Late Pleistocene To Holocene Paleoceanographic And Paleo-climatic Changes In Gulf Of Gemlik, Sea Of Marmara, Turkey

Tez (Yüksek Lisans) -- İstanbul Teknik Üniversitesi, Fen Bilimleri Enstitüsü, 2016Thesis (M.Sc.) -- İstanbul Technical University, Instıtute of Science and Technology, 2016Marmara Denizi’nin doğusunda yer alan Gemlik Körfezi, İzmit Körfezi gibi yarı kapalı bir havza olup, benzer oşinografik özelliklere sahiptir. Gemlik Körfezi, Kuzey Anadolu Fay Zonu içerisinde bulunması nedeniyle tektonik olarak aktif bir havza olmasına rağmen, İzmit Körfezinden farkı, 17 Ağustos 1999 İzmit depreminden fazlaca etkilenmemiş olmasıdır. Ancak Gemlik Körfezi, Kuzey Anadolu Fayı’nın (KAF) orta kolu üzerinde yer alması nedeniyle, olası bir deprem ile karşı karşıyadır. Bu nedenle Gemlik Körfezi’nin de Marmara Denizi’ndeki diğer havzalar kadar araştırılması gerekmektedir. Bu durumun en önemli sebebi ise Marmara Denizi’nde paleoşinografik çalışmaların genelde kuzey şelf ve yakınlarında yoğunlaşmış olmasıdır. URANIA araştırma gemisi ile 2013 yılında Gemlik Körfezi’nde tamamlanan deniz çalışmalarında sediment karotları ile birlikte sismik veriler alınmıştır. Bu tez çalışmasında körfezin kuzey yamacına yakın alınmış bir adet piston karotu (M13-08) ilk defa bu tez kapsamında çalışılmış olup, Gemlik Körfezi’nde Geç Pleistosen-Holosen dönemi paleoşinografik değişimler ortaya çıkarılmaya çalışılmıştır. Bu tez çalışmasında ayrıca daha önce farklı araştırmacılar tarafından incelenmiş olan bir piston karotu da sismik-karot eşleşmesi için ilave olarak kullanılmış ve böylece sismik yansıma görüntülerindeki çökellerin son 18 bin yıldaki kronostratigrafileri detaylı bir şekilde ortaya çıkarılmıştır. Bu tez kapsamında oluşturulan çok ışınlı batimetri haritası ile Gemlik Körfezi’nin morfo-tektonik yapısı incelenmiştir. Körfezde ortası elips şeklinde KB-GD yönünde uzanan, en derin yerinin -113 m’ye ulaştığı bir havza olduğunu belirlemenin yanı sıra, havza ortasında bulunan Burgaz çökelimi, kuzeydeki Kapaklı çıkıntısı, doğudaki Gençali Deltası, Gençali ve Kurşunlu arasındaki Gemlik Yükseltisi ve çeşitli lokasyonlarda bulunan fay kırıkları gibi birçok morfolojik yapı gösterilmiştir. Ayrıca Geç Pleistosen-Holosen dönemine ait çökellerinin stratigrafisi de detaylı bir şekilde ortaya konulmuştur. M13-08 karotunun başlıca iki farklı litolojik birimden oluştuğu saptanmıştır. Bunlar, altta daha yaşlı olan Birim-L2 ve bunu uyumsuzluk ile üzerleyen Birim-L1’dir. Karotun tabandan 257 cm’ine kadar devam eden Birim-L2 genel olarak tatlı su molluskları içeren gri renkte iyi boylanmış orta kumdan oluşmaktadır. Bu birim içerisinde, karot tabanı G.Ö. 14.1 bin yıl kalibre yaşını verirken, birimin tavanından yaklaşık G.Ö. 12.2 bin yıl kalibre yaşı elde edilmiştir. Birim-L1 incelendiğinde ise genel olarak yeşil-kahve renklerde, kil ve siltten oluştuğu, yer yer kavkı yığışımları ve denizel mollusklar içerdiği söylenebilir. Birim-L1 için ortalama sedimantasyon hızı 0.02 cm/yıl iken Birim-L2 için bu sedimantasyon hızı ortalama 0.2 cm/yıl olarak belirlenmiştir. Bu farkın en önemli nedeninin, Birim-L1 denizel ortamda çökelirken, Birim L2’nin delta ortamında çökelmiş olduğu söylenebilir. Günümüzde Gemlik Körfezi’ndeki deniz seviyesi değişimleri ile Marmara Denizi’ndeki değişimlerin eş zamanlı gerçekleştiği bilinmektedir. Ancak günümüzde körfezin, Marmara Denizi ile geçişinde -55 m eşiğinin bulunması, Marmara Denizi ile su geçişlerinin farklı zamanlarda kesilmiş olabileceğini göstermektedir. M13–08 karotunda yapılan sedimantolojik çalışmalar ile Marmara Denizi’nde de gerçekleşmiş olan göl/deniz geçişi belirlenmiş ancak karottan elde edilen radyokarbon yaşlar ile bu geçişin G.Ö. 12 bin kalibre yılında gerçekleştiği ve Gemlik Körfezi’nde göl/deniz geçişinin, Marmara Denizi’nden 550 yıl sonra gerçekleştiği tespit edilmiştir. Global deniz seviyesi değişimleri göz önünde bulundurularak -55 m su derinliğinde bulunan eşiğin bu dönemde -65 m’de olması gerektiği ve eşik derinliğindeki yükselmenin, eşik üzerinde sediman birikmesi ile gerçekleştiği düşünülmektedir. İncelenen karotlar ve sığ-sismik yansıma görüntüleri sayesinde Gemlik Körfezi Geç Pleistosen-Holosen dönemi eski iklim ve deniz seviyesi değişimleri tespit edilmiştir. Sismik profillerde günümüzden önce 18 bin yılı kapsayan 3 farklı sismik birim ayırtlanmıştır. Karot ile kesilebilen en yaşlı birim Birim-S3'ün G.