Marmara Denizi'nde son buzul döneminden günümüze deniz seviyesi değişimleri

High-resolution reflection seismic profiles and core analyses of sedimentary sequence near the Sea of Marmara (SoM) entrance to the Strait of İstanbul (SoI, Bosphorus) provide detailed record of sealevel changes since the Late Glacial Maximum (LGM). The sequence is deposited over the LGM erosional surface at a maximum depth of -105 m. It includes seven seismic units that can be confidently correlated with 14C-dated (uncalibrated) sedimentary units in the cores. Unit-S1f consists of conformable beds constituting a ridge-like future in the middle of the main channel and dipping in opposite directions. We interpret this unit as a levee deposited in the main paleo-strait’s channel. The lowermost 175 cm part of Core MD-2750 represents the upper part of the levee deposit (Unit-S1f), which comprises silty and sandy beds with marine molluscan shells. This unit is differentiated from the overlaying other units by its relatively coarse-grained nature. Unit-S1f represents levee deposits of the paleo-SoI channel that was deposited by a Black Sea outflow during 12000-10 600 yr BP. The levee unit is overlaid by Unit-S1e over ersoional surface E, which corresponds to the onset of the early Holocene Sapropel (Çağatay et al., 2000) as indicated by high Corg values (>1.5 wt%) and its stratigraphic position in Core MD-2750. This sapropelic layer was previously dated 10 600-6400 yr BP by Çağatay et al. (2000). Its onset age is consistent with the 14C ages of 10 950 yr BP and 10 450 yr BP obtained below and above truncation surface E, therefore, age of the reflector E can be estimated as 10 600 yr BP. The reflection surfaces at the base of Unit-S1d and S1c correspond to wave-cut terraces at -71 and -63 m, respectively, and are estimated to be 10 600 and 9800 yr BP. The seismic Unit-S1b comprises deltaic sediments that were deposited during 6400-3200 yr BP. Isopach map and forset directions of the deltaic unit indicate that the delta was sourced from Kurbağalıdere River. Contrary to the interpretations of Hiscott et al. (2002) and Gökaşan et al. (2005) that this deposit formed during the course of a rapid sealevel rise, our dating places its growth after sealevel had nearly reached its modern level about 6500 yr BP. As mapped out in plan view the outline of the topsets and foresets of Unit-S1b coincides well with the same deposit mapped by Gökaşan et al. (2005). We propose the following sequence of events leading to the delta formation. According to global sealevel curves (e.g. Fairbanks, 1989), the rate of sealevel rise decreased after 6500 yr BP. This decrease combined with an increase in sediment supply during mid-Holocene climatic optimum resulted in the deltaic progradational sedimentation on the shelf of southern enterance of SoM to SoI. This conclusion is in agreement with the fact that the mid Holocene is a well known period for widespread delta development (Stanley and Hait, 2000). The change from the progradational stage to the aggradational stage of delta development at 4700 yr BP is marked by a relatively rapid rise of sealevel as indicated by climbing delta foreset-topset transition. The onset of aggradational period also coincides with establishment of the present dual-flow regime (Çağatay et al., 2000) with the Mediterranean under-current in the SoI, forming