PYCNOCLINE VARIABILITY IN THE NORTH CENTRAL BLACK SEA AND ITS INFLUENCE ON SHIPWRECKS AND BEDFORMS

The oxygenated upper layer of the Black Sea has an average salinity of 17.5 psu while the deep anoxic water has an average salinity of 23 psu. The two layers are separated by a pycnocline at approximately 150m depth. A Byzantine shipwreck, Chersonesos A, which was found off the southwest coast of the Crimean Peninsula in 2006, is located at approximately 140m depth and above the typical level of the pycnocline. Nevertheless it has been influenced by little biotic activity, thereby affecting its preservation. It is hypothesized that the pycnocline intermittently elevates to shallower levels, bringing anoxic water to the site and eradicating harmful biota. There are two alternative mechanisms that could be the cause of such intermittent elevation of the local pycnocline: (1) internal waves that propagate along the pycnocline interface, and (2) meanders in the rim current circulation that spatially shifts the geostrophically tilted pycnocline, thereby influencing its depth. Hydrographic profiles of oceanographic data collected during the 2006 Black Sea Expedition onboard the R/V Endeavor near the Chersonesos A site was used to calculate differences in hydrographic properties of the water column during ascents and descents of a remotely operated vehicle (ROV). Analysis of this data will help determine which mechanism more strongly influences pycnocline depth. Significant changes in just a few hours would suggest the presence of internal waves, but slower changes over a few days or weeks would suggest current meanders. Also, internal wave activity may be absent in summer but present in winter, when there are stronger winds, so if short period variations are not found in the presently available data this does not preclude the possibility of an important role for internal waves. These internal waves could be responsible for developing sedimentary bedforms on the seafloor in this region.