A.E. Aksu

Oscillating Quaternary water levels of the Marmara Sea and vigorous outflow into the Aegean Sea from the Marmara Sea–Black Sea drainage corridor

Abstract Detailed interpretation of single-channel air-gun and deep-tow boomer profiles demonstrates that the Marmara Sea, Turkey, experienced small-amplitude (∼70 m) fluctuations in sea level during the later Quaternary, limited in magnitude by the sill depth of the Strait of Dardanelles. Moderate subsidence along the southern shelf and Quaternary glacio-eustatic sea-level variations created several stacked deltaic successions, separated by major shelf-crossing unconformities, which developed during the transitions from global glacial to interglacial periods. Near the Strait of Dardanelles, a series of sand-prone deposits are identified beneath an uppermost (Holocene) transparent mud drape. The sandy deposits thicken into mounds with the morphology and cross-sectional geometries of barrier islands, sand waves, and current-generated marine bars. All cross-stratification indicates unidirectional flow towards the Dardanelles prior to the deposition of the transparent drape which began ∼7000 years BP, in strong support of the notion that the Marmara Sea flowed westwards into the Aegean Sea through the Dardanelles at times of deglaciation in northern Europe. The global sea-level curve shows that, at ∼11,000 and ∼9500 years BP, sea level rose to the sill depths of the Straits of Dardanelles and Bosphorus, respectively. The effect from ∼11,000 to ∼9500 years BP was seawater incursion into the Marmara Sea, drowning and formation of algal-serpulid bioherms atop lowstand barrier islands, and transgression of shelves and lowstand deltas. At ∼9500 years BP, glacial meltwater temporarily stored in the Black Sea lake, developed into a vigorous southward flow toward the Aegean Sea, forming west-directed sandy bedforms in the western Marmara Sea and initiating deposition of sapropel S1 in the Aegean Sea. This strong outflow persisted until ∼7000 years BP, after which a mud drape began to accumulate in the Marmara Sea and euryhaline Mediterranean mollusks successfully migrated into a progressively more saline Black Sea where sapropel deposition began. Most eastern Mediterranean sapropels from S1 to S11 appear to correlate with periods of rising sea level and breaching, or near-breaching, of the Bosphorus sill. These events are believed to coincide with times of vigorous outflow of low-salinity (?fresh) surface waters transiting the Black Sea–Marmara Sea corridor, and ultimately derived from melting of northern European ice sheets.

Last glacial–Holocene paleoceanography of the Black Sea and Marmara Sea: stable isotopic, foraminiferal and coccolith evidence

Multi-proxy data and radiocarbon dates from several key cores from the Black Sea and Marmara Sea document a complex paleoceanographic history for the last V30 000 yr. The Marmara Sea was isolated from both the Black Sea and the Aegean Sea during glacial periods when global sea-level lowering subaerially exposed the shallow sills at the Straits of Bosphorus and Dardanelles (i.e. lake stage), and reconnected through both straits during interglacial periods, when rise of global sea level breached the shallow sills (i.e. gateway stage). Micropaleontological data show that during the ‘lake stage’ the surface-water masses in both the Marmara Sea and Black Sea became notably brackish; however, during the ‘gateway stages’ there was a low-salinity surface layer and normal marine water mass beneath. Two sapropel layers are identified in the Marmara Sea cores: sapropels M2 and M1 were deposited between V29.5 and 23.5 ka, and V10.5 and 6.0 ka, respectively. Micropaleontological and stable isotopic data show that the surface-water salinities were reduced considerably during the deposition of both sapropel layers M2 and M1, and calculation using planktonic foraminiferal transfer functions shows that sea-surface temperatures were notably lower during these intervals. The presence of fauna and flora with Black Sea affinities and the absence of Mediterranean fauna and flora in sapropels M1 and M2 strongly suggest that communication existed with the Black Sea during these times. A benthic foraminiferal oxygen index shows that the onset of suboxic conditions in the Marmara Sea rapidly followed the establishment of fully marine conditions at V11^10.5 ka, and are attributed to Black Sea outflow into the Marmara Sea since 10.5 ka. These suboxic conditions have persisted to the present. The data discussed in this paper are completely at odds with the ‘Flood Hypothesis’ of Ryan et al. (1997), and Ryan and Pitman (1999). Crown Copyright > 2002 Elsevier Science B.V. All rights reserved.

