SEASONAL TRENDS IN THE URANIUM-SERIES ISOTOPIC SIGNATURES OF LAKE WATERS AND SEDIMENTS - HAWES WATER, NORTHWEST ENGLAND

Potential seasonal variation in the U-series radioisotopic signatures of lake and input waters was investigated in Hawes Water, north-west E ngland, and the sedimentary record was examined to establish whether a ny observed seasonality is presented in the sediment U and Th radioiso topic activity ratios. U-series disequilibrium in the waters is indica tive of both surface and shallow sub-surface origins for the dissolved U, and that this supply is influenced by changes in the prevailing hy drological regime. However, the lake is buffered from this seasonality in that the U-234/U-238 activity ratio remains constant during the ye ar, and the U content undergoes only a marginal decrease during summer . The change in the mechanism of lake sedimentation from allochthonous organic input during winter to autochthonous carbonate formation duri ng summer has a more significant influence on the U-series radioisotop ic characteristics of sedimentary record in that the proportion of org anic and carbonate material both exhibit a strong control on the U-234 /U-238 activity ratio and U content. In addition, significant proporti ons of 'sediment' U and Th are associated with ferromanganese oxyhydro xides. Given this seasonality, it is of some significance that the U-2 34/U-238 activity ratios of the bottom sediments preserve a reasonable record of lake U-series disequilibrium and are, therefore, potential indicators of long-term changes in catchment hydrology.