Late Holocene climate of the Eastern Mediterranean inferred from diatom analysis of annually-laminated lake sediments

Quaternary Science Reviews 30 (2011) 3381e3392

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Late Holocene climate of the Eastern Mediterranean inferred from diatom analysis of annually-laminated lake sediments Jessie Woodbridge*, Neil Roberts School of Geography, Earth and Environmental Sciences, University of Plymouth, Drake Circus, Plymouth PL4 8AA, UK

a r t i c l e i n f o

a b s t r a c t

Article history: Received 6 April 2011 Received in revised form 23 August 2011 Accepted 25 August 2011 Available online 2 October 2011

Diatoms from the annually-laminated sediments of Nar crater lake in central Turkey are used to investigate climatic changes throughout the last 1720 years at decadal time resolution. A diatom-conductivity transfer function is employed to infer past water balance. Further information has been extracted from the palaeo-record through calculation of diatom biovolume, rarefaction (species diversity) and concentration, and through the identification of diatom bloom events in core thin sections. The Nar diatom sequence is compared with oxygen isotope (d18O) and pollen records from the same sediment cores in order to test the respective roles of changes in climate and land cover. Diatom-inferred (DI) conductivity excluding bloom taxa and d18O show very good correspondence for the first half of the record and demonstrate that this region experienced a period of century-scale drought prior to AD 540, with a subsequent rapid and simultaneous shift to fresher lake conditions and wetter climate. After a drier phase in the Nar record from AD 800e950, the period of the Medieval Climate Anomaly (AD 950 e1400) was generally well watered. During the subsequent Little Ice Age (wAD 1700e1900), DI-conductivity and d18O become decoupled. Thin sections reveal between 20 and 40 distinct diatom bloom events per century since AD 1100, with increasing frequency between AD 1700 and 2000. Human land-use changes evident in the pollen sequence may have influenced the diatom relationship with lake water conductivity in the later part of the record. None the less, diatom DCA axes do show a clear response to multi-decadal drought events within the last six centuries. Differences between the proxy-climate records from Nar Lake may be associated with the dissimilar thresholds to environmental fluctuations and non-stationarity in the response of different proxies through time. The palaeoclimate records from Nar show that arid periods occurred in the Eastern Mediterranean during the last two millennia that were more prolonged and extreme than those experienced in the last century. Ó 2011 Elsevier Ltd. All rights reserved.

Keywords: Diatom Eastern Mediterranean Turkey Palaeoclimate Salinity Transfer function Crater lake

1. Introduction Droughts are a critical and persistent feature of the world’s drylands. It is consequently important to understand their longterm frequency, magnitude and duration in order that droughtsensitive regions such as the Mediterranean and south-western Asia can be managed and developed sustainably. A key time period for study is the last 2000 years, during which time underlying orbital forcing of the Earth’s climate system has been almost the same as today (Wanner et al., 2008), making it easier to identify other forcings. Furthermore, knowledge of hydrological changes during the last two millennia is valuable for understanding current

* Corresponding author. Tel.: þ44 1752 585920. E-mail addresses: [email protected] (J. Woodbridge), C.N. [email protected] (N. Roberts). 0277-3791/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.quascirev.2011.08.013

and future anthropogenic impacts on climate (Luterbacher et al., 2006; NRC, 2006). Hydrologically-closed (i.e. non-outlet) lakes can act as valuable registers of climatically-induced changes in regional water balance. They respond dynamically to changes in precipitation and evaporation by adjusting their surface area and water level, which in turn alters their hydrochemistry (Wetzel, 2001). During drought events, the area of a closed lake shrinks, water levels fall and salinity increases. Water level and salinity variations in crater lakes and other ‘simple’ closed lacustrine water bodies therefore reflect past periods of drought and flood (Battarbee et al., 2001). This environmental information is recorded in lake sediments, which can provide a record of hydro-climatic variability over decadal to millennial timescales via a wide range of palaeoecological, geochemical and geomorphological proxies. Shifts in lake water balance are preserved in geochemical proxies, such as the SreCa ratio and oxygen isotope (d18O) composition of lacustrine

