Records of repeated drought stages during the Holocene, Lake Iznik
(Turkey) with reference to beachrock
Muhammed Zeynel Ozturk
a, Ahmet Evren Erginal
b,*, Na
fiye Gunec Kiyak
c,
Alper Demirci
d, Yunus Levent Ekinci
e, _Isa Curebal
f, Mustafa Avc
ıoglu
g, Tugba Ozturk
c aNigde University, Department of Geography, TR-51240, Nigde, TurkeybArdahan University, Department of Geography, TR-75000, Ardahan, Turkey cIsik University, Department of Physics, TR-34980, Istanbul, Turkey
dBitlis Eren University, Department of Geophysical Engineering, TR-13000, Bitlis, Turkey eBitlis Eren University, Department of Archaeology, TR-13000, Bitlis, Turkey
fBalıkesir University, Department of Geography, TR-10145, Turkey
gCanakkale Onsekiz Mart University, Department of Geological Engineering, TR-17020, Canakkale, Turkey
a r t i c l e i n f o
Article history:
Available online 6 November 2015
Keywords: Beachrock Cementation Late Holocene OSL Lake Iznik
a b s t r a c t
The cement fabrics, subsurface nature and optically stimulated luminescence age of beachrocks along the shores of Lake Iznik in NW Turkey were studied within the context of Holocene lake level changes. With a maximum thickness of 1.5 m, the low-angle (average 5e10) beds are composed of coarse grains and
small gravels and extend up to 5 m offshore at their most lakeward extremities. Cement textures on and around the poorly-rounded grains are made up of micrite envelopes and meniscus bridges as well as acicular aragonite rims. Geoelectrical resistivity sections taken from a representative location along the beach where the beds have maximum thickness showed that the sand-buried beds are followed up to about 24 m landward. Based on the OSL ages of 33 samples, the cemented beds occurred at four drier periods of the following: Pre- and Early Holocene (dated to 15e9 ka), Holocene Climatic Optimum (7.9 e5.6 ka), Middle Holocene (4.9 kae2.8 ka) and Late Holocene (2.0 kae0.9 ka).
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1. Introduction
As a dependable marker of environmental change, temporal variations in lake levels leave a record of changes in hydrological cycle and climate during the Late Quaternary (Street-Perrott and Harrison, 1984). The amounts of precipitation and evaporation as well as input by runoff are among the primary factors controlling the water level in lakes. Temporal changes in lake levels can be reconstructed using geomorphic and sedimentological methods (Harrison and Digerfeldt, 1993).
Precipitating from sea or lake waters, the connective carbonate cements of beachrocks, consisting of mainly aragonite and high-Mg calcite, bear a record of paleo-climatic changes favoured mostly by increased evaporation under dry climatic conditions. Beachrock has been mostly attributed to intertidal environments along tropical and subtropical coasts (Vousdoukas et al., 2007). There has been a common belief in its intertidal origin since early studies (Ginsburg,
1953; Bernier and Dalongeville, 1996; Neumeier, 1998), as opposed to other studies underlining the importance of upper subtidal (Alexandersson, 1972) or supratidal zones (Kelletat, 2006). Given that intertidal cementation makes beachrock favourable for deter-mining sea-level changes (Hopley, 1986; Desruelles et al., 2009; Mourtzas, 2012; Mourtzas and Kolaiti, 2014), which can be confirmed by the stable isotope composition and petrography of the connective cements (Vieira and Ros, 2006), it can be envisioned that the beachrock on lake shorelines also provides substantial hints inferring the paleo-climatic changes that have prevailed in the lake environment.
Knowledge of the existence of beachrock on lake shorelines is limited to just a few studies, as suggested byBinkley et al. (1980)
andJones et al. (1997). These authors demonstrated that beach-rock cements precipitated from the low magnesian-calcite-dominated waters of Marl Lake in Michigan, USA and the silica-rich waters of Lake Taupo in New Zealand. Recent publications concerning Lake Iznik, NW Turkey (Erginal et al., 2012a,b) showed its presence along the fresh-water lake shorelines. In this paper, on the basis of cementation fabrics, subsurface geometry and optical luminescence ages, we discuss the implications of beachrocks on * Corresponding author.
