The geological and geodynamic evolution of the eastern Black Sea basin: insights from 2-D and 3-D tectonic modelling

School of Earth Sciences and Geography, University of Keele, Keele, Staffs, ST5 5BG, UK
Tectonophysics (Impact Factor: 2.87). 05/2002; DOI: 10.1016/S0040-1951(02)00121-X

ABSTRACT Subsidence mechanisms that may have controlled the evolution of the eastern Black Sea have been studied and simulated using a numerical model that integrates structural, thermal, isostatic and surface processes in both two- (2-D) and three-dimensions (3-D). The model enables the forward modelling of extensional basin evolution followed by deformation due to subsequent extensional and compressional events. Seismic data show that the eastern Black Sea has evolved via a sequence of interrelated tectonic events that began with early Tertiary rifting followed by several phases of compression, mainly confined to the edges of the basin. A large magnitude (approximately 12 km) of regional subsidence also occurred in the central basin throughout the Tertiary. Models that simulate the magnitude of observed fault controlled extension (β=1.13) do not reproduce the total depth of the basin. Similarly, the modelling of compressional deformation around the edges of the basin does little to enhance subsidence in the central basin. A modelling approach that quantifies lithosphere extension according to the amount of observed crustal thinning and thickening across the basin provides the closest match to overall subsidence. The modelling also shows that deep crustal and mantle–lithosphere processes can significantly influence the rate and magnitude of syn- to post-rift subsidence and shows that such mechanisms may have played an important role in forming the anomalously thin syn-rift and thick Miocene–Quaternary sequences observed in the basin. It is also suggested that extension of a 40–45 km thick pre-rift crust is required to generate the observed magnitude of total subsidence when considering a realistic bathymetry.

