Concordant 210Pb and 137Cs ages of black sea anoxic unconsolidated sediments

Abstract

Two independent geochronologic radiometric methods:137 Cs and 210 Pb were used to determine the sedimentation rate and subsequently to date the oldest sediments of a 50 cm core containing unconsolidated sediments collected at a depth of 600 m below sea surface in the anoxic zone of the Western Black Sea, off the Romanian town of Constanta. Both methods gave coincident value within experimental uncertainties of 0.42±0.20 mm·y -1 for 137 Cs and 0.49±0.03 mm·y -1 in the case of 210 Pb, within the Constant Initial Concentration model, which gave for the lower most sediments an age of 1.00±0.01 ka. The constant sedimentation model as well as the possible causes of observed discrepancy between experimental uncertainties are discussed.

Global Boundary Stratotype Section and Point (GSSP) for the Anthropocene Series: Where and how to look for potential candidates

“…The Black Sea is an enclosed silled basin with an area of 422,000 km 2 and a maximum depth of 2212 m (Florea et al 2011). The water column is stratified, with the pycnocline and halocline both occurring at about 75 m depth in the centre of the basin and 200 m at the margins, and marking the boundary between oxygenated water above and euxinic, H2Ssaturated and saline deeper waters below (Arthur et al 1994, Florea et al 2011. At the seafloor, there is a topmost gelatinous flocculent surface layer about 2 cm thick overlying a further 2 cm of unlaminated grey/white carbonate-rich sediment (Pilskaln and Pike 2001).…”

“…Black organicrich laminae with biogenic metal sulphides occur between the carbonate and terrigenous laminae(Pilskaln and Pike 2001, Schimmelmann et al 2016). The top flocculent layer is thought to be a permanent transition zone that hydraulically sorts particles by density(Pilskaln and Pike 2001).The varves have been independently dated using 210 Pb, 137 Cs and 14 C AMS, although the latter commonly gives older ages due either to the presence of "dead" carbon in Black Sea sediments or missing varves(Arthur et al 1994, Pilskaln and Pike 2001, Florea et al 2011.Although the highest countable varves beneath the flocculent layer date to about 1918 CE(Arthur et al 1994),Florea et al (2011) discerned the 1986 CE Chernobyl 137 Cs signal at between 5 and 10 mm depth (in samples collected in 2004 CE), which agrees with a 210 Pbdated accumulation rate of 0.49 ± 0.03 mm yr -1 . Groups of varves in Unit I have been correlated across the entire deeper basin…”

The Geochemistry of 1 ky Old Euxinic Sediments of the Western Black Sea

“…Previous analysis of this core demonstrated that, by assuming a constant depositional rate, the maximum age of the sediments is about 1 ± 0.1 ky [18]. Given the quasi-regularity of lamina thickness (Figure 1), we have used the 210 Pb data for determination of an estimated age to different sections of the core.…”

“…Another peculiarity observed for the EF vertical profiles (Figure 3) is the significant fluctuation at depths varying between 15 to 25-30 cm, the same depths where the clay and carbonate contents reached the maximum variability (Figure 1b). By assuming for the entire column an age of about 1 ky [18], this sector could have an age between 300 and 600 y BP, which partially coincides with the Little Ice Age. Another possible explanation could be linked with the onset of the Industrial Revolution and its significant environmental impact.…”

“…In order to assess the past geochemical evolution as reflected in a 50-cm-long core, of which estimated age was about 1 ky [18], and to increase the accuracy of measurements, we have used two complementary analytical techniques: Epithermal Neutron Activation Analysis (ENAA) and Prompt Gamma-ray Activation Analysis (PGAA).…”