Stable isotope variations in stalagmites from northwestern Sweden document changes in temperature and vegetation during the early Holocene: a reply to Hammarlund and Edwards

Abstract

In a comment on the paper by Sundqvist et al. (2007a), Hammarlund and Edwards suggest that the oxygen isotope signal of the speleothem records K1 and L4 from northwestern Sweden as well as for the earlier published stalagmite SG93 from northern Norway should mainly reflect variation in the δ 18O of mean annual precipitation. The main argument used by the authors is that the SG93 δ18O record should show remarkable similarities with the lacustrine carbonate δ18O record from Lake Tibetanus. The similarities between SG93 and Lake Tibetanus are mainly seen for the period between 10 and 9 ka BP, which is a period where both records have higher δ18O values compared with later in the Holocene. If true, this would indicate that temperatures in northern Scandinavia would have reached their Holocene maximum between 10 and 9 ka BP, which is contrary to what has been reported from other archives in the region which place the Holocene climatic optimum somewhere between 8 and 5 ka BP. Several factors affect the δ18O of speleothems and definitely also by annual mean δ18O of the local precipitation. However, for Holocene speleothems from northern Scandinavia as well as from the Alps this signal seems to be overridden by other factors, such as variable mixing of summer and winter precipitation of cave drip water as well as the temperature-dependent fractionation between calcite and water. But the relative importance of the factors can differ between different timescales, caves and even speleothems from the same cave, depending on hydrological pathways. To better understand the factors controlling the stable oxygen isotopes in precipitation and proxy archives back in time there is a need for more highly resolved records as well as for more atmospheric and hydrological modelling.