E-mail address:aerginal@gmail.com(A.E. Erginal).
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http://dx.doi.org/10.1016/j.quaint.2015.08.077
the shores of Lake Iznik with regard to Holocene climatic and lake level changes.
2. Study area
Lake Iznik is the fifth largest lake in Turkey, lying at 40300e40220N and 29200e29420 E in the eastern part of the
Marmara Region, NW Turkey (Fig. 1a, b). It is roughly rectangular (32 km long, 12 km wide;Fig. 1c), and has a maximum depth of 80 m. Found at a level of about 85 m above the present sea-level, the mean annual variation in lake level is ~50 cm (Ülgen et al., 2012), influenced by anthropogenic water usage, evaporation and seasonal changes in precipitation. The lake has a surface area of 313 km2(Ozturk et al., 2009) and occupies an east-west-aligned depression formed by the middle segment of the North Anatolian Fault. Its present connection with the Sea of Marmara to the west is through the Garsak Gorge, carved deeply (up to 400 m) in Paleozoic and Triassic metamorphic rocks. This valley is at present crossed by the Garsak Stream thatflows into the Gulf of Gemlik (Fig. 1c). A Mediterranean climate prevails in the study area, characterized by arid and semiarid conditions from May to October. According to
Viehberg et al. (2012), it is a warm monomictic lake. The lake area receives an average total precipitation of 737.9 mm. The average annual air and water temperatures are 15.1 and 20C, respectively. High evaporation occurs when maximum temperatures reach 45C during the summer months.
3. Methods
A total of 33 samples of beachrock were collected from 8 different sites (L1-L8) along the southern, northern and western shores of Lake Iznik for petrographic and electron microscopy analysis as well as luminescence dating. The micro-fabrics and
elemental composition of samples were examined using Scanning Electron Microscopy (SEM-ZEISS EVO 50 EP) coupled with Energy Dispersive Spectroscopy (EDX-Bruker AXS XFlash). The precipitated carbonate minerals were determined using X-ray diffractometry (XRD). Using a Scheibler calcimeter, CaCO3 content (%) was measured after extracting particles larger in size than 2 mm. The U, Th and K concentrations were analyzed using Inductively Coupled Atomic Mass Spectroscopy (ICP-MS).
3.1. Geoelectrical imaging survey
An ERT (Electrical Resistivity Tomography) survey, which has been widely used to describe the subsurface nature of cemented coastal deposits (David et al., 2009; Erginal et al., 2012c, 2013a, 2013b, 2013c; Ertek et al., 2015), was carried out on one of the studied beaches where the beds have maximum thickness (Fig. 1c). Thereby, we aimed at both determining the thickness of the cemented beds vertically and also perceiving their contact rela-tionship with the underlying unit. For this purpose, a total of 11 depth levels on three ERT lines oriented perpendicular to the lake shoreline were measured using dipoleedipole electrode configu-ration with electrodes spaced every 1 m. Apparent resistivity measurements were carried out via the GF ARES multi-electrode resistivity-meter system. Due to the limited width of the shore-line, the lengths of profiles were selected as 24, 21 and 33 m for Line-1, Line-2 and Line-3, respectively (Fig. 1d). The measured apparent resistivities were then inverted to true resistivities using the tomographic inversion software package RES2DINV, which is based on smoothness-constrained least-squares (Sasaki, 1992) implemented by a quasi-Newton optimization technique (Loke and Barker, 1996). Significant topographical relief data obtained by optical levelling were also incorporated into the inversion proce-dure to achieve more accurate results.
