HR: 1330h
AN: GC12A-0159    [PDF]
TI: Comparison Between Speleothem Isotopic And Instrumental Climate Records: Annual $\delta$$^1$$^8$O Cycles, And Inter-annual $\delta$$^1$$^8$O And $\delta$$^1$$^3$C isotopes
AU: * Treble, P C
EM: Pauline.Treble@ess.ucla.edu
AF: Dept. Earth and Space Sciences & IGPP, University of California, Los Angeles, CA 90095-1567 United States
AU: * Treble, P C
EM: Pauline.Treble@ess.ucla.edu
AF: Research School of Earth Sciences, Australian National University, Canberra, ACT 0200 Australia
AU: Chappell, J
EM: john.chappell@anu.edu.au
AF: Research School of Earth Sciences, Australian National University, Canberra, ACT 0200 Australia
AU: Harrison, T M
EM: mark.harrison@anu.edu.au
AF: Dept. Earth and Space Sciences & IGPP, University of California, Los Angeles, CA 90095-1567 United States
AU: Harrison, T M
EM: mark.harrison@anu.edu.au
AF: Research School of Earth Sciences, Australian National University, Canberra, ACT 0200 Australia
AU: McKeegan, K D
EM: kdm@ess.ucla.edu
AF: Dept. Earth and Space Sciences & IGPP, University of California, Los Angeles, CA 90095-1567 United States
AB: An intra and inter-annual stable isotope record has been obtained from a southwest Australian speleothem whose age is confidently confined by the boardwalk on which it grew (1911-1992). This provides an excellent test of speleothem climate proxies because the regional climate is strongly seasonal (wet winter/dry summer), and has experienced a 20% reduction of mean rainfall since 1964 (Smith et al., 2001) and $0.8\deg$C temperature rise since 1953. Inter-annual variations of oxygen ($\delta$$^1$$^8$O) and carbon ($\delta$$^1$$^3$C) isotopes (1.5-2 year increments) were measured by conventional mass spectrometry whilst intra-annual $\delta$$^1$$^8$O was measured using a high spatial resolution ion microprobe. Comparing the speleothem O and C isotope and instrumental climate records reveals surprising trends. Speleothem $\delta$$^1$$^8$O varies positively with temperature on both intra- and inter-annual timescales, eliminating a cave temperature effect (McCrea, 1950). On the inter-annual scale, $\delta$$^1$$^8$O rises 0.3% after 1974. This occurs 10 years after the regional rainfall decrease but synchronous with a delayed response seen in P and Mg concentrations, which otherwise vary strongly with rainfall, indicating that speleothem $\delta$$^1$$^8$O is responding to rainfall $\delta$$^1$$^8$O. This is confirmed by the detection of annual 1-2% $\delta$$^1$$^8$O cycles which record rainfall $\delta$$^1$$^8$O, smoothed by storage in the overlying limestone. Rainfall in the region is isotopically lightest in the wet winters and heaviest in the relatively dry summers, driven by the intensity and proximity of passing low pressure systems. Preservation of the annual rainfall $\delta$$^1$$^8$O signal suggests that rainfall amount is recorded by speleothems in this region. A 2.5% rise in $\delta$$^1$$^3$C since 1934 cannot be related either to rainfall or temperature. Furthermore, there are no consistent relationships between $\delta$$^1$$^3$C with speleothem growth rate or with trace elements leached from the limestone, but a kinetic control for $\delta$$^1$$^3$C is not ruled out. Mean $\delta$$^1$$^8$O rises by 0.5% between 1929-1957, which coincides with faster speleothem growth; however, the results do not support accepted theory (Hendy, 1971), which suggests $\delta$$^1$$^8$O should be less sensitive to precipitation kinetics than $\delta$$^1$$^3$C. McCrea, J. 1950. J. Chem. Phys. 18: 849; Hendy, C. 1971. GCA 