DIRECT U-TH DATING OF ORGANIC-RICH AND CARBONATE-RICH LAKE-SEDIMENTS FROM SOUTHERN SCANDINAVIA

U-Th ages of Holocene lacustrine sediments from southern Sweden have b een determined to assess the reliability of this technique for dating climatic events from the last interglacial-glacial-interglacial cycle, and for calibration of the C-14 dating method, The sediments are orga nic-and carbonate-rich, and they are shown to be very well suited to h igh precision U-238-U-234-Th-230 dating by thermal ionisation mass spe ctrometry (TIMS). Twenty-one samples from 2.58 m of sediment from Lake Igelsjon, southern Sweden, yield ages between 3058 +/- 89 and 9426 +/ - 262 yr (2 sigma error). The sediments consist of hydrogenous calcium carbonate (50-90%), organic matter (5-50%), and siliceous residue (<5 %). The high U contents of the lake sediments (between 6 and 76 ppm) a re in contrast to the low concentrations in the lake-and groundwater ( 1.9 and 3.2 ppb, respectively), and mass balance calculations based on U content of water and sediments can be used to calculate a lakewater residence time of 24 days which is close to the residence time calcul ated using hydrologic data. Recent ground and lakewater ((U-234/U-238) = 0.920) is enriched in U-234 compared to soils from the catchment ar ea ((U-234/U-238 = 0.587), and (U-234/U-238) in the lake sediments sho ws a marked decrease with time from 0.958 at 9426 yr BP to 0.919 at 30 58 yr BP. This is attributed to progressive leaching of U from soils w hich have experienced a previous history of recoil-related U loss, and it indicates that the assumption of constant initial (U-234/U-238) ra tios may be a source of significant error in the calculation of (U-234 /U-238) ages for continental deposits. (C) 1997 Elsevier Science B.V.