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Recent sedimentation in the Black Sea: New insights from radionuclide distributions and sulfur isotopes

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2012-08-01
Yücel, Mustafa
Moore, Willard S.
Butler, Ian B.
BOYCE, Adrian
Luther, George W.
The Black Sea is the world's largest anoxic-sulfidic marine basin and has unique sedimentation conditions. Recent studies suggested that mass accumulation rates (MAR) in this environment have increased in the past century when compared to the last 2000 years (Unit 1 period). In this paper we test this hypothesis with new MAR data and further explore the relationship between the depositional pattern and pyrite-sulfur isotopic signature. Based on 15 cores sampled in 2001 and 2003, our dataset comprises radioactive isotopes (Pb-210, Ra-226, Cs-137 Cs-137) and sulfur stable isotopes (delta S-34(VCDT)) along with organic, inorganic carbon and pyrite-sulfur. We calculated MARs using Pb-210 profiles and/or Chernobyl-derived Cs-137 horizon buried in the sediment column. Our turbidite-free deep basin sediment MARs (61 to 76 g m(-2) yr(-1)) agreed with the previous results (50-100 g m(-2) yr(-1)) and confirm the view that MARs of the deep Black Sea basin have been increasing. A unique feature of our dataset was the presence of Chernobyl-derived radionuclides below up to 20 cm thick turbidite layers (deposited between 1986 and 2003), which enabled us to compute MARS for these coring locations. MARs were 1120 +/- 103 and 5230 +/- 125 g m(-2) yr(-1) for the last two decades in two turbidite-impacted western central basin cores, 20-100 times the long-term rates of the deep basin. This fast depositional pattern was reflected in the geochemical and isotopic data as well. Turbidites had isotopically heavier pyrite-sulfur compared to the Unit 1-type water column formed pyrite. This is probably because the turbidites originated from slope and transported slope pyrite isotopic signature to the deep basin. Diagenetic effects within the turbidite can make pyrite-sulfur even heavier. These tightly linked results demonstrate the importance of turbidites in recent sedimentation of the Black Sea.