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carbonates (Leng and Marshall, 2004), and in the species composition of aquatic organisms, such as ostracods and diatoms. Past changes in lake salinity (expressed as conductivity) have been quantified via transfer functions that model statistically the relationship between modern water chemistry and species assemblages (e.g. Reed et al., 1999). Aquatic organisms can therefore provide a powerful tool to infer past limnological conditions quantitatively and diatoms, in particular, have frequently been used to reconstruct lake salinity and to infer changes in climate (Fritz et al., 1999). The Late Holocene has witnessed increasing human impact upon terrestrial ecosystems, notably by deforestation and land-use conversion for agriculture. Land cover change in lake watersheds has indirectly caused significant alterations in freshwater aquatic ecosystems. Increased flux of mineral sediments, organic matter and nutrients from catchments has led to shifts in limnic productivity and pH that are clearly indicated in lake sediment records from the temperate zone (e.g. Renberg et al., 1993; Bradshaw et al., 2005). Many dryland lake catchments have also been impacted by human activities during the last 2000 years (e.g. Flower et al., 1989; Lamb and van der Kaars, 1995; Lamb et al., 1999; Wick et al., 2003), and this is likely to have caused multiple forcing of ecosystem states, with site-specific human impacts superimposed on regional-scale hydro-climatic drivers. Non-stationarity in ecosystem state over the Late Holocene may have generated compound signals, in which it can be difficult to disentangle climatic variations from humaninduced catchment disturbance. This complexity may be especially significant if palaeolimnological data are transformed statistically to derive quantitative estimates of environmental parameters in the past, such as lake salinity or drought indices. In this study, we investigate the issue of multi-causality during the Late Holocene in a high-resolution diatom record from a climatesensitive crater lake in a dryland region, Nar Gölü (Nar Lake), central Turkey. We compare the lake diatom record against other proxy data from the same sediment core record, specifically d18O as an index of hydro-climatic variability (Jones et al., 2006) and a pollen profile as a record of changes in land use and vegetation cover (England et al., 2008) over the last 1720 years. This multi-proxy approach allows different drivers of the lake diatom record to be identified and separated. Diatoms have been utilised as a climate proxy in Turkish lakes by several researchers including Reed et al. (1999), Roberts et al. (2001) and Kashima (1996, 2003). However, these studies involved multi-millennial timescales with limited chronological precision and were disadvantaged in some cases by poor diatom preservation. By contrast, Nar Gölü has excellent diatom preservation and has deposited annually-laminated lake sediments that allow high-resolution analyses and good dating accuracy (Jones et al., 2005). 1.1. Regional and site description Nar Gölü is a relatively small and deep lake located in the Cappadocia region of central Anatolia in central Turkey (38 200 25.0800 N; 34 270 24.1700 E) (Fig. 1, Table 1). The lake lies at 1363 masl and the crater rim reaches w200 m above the lake. Isotopic hydrological modelling (Jones et al., 2005) and lake monitoring indicate that the lake level and isotope chemistry are linked to the balance between regional precipitation and evaporation. Groundwater accounts for around two-thirds of water inflow and about half of outflow (Jones et al., 2005). Nar water is weakly alkaline and oligosaline-mesosaline with sodium, chloride and bicarbonate as the major ions in the system. Summer nitrate and phosphate concentrations are low, implying that these may be limiting nutrients. Temperature, pH, dissolved oxygen content and conductivity measurements through the water profile indicate that the lake is monomictic.

Fig. 1. Nar Gölü catchment map showing sampling locations with inset map of the Eastern Mediterranean showing the locations of other sites discussed.

The climate of central Turkey is continental Mediterranean (Kutiel and Türkes¸, 2005). Combined with the elevation of Cappadocia, this leads to substantial temperature variations between summer and winter. Average Nevs¸ehir (w50 km from Nar Gölü) summer (Jun., Jul. and Aug.) and winter (Dec., Jan. and Feb.) temperatures are 20.3  C and 0.4  C respectively. Precipitation mainly falls in winter and spring and is associated with westerly cyclonic depressions. Spring precipitation accounts for over 30% of the annual total (300e400 mm y1), while summer accounts for less than 5% (Türkes¸, 2003). Dry steppe-forest is the dominant natural vegetation at present, with degraded deciduous oak (Quercus cerris) woodland above 1400 masl (England et al., 2008). There is extensive evidence of human-induced alteration to natural vegetation in pollen records from central Anatolia over the last three-four millennia (Woldring and Bottema, 2003). Table 1 Nar Gölü characteristics and water chemistry. Further details available in Woodbridge and Roberts (2010, Table 1). Lake area (m2)

Lake volume (m3)

Catchment area (m2)

Surface pH

Surface conductivity mScm1

Water depth (m)

5.6  105

7.7  106

2.4  106

7.9 (07/01) 8.2 (07/09)

3300 (07/01) 3370 (07/09)

26.0 (07/01) 22.5 (07/09)

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2. Methods

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The NAR01/02 sequence is entirely laminated with sporadic grey clastic layers, which represent homogenites/turbidites and relate to sub-lacustrine mass movements of sediment. Annual varve couplets are composed of light coloured (white) lamina comprising endogenic carbonate and dark brown layers comprising organic material and diatoms. The age-depth relationship reveals that sedimentation at Nar is almost constant when clastic layers are excluded, and that the 376 cm-long master sequence covers the last 1720 years (AD 280e2001).

to existing transfer function training sets provided by EDDI (Juggins, 2011). Training sets and models have been selected based on the percentage of fossil sample species represented in the modern data set, the number of sites in which these species are present and the model performance (r and RMSEP). The combined salinity training set (comprising data from East Africa, North Africa and Spain) by weighted averaging with inverse deshrinking was identified as satisfying these criteria most suitably. Within a new diatom/ostracod-salinity training set from Turkish lakes (Reed et al., in press) similar diatom species optima were identified to those of adjacent biogeographic regions; therefore the combined salinity training set employed here is thought to be suitable for reconstructing conductivity from the Nar sediments.

