ohrid, and form the basis of more detailed work in the future. Furthermore, our results enable a first characterization of glacial and interglacial severity since marine isotope stage (MIS) 16.
Lake Ohrid is located at the border of the Former Yugoslav Republic of
Macedonia (FYROM) and Albania at an altitude of 693 m above sea level
(m a.s.l., Fig. 1a). The lake is approximately 30 km long, 15 km wide, and
covers a surface area of 358 km
The catchment of Lake Ohrid comprises 2393 km
The oldest bedrock in the catchment of Lake Ohrid is of Devonian age, consists of metasedimentary rocks (phyllites), and occurs in the northeastern part of the basin. Triassic carbonate and siliciclastic rocks occur in the southeast, east, and northwest (e.g., Wagner et al., 2009; Hoffmann et al., 2010; Vogel et al., 2010b). Ultramafic metamorphic and magmatic rocks including ophiolites of Jurassic and Cretaceous age crop out in the west (Hoffmann et al., 2010). Quaternary lacustrine and fluvial deposits cover the plains to the north and to the south of Lake Ohrid (Hoffmann et al., 2010; Vogel et al., 2010b).
The climate at Lake Ohrid is influenced both by continental and
Mediterranean climate conditions (Watzin et al., 2002). Between summer and
winter, monthly average air temperatures range between
The DEEP site (5045-1) is the main drill site in the central part of the lake
at a water depth of 243 m (Fig. 1b; 41
During the drilling campaign in 2013, onsite core processing comprised smear-slide analyses of core catcher material and magnetic susceptibility measurements on the whole cores in 2 cm resolution using a Multi-Sensor Core Logger (MSCL, GEOTEK Co.) equipped with a Bartington MS2C loop sensor (see also Wagner et al., 2014). Following the field campaign, the cores were shipped to the University of Cologne for further analyses.
A first correlation of the individual core segments to provide a preliminary composite profile for the DEEP site sequence was established based on the magnetic susceptibility data of the whole cores from holes 5045-1B, 5045-1C, 5045-1D, and 5045-1F. Cores incorporated into the composite profile were then split lengthwise and described for color, grain size, structure, macroscopic components, and calcite content (reaction with 10 % HCl). High-resolution line scan images were taken using the MSCL (GEOTEK Co.). X-ray fluorescence (XRF) scanning was carried out at 2.5 mm resolution and with an integration time of 10 s using an ITRAX core scanner (Cox Analytical, Sweden). The ITRAX core scanner was equipped with a chromium (Cr) X-ray source and was run at 30 kV and 30 mA. Data processing was performed with the QSpec 6.5 software (Cox Analytical, Sweden; cf. Wennrich et al., 2014). In order to account for inaccuracies and to validate the quality of the XRF scanning data, conventional wavelength dispersive XRF (WDXRF, Philips PW 2400, Panalytical Cor., the Netherlands) was conducted at 2.56 m resolution. The optical and lithological information (layer by layer correlation) was then combined with XRF scanning data to fine-tune the core correlation by using the Corewall software package (Correlator 1.695 and Corelyzer 2.0.1).
If an unequivocal core correlation was not possible, additional core sections from other drill holes in the respective depths were opened, likewise analyzed, and used to refine the core correlation. In the composite profile, the field depth measurements based on “meters below lake floor” (m b.l.f.) were replaced by “meters composite depth” (mcd). The DEEP site composite profile down to 247.8 mcd comprises two sections of core Co1261 for the uppermost 0.93 mcd and a total of 386 core sections from holes 5045-1B, 5045-1C, 5045-1D, and 5045-1F (Fig. 2, Table 1). The overall recovery of the composite profile calculates to 99.97 %, as no overlapping sequences were found between core run numbers 80 and 81 in hole 5045-1C. The length of the core catcher (8.5 cm) between these two runs led to one gap between 204.719 and 204.804 mcd.