3.2. OSL analyses
3.2.1. Sample preparation and luminescence measurements For OSL dating, the outer surface of samples was removed to eliminate parts exposed to light. The inner parts, well preserved against light, were crushed in a mortar. After wet sieving to 90e180
m
m, carbonates and organic matter were removed from the sample using HCl and H2O2, respectively. Clean quartz was extractedfrom this fraction using HF followed by HCL rinse and re-sieving. The purified quartz was held as a monolayer on 10 mm-diameter aluminium discs using Silkospray silicone oil. The purity of the quartz grains was tested by the absence of luminescence signal during infrared stimulation. All luminescence measurements were performed with an automated Risø TL/OSL reader, model TL/OSL-DA-15, equipped with an internal90Sr/90Y beta source (~0.1 Gy s1), blue light emitting diodes (LEDs) (470 nm, ~40 mW cm2) and IR LEDs (880 nm, ~135 mW cm2). Luminescence signals were detected us-ing an EMI 9635QA photomultiplier tubefitted with 7.5 mm-thick-ness Hoya U-340filters (Bøtter-Jensen, 1997).
3.2.2. OSL analysis for equivalent dose (De) estimate
The single-aliquot regenerative-dose (SAR) method was used to determine the equivalent dose (De) for each aliquot, based on
com-parison of the natural OSL signal with regenerative OSL signals produced by known laboratory doses (Murray and Mejdahl, 1999; Murray and Wintle, 2000). Using the SAR protocol, a natural aliquot wasfirst preheated at 260C for 10 s and then recorded with
blue light stimulation at 125C for 40 s to obtain the natural OSL signal (L0). In order to monitor and correct possible sensitivity
changes in the OSL signal, a test dose was administered (10e20% of the natural dose) to the same aliquot prior to heating to 190C. Then, the test dose OSL signal (T0) was measured. For the next three cycles,
following the same sequence of treatments described in thefirst cycle, three regeneration doses were applied to the same aliquot to obtain regenerated OSL signals (Li, i¼ 1,2,3). The corresponding test
dose OSL signal (Ti) was recorded after a cut heat to 190C and used
for sensitivity correction of the relevant OSL signal (Li/Ti, i¼ 1,2,3).
Using these corrected dose points for a representative sample coded L3-9.1, a growth curve was constructed (Fig. 2). The sensitivity-corrected natural signal (L0/T0) was interpolated onto
the growth curve to obtain the accumulated dose De. The corrected
experimental points of growth curves for all samplesfit well using an exponential function, as shown inFig. 2, and the equivalent dose was measured before saturation. As an internal test for reliability of the OSL measurements, a zero regeneration dose and a regenerative dose equal to thefirst regeneration dose were given to the aliquot to observe the bleachability of the OSL signal (recuperation), which is expected to be below the upper suggested limit of 5% of natural signal. It was also monitored the sensitivity change in OSL signals recorded at two different cycles of the same dose, namely, the recycling ratio, which is expected to be close to unity.
3.2.3. Dose rate estimation
The annual dose of the radiation environment was obtained from the concentrations of major radioactive isotopes of the ura-nium and thorium series and of potassium. The alpha irradiated rind of the quartz grains was assumed to have been removed by HF etching for 40 min and therefore the alpha dose rate was not taken into account. The contribution of cosmic rays to the dose rate was obtained using the formula given byPrescott and Hutton (1988). Beta and gamma dose rates were calculated from radioisotope concentrations of U, Th and K, as given inTable 1(Adamiec and Aitken, 1998), where dose rates and other related parameters based on dose rates for each sample are also presented.
Table 1
Parameters used for dose rate estimation.
