Gas and hydrate occurences in the western Black Sea

Here we present the preliminary results of the ongoing project ¸SAssessment of the gas-bearing sediments in the north-western Black Sea?T supported through a EC Marie Curie Fellowship. The project aims to integrate information from geophysical and geomorphological data in order to define a general characterization of the gas accumulations in the recent sediments of the Black Sea. Investigation is based mainly on high-resolution seismic reflection data and swath bathymetry acquired during the BlaSON and BlaSON 2 surveys by IFREMER (France) in cooperation with GeoEcomar (Romania). Sub-bottom profiles (3,5 KHz) acquired by GeoEcoMar have been also used, as well as seismic and bathymetric data from the CRIMEA EC project.The western margin of the Black Sea contains evidence of abundant subsurface gas accumulations that appear in a complete suite of settings from the coastal zone to the deep basin. Considering the variation of their features from one setting to another, we defined several distinct gas provinces:- the coastal zone in front of the Danube Delta and along the Bulgarian shore is marked by the presence of a shallow gas accumulation that causes a widespread acoustic mask up to 1-3 m depth below the seafloor.- on the shelf the typical gas facies is represented by acoustic columns from a deep source (below the penetration limit of our profiles), either reaching the seafloor or being buried under the upper shelf deposits.- the shelf-edge zone (including the external shelf and the upper slope down to about 750 m water depth) shows significant gas discharge and numerous gas seeps have already been mapped. Gas escape zones are commonly associated with subsurface acoustic turbidity and acoustic columns.- on the slope below 750 m depth, bottom simulating reflectors (BSR) were detected and indicate the probable presence of gas hydrates. A remarkable BSR pattern consisting of multiple BSR-type reflectors occurs in the Danube fan.Zones of significant gas accumulation (especially the coastal zone and the shelf edge) seem to be associated with high sedimentation rates, which may indicate a possible biogenic origin for this gas. Nevertheless, there is evidence that deep thermogenic gas sources could have contributed as well

Terrestrial implications for the maritime geoarchaeological resource: A view from the Lower Palaeolithic

Stone tools and faunal remains have been recovered from the English Channel and the North Sea through trawling, dredging for aggregates, channel clearance, and coring. These finds highlight the potential for a maritime Lower Palaeolithic archaeological resource. It is proposed here that any Lower Palaeolithic artefacts, faunal remains, and sediments deposited in the maritime zone during dry, low-stand phases were once (and may still be) contextually similar to their counterparts in the terrestrial Lower Palaeolithic records of north-western Europe. Given these similarities, can interpretive models and analytical frameworks developed for terrestrial archaeology be profitably applied to an assessment of the potential value of any maritime resource? The terrestrial geoarchaeological resource for the Lower Palaeolithic is dominated by artefacts and ecofacts that have been fluvially reworked. The spatio-temporal resolution of these data varies from entire river valleys and marine isotope stages to river channel gravel bar surfaces and decadal timescales, thus supporting a variety of questions and approaches. However, the structure of the terrestrial resource also highlights two fundamental limitations in current maritime knowledge that can restrict the application of terrestrial approaches to any potential maritime resource: (i) how have the repetitive transgressions and regressions of the Middle and Late Pleistocene modified the terrace landforms and sediments associated with the river systems of the English Channel and southern North Sea basins?; and (ii) do the surviving submerged terrace landforms and fluvial sedimentary deposits support robust geochronological models, as is the case with the classical terrestrial terrace sequences? This paper highlights potential approaches to these questions, and concludes that the fluvial palaeogeography, Pleistocene fossils, and potential Lower Palaeolithic artefacts of the maritime geoarchaeological resource can be profitably investigated in future as derived, low-resolution data sets, facilitating questions of colonisation, occupation, demography, and material culture

A revised calendar age for the last reconnection of the Black Sea to the global ocean

During the last decade, a debate arose regarding the timing and pattern of the last reconnection of the Black Sea "Lake" with the global ocean. On a geochemical basis, the radiocarbon age of Black Sea "Lake" surface water, during the time of reconnection, was determined to be similar to 8400 C-14 a. BP. Despite the potential bias induced by the hard water effect in lakes, the reconnection age was calibrated without any reservoir age correction, which led to an estimate of similar to 9400 cal a. BR Since knowledge for the actual calendar age has important hydrologic implications that would provide new insights regarding the mechanism of reconnection, determining the actual calendar age for the last reconnection remains important. Based upon modeling experiments and micropaleontological reconstructions, the Black Sea "Lake" reconnection occurred in two steps, as follows: 1) Initial Marine Inflow (IMI) followed by 2) a period of increasing basin salinity that led to the Disappearance of Lacustrine Species (DLS). In order to better define the actual calendar age for the last reconnection, a review of the sedimentary expressions of the IMI and DLS boundaries was performed in order to correlate them throughout the Black Sea sedimentary environments. This correlation reconciles the apparent inconsistency in the published radiocarbon dataset, and provides the atmospheric radiocarbon age of the last reconnection, which represents the reference for reservoir age calculations and which can be directly calibrated. We determine reservoir ages for the water column, as well as the reconnection calendar age to be similar to 9000 cal a. BP. At the reconnection with the global ocean, Black Sea "Lake" reservoir ages were non-negligible and water-depth-dependent, consistent with a weak water column stratification during the Early Holocene. The calibrated age of Initial Marine Inflow (9000 cal a. BP) implies that the former Bosphorus sill was shallower by similar to 10 m than is commonly assumed in the literature. Compared to the sedimentary context of the Sakarya coastal plain, this result suggests that the level of the isolated Black Sea was below the former Bosphorus sill depth at the time of the last reconnection. Furthermore, a lag of 900 yr between Initial Marine Inflow and the Disappearance of Lacustrine Species indicates that approximately ten centuries were needed to establish the currently observed two-way flow exchange with the global ocean. (C) 2011 Elsevier Ltd. All rights reserved

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

International audienceThis 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