Ανασύνθεση παλαιοπεριβάλλοντος σε παράκτιες γεωμορφές της νότιας Κύπρου με την χρήση τεχνικών φωταύγειας
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Τσάκαλος, Ευάγγελος
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Abstract
Absolute dating methods have been developed during the last five decades (Jull and Scott, 2007). The most common methods applied to minerals’ dating are cosmogenic radionuclides, Electron Spin Resonance (ESR) and luminescence techniques. The latter were initially used to date burned minerals from archaeological artefacts (Thermoluminescence (TL) method). However, advancements of this technique led to the development, of the optical dating method (usually referred to as Optically Stimulated Luminescence (OSL)) which is now applied to sediments from various origins (aeolian, fluvial, marine and colluvial) and has been proven to be increasingly advantageous, especially for dating sediments that were deposited since the Late Quaternary (Wintle, 2008).
The importance of luminescence dating lies on the fact that the method determines how long ago sediments were last exposed to daylight or heat rather than sediment’s comminution from parent rock which usually took place a very long time before. This characteristic makes luminescence a very advantageous technique in dating sedimentary formations, in which their luminescence signal has restarted during their formation.
Sedimentary environments contain amounts of radioactive isotopes of elements such as potassium, uranium and thorium. Their decay produces ionizing radiation which is absorbed by common mineral grains such as quartz and feldspar which are found in the sediments. This radiation is “stored” within the mineral grains in structurally unstable “electron traps”. The trapped radiation “builds up” over time at a rate determined by the amount of the available radiation at the spot where the sample was buried. Stimulating the electon traps using either light or heat causes the release of the trapped radiation and the emission of a luminescence signal, the intensity of which depends on the amount of radiation stored during burial and the specific properties of the mineral. The date of a sample is obtained from the ratio of the palaeodose (the radiation dose that has been accumulated in a sample) to the dose-rate that the sample - to be dated - has been exposed to. Accurate determination of both the palaeodose and the dose-rate is of great importance in luminescence dating. Regarding the determination of the palaeodose, a number of different methodologies are available that can assist in obtaining the best palaeodose estimates (e.g., Olley et al., 1998; Stokes et al., 2001; Lepper and McKeever, 2002, Grün, 1989; Schellmann et al., 2008) which are usually based on their statistical treatment (Galbraith et al., 1999).
Regarding the second quantity that has to be determined, in spite of the fact that dose-rate determination has become quite routine in luminescence dating, there still exists a complex situation, due to the conversion of elemental concentrations into dose rates using conversion factors derived from up to date nuclear data as well as updated absorb dose fractions for the different radioelements of materials having different densities.
Luminescence dating relies on the assumption that the mineral grains were sufficiently exposed to daylight or heat at the time of the event being dated. For instance, when quartz grains are used, a daylight exposure in the range of 1-100 seconds before burial is considered sufficient to effectively “reset” the OSL chronometer (e.g. Rhodes, 2011). This is usually the case with aeolian deposits. OSL ages using quartz range from 100 to 350000 years BP, and can be reliable when appropriate procedures are used and proper validation checks have been performed. Luminescence dating using feldspar can extend the datable range up to a million years as feldspars have considerably higher dose saturation levels than quartz, however issues regarding “anomalous fading” (signal loss) need to be dealt with first (Rhodes, 2011). This signal loss over burial time often gives rise to significant age underestimation, with many fading correction procedures being developed (e.g., Lamothe and Auclair, 1999; Huntley and Lamothe, 2001) to correct for these underestimations. Luminescence ages are commonly reported with associated errors which reflect random and systematic uncertainties. The uncertainty at one sigma (68 % confidence interval) associated with an OSL age is usually 5-10% of the sample age (Roberts et al., 2015).
Despite these limitations, nowadays luminescence dating is considered a key geochronological technique, significantly contributing to our understanding regarding the timing, rates and nature of many palaeoenvironmental processes, and their implications for landform and landscape development.
Coastal zones represent complex environments in the transition between sea and land. From a geological viewpoint, coastal zones are very dynamic areas with daily low and high tides as well as sporadic storm events having a high impact on coastal geomorphology. Long term changes are due to rising or falling sea levels caused by global climate change, specifically, sea level fluctuations during the Quaternary (last 2.5 million years) as well as uplifting or sinking regimes as a result of local tectonic activity.
The coastal zone of southern Cyprus is of great palaeoenvironmental and archaeological interest (Ammerman et al., 2008; Bar-Yosef, 2001). In particular for Cyprus, eustatic changes in sea level and the interaction with the land, which in places characterized by distinct morphodynamic and tectonic features, resulted in the formation of palaeoenvironmental and paleogeographical conditions which were very different than today’s (Bailey, 2004; Bailey and Milner, 2002; 2003). It is possible positions that today are coastal or even underwater, once being part of the mainland of Cyprus, when the sea level was very low; during the glacial periods of the Quaternary. These changes are mainly reflected in coastal sedimentary deposits, occurring along the modern Cypriot coastline. In addition to the palaeoenvironmental interest, Cyprus is of great archaeological significance. Archaeological studies have revealed human presence on several sites of the island, with Akrotiri Aetokremmos, Nissi Beach, and Akamas Aspros being dated to the Late Epipalaeolithic (ca 11,000-10,000 cal BC) (Ammerman, 2010; Knapp, 2010; Simmons, 2013) and considered as the oldest evidence for seagoing in the eastern Mediterranean. Therefore, the establishment of a reliable geochronological framework is fundamental not only for a better geological and climatological understanding of the coastal paleoenvironment but also provides an improved framework for understanding human evolution of eastern Mediterranean.
