High frequency sea level fluctuations recorded in the Black Sea since the LGM

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Abstract

This paper presents a 3D geometric interpretation of very high resolution seismic Chirp profiles acquired on the Romanian shelf during ASSEMBLAGE European Project. The results provide a solid record of the Black Sea Last Glacial Maximum (LGM) water level fluctuations. This pseudo-3D seismic interpretation shows that the Black Sea lacustrine shelf deposits form a significant basinward-prograding wedge system. On top of these prograding sequences is a set of sand dunes that delineates a wave cut-terrace like feature around the isobath − 100 m. Landward of this dune field are small depressions containing barkhan-like bodies. The upper part of the last prograding sequence is incised by anastomosed channels which end in the Danube (Viteaz) canyon which are also built on the lacustrine prograding wedge. Ten stratigraphic units were distinguished and correlated with analyses of cores retrieved from this area. The results demonstrate that the first eight sequences represent lacustrine prograding wedges, the ninth sequence is the dune system itself and the tenth is a marine mud drape which covers the entire continental shelf. The lacustrine prograding wedges represent a lowstand deposit characterised by forced regression-like reflectors mapped from the pseudo-3D seismic data. Their hinge point corresponds to the wave erosion surface mapped around − 100 m isobath on the multibeam mosaic. Dated cores give age control on this lowstand period, which lasted from 11 to 8.5 kyr 14C BP as implied by: (1) the continuously dry climatic conditions in the region inferred by high percentages of herbs and steppe elements determined from ASSEMBLAGE cores and, (2) the formation of dunes between 10 and 8.5 kyr 14C BP on the desiccated north-western Black Sea shelf at − 100 m and (3) all of these covered by a marine mud drape confirming that the dune system is no longer active. The buried, anastomosed fluvial channels that suddenly disappear below 90 m depth, and a unique wave-cut terrace between 95 and 100 mbsl on the outer shelf are also consistent with a major lowstand base-level at around − 100 m water depth. Preservation of sand dunes and the occurrence of small, buried incised valleys mark a rapid transgression within less than a century during which ravinement processes related to the water level rise had no time to significantly erode the seafloor.

Introduction

As part of its evolution, the Paratethys basin went through a small, semi-isolated basin phase at the end of the Oligocene. The link between resulting marginal basins and the ocean was episodically restricted or even severed in response to sea level changes. The Black Sea is a present-day example of such a marginal basin where the connection changes dramatically with sea level (Ross et al., 1970, Ross, 1971, Ross and Degens, 1974, Ross, 1978, Ryan, 1997, Ryan et al., 2003). The Black Sea is at present the world's largest anoxic basin, making it an important modern analogue for past anoxic conditions, while during the last glacial period, it was a low salinity oxygenated lake, isolated from the Mediterranean (Deuser, 1972, Deuser, 1974, Wall and Dale, 1974, Lericolais, 2006). The location of this inland sea, between Europe and Asia, makes its water level dependent on Eurasian climatic fluctuations. As the Black Sea has experienced a limnic situation, the deposited lake sediments are valuable archives for the study of past climate changes. During glacial periods, the ice cap prevented major rivers flowing north as they do today. Then, these rivers were diverted to the south in the direction of the Black Sea and Caspian Sea receiving basins and consequently have increased the size of these Drainage Basins (Arkhipov et al., 1995). Isolation of the Black Sea from the Global Ocean led to the establishment of unique conditions specific to the Black Sea, and avoiding the hysteresis effect which corresponds to the latent period needed by the Global Ocean to respond to the consequences of ice melting. During these isolation phases, the Black Sea was more sensitive to climate changes than the Caspian Sea is today. Some authors interpret the Caspian Sea fluctuations as opposed to those of the Global Ocean to have been caused the possible connection between the Black Sea and the Caspian Sea through the Manych Strait (Chepalyga, 1984, Arkhipov et al., 1995). During the glacial periods, when the Black Sea was isolated, both the lack of saltwater input and the increase of fresh water run-off from the rivers, led to reduced salinity levels in the Black Sea. This process, linked to water level fluctuation, is measured in the fauna succession showing an abrupt change from salt-to fresh/brackish-water species. However, since 1997, a still unsolved controversy concerned the last re-connection of the Black Sea to the Marmara Sea. This question is still under debate, even though each opposing view is supported with data representing only a minute part of the Black Sea floor. Yet, not all of the 420 000 km2 have been surveyed using modern scientific equipment and technology to precisely understand the water level fluctuation of the Black Sea in the Quaternary. Recently, an assessment of the North-western part of the Black Sea sedimentary systems from the continental shelf and slope down to the deep-sea zone, was provided by the ASSEMBLAGE European Project (EVK3-CT-2002-00090). Part of this project's deliverables consisted of applying sequence stratigraphic models to seismic data recorded on the north-western Black Sea shelf in order to correlate the sequences interpreted using seismic stratigraphy methods to sea-level fluctuations. In this paper, seismic sequences recognized on seismic profiles are interpreted in terms of systems tracts, by using the character of the erosional unconformity and of the strata truncated below it as well as the nature of the sediments above it.