Seismic stratigraphy of Late Quaternary deposits from the southwestern Black Sea shelf: evidence for non-catastrophic variations in sea-level during the last ∼10 000 yr

Abstract Detailed interpretation of single channel seismic reflection and Huntec deep-tow boomer and sparker profiles demonstrates that the southwestern Black Sea shelf formed by a protracted shelf-edge progradation since the Miocene–Pliocene. Five seismic–stratigraphic units are recognized. Unit 1 represents the last phase of the progradational history, and was deposited during the last glacial lowstand and Holocene. It is divided into four subunits: Subunit 1A is interpreted as a lowstand systems tract, 1B and 1C are interpreted as a transgressive systems tract, and Subunit 1D is interpreted as a highstand systems tract. The lowstand systems tract deposits consist of overlapping and seaward-prograding shelf-edge wedges deposited during the lowstand and the subsequent initial rise of sea level. These shelf-edge wedges are best developed along the westernmost and easternmost segments of the study area, off the mouths of rivers. The transgressive systems tract deposits consist of a set of shingled, shore-parallel, back-stepping parasequences, deposited during a phase of relatively rapid sea-level rise, and include a number of prograded sediment bodies (including barrier islands, beach deposits) and thin veneers of seismically transparent muds showing onlap onto the flanks of older sedimentary features. A number of radiocarbon dates from gravity cores show that the sedimentary architecture of Unit 1 contain a detailed sedimentary record for the post-glacial sea-level rise along the southwestern Black Sea shelf. These data do not support the catastrophic refilling of the Black Sea by waters from the Mediterranean Sea at 7.1 ka postulated by [Ryan, Pitman, Major, Shimkus, Maskalenko, Jones, Dimitrov, Gorur, Sakinc, Yuce, Mar. Geol. 138 (1997) 119–126], [Ryan, Pitman, Touchstone Book (1999) 319 pp.], and [Ballard, Coleman, Rosenberg, Mar. Geol. 170 (2000) 253–261].

Dinoflagellate cysts, freshwater algae and fungal spores as salinity indicators in Late Quaternary cores from Marmara and Black seas

Abstract Seismic profiles and mollusks have been used to suggest that from ∼12 500 to 7000 yr BP, the Black Sea was an isolated freshwater lake containing potable water and implying a surface salinity of 24, and that Spiniferites cruciformis, Spiniferites inaequalis, Peridinium ponticum, Polykrikos spp. and Quinquecuspis concreta characterize the lower salinity of the Marmara and/or Black seas. The core-top data and correlatable down-core assemblage changes in time-equivalent sapropelic and brown muds show that there is no evidence for differential aerobic decay of dinocysts in the study area. The main acritarchs are Sigmopollis psilatum, Concentricystes cf. C. rubinus and cf. Acritarch-8 of Traverse (1978), all of which are absent from the Aegean Sea and decrease in abundance with increasing salinity; the first two taxa have been reported previously as freshwater species. Fungal remains show a similar distribution pattern to the freshwater acritarchs, indicating their origin from terrestrial environments. Freshwater Chlorococcales are almost confined to the Black Sea but they have rare occurrences in the Aegean, indicating long-distance transport. Microforaminiferal linings are abundant in the Marmara Sea but are absent in deep water of the Black Sea. In the Marmara Sea, mid–late Holocene assemblages (

Pollen stratigraphy of Late Quaternary cores from Marmara Sea: land^sea correlation and paleoclimatic history

Marine pollen analysis is an important tool for paleoclimatic reconstruction of regions like the eastern Mediterranean and Near East where few onshore sites provide long pollen records. First, a compilation was made of core-top data from the eastern Mediterranean to BlackSeas to map regional variations in modern marine pollenspore concentrations. These data show a strong linkbetween pollen-spore concentrations and sea surface salinity, with minima of 5 grains/g in the southern Mediterranean and maxima of 160 000 grains/g in the BlackSea. Despite under-representation of some forest and herbaceous vegetation indicators, e.g. Fagus and Pistacia, variations in species composition of the marine assemblages correspond closely with the distribution of regional vegetation zones and can be used as proxies for spatial differences in seasonal and total temperature and rainfall. Pollen-spore assemblages in five cores from the Marmara Sea with multiple ages ranging from 33 550 to 1990 yr BP and one Holocene core from southeastern BlackSea were used to compile a Late Quaternary marine pollen stratigraphy. These data were then compared with eastern Mediterranean onshore reference sites in order to reconstruct a vegetation and paleoclimate history for the BlackSea^Aegean corridor from the Pleniglacial interval to present. Five marine pollen zones are recognized. PZ-5 (V33.6^24 ka) corresponds to the late Wu « rm Pleniglacial; PZ-4 (V24^ 13 ka) spans the Late Glacial Maximum (LGM); PZ-3 (V13^10.2 ka) includes the Allero « d^Younger Dryas glacial^ interglacial transition; PZ-2 (10.2^4 ka) marks the early Holocene interglacial warm, wet interval; and PZ-1 covers the latest Holocene colonization phase, which may also have been cooler and drier. Using the steppe^forest index of Traverse (1975), it is shown that the only intervals of severely dry conditions occurred briefly during the LGM and its transition; during most of the Pleniglacial and all of the Postglacial time, precipitation versus evaporation rates were sufficiently high to permit persistence of oro-Mediterranean forest vegetation. Furthermore, there is no evidence for environmental conditions in the BlackSea^Marmara region that would have encouraged pastoral or agricultural settlement in the littoral region prior to the Bronze Age, commencing 4600 years ago. Crown Copyright A 2002 Elsevier Science B.V. All rights reserved.

