Northern Mediterranean climate since the Middle Pleistocene: a 637 ka stable isotope record from Lake Ohrid (Albania/Macedonia)

. Lake Ohrid (Macedonia/Albania) is an ancient lake with unique biodiversity and a site of global signiﬁcance for investigating the inﬂuence of climate, geological, and tectonic events on the generation of endemic populations. Here, we present oxygen ( δ 18 O) and carbon ( δ 13 C) isotope data from carbonate over the upper 243 m of a composite core proﬁle recovered as part of the Scientiﬁc Collaboration on Past Speciation Conditions in Lake Ohrid (SCOP-SCO) project. The investigated sediment succession covers the past ca. 637 ka. Previous studies on short cores from the lake (up to 15 m, < 140 ka) have indicated the total inorganic carbon (TIC) content of sediments to be highly sensitive to climate change over the last glacial–interglacial cycle. Sediments corresponding to warmer periods contain abundant endogenic calcite; however, an overall low TIC content in glacial sediments is punctuated by discrete bands of early diagenetic authigenic siderite. Isotope measurements on endogenic calcite ( δ 18 O c and δ 13 C c ) reveal variations both between and within interglacials that suggest the lake has been subject to palaeoenvironmental change on orbital and millen-nial timescales. We also measured isotope ratios from authigenic siderite ( δ 18 O s and δ 13 C s ) and, with the oxygen isotope composition of calcite and siderite, reconstruct δ 18 O of lake water ( δ 18 O lw ) over the last 637 ka. Interglacials have higher δ 18 O lw values when compared to glacial periods most likely due to changes in evaporation, summer temperature, the proportion of winter precipitation (snowfall), and inﬂow from adjacent Lake Prespa. The isotope stratigraphy suggests Lake Ohrid experienced a period of general stability from marine isotope stage (MIS) 15 to MIS 13, highlighting MIS 14 as a particularly warm glacial. Climate conditions became progressively wetter during MIS 11 and MIS 9. Interglacial periods after MIS 9 are characterised by increasingly evaporated and drier conditions through MIS 7, MIS 5, and the Holocene. Our results provide new evidence for long-term climate change in the northern Mediterranean region, which will form the basis to better understand the inﬂuence of major environmental events on biological evolution within Lake Ohrid.


Introduction
Global climate models indicate the Mediterranean to be a highly vulnerable area with respect to predicted future changes in temperature and precipitation regimes (Giorgi, 2006;Giannakopoulos et al., 2009), and the associated stress on water resources may have important socioeconomic im-Published by Copernicus Publications on behalf of the European Geosciences Union. source: https://doi.org/10.7892/boris.80329 | downloaded: 24.2.2020 pacts across the region (García-Ruiz et al., 2011). It is therefore vital to investigate the regional response to past climate fluctuations and improve our understanding of global climate dynamics as a prerequisite for establishing future scenarios (Leng et al., 2010a). Stable isotope ratios preserved in sedimentary lacustrine carbonates are a proxy for past climate and hydrological change (Leng and Marshall, 2004), and combinations of lake records can be used to assess the spatial coherence of isotope variations (Roberts et al., 2008). Although there are numerous stable isotope records from the Mediterranean, for example from marine sediment cores (Piva et al., 2008;Maiorano et al., 2013;Regattieri et al., 2014) and speleothems (Bar-Matthews et al., 2003;Antonioli et al., 2004), those from lacustrine carbonate typically are Late Glacial-Holocene in age (Dean et al., 2013;Francke et al., 2013) and only a limited number of extend beyond the Last Glacial (Frogley et al., 1999;Kwiecien et al., 2014;Giaccio et al., 2015;Regattieri et al., 2016). Lake Ohrid, located on the Balkan Peninsula in southeastern Europe, is thought to be among the oldest extant lakes on Earth with a limnological age in excess of 1.2 million years Lindhorst et al., 2015). So-called ancient lakes are often associated with an outstanding degree of natural biodiversity, and Ohrid is one of only a few lakes worldwide to contain such a varied assemblage with over 300 endemic species (Albrecht and Wilke, 2008;Föller et al., 2015). Previous core sequences span the past 140 000 years and proxies (e.g. geochemical, pollen, diatoms) indicate Lake Ohrid to be highly sensitive to both long-and short-term environmental change (Wagner et al., 2008(Wagner et al., , 2009Vogel et al., 2010). Based on the potential for extended palaeoenvironmental reconstructions, the Scientific Collaboration on Past Speciation Conditions in Lake Ohrid (SCOPSCO) project was established within the framework of the International Continental scientific Drilling Program (ICDP). The principle aims of the SCOPSCO project are (1) to obtain precise information about the age and origin of the lake, (2) to unravel the regional seismotectonic history, (3) to obtain a continuous record containing information on Quaternary climate change and volcanic activity in the central northern Mediterranean region, and (4) to evaluate the influence of major geological events on evolution and the generation of the observed extraordinary degree of endemic biodiversity (see Wagner et al., 2014).
Existing records from Lake Ohrid have been analysed for the isotope composition of carbonate over the last glacialinterglacial cycle, including fine-grained calcite (endogenic) from bulk sediment (Leng et al., 2010a;Lacey et al., 2015) and benthic ostracods . However, the current isotope data sets do not have the temporal range necessary to meet the primary research aims of the SCOP-SCO project . Here, we present new stable isotope data from carbonates (endogenic calcite and authigenic siderite) from SCOPSCO cores covering ca. 637 ka (Baumgarten et al., 2015;Francke et al., 2016) and also reconstruct the oxygen isotope composition of lake water (δ 18 O lw ). These data represent an extensive isotope stratigraphy covering multiple orbital cycles that provides valuable information on long-term palaeoenvironmental change between interglacial and glacial periods and on millennialscale variability within interglacial stages. The isotope data presented here will ultimately act as a reference record for climate change in the Mediterranean and across the Northern Hemisphere, provide a better understanding of the magnitude and timing of Late Quaternary climate oscillations, and deliver a robust framework to investigate SCOPSCO aims (3) and (4).

General setting
Lake Ohrid (Former Yugoslav Republic of Macedonia/Republic of Albania) is situated at 693 m above sea level (a.s.l.) and formed in a tectonic graben bounded by high mountain chains to the west and east (Fig. 1). The lake has a maximum length of 30.8 km, a maximum width of 14.8 km, an area of 358 km 2 , and a volume of 50.7 km 3 (Stankovic, 1960;Popovska and Bonacci, 2007); the basin has a simple bath tub-shaped morphology with a maximum and average water depth of 293 and 150 m respectively (Lindhorst et al., 2015). There is a relatively small catchment area of 2600 km 2 , even accounting for input from neighbouring Lake Prespa, which delivers water through a network of karst aquifers thought to correspond to 53 % of total water input (Matzinger et al., 2006a). The subterranean connection has been confirmed using tracer experiments (Anovski et al., 1991;Amataj et al., 2007) and feeds spring complexes mainly to the south-east of the lake (Eftimi and Zoto, 1997;Matzinger et al., 2006a). The remaining input comprises river inflow (24 %) and direct precipitation on the lake's surface (23 %). Water output is via the river Crim Drim on the northern margin (66 %) and by means of evaporation (34 %) (Matzinger et al., 2006b). Lake Ohrid has a hydraulic residence time of around 70 years and complete overturn is thought to occur approximately every 7 years (Hadzisce, 1966), which leads to de-stratification of the water column during deep convective winter mixing (Matzinger et al., 2006b).
Mediterranean climate is generally influenced by the subtropical anticyclone in summer and mid-latitude westerlies during winter, providing a complex and sensitive climatology at a major transition zone between temperate and arid domains (Lionello et al., 2012). This leads to precipitation seasonality controlled by the southward migration of the Intertropical Convergence Zone during winter, allowing the influence of westerlies to be established, and the development of cyclogenesis across the Mediterranean (Harding et al., 2009). Local orography produces climatic sub-zones with variable distributions of precipitation and temperature across the Mediterranean (Zanchetta et al., 2007), and the climate  (Lindhorst et al., 2015) and indicating the locations of coring sites (a) DEEP 5045-1 and (b) Lini Co1262 (Wagner et al., 2012;Lacey et al., 2015).
of Lake Ohrid and its watershed is controlled by both sub-Mediterranean and continental influences owing to its location in a deep basin sheltered by mountains and its proximity to the Adriatic Sea . Today, air temperatures range between a minimum of −6 • C and a maximum of +32 • C and have an annual average of around +10 • C ( Fig. 2; Popovska and Bonacci, 2007). Lake Ohrid surface water temperature remains between +6 and +26 • C and bottom water temperature is constant between +5 and +6 • C (Popovska and Bonacci, 2007). The catchment receives an average annual rainfall of around 900 mm and the prevailing northerly-southerly winds trace the Ohrid valley (Stankovic, 1960).

