High resolution trace element and oxygen isotope ((18O) analyses of a modern speleothem

High resolution trace element and oxygen isotope ((18O) analyses of a modern speleothem

 P. Treble

Annual resolution of speleothem proxy climate records is an attractive goal in Quaternary palaeoclimate studies. This is now attainable following advances in LA-ICP-MS and SIMS technology. In this study, 5 and 32 micron spatial resolution LA-ICP-MS trace element data were acquired alongside 20 micron resolution SIMS 18O data for a modern stalagmite from the Margaret River region of southwestern Australia.

 The trace element data show clear cycles of positively correlated Sr, Ba, U, P and Na concentrations that are negatively correlated with Mg concentrations (Figure 23). These cycles are known to be annual since the age of the stalagmite is well-constrained by the date of placement of the cave tourist boardwalk on which it grew (1911-1992). These findings support earlier studies using lower resolution SIMS data (Roberts et al., 1998; Huang et al., 2001; Fairchild et al., 2001).

Figure 23: A portion of annual trace element cycles for
the modern stalagmite from southwestern Australia.

Unique to this study, the relatively fast growth rate, (approx. 350 microns per year) has enabled SIMS 18O analyses to be performed alongside a portion of the trace element data using the Cameca ims1270 ion microprobe at the University of California, Los Angeles, in collaboration with Profs Mark Harrison (RSES, ANU), Kevin McKeegan (UCLA) and Dr Marty  Grove (UCLA). These 18O data show an annual cycle of approximately 1, in sympathy with the magnesium concentration variations. 18O is the most commonly used proxy in speleothem palaeo-environment reconstructions and is also the best tracer for hydrological processes.

Figure 24: Annual cycles of (18O, Ba, Mg and U.

 The superior age control provided by the cave tourist boardwalk together with the excellent annual growth rate control provided by the trace element data, make this sample ideal for conducting a rigorous examination of the trace element and 18O cycles with the instrumental climate data. This should bring us closer to decoupling the cave temperature and precipitation signal in speleothem records, and thus verification and validation of these climatic proxies. To date, these trace element cycles have been stacked into a master record which allows the proxy data to be compared with the instrumental rainfall record (Figure 25).

Figure 25: Comparison of stacked maximum values of Ba, Sr, U, Na and P (and minimum Mg) occurring for each year with the amount of winter rainfall (June-August). All element values have the mean of the period A.D.1961-1992 subtracted while rainfall is standardised (by subtracting the mean and dividing by the standard deviation). Darker envelopes represent the standard error of stacked master record.
Southwestern Australia has suffered a dramatic 20% decrease in rainfall since ~A.D.1965. To examine the response of the trace elements to this rainfall shift, variations are conveniently compared in four episodes: (i) pre-A.D.1930, (ii) A.D.1931-1956, (iii) A.D.1957-1969 and (iv) A.D.1970-1991. Figure 3 shows that Mg, P and U in particular, resemble many features of the rainfall record suggesting that Mg, P and U may be useful palaeo-hydrological indicators. Ba, Na and rainfall show similar trends during A.D.1930-1992, with higher values during A.D.1931-1956 (ii), however the trend towards  lower values after A.D.1970 (iv) is not as clear as it is for U, P and Mg. Ba, Na and also Sr shows an opposite trend to rainfall prior to A.D.1930 (i), where Ba, Sr and Na are all below the mean and rainfall is above the mean. Overall, Sr shows the least

resemblance to the rainfall record. Investigation of the promising behaviour of annually resolvable speleothem trace elements and (18O as palaeoclimate proxies is ongoing.

 

References
Huang Y., Fairchild I. J., Borsato A., et al., (2001) Chemical Geology 175, 429-448.
Roberts M.S., Smart P.L. and Baker A., (1998) Earth and Planetary Science Letters 154, 237-246.
Fairchild, I. J., Baker, A., Borsato, A., Frisia, S., Hinton, R. W., McDermott, F. and Tooth, A. F. (2001) Journal of the Geological Society, London, 158, 831-841.