Direct U-Th dating of organic- and carbonate-rich lake sediments from southern Scandinavia

U-Th ages of Holocene lacustrine sediments from southern Sweden have been determined to assess the reliability of this technique for dating climatic events from the last interglacial-glacial-interglacial cycle, and for calibration of the14C dating method. The sediments are organic- and carbonate-rich, and they are shown to be very well suited to high precision238U-234U-230Th dating by thermal ionisation mass spectrometry (TIMS). Twenty-one samples from 2.58 m of sediment from Lake Igelsjön, southern Sweden, yield ages between 3058 ± 89 and 9426 ± 262 yr (2σ 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) are in contrast to the low concentrations in the lake- and groudwater (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 lakewter residence time of 24 days which is close to the residence time calculated using hydrologic data. Recent ground and lakewater ((234U/238U) = 0.920) is enriched in 234U compared to soils from the catchment area ((234U/238U= 0.587), and (234U/238U) in the lake sediments shows a marked decrease with time from 0.958 at 9426 yr BP to 0.919 at 3058 yr BP. This is attributed to progressive leaching of U from soils which have experienced a previous history of recoil-related U loss, and it indicates that the assumption of constant initial (234U/238U) ratios may be a source of significant error in the calculation of (234U/238U) ages for continental deposits.