İstanbul-Şile kumullarının manyetik ve sedimantolojik analizi: Paleoçevresel çıkarımlar
Magnetic and sedimentological analysis of İstanbul-Şile dunes: Paleoenvironmental inferences
Yazar: MEDİHA SEYHAN
Danışman: PROF. DR. CENGİZ YILDIRIM ; DR. ÖĞR. ÜYESİ ÖZLEM MAKAROĞLU
İstanbul Teknik Üniversitesi / Lisansüstü Eğitim Enstitüsü / Katı Yer Bilimleri Ana Bilim Dalı / Jeodinamik Bilim Dalı
Derece: Yüksek Lisans, 2022
Konu: Coğrafya = Geography ; Jeofizik Mühendisliği = Geophysics Engineering ; Jeoloji Mühendisliği = Geological Engineering
Dizin: Kumullar = Dunes ; Manyetik analiz = Magnetic analysis ; Paleoçevresel = Palaeoenvironmental ; Tane boyu = Grain size ; İstanbul-Şile = İstanbul-Şile

Özet:
Kumullar sahip oldukları stratigrafik, manyetik ve kronolojik özellikleri ile Kuvaterner döneminde meydana gelen ortamsal (iklim, çevre) değişimlerle ilgili çıkarım yapılmasına imkân veren önemli çökel kayıtlarıdır. Bu çalışmada İstanbul'un Şile İlçesi'nin batısında bulunan Sofular Köyü'nün sahil kısmında yer alan falezler boyunca yüzlek veren 3 ayrı kesit boyunca paleosol ve kumul çökelleri incelenmiştir. Çevresel manyetik özellikleri analiz edilmiş ve Işık Uyarmalı Lüminesans (OSL) yöntemi ile tarihlendirilerek kronolojiler elde edilmiştir. Bir çökelim alanının karakteristik özellikleri tortunun bünyesine yansıdığından, ortamsal karşılaştırma amacıyla Passega Diyagramı çizilmiş olup, ortamsal anlamda kum ağırlıklı kesitler olduğu ve taneboyu analizi ile uyumlu olduğu görülmektedir. κlf (manyetik duyarlılık), ARM (Anhisteretik kalıntı mıknatıslanma), IRM (Eş-ısıl kalıntı mıknatıslanma) ve SIRM (Doymuş eş-ısıl kalıntı mıknatıslanma) gibi parametrelerin ölçümlerini içeren çevre manyetizması analizleri ile kesitler boyunca manyetik minerallerin yoğunluk ve tane boyu değişimleri incelenmiştir. Farklı derinliklerinde ani değişimler veya yüksek değerlerin ölçüldüğü bazı seviyelerde ölçülen yüksek manyetik duyarlılık ve kalıntı mıknatıslanma şiddetleri (NRM, ARM ve SIRM) kesit boyunca görece, manyetik minerallerin bu seviyelerde yoğunlaştığını açık olarak göstermektedir. Birinci ve ikinci kesitte çoğunlukla düşük olarak hesaplanan S-oranı bu iki kesitte üçüncü kesite oranla daha yüksek oranda yüksek koersiviteli manyetik minerallerin olduğunu göstermektedir. Yüksek HIRM değerleri de bu durumu desteklemektedir. Üçüncü kesitte elde edilen tane boyu analizleri bu kesit boyunca kil içeriğinin diğer iki kesite göre önemli derecede düşük olduğunu buna karşılık, kum içeriğinin en yüksek bu kesitte bulunduğunu göstermektedir. Diğer iki kesite oranla, üçüncü kesitte elde edilen düşük manyetik duyarlılık ve kalıntı mıknastıslanma şiddeti tane boyu analizi ile de desteklenen düşük kil içeriği ile ilişkilendirilmiştir.  OSL analizlerinde çapraz tabakalı eolinitin hemen altında yer alan paleosol olan birinci kesitin yaşı 121.3±7.66 bin yıl, yine paleosol olan ikinci kesitin yaşı 54.94±3.77 bin yıl ve tamamı eolinitten oluşan üçüncü kesitin yaşı 139.1±22.46 bin yıl olarak belirlenmiştir. Birinci ve üçüncü  kesitlerin denizel izotopik evre DİE 5, ikinci kesitin ise DİE 3'e karşılık geldiği görülmüştür. Bu veriler sahada buzullararası dönemde (DİE 5) kumul oluşumuna uygun iklim şartlarına sahip olduğunu göstermektedir. İkinci kesitin oluştuğu DİE 3'te yani 80 bin yıl önce başlayan buzul döneminin ortalarında bölgede toprak gelişimi için daha uygun ve kimyasal ayrışma için daha elverişli çevre koşullarının bulunduğuna işaret etmektedir.

