Geochronology of Lake Baikal from Pb-210 and Cs-137 radioisotopes

Two box cores of near surface sediments were obtained from Lake Baikal in S outheastern Siberia, Russia. The cores were taken from the northern and sou thern basins of the lake during a joint American-Russian research expeditio n in the summer of 1994. The cores were analyzed for Pb-210, Cs-137 and tot al organic carbon (TOC). Organic carbon is an indicator of photosynthetic p roduction by phytoplankton, taking place primarily in the euphotic zone of the water column. Accumulation rates of TOC may be used as indicators of pa leo-productivity when sedimentation rates are determined using the Pb-210 d ating method and combined with both the density of sediment and organic car bon content. Accordingly, the lake is characterized by changes in accumulat ions of TOC, which may be linked to rates of sedimentation. Accumulations o f TOC and sedimentation rates were higher in the southern basin site than i n the northern basin site. The southern station core was taken from an area in close proximity to the Selenga River delta, which carries 50% of the wa ter input to Lake Baikal, Productivity should thus be higher in this region due to the high nutrient input and sediment accumulation higher due to inf lux of riverine sediment input. Traces of Cs-137 (anthropogenic product) we re found in both cores. However, activities of Cs-137 were significantly hi gher in the southern basin, likely due to the input of the Selenga River in the southern region, which extends to a region in Mongolia in close proxim ity to the area of the Chinese atomic-bomb atmospheric testing of the 1970s . Application of a quantitative inverse model to the Pb-210 profiles yielde d the following results. (i) station 12, near the Selenga Delta, had an acc umulation rate of about 0.38 cm/y in 1957 but this rate was halved by 1980 to about 0.22 cm/y and has been roughly steady since that time; Cs-137 valu es are consistent with the age-to-depth determination from Pb-210 for stati on 12; (ii) since about 1960, station 5A in the northern basin had an accum ulation rate lower by a factor 2-4 than that of the station near the Seleng a Delta; the Cs-137 values are consistent with the Pb-210 age-to-depth dete rmination for station 5A; (iii) the Cs-137 activities for station 12 system atically increase with time towards the present day and are about a factor 6-10 higher than Cs-137 activities recorded for station 5A, which do not sh ow a corresponding systematic increase with time. (C) 1999 Elsevier Science Ltd. All rights reserved.