2.2. Field methods

3. Results

Core sediment sequences were collected in 2001 and 2002 using a combination of Livingstone (1955) and Mackereth (1969) corers to generate a 376 cm-long master sequence (NAR01/02), supplemented by a 36 cm-long short core (NAR06) collected using a Glew corer (Glew et al., 2001) during summer 2006. Cores were extruded, cut into half lengths and stored in guttering below 4  C. Modern diatom samples have been collected from habitats including lake macrophytes, bottom mud, surface gravel and plankton, along with sediment traps collected and replaced annually each summer. For information on these and the detailed diatom record for the NAR06 short core, which covers the period from 1926 to 2006, see Woodbridge (2009) and Woodbridge and Roberts (2010).

3.1. Nar diatom assemblages

2.1. Chronology

2.3. Laboratory methods A master sequence has been compiled from the Nar core sections through replicated varve counting of multiple cores and radioisotope dating (Jones et al., 2006). 210Pb and 137Cs analysis of the top 50 cm of the core and sediment trap material identified that varve couplets are deposited annually. Comparisons of counts suggest that varve ages from the NAR01/02 sequence have a maximum possible uncertainty of 2.5% of the given age. Subsamples for diatom analysis were obtained by cutting fragments of laminations from core half sections using a scalpel. Three varve year (VY) samples, of approximately 0.5 cm3, were taken at ten varve year intervals for the NAR01/02 master sequence. Annual sub-samples, representing the most recent 80 VY (AD 1926e2006), were taken from the NAR06 core (results described in Woodbridge and Roberts, 2010). The diatom preparation procedure follows standard methods adapted from Battarbee (1986) and Battarbee et al. (2001), with 300e400 diatom valves counted in transects on each microscope slide. Literature sources for species identification primarily included Krammer and Lange-Bertalot (1991a,b, 1997a,b, 2000) and the European Diatom Database Initiative (EDDI) (Juggins, 2011). In addition, thin sections were prepared from the upper half of the NAR01/02 sequence. For thin section preparation 7 cm-long core sections were impregnated in epoxy resin, glued onto microscope slides, ground to 30 mm thickness and enclosed with a coverslip. Thin sections were viewed using a light microscope, which allowed individual varves and diatom-rich layers to be analysed. 2.4. Data analysis Diatom count and percentage data have been used to calculate valve concentration, biovolume (Kirschtel, 1996) and rarefaction (Holland, 2003). Detrended Correspondence Analysis (DCA) was performed on percentage and biovolume data and axis scores have been plotted against sediment depth. The diatom data were applied

The Nar diatom assemblages are diverse and issues have arisen regarding the taxonomy of certain taxa. For example, numerous extremely variable Nitzschia diatoms were difficult to attribute precisely due to the presence of intermediate features that appeared to belong to different species and forms. In addition, a previously undescribed taxon, newly named as Clipeoparvus anatolicus (Woodbridge, E.J. Cox and Roberts, 2010), is abundant in the modern and palaeoenvironment at Nar (Woodbridge et al., 2010). Through annual spot sampling of modern diatoms C. anatolicus and Achnanthidium minutissimum were identified in high abundance on macrophyte samples, while abundant species in the lake marginal gravel included C. anatolicus, Navicula cincta and Achnanthes lanceolata. A comparison of the contemporary and fossil diatom assemblages reveals that the majority of diatom species are represented in both the modern and palaeoenvironment; this implies that dissolution is not significant for most diatom taxa at Nar. Diatoms inhabiting different lake habitats at Nar today have contrasting seasonal patterns of change and relationships with lake water chemistry, and can therefore provide information about individual species’ life-forms when interpreting the palaeo-record. Nitzschia paleacea, Synedra acus and Stephanodiscus parvus valves have been identified in core thin sections as discrete layers, indicating that these taxa bloom for short time periods in extremely high cell numbers at specific times of the year. Differences may exist in the climate signal from these ‘bloom’ species and that from the remainder of the diatom assemblage, which may approximate more closely to the mean lake state (Woodbridge and Roberts, 2010). Consequently, conductivity reconstructions have been performed both with and without the inclusion of N. paleacea, S. acus and S. parvus. The overall down-core trends are similar for both reconstructions. However, the key difference between the reconstruction based on all species and non-bloom species is the dissimilarity in the absolute magnitude of conductivity values; the reconstruction based on non-bloom species results in higher values that are closer to measured modern lake conductivity levels, although reconstructed values for the most recent part of the record based on non-bloom species are still lower than measured conductivity values. Throughout the sediment sequence overall diatom concentration increases when N. paleacea is dominant and total biovolume increases when S. acus is abundant (Fig. 2). This is associated with the larger size of S. acus and the fact that N. paleacea blooms with extremely high cell numbers. Species diversity (rarefaction) is also closely associated with the dominant species N. paleacea (r ¼ 0.813, p ¼ 0) and diatom concentration (r ¼ 0.738, p ¼ 0). This implies that N. paleacea blooms swamp the community and drive diversity

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Fig. 2. a) NAR01/02 diatom percentages (AD 280e2001) plotted with DCA axes 1 and 2, rarefaction and concentration (log10), b) NAR01/02 diatom biovolume plotted with DCA axes 1 and 2, and total biovolume (mm3, log10).