Variations of the lithological and (bio-)geochemical proxy data of
the DEEP site sequence plotted against meter composite depth (mcd). The core
composite profile of the DEEP site sediment sequence consists of core
sections from core Co1261 (upper 0.93 mcd) and of core sections from holes
5045-1B, 5045-1C, 5045-1D, and 5045-1F (cf. legend “Composite”). The gap in
the composite profile between 204.719 and 204.804 mcd is also marked where
no overlapping core segments are available. The lithological information
includes the classification of the sediments into the three lithotypes (for
the color code, see legend “Lithology”) and information about the water
content, TIC, TOC, bSi, TOC/TS, TOC/TN, K, Fe, Zr/K, and grain size
variability (< 4
The contributions of the different core sections to the composite DEEP site profile.
At 16 cm resolution, 2 cm thick slices (40.7 cm
All sub-samples (8 cm resolution) were freeze-dried, and the water content
was calculated by the difference in weight before and after drying. For every
other sample, an aliquot of about 100 mg was homogenized and ground to
< 63
Biogenic silica (bSi) concentrations were determined at 32 cm resolution by
means of Fourier transform infrared spectroscopy (FTIRS) at the Institute of
Geological Sciences, University of Bern, Switzerland. For sample preparation,
11 mg of each sample were mixed with 500 mg of oven-dried spectroscopic
grade potassium bromide (KBr, Uvasol®, Merck
Corp.) and subsequently homogenized using a mortar and pestle. A Bruker
Vertex 70 equipped with a liquid nitrogen cooled MCT
(mercury–cadmium–telluride) detector, a KBr beam splitter, and a HTS-XT
accessory unit (multisampler) was used for the measurement. Each sample was
scanned 64 times at a resolution of 4 cm
For grain size analyses at 64 cm resolution, 1.5 g of the sample material were
treated with hydrogen peroxide (H
The sediments from the DEEP site sequence down to 247.8 mcd consist of fine-grained hemipelagic sediments, which are sporadically interspersed by more coarse-grained event layers. From the top to the bottom of the sequence, the water content decreases from a maximum of 70 % to a minimum of 32 % due to compaction by overlying deposits (cf. Fig. 2, for detailed studies on the sediment compaction at the DEEP site, see Baumgarten et al., 2015).
The hemipelagic deposits of the DEEP site sequence were subdivided into three lithotypes (Figs. 2 and 3) based on information from the visual core descriptions. This includes variations in the sediment color and structure.
The sediments of lithotype 1 (calcareous silty clay, Fig. 2) have very dark greenish grey to greenish grey colors, and appear massive or mottled (cf. Fig. 3a to d). Silt to gravel-sized vivianite concretions occur irregularly distributed within lithofacies 1 and can be identified by a color change from grey/white to blue after core opening.
TIC contents between 2.0 and 9.7 % imply that calcite (CaCO
Lithotype 2 (slightly calcareous silty clay) sediments are greenish black and very dark greenish grey in color, and appear mottled or massive (cf. Fig. 3e to h). Vivianite concretions occur irregularly, and yellowish brown layers exhibit high amounts of siderite crystals in smear slides (Figs. 2 and 4). The occurrence of siderite in the DEEP site sediments was confirmed by means of XRD, EDX, and FTIRS spectroscopy (Lacey et al., 2015b).
TIC contents between 0.5 and 2 % indicate that calcite is less abundant in lithotype 2 sediments. Distinct peaks in the TIC content correspond to peaks in Fe and Mn counts and to the occurrence of the yellowish brown siderite layers (Fig. 4). The greenish black sediment successions of lithotype 2 sediments are mottled and have high amounts of OM, as indicated by TOC contents of up to 4.5 %. Brighter, very dark greenish grey sections can be massive or mottled and have lower TOC contents (Fig. 3). The number of diatom frustules is moderate to high, as inferred from bSi contents between 2 and 27.9 %. The bulk sediment composition is balanced by moderate amounts of clastic material (Fig. 2, K intensities).
The bright, greenish grey sediments of lithotype 3 (silty clay) are mottled and intercalated with massive sections of up to several decimeters in thickness (Fig. 3i to l). Vivianite concretions occur irregularly, and yellowish brown siderite layers are abundant (Fig. 2).
