Implications for Late Holocene Sea-Level Fluctuations
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Sedimentary Geology 294 (2013) 294–302 Contents lists available at ScienceDirect Sedimentary Geology journal homepage: www.elsevier.com/locate/sedgeo First record of beachrock on Black Sea coast of Turkey: Implications for Late Holocene sea-level fluctuations Ahmet Evren Erginal a,⁎, Yunus Levent Ekinci b, Alper Demirci b, Mustafa Bozcu c, Muhammed Zeynel Ozturk d, Mustafa Avcioglu c, Erdal Oztura e a Ardahan University, Department of Geography, TR-75000 Ardahan, Turkey b Çanakkale Onsekiz Mart University, Department of Geophysical Engineering, TR-17020 Çanakkale, Turkey c Çanakkale Onsekiz Mart University, Department of Geology Engineering, TR-17020 Çanakkale, Turkey d Çanakkale Onsekiz Mart University, Department of Geography, TR-17020 Çanakkale, Turkey e Çanakkale Onsekiz Mart University, Department of Geography Education, TR-17020 Çanakkale, Turkey article info abstract Article history: We present new data on the diagenetic characteristics, subsurface nature and radiocarbon ages of beachrock Received 9 May 2013 from the Thracian Black Sea coast of Turkey, indicative of sea-level changes and climatic conditions favoring Received in revised form 14 June 2013 lithification of beach sands between 5.4 ka and 3.5 ka cal BP. Micrite coatings and succeeding meniscus ce- Accepted 15 June 2013 ments typify diagenetic history and suggest a two-stage cementation over this timeframe. The early cements Available online 24 June 2013 are typical of upper intertidal zone when the sea-level was likely similar to that of today. The ensuing Editor: Dr. J. Knight intergranular bridges refer to an approximate 2 m decline in sea-level, favoring downward percolation of meteoric waters related to subaerial exposure, marked by a reduction in Mg concentration and dissolution Keywords: pits on early cement coatings. Formation of beachrock during this bimillennial period could be associated Beachrock with relatively drier conditions promoting the precipitation of connective cements. Sequential cementation © 2013 Elsevier B.V. All rights reserved. Sea-level changes Thrace Black Sea 1. Introduction as a key indicator for sea-level changes due to differing viewpoints (Kelletat, 2006; Knight, 2007). As a kind of cemented beach deposit frequently found in the form In contrast to authors who have focused on geomorphological of blocks or slabs, beachrock offers a record of the morphodynamic, data (Kelletat, 2006), many researchers have recently gained benefit climatic and oceanographic changes that have occurred in coastal from studying cementation characteristics (Kneale and Viles, 2000; areas. Amalgamation of sediments as well as fossil shells and micro- Hillgaèrtner et al., 2001; Calvet et al., 2003; Rey et al., 2004; Vieira fossils entombed within cement or intergranular spaces provide a re- and De Ros, 2006) and stable isotope data (Holail and Rashed, 1992; cord of the diagenetic history of the beachrock. Since the very earliest Calvet et al., 2003; Friedman, 2005; Vieira and De Ros, 2006). One of studies, attempts have been made to pinpoint the origin, age, distri- the main problems regarding the solidity of intertidal beachrock in bution and significance of beachrock on various coastal outcrops real terms is lack of information on the continuation of beachrock around the world (see Vousdoukas et al. (2007) and references there- backshore under beach materials, since samples are mostly taken in). Since abiotic cement, i.e., aragonite and high Mg-calcite, is precip- from slabs exposed at the coast. Along the 8333 km-long coastline itated as a direct result of the evaporation of sea-water, predominantly of Turkey, beachrocks display a wide distribution, from the eastern as a connective carbonate, beachrock has been generally attributed to end of the Mediterranean to the Gulf of Saros, comprising the northern intertidal cementation and regarded as giving explicit indications of limit of the Aegean Sea (Avşarcan, 1997; Desruelles et al., 2009). sea-level changes. Nevertheless, contribution of different specific pro- In this study, results obtained from the first recorded beachrock on cesses regarding to precipitation of binding carbonates have been sug- the Thracian Black Sea coast of Turkey are discussed (Fig. 1a, b). We gested (Vieira and De Ros, 2006 and references therein). Until considered that the formation of beachrock on the coast of the Black recently, however, there was debate on the credibility of beachrock Sea is substantive in that this anoxic sea is presently characterized by low (17–20‰) surface salinity and a nominal amount of evaporation compared to the much greater amount of freshwater input by precipita- ş ⁎ Corresponding author. Tel.: +90 478 2114132; fax: +90 478 2113275. tion and rivers (Be iktepe et al., 1994; Mertens et al., 2012). Based on E-mail address: [email protected] (A.E. Erginal). diagenetic characteristics in conjunction with radiocarbon ages