down. The percentage of variation within the data explained by DCA axes 1 and 2 (Fig. 3) is 51.1 and 19.8% respectively. Axis 1 accounts for the major change in the assemblage between zones ND1 and ND2, whereas axis 2 appears to represent more recent variability in the record. Comparison of the diatom percentage and biovolume diagrams (Fig. 2) reveals that certain taxa (e.g. Campylodiscus clypeus and C. anatolicus) contribute more significantly to the assemblage than percentage calculations imply. DCA axis 1 based on biovolume and percentage data show similar broad trends, however, axis 2 scores show greater variability according to percentage data, which is associated with the decreased weighting of N. paleacea according to biovolume. 3.2. Nar diatom stratigraphy The 1720-year NAR01/02 diatom percentage record has been divided into four zones (Fig. 2) informed by stratigraphicallyconstrained cluster analysis (Grimm, 2004). For the conductivity reconstruction the combined salinity EDDI modern training set was identified as possessing the highest number of matching analogue diatom species in the Nar fossil assemblage (74.4%) using the modern analogue technique (MAT). Weighted Averaging (WA) with inverse deshrinking was identified as the model with highest predictive ability (r ¼ 0.85) and lowest prediction errors (RMSEP ¼ 0.47). Diatom-inferred (DI) conductivity values for the full data set vary between a maximum of w5500 mS1 at 1560 VY (AD 450) and a minimum of w120 mS1 at the core top (Fig. 4). Equivalent reconstructions excluding bloom species gave DIconductivity values between w7100 mS1 at 1580 VY (AD 430) and w620 mS1 at 20 VY (AD 1980). DI-conductivity values from the NAR06 short core for the period since 1998, when recording of direct measurements began, range from w930 to w2900 mS1 if bloom taxa are excluded (Woodbridge and Roberts, 2010). Modern conductivity levels measured at the lake (w3300 mS1) therefore disagree with the diatom-inferred values (see Section 4 below). Between 7 and 40 distinct mono-specific diatom bloom events have occurred per century since AD 1100, with highest frequencies from AD 1700 until 2000 (Fig. 5). Significant S. parvus blooms occur

Fig. 3. DCA ordination plot of the NAR01/02 diatom percentage record.

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Fig. 4. Selected diatom species from the NAR01/02 % record plotted with DI-conductivity based on all species and non-bloom species presented with NAR01/02 d18O and selected pollen taxa.

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eutrophic, planktonic species S. parvus at w750 VY (AD 1250) may be indicative of higher nutrient levels and implies an abrupt, shortlived change in environmental conditions. This is confirmed in thin sections, where S. parvus was found to bloom on nine occasions during a 20 year time period (Fig. 5). In comparison with the other two known bloom taxa, S. parvus has a narrow conductivity range (Juggins, 2011), which may account for the absence of this species throughout the rest of the record. Bloom species increase in overall abundance throughout zone ND3, but with strongly fluctuating percentages between samples, which highlights seasonal and interannual variability in the assemblages. Diatom rarefaction, concentration and total biovolume remain relatively consistent through this zone. Fig. 5. Frequency of diatom ‘bloom’ events of Synedra acus, Nitzschia paleacea and Stephanodiscus parvus per 100 years identified on NAR01/02 core thin sections spanning the last nine centuries (AD 1100e2000).

earlier in the sequence for w20 varve years (AD 1205e1246), S. acus blooms have become increasingly common since w500 VY (AD 1500), and N. paleacea blooms have increased in frequency since w230 VY (AD 1770). The appearance of bloom species is likely to represent specific changes in the lake system over short time periods as well as longer-term shifts in baseline state. 3.2.1. Zone ND1 (AD 290e540, 1720e1470 VY) Zone ND1 is markedly different from all successive time periods (Fig. 2), a pattern confirmed by the DCA ordination plot (Fig. 3). The diatom assemblage is dominated by the widespread pioneering species Fragilaria construens var. venter, which tolerates a wide range of water chemistry conditions and is associated with disturbance and increased turbidity (Gasse, 1986). The centric, widespread planktonic species Cyclotella meneghiniana, which is an indicator of higher salinity levels (Juggins, 2011) and tolerates a wide range of alkalinity (Gasse, 1986), also dominates this period and may have been a ‘bloom’ species at this time. Zone ND1 remains distinct when the data are plotted according to species biovolume (Fig. 2, b) with C. clypeus, C. meneghiniana, Navicula halophila and Rhopalodia operculata being the dominant taxa. The dominance of taxa with high conductivity optima during this period implies that the lake level was low and watershed disturbance was high, which may be associated with an arid climate and/ or catchment disturbance. DI-conductivity reveals a period of elevated values throughout this zone, which peaked from AD 400e540 (Fig. 4). 3.2.2. Zone ND2 (AD 540e790, 1470e1210 VY) The transition to zone ND2 involves a major diatom assemblage shift, with high values of C. anatolicus, while biovolume data show increases in Navicula oblonga and Rhopalodia gibba. The shift from zone ND1 to ND2 is reflected by an increase in species diversity and is evident in the significant shift in DCA axis 1 (Fig. 2). DIconductivity decreases through this zone with increasing abundance of N. paleacea, indicating freshening of the system (Fig. 4). However, the statistical reliability of the reconstruction is reduced by the presence of the newly described non-analogue species C. anatolicus during this period. 3.2.3. Zone ND3 (AD 790e1440, 1210e560 VY) In percentage terms, this zone is dominated by the bloom taxon N. paleacea, which may be indicative of higher nutrient conditions (Baier et al., 2004), implying that the lake may have become more productive. Although DI-conductivity values are generally low throughout this period, reconstructed values omitting bloom species reveal a rise in conductivity between 1200 and 1100 VY (AD 800e900) (Fig. 4). The sudden bloom peak of the freshwater,