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    ABSTRACT: The last glacial–interglacial transition or Termination I (T I) is well documented in the Black Sea, whereas little is known about climate and environmental dynamics during the penultimate Termination (T II). Here we present a multi-proxy study based on a sediment core from the SE Black Sea covering the penultimate glacial and almost the entire Eemian interglacial ( ( 133.5 ± 0.7 ) – ( 122.5 ± 1.7 ) ka BP ). Proxies comprise ice-rafted debris (IRD), O and Sr isotopes as well as Sr/Ca, Mg/Ca, and U/Ca ratios of benthic ostracods, organic and inorganic sediment geochemistry, as well as TEX86 and UK′37 derived water temperatures. The ending penultimate glacial (MIS 6, 133.5 to 129.9 ± 0.7 ka BP ) is characterised by mean annual lake surface temperatures of about 9 °C as estimated from the TEX86 palaeothermometer. This period is impacted by two Black Sea melt water pulses (BSWP-II-1 and 2) as indicated by very low Sr/Caostracods but high sedimentary K/Al values. Anomalously high radiogenic 87Sr/86Srostracod values (max. 0.70945) during BSWP-II-2 suggest a potential Himalayan source communicated via the Caspian Sea. The T II warming started at 129.9 ± 0.7 ka BP , witnessed by abrupt disappearance of IRD, increasing δ18Oostracod values, and a first TEX86 derived temperature rise of about 2.5 °C. A second, abrupt warming step to ca. 15.5 °C as the prelude of the Eemian warm period is documented at 128.3 ka BP. The Mediterranean–Black Sea reconnection most likely occurred at 128.1 ± 0.7 ka BP as demonstrated by increasing Sr/Caostracods and U/Caostracods values. The disappearance of ostracods and TOC contents > 2 % document the onset of Eemian sapropel formation at 127.6 ka BP. During sapropel formation, TEX86 temperatures dropped and stabilised at around 9 °C, while UK′37 temperatures remain on average 17 °C. This difference is possibly caused by a habitat shift of Thaumarchaeota communities from surface towards nutrient-rich deeper and colder waters located above the gradually establishing halo- and redoxcline.
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    ABSTRACT: We investigated a high-flux seepage site in the Eastern Black Sea in order to quantify the content of shallow gas hydrates and to elucidate their physico-chemical behavior. Pressure and non-pressure sediment cores, as well as venting gas were collected at the Batumi seep area (BSA) in about 845 m water depth. Sediments represented late glacial to Holocene deposits. In gravity cores, hydrates were absent in top sediments (lithological Unit 1) but abundant below ca. 0.9 mbsf. Here, hydrates occurred as massive aggregates in deeper sections of the Unit 2 and as disseminated pieces in the underlying Unit 3. Gas from the degassing of pressure cores and from hydrates as well as vent gas were dominated by CH 4 (>99.9 mol-% of light hydrocarbons, LHC). Enrichments in CH 4 and C 2 H 6 accompanied by depletions in C 3 H 8 and C 4 -isomers in hydrate-associated gas relative to vent gas resulted from molecular fractionation during hydrate precipitation. Volumetric gas/bulk sediment ratios determined by pressure core degassing approached 20.3. CH 4 concentrations reflected hydrate saturations of 5.2% in Unit 2 and 21% of pore volume in Unit 3. It is calculated that over the entire BSA covering 0.5 km 2 about 11.3 kt of hydrate-bound CH 4 exist in shallow sediments. X-ray diffraction showed structure I hydrate to prevail. Stable O isotope ratios of authigenic carbonates signify that hydrate decomposition along with gas discharge into overlying sediments occurs episodically. From the rough seafloor topography and carbonate data we conclude that in situ dissociation and/or upfloating of shallow-buried hydrates are a typical feature of the BSA. INTRODUCTION The Black Sea basin comprises the world's largest reservoir of dissolved methane (9.6 × 10 4 kt [1]), which is primarily supplied from seeps and de-composing hydrates [2]. It is estimated to contain ca. 10–50*10 3 km 3 of hydrate-bound methane [3]. So far, numerous hydrocarbon seepage sites fuel-led from reservoirs in the deeper subsurface were discovered mainly above the upper boundary of the gas hydrate stability zone (GHSZ), which for pure methane hydrates (structure I, sI) is located at about 710 to 720 m water depth [4-5]. In addition, distinct areas of intense hydrocarbon seepage within the GHSZ and associated with shallow hydrates were recognized in recent years (e.g. [6] for refs.). Especially shallow-buried hydrates are sensitive to changes in the environmental conditions control-ling hydrate stability (i.e. temperature, pore water salinity, hydrocarbon availability, hydrostatic pressure; e.g. [7]) compared to their deeply buried counterparts. In the case one or more of these factors change, submarine hydrates might disso-ciate and release significant amounts of light hydrocarbons (LHC) to the hydrosphere with con-sequences for the seafloor topography, biogeo-chemical carbon cycling, and global climate. However, information on total methane amounts trapped in hydrates at individual seepage sites in the Black Sea is sparse. This is mainly due to the technical effort required to determine true gas and hydrate concentrations (i.e. pressure sampling techniques [5, 8]) in deep sea sediments. In sediments overlying hydrates the anaerobic oxidation of methane (AOM) typically takes place and affects the vertical hydrate distribution. The AOM is mediated by a consortium of methano-trophic archaea and sulfate-reducing bacteria in a transition zone where methane ascending towards the seabed and seawater-derived sulfate meet. Authigenic methane-derived carbonates, formed as by-products of the AOM, might be used as archives of biogeochemical processes since they preserve the geochemical signature of the inter-stitial water and therefore reflect varying methane seepage activity and hydrate decomposition (e.g. [9-11]). However, authigenic carbonates from deep sea hydrocarbon seep site in the Black Sea have barely used to evaluate the long-term stability of associated hydrates, so far [12]. An aspect of parti-cular interest is whether hydrate-derived methane is released constantly over time or mostly in form of huge bursts rapidly exhausting the hydrate re-servoir. An improved understanding of processes affecting hydrate formation and dissociation in the past will contribute to a refined prediction of similar processes in the future if global warming leads to an increase of deep water temperatures. In this study we present total amounts of hydrate-bound methane contained in surface sediments of a highly active hydrocarbon seepage area, the Batumi seep area, in the Eastern Black Sea. More-over, we discuss the stability of hydrates associated to this seepage site in the past. For the study pre-sented here, we analyzed pressurized and non-pressurized sediment cores as well as authigenic methane-derived carbonates using several state-of-the art techniques.