Ö. 18–15.5 bin yılları arasında Oldest Dryas döneminde bölgede kurak bir iklim hüküm sürerken çökeldiği, bu birimin tabanında yer alan SB–3 sismik yansıma yüzeyinin erozyonel kanal özelliği gösterdiği ve bu yüzey üzerinde bulunan Birim-S3 tarafından doldurulduğu belirlenmiştir. Bunu üzerleyen Birim-S2 ise dik eğimli klinoformlara sahiptir ve G.Ö. 12.7 bin yılına kadar olan göl seviyesinin yükselmeye devam ettiği Bolling Allerod dönemi ve bunun ardından gerçekleşen 1000 yıllık bir dönem olan Genç Kuruma döneminde çökeldiği belirlenmiştir. Bunun ardından gerçekleşen durağan bir dönemin sonunda Marmara Denizi ile bağlantının sağlandığı Holosen döneminde çökelen Birim-S1’in, altındaki tüm diğer birimleri uyumsuz olarak üzerlediği tespit edilmiştir. Bu çalışmada sığ-sismik ve karot eşleşmesinin yanı sıra, çökellerde Manyetik duyarlılık analizi, tane boyu analizi yapılmıştır. Bunun yanı sıra, XRF karot tarayıcı analiz yöntemi ile karot boyunca element profilleri elde edilmiştir. Bu analizler sayesinde çökellerin zaman içerisinde değişen fiziksel ve jeokimyasal özellikleri belirlenerek paleo-iklimsel değişimlerin yanı sıra su kolonunda meydana gelen jeokimyasal değişimlerin de deniz seviyesi ile olan ilişkileri kurulmuştur. Son 18 bin yıldan günümüze Bolling Allerod ve Genç Kuruma (YD) dönemleri belirlenmiş ve Genç Kuruma döneminde bölgede ciddi bir kuraklık gerçekleştiği, bunun ardından Erken Holosen döneminde kuraklık, orta Holosen’de bir ısınma ve bunu takip eden Geç Holosen döneminde ise tekrar bir kuraklık evresinin bölgede gerçekleştiği belirlenmiştir.Taneboyu analizleri incelendiğinde ise gölsel sedimanların fluviyal etkide ve yatak yükü şeklinde, denizel sedimanların ise kapalı bir haliç ortamında, genelde süspansiyon şeklinde çökeldiği belirlenmiştir. Ayrıca karot çökellerinde Toplam Organik Karbon (TOK) ve Toplam İnorganik Karbon (TİK) oranlarının tespiti sayesinde sapropel seviyeleri belirlenmiş ve bu çökelme yaşları iyi bilinen özel seviyeler, Alt Sapropel G.Ö. 10.8-6.6 bin kalibre yılları arasında ve Üst Sapropel G.Ö. 2.7-5.4 bin kalibe yılları arasında olmak üzere yaşlandırılmış ve bu dönemlerde dip suyunun oksijence fakir olduğu belirlenmiştir.The Gemlik Gulf situated in the eastern part of the Sea of Marmara (SoM) is an oval shaped pull-apart basin on the middle strand of the North Anatolian Fault. The gulf has a maximum depth of 113m, a length of 36km and a width of 11km, which is separated from the southern shelf of the SoM with a sill depth of −50m. There were presumably several disconnections during the Late Pleistocene to Holocene, but the timing of the youngest connection around the onset of the Holocene is still conversial. Since the paleo-climatic and paleo-evironmental changes in the Gulf of Gemlik are poorly known, the multi-proxies of sedimentary records are needed to be provided with a reboust chronology. In this study, we present bathymetric seismic reflections and multiproxy analyses of 6.70 m-long core, recovered from -73.62 m water depth in the Gemlik Gulf. The analyses include total organic (TOC) and inorganic carbon (TIC), high-resolution µ-XRF core scanner elemental, grain size, and magnetic susceptibility determinations together with 14C ages obtained for a precise chronology of the Late Pleistocene to Holocene. The main objective of the study is to investigate environmental changes over the last 18 cal ka BP. Radiocarbon dated sediment core together with compared seismic data enable us to differentiate chronostratigraphic units of Gemlik Gulf for the Late Pleistocene to Holocene. In this context, we differentiated two main stratigraphic units covering the last 14 ka BP. A dark gray, well sorted medium sand with rare