Core recovery
The ICDP SCOPSCO coring campaign of spring 2013 was a resounding success with over 2100 m of sediment recovered from four different sites; a full overview of coring locations, processes, and initial data is given by . . The cores were subsequently processed and correlated at the University of Cologne to provide a composite profile for the DEEP site sequence, which currently extends down to 247.8 m core depth (described by Francke et al., 2016). Total inorganic carbon (TIC) data were measured by Francke et al. (2016) using a DIMATOC 100 carbon analyser (Dimatec Corp., Germany).

Chronology
The age model for the upper 247.8 m of the DEEP site sequence was established by (1) using tephrostratigraphical information (first-order tie points) and (2) tuning total organic carbon (TOC) and TOC/total nitrogen (TN) to trends in local daily insolation patterns (26 June at 41 • N; Laskar et al., 2004) and the winter season length (second-order tie points; cf. Francke et al., 2016). The tie points comprise 11 tephra layers, correlated to well-known Italian volcanic eruptions by geochemical fingerprint analysis (age and error is based on recalibration of Ar / Ar ages from the literature by Leicher et al., 2015), and 30 tuning points. The tuning points are based on TOC and TOC / TN minima, which are observed to be coincident with inflection points in summer insolation and winter season length. For each TOC tuning point, an error of ±2000 years was included in the age-depth calculation to account for inaccuracies in the tuning process (Francke et al., 2016). Finally, the age model for the sediment cores was cross-evaluated with the age model of the borehole logging data (Baumgarten et al., 2015). The latter is based on tuning K concentration from downhole spectral gamma ray to LR04 and cyclostratigraphic analysis on gamma ray data. Chronological information and tie points are presented and discussed in detail in Francke et al. (2016); Leicher et al. (2015) and Baumgarten et al. (2015). The age model implies that the upper 247.8 m of the DEEP site composite profile represents the last ca. 637 ka, which broadly corresponds to marine isotope stage (MIS) 16 to MIS 1 (Lisiecki and Raymo, 2005;Railsback et al., 2015).

Analytical work
The DEEP site composite profile was sampled for oxygen and carbon isotope ratios (δ 18 O and δ 13 C) on carbonate at 16 cm intervals from the surface to 242.98 m throughout zones with a high carbonate content (up to 10 % TIC; thought to represent interglacials). A previous record (Lini core Co1262; Lacey et al., 2015) provides the most extensive Holocene sequence recovered from Lake Ohrid to date, and so it is utilised in place of the uppermost sediments of the composite profile. The carbonate found within zones of high TIC predominantly consists of calcite. Idiomorphic calcite crystals and crystal clusters between 20 and 100 µm have been reported from previous scanning electron microscopy investigations, which show the crystals to be dominantly CaCO 3 (Wagner et al., 2008;Matter et al., 2010). The size and shape of the crystals are typical of endogenic precipitation (Leng et al., 2010b;Lézine et al., 2010), and although CaCO 3 crystals recovered from sediment traps are generally pristine (Matter et al., 2010), those from core material are typically characterised by partial dissolution (Wagner et al., 2009). Within zones of overall low TIC, intermittent spikes to higher TIC were also sampled and the constituent carbonate species investigated using X-ray diffraction (XRD), as X-ray fluorescence (XRF) showed the spikes were high in Fe and Mn (Francke et al., 2016). XRD was conducted on a PANalytical X'Pert Pro powder diffractometer, with Cobalt Kα 1 radiation over the scan range 4.5-85 • 2θ and a step size of 2.06 • 2θ min −1 . Phase identification was conducted using PANalytical HighScore Plus version 4.0 analytical software interfaced with the latest version of the International Centre for Diffraction Data (ICDD) database. The XRD analysis showed the carbonate in the samples to consist of siderite, which was confirmed using energy-dispersive X-ray spectroscopy (EDX) on epoxy resin-embedded thin sections.
Relative concentration changes of the carbonate phases (calcite and siderite) were determined at 32 cm intervals from the surface to a correlated depth of 247.8 m using Fourier transform infrared spectroscopy (FTIR). For FTIR analysis, 0.011 ± 0.0001g of each sample was mixed with 0.5 ± 0.0001g of oven-dried spectroscopic-grade potassium bromide (KBr) (Uvasol ® , Merck Corp.) and subsequently homogenised using a mortar and pestle. A Bruker Vertex 70 equipped with an MCT (mercury-cadmium-telluride) detector, a KBr beam splitter, and an HTS-XT accessory unit (multi-sampler) was used for the measurement. Each sample was scanned 64 times at a resolution of 4 cm −1 (reciprocal centimetres) for the wave number range from 3750 to 520 cm −1 in diffuse reflectance mode. FTIR analysis was performed at the Institute of Geological Sciences, University of Bern, Switzerland. Linear baseline correction was applied to normalise the recorded FTIR spectra and to remove baseline shifts and tilts by setting two points of the recorded spectrum to 0 (3750 and 2210-2200 cm −1 ). Peak areas diagnostic for bending vibrations of the carbonate ion in calcite (707-719 cm −1 ) and siderite (854-867 cm −1 ) and representative for their relative abundance (White, 1974;Chukanov, 2014) were integrated using the OPUS (Bruker Corp.) software package.
For the isotope analysis, approximately 250 mg (calcite) or 1000 mg (siderite) of sample was disaggregated in 5 % sodium hypochlorite solution for 24 h to oxidise reactive organic material, then washed in deionised water to neutral pH, dried at 40 • C and ground to a fine powder. To evolve CO 2 for isotope analysis, calcite-bearing samples were reacted overnight inside a vacuum with anhydrous phosphoric acid at a constant 25 • C, and siderite-bearing samples were reacted with anhydrous phosphoric acid within a vacuum for 96 h at 100 • C. For both types of sample, CO 2 was cryogenically separated from water vapour under vacuum and analysed using a VG Optima dual inlet mass spectrometer. The mineral-gas fractionation factor used for calcite was 1.01025 and for siderite was 1.00881 (Rosenbaum and Sheppard, 1986). The oxygen and carbon isotope composition of calcite (δ 18 O c and δ 13 C c ) and siderite (δ 18 O s and δ 13 C s ) are reported as per mille (‰) deviations of the isotope ratios ( 18 O / 16 O and 13 C / 12 C) calculated to the VPDB scale. Within-run laboratory standards were utilised for which analytical reproducibility was < 0.1 ‰ for δ 18 O and δ 13 C.