Lab code Depth (cm) Cosmic (Gy/ka) Moisture content (%) Carbonate (%) U (ppm) Th (ppm) K (%) Gamma (Gy/ka) Beta (Gy/ka)
L1-1.1 5 0.3 7.7 22.49 0.7 1.4 0.16 0.1839 0.2634 L1-1.2 15 0.26 4.21 21.57 0.5 1.7 0.13 0.1685 0.2196 L1-2.1 5 0.3 5.01 25.16 0.7 0.8 0.1 0.1408 0.2001 L1-3.1 5 0.3 7.49 23.29 0.8 1.3 0.16 0.1904 0.275 L1-3.2 20 0.25 4.94 23.88 1 1.2 0.11 0.1959 0.2618 L1-4.1 40 0.22 6.95 29.55 0.9 1.1 0.14 0.1872 0.2682 L1-4.2 5 0.3 8.72 27.68 1.5 1.5 0.16 0.2783 0.3806 L1-4.3 50 0.21 8.45 27.74 1.1 1.1 0.13 0.2072 0.289 L1-5.1 10 0.27 12.76 19.23 0.7 1.1 0.17 0.1721 0.2631 L1-5.2 80 0.21 6.56 22.49 0.8 0.9 0.17 0.1738 0.2719 L1-6.1 5 0.28 6.64 24.43 1.3 0.9 0.13 0.22 0.3121 L2-7.1 30 0.23 8.72 26.07 0.5 0.6 0.1 0.1089 0.1661 L2-7.2 5 0.3 8.45 22.06 0.5 0.6 0.14 0.1186 0.1974 L2-7.3 30 0.23 12.76 27.47 0.7 0.7 0.1 0.136 0.1974 L2-7.4 5 0.31 6.56 31.15 0.6 0.8 0.1 0.1296 0.1858 L3-8.1 5 0.3 7.7 20.44 0.5 2 0.11 0.1779 0.2121 L3-9.1 5 0.3 4.21 18.35 0.5 2.1 0.11 0.1827 0.2149 L3-9.2 30 0.24 5.01 17.49 0.5 1.7 0.09 0.1588 0.1883 L4-10.1 5 0.3 7.49 26.87 0.5 2.2 0.11 0.1875 0.2176 L4-11.1 5 0.3 4.94 16.42 0.5 1.3 0.09 0.1398 0.1774 L4-11.2 10 0.28 6.95 18.09 0.5 1.5 0.08 0.1468 0.175 L4-12.1 5 0.3 8.72 12.73 0.5 1.2 0.11 0.1399 0.1903 L4-12.2 5 0.3 8.45 18.09 0.5 1.4 0.1 0.1469 0.1879 L5-13.1 5 0.29 12.76 25 0.5 1.6 0.11 0.1589 0.2012 L6-14.1 5 0.3 7.7 36.2 0.7 1.6 0.11 0.1813 0.2298 L6-14.2 20 0.25 4.21 47.2 0.7 2 0.11 0.2003 0.2407 L6-15.1 5 0.3 5.01 31.1 0.8 2 0.1 0.2091 0.2472 L6-15.2 5 0.3 7.49 52.88 0.7 1.9 0.13 0.2004 0.2536 L7-16.1 5 0.3 4.94 20.3 0.5 2.6 0.13 0.2114 0.2441 L7-17.1 5 0.3 6.95 17.6 0.5 2.7 0.13 0.2161 0.2469 L8-18.1 5 0.3 8.72 26.1 0.5 2.6 0.11 0.2065 0.2285 L8-18.2 5 0.3 8.45 29.6 0.5 2.3 0.12 0.1946 0.2281 L8-18.3 5 0.29 12.76 27.87 0.5 2.1 0.09 0.1778 0.1992
4. Results and discussion
4.1. Morphology and subsurface nature of beachrock
The studied beachrocks extend along the southern, northern and western shores of the lake. Visible thicknesses of beds reach 1.2 m (Fig. 3a and b). Observations in the well-protected thickest section at backshore showed that the composition of beds is notably similar to that of the present beach, being composed of coarse grains and small gravels derived from the surrounding highlands, dipping at angles between 5and 10towards the lake. The maximum length and width of the beachrocks are 800 m and
10 m, respectively. In many cases, submerged edges of beds are followed up to 4 m offshore and the landward edges of beds covered by herbaceous plants are decomposed (Fig. 3c). Along the shoreline, the beds are rather hackly as they have undergone vio-lent erosion by wave action (Fig 3d).
