A gradual drowning of the southwestern Black Sea shelf: Evidence for a progressive rather than abrupt Holocene reconnection with the eastern Mediterranean Sea through the Marmara Sea Gateway

Abstract

Core M02-45 recovered 9.5m of a ∼12m-thick transgressive succession on the SW Black Sea shelf. The underlying transgressive unconformity, α, deepens toward the shelf edge, so that the coresite was never isolated from the open Black Sea. Fourteen radiocarbon dates indicate sedimentation from ∼9.3ka to the present, with only one hiatus at ∼270cm depth spanning ∼4.5–2.5ka. Three units are present in the core: Unit A (0–270cm)=burrowed mud with laminated silt beds and mollusc shells of Mediterranean affinity (accumulation rate ∼125cm/ky); Unit B (270–525cm)=silty mud with shelly interbeds containing Truncatella subcylindrica, Mytilus galloprovincialis, Parvicardium exiguum, Rissoa spp. and Modiolula phaseolina (rate ∼85cm/ky); Unit C (525–950cm)=burrowed silty mud with graded beds of silt and fine sand, and shells of T. subcylindrica, P. exiguum and Dreissena polymorpha (rate ∼360cm/ky). Unit C developed below storm wave base at a time when proponents of a catastrophic flood in the Black Sea claim that the shelf was subaerially exposed. Clearly it was not. Ostracoda of Caspian affinity indicate ∼5‰ salinity until ∼7.5ka. Dinocysts and foraminifera confirm a low but rising salinity after ∼8.6ka. An increase of δ34S from ∼5–30‰ through 8.4–7.6ka is attributed to a first pulse of sulfate-rich Aegean water into an already high Black Sea, after which this sulfate was quantitatively precipitated as sulfide. δ34S then dropped at ∼8ka to ∼−20‰ as dysoxia and water-column stratification were established because of the initiation of two-way flow through the Bosphorus. Earlier water exchange with the Mediterranean was likely impeded by strong Black Sea outflow which prevented easy access of the Aegean water mass.