To achieve these objectives, very high resolution seismic data were acquired during the BlaSON cruises (1998 and 2002) on board the research vessel “LE SUROÎT” and during the ASSEMBLAGE 2 (2004) cruise on board the research vessel “LE MARION DUFRESNE”. During the early cruises paleo-shorelines and sand ridges were identified and a set of seismic data was acquired on these targets to support pseudo 3D analyses. Digital Terrain Models (DTM) were made to determine the paleo-bathymetry and to establish a sequence stratigraphy model to identify age and onset of observed paleo-shorelines and ridges. Digital Terrain Models also help to define the internal structure of these sand ridges and to decipher the nature and origin of paleo-valley fills. This coupled with a multiproxy approach emphasizes that the Black Sea water level is dependent on Eurasian climatic fluctuations. This sequence stratigraphy study is validated by dated samples obtained from long cores (up to 50 m long) providing a firm calibration of Black Sea water level fluctuation since the LGM. It especially shows that the Black Sea experienced a contemporary rise in water level with the melting of the Fennoscandian Ice sheet followed by a drop of the water level from the Younger Dryas to the PreBoreal. This recent lowstand is confirmed by the presence of the forced regression sequences, the wave cut terrace and the coastal dunes still preserved on the shelf, even after the Black Sea was rapidly invaded by Mediterranean/Marmara marine waters.

Section snippets

Black Sea Paleo-shorelines

Since the 1970's Russian scientists have documented Pleistocene lowstand shorelines on the north-western Black Sea shelf (Kuprin et al., 1974, Shcherbakov, 1978). Actually, many Soviet surveys were carried out in the Black Sea, and paleo-littoral zone near the shelf edge was identified and sampled from coring. Although these researchers had not published reflection profiles to document the exposed margin of the lake, their numerous piston and drill cores confirmed the ancient coast. Some of

Pseudo 3D methodology

ASSEMBLAGE very-high-resolution seismic data was acquired with sub-meter resolution and penetration of a few meters to several tens of meters. Representative areas were selected to apply specific seismic data processing in order to build digital terrain models (DTM), to determine the paleo-bathymetry and to establish a sequence stratigraphic framework. Pseudo 3D seismic interpretation has allowed us to identify paleo-shorelines, prograding features, and indicators of water level fluctuation.

Localization of the selected area

The area of interest was chosen from the area where a high density of seismic profiles was acquired to realize the DTM and where a bathymetric study is available (Lericolais et al., 2007a) (Fig. 2). In the north-eastern part of this mosaic, linear ridges 4 to 5 m in relief and with an average spacing of 250 m are prominent. They strike almost uniformly obliquely to a berm-like step along a north–south axis of the mosaic. In addition, depressions with diameter from 100 to 500 m and negative

Discussion

The analysis of the very high-resolution seismic data in pseudo3D mode demonstrates that the last climate change known during Holocene times was well recorded in the Black Sea basin. A particularly good record is the water level fluctuations of the Black Sea itself.

The 3D geometric interpretation of Chirp profiles acquired on the Romanian shelf shows that the lacustrine shelf deposits form an important basinward-prograding wedge system. On top of these prograding units is a set of sand dunes

Conclusions

The transition of the Black Sea system from a lacustrine to a marine environment is perhaps one of the best records of climate change on the European continent. During the Last Glacial Maximum, 21,000 yr ago, the Black Sea was probably a giant freshwater lake as proposed by (Arkhangelskiy and Strakhov, 1938) or at least a brackish enclosed basin. Its water level stood more than 120 m below than today's level. During the ASSEMBLAGE (EVK3-CT-2002-00090) European project, analysis of chirp

Acknowledgments

Our research was supported by the French ministry of foreign affairs within the frame of a bi-lateral collaboration between France and Romania, and prolonged by a European project of the 5th framework program called ASSEMBLAGE (EVK3-CT-2002-00090). Special acknowledgement goes to Nicolae Panin who started and supported the project. We also thank Eliane Le Drezen, Alain Normand for their work in multibeam data processing, Hervé Nouzé for seismic processing, the crew of the vessels “Le Suroît”

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