The NWeSE trending ERT images are shown inFig. 4. The to-mograms, having RMS errors of 6.7, 7 and 3.6 for Line-1, Line-2 and Line-3, respectively, indicated that the resistivity values vary on a large scale and therefore the resistivity discrepancies were thought to be sufficient to detect the anomalies which are associated with buried beachrocks. Thus, the beachrock unit was distinctly identi-fied in the tomograms as having very high resistivities in propor-tion to the underlying conductive beach material. Addipropor-tionally, it can be clearly seen that the three tomograms showed similar anomaly characterization in terms of thickness and shape. The sharp resistivity transitions in the images marked with dashed black lines show the lithological boundary between the beachrocks and water-saturated beach material. The relatively high resistivity zone at the end of Line-3, after a horizontal distance of ~24 m, is due to backfill material, which was also observed during the field study. Based on the tomographic resistivity images, beds become thicker landwards and reach a maximum thickness of 1e1.5 m and width of ~24 m.
4.2. Composition and cement textures
The composition and texture of samples examined by micro-scopic study of the petrographic thin sections and SEM images revealed that the studied beachrocks are sandstone or conglom-eratic sandstone with high void ratio. Comprised of poorly sorted sub-angular grains and lithoclasts derived from clastic, magmatic and metamorphic terrains cropping out in the lake basin, the pre-dominant components are made up of basalt and andesite as well as limestone, quartz arenite and lithic arenite.
Meniscus-style cemented polygenic grains derived from basalt, quartzite, micaschist, marble and chert were also observed in many samples. Poor roundness is suggestive of short-distance alongshore
Fig. 3. View of beachrocks at (a, b) locality 1, (c) locality 4, (d) locality 6. Fig. 2. Dose-response curve of representative sample L3-9.1. Curve isfitted with
drifts of grains and gravels. The mineral components are made of serisitized plagioclase and coarse grains of quartz, the latter of which display wavy-extinction typical of a metamorphic origin. Other accessory minerals are composed of mica, epidote, ogite, hornblende and biotite lamellae. In many samples, precipitation of iron oxides acting as a cement within voids as well as in the for-mation of opaque minerals at grain boundaries, is also very common.
Calcimetric measurements revealed that beachrock samples contain CaCO3ranging between 13% and 53%, typical of a high void
ratio, as confirmed by SEM images showing the predominance of poorly sorted sub-angular grains dispersed within a voided texture. As the connective material, aragonite is the predominant carbonate polymorph according to XRD results (Fig. 5a). Dominant cement fabrics are two-fold, i.e. micritic coatings on and around the grains (Fig. 5b) and meniscus bridges (Fig. 5c, d and f), the latter of which consists almost solely of aragonite needles (Fig. 5e) and algal fila-ments to a lesser extent (Fig. 5f and g). Acicular aragonite rims (Fig. 5h) and porefillings (Fig. 5i and j) are other subordinate types of cements.
EDX analyses obtained from 43 images of micritic coatings and meniscus bridges detected elemental composition in the following descending order; O> Ca > C > Si > Mg > Fe > Al > Na > K. The average and maximum values detected for main elements (Ca, C and O) were 79.6% and 97%, respectively (Fig. 6). These marine-like cement fabrics, albeit typical of a meteoric environment, were previously detected byErginal et al. (2012a,b). Ever-increasingly precipitation of carbonates has been effective since the last glacial maximum in consequence of the input of melting waters flowing into the lake and the onset of lake productivity (Roeser et al., 2012). The studied beachrocks contain abundant freshwater diatoms in both fossil and modern forms (Fig. 5k), some of which arefilled with clumps of calcium carbonate precipitated from lake water (Fig. 5l).
4.3. OSL ages and main periods of beachrock formation
The OSL measurements indicate that the quartz grains are sensitive to both dose and light exposure, yielding very bright OSL signals, and that the SAR method produces reliable results when applied to these samples. Briefly, aliquots yield a recuperated value below 5% and recycling ratios close to unity (Murray and Wintle, 2000). A very small number of aliquots, yielding recycling ratios differing from unity by greater than 10%, were not evaluated. The OSL ages produced from the equivalent dose and environmental dose rates are presented in Table 2, in which (n) indicates the number of aliquots evaluated for each sample by the OSL SAR technique. Uncertainties given in the Table are based on the propagation of errors associated with individual errors for all measured quantities.
