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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.