brackish water shells (Dreissenapolymorpha, Plagiocardium) represents the lower part of the oldest lacustrine Unit L2 until Unit L1, which contains different lithological layers, overlies 257 cm. Unit L2. The lowermost part of unit L1 is observed as homogeneous dark gray silty laminated mud abound with mollusk shell. This layer is overlain by 2-3cm shell debris over 233.5cm and continues to 177cm with greenish gray mud. Dark green mud including coal fragments appear between 177-152cm that is overlain by 17.5cm thick green clay with marine shells and extensive bioturbations. Abundant marine mollusks and coal fragments are presented in grayish green homogenous silty clay between 134.5-60cm in the core. The uppermost part of unit L1 consists of interbedded dark green fine clay and light greenish brown homogenous clay with mollusks. In this study, detailed seismic stratigraphy together with sediment cores retrieved from the Gulf of Gemlik enable us to evaluate chronostratigraphy of the gulf during the Late Pleistocene to Holocene. In the seismics, we have differentiated five different seismic units, three of them are well correlated with sediment cores on the basis of seismic to core comparision. Based on radiocarbon dates, the studied piston cores covers the time period for the last 18 ka BP. The oldest seismic unit (Unit-S5) has been described as deltaic succession based on having highly-reflective of foresets and bottomsets, typically evidence of prodelta formation along the seafloor. In the seismics, the foreset to topset transition occurs at ca. -65 m, whereas the topsets were truncated by the main unconformity surface SB-5. On the basis of radiocarbon dated sedimentary units and their correlation with seismics, relative stratigraphic position of such deltaic complex reveals that the deposition occurred during the Last Glacial Maximum (LGM) between 23-21 ka cal BP. The deltaic sediments (Unit-S5) are unconformably overlaid by Unit-S4 that is recognized by highly-reflective clinoforms below -75 m, implying shelf-edge progradation. According to relative stratigraphic position of Unit-S4, it can be assumed to be deposited in the Gulf Gemlik between 21-18 ka cal BP. On the shelf margin, the prograded clinoforms of seismic unit-S4 are truncated along the main unconformity surface (SB-5) below depth of -75 m, whereas the same sediments are locally incised by seismic surface SB-3 in the deeper part due to formation of erosional gullies along the seismic surface. On the basis of seismic to core comparison, this erosional surface corresponds to the base of unit L3 in core GE-124, which has been dated as 18 ka cal BP. Seismic unit-S3 over the surface SB-3 consists of highly-reflective parallel bedded sediments in the deeper basin, infilingthe gullies in the seismics. On the shelf margin, it displays onlapping mud drape over seismic surface SB-3. Its all seismic architectures are interpreted as transgressive sediments deposited after 18 ka cal BP in the Gulf. These sediments were penetrated in core GE-124 that comprises sandy and silty intercalations with clays including abundant Neouxine fauna, implying a shallowing-up sequence. Based on 14C dating in the same core and its comparision with the seismic, deposition of Unit-S3 took place between 18-15.5 ka cal BP that corresponds to the Oldest Dryas. In the seismic profiles, the youngest sediment sequence prior to the Holocene is defined as Unit-S2 that is sandwiched between surfaces SB-4 and SB-5 in the seismics. On the shelf margin, this unit is documented as having highly reflective prograding clinoforms with very steep inclinations, whereas it consists of very thin mud drape covering the older sedimentary units below. Prograding nature of these sediments explains well its regressive facies during sealevel history of the gulf prior to the onset of the Holocene. In the seismics, the formation of berms at -65 m between surfaces SB-4 and SB-5 on the inner shelf can be attributed to existence of paleoshoreline. Unit-S2 can be calibrated by