Late-glacial and Early Holocene climate and environment from stable isotopes in Welsh tufa

Abstract

The Caerwys tufa in North Wales (UK) contains basal deposits thought to represent carbonate precipitation during the Late-glacial interstadial. These deposits are used to test whether stable isotope data record all or part of a warming-cooling-warming trend through the transition of the Late-glacial interstadial, Younger Dryas stadial and early Holocene. The d18O values and molluscan abundance data suggest that deposition of the Late-glacial tufa occurred mainly during a relatively warm period (? GL-1c or 1e) followed by cooling, the latter likely to be the transition into the Younger Dryas stadial. Tufa deposition is not recorded in the coldest part of the stadial: it is replaced by a sandy horizon. d13C values around -8.5‰ in the basal Late-glacial interstadial tufas show there was a significant influence from isotopically light soil-zone CO2, consistent with development of birch scrub and woodland further south in North Wales. During climatic cooling into the Younger Dryas stadial a 1‰ decrease in tufa d13C is interpreted to represent decreasing phytoplankton photosynthetic activity in pools causing less isotopic enrichment of the tufa d13C. In the (pre-9000 cal. years BP) early Holocene tufas at Caerwys, palaeo-water temperatures calculated from d18O in tufa calcite and from shell carbonate of Lymnaea peregra agree well and suggest summer water temperatures in the range 13 to 16.5°C. d13C data from these early Holocene pool-micrite tufas demonstrate that phytoplanktonic photosynthetic activity within the water column resumed under the warmer conditions causing isotopic enrichment of the tufa d13C values.