Sedimentological, geochemical and biological proxies as indicators for hypoxia in the northwestern-shelf of the Black Sea (BENTHOX project)

The northwestern shelf of the Black Sea has been submitted to seasonal hypoxia due to eutrophication since 1980’s, leading to changes in the ecological structures of benthic communities. The present study tests a multiproxy approach in order to identify the hypoxic events. The first part of a gravity core (MN16-GC6-1) collected near to Ukranian shelf during a research cruise in mid-May 2016 was analyzed. The sediments of the core were submitted to different sedimentological (grain size, XRD, SCOPIX), geochemical (LOI, XRF) and biological (benthic foraminifera) analyses. Sediments are mainly made by silted particles while the sandy and the fine-size fraction (clays) did not have a really significance presence. Regarding the mineralogy, quartz is the predominant component in all the samples analyzed (50-80%). Silicates are the major component of the sediments (80-90%) while organic matter content is low. Four clusters of elements (lithogenic, biogenic, organic matter related and metallic) were evidenced in the XRF results and compare with the ratios of the different elements. Benthic foraminifera were analyzed in two fractions, >150 μm and >63 μm mesh diameter. The distribution of the specimens was uneven along the core and species richness (D) and diversity index (H’) were very low. Rotaliida order has been found as the major abundant specimen along the core. Those foraminifera in particular lives at the oxic-anoxic boundary throughout the sediment. Therefore it must occasionally be subjected to completely anoxic conditions and could be consider as a proxy for hypoxia records. Further analyses on the sedimentary core material collected during the MN16 cruise along the Black Sea platform would allow to confirm the trends observed in the upper part of core MN16-GC6-1.