two piston cores, M13-08 and GE-124, that are all well dated and correlated with seismic data. On the basis of 14C datings and together with seismic to core comparisions, Unit-S2 was presumably deposited during 15.5-12 ka cal BP. This timing is represented by two different climatic periods, the Bolling-Allerod and Younger Dryas. According to lithologic definitions from core M13-08, Unit-L2 (lithostratigraphic equivalent of seismic unit-S2) consists of medium well sorted sands with abundant Neouxine mollusks, whereas the deeper core GE-124 reveals much finer sediments with the same faunal content. The transition from lacustrine to marine sediments in the seismics is represented by the main unconformity surface SB-5 that can be well traced in core M13-08 between units L2 and L1. This remarkable surface in the core has been dated at 12 ka cal BP. The Holocene unit S1 in the seismics covers unconformably all the other units below. The highly-reflective parallel bedded mud drapes along the surface SB-5 and the existence of coastal onlapping nature imply that Unit-S1 present transgressive sediments deposited during the Holocene in the Gulf. The lithologic equivalent of this unit has been differentiated as Unit-L1 in cores M13-08 and GE-124. Numerous radiocarbon dates obtained from cores GE-124 and M13-08 provide us to compare multi-proxies to infer different paleoclimatic periods that are assigned to various lithologic layers in cores. While there was no connection between the Sea of Marmara (SoM) and the Gulf of Gemlik due to existence of a sill at -65 m, the gulf might be regarded as closed inland lake that was feed by only surrounding catchment area, controlling the water budget prior to the Holocene. Such a phenomenan implies that the water level fluctuations in the gulf can not be affected by sealevel changes in the SoM until the inundation by the Marmara waters. The specific ıssues in this study to document an exact timing for connection with the salty waters provided by the SoM inflow, and to evaluate water level changes when the gulf prevailed as isolated lake during the Late Pleistocene. This study is the first attemp to document and discuss the Late Pleistocene to Holocene waterlevel and climatic changes based on the core and seismic data together with precise chronology of the sediment sequence. The pre-Holocene sequence of the Gulf of Gemlik can be subdivided into four different depositional units, labelled as units-S3 to -S2, each of them corresponds to various water levels and depositional facies. In the seismic data, the minimum depth of reflector surface BS-3 indicates that the water level was well below -75 m, implying a 10 m water level decrease comperative to the initial stage of the LGM period. This finding suggests that the paleoclimate around the Gulf during the LGM was similarly cold and dry, giving rise to negative water balance in the Gemlik Lake. Similar global climatic deterioration during the LGM has been documented in the SoM, when the water level decreased to below -110 m. The onset of the Oldest Dryas was associated with generation of dense turbidity currents along the slopes that gave rise to formation of erosional gullies at 18 ka cal BP. During the OD, deposition of Unit-S3 as infilling of incised-gullies in the basin can be attributed to a transgressive water level in the gulf between 18 ka cal BP and 15.5 ka cal BP. While the minimum water depth of reflector surface BS-4 covering the top of the unit is observed above -50 m in the seismics, the water level might be increased due to high river discharges into the the lake as a result of wetter paleoclimate during the OD. The Bolling-Allerod (B-A) and the Younger Dryas (YD) paleoclimatic periods are represented by Unit-S2 in the seismic data that can be recognized by prograded clinoforms on the shelf margin. Such seismic architecture of this unit implies forced water level decrease in the gulf due to relatively dry paleoclimate, particularly during the YD. According to seismic to core comparision, Unit-S2 