Additionally, the study of sand-grains surface marks (exoscopy) using Scanning Electron Microscopy (SEM) has been developed into a method for linking sand-grains microtextures to particular sedimentary environments (Krinsley and Marshall, 1987; Legigan et al. 1989). Different types of microtextures on the quartz grain surfaces can be used to differentiate between aeolian, marine, and glacial depositional environments. If a grain has transited different environments, the surface signatures may comprise a mixture of different textures produced during sediment transport, deposition and diagenesis. Principally, mechanical processes leaving signatures as different impact and abrasion marks on the grain surfaces during transportation in different dynamic environments are the profound diagnostic features which record those mechanical processes. Marks as a result of chemical processes consisting of a range of different overgrowth and etching types, can further facilitate the identification of the diverse post-depositional course of action that grains have experienced.
Purpose of this thesis is to examine the chronology of coastal deposits of southeast Cyprus by employing up-to-date luminescence methods and evaluate from the palaeoenvironmental viewpoint the episodes/phases of development of coastal areas of south Cyprus during the Upper Quaternary. In this work, special focus is given to the coastal deposits of the southeast coast, between Agia Napa and Cape Greco. This research work also presents inherited quartz features observed on quartz grains of the Quaternary coastal deposits from Agia Napa area with the purpose to uncover their depositional history.
In the frame of this thesis an attempt was also made to determine the burial age of sediment horizons within a sequence containing chipped stone artefacts on an upland site, located in Troodos Mountain at the site of Vretsia-Roudias. The geoarchaeological investigation conducted at the site revealed the existence of clearly defined depositional episodes (palaeosols) leading to the identification of paleosurfaces. The finds, mostly lithic tools reinforce the view that this Pleistocene river terrace in the Troodos mountains, was repeatedly visited by groups of hunter-gatherers who remained at the site for an unknown period each time, and whose trip was part of a route from the coast to the mountains and vice-versa. The main issue with Roudias is that it lacked absolute dates. Attempts have been made to date bone recovered from the site; unfortunately, these samples did not contain enough collagen to return any dates. In this regard, OSL dating was attempted.
Purpose of this thesis is also the enhancement of the accuracy and reliability of luminescence ages through the development of two computer software packages; a) The Dose Rate calculator (DRc) which facilitates the calculation of dose-rates and ages determination of materials, used in palaeodosimetric dating methods using updated conversion and attenuation factors and b) the Palaeodose Statistical Tool (PST), which brings together the statistical models available for palaeodose determination that are widely dispersed in the literature. PST produces a single palaeodose value which is representative of the event (last exposure to the light or heat) to be dated.
The proposed approach, especially the use of luminescence dating is innovative for the area, since no studies have been performed with this particular dating technique so far in Cyprus. Furthermore, the the production of reliable and accurate palaeodose and dose-rate values will be a great advancement in luminescence dating methodology.
Chapter one presents an introduction to the principals of the luminescence dating method along with its different techniques. Chapter one also discusses the application of the luminescence dating method to different sedimentary environments.
Chapter two discusses issues associated with the accurate determination of the equivalent dose (De). In this regard, a computer program written in Java was developed which assist the selection of the most representative equivalent dose estimate. The Palaeodose Statistical Tool (PST), automatically decides on the use of the appropriate age model and produces a single De value. This is followed by Chapter three where second software was developed. The Dose Rate calculator (DRc) is a Java application which facilitates the calculation of dose rates and ages determination of materials, used in palaeodosimetric dating methods. In DRc, dose rates are calculated using updated conversion and attenuation factors.
Chapter four is an introduction to the study of sand-grains surface marks (exoscopy) using SEM for linking sand-grains microtextures to particular sedimentary environments.
Chapter five presents the geology and evolution of Cyprus including quaternary stratigraphy, palaeoclimate, and sea level changes. This is followed by Chapter six where the geomorphology of Cyprus is discussed. This chapter focuses on coastal geomorphology and the different coastal deposits developed along the south east coastal areas of the island. Special, attention is given to chronological framework of the aeolianites and other coastal formations. In this regard, a lithological description of Cyprian coastal deposits is given based on field observations as well as petrographic thin sections from samples collected.
Chapter seven presents the methodology used in this research which includes the sampling strategy, treatment of samples, analytical procedures used in luminescence dating of quartz and feldspar as well as exoscopy.
Chapter eight presents and discusses the findings of the study. It gives absolute ages for the formation of the coastal deposits, as well as the burial age of the sediment horizons within the sequence containing the chipped stone artefacts. It also presents and discusses the findings of the examination of quartz-grains surface features. This chapter also compares the chronological findings of the study with other relevant studies in the area and tries to build up the time-frame for their development.
Finally, the thesis finishes with Chapter nine where conclusions are given.
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Keywords
Palaeoenvironmental reconstraction, Luminescence dating techniques