Modern isotope data
Understanding how the isotope composition of contemporary lake water relates to the measured signal from a mineral precipitate is fundamental in resolving the past systematics of hydroclimate variation from lacustrine records (Leng and Marshall, 2004). The isotope composition of lake water (δ 18 O lw ) from Ohrid and Prespa, as well as spring inflows, have been previously investigated (water samples collected 1984collected -2011Eftimi and Zoto, 1997;Anovski et al., 2001;Matzinger et al., 2006a;Leng et al., 2010aLeng et al., , 2013. Modern waters from Lake Ohrid fall on a local evaporation line (LEL) away from the local meteoric water line (LMWL) inferring that they have undergone kinetic fractionation (average δ 18 O lw = −3.5 ‰; Leng and Marshall, 2004). Lake Prespa has a reduced surface area to volume ratio in comparison to Ohrid and is highly sensitive to seasonal variations in moisture balance (Popovska and Bonacci, 2007;Leng et al., 2010a); hence its waters fall higher on the LEL (average δ 18 O lw = −1.5 ‰). The initial water composition at the LMWL-LEL intersect suggests that both lakes are principally recharged from meteoric water, assumed to be similar to the mean annual isotope composition of precipitation (δ 18 O p ) across the catchment (δ 18 O p = −10.2 ‰; Anovski, 2001), which falls close to the average value for precipitation-fed spring waters (−10.1 ‰). The spring complexes are split between those with lower δ 18 O (fed predominantly by isotopically depleted winter precipitation) and those with higher δ 18 O (having an evaporated component). Mixing analysis at spring complexes primarily to the south-east of Ohrid indicates they receive up to 53 % of their incoming water budget from Prespa Eftimi et al., 2001;Matzinger et al., 2006a). Springs deliver around half of total inflow to Lake Ohrid, and therefore a large proportion of water input will be seasonally variable as it is derived from Lake Prespa (lower δ 18 O lw in winter and higher during summer). Contemporary waters from Lake Ohrid show uniform δ 18 O lw values over the ca. 30-year sampling period, signifying that seasonal variations in the water contribution from Lake Prespa have a negligible overall effect, most probably due to Ohrid's large volume and long lake water residence time (Leng et al., 2010a). This suggests that, in combination with modern lake water that has higher δ 18 O than local meteoric water, changes in the isotope composition of lake water (δ 18 O lw ) are principally driven by regional water balance and most likely represent lower frequency changes in climate.

Late Glacial to Holocene isotope data
A 10 m core (Co1262), recovered from the western margin of Lake Ohrid at the "Lini" drill site ( Fig. 1), has been analysed for δ 18 O c and δ 13 C c at high resolution over the Late Glacial to Holocene (Lacey et al., 2015); the study is utilised here as a recent comparison for the longer-term reconstruction in combination with the modern water isotope data. Lacey et al. (2015) highlighted the significance of Lake Ohrid as a sensitive recorder of climate change and confirmed that Ohrid responds to regional changes in water balance over the Holocene. Core Co1262 has δ 18 O c ranging between −6.5 and −2.1 ‰, being higher following the Late Glacial to Holocene transition, reaching a minimum between approximately 9 and 7 ka, and subsequently undergoing a step-wise increase to present-day values (Fig. 5). This pattern of δ 18 O c variability is similar to other lake sediment sequences from Greece (Lake Pamvotis; Frogley et al., 2001) and Turkey (Lake Acıgöl; Roberts et al., 2001), as well as in speleothem records from Israel (Soreq Cave; Bar-Matthews et al., 1999). A period of sustained moisture availability above that of present-day values is recorded between 8 and 6.5 ka from Lake Acıgöl (Roberts et al., 2001), which is likewise identified at Lake Pamvotis where higher lake levels are inferred by a reduction in the quantity of shallow water ostracod taxa (Frogley et al., 2001). The Soreq Cave speleothem record indicates a greater annual number of heavy rainstorms throughout the period 10 to 7 ka, with rainfall estimated to have been up to twice that of present day (Bar-Matthews et al., 1997), and similarly early Holocene rainfall is calculated to have increased by around 20 % in central Anatolia (Jones Isotope results from calcite (δ 18 O c , δ 13 C c ), siderite (δ 18 O s , δ 13 C s ), and calculated lake water (δ 18 O lw ) from Lake Ohrid, also showing TIC (Francke et al., 2016), the Holocene Co1262 calibration data set (δ 18 O c , δ 13 C c ; Lacey et al., 2015), and MIS stratigraphy (Railsback et al., 2015). FTIR results are shown for calcite (grey bars; calcite area = 707-719 cm −1 ) and siderite (blue bars; siderite area = 854-867 cm −1 ). Calcite data are given as raw (grey line) and lowess as smooth (span = 0.02; black line); siderite data are presented as individual points (black dots). For calculation of δ 18 O lw , +18 • C was assumed for calcite data (red shaded area = ±3 • C) and +6 • C for siderite data (red shaded area = ±2 • C); see text for further detail. et al., 2007). A wetter early Holocene is suggested by several other central-eastern Mediterranean records, such as that from Lake Pergusa (Zanchetta et al., 2007), Lake Van (Wick et al., 2003), Lake Zeribar (Stevens et al., 2001), and Lake Göhlisar (Eastwood et al., 2007). The transition to drier climate conditions in the late Holocene is reflected across these records as a progressive shift to higher δ 18 O c . Therefore, the Holocene calibration data set confirms that Lake Ohrid δ 18 O c is primarily driven by millennial-scale changes in regional water balance.