is penetrated at the lower part of core M13-08 that is defined here as Unit-L2. Magnetic succeptibility analysis performed on the core reveals that the B-A is associated with high magnetic succeptibility value, therefore, implies high detrital input due to relatively wetter climate, whereas its value gradually decreases within the YD part of the core. This indicates that the wetter climate during the B-A period was turned to a dry and cold climate during the YD. Numereous lake records from the western and central Anatolia, the YD period has been documented by dry and cold climate that gave rise to an abrupt lake level decrease until the Holocene. While our geochemical proxies in core M13-08 only cover the upper part of the YD, the termination of this cold period seems to be associated by high soil erosion due to exposed shelves during the lowstand of lake level. High Fe/Mn ratio implies oxic deep water condition that is consistent of low TOC content, suggesting a low organic productivity at this time. The formation of berm and wave-cut platforms along the main unconformity surface (SB-5) indicates an existence of paleoshoreline at -65 m prior to the Holocene. This also confirms that the water level decrease was modulated by stillstand untill the lake to marine transition that has been dated at 12 ka cal BP by core M13-08. According to global sealevel curve, the sealevel was around -63 m during the marine connection of the gulf, therefore, the bedrock sill might be much below this thereshold. It can explain existence of stillstand lake level that gave rise to well-formed berm prior to the inundation of the gulf by the SoM water. Transition from lake to marine condition in the Gulf of Gemlik is represented by appearence of eurlyhaline molluscs in Unit-L1 in core M13-08. The similar evidences of marine incursion into the gulf can be documented in our multi-proxies in the core by a gradual lowering in magnetic succeptibility that indicates decreasing detrital input due to flooding of the exposed shelves by marine transgression at 12 ka cal BP. However, a gradual decrease in Ca/Sr ratio in the lower part of Unit-L1 until ca. 11 ka cal BP supports the elevated salinity in the gulf as a result of marine incursion. Deepening of the gulf water level gave rise to sub-oxic to anoxic deep water condition and concominent of water stratification that is well consistent with gradual increase in TOC content within the same part of the core. During the Holocene, further deepening in the gulf promoted water stratification with an anoxic deep water condition and organic productivity, therefore, resulted in formation of lower sapropel layer in the gulf. This special layer has been previously defined and dated between 10.8 ka cal BP and 6.6 ka cal BP, and can be infered in core M13-08 by a high TOC content. The most prominent change in our multi-proxies occurs at ca. 8.2 ka cal BP that can be documented by an abrupt decrease in Fe/Mn and increase in Ca/Sr, implying existence of an oxic deep water condition together with rising sea surface temperature due to global warming around the middle-Holocene. The late Holocene is represented in the core by formation of upper sapropel between 5.4 ka cal BP and 2.7 ka cal BP. In contrast to lower sapropel, relatively low K and Ti contents and Ca/Sr ratio can be assigned to drying climate that gave rise to cessation of detritic input from the catchment area around the gulf. It is well known that middle to late Holocene transition is marked by general drying around the most of Anatolian lakes (refere et). At about 2.7 ka cal BP, remarkable increases in K and Ti contents show high clastic inputs from the catchment areas around the gulf due to existence of the wetter climate. During this milder climatic period, deep water presumably became an oxic on the basis of very low Fe/Mn ratio, since high river discharges around the gulf generate turbidity currents along the bottom due to hyperpycnal mixing that promote oxygenated deep water conditions.Yüksek LisansM.Sc