Abstract:
One of the geomorphic and sedimentary markers of the paleo-environment are paleosols and eolinites. Their stratigraphic, magnetic and chronological features allow to quantify environmental (climate, environmental) changes that occurred in the Quaternary period. Sand dune fields have long been recognized as important sites in both the modern and geological records. Paleosols provide important data revealing paleo-climatic processes as they can keep records of past climatic changes in the coastal environment in which they were formed. The eolinite composition reflects the composition of the source area and the cross-bedded structure reflects the paleowind systems that affect the dune formation. Within the eolinite layers, paleosols reflect relatively humid rainy interim periods between dry periods. Soils are formed relatively during moist, hot and rainy interim periods, while dunes reflect strong windy environments during cold/hot dry periods. The presence of a sharp transition from the fossil soil level to the cross-bedded fossil dunes in the sections from the bottom to the top is an important stratigraphic indicator showing the presence of a sudden environmental change on these coasts. The main purpose of this study is to learn the paleoenvironmental conditions and to have information about the formation-development processes.
In this study, paleosol and dune deposits were investigated along 3 separate sections outcropping along the cliffs located on the coast of Sofular Village, west of the Şile District of Istanbul. The first section is the East-West oriented paleosol section just below the area formed by the cross-bedded eolinites 30 m behind the shoreline. The second section is a North-South directional paleosol section with distinct color and level changes, with intense chemical weathering of brown-red colored sand, silt and clay, 200 m away from the shoreline, under the dunes climbing south along the low topography to the east of the first section. The third section is a section of approximately 9 m consisting of cross-bedded eolinite 40 m west of the area where the first section is located. The coast is bordered by a topography consisting of cliffs consisting of bedrock in places and eolinites behind a wide beach. Between the cliffs, along the valleys or inland along the low topography, the current dunes climb from sea level to the forest border. In the field studies we carried out within the scope of the thesis; a total of 106 pieces of three different sections for environmental magnetism in accordance with each method; 22 samples were taken for grain size analysis and one sample from each section was taken for OSL dating.
Environmental magnetism is based on measuring the magnetic properties of materials found in nature, such as sediment, soil and dust. Environmental magnetism is a geophysical method suitable for high resolution and sensitive measurement. If different properties of different magnetic minerals such as stage, origin, density and composition can be determined, these minerals can be shown as environmental proxies. In this method, the aim is to determine the density, grain size and mineralogy of the magnetic minerals possessed by the materials and to link the magnetic properties of the mineral assemblages to the environmental processes that control them. Environmental magnetic properties were analyzed and chronologies were obtained by dating using the Optically Stimulated Luminescence (OSL) method.
The Passega diagram, also known as the CM diagram, shows four different sets of points on the line C=M. Although a direct climatic inference cannot be made here, the transport mechanism and grain size distributions can be interpreted. The aim is to compare different depths and different areas from an environmental perspective. In other words, it is to learn about the transport mechanism. The characteristic features of a depositional area are reflected in the texture of the sediment. Since the characteristics of a deposition area are reflected in the sediment, the Passega Diagram has been drawn for the purpose of environmental comparison, and it is seen that there are sand-weighted sections in the ambient sense and are compatible with the grain size analysis. Particle size analyzes are used to determine wear of particles, degradation processes and environmental conditions during transport and precipitation. According to the Udden-Wentworth scale, changes in grain size with depth are observed.