Oxygen isotope composition of calcite
δ 18 O c is dependent on δ 18 O lw and the temperature of lake water at the time of mineral precipitation, assuming equilibrium conditions (Leng and Marshall, 2004). Sediment trap data from Lake Ohrid show that calcite precipitation is seasonally induced, with up to 3 times more TIC formed during summer months in comparison to winter months (Matzinger et al., 2007). The precipitation of calcite is thought to be  (Anovski et al., 1980(Anovski et al., , 1991Eftimi and Zoto, 1997;Matzinger et al., 2006a;Jordanoska et al., 2010;Leng et al., 2010a). The Global Meteoric Water Line (GMWL; Craig, 1961), local meteoric water line (LMWL; Anovski et al., 1991;Eftimi and Zoto, 1997), and calculated local evaporation line (LEL) are given. The annual distribution of δ 18 O was calculated using the Online Isotopes in Precipitation Calculator (OIPC; Bowen et al., 2005;Bowen, 2015). associated with increased temperatures and the photosynthetic removal of CO 2 within the epilimnion, providing there is sufficient supply of calcium (Ca 2+ ) and bicarbonate (HCO − 3 ) ions, which are mainly sourced from spring inflows (Matzinger et al., 2006a). The production of phytoplankton in the lake reaches a maximum between June and August, during which the temperature of the main productivity zone ranges between approximately 12 and 22 • C (Stankovic, 1960). If the average temperature of the photic zone during summer months is approximately 18 • C, and we take the average modern δ 18 O lw of −3.5 ‰ (Fig. 4), the calculated δ 18 O c of contemporary calcite precipitation should be approximately −4.0 ‰, using the equation of O'Neil et al. (1969), or -4.4 ‰, using the Leng and Marshall (2004) expression of Kim and O'Neil (1997). This is similar to the average δ 18 O c through the Holocene (−4.9 ‰) and to the most recent measurement of δ 18 O c (−4.5 ‰) from core Co1262 (Lacey et al., 2015), suggesting that δ 18 O c most likely corresponds to summer lake water conditions.
Calcite may comprise up to 80 % of the total sediment composition (assuming TIC mainly represents CaCO 3 ; Wagner et al., 2008), with only a minor biogenic component (< 0.1 %) and limited terrigenous contribution . As Lake Ohrid is located within a karst catchment, a proportion of the carbonate could be of detrital origin, which commonly has a different isotope composition to the endogenic fraction (Leng et al., 2010b). The catchment geology has variable δ 18 O (−9.7 to −2.6 ‰; Leng et al., 2010a); how-δ¹⁸Oc (‰) -6.5 -2 δ¹⁸Oc (‰) -5 5 Lake Acıgöl Lake Ohrid . Comparison between the high-resolution Holocene δ 18 O c calibration data set from Lake Ohrid Lini core Co1262 (Lacey et al., 2015) and δ 18 O from other regional records, including Lake Pamvotis, Greece (Frogley et al., 2001), Lake Acıgöl, Turkey (Roberts et al., 2001), and Soreq Cave, Israel (Bar-Matthews et al., 1999). ever, previous investigations on sediment trap and core material have shown that the calcite crystals have morphological characteristics (for example size and shape) typical of an endogenic origin Matter et al., 2010). The modern isotope data from Lake Ohrid indicate a clear evaporative disparity between the isotope composition of lake water and that from meteoric and groundwater sources (Fig. 4). The calcite precipitated from a hypothetical, exclusively meteoric, water source would have δ 18 O c = −10.6 ‰ (using the mean summer lake water temperature of +18 • C and inflow δ 18 O of −10.1 ‰). Similarly low δ 18 O c is not observed in any isotope data from Lake Ohrid to date, including core catcher data covering the entire 1.2 Ma sediment sequence , which indicates that lake water has always been subject to a varying extent of evaporative fractionation. This suggests that δ 18 O lw is primarily influenced by long-term changes in the precipitation/evaporation ratio (P / E). Although temperature changes will influence δ 18 O lw values, the effect is reported to be roughly +0.2 ‰ • C −1 for the central Mediterranean region (Bard et al., 2002), which is quantitatively compensated for by the equilibrium isotope fractionation between carbonate and water (−0.2 ‰ • C −1 ; Leng and Marshall, 2004).
The δ 18 O c data from calcite in Lake Ohrid are largely restricted to the interglacial (or interstadial) periods. Interglacial sediments are characterised by concomitant increases in both TIC and TOC (Francke et al., 2016), suggested to be the result of enhanced primary productivity associated with a warmer climate . Calcite precipitation is favoured by elevated temperatures during interglacials, which drives higher evaporation rates, thereby concentrating Ca 2+ and HCO − 3 ions. Further, elevated catchment soil activity and temperature will enhance the dissolution of carbonate rocks leading to a greater concentration of dissolved ions in karst spring water. Warmer surface waters also lower the calcite saturation threshold . Conversely, glacial sediments typically have low TIC and TOC that are inversely correlated with K and Ti concentrations, indicating low productivity and increased clastic input . Lower temperatures during glacial periods would lead to a more oxygenated water column through increased vertical mixing and more frequent complete deep convective overturn. Enhanced levels of mixing breaks down water column stratification and associated oxygenation of the water column increases the rate of aerobic decomposition of organic matter, releasing CO 2 that reduces pH levels and increases calcite dissolution . Following extensive organic matter degradation the C / N ratio of sediments may be significantly reduced, as observed in the DEEP site cores (Francke et al., 2016) and in previous cores from Lake Ohrid where during the Last Glacial C / N values were typically very low (4-5) compared to higher values (8-12) in both the Holocene and MIS 5 (Wagner et al., 2009;Leng et al., 2010a). Catchment permafrost may have also been prevalent in glacial periods, limiting the supply of Ca 2+ and HCO − 3 ions to the lake by reducing the volume of karstic spring inflow (Belmecheri et al., 2009), which is supported by pollen-inferred mean annual temperatures during the last glacial period of between −3 and +1 • C (Bordon et al., 2009). Although there is no (or limited) calcite in the glacials, previous work on Lake Ohrid has shown spikes in TIC during MIS 2-3 , and similar in-creases in glacial TIC are observed throughout the 5045-1 composite profile (Francke et al., 2016). These TIC spikes are most likely analogous to those found in Lake Prespa glacial sediments during MIS 4-2, which comprise siderite .

Oxygen isotope composition of siderite
Thin sections from discrete higher-TIC glacial intervals reveal individual siderite crystals (< 5 µm) and siderite crystal clusters (50-100 µm) nucleating within an uncompacted clay matrix (Fig. 6). The distribution of siderite within each thin section is variable; a higher concentration of siderite crystals is contained within burrow-like structures that impart a mottled texture to the sediment. Occasional dolomite grains, large (> 20 µm) and distinct from the fine clay matrix, are fringed by 5 µm grains of siderite. The dolomite crystals are thought to be detrital as they are larger than the individual siderite grains and have irregular margins. Siderite comprises the principal carbonate component in these horizons, and apart from the occasional dolomite crystals, no other type of carbonate was observed. Individual siderite crystals appear to predominantly form within the open framework of the clay matrix, which suggests they precipitated in situ within the available pore space. The siderite is therefore most likely to be early diagenetic and formed before compaction within the sediment. Discrete horizons enriched in Fe have been previously observed in Lake Ohrid , neighbouring Lake Prespa Leng et al., 2013), and in other ancient lakes, such as Lake Baikal (Granina et al., 2004), where the formation of Fe-enriched layers up to approximately 25 cm below the sediment-water interface is thought to be related to bottom water redox conditions and significant changes in sedimentation regime. Assuming the siderite is formed in superficial sediments during the initial stages of diagenesis, like calcite, its isotope composition can be used as an indicator of depositional environment (Mozley and Wersin, 1992).