Deltas south of the Bosphorus Strait record persistent Black Sea outflow to the Marmara Sea since ∼10 ka

At the southern exit of the Bosphorus Strait in the northeastern Marmara Sea, high-resolution seismic profiles reveal two lobate, progradational delta lobes in modern water depths of V40^65 m. The younger delta was active from V10 to 9 ka based on radiocarbon dates of equivalent prodelta deposits and the elevation of its topset-to-foreset transition. The topset-to-foreset transition climbs in the seaward direction because the delta prograded into a rising sea. Low abundances of marine fauna and flora in the 10^9-ka interval support a deltaic interpretation. There are no rivers in the area that could have fed the delta; instead, all evidence points to the strait itself as the source of sediment and water. When this outflow was strongest (V10.6^6.0 ka), sapropels accumulated in basinal areas of both the Aegean and Marmara seas. Benthic foraminiferal and dinoflagellate cyst data from contemporary deposits elsewhere in the Marmara Sea point to the continual presence through the Holocene of a surface layer of brackish water that we ascribe to this same outflow from the Black Sea through the Bosphorus Strait. By V9.1^8.5 ka, two-layer flow developed in the Bosphorus Strait as global sea level continued to rise, and the sediment supply to the younger delta was cut off because the outflowing Black Sea water ceased to be in contact with the floor of the strait. The older delta lobe lies below a prominent lowstand unconformity and is tentatively interpreted to have formed from V29.5 to 23.5 ka (oxygen-isotopic stage 3) when the Marmara Sea stood at V355 m and a second sapropel accumulated in deep basinal areas. Crown Copyright ? 2002 Elsevier Science B.V. All rights reserved.

Late Glacial to Holocene benthic foraminifera in the Marmara Sea: implications for Black Sea^Mediterranean Sea connections following the last deglaciation

Benthic foraminifera were studied from four gravity cores that penetrated Holocene marine sediments in the Marmara Sea. Morphogroup and assemblage analyses reveal that the Holocene sea-level rise did not result in a catastrophic flooding event as proposed by W.B.F. Ryan and others, whereby well-oxygenated, saline Mediterranean waters rapidly inundated a low-lying low salinity ‘Black Sea Lake’ at V7.15 ka (popularly known as the ‘Noah’s Flood Hypothesis’). Rather, the benthic foraminiferal data confirm the hypothesis that the Dardanelles sill was breached by the Mediterranean at V12 ka, allowing saline waters to penetrate the Marmara Sea. These saline waters reached the level of the Bosphorus sill at V9.5 ka, but were unable to penetrate into the Black Sea until after V9.1 ka because of the persistent strong outflow of brackish to fresh water from the Black Sea. The initial colonisation of the Marmara Sea by benthic foraminifera is essentially synchronous with the re-establishment of marine connections through the Dardanelles Strait at V12 ka. By V10 ka, Ammonia-dominated faunas developed on the strait-exit delta (v1) at the southern end of the Bosphorus, and at V9.1 ka the appearance of fully marine species documents the establishment of a more stratified water column over v1. Finally, the increase in abundance of planktonic foraminifera at the southern exit of the Bosphorus after V6.1 ka reflects a decreased volume of outflow water from the Black Sea. Quantitative analysis of benthic foraminiferal morphogroups reveals that the oxygen content of subhalocline water was low (below V1.5 ml/l) throughout the Holocene, and the occurrence of sapropel sediment in the deeper part of the basin suggests bottom waters may have been anoxic at times. After V4.5 ka, an increase in benthic foraminiferal oxic morphotypes suggests a reduction in Black Sea outflow and weakening of the halocline. The strong and persistent stratification of the water column in the Marmara Sea throughout the Holocene is entirely incompatible with the ‘Noah’s Flood Hypothesis’. : 2002 Elsevier Science B.V. All rights reserved.

