Climate related records revealed in radiometrically dated using 210Pb, 137Cs and 241Am recent sediment of Transcarpathian Lake Synevyr, Ukraine

Undisturbed sediment cores have been obtained for the first time in 2019 from mountain Lake Synevyr located at 989 ASL on the territory of eponymous National Nature Park in the Ukrainian Transcarpathian region. The sediment cores were collected from the northern and southern subbasin of the lake with the maximal depths ~22m verified by the bathymetric survey of the lake bottom.

Testing the water column physico-chemical profile (ToC, DO, TDS, pH, ORP) allowed to show that the Lake Synevyr has strong vertical  stratification as it was established by apparent thermocline and absence of DO at depth below ca. 10 m. Most likely this stratification occurs permanently whereby the bottom sediment showed quasi-laminated (varved) structure, with clearly distinguished fragments of sediment  accumulated on the bottom of the lake at different episodes of times.

Other local natural archives under study were small mountain swampy lake Diike, sphagnum oligotrophic swamps Zamshatka and Gluhanya, where samples of the sphagnum moss were collected and subjected for radiometric analysis to establish regional reference atmospheric flux of airborne radionuclides. 

Radiometric dating of collected sediment cores using CRS and CIC models (1) applied to vertical profile of unsupported 210Pb was successfully carried out, allowing to quantify variations in sediment accumulation rate at the bottom of Lake Synevyr over the last 120 years (2).

To corroborate the validity of the results derived by 210Pb dating 137Cs and 241Am - anthropogenic radionuclides, products of nuclear testing in the atmosphere or due to major nuclear accidents, were used as the time markers. Radiometric chronological dates suggested by the 210Pb models  satisfactorily coincide with the vertical distribution of 137Cs and traces of 241Am, which clearly showed two resolved peaks dated as 1963 (global nuclear weapons tests) and 1986 (Chernobyl accident).

It has been established that variation of sedimentation rates in Lake Synevyr in general follow the regional atmospheric precipitation time pattern, instrumental records of which are available from 1947 on and show apparent ∼20y cycling oscillation. Most noticeable was presence in the sediment column records of impulse ingression of erosion products from the catchment due to mudflows caused by combination of extreme rainfall and snowmelt dated back to 1998 and 2001 when catastrophic floods devastated the entire Transcarpathia bringing numerous human losses. According to the sediment records these were the only episodes of extensive erosion that happened in the last century.

Taking into account that Lake Synevyr was formed as a result of massive landslide happened 10 thousand years ago, and located in pristine remote location far away from industrial sites with intensive human activity, one can consider it as a prospectus geochronological archive for studying the past environmental history and ongoing climate change (3).

1. Appleby P. G.Radiometric dating of sediment records in European mountain lakes. Limnol., 59 (Suppl. 1): 1-14, 2000.

2. Laptev, G. V., et al..Radiometric dating of bottom deposits of mountain lake Synevyr (Ukrainian Carpathians) by 210Pb. Kiev: INS, 2020 

3. Laptev G.V. The Black sea SST reconstruction for the past 2000 years using radiometrically dated sediment records from abyssal plain and continental slope. ISIH Monaco, 2011.