Comparison of the oxygen isotope composition of calcite and siderite
To enable comparison between δ 18 O c and δ 18 O s , we convert both to δ 18 O lw using specific mineral fractionation equations and different estimates of temperature. For calcite data we use the equation of O'Neil et al. (1969) and a precipitation temperature of +18 • C (±3 • C) to represent average summer conditions within the photic zone during the period of maximum phytoplankton activity. For siderite data we use the equation of Zhang et al. (2001), which is considered robust for defining equilibrium precipitation at lower temperatures (Ludvigson et al., 2013), and assume a bottom water temperature of +6 • C (±2 • C; Stankovic, 1960). The calculated δ 18 O lw is given in Fig. 3 and the averages for each MIS are compared to those of δ 13 C c and δ 13 C s in Fig. 7a.
The calculated δ 18 O lw from glacial siderite is generally lower compared that from calcite (higher δ 18 O lw ) during warmer interglacial periods (Fig. 7). Although siderite horizons probably represent distinct rapid and recurrent events in Lake Ohrid , overall lower δ 18 O lw may nevertheless be expected through glacial periods due to reduced lake water evaporation as a result of decreased temperatures. Jones et al. (2007) calculated evaporation rates at Eski Acıgöl in central Turkey and showed that glacial evaporation was around 3 times lower compared to that of the Late Holocene (0.4 m yr −1 vs. 1.1 m yr −1 ). If a similar calculation is conducted for Lake Ohrid, glacial evaporation may have been over 4 times lower than during the present interglacial (0.4 m yr −1 vs. 1.8 m yr −1 ), after Jones et al. (2007) using the equation of Linacre (1992). Higher evaporation rates are typically associated with closed lake basins and covariance between δ 18 O and δ 13 C (Talbot, 1990;Li and Ku, 1997), which is observed in the Lake Ohrid data as interglacial δ 18 O lw from calcite has a moderate covariance (r = 0.30; Fig. 7b), corroborating that interglacial periods were characterised by higher evaporation. As rates of evaporation reduce during colder intervals, the influence of other controlling factors, such as δ 18 O p , may have had a greater importance in determining δ 18 O lw during glacial periods.
During colder intervals, δ 18 O p would have been lower as a direct correlation exists between annual precipitation and temperature of +0.6 ‰ • C −1 at mid-high latitudes (Dansgaard, 1964) and +0.2 ‰ • C −1 in the central Mediterranean (Bard et al., 2002). If a mean annual temperature difference of up to 9 • C is assumed between interglacial and glacial periods, based on pollen-inferred temperature data from nearby Lake Maliq (Bordon et al., 2009), δ 18 O p may have decreased by between −5.4 and −1.8 ‰ in glacial periods. When considering interglacial-glacial timescales, changes to the oxygen isotope composition of seawater may also influence δ 18 O p . The isotope composition of mean global seawater is reported to be 1.0 ‰ higher during the last glacial due to the expansion of global ice volume  and up to 1.2 ‰ in the Mediterranean due to local evaporative enrichment (Paul et al., 2001). Therefore, the net effect of temperature and source changes during glacial periods results in lower δ 18 O p .
In addition to more regional effects on δ 18 O lw , local influences may also contribute to lower isotope values through glacial periods. Today, a significant proportion of winter precipitation occurs as snowfall at higher altitudes in the Ohrid-Prespa catchment, which is ultimately transferred to the lakes during spring when temperatures remain high enough for the snow to melt (Hollis and Stevenson, 1997;Popovska and Bonacci, 2007). Average winter temperatures at present are around 2 • C (Stankovic, 1960); however, winter temperatures would have been considerably reduced during glacial periods and temperatures during summer months may also have been lower (Bordon et al., 2009). If lower temperatures persisted throughout much of the year, a higher proportion of annual precipitation may have fallen in winter as snow. Snow is typically characterised as having much lower δ 18 O than rainfall, which reflects in-cloud equilibrium conditions and cooler condensation temperatures (Darling et al., 2006), and so would provide a further potential source for low δ 18 O (Dean et al., 2013).
Temperatures may have been sufficiently reduced during glacials to also allow (at least discontinuous) permafrost to form in the Ohrid catchment, thereby decreasing input from karst waters and perhaps restricting the inflow of water from Lake Prespa (Belmecheri et al., 2009). Lake Prespa provides a large proportion of water input to Ohrid through the underground network of karst channels, which has higher δ 18 O lw when compared to measured precipitation ( Fig. 4; Leng et al., 2010a). This infers that during periods where glacial conditions were prevalent in the catchment, the inflow of water comprising high δ 18 O from Lake Prespa may have reduced,   Lisiecki and Raymo, 2005), Antarctic EPICA Dome C CH 4 and CO 2 (Loulergue et al., 2008;Lüthi et al., 2008), and MIS stratigraphy (Railsback et al., 2015). and input would have instead been principally sourced from a combination of direct precipitation and surface run-off, both of which would result in lower δ 18 O lw .

Carbon isotope composition of carbonate
When a carbonate mineral precipitates under equilibrium conditions it captures the δ 13 C of the total dissolved inorganic carbon (TDIC) of lake water. TDIC in most lakes (at neutral pH) can be approximated to dissolved HCO − 3 , which is principally derived from the dissolution of carbonate catchment rocks, soils, and atmospheric CO 2 (Cohen, 2003). Consequently there are several carbon reservoirs that may influence δ 13 C TDIC , in addition to two major fractionation effects: (1) the chemical exchange between atmospheric CO 2 and dissolved HCO − 3 and (2) kinetic processes during the formation of organic matter (Hoefs, 1980;McKenzie, 1985). In Lake Ohrid, endogenic calcite precipitated within the epilimnion is thought to form in equilibrium with surface waters; thus δ 13 C c can provide information on past variations in δ 13 C TDIC and associated carbon-cycle transitions (Leng and Marshall, 2004). The DEEP site record (Fig. 3) shows overall high and consistent δ 13 C c throughout the core (+0.4 ± 0.6 ‰, 1σ , n = 924), which is most likely driven in part by the carbon isotope composition of inflow (δ 13 C TDIC ). Within karst catchments the local and groundwater chemistry will be dominated by Ca 2+ and HCO − 3 ions (Cohen, 2003), and δ 13 C TDIC will be high as catchment limestones usually comprise ancient marine carbonates (average δ 13 C = 0 ‰; Hudson, 1977;Jin et al., 2009) and typically range between −3 and +3 ‰ (Andrews et al., 1993;Hammarlund et al., 1997). Although δ 13 C TDIC has not been measured for Lake Ohrid, analysis conducted on several of the main geological units in the catchment provides average δ 13 C of +1 ‰ (Leng et al., 2010a), which confirms that δ 13 C TDIC will most probably be high as over 50 % of water input to Ohrid is derived from springs fed by karst aquifers (Matzinger et al., 2006b). Over glacial-interglacial timescales, the extent to which geological sources of carbon contribute to TDIC will primarily be determined by hydrological balance and associated changes in the residence time of water passing through the karst system; lower δ 13 C TDIC may occur during wetter periods with a lower residence and higher δ 13 C TDIC in more arid periods with a higher residence time.
In opposition to geological sources of high δ 13 C, a major source of carbon in ground-and river water typically derives from CO 2 liberated during the decay of terrestrial organic matter (low δ 13 C). Low δ 13 C TDIC is measured for Lake Prespa water inputs (average δ 13 C TDIC = −11.5 ‰;  and may be similar to the inflow to Lake Ohrid. Over glacial-interglacial timescales, variations in the proportion of soil-derived CO 2 incorporated into catchment waters will likely influence Lake Ohrid δ 13 C TDIC . In colder periods, δ 13 C TDIC would be higher due to poor soil development (Panagiotopoulos et al., 2014;Sadori et al., 2016); conversely, during wetter and warmer intervals the development of dense forests would promote well-developed soils (lower δ 13 C TDIC ) and encourage the delivery of Ca 2+ and HCO − 3 through the dissolution of carbonate catchment rocks.
Equilibrium exchange between atmospheric CO 2 and lake water will result in δ 13 C TDIC of approximately +3 ‰ (fractionation factor +10 ‰ when in equilibrium with CO 2 gas). As δ 13 C c is thought to reflect changes in δ 13 C TDIC , higher δ 13 C c in the Ohrid record may also reflect variable degrees of equilibration between atmospheric CO 2 and dissolved HCO − 3 . This process is also observed in isotope data from Lake Prespa, where low δ 13 C TDIC entering the lake (−11.5 ‰) is modified by within-lake processes to increase lake water δ 13 C TDIC to give the average value of −5.2 ‰ . In addition to evaporation, it is also likely that biogenic productivity drives higher δ 13 C TDIC in Prespa (and Ohrid) due to the preferential incorporation of 12 C dur-ing photosynthesis, assuming the organic carbon is exported to the lake floor and buried (Meyers and Teranes, 2001).
Evaporative drawdown will also affect δ 18 O as the preferential loss of 16 O during evaporation can drive higher δ 18 O resulting in covariance between δ 18 O and δ 13 C. The signal of covariance will be recorded in primary lacustrine carbonates and can potentially be used to determine the degree of past hydrological closure (Talbot, 1990). The extent to which isotope measurements covary can depend on several factors, including hydrological balance, stability of the lake volume, vapour exchange, and evaporation (Li and Ku, 1997). Covariance may therefore not simply be a function of residence time or hydrological closure (Leng et al., 2010a) and has been shown to be spatially inconsistent between Mediterranean lake sediment records (Roberts et al., 2008). Nevertheless, lake level fluctuations are thought to have occurred in Lake Ohrid, at least during MIS 6, as evidenced by the presence of subaquatic terraces on the northeast shore of the lake (Lindhorst et al., 2010). Reductions in lake level, most probably coincident with periods of regional aridity and generally lower P / E, may limit surface outflow, solely met at present by the river Crim Drim (66 %; Matzinger et al., 2006b), which in turn would extend lake water residence time and increase the possibility of evaporation and isotope exchange, resulting in higher δ 18 O and δ 13 C. However, periods of higher covariance are generally restricted to certain intervals, for example MIS 5 (r = 0.53, p = < 0.001, n = 104), as throughout the whole core correlation between δ 18 O and δ 13 C is generally weak (r = 0.30, p = < 0.001, n = 924; Fig. 7). In those areas where δ 18 O and δ 13 C are decoupled, local in situ process are likely to dominate the evolution of δ 13 C TDIC and act to buffer any climate signal .
In contrast to δ 13 C c , δ 13 C s is higher in Lake Ohrid sediments by > 8 ‰ and has a mean value of +12.3 ± 0.5 ‰ (1σ , n = 22). Higher δ 13 C is characteristic of siderite formed in non-marine sediments and is most probably associated with the incorporation of 13 C-enriched bicarbonate derived from methanogenesis. The metabolic pathway utilised by bacteria during the reduction of organic matter strongly fractionates in favour of 12 C, which, for isotopic mass balance, produces methane (low δ 13 C) and proportionally enriches the bicarbonate ion in 13 C. The methane is subsequently removed by ebullition or by emission through the stems of aquatic macrophytes, and the enriched bicarbonate is incorporated into TDIC (Curry et al., 1997).