3.2.4. Zone ND4 (AD 1440e2000, 560e1 VY) The transition to zone ND4 saw the disappearance of C. anatolicus and significantly greater abundance of S. acus and A. minutissimum, along with N. paleacea. Although S. acus and N. paleacea have low conductivity optima in the combined salinity training set, the upper range of these species reaches well above their optima, and reconstructed values may therefore be misleading. Synedra acus is an opportunistic species (Mackay et al., 2005) and can dominate when conditions become unfavourable for other species. Achnanthidium minutissimum inhabits various environments within the littoral zone, tolerates moderate nutrient levels and low conductivity (Gasse, 1986). Increased values of this species may represent a change in nutrient levels, as well as the existence of suitable lake marginal habitats (e.g. reed beds). This may relate to environmental variability associated with changes in habitat availability linked, for example, to fluctuating lake water levels. DI-conductivity remains consistently low throughout zone ND4 and mono-specific diatom bloom events become increasingly common (w30 per 100 years) during this time (Fig. 5). 4. Discussion 4.1. Late Holocene environmental change The multi-proxy diatom, d18O and pollen records from Nar Gölü, shown in Fig. 4, reveal that the local environment and regional climate of Cappadocia have undergone major alterations throughout the last 1720 years. Fluctuations in DI-conductivity values through the record would, a priori, be expected to relate to lake water depth and to the regional precipitation-evaporation (PE) balance (Fritz et al., 1999). In the basal diatom zone ND1 (AD 280e540) strongly saline conditions were accompanied by highly positive d18O values, which imply low lake levels, although not so low as to lead to a breakdown of lake stratification, because the sediments remained varved. Saline, evaporated lake waters in turn would have been the result of a water balance deficit and a drier climate, which seem likely to have included a reduction in winterspring precipitation, the main rainfall seasons at present. According to the diatom record, the main period of elevated lake conductivity e and inferred regional drought e lasted for around a century and a half (AD 400e540). Elsewhere in the Eastern Mediterranean region, the 14C-dated sediment core sequence from Tecer Lake (Fig. 1) shows high aragonite and sand content, indicative of low water levels and lu elevated salinity values, between wAD 250 and 500 (Kuzucuog et al., 2011) (Fig. 6). This period has the lowest lake levels identified during the last two millennia at Tecer. This lake lies in the same central Anatolian precipitation regime as Nar (Türkes¸, 2003) and must have experienced the same climate history. It is therefore particularly significant that both sites register the period

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lu Fig. 6. a) Nar d18O (decadal) and DI-conductivity (non-bloom species) for the period AD 290e2000, along with an aragonite-inferred aridity record from Tecer Lake (Kuzucuog et al., 2011); b) Nar d18O (annual) and diatom DCA axis 1 plotted alongside tree-ring inferred MayeJune precipitation (central Turkey) (Touchan et al., 2007) for the period AD 1450e1960. All data are plotted to show increasing aridity to the right on the graphs.

centred on the 5th-century AD as the most extreme drought phase of the last two millennia. Further a field, there is evidence from the southern Levant of significant moisture shifts at this time. The Dead Sea experienced falling lake levels during the first millennium AD, interrupted by a reversal to higher water levels 14 C-dated to around AD 400 (Bookman et al., 2004). In the nearby Soreq Cave (Fig. 1), very high-resolution speleothem d18O analysis was undertaken by Orland et al. (2009) for the interval 2140 to 1480 BP (190 BCeAD 470), dated by MC-ICP-MS U-Th. This showed a stepwise decline in precipitation, especially around AD 100, and with a brief reversal to wetter conditions around AD 500, a pattern that broadly matches the lower-resolution marine isotope record of Schilman et al. (2001) from the southeast Mediterranean Basin. These differences in the timing of changes in P-E balance between the southern Levant and central Turkey during the early first millennium AD are unlikely to be due simply to dating uncertainties. Despite their geographical proximity, the northern and southern sectors of the Eastern Mediterranean do not show a good correlation in inter-annual precipitation variability during the period of instrumental records (Xoplaki et al., 2003). The most significant diatom assemblage change in the entire Late Holocene record occurred at the end of zone ND1, simultaneously with the most important shift to more negative isotopes (Fig. 4). Both diatoms and d18O therefore indicate a very marked shift from drier conditions prior to wAD 540 to more dilute lake waters and wetter hydro-climatic conditions after this date (zone ND2). According to Gasse et al. (1997), rapid species shifts can occur when an internal threshold has been reached, which could therefore represent a biological response to a shift in climate that was gradual rather than abrupt. However, because a large shift to negative values occurred in the d18O record simultaneously with

