A Multi-Proxy Approach to Reconstruct Hypoxia on the NW Black Sea Shelf over the Holocene

Abstract

Coastal hypoxia is a worldwide concern. Even though seasonal hypoxia has been reported on the northwestern Black Sea shelf since the 1970s, little is known about oxygenation in this area over the Holocene. With a multiproxy approach, this work aimed to detect potential hypoxic events in two gravity cores. Our results demonstrate that the most common proxies of hypoxia are irrelevant for the Black Sea coastal environment. For instance, the Ammonia-Elphidium index appears to be inappropriate. We assume a salinity bias due to the influence of freshwater inputs. Likely, the redox sensitivity of vanadium is obscured by the dominant supply of detrital elements. However, the size distribution of framboidal pyrites gives clear indications of the oxygenation of the water column. Indeed, the absence of framboidal pyrites in the core located at the Danube mouth suggests permanent oxic conditions over the last 2 ka. On the contrary, in the core located in the Odessa Bay, the two observed ranges of pyrite sizes evidence an alternation between anoxic (mean diameter 3–6 ± 1–2 µm) and hypoxic or oxic (mean diameter 6–10 ± 3–5 µm) conditions in bottom waters over the last 7 ka. Therefore, this proxy reveals different oxygenation levels for these proximal sites.