Climate and interglacial variability at Lake Ohrid over the past 637 ka
The Late Quaternary is characterised by cyclic alternations between colder glacial and warmer interglacial periods, the timing and magnitude of which are principally determined by orbital-induced climate oscillations and variations in atmospheric greenhouse gas concentrations (Im-  , 1984;Shackleton, 2000). This glacial-interglacial climate signal has been globally observed in deep marine sediments (Lisiecki and Raymo, 2005), ice cores (Jouzel et al., 2007), and continental sequences (Sun and An, 2005), which, when compared, indicate a broad correspondence over orbital timescales (Tzedakis et al., 1997;Lang and Wolff, 2011). However, comprehensive terrestrial sequences covering multiple glacial-interglacial cycles are still rare (Prokopenko et al., 2006;Tzedakis et al., 2006), especially when isotope stratigraphies are considered, and consequently the 5045-1 record can provide valuable information on climate evolution over an extended time frame. Although it is beyond the scope of this paper to look in detail at each interglacial over the last 637 ka (MIS 15 to MIS 5), some preliminary observations can be made about their structure and consistency.

MIS 16-13 (637-474 ka)
At the transition between MIS 16 and 15, δ 13 C c shows a prolonged trend to lower values between ca. 625 and 615 ka, whereas a δ 18 O c minima occurs at ca. 622 ka, following which values increase (Fig. 8) concomitant with increasing TIC and biogenic silica (BSi; Francke et al., 2016). This suggests an initial phase of higher precipitation (elevated P / E) may have been associated with a mediated catchment soil development and gradual climate warming following the MIS 16 glacial, and after ca. 622 ka lake waters became more evaporated. Excursions to lower δ 18 O c and δ 13 C c within MIS 15, centred on ca. 615, 600, and 577 ka, are most likely related to lower δ 18 O in LR04 ( Fig. 8; Lisiecki and Raymo, 2005), the Mediterranean Sea (Kroon et al., 1998), and the Ionian Sea (Rossignol-Strick and Paterne, 1999), representing an influx of glacial meltwater into the oceans and warmer conditions during MIS 15e, 15c, and 15a respectively (Fig. 8). In addition, this is mirrored by high arboreal pollen (AP) at Tenaghi Philippon (Tzedakis et al., 2006) and warmer temperatures during these times also promoted vegetation growth in the catchment at Lake Ohrid (Sadori et al., 2016), leading to enhanced soil development and lower δ 13 C c . MIS 14 is the only glacial of the record to contain a higher proportion of TIC and TOC throughout the majority of the stage (Francke et al., 2016), indicating temperatures at Lake Ohrid did not decrease significantly to abate productivity and calcite production (therefore calcite isotope data are available for the majority of MIS 14). Hydroclimate conditions are assumed to have remained fairly similar to MIS 15 as average δ 18 O c remains consistent between the two stages (Fig. 8), suggesting that MIS 14 was a particularly weak glacial. Sustained warmth through MIS 14 is in agreement with a range of global records, including in sea surface temperature (SST) estimates from the Iberian margin ( Fig. 8; Rodrigues et al., 2011) and South China Sea (Yu and Chen, 2011), BSi and magnetic susceptibility from Lake Baikal (Prokopenko et al., 2002(Prokopenko et al., , 2006, and several proxies from Antarctic ice cores (Jouzel et al., 2007;Masson-Delmotte et al., 2010). Although overall warmer conditions may have prevailed during MIS 14, the Ohrid record suggests that colder glacial-like conditions occurred between ca. 540 and 531 ka, which is supported by the presence of siderite (Fig. 3). δ 13 C c increases throughout MIS 14 imply a progressive decline in catchment soil development. A reduction in the amplitude of δ 13 C c oscillations, for example when compared to MIS 15, may be due to low orbital eccentricity reducing the influence of precession and insolation variability (Fig. 8).
MIS 13 in Lake Ohrid δ 18 O c and δ 13 C c represents a relatively stable period, experiencing only minor oscillations through the stage. The average isotope composition is comparable to that observed during MIS 14 and 15 (Fig. 7); however MIS 13 is generally considered to be one of the weakest interglacials of the last 800 ka (Lang and Wolff, 2011). Similar to the onset of MIS 15, MIS 13c is characterised by relatively rapid transition to lower δ 18 O c at ca. 529 ka followed by increasing values through to ca. 521 ka, associated with a gradual concomitant trend to lower δ 13 C c (Fig. 8). An excursion to higher δ 13 C c centred around ca. 510 ka is probably linked to cooler conditions and the onset of MIS 13b, and a transition to higher δ 18 O c suggests lower P / E, which is similar to observations from Tenaghi Philippon and Antarctic ice cores that indicate reduced AP and lower reconstructed temperatures (Tzedakis et al., 2006;Jouzel et al., 2007). The LR04 record also exhibits higher δ 18 O at this time, signifying an expansion of global ice volume ( Fig. 8; Lisiecki and Raymo, 2005). A shift to lower δ 18 O c and δ 13 C c at ca. 502 ka most likely represents the onset of MIS 13a, where δ 13 C c is the lowest of the stage (ca. 530 to 474 ka), suggesting MIS 13a most probably experienced warmer conditions than MIS 13c. The timing of interglacial conditions is unique within the Lake Ohrid record, and correspondingly in global sequences, as minimum ice volume (Lisiecki and Raymo, 2005) and maximum warmth (Jouzel et al., 2007;Loulergue et al., 2008) occurred in the final substage of the MIS rather than directly following the glacial termination (Fig. 8). This was most probably due to MIS 14 experiencing only weak glacial conditions resulting in a low-amplitude glacial termination (Voelker et al., 2010). Peaks to higher δ 18 O c and δ 13 C c toward the end of MIS 13a are coincident with an increased proportion of siderite in sediments (Fig. 3), and so fluctuations may be the result of a mixed carbonate composition rather than due to environmental change.

