High-resolution seismic and core-based stratigraphy of the Southwestern Black Sea shelf during the Holocene

Ankindinova, Olga (2020) High-resolution seismic and core-based stratigraphy of the Southwestern Black Sea shelf during the Holocene. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

The paleo-environmental evolution of the Black Sea during the last 12,000 years is studied using high-resolution seismic-stratigraphic and geochemical results for two long composite cores from the southwestern shelf, constrained by 56 radiocarbon dates on mollusc shells. A grid of Huntec deep-tow seismic-reflection profiles (~9150 line-km), complemented by 72 gravity cores and 25 piston cores reveals a prominent shelf-crossing transgressive unconformity α which is overlain in most areas by, in succession, allounit 1b (~12.5–7.4 cal ka BP) between α and the α1 submarine unconformity/correlative conformity, allounit 1c (~7.4–5.3 cal ka BP) between the α1 and α2 submarine unconformities/correlative conformities, and allounit 1d (deposited since ~5.3 cal ka BP). The ⁸⁷Sr/⁸⁶Sr isotopic signature in molluscs shows four stages of increase corresponding to salination associated with the reconnection of the Black Sea to the global ocean: 12,145–9580 cal yr BP (stage A; stable, brackish lacustrine conditions); 9580–9490 cal yr BP (stage B: freshish waters); 9465–9380 cal yr BP (stage C; sharp ⁸⁷Sr/⁸⁶Sr increase to a plateau spanning 350–400 years); ~8985 cal yr BP to the present day (stage D; step-wise salination). The existence of the first plateau provides strong evidence against catastrophic flooding of a lowstand Black Sea as proposed by other workers. δ¹⁸O and δ¹³C values in ostracod species Palmoconcha agilis and Loxoconcha lepida define three isotopic substages corresponding to a transition from the Neoeuxine Lake to modern conditions. These are mirrored in trace-element/Ca data, which reveal a Neoeuxine Lake stage (12.1–9.4 cal ka BP), the transition from lacustrine to marine conditions (9.4–6.3 cal ka BP) and the establishment of marine conditions (6.3–0 cal ka BP). At ~6.3 cal ka BP, isotopic data and trace-element/Ca ratios exhibit a major shift contemporaneous with a sharp increase in water temperature from 12–14°C to 17–19°C. This shift coincides with the weakening of Black Sea outflow and the proliferation of marine organisms. This study proves that the level of the Neoeuxine Lake must have been significantly higher than −70 m at the time of the reconnection at 9.4 cal ka BP with the gradual step-like transition to present-day conditions.

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