a northward sediment drift. The delta deposition was terminated when increased base level due to further global sealevel rise combined with a probable decrease in sediment input by the Kurbağalıdere River at 3200 yr BP. This date is supported by the correlation with the age of the late Holocene Sapropel layer and by a 14C age of 4400 yr BP in Core TSU03-13 below the eroded uppermost topsett bed. The Kurbagalıdere River, with a present day annual water and sediment discharges of 80 m³ and 60 000 tons (EİE, 1993) draining a 180 m high ground southeast of the SoI, is large enough to form such a delta having about 290 x 106 tons of sediment. In this study, we present detailed seismic stratigraphic analysis of high resolution shallow seismic lines crossing an area near the southern entrance of SoI, together with the core stratigraphy. Our main objectives are to discuss the chronostratigraphic evolution of the Holocene sediments including delta formation in the paleo-SoI channel, and shed light on the water exchange problem between the Marmara and the Black Seas during the Holocene (last 12 000 years).  Keywords: Marmara Sea, core stratigraphy, seismic stratigraphy, sealevel changes, delta, sapropel. Marmara Denizi İstanbul Boğazı girişinde yeralan sismik profil ve karotlarda yapılan ayrıntılı stratigrafik ve kronolojik çalışmalar Son Buzul Maksimum (Late Glacial Maximum) döneminden günümüze deniz seviyesi değişimlerini ortaya çıkarmaya yardımcı olmuştur. Bu dönem süresince Marmara Denizi’nde çökelen birimler -105 m su derinliğine kadar yüzeyleyen uyumsuz yüzey üzerinde çökelmiştir. Sismik profillerde yapılan stratigrafik çalışmalarda 7 farklı sismik birim ayırtlanmış ve bu birimleri ayıran 7 sismik yansıma yüzeyi belirlenmiştir. Sismik profillerde ayırtlanan birimlerin yaşları ve fasiyesleri karotlardan alınan 14C yaşları ile saptanmıştır. Sismik profillerde ayırtlanan en yaşlı çökel birimi Birim-S2 olarak tanımlanmıştır. Bu birim Holosen (G.Ö. 12 bin yıl öncesi) öncesinde Marmara Denizi’nde başlayan transgresyon sonucunda vadi-dolgusu fasiyesinde gelişmiştir. Bu transgresyon ile göl seviyesi yükselerek -81 m’ye ulaştığı Birim-S2’yi üzerleyen sismik yansma yüzeyi üzerinde gelişen taraça düzlüklerinin varlığından anlaşılmaktadır. Birim-S2’yi uyumsuzlukla üzerleyen Birim-S1f kanal-bank kenarı çökelleri olarak yorumlanmıştır. Holosen içerisinde Karadeniz’den Marmara Denizi’ne G.Ö. 12 000-10 600 yılları arasında su akışı sonucu İstanbul Boğazı paleo-kanalı içerisinde bu çökeller oluşmuştur. Ayırtlanan Birim-S1d ve S1c’nin tabanlarını oluşturan sismik yansıma yüzeyleri -71 m ve -63 m’lerde gelişen kıyı aşındırma düzlüklerini temsil etmektedir. Holosen başı olan G.Ö. 12 bin yılında deniz seviyesinin yükselmeye başlamasının ardından bu durağan deniz seviyesi dönemleri G.Ö. 10 600 ve 9800 yıllarında gelişmiştir. Sismik birimlerden Birim-S1f’in eğimli klinoform tabakalar içermesi bu birimin delta çökeli olduğunu göstermektedir. Delta çökellerinde bu çalışma kapsamında yapılan sismik stratigrafik ve kronolojik çalışmalara göre G.Ö. 6400 ile 3200 yılları arasında çökeldiği tespit edilmiştir. Bu çökellere ait yapılan kalınlık haritası ve foresetlerin eğim yönleri bu deltanın Kurbağalıdere Nehri’nden geliştiğini göstermektedir. Anahtar Kelimeler: Marmara Denizi, karot stratigrafisi, sismik stratigrafi, deniz seviyesi, delta, sapropel