The Passega analysis is comparable with grain size analysis and indicates dominant sand settings along the sections. κlf (magnetic susceptibility), ARM (Anhysteretic remanent magnetization), IRM (Isothermal remanent magnetization) and SIRM (Saturated IRM) measurements were applied within the scope of environmental magnetism. It was observed that sudden changes or high values were measured at different depths of the sections. The high magnetic susceptibility and remanent magnetization intensities (NRM, ARM, and SIRM) measured at these levels clearly indicate that relatively magnetic minerals are concentrated at these levels throughout the section. The S-ratio, which was calculated to be mostly low in the first and second sections, shows that there are higher coercivity magnetic minerals in these two sections compared to the third section. High HIRM values also support this situation.The grain size analyzes obtained in the third section show that the clay content along this section is significantly lower than the other two sections, while the sand content is the highest in this section. Compared to the other two sections, the low magnetic susceptibility and remanent magnetization intensity obtained in the third section were associated with the low clay content, which was also supported by the grain size analysis. In recent years, one of the most used methods in earth science research is the luminescence method. This method is mainly used for quartz, feldspar, etc. It is based on the natural radioactivity of ionizing radiation and the cosmic radiation it emits (annual dose). The dating technique is crucial for establishing chronologies and calculating sedimentation rates as an indicator of paleo-environmental change. In OSL analyzes, the age of the first section, which is paleosol, located just below the cross-bedded eolinite, is 121.3±7.66 thousand years, the age of the second section, which is also paleosol, is 54.94±3.77 thousand years, and the age of the third section, which was entirely composed of eolinite, was 139.1±22.46 thousand years. Age analysis, grain size and environmental magnetism studies conducted within the scope of the thesis show that paleosol (1st Section) and fossil dunes (3rd Section) were formed in the interglacial period corresponding to Marine Isotope Stage (MIS 5) (134-114 thousand years ago). It is possible to see paleosol levels above and below the dunes. The OSL ages we obtained from the upper levels show that these paleosols were formed approximately 55 thousand years ago towards the middle of the MIS 3 (80-20 thousand years ago) phase and that there were climatic conditions suitable for soil development in this region during this period.
Sedimentological analyzes revealed that the samples taken from Sections 1 and 2, which are paleosols, contain clay contents, while samples from Section 3, which is eolinite, are dune deposits with the lowest clay content but high sand content, and generally consist of relatively well sorted, fine to very fine grained sands. The amount of fine silt and clay sized grains in the 3rd section is quite low. In the 1st and 2nd sections, more clay content, which varies with depth, was observed. When the Passega diagram is examined, it is seen that the areas where the sections are taken are clustered in the same area in general terms. This shows that the areas where the samples were taken represent the same area, and in addition, these sections have a similar transport process from an environmental point of view. Again, it can be deduced from the diagram that the average grain sizes are close, and it is observed that it is compatible with the grain size analysis.
In the environmental magnetism results, it is seen that sudden changes or high values are measured at different depths. On the basis of these changes, when we distinguish the magnetic units along the section; For section 1, from 180 cm to the end of the section; It is thought that there are different units between 70-110 cm for the 2nd section and between 1-3 m and 6.5-8.5 m for the 3rd section. The high magnetic susceptibility and remanent magnetization intensities (NRM, ARM, and SIRM) measured at these levels clearly indicate that relatively magnetic minerals are concentrated at these levels throughout the section. Among the sections, the highest magnetic mineral density was found in the 1st and 2nd sections, while the lowest magnetic mineral density was found in the 3rd section. This relationship is very useful for distinguishing paleosol and eolinite levels in the study area. The S-ratio, which was calculated to be mostly low in the 1st and 2nd sections, shows that there are higher coercivity magnetic minerals in these two sections compared to the 3rd section. HIRM values also support this situation. The high magnetic mineral density obtained from 180 cm to the end of the section in the first section is thought to be related to the increase in clay content at these levels, as seen from the grain size analysis. A similar relationship is seen in the 2nd section. The grain size analyzes obtained in section 3 show that the clay content along this section is significantly lower than the other two sections, while the sand content is the highest in this section. Compared to the other two sections, the low magnetic susceptibility and remanent magnetization intensity obtained in the 3rd section were associated with the low clay content, which was also supported by the grain size analysis. Our data indicate abrubt transition from paleosol to dune formations around 120 ka (MIS5e) and second phase of paleosol formation around 50 ka BP.