the shift in the diatom record during the 6th century AD, this rapid limnological shift is less likely to reflect a threshold effect and more likely to indicate a major centennial-scale re-organisation of regional climate. Between AD 540 and 800 (diatom zone ND2), lower DIconductivity values match well with negative d18O values on Nar carbonates, both being indicative of wetter moisture balance conditions. After AD 800, known ‘bloom’ diatom taxa become increasingly important, especially N. paleacea, which can indicate higher nutrient levels. Availability of phosphate and nitrate as limiting nutrients in the lake system may account for the substantial fluctuations in the percentage of N. paleacea through time. There is a significant divergence in diatom zone ND3 between DI-conductivity reconstructions when bloom taxa are included and excluded (Fig. 4). As discussed by Woodbridge and Roberts (2010), N. paleacea blooms occur in spring, immediately prior to carbonate precipitation, and may be linked to the formation of a freshwater lid on the lake following heavy spring rains or snowmelt. In consequence, this taxon may not represent the annually-averaged lake conductivity values. Similarly to the Nar diatom assemblage, Hobbs et al. (2011) identified presence of N. paleacea and S. acus var. angustissima during alternating time periods in sediment records from the Northern Great Plains (USA). They inferred that during periods of the salinity reconstruction when these two diatoms dominate, the inferred lake environment is not represented by the training set and may actually represent much more arid periods, and suggest that these taxa are likely to be growing in a fresh mixolimnion lens overlying a highly saline monimolimnion (Hobbs et al., 2011). In zone ND3 DI-conductivities excluding bloom species (N. paleacea and S. acus) match well with the d18O profile (Fig. 6), suggesting that this conductivity reconstruction is more reliable.

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Both DI-conductivity (non-bloom) and d18O have elevated values from AD 800e950, implying drier climatic conditions, although not as intensely arid as during diatom zone ND1. This secondary arid phase is also evident in the Tecer record between AD 800e1000 (Fig. 6). For the remainder of ND3, after AD 950, DI-conductivity and d18O indicate a return to less saline and less evaporated lake waters, signifying that hydro-climatic conditions at the time of the Medieval Climate Anomaly in this region were relatively humid. At the start of zone ND4, around AD 1400e1450, there was a shift to more positive d18O values and a switch from calcite to aragonite deposition, indicating drier conditions. This trend is not evident in either of the DI-conductivity reconstructions (Fig. 4) although there was a shift in the diatom assemblage marked by decreased C. anatolicus and increased S. acus and A. minutissimum, which is also evident in the DCA axis scores (Fig. 2). The reasons for this decoupling between inferred lake conductivity and isotopeinferred P-E are not fully understood. However, they may be related to a non-linear relationship between water level and conductivity in Nar Gölü and/or to the confounding influence of human-induced catchment changes during the last six centuries (discussed below). Recordings made between 2001 and 2009 show a lake level fall of 2.4 m at Nar. However, the fact that conductivity recordings at the lake did not increase during this time implies that the link between water depth and conductivity is not straightforward, and suggests a threshold effect within this relationship. In a groundwater lake such as Nar, solute concentrations are only likely to change significantly at times of low water level, when evaporative losses strongly exceed those from groundwater outflow. Lake levels seem to have remained high enough during the Little Ice Age (LIA; zone ND4) for solutes to be removed through groundwater outflow, rather than increasing in concentration. In contrast, during the late Roman lake low-stand prior to AD 540 (zone ND1) solute loss via groundwater flow must have been restricted sufficiently for lake conductivity to increase, leading to a response in the diatom assemblage. Despite the lack of response in halophytic diatom taxa in zone ND4, diatom assemblages none the less continue to show a good correspondence with d18O values. This is evident, for example, in the relationship between diatom DCA axis 1 and d18O for the time period AD 1450e1960 (Fig. 6). This climatic interpretation involving a shift to drier conditions after AD 1400e1450 is given independent corroboration from tree-ring inferred precipitation (Kuniholm, 1990; Touchan et al., 2007) (Fig. 6). In particular, there is wellattested evidence from central Anatolia of a drought period at the end of the 16th century, a time of socio-political crisis during the Ottoman Empire associated with the Celâli riots (Kuniholm, 1990). This drought event is clearly evident in both d18O and diatom (DCA axis 1) data from Nar Gölü (Fig. 6), which also coincides with an agricultural crisis identified in the Nar pollen record between AD 1500e1700 (England et al., 2008). Because diatom zone boundaries coincide with the major shifts in d18O values, this implies that climate variability has been the main pacemaker of changes in diatom assemblages during the last 1720 years. The diatom record comprises numerous species originating from diverse lake habitats, which have differing seasonal relationships with climate and other limnological variables. In comparison, d18O provides a smoothed record of environmental change, related to the 8e11 year residence time of the lake, and represents spring/ summer conditions when carbonates are precipitated. A movingwindow Pearson’s correlation based on 200-year time-slices has been used to identify periods of correspondence and divergence between the d18O and diatom records (Fig. 7). The most prolonged and consistently highly significant (p < 0.01) period of positive correlation between the d18O and DI-conductivity and DCA records occurs during the period of mega-drought and the subsequent shift