The TIC values of lithotype 3 sediments rarely exceed 0.5 %, which infers
negligible calcite contents. Occasional peaks in TIC > 0.5 %
can be attributed to the occurrence of siderite layers (Figs. 2 and 4). TOC
ranges between 0.4 and 4.8 % (Fig. 2), with higher values
> 2.5 % close to the lower and upper boundaries of lithotype
3 sediment sections, and between 3.21 and 2.89 mcd (Fig. 2). The amount of
bSi is mostly between 1.68 and 14.5 %, except for several peaks of up to
41.3 % just above tephra layers. High potassium intensities throughout
most parts of lithotype 3 sediments indicate high clastic matter contents and
correspond to high percentages of the fine fraction
(< 4
The macroscopic event layers were classified as tephra deposit if a high proportion of glass shards was observed in the smear slides, and as mass movement deposit (MMD) if predominantly coarse grains and detrital siliciclastic components occurred (cf. Figs. 2 and 3). Tephra layers in the DEEP site sequence appear as up to 15 cm thick layers and as lenses (cf. Leicher et al., 2015). Most of the tephra layers are between 0.5 cm and 5 cm thick (e.g., Fig. 3h and p). In addition, a distinct peak in the K intensities in the DEEP site sequence at 2.775 mcd was identified as the Mercato crypto tephra layer by a correlation of the K XRF curve from the DEEP site with the curve of core Co1262 (Wagner et al., 2012; for location of the core, see Fig. 1). Tephrostratigraphic investigation including geochemical and morphological analyses of glass shards enabled the correlation of 13 tephra layers from the DEEP site sequence with known volcanic eruptions or distal tephra from the central Mediterranean region (Leicher et al., 2015).
High-resolution line-scan images showing characteristic core segments from deposits of lithotypes 1 to 3, and of mass movement deposits (MMDs) and tephra layers. The vertical scale is in centimeter section depth. For composite depths of the line scan image, see the Supplement.
The MMDs in the DEEP site sequence are between 0.1 cm and 3 cm thick, and consist of very coarse silt to fine sand-sized material (cf. also Fig. 3m, n, and o). A higher frequency of MMDs occurs between 117 and 107 mcd, and between 55 and 50 mcd, respectively. Most of the MMDs show normal gradation (Fig. 3n) or appear as lenses (Fig. 3o). In some very thin MMDs, the gradation is only weakly expressed. The MMD in Fig. 3m differs from all other MMDs in the DEEP site sequence as it is the only one with a clay layer at the top and a 1.5 cm thick, poorly sorted, clay to fine sand-sized section at the bottom.
Siderite layer in core 1F-11H-3 (ca. 60 cm section depth) at 22.56 to 23.57 mcd. The yellowish brown siderite layer correlates with enhanced TIC, iron (Fe), and manganese (Mn) intensities in the sediments. For SEM images of the siderite, please see Lacey et al. (2015b).
Although a detailed examination of the sediment bedding structures in the DEEP site sediments was frequently difficult due to secondary oxidation structures (cf. Fig. 3), the mottled and massive appearance and the lack of lamination imply that anoxic bottom water conditions did not occur. As massive structures commonly correspond to high calcite concentrations in the sediments (high TIC), they can be explained by a high abundance of calcite crystals in the sediments.
Scanning electron microscope (SEM) and X-ray diffraction (XRD) analyses of
carbonate crystals from sediment traps (Matter et al., 2010) and from
sediment cores spanning the last 40 000 kyr (e.g., Wagner et al., 2009;
Leng et al., 2010) show that the majority of carbonates in Lake Ohrid
sediments are endogenic calcite. Only minor contributions to the calcite
content come from biogenic sources; for example, from ostracod valves (Vogel
et al., 2010a), detrital carbonates only occur in trace amounts (Lacey et
al., 2015b). Endogenic calcite deposition in the sediments of Lake Ohrid is
predominately triggered by photosynthesis-induced formation of calcite
crystals in the epilimnion (e.g., Wagner et al., 2009; Vogel et al., 2010a).
