DATING OF COASTAL MARINE SEDIMENTS: 210Pb AND 137Cs IN DANUBE-INFLUENCED BLACK SEA SHELF SEDIMENTS

Coastal marine sediments are natural archives of environmental change due to anthropogenic impact and natural variability in aquatic ecosystems, e.g. coastal erosion, changing river discharge, marine productivity and pollution. Dating of those sediments is a prerequisite for recovering records of change. The natural occurring radionuclide ²¹⁰Pb and the artificial fallout radionuclides ¹³⁷Cs and ²⁴¹Am are applied in dating of recent sediments, i.e. deposited since the beginning of the industrial period. ¹³⁷Cs serves as an independent time marker for end of the atmospheric bomb test fallout in 1963 and the Chernobyl accident in 1986. Due to its chemical mobility in sediments, the ¹³⁷Cs signal is often weakened. Complementary, the less mobile ²⁴¹Am may be used. ²⁴¹Am originates from decay of the bomb fallout of ²⁴¹Pu, and is used as a second time marker of the 1963 event.

The northwestern shelf ecosystem of the Black Sea has been hit by eutrophication and pollution from the late 1960’s to the mid-1990’s, largely triggered by Danube River input of nutrients and pollutants. The aim of our study is to reconstruct the eutrophication history and deposition of nutrients in the sediments. The ‘memory effect’ of the sediment for recycling of nutrients plays a critical role in fuelling pelagic productivity and thus maintaining eutrophic conditions in enclosed seas with long water residence times such as the Black Sea.

Here we present results from sediment cores taken in the Danube River plume on shelf of the Black Sea. The dating of the sediment is somewhat hindered by irregularities in the unsupported ²¹⁰Pb and ¹³⁷Cs profiles. The sediment records are repeatedly interrupted by layers of stiff clay. Those clay layers show a drop in unsupported ²¹⁰Pb and ¹³⁷Cs and higher values of supported ²¹⁰Pb. In between and below the clay layers, unsupported ²¹⁰Pb and ¹³⁷Cs increase again. Low values of the fallout radionuclide and of unsupported ²¹⁰Pb combined with higher supported ²¹⁰Pb point to a terrestrial origin of the clay. We hypothesise that the clay represents material eroded from the Danube Delta and transported to the sea in pulse-like events during flash floods of the Danube River.