to a wetter climate (wAD 500e600). In the latter part of the record this relationship breaks down and at times is reversed with significant negative correlation between the records. This indicates a change in the diatom-climate relationship before and after Medieval times. The clearest example of this changing relationship is provided by the graph showing the correlation between DIconductivity (non-bloom species) and d18O, where there is positive correlation before AD 1100 and negative correlation after AD 1300, which is evident at the 0.05 and 0.01 significance levels (Fig. 7). Fig. 8 illustrates the strongly changing relationship between diatom DCA axis 1, DI-conductivity and d18O through time and demonstrates that positive d18O values need to persist for long enough to allow solutes to accumulate and initiate a response in the diatom assemblage. This provides evidence of a chemical threshold effect as ions only start to accumulate significantly at Nar when evaporative concentration is extreme (i.e. when d18O values become positive) and long-lasting (of at least 20e30 years duration), which is evident during the arid phase prior to AD 540. Dry periods following this at Nar seem to have been too short and/or not sufficiently arid for salts to accumulate in the lake water and initiate a diatom response. One of the arid phases that came nearest to initiating a diatom-salinity response, according to the d18O record, was during the 1860e1870s. Drought conditions in central Turkey in AD 1873e1874 are reported to have killed 250,000 people and 100,000 head of livestock (40% of all herds) (Naumann, C. 1893 cited in Kuniholm, 1990). Following this dry phase, a short-lived peak in DI-conductivity is recorded at AD 1880, suggesting that it may have taken a couple of decades for lake salinity levels to respond. Fig. 8 also implies that a hysteretic climate-salinity-lake level relationship has existed at Nar, as in some other closed lake systems (Langbein, 1961). The solute concentration of closed-basin lake water increases as lake volume decreases, but when the water level returns back to the same level, this does not always result in a return to the previous solute concentration; this relationship was demonstrated by Langbein (1961) for two dryland lakes (Devil’s Lake (North Dakota) and Lake Eyre (Australia)). At Nar the temporal DI-conductivity vs d18O trajectory (Fig. 8) indicates that the switch from fresh to saline lake conditions occurred at higher d18O values than the switch back from saline to freshwater. A similar hysteretic relationship was found over Holocene timescales at nearby Eski Acıgöl (central Turkey), which also showed a DI-conductivity threshold when d18O values were above w0& (Roberts et al., 2008). The Nar pollen profile has revealed human activity to be the main driver of vegetation change in the catchment throughout the last 1720 years (England et al., 2008). Comparison of the pollen zones with the diatom stratigraphy reveals no obvious relationship in the timing of changes in these proxies, and pollen and diatom zone boundaries do not correspond (Fig. 4). England et al. (2008) also recognised that d18O data from Nar do not coincide with pollen-inferred land-use and landscape changes. Among the most significant shifts in the pollen record were a period of human abandonment around AD 670e950 and agricultural intensification after AD 1830, neither of which is reflected directly in the diatom record. According to the Nar pollen sequence the arid episode prior to AD 540 does not appear to have substantially inhibited human occupation and agriculture in the region (England et al., 2008). Human occupants were presumably able to adapt to climatic conditions and continued to utilise the land during this time. Even during non-drought conditions, the Mediterranean climate still experienced strong seasonality, with dry summers, and past societies were well adapted to this regime (Roberts et al., 2004). Although pollen-inferred changes in regional land use and Nar diatoms do not show any clear relationship through time, longterm shifts in catchment state may have altered underlying lake conditions, for example, via changing nutrient supply. During

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Fig. 7. Pearson’s correlation (r and p-values < 0.05) between d18O and DI-conductivity/DCA axis 1 (for all diatom species and non-bloom species) based on a 200-year movingwindow correlation (periods of significant correlation are shaded). The line graph presents the r-values and the dots indicate the corresponding p-values.

Fig. 8. d18O plotted against a) DI-conductivity and b) DCA axis 1 (based on all species) for the 1720 year Nar record (arrows indicate the direction of time) and shaded areas represent different time periods.