OSL ages revealed that the beachrocks formed at four main periods on the shoreline of Lake Iznik; namely, Pre- and Early Ho-locene (P-EH) before 9 ka, HoHo-locene Climatic Optimum (HCO) be-tween 7.9 ka and 5.6 ka, Middle Holocene (MH) bebe-tween 4.9 ka and 2.8 ka, and Late Holocene (LH) between 2 ka and 0.9 ka. These periods match quite well with several Bond events (Bond et al., 1997), discussed below.
4.3.1. Pre and Early Holocene (P-EH) period (15 kae9.4 ka) In harmony with previous data regarding formation of beach-rock at between 20.2 ka and 18.4 ka, matching the last glacial period (Erginal et al., 2012a), the oldest OSL ages obtained from buried quartz grains embedded within the studied beds date back to 15.18 ka. The four lowermost levels of the beds of beachrock yielded ages between 15.18 ka and 9.44 ka, spanning a period from
just before BøllingeAllerød interstadial, dominated by warm and moist conditions, to Early Holocene. With regard to the period that encompasses the short-duration Younger Dryas stadial, in the ensuing transition period to Holocene between 11.5 ka BP and 11.6 ka BP (Gulliksen et al., 1998) and warmer Early Holocene, cli-matic conditions were at variance in terms of unstable temperature conditions. Increasing temperatures had an important influence over European lakes by way of raising lake levels (Magny, 2004). On the other hand, for the period between 17.4 ± 0.3 to 12.2± 0.26 ka BP,Roeser et al. (2012) suggested that the sedi-mentary environment in Lake Iznik was characterized by increased carbonate accumulation overlying the detrital input and a decreased lake level. This period was followed by a relatively drier stage during the earlier Holocene, as confirmed also by vegetation records in various lacustrine environments in the near east (van Zeist and Bottema, 1991), Lake Van in eastern Anatolia (Wick et al., 2003; Litt et al., 2009) as well as by pollen and stable isotope data from Lake G€olhisar (Eastwood et al., 2007). Even though conditions congenial to carbonate deposition existed since 18 ka in Lake Iznik (Roeser et al., 2012), formation of beachrock since then (Erginal et al., 2012a) during that early phase was possibly related to rising temperatures and thereby resulted in cementation by the precipitated aragonites under the aforemen-tioned arid conditions.
4.3.2. Holocene Climatic Optimum (HCO) period (7.9 kae5.6 ka) The OSL ages of nine samples revealed that beachrock forma-tion continued during the HCO, represented by ages varying be-tween 7.9 ka and 5.6 ka. Such old ages were obtained from beds Table 2
OSL-SAR ages, equivalent doses, and dose rates for samples taken from different profiles.
