Untitled Document
Calibrating the speleothem O isotope signal to rainfall
Pauline C. Treble1,2 and Matthew J. Fischer1
1 Australian Nuclear Science and Technology Organisation,
Lucas Heights, NSW 2234, Australia
2 Research School of Earth Sciences, Australian National University,
Canberra, ACT 0200, Australia
Figure 1.
Oxygen isotopes (δ180) are the most commonly used paleoclimate proxies
in speleothems (see McDermott, 2004) and are being used in current research
by Dr Treble to reconstruct records of natural rainfall variability for
southwest Western Australia. This region has suffered a significant decrease
in rainfall since the 1970s and longer records are needed to understand
why. The relationship between speleothem δ180 and rainfall was reported
in a previous publication (Treble et al., 2005) and was further examined
in detail by Fischer and Treble (2008). The findings of this latter study
challenge the common expectation with climate proxies is that we can
apply short-interval calibrations to interpret trends in proxy data back
in time. These findings have wide application to the interpretation of
speleothem δ180 records in general, and are summarised here.
Two datasets were originally used to examine the relationship between
speleothem δ180 and rainfall. The first are measurements of δ180 in daily
rainfall events for 2001 which show that larger rainfall events contain
less of the heavier isotope, 18O. The second are measurements of speleothem
δ180 between 1911-1992 which can be closely compared with changes in
the instrumental period (Figure 1, upper panel; Treble et al., 2005).
The speleothem record showed that δ180 rose after 1970, consistent with
the decrease in the frequency of large rainfall events. But importantly,
the speleothem record showed that this simple inverse relationship between
rainfall amount and δ180 did not hold, evidenced by the higher δ180 values
between 1930-55 when there was no decrease in rainfall.
This key finding led to the publication, Fischer and Treble (2008),
where the simple rainfall amount-δ180 regression model was improved to
include inter-annual climate variance. The new model was modulated by
dominant modes of inter-annual climate variability or climate indices
(calculated as the principal components of sea level pressure over the
study period). The new model produced a positive shift in δ180, similar
to that in the speleothem record between 1930-55. This suggests that
the dominant modes of interannual variability can cause shifts in vapour
source regions (or other effects), which can affect δ180 independent
of amount-type effects. In southwest Australia, it appears that the interannual
mode most responsible for isotopic changes related to vapour source,
is Zonal Wave 1 (ZW1). An EOF-based reconstruction of ZW1 over the last
century suggests that a negative ZW1 state from 1930-55 favoured the
advection of 18O-enriched moisture from low latitudes, while a positive
ZW1 state post-1970 resulted in more 18O-depleted moisture advected from
the sub-polar region (Figure 1, lower panel).
As a result of the study summarised here, we now have an improved regression
model for δ180 rainfall in southwest Australia that replicates key patterns
at daily to interdecadal timescales. This is a statistical forward model
and thus it can be used to compare paleoclimate simulations to proxy
data. The inversion of the model will require a multi-proxy approach
(e.g., isotopes and rainfall amount-sensitive trace elements), because
the new model relies on two predictors (δ180 depends on both precipitation
amount and vapour source). In general terms, this study illustrates the
importance of understanding the multiple factors which influence speleothem
δ180 and demonstrate for the first time, the effect of interannual climate
modes on rainfall δ180.
Fischer, M.J., Treble, P.C. (2008) Calibrating climate-delta O-18 regression
models for the interpretation of high-resolution speleothem delta O-18
time series. Journal Of Geophysical Research-Atmospheres 113(D17): D17103.
McDermott, F. (2004) Palaeo-climate reconstruction from stable isotope
variations in speleothems: a review. Quaternary Science Reviews 23(7-8):
901-918.
Treble, P.C., Chappell, J., Gagan, M.K., McKeegan, K.D., Harrison,
T.M. (2005) In situ measurement of seasonal delta O-18 variations and
analysis of isotopic trends in a modem speleothem from southwest Australia.
Earth And Planetary Science Letters 233(1-2): 17-32.