Geological evidence for non-catastrophic sea-level rise in the northwestern Black Sea over the past 25 ky

Category Tectonic & Seismotectonic
Group GSI.IR
Location International Geological Congress,oslo 2008
Author Larchenkov, Evgeny; Kadurin, Sergej
Holding Date 11 October 2008

At present, there is an ongoing debate about the rates of sea-level change in the Black Sea during the Early Holocene. Based on some radiometric dates, sea-level fluctuations did not exceed 20 m, whereas others suggest that they may have reached 100 m or more. To estimate the characteristics of sea-level changes over the past 25 ky, we used a detailed analysis of stratigraphy, sedimentology, mineralogy, geomorphology, regional tectonics and depositional environments. An increase in stratigraphic thickness can be traced in bottom sediments from the outer shelf to the coastline: Neoeuxinian horizons are overlain by New Chernomorian, which reflects a transgressive succession in the formation of the Late Pleistocene-Holocene deposits. This transgression was gradual and had an oscillating character. The thickness of recent sediments along the inner and middle parts of the shelf is 3-4 m, with an increase in thickness in antecedent depressions and a decrease in topographic highs. A statistical estimate of the distribution of granulometric fractions of the Holocene sediments shows that mud dominates most of the inner shelf depressions, whereas coarse-grained deposits occur on the raised areas of the seafloor. Along the middle and outer shelf, beyond the 40 m depth there is a relatively uniform occurrence of all textural fractions.
In general, the shelf seafloor is a gently tilted surface, but according to statistical analysis there are several depth intervals where the surface is relatively flat or steep. Steep surfaces correspond to a shoreline position reflecting the non-uniformity of the transgression. Several levels of stillstands and stable coastlines have been detected that existed during a sea-level rise from -87-85 m (25 ka BP) to the present. The bottom sediments and the nearshore regions reveal signs of active tectonics. The amplitudes of the most recent vertical movements are comparable to eustatic Black Sea level changes and do not exceed millimeters per year.
The absence of abrupt sea-level fluctuations is substantiated by a sequence of depositional scenarios along the outer shelf. Here, the Neoeuxinian stage is characterized by neritic, inner shelf, nearshore, and pro-delta environments. During the Drevnechernomorian stage, they are replaced by neritic facies of the inner and middle shelf where sedimentation was largely precluded due to active wave regime. During the New Chernomorian stage, the area was dominated by the currents characterizing the outer shelf. The aforementioned data do not show evidence of sea-level amplitude exceeding 20 m and suggest that its rate was less than 10 cm/year. The oscillating and weakly contrasting sea-level rise along the northwestern Black Sea over the past 25 ky essentially excludes the possibility of catastrophic consequences (e.g., consequent mass casualties of the people in this region).