and 0037-0738/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.sedgeo.2013.06.003 A.E. Erginal et al. / Sedimentary Geology 294 (2013) 294–302 295 Fig. 1. (a–b) Location maps of study area; (c) geology map of the study area and its environs; (d) ERT lines taken perpendicular to the coastline. geophysical imaging data, new findings in respect of Late Holocene the northern coast is the mouth of the Kazandere River which flows sea-level fluctuations on the Turkish Black Sea coast are presented. into the sea from between Kıyıköy port and a coastal spit (Fig. 1c). On the backshore, a low ridge made up of a 5-m thick coastal dune stands between the river mouth and the present sandy beach. Meteoro- 2. Study area logical data (1975–2006) from Kumköy meteorology station 85 km to the southeast indicate that the area receives a yearly precipitation of On the eastern coast of Kıyıköy, Kırklareli, NW Turkey, the studied 831.4 mm. The average annual air and sea-water temperatures were deposits constitute the first record of beachrock from the northwestern recorded as 13.8 °C and 14.3 °C, respectively. During the hot period be- Thracian Black Sea coast of Turkey (Fig. 1a, b). Comprising the European tween May and September, evaporation values from sea water reach a part of the country, Thrace is bounded by the Sea of Marmara to the totalof600mm,whichislowerthantheMediterranean(approximate- south and Black Sea to the north and has a coastline of about 786 km. ly 2000 mm). Climatically, a humid-temperate Black Sea climate domi- The Black Sea coast of Thrace starts from the mouth of the River Rezve nates (Türkeş,1996). Tidal range is nominal with an average of 10 cm (Rezovo) at the Bulgarian border and extends in a northwest–southeast (Goudie, 2001). direction down to the strait of Istanbul (Bosphorus) (Fig. 1b). The eastern part of Thrace comprises the Çatalca Peninsula and is character- ized by a nearly straight coastline. However, in the northwestern section, 3. Methods the Thracian coast is rather indented as the coastal area forms a seaward (northeast) prolongation of the Strandja Mountains, a massif consisting 3.1. Analyses of beachrock materials of basement rocks such as gneiss and amphibolite, and cover deposits formed of schist, limestone, sandstone and igneous rocks (Pamir Five samples of beachrock were collected for analysis and radio- and Baykal, 1947). carbon dating. Petrographic analysis was carried out based on thin The geology of the backshore at the southern end of the cemented section interpretations. Energy Dispersive Spectroscopy (EDX-Bruker beach in this study is composed of Middle to Late Eocene fossiliferous AXS XFlash) analysis of the beachrock was performed to shed light on limestones with sandstone and claystone intercalations (Fig. 1c). On elemental composition of connective cements. Using Scanning Electron 296 A.E. Erginal et al. / Sedimentary Geology 294 (2013) 294–302 Microscopy (SEM-Zeiss EVO 50 EP), the same sub-samples were used depth of −2 m extends up to 20 m offshore. Following the assump- for determination of sequential cement fabrics on and around the bond- tion that beachrock is suggestive of intertidal cementation, and its po- ed grains. Analysis was carried out at Izmir Institute of Technology, tential for highlighting sea-level changes, knowledge of the total Turkey. To determine foraminiferal contents, samples were treated thickness of beds, including buried slabs, is of importance. For this with 10% H2O2 for one day and wet sieved through a 63 μmsieve.The purpose, ERT images (Fig. 3) were obtained from the beach. Taken residue was then dried in air. In the sediment fraction above the 63 μm perpendicular to the coastline (Fig. 1d), resistivity tomograms pro- sieve, benthic foraminiferal fauna was examined under a binocular mi- duced by the 2D tomographic inversion process displayed a depth croscope. Five bulk samples of beachrock were dated by conventional range of about 2.7 m for ERT-1 and ERT-2 and about 4 m for ERT-3 and AMS radiocarbon methods at BETA, Miami, USA. and ERT-4 (Fig. 3). The resistivity values in the survey area vary between about 1 and 550 ohm.m. Thus, the tomographic images clearly 3.2. Geophysical survey show a sharp resistivity contrast between the high-resistivity beachrocks and high-conductivity beach material. The abnormal decrease in the A geoelectrical survey using electrical resistivity tomography resistivity values of the beach material might have resulted from an in- (ERT) technique was conducted along 4 coastline-perpendicular lines crease in salinity due to seawater intrusion. These images also show (Fig. 1d), 10 m apart from each other, in order to depict the internal that the buried slabs of beachrock are about 1 m thick, showing a structure and contact relationship of the beachrocks and also to monitor sharp termination at this depth. Additionally, a high-resistivity zone is their landward extent. The ERT measurements were carried out with a observed at the end of ERT-3 and ERT-4 lines.