The precipitation occurs at warm temperatures during spring and summer, as
long Ca
The high TIC contents in lithotype 1 imply high photosynthesis-induced precipitation of endogenic calcite, high temperatures during spring and summer, good calcite preservation in the sediments, and a somewhat higher lake-water pH. Lower primary productivity, lower temperatures, and a somewhat stronger dissolution of calcite can be inferred from the TIC content in lithotype 2 and 3 sediments. In lithotypes 2 and 3, siderite layers also contribute to the TIC content (cf. Figs. 2 and 4). In neighboring Lake Prespa, siderite formation has been reported to occur in the surface sediments close to the redox boundary under rather acidic and reducing conditions (Leng et al., 2013). In Lake Ohrid, DEEP site lithotype 2 and 3 sediments contain discrete horizons of authigenic siderite crystals and crystal clusters nucleating within an unconsolidated clay matrix (Lacey et al., 2015b). The open-packed nature of the matrix and growth relationships between crystals suggest that, as also observed in Lake Prespa, siderite formed in the pore spaces of the surface sediments close to the sediment–water interface, similar to other ancient lakes such as Lake Baikal (Berner, 1981; Lacey et al., 2015b).
The OM in lithotype 2 and 3 sediments is predominately of aquatic origin, as indicated by TOC / TN ratios below 10 (cf. Meyers and Ishiwatari, 1995; Wagner et al., 2009). In lithotype 1 sediments, TOC / TN occasionally exceeds 10, which could imply some contributions of terrestrial OM. However, due to the coring location in the central part of Lake Ohrid and the relatively small inlet streams, a substantial supply of allochthonous OM to the DEEP site is rather unlikely. The high TOC / TN ratios are therefore most likely a result of early digenetic selective loss of N (cf. Cohen, 2003). The aquatic origin of the OM in the sediments of the deep basin of Lake Ohrid is also in agreement with the results of Rock Eval pyrolysis from the nearby LINI drill site (Lacey et al., 2015a; see Fig. 1 for the coring location). This implies that phases characterized by higher TOC contents are representative of a somewhat elevated primary productivity. This finding is confirmed by high amounts of diatom frustules in the sediments (Wagner et al., 2009) and, accordingly, in high biogenic silica contents. Enhanced productivity in the lake requires high temperatures and sufficient nutrient supply to the epilimnion. The nutrient supply to Lake Ohrid is mainly triggered by river inflow (e.g., Matzinger et al., 2006a, b, 2007; Wagner et al., 2009; Vogel et al., 2010a), karstic inflow from Lake Prespa (Matzinger et al., 2006a; Wagner et al., 2009), and by nutrient recycling from the surface sediments (Wagner et al., 2009). Phosphorous recycling from the surface sediments is promoted by anoxic bottom water conditions and mixing can transport phosphorous from the bottom water to the epilimnion (e.g., Wagner et al., 2009). Mixing also leads to oxidation of OM at the sediment surface and, thus, to lower TOC contents. TOC preservation in the sediments can also be modified by lake-level fluctuations, as oxidation of OM starts in the water column during settling (Cohen, 2003; Stockhecke et al., 2014b). However, distinct climate-induced lake-level fluctuations such as, for example, described for the Younger Dryas at Lake Van in Turkey (Wick et al., 2003; Stockhecke et al., 2014b, and references therein) have not been observed at Lake Ohrid in previous studies covering the last glacial–interglacial cycle (cf. Vogel et al., 2010a; Wagner et al., 2010). This can potentially be explained by the hydrological conditions at the lake, the large water volume, and the relatively high contribution of karstic groundwater inflow to the hydrological budget of Lake Ohrid. Hence, lower (higher) TOC content can be related to an overall lower (higher) productivity and/or to more (less) oxidation of OM and improved (restricted) mixing conditions.
Overall high TOC and bSi contents in lithotype 1 sediments imply high
productivity as a result of high temperatures in Lake Ohrid. Less
productivity and/or oxidation of OM can be inferred for sediments of
lithotypes 2 and 3 from low TOC and bSi contents, and from TOC
Good OM preservation, low oxygen availability, and overall poor mixing
conditions could have favored sulfide formation in lithotype 1 sediments.
