| Speleothems can provide reliable well-dated archives of past climatic conditions on
the continents. However, unambiguous attribution of temporal changes in δ18O
and δ13C to specific climate variables such as air temperature, rainfall amount and
palaeo-aridity remains difficult. This issue is particularly acute in the case of Holocene
speleothems, because their low-amplitude δ18O and δ13C signals can be superimposed on
variability that is unrelated to climate (e.g. stochastic water-routing effects, isotope
disequilibrium effects). Methodologies to distinguish between true climate signals and such
extraneous ‘noise’ have not yet received much attention. Replication of records
from the same cave or region is one approach, but in practice is often limited for
conservation and resource reasons. Two other methodologies can reduce the uncertainties.
Traditionally, a multi-proxy approach of using mineralogical, textural, growth-rate and
trace element proxies within speleothems (e.g. calcite/aragonite transitions, trace
element ratios, δ13C, Î14C) has been used to provide additional constraints on
interpretations. Some speleothems exhibit highly correlated Mg/Ca, δ13C and δ18O
time-series trends for example, indicating strong prior calcite precipitation (PCP) and/or
kinetic isotope fractionation effects that complicate palaeoclimate interpretations.
Crucially, all such processes lead to elevated δ13C and δ18O values suggesting that
sub-samples with the lowest δ18O and δ13C values within noisy time-series data may best
reflect the climate-related signal, but this approach may be adversely affected by
site-specific effects. A second, potentially more robust, but under-utilised approach is to
consider individual stable isotope time-series datasets in the context of regional scale
spatial isotope gradients. A recent compilation of data from >50 European Holocene
speleothems for example has demonstrated persistent systematic zonal trends of
decreasing δ18Ospel consistent with progressive rainout from a predominantly westerly
(Atlantic) derived moisture source through the Holocene. Crucially, the slope of this
rainout trend (dδ18Ospel/dx) changes markedly from the late glacial period to the late
Holocene, with steeper eastward decreases in δ18O across Europe in the period
12-8 ka. Steeper dδ18Ospel/dx trends are a reflection both of (i) higher δ18Ospel in
samples from the western margin of Europe in the early Holocene, attributed to
higher δ18O in the Atlantic oceanic source and colder conditions (ii) lower values
of δ18Ospel in the mid- to east-European sites in the later part of the Holocene.
Deviations from these regional-scale gradients may be used to; (i) identify cave sites
that are sensitive to different vapour sources (e.g. Mediterranean sources) and (ii)
isolate site-specific effects such as disequilibrium or evaporative effects that cause
local enrichment in δ18O above the regional rainout trends. The significance of
temporal changes in the O isotopic gradient during the Holocene will be modelled
in terms of climate parameters such as air temperature and humidity gradients. |