High-resolution palaeomagnetic record from Sea of Marmara sediments for the last 70 ka

Magnetostratigraphic and geochemical analyses were performed on two sediment cores recovered from the Sea of Marmara to investigate geomagnetic field variations over the last 70 ka. A chronology for each of the two cores was developed from eight AMS 14C datings, tephrochronology, and tuning of Ca concentrations with stadials and interstadials observed in Greenland ice core oxygen isotope data. Based on the age models, cores MD01–2430 and MRS-CS19 reach back to 70 ka and 32 ka, respectively. High average sedimentation rates of 43 cm/kyr for core MD01–2430 and 68 cm/kyr for core MRS-CS19 allow high-resolution reconstruction of geomagnetic field variations for the Sea of Marmara. Mineral magnetic properties are sensitive to glacioeustatic sea-level changes and palaeoclimate variations in this region, reflecting the variable palaeoenvironmental conditions of the Sea of Marmara during last 70 ka. Despite the impairment of the palaeomagnetic record in some stratigraphic intervals due to early diagenesis, relative palaeointensity variations in the Sea of Marmara sediments correlate well with similar records derived from other regions, such as the nearby Black Sea and the GLOPIS-75 stack. The directional record derived from the Sea of Marmara cores exhibits typical palaeosecular variation patterns, with directional anomalies at 41 ka and 18 ka, representing the Laschamps and postulated Hilina Pali excursions, respectively. Both directional anomalies are also associated with palaeointensity minima. A further palaeointensity minimum at 34.5 ka is likely related to the Mono Lake excursion, with no directional deviation documented in the Sea of Marmara palaeomagnetic record so far

Defining the Upper Nisyros Pumice (57.1 ± 1.5 ka) as new tephra isochrone for linking early MIS-3 palaeoenvironmental records in the Aegean-Black Sea gateway: New evidence from the Sea of Marmara

International audienceThe rhyolitic Upper Nisyros Pumice (UNP) from the Kos-Yali-Nisyros volcanic system has been detected as a cryptotephra layer in lacustrine sediments from the Sea of Marmara (SoM). A new independent age of the UNP eruption at 57.1 ± 1.5 cal ka BP has been interpolated using a combination of radiocarbon dating, tephrochronology and wiggle-matching of the SoM proxy record (Ca-curves) with Greenland oxygen isotope data, therewith confirming recently published radioisotopic dates of UNP land deposits. The UNP tephra in the SoM was identified by comparisons of the SoM tephra glass chemical dataset with published data of other marine tephra records from the Aegean Sea and the Megali Limni lacustrine sediment sequence (Lesvos Island). The stratigraphic position of the UNP tephra in these records verified its deposition in the SoM at the onset of MIS-3 and specifically at the termination of Greenland Interstadial GI-16. The new findings define the UNP tephra as a valuable time marker for the synchronisation of palaeoenvironmental data for this time period and help spurring the establishment of a robust tephrostratigraphical framework for the last ~70 kyr in the Aegean-Black Sea region

Sedimentological and Geochemical evidence for Seismoturbidite Generation in the Kumburgaz Basin, Sea of Marmara: Implications for Earthquake Recurrence along the Central High Segment of the North Anatolian Fault

International audience9 Holocene earthquake history of the Central High Segment of the North Anatolian Fault is 10 examined here for the first time based on analysis of seismoturbidites within a 21-m-long piston 11 core recovered from the Kumburgaz Basin in the Sea of Marmara. The visual lithological 12 description combined with detailed grainsize analyses indicate that the deep basin hemipelagic 13 sediments are interrupted by 28 turbidite units during the last 6.1 cal kyrs BP. The turbidites 14 show strong segregation and a sharp boundary between a coarse basal part and overlying 15 homogenite as inferred from detailed sedimentological and geochemical data. Several 16 amalgamated turbidites are recognized by repeated fining upward sequences with no 17 intervening homogenite indicating multiple episodes of traction and deposition as a result of 18 various slope failures and turbidity currents. Each unit was possibly triggered by the same 19 earthquake event rupturing in the Sea of Marmara. The most common sedimentary feature is 20 the continuous parallel lamination that was presumably introduced by long lasting water 21 oscillations on suspended sediments due to the seiche effect. The establishment of geochemical 22 criteria and exclusive sedimentary processes distinguish earthquake triggered turbidites 23 (seismoturbidites) from other trigger factors. Moreover, such distinction allows us to evaluate 24 2 hydrodynamic sedimentary conditions and processes in the Kumburgaz Basin. The base of most 25 seismoturbidites are associated with a sharp increase in Mn concentration that can be explained 26 by a diagenetic enrichment of Mn at the oxic/anoxic interface of the sediments near the seafloor 27 prior to the deposition of the turbidite. An age-depth model of the studied core based on seven 28 AMS 14 C ages allows precise correlation between historical earthquakes and seismoturbidites 29 in the Kumburgaz Basin. At least the latest nine of them fit well with the previously recorded 30 major earthquake events between ca. ~500 cal yrs BP and 2.5 cal kyrs BP. 3