Lab code Depth (cm)
OSL age (ka) Paleodose (Gy) (n) Dose rate (Gy/ka) L1-1.1 5 2.63± 0.25 4± 0.37 8 1.52± 0.03 L1-1.2 15 7.24± 0.44 8.94± 0.49 11 1.24± 0.03 L1-2.1 5 1.98± 0.26 2.4± 0.3 13 1.21± 0.03 L1-3.1 5 5.58± 0.55 7.2± 0.68 11 1.29± 0.03 L1-3.2 20 7.41± 0.95 9.29± 1.17 13 1.25± 0.03 L1-4.1 40 5.86± 0.89 3.07± 0.42 10 0.52± 0.03 L1-4.2 5 1.47± 0.2 2.01± 0.27 18 1.37± 0.03 L1-4.3 50 4.91± 1.34 2.65± 0.71 7 0.54± 0 L1-5.1 10 1.54± 0.21 1.9± 0.26 8 1.23± 0.03 L1-5.2 80 6.09± 1.09 3.17± 0.46 8 0.52± 0.04 L1-6.1 5 1.65± 0.47 2.16± 0.61 8 1.31± 0.04 L2-7.1 30 10.73± 1.26 4.47± 0.4 12 0.42± 0.03 L2-7.2 5 4.2± 0.35 5.83± 0.46 12 1.39± 0 L2-7.3 30 3.33± 0.37 1.48± 0.13 10 0.44± 0.03 L2-7.4 5 3.78± 0.61 4.34± 0.68 7 1.15± 0.04 L3-8.1 5 1.28± 0.19 1.69± 0.25 13 1.32± 0.03 L3-9.1 5 6.21± 0.36 8.37± 0.42 14 1.35± 0.04 L3-9.2 30 7.88± 0.89 4± 0.35 13 0.51± 0.04 L4-10.1 5 3.41± 0.56 4.31± 0.7 14 1.26± 0.03 L4-11.1 5 1.72± 0.15 2.23± 0.18 13 1.3± 0.04 L4-11.2 10 3.01± 0.41 3.72± 0.49 11 1.23± 0.04 L4-12.1 5 1.19± 0.1 1.54± 0.12 12 1.3± 0.04 L4-12.2 5 1.33± 0.15 1.69± 0.18 12 1.27± 0 L5-13.1 5 9.44± 0.53 11.38± 0.55 13 1.2± 0.03 L6-14.1 5 1.52± 0.25 1.83± 0.3 12 1.2± 0.03 L6-14.2 20 1.42± 0.19 1.55± 0.21 9 1.09± 0.03 L6-15.1 5 0.87± 0.1 1.13± 0.12 11 1.27± 0.03 L6-15.2 5 2.01± 0.32 2.24± 0.35 10 1.12± 0.03 L7-16.1 5 15.18± 1.13 20.74± 1.44 13 1.37± 0.04 L7-17.1 5 12.24± 1.15 16.71± 1.5 11 1.37± 0.04 L8-18.1 5 6.45± 0.57 8.11± 0.68 10 1.26± 0.03 L8-18.2 5 6.38± 0.5 7.87± 0.58 12 1.23± 0 L8-18.3 5 3.66± 0.37 4.38± 0.43 11 1.2± 0.03
varying in depth between 5 cm and 80 cm as result of removal of much of the beds due to wave erosion, as in the Pre and Early Holocene-aged lowermost beds. This period coincides with the Holocene Climatic Optimum, a warm period with temperature anomalies, dominated by humid and rainy conditions alternating with short-lived drier conditions in mid-latitudes. Causing suc-cessive changes in lake level at four different periods from 8.3 ka to 5.2 ka, the humid and rainy conditions were followed by drier and warmer periods recorded in central Europe (Magny, 2004) as in the case of many Turkish lakes. It can be suggested that, based on the assumption that authigenic precipitation of amalgamating carbonate is favoured by drier conditions, the formation of beachrock in that time span was associated with the intervening drier stages. Similar data with regard to alternating arid and hu-mid stages between 8.8 ka and 5.1 ka was recorded at Lake G€olhisar in southwest Turkey based on isotopic fluctuations (Eastwood et al., 2007). In central Anatolia, the level of Lake Tuz, that began to fall after 10 ka BP, allowed the formation of lake-side terraces and lakeward-prograding alluvial fan deposits between 7 ka and 5.5 ka BP (Kashima, 2002). Similar records for aridity between 6 ka and 4 ka BP were ascertained at Lake Tecer in central Anatolia (Kuzucuoglu et al., 2011) as well as further east, e.g. Lake Zeribar in Iran between 7.8 ka and7.5 ka BP (Wasylikowa et al., 2006).
4.3.3. Mid-Holocene period (4.9 kae2.8 ka)
A 4700 year record of core sediments from Lake Iznik demon-strated abrupt shifts in climate towards an increase in drought conditions at 4.2 and 3.3 ka BP (Ülgen et al., 2012). This period including two main intervalse between 4.5 ka and 3.9 ka, and 3.1 ka and 2.8 ka BPe also matches up with arid conditions in the eastern Mediterranean (Roberts et al., 2011).