MIS 11 (425-380 ka)
MIS 11 is thought to have a characteristic orbital and climate configuration potentially analogous to that of the Holocene (Loutre and Berger, 2003) and follows a strong and relatively wet glacial MIS 12 at Lake Ohrid (Sadori et al., 2016). Following the glacial termination δ 18 O c is low (around −6 ‰) and increases over the next 15 ka to a maximum (−4 ‰) at ca. 410 ka, whereas δ 13 C c transitions to lower values between ca. 425 and 410 ka, suggesting a prolonged period of warm and relatively stable conditions. This period most likely corresponds to MIS 11c, where warmer, wetter conditions are supported by high TIC and AP at Lake Ohrid (Francke et al., 2016;Sadori et al., 2016). The overall progression of δ 18 O c and δ 13 C c through MIS 11c corresponds to high SST at the Iberian Margin ( Fig. 8; Rodrigues et al., 2011) and compares well with the development of CO 2 and CH 4 measured from the EDC Antarctic ice core record (Loulergue et al., 2008;Lüthi et al., 2008). A distinct δ 18 O c and δ 13 C c maxima centred around ca. 405 ka is likely associated with colder conditions and a drier environment (lower P / E) through stadial phase MIS 11b, which is supported by higher K intensity, lower TIC, and a substantial decrease in AP (Francke et al., 2016;Sadori et al., 2016). An overall trend to lower δ 18 O c and δ 13 C c through MIS 11a (ca. 402 to 385 ka) shows greater variability in comparison to MIS 11c, suggesting that the stability of hydroclimate conditions changed after peak interglacial conditions. Greater variability after ca. 400 ka is also seen in other proxies from Lake Ohrid (TIC, BSi;Francke et al., 2016), as well as in reconstructed SST at the Iberian Margin (Rodrigues et al., 2011) and atmospheric CH 4 and temperature profiles from Antarctic ice cores ( Fig. 8; Petit et al., 1999;Jouzel et al., 2007). Following the climatic optimum of MIS 11c, δ 18 O c transitions to the lowest value of the record at ca. 384 ka (−7.6 ‰), over which time there is an increase in global ice volume and decrease in atmospheric greenhouse gas concentrations observed through MIS 11b and 11a (Lisiecki and Raymo, 2005;Loulergue et al., 2008). A trend to lower δ 18 O c at Ohrid also traces reducing SST in the North Atlantic and on the Iberian Margin (Stein et al., 2009;Rodrigues et al., 2011). The progression to lower δ 18 O c during a period of overall cooling is expected, given that the reconstructed δ 18 O lw from glacial siderite is typically lower than interglacial δ 18 O lw , as calculated from calcite (Fig. 3).

MIS 9 (335-285 ka)
The initial ca. 4 ka of MIS 9 is marked by a transition to higher δ 18 O c and lower δ 13 C c , similar to previous interglacial stages. After ca. 330 ka variations in δ 18 O c and δ 13 C c are coupled and a minimum in both values is observed around ca. 329 ka (Fig. 8), which is assumed to correspond to peak interglacial conditions during MIS 9e. The onset of full interglacial conditions during MIS 9e is indicated to be relatively rapid, given that a maximum of ca. 6 ka elapses between the onset of calcite precipitation and peak interglacial conditions, which is consistent with warming at the start of MIS 9 observed in AP from Tenaghi Philippon (Tzedakis et al., 2006) and SST from the Iberian Margin ( Fig. 8; Martrat et al., 2007). An abrupt transition to the highest δ 18 O c and δ 13 C c of MIS 9 between ca. 321 and 318 ka is coincident with lower TIC, BSi, and AP at Lake Ohrid (Francke et al., 2016;Sadori et al., 2016). A reduction in Pinus populations in the Ohrid catchment alongside lower P / E indicates an overall drier climate, which is also documented at Tenaghi Philippon (Tzedakis et al., 2006) and most likely associated with stadial conditions during MIS 9d. The subsequent transition to lower δ 18 O c and δ 13 C c towards ca. 310 ka is probably associated with a warmer climate, higher precipitation and P / E during interstadial MIS 9c, reflected at Tenaghi Philippon by higher AP (Tzedakis et al., 2006) and moderately lower δ 18 O in the LR04 stack ( Fig. 8; Lisiecki and Raymo, 2005). A subsequent break in TIC preservation occurs between ca. 308 and 293 ka, which is preceded by a transition to higher δ 18 O c and δ 13 C c , suggesting a drier climate prevailed, and is most likely associated with stadial MIS 9b. This interval is also indicated to be an extended period of cold and dry conditions at Lake Ohrid by low TIC and BSi (Francke et al., 2016) and low AP (Sadori et al. 2016), where the presence of siderite advocates that a glacial-like climate state persisted throughout much of the stadial phase (Fig. 3). A trend to lower δ 13 C c between ca. 293 and 286 ka closely corresponds to an increase in TIC, BSi, and AP, and a rapid shift to lower δ 18 O c after ca. 293 ka indicates the onset of warmer and wetter conditions during interstadial MIS 9a. Low and relatively stable δ 18 O c values between ca. 292 and 288 ka imply a fresh lake system and higher P / E, which may be driven by increased precipitation in association with the deposition of sapropel S in the Mediterranean Sea (Ziegler et al., 2010) and summer insolation maxima ( Fig. 8; Laskar et al., 2004). Overall, low δ 18 O c through MIS 9 and its calculated δ 18 O lw are coincident with similar δ 18 O lw values from siderite during MIS 10 and MIS 8 (Fig. 3), indicating glacial and interglacial lake water broadly converge through this interval.