New records of vegetation and climate changes in the Sea of Marmara during the Marine Isotope Stages 3, 4 and 5 (a-c)

A giant piston core MRS-CS27 sedimentary sequence in the Sea of Marmara (SoM) was investigated by palynological analysis to evaluate the paleoenvironmental and climate changes, corresponding to the Marine Isotope Stages MIS-5 (c-a), MIS-4, MIS-3, and the earliest part of the MIS-2. During MIS-5c, the palynological analysis indicates a warm and humid climate in the SoM. The pollen assemblage is characterized by the presence of broad-leaved deciduous woody taxa, with a predominance of deciduous Quercus. Following MIS-5c, a relatively colder and drier climate is inferred during MIS-5b, as indicated by the herbaceous assemblages found in the sedimentary sequence. The presence of herbaceous plants in the pollen record suggests less favorable conditions for woody vegetation and a shift towards a cooler and drier climate compared to the previous phase. Within the MIS-5a interval, the palynological analysis reveals the presence of abundant mesophyllous woody taxa. This indicates a return to a warm and humid climate during this period, supporting the growth of diverse forest vegetation. Distinct changes in vegetation are observed during MIS-4, with the presence of abundant Cedrus pollen. This suggests the survival of montane trees in the region under a cooler climate compared to the previous stages. Relict plants such as Cathaya, Tsuga, and Carya are identified during MIS-3 and MIS-4. Their presence can be attributed to the reworking of older sediments during these periods, indicating that these plants were not contemporary with the sediment but were preserved from earlier time periods

Factors affecting thickness and frequency of turbidites triggered by earthquakes in Kumburgaz Basin, Sea of Marmara

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The Sea of Marmara during Marine Isotope Stages 5 and 6

Multi-proxy analyses and lithology of two cores, MRS-CS18 and MRS-CS27, from the İmralı Basin of the Sea of Marmara (SoM) provide novel information on environmental conditions, relative sea level, and sill depths of the straits of Bosporus and Dardanelles during the Marine Isotope Stages (MIS) 5 and 6. The fossil and multi-proxy geochemical records show that lacustrine conditions prevailed in the SoM during most of MIS 6, from 171 to 134 ka BP, and that the transition to marine conditions during Termination II took place at ∼134.06 ± 1.10 ka BP. MIS 5 interstadials a, c, and e witnessed the formation of three sapropels (MSAP-2, MSAP-3 and MSAP-4) under suboxic to anoxic marine conditions, whereas during stadials MIS 5b (∼94–86) and MIS 5d (∼112–105 ka BP), lacustrine and marine conditions with deposition of sediments having relatively low TOC contents (<2%) prevailed, respectively. Consideration of the global sea level, together with the timing of the marine reconnection of the SoM during Termination II and persistence of the marine conditions during MIS 5, except for MIS 5b and later part of MIS 5a, suggests that the Dardanelles sill depth was at ∼ -75 ± 5 m during the reconnection at Termination II and at −55 ± 5 m during most of MIS 5. On similar considerations of the Black Sea marine reconnections and disruptions during the MIS 5, a sill depth of −35 to −40 m (similar to the present day depth) is indicated for the Bosporus Strait. The SoM geochemical proxy records correlate well with the regional terrestrial and marine records and the NGRIP oxygen isotope record with its Stadial and Interstadial phases, showing the common effect of the North Atlantic climatic events triggered by the perturbations in the Atlantic Meridional Overturning Circulation. However, the amplitude of the oscillations recorded in the SoM during MIS 6 (Penultimate Glacial Period) is relatively small compared to the MIS 4 to MIS 2 (Last Glacial Period)

Pelagic diatoms communicate through synchronized beacon natural fluorescence signaling

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