The Mid-Holocene period in Lake Iznik beachrocks is repre-sented by OSL ages (seven samples) ranging from 4.9 ka to 2.6 ka,
following a gap for beachrock formation spanning 5.6 ka to 4.9 ka, likely related to a higher lake level when humidity increased, as confirmed byÜlgen et al. (2012). The most significant period for this time span is the 4.2 ka aridity, which was recorded not only in Lake Iznik (Erginal et al., 2012b; Ülgen et al., 2012) but also in Lake Van from 4 ka BP (Wick et al., 2003), and Lake Zeribar, Iran between 4.5 and 3.8 ka BP (Wasylikowa et al., 2006), albeit with intervening humid intervals such as in lakes in Central Europe (Magny, 2004) and Lake Purwin, NE Poland (Galka and Apolinarska, 2014). 4.3.4. Late Holocene period (2 kae0.9 ka)
Many of the beachrock beds (13 samples) ranging in age be-tween 2 ka and 0.9 ka were formed during the Late Holocene as confirmed by previous data (Erginal et al. (2012b), when warmer conditions including the 1.8 ka aridity event prevailed.Ülgen et al. (2012)reported in the light of data from core sediments that the lake sediments deposited between 1.95 ka and 1.35 ka BP and 1.1 ka and 0.7 ka BP were deposited under humid conditions to the con-trary of those deposited between 1.35 ka and 1.1 ka BP, which were typical of a drier stage. When this comprehensive downcore sedi-ment data is compared with beachrock ages, it can be suggested that, except for two samples dated to 2 ka, beds formed and then cemented in the last 1.5 ka. On the other hand, previous climatic proxy data obtained from several lakes in Turkey verified the prevalence of aridity both between 2 ka and 1.4 ka (Jones et al., 2006; Kuzucuoglu et al., 2011), followed by a bicentennial dry period during the so-called Event 1 in the Bond cycles (Bond et al., 1997), as emphasized byÜlgen et al. (2012).
5. Conclusion
The sequential cement fabrics, subsurface nature and OSL age of beachrocks at eight different sites along the shoreline of Lake Iznik were investigated with regard to Holocene lake level changes and Fig. 4. Interpreted inverse model resistivity sections obtained from locality 1.
its relation with periods of aridity. The low-angle cemented beds having a thickness between 50 cm and 1.5 m are sandstone and conglomerate in composition and are followed up to 5 m offshore terminating at a maximum depth of 1 m. The maximum width of the shingle beaches prior to cementation was 24 m, as confirmed by geoelectrical resistivity data. The sequential cement fabrics, identical in all samples, are composed of micrite envelops and meniscus bridges and, to a lesser extent, acicular aragonite rims.
OSL ages of the 33 samples revealed deposition and ensuing cementation of the beachrock materials during three main pe-riods; the Holocene Climatic Optimum (HCO) and the Middle and Late Holocene. Cementation of beds is likely associated with drier periods when the lake level varied between 1 and þ1 m as against the present lake level, which is confirmed by the sub-merged and buried beds at the most lakeward and landward edges of the cemented slabs.
Fig. 5. SEM images of beachrock cements. (a) General view, (b) micritic coating, (c, d, f) meniscus bridge, (e) closer view of this bridge, (g) closer view of algalfilaments, (d, e) aragonite crystals, (i, j) pore-filling cement consisting of aragonite crystals, (k, l) freshwater diatoms.
Acknowledgements
AEE wishes to thank the Scientific and Technological Research Council of Turkey (TUBITAK) forfinancial support (Project number: 109Y143) and the Turkish Academy of Sciences for support in the framework of their Distinguished Young Scientist Award Pro-gramme (TÜBA-GEB_IP). Thanks are due to Graham H. Lee for checking the English of the earlier version of the paper. We also thank journal reviewers for their constructive comments that have greatly improved our paper.
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