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diatom zone ND4 there are significant rises in A. minutissimum and Synedra spp., both of which were rare or absent in zones ND1 and ND2. At the same time, pollen data indicate an increase in agropastoral activity in the regional landscape, while mineral in-wash layers become more common in the lake sediments (Woodbridge, 2009). This may therefore be partly responsible for the divergence observed between the DI-conductivity and d18O records through this part of the sequence. Although the small and steeply sloping lake catchment would never have been able to support extensive arable land-use activity, the lake and surrounding springs have attracted herds of grazing livestock and flocks of migratory wildfowl in the past, which would have increased nutrient flux and/ or erosive in-wash into the lake. Populations of migratory waterfowl caused pre-European eutrophication of some North American lakes (e.g. Donovan and Grimm, 2007; McAndrews and Turton, 2007), while human impacts on lake ecosystems have been identified in numerous other palaeolimnological records from the Mediterranean (e.g. Lamb and van der Kaars, 1995; Reed et al., 2008; Roberts and Reed, 2009). At Lake Van in eastern Turkey, Wick et al. (2003) recognised divergence between isotope and pollen data as related to increasing human modification of vegetation cover during the Late Holocene, particularly during the last 600 years. The Nar lake catchment is therefore likely to have been subject to increasing human disturbance during the later part of the record, which in turn appears to have impacted upon lake ecology. 4.2. Implications for Quaternary palaeoclimate reconstructions Gasse et al. (1997) and Fritz (2008) have emphasised that changes in a lake system associated with threshold effects and nonclimatic influences may alter the diatom response to a given climate perturbation. The palaeoclimate record from Nar demonstrates that temporal non-stationarity in diatom-lake salinityclimate relationships and changing human impacts through time can present complexities in interpreting Late Holocene proxyclimate records. Such issues are frequently encountered in other palaeoenvironmental records where human impacts confound the climate signal in lake sediment archives. For example, through analysis of diatom-inferred conductivity and tree-ring inferred precipitation records for the last 2000 years in lakes from arid Mongolia Shinneman et al. (2010) identified significant shifts between cooler and warmer climate, but highlighted that recent changes in the diatom assemblages have been linked to rapid landuse change with evidence of eutrophication. Similarly, StoofLeichsenring et al. (2011) identified two phases in a diatompalaeoenvironmental reconstruction from Lake Naivasha (Kenya) sediments. This included a period during which the lake was mainly affected by climate variations, and a more recent phase when anthropogenic activity over-printed natural climate variation. Gell et al. (2007) also showed that diatom-climate reconstructions from lakes along the lower River Murray (Australia) are subject to a wide range of recent human impacts (e.g. saline groundwater tables and increased nutrient and sediment fluxes associated with European settlement), on which are superimposed shorter-term, inter-annual rainfall variability. Changes in land use, which cause nutrient enrichment or catchment erosion, can lead to long-term shifts in the diatom response to climate through time. The record from Nar is consequently not unique with regard to issues of non-stationarity, and it highlights the need to avoid mechanistic interpretations of diatominferred climate changes on Quaternary timescales. This also has implications for the transfer function approach, which relies on the assumption that the same relationships between diatom taxa and salinity/climate which exist at present also existed in the past. Through development of a diatom-salinity transfer function for

lakes in the volcanic highlands of central Mexico Davies et al. (2002) demonstrated that a lack of modern analogue diatoms may reflect a high degree of recent anthropogenic disturbance. Thus the effects of changing human land use on diatom species assemblages through time can reduce the capacity of transfer functions to reconstruct palaeoclimate reliably. Climate reconstructions on Quaternary timescales therefore require careful interpretation and a multi-proxy approach should be applied where possible, especially as diatom species responses and relationships with water chemistry may be temporally nonlinear. A multi-proxy approach permits climate and human impacts to be separated and a more thorough understanding of palaeoenvironmental change during the Late Holocene, when anthropogenic activity increased. Legesse et al. (2002) highlighted the value of a multi-proxy approach by using combined diatom and pollen records to separate climate and human impacts in a lake sediment sequence from the Ethiopian Rift Valley (East Africa) for the last few centuries. The Nar record similarly demonstrates the potential of multiple proxies to record different and complementary information about past environmental change. 5. Conclusion The diatom record indicates that Nar lake water was much more saline and the climate of central Anatolia was significantly more arid during the decades prior to AD 540 than at any other time during the last 1720 years. The same dry episode is also clearly evident in the d18O climate signal from Nar (Jones et al., 2006) and lu et al., in other lake records from the same region (Kuzucuog 2011). Furthermore, the d18O record from speleothems in the southern Levant reveals significant shifts in climate during the period prior to wAD 500, although the precise timing of these changes differs from the lake records in Turkey. The extreme and prolonged arid episode centred on the 5th century AD that has been identified at Nar may be the last in a series of mega-droughts, which affected Anatolia and adjacent regions during the second half of the Holocene (Roberts et al., 2001; Wick et al., 2003; lu et al., 2011). Further research is required to confirm Kuzucuog the nature and synchronicity of earlier mega-drought periods given that understanding these events is important for sustainable management of drylands in the future. Our findings highlight the complementary characteristics of different palaeolimnological proxies. The pollen-climate signal at Nar has been masked by human land use changes since late Roman times, while diatoms and d18O differ in their response to climatic change as a result of dissimilar limnological thresholds. At AD 1400 a critical hydro-climatic threshold was reached at Nar Gölü to initiate a shift to more positive d18O values, but this climatic shift at the start of the LIA was not sufficiently large to initiate a salinity response in the diatom record. Instead post-Medieval diatom assemblages appear to have responded to climate in other ways. The DI-conductivity record has therefore been subject to nonstationarity, partly because changes in human land use may have altered the primary proxy signal through time. Analysis of these multiple proxies from the same lake sediment sequence can allow different forcing factors to be “teased apart”. Acknowledgements This research was funded by a University of Plymouth PhD scholarship. The authors would like to thank the British Institute at Ankara, National Geographic and MTA Institute for supporting fieldwork in Turkey and the UoP Geography academic, technical and cartographic staff who have provided advice, laboratory and IT assistance throughout the duration of this research. Thanks are also

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