Paleoclimatic and paleoceanographic conditions leading to development of sapropel layer S1 in the Aegean Sea

Abstract Sapropel S1 occurs as 25–35 cm-thick black, weakly laminated muds in Aegean Sea cores. S1 was deposited between 9600 and 6400 yr B.P., during a period of isotopically depleted and relatively cool surface waters. Micro-faunal and -floral data indicate a major reduction in surface waters salinity during the deposition of S1, and oxygen isotopic data show a northerly fresh water source. Relatively light δ 13 C org and high pollen-spore concentrations in S1 suggest increased influx of terrestrial organic carbon, probably supplied by major rivers draining into the northern Aegean Sea. Benthic foraminifera indicate high-nutrient, low oxygen bottom waters for S1, and together with silt-sized hematite and manganese coatings suggest that during the deposition of S1 surface sediments were oxic. Visual and XRD evidence of pyrite in S1, together with enrichments in S, Cu, Zn, As, Ni, Cr and Fe suggest that subsurface conditions were sufficiently reducing for SO 4 2− reduction to occur, probably by diffusion from surface oxic into subsurface anoxic sediments. Palynomorphs in S1 show large increases in terrestrial pollen and spores, with the floral assemblage indicating significant influx from northern European rivers, and minor African components associated with increased summer monsoonal rain. Abundance of dinoflagellates and amorphogen suggests some increase in primary productivity in response to increased influx of humic compounds, however, there is no evidence of upwelling. The clay fraction in S1 shows notable decreases in smectite and kaolinite and reciprocal increases in illite and chlorite. The combined data suggested that the evolution of S1 in the Aegean Sea largely resulted from stagnation of the surface waters during the final disintegration of the continental ice sheets, rather than an increase in primary productivity and higher preservation of organic carbon on the sea floor.

The source and origin of the 1929 Grand Banks turbidity current inferred from sediment budgets

Side-scan data from the epicentral area of the 1929 Grand Banks earthquake and cores from the resulting turbidite are used to determine a sediment budget for the event. The 1929 turbidite has a volume of about 185 cubic km, which is mostly sand. Features indicating failure on the continental slope are observed only in muddy sediment. A major source of sand in the heads of the fan valleys of the Laurentian Fan is postulated to balance the sediment budget. This sand accumulated proglacially during the Wisconsinan glaciation and probably failed through liquefaction in the 1929 earthquake.

Late Pleistocene uplift history along the southwestern Marmara Sea determined from raised coastal deposits and global sea-level variations

Pleistocene raised coastal deposits characterized by locally abundant shells, aragonite-cemented beachrock and associated nearshore deposits border the western Marmara Sea at elevations of 0–50 m. Field observations confirm that these deposits formed during a series of transgressive and regressive events. U/Th dates in 16 in situ shells from four localities show that the peak of the transgressions occurred during the highstands of oxygen isotopic stages 7 and 5, between ∼53 and ∼210 ka. The elevations of these dated deposits can be used to quantify their post-depositional uplift and indicate that the entire western Marmara shelf, including the Strait of Canakkale (Dardanelles) has been rising at an average rate of ∼0.40 mm yr−1 since ∼225 ka. The primary cause of uplift is the local compression associated with a restraining bend in the western segment of the North Anatolian Fault. Paleogeographic maps constructed using the average rate of tectonic uplift and detailed topographic and bathymetric maps reveal that prior to glacial oxygen isotopic stage 8 the Marmara Sea was never isolated from the Aegean Sea, even when global sea level was low, because the floor of the Strait of Canakkale was too deep early in its uplift history. The dominance of Mytilus edulis in raised coastal terraces dating from the lowstand of glacial oxygen isotopic stage 8 suggests that the degree of communication between the Aegean Sea and Marmara Sea was comparable to that of the present. During the peak of glacial oxygen isotopic stage 6, the floor of the strait was subaerially exposed, isolating the Marmara Sea for the first time in the Pleistocene from the higher salinity Mediterranean water inflow and possibly causing it to become a blackish-water lake. During interglacial isotopic stages 9, 7 and 5, the Strait of Canakkale was very wide and deep (∼100–125 m), and there were two subsidiary channels (Bolayir and Eceabat channels), providing additional links between the Aegean Sea the Marmara Sea, further promoting significant water exchange between these basins. The dominance of Ostrea edulis in the raised terraces dating from isotopic stages 5 and 7 confirms an enhanced penetration of the Mediterranean water mass into the Marmara Sea. The history of communication between the Black Sea, fed by central and northern European drainage systems, and the low-latitude Mediterranean Sea is of fundamental importance in understanding the genesis of organic-rich sapropel deposits throughout the region. The results presented in this paper caution against the simple assumption that sea-level change alone controlled the degree of connection across this oceanographic gateway. Instead, the physiography and paleoceanography of the region were controlled both by variations in global sea level and the rate of uplift in an area of active transpression.