MIS 7 (243-191 ka)
MIS 7 at Lake Ohrid is characterised by three distinct phases of calcite preservation ( Fig. 3; Francke et al., 2016), which most likely correspond to MIS sub-stages 7e, 7c, and 7a (Railsback et al., 2015). The first peak in TIC between ca. 245 and 238 ka is associated with initially low and increasing δ 18 O c and high but decreasing δ 13 C c , which both show a higher amplitude of variability after ca. 242 ka. Increasing δ 18 O c suggests a transition from wetter to drier climate through to ca. 238 ka; however, warm and wet conditions are indicated by overall low δ 18 O c coincident with high TIC, BSi, and AP (Francke et al., 2016;Sadori et al., 2016). This period most probably corresponds to interglacial MIS 7e and is coincident with warming indicated by arboreal expansion at the Ioannina (Roucoux et al., 2008) and Tenaghi Philippon ( Fig. 8; Tzedakis et al., 2006) basins in Greece and at Lake Van in Turkey . Overall warming is supported by higher reconstructed SST for the Adriatic Sea (Piva et al., 2008) and on the Iberian Margin ( Fig. 8; Martrat et al., 2007). A cessation of TIC production between ca. 238 and 221 ka indicates a colder, extended glacial-like climate state during stadial MIS 7d, which is also suggested by the presence of siderite (Fig. 3). A relatively abrupt decrease in δ 13 C c after ca. 221 ka marks the onset of warmer conditions at Lake Ohrid and the transition to interstadial MIS 7c. An interval of low δ 13 C c between ca. 221 and 212 ka represents the lowermost values of the core, which suggests MIS 7c may have experienced warmer conditions than during the full interglacial (MIS 7e) and is supported by a higher TIC plateau (Francke et al., 2016). In addition, a greater diversity of AP is observed at Tenaghi Philippon alongside a greater abundance of thermophilous taxa (Tzedakis et al., 2003b) and higher BSi at Lake Baikal (Prokopenko et al., 2006). Although CO 2 and CH 4 concentrations were lower in MIS 7c (Loulergue et al., 2008;Lüthi et al., 2008), higher summer insolation during MIS 7c most likely promoted warmer climate conditions (Fig. 8). A change to stadial conditions is suggested around ca. 211 ka by a shift to higher δ 18 O c and δ 13 C c , as well as the presence of siderite (Fig. 3), which likely corresponds to MIS 7b. Decreasing δ 13 C c and an increasing δ 18 O c trend between ca. 210 and 202 ka most probably correspond to interstadial MIS 7a; however, unlike substages 7e and 7c, the interval is characterised by large amplitude coupled oscillations in both δ 18 O c and δ 13 C c . A change in climate stability is also reflected in AP at Tenaghi Philippon (Tzedakis et al., 2003b) and in speleothem δ 18 O from northern Israel (Bar-Matthews et al., 2003). After ca. 202 ka siderite is observed to be abundant through MIS 6.

MIS 5 (130-80 ka)
MIS 5 has been characterised as one of the strongest warm periods of the last 800 ka (Lisiecki and Raymo, 2005;Jouzel et al., 2007;Lang and Wolff, 2011). At Lake Ohrid, the highest δ 18 O c and δ 13 C c of the record are observed during MIS 5 (Fig. 8), suggesting low P / E and high rates of evaporation, which may be related to severe lake level changes inferred during MIS 5 (Lindhorst et al., 2010). A rapid transition to lower δ 18 O c and δ 13 C c after ca. 129 ka is most likely associated with the onset of full interglacial conditions during MIS 5e. A climatic optimum is probably associated with the point of lowest isotope values at ca. 124 ka; however, δ 18 O c decreases at a faster rate in comparison to δ 13 C c (Fig. 8). This may be due to terrestrial and lacustrine proxies decoupling due to local effects of ice cap and snowfield meltwater entering the lake, as is observed at Lake Ioannina (Wilson et al., 2015). Local ice caps on the mountains surrounding Lake Ohrid are indicated for the Last Glacial by catchment moraine deposits (Ribolini et al., 2011) and are likely to have been present during early glacials, such as MIS 6 (Francke et al., 2016). Overall, the interval of lower δ 18 O c and δ 13 C c is coincident with higher Mediterranean SST (Piva et al., 2008;Martrat et al., 2014), the deposition of sapropel S5 (Ziegler et al., 2010), and greater regional precipitation (Bar-Matthews et al., 2003;Drysdale et al., 2005). An increase in δ 18 O c and δ 13 C c values after ca. 114 ka signals the onset of stadial conditions (MIS 5d) and infers that interglacial conditions persisted for approximately 15 ka, which is in broad agreement with reported durations from other regional sequences (Tzedakis et al., 2003a;Brauer et al., 2007;Pickarski et al., 2015). High δ 18 O c and δ 13 C c between ca. 114 and 108 ka suggest drier conditions during MIS 5d, which is supported by the presence of siderite and a reduction in AP (Sadori et al., 2016). An excursion to lower δ 13 C c between ca. 108 and 91 ka is coincident with higher AP at both Lake Ohrid and Tenaghi Philippon (Tzedakis et al., 2003b;Sadori et al., 2016) and higher SST at the Iberian Margin ( Fig. 8; Martrat et al., 2007), which suggests warmer temperatures during MIS 5c. δ 18 O c is observed to be lower in MIS 5c than during full interglacial conditions through MIS 5e (Fig. 8), which could be due to refilling after a lake water lowstand (Lindhorst et al., 2010). Alternatively, given higher insolation and atmospheric CO 2 and CH 4 concentrations during MIS 5e (Petit et al., 1999;Laskar et al., 2004;Loulergue et al., 2008), evaporation may have been stronger and driven higher δ 18 O c , even under a regime of elevated regional precipitation (Bar-Matthews et al., 2003;Drysdale et al., 2005). A transition to higher δ 18 O c and δ 13 C c after ca. 91 ka corresponds to reduced TIC, BSi, and AP (Francke et al., 2016;Sadori et al., 2016) and high siderite abundance, which are assumed to be associated with stadial conditions during MIS 5b. The transition to higher isotope values occurs over a shorter time interval in comparison to MIS 5d, suggesting MIS 5b probably experienced a more produced change to cold and dry climate conditions. This is supported by higher speleothem δ 18 O at Soreq Cave (Bar-Matthews et al., 2003), lower reconstructed SST from the Adriatic and Alboran seas (Martrat et al. 2004;Piva et al., 2008), and lower atmospheric CO 2 ( Fig. 8; Petit et al., 1999). A decrease in δ 18 O c and δ 13 C c after ca. 86 ka is thought to be related to the onset of interstadial conditions during MIS 5a. A minimum in both isotope values centred around ca. 82 ka infers climate conditions may have been wetter and warmer toward the end of the stage. A transition to lower δ 18 O is also observed at Soreq Cave ( Bar-Matthews et al., 2003), suggesting regional rainfall may have been enhanced toward the end of MIS 5a, which may be related to a coincident peal in summer insolation ( Fig. 8; Laskar et al., 2004).

Conclusions
Here, new stable isotope data from the ICDP SCOPSCO 5045-1 composite core provide information on hydroclimate variability in the northern Mediterranean over the last 637 ka and represent one of the most extensive terrestrial isotope records available for the region. Modern lake water data (Leng et al., 2010a) and a high-resolution Holocene calibration data set (Lacey et al., 2015) show that contemporary lake water is evaporated and that variations in δ 18 O principally reflect changes in P / E driven by regional water balance. Iso-tope data from calcite are continuous through intervals associated with high TIC (interglacials and interstadials), and discrete bands of siderite are present during periods characterised by low TIC (glacials and stadial). The siderite is considered to be early diagenetic and therefore, like calcite, can be used as a proxy for past lake water conditions, assuming at shallow depths δ 18 O of the lake water and pore water at the same. Overall, calculated δ 18 O lw is lower during glacial periods indicating lake water was fresher in comparison to interglacials most probably due to a change in summer temperature, evaporation rates, and the proportion of winter precipitation falling as snow. The isotope data suggest largely stable conditions persisted through MIS 15-13, inferring MIS 14 to be a particular weak glacial. A transition to lower δ 18 O c is observed in the later stages of MIS 11 through to MIS 9, followed by a change to higher values through MIS 7 and evaporated conditions during MIS 5. The pattern of variability observed in the Lake Ohrid sequence reflects comparable changes in both regional and global palaeoclimate records, and our data highlight the potential for future work on the 5045-1 composite profile to provide evidence for long-term climate change in the Mediterranean, as a prerequisite for better understanding the influence of major environmental events on biological evolution within the lake.