Geochronology of Lake Baikal from 210Pb and 137Cs radioisotopes

Two box cores of near surface sediments were obtained from Lake Baikal in Southeastern Siberia, Russia. The cores were taken from the northern and southern basins of the lake during a joint American-Russian research expedition in the summer of 1994. The cores were analyzed for Pb-210, Cs-137 and total organic carbon (TOC). Organic carbon is an indicator of photosynthetic production by phytoplankton, taking place primarily in the euphotic zone of the water column. Accumulation rates of TOC may be used as indicators of paleo-productivity when sedimentation rates are determined using the Pb-210 dating method and combined with both the density of sediment and organic carbon content. Accordingly, the lake is characterized by changes in accumulations of TOC, which may be linked to rates of sedimentation. Accumulations of TOC and sedimentation rates were higher in the southern basin site than in 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 water input to Lake Baikal, Productivity should thus be higher in this region due to the high nutrient input and sediment accumulation higher due to influx of riverine sediment input. Traces of Cs-137 (anthropogenic product) were found in both cores. However, activities of Cs-137 were significantly higher 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 proximity to the area of the Chinese atomic-bomb atmospheric testing of the 1970s. Application of a quantitative inverse model to the Pb-210 profiles yielded the following results. (i) station 12, near the Selenga Delta, had an accumulation 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 values are consistent with the age-to-depth determination from Pb-210 for station 12; (ii) since about 1960, station 5A in the northern basin had an accumulation rate lower by a factor 2-4 than that of the station near the Selenga Delta; the Cs-137 values are consistent with the Pb-210 age-to-depth determination for station 5A; (iii) the Cs-137 activities for station 12 systematically 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 show a corresponding systematic increase with time. (C) 1999 Elsevier Science Ltd. All rights reserved.