Benthic hypoxia and early diagenesis in the Black Sea shelf sediments

Marine waters of semi-enclosed seas are affected by a major environmental issue which is oxygen depletion in bottom
waters. Deoxygenation is one of the most widespread man-induced consequences which can be catastrophic
for living species. Between 1970 and 1990, the benthic compartment of the Black Sea underwent modifications due
to the occurrence and increase of hypoxia. Indeed, these changes might cause a deterioration of the structure and
functioning of the ecosystems. Nowadays, some regions, such as the north-western shelf, are still affected seasonally
by this phenomenon.Within the framework of the BENTHOX project, a biogeochemical study focusing on the
early diagenesis is conducted in the Black Sea. It aims (1) to obtain a better understanding of the impact of benthic
hypoxia on the diagenetic pathways, (2) to contribute to a new dataset of biogeochemical measurements in the
sediments including porewaters. During a cruise (Emblas II – May 2016), on board the RV Mare Nigrum, sediment
cores were taken at 4 stations on the Ukrainian shelf. Porewaters were extracted on board the ship using Rhizon
technique under N2 atmosphere and will be analyzed for dissolved nutrients and major ions. In addition, sediments
were sliced and will be determined for major solid phases and trace element contents. A multi-proxies (biological,
sedimentological, mineralogical and geochemical) approach will be used to identify the hypoxic events and to
reconstruct the history of bottom hypoxia. The results obtained will be presented and discussed with emphasis on
the first outcomes and the major biogeochemical processes involved in the early diagenesis.