Sulfide formation can be indicated by a low TOC
Elemental intensities of the clastic matter, obtained from high-resolution
XRF scanning, can provide information about the sedimentological composition
of the deposits, and about erosional processes in the catchment. Variations
in the potassium intensities (K, Fig. 2) and in the clastic matter content of
DEEP site sequence sediments could primarily be a result of changing erosion
in the catchment, such
Potassium can occur in K-feldspars, micas, and clay minerals, and is
mobilized particularly during chemical weathering and pedogenesis, and the
residual soils in the catchment become depleted in potassium (Chen et al.,
1999). In contrast to K, Zirconium (Zr) mostly occurs in the mineral zircon,
which has a high density and resistance against physical and chemical
weathering and is therefore commonly enriched in coarse-grained (aeolian)
sediments. However, in lithotype 1 and 2 sediments, low Zr
Probable trigger mechanisms for MMDs have been widely discussed and encompass earthquakes, delta collapses, flooding events, over-steepening of slopes, rockfalls, and lake-level fluctuations (e.g., Cohen, 2003; Schnellmann et al., 2006; Girardclos et al., 2007; Sauerbrey et al., 2013). At Lake Ohrid, MMDs in front of the Lini Peninsula (Fig. 1) and in the southwestern part of the lake were likely triggered by earthquakes (Lindhorst et al., 2012, 2015; Wagner et al., 2012). An earthquake might have also triggered the deposition of the MMD in Fig. 3m, which is composed of a turbidite succession and an underlying, poorly sorted debrite (after the classification of Mulder and Alexander, 2001). The disturbance generated by a debris flow can cause co-genetic turbidity currents of fine-grained material in front of and above the mass movement (Schnellmann et al., 2005; Sauerbrey et al., 2013). As the debrite–turbidite succession occurs at 7.87 mcd in hole 5045-1F, it likely corresponds to a massive slide complex north of the DEEP site (cf. the hydro-acoustic profile of Fig. 2 in Wagner et al., 2014). Density flows that enter the central part of the Lake Ohrid basin close to the DEEP site from the eastern or southern directions have not been observed in the upper parts of hydro-acoustic profiles (cf. Figs. 2 and 3 in Wagner et al., 2014). The three massive MMDs that occur in front of the Lini Peninsula to the west of the DEEP site (cf. Wagner et al., 2012) are likely not related to the debrite–turbidite succession in Fig. 3m. The underlying debrite does not occur in overlapping segments of holes 5045-1B, 5045-1C, and 5045-1D. Holes 5045-1B, 5045-1C, and 5045-1D form a N–S transect, whereas hole 5045-1F is located to the east. Due to the absence of the debrite deposits in most drill holes, the relatively low thickness in hole 5045-1F, and hydroplaning that generates a basal water layer below a debris flow (Mohrig et al., 1998; Mulder and Alexander, 2001), erosional processes at the DEEP site are likely low.
The sand lenses and normal graded MMDs (cf. Figs. 2, 3) can be classified as grain-flow deposits (after the classification of Mulder and Alexander, 2001; Sauerbrey et al., 2013) and are composed of reworked lacustrine sediments from shallow water depths or subaquatic slopes close to riverine inflows. Grain flows that enter the deep parts of the Lake Ohrid basin via the steep slopes might transform into a mesopycnal flow at the boundary of the hypolimmion (cf. also Mulder and Alexander, 2001; Juschus et al., 2009), which would have prevented erosion of the underlying sediments.
Correlated tephra layers in the DEEP site sequence according to
Leicher et al. (2015).
The chronology for the sediments of the DEEP site sequence down to 247.8 mcd
was established by using tephrochronological information from 11 out of 13
tephra layers (cf. Table 2 and Leicher et al., 2015) and by cross-correlation
with orbital parameters. The tephra layers were correlated with well-known
eruptions from Italian volcanoes or central Mediterranean marker tephra by
geochemical and morphological analyses of glass shards. Radiocarbon and
The chronological information from the 11 tephra layers was also used to
define cross-correlation points with orbital parameters, which were included
in the age–depth model as second-order tie points. The tephra layers Y-5,
X-6, P-11, and A11/12 were deposited when there are minima in the TOC content
and in the TOC
Left: comparison between TOC (DEEP site) and arboreal pollen
percentages (AP, Tenaghi Philippon, Tzedakis et al., 2006) from 636.69 ka to
the present. Right: comparison between TOC (DEEP site) and
The 11 tephra layers and 31 cross-correlation points of second order
(Supplement 1) were used for the establishment of an age–depth model. An
uncertainty of
On the basis of the established core chronology for the DEEP site sequence, glacial–interglacial environmental variability at Lake Ohrid inferred from the TOC record is in agreement with other paleoclimate records from the Mediterranean region such as the Tenaghi Philippon (Tzedakis et al., 2006) and the Soreq Cave records (Grant et al., 2012; cf. Fig. 6). This supports the quality of the chronology for the DEEP site sediments. Whereas the chronology of the Tenaghi Philippon record is, similar to the DEEP site sequence, based on tuning against orbital parameters, the Soreq Cave speleothem record is the longest absolute dated (U-Th) paleoclimate record currently available for the Mediterranean region.
Age model of the DEEP site sequence down to 247.8 mcd (637 ka).
Ages were calculated using the Bacon 2.2 software package (Blaauw and
Christen, 2011). Overall stable sedimentation rates at the DEEP site
(mem.strength
Proxy data from the DEEP site sediments plotted versus age and
compared to the global benthic isotope stack LR04 (Lisiecki and Raymo, 2005),
the local (41
Variations in the TIC, bSi, TOC, K, and Zr
Between 637 ka and the present day, the DEEP site sediments deposited during
the interglacial periods (MIS boundaries after Lisiecki and Raymo, 2005; see
Fig. 7) mainly consist of lithotype 1 and 2 sediments. Moderate to high TIC,
TOC, and bSi contents in lithotype 1 and 2 sediments imply a moderate to
strong primary productivity and, thus, moderate to high temperatures during
spring and summer. The overall high temperatures during spring and summer and
a longer summer season likely resulted in an incomplete and restricted mixing
of the water column during winter, such it as also persists today. Poor mixing
hampers the oxidative mineralization of OM and, thus, promotes the
preservation of TOC and restricts the bacterial CO
Low supply of clastic matter from the catchment can be inferred for lithotype
1 and 2 sediments, where K intensities and the sedimentation rates are low
(
Lithotype 3 deposits with negligible TIC contents only occur at the onsets and terminations of interglacial periods, and during MIS7 and 3 (cf. Fig. 7). The low TIC, TOC, and bSi contents of these sediments correspond to colder periods with a restricted primary productivity. In addition, low temperatures during winter would have improved the mixing, which could promote decomposition of OM in the surface sediments and lead to lower TOC, and to lower TIC by dissolution of calcite.
Differing OM, bSi, and TIC contents in
the sediments of the DEEP site sequence corresponding to interglacial time
periods imply different intensities of interglacials at Lake Ohrid.
Comparable conditions can be inferred for MIS15 and 13 with high TIC and bSi,
and low OM, which indicate strong primary productivity and high temperatures
during spring and summer, and decomposition of OM during the mixing season.
However, MIS15 and 13 are regarded as relatively weak interglacials based on
the synthetic Greenland isotope record (Barker et al., 2011) and the LR04
global benthic isotope stack (Lisiecki and Raymo, 2005; cf. also Fig. 7).
Possible explanations could be that the inferred high intensity of these
interglacials at Lake Ohrid is due to a strong seasonality or a lower water
volume, which promotes a high ion concentration (high TIC, bSi) and high
During the first part of MIS11, between 420 and 400 ka, highest TIC concentrations along with moderate and high bSi and TOC imply highest productivity (high TOC) and highest temperatures (highest TIC), whereby the moderate bSi concentrations can be explained by mutual dilution with calcite. This is consistent with other records, where the strongest interglacial conditions and highest temperatures since 637 ka are reported for the onset of MIS11 (Lang and Wolff, 2011).
TIC concentrations during the second phase of MIS11, between 400 and 374 ka,
and during MIS9 and 7 are generally lower and mostly restricted to confined
peaks, which implies overall less calcite precipitation, less primary
productivity, and lower temperatures at Lake Ohrid. This is consistent with
the low bSi concentrations, but not with the high TOC contents, and with
relatively stronger interglacials subsequent to the MBE (Mid-Brunhes Event)
inferred from the synthetic North Greenland isotope record (Barker et al.,
2011) and the LR04 stack (Lisiecki and Raymo, 2005). The temperatures during
the second phase of MIS11 and during MIS9 and 7 were likely lower compared to
the first phase of MIS11 as indicated by the TIC record (cf. Fig. 7). In
addition, the somewhat lower TIC and bSi contents also correspond to lower
Overall high TIC, TOC, and bSi concentrations during MIS5 imply a strong
primary productivity in the epilimnion and high temperatures during spring
and summer.
Glacial periods between 637 ka and today are characterized by predominant deposition of lithotype 3 sediments, with rare occurrence of lithotype 1 and 2 sediments in MIS14 and 8, when TIC contents are higher. Low TOC and bSi and negligible TIC contents in lithotype 3 sediments imply low primary productivity and overall low temperatures during glacial periods. Some minor fluctuations in productivity and temperature are indicated by TOC and bSi. They are not documented in TIC, because restricted ion supply from the catchment and oxidation of OM at the sediment surface due to intensified and prolonged mixing may have led to a slight decrease in the bottom water pH and dissolution of calcite precipitated from the epilimnion. Dissolution of calcite and the existence of a threshold can also explain the delayed increase in TIC compared to TOC and bSi at the transitions of MIS16, MIS12, MIS10, MIS8, MIS6, and MIS2 into the following interglacials.
High K, a high proportion of the fine fraction < 4
As TIC is affected by dissolution, information about the severity of the individual glacials at Lake Ohrid can only be inferred from TOC and bSi. Minima in the TOC and bSi imply that most severe glacial conditions at Lake Ohrid occurred at the end of MIS16, and during MIS12, 10, and 6. Somewhat higher bSi and TOC in parts of MIS14 and 8, and in MIS6, 4, and 2, imply less severe glacial conditions. This suggests that the finding of glacial moraines from MIS2 (Ribolini et al., 2011) is probably only due to better preservation of these glacial features compared to the older glacials. Interglacial-like conditions with higher primary productivity and reduced oxidation of OM in the surface sediments prevailed at the occurrence of lithotype 1 and 2 sedimentation, i.e., between 563 and 540 ka during MIS14, and between 292 ka and 282 ka during MIS8. Interglacial-like conditions along with a forest expansion and more warm conditions between 292 and 282 ka can also be seen in the pollen records of Lake Ohrid (cf. Fig. 7 and Sadori et al., 2015) and Tenaghi Philippon (Fletcher et al., 2013).
The general observation that MIS16 and 12 were more severe glacials is in broad agreement with other records, such as the North Greenland isotope record (Barker et al., 2011), the LR04 global benthic stack (Lisiecki and Raymo, 2005), and the Tenaghi Philippon pollen record (Tzedakis et al., 2006). Thereby, the comparable low sedimentation rates in combination with the negligible TIC, bSi, and TOC concentrations between 460 and 430 ka (MIS12; cf. Fig. 7) imply low supply of clastic matter and, thus, low erosion in the catchment despite an open vegetation cover in the catchment (cf. Fig. 7 and Sadori et al., 2015). One potential explanation for the low clastic matter supply could be dry conditions and associated reductions in terrestrial runoff compared to other glacials.
The frequent occurrence of MMDs between 280 and 241 ka and between 160 and 130 ka could reflect significant lake-level fluctuations during MIS8 and 6. During the first period (MIS8), distinct fluctuations in the pollen concentrations of the DEEP site sediments (cf. Fig. 7 and Sadori et al., 2015) correspond to similar fluctuations in the AP pollen percentages of the Tenaghi Philippon pollen record (Tzedakis et al., 2006) and probably indicate a shift from cold and dry to more warm and humid conditions in northern Greece and at Lake Ohrid. During MIS6, a 60 m lower lake level compared to present conditions and a subsequent lake-level rise during late MIS6 or during the transition from MIS6 to MIS5 are reported from hydro-acoustic and sediment core data from the northeastern corner of Lake Ohrid (Lindhorst et al., 2010). In the sediments of the DEEP site, the MIS6 to MIS5 transition occurs at ca. 50 mcd, which could indicate that the water depth might have not changed significantly compared to the present conditions. This can be explained by the ongoing subsidence (Lindhorst et al., 2015). However, the pollen record from the DEEP site also implies a phase of strong aridity during MIS6 (Sadori et al., 2015), which might imply that climate-induced lake-level fluctuations at Lake Ohrid were probably less severe compared for example to Lake Van in Turkey, where a 260 m lower lake level has been reported for the Younger Dryas (e.g., Wick et al., 2003, and references therein; Stockhecke et al., 2014b). Thereby, the extraordinarily high sedimentation rates in particular during the first part of MIS6 in combination with high K intensities and low bSi, TIC, and TOC imply intensive erosion.
The investigated sediment succession between 247.8 mcd and the sediment surface from the DEEP site in the central part of Lake Ohrid provides a valuable archive of environmental and climatological change for the last 637 kyr. An age model was established by using chronological tie points from 11 tephra layers, and by tuning bio-geochemical proxy data to orbital parameters. The imprint of environmental change on the lithological, sedimentological, and (bio-)geochemistry data can be used to unravel the lake's history including the development of the Lake Ohrid basin and the climatological variability on the Balkan Peninsula.
The lithological, sedimentological, and geochemical data from the DEEP site sequence imply that Lake Ohrid did not experience major catastrophic events such as extreme lake-level low stands or desiccation events during the last 637 kyr. Hiatuses are absent and the DEEP site sequence provides an undisturbed and continuous archive of environmental and climatological change.
Based on the initial core description and the calcite content, the hemipelagic sediments from the DEEP site sequence can be classified into three different lithotypes. This classification is supported by variations in the (bio-)geochemistry proxies and matches climate variations on glacial–interglacial timescales. Overall, interglacial periods are characterized by high primary productivity during summer, restricted mixing during winter, and low erosion in the catchment. During glacial periods, the primary productivity is low, and intense mixing of the water column promotes the decomposition of OM, which may have lowered the water pH and led to dissolution of calcite. Enhanced erosion of chemically altered siliciclastics and an overall higher clastic matter input into the lake during glacial periods can be a result of a less dense vegetation cover in the catchment and meltwater run-off from local glaciers on the surrounding mountains.
Following a strong primary productivity during spring and summer, the highest interglacial temperatures can be inferred for the first part of MIS11 and for MIS5. In contrast, somewhat lower spring and summer temperatures are observed for MIS15, 13, 9, and 7. The data also suggest that high ion and nutrient concentrations in the lake water promote calcite precipitation and diatom growth in the epilimnion during MIS15, 13, and 5, whereas less evaporated interglacial periods exhibit lower TIC and bSi contents (MIS9 and 7).
The most severe glacial conditions at Lake Ohrid occurred during MIS16, 12, 10, and 6, whereas somewhat warmer temperatures can be inferred for MIS14, 8, 4, and 2. Interglacial-like conditions occurred during parts of MIS14 and 8, respectively.
The SCOPSCO Lake Ohrid drilling campaign was funded by ICDP, the German Ministry of Higher Education and Research, the German Research Foundation, the University of Cologne, the British Geological Survey, the INGV and CNR (both Italy), and the governments of the republics of Macedonia (FYROM) and Albania. Logistic support was provided by the Hydrobiological Institute in Ohrid. Drilling was carried out by Drilling, Observation and Sampling of the Earth's Continental Crust (DOSECC) and using the Deep Lake Drilling System (DLDS). Special thanks are due to Beau Marshall and the drilling team. Ali Skinner and Martin Melles provided immense help and advice during logistic preparation and the drilling operation. Furthermore, the authors thank Ronald Conze (GFZ Potsdam), Dorothea Klinghardt (University of Cologne), Nicole Mantke (University of Cologne), Volker Wennrich (University of Cologne), and various student assistants for their immense support during the core processing and the analytical work. The palynological specialists of the Lake Ohrid community contributed with immense work to the pollen analyses on the DEEP site sequence; the reader is referred to Sadori et al. (2015) for a detailed list of all involved scientists. Acknowledged are also Andreas Koutsodendris (University of Heidelberg) and Alessia Masi (University of Rome) for the pollen plot in Fig. 7 and for helpful discussions. The authors thank Mona Stockhecke, Chronis Tzedakis, and two anonymous reviewers for their suggestions and comments, which improved the quality of the manuscript, and Jack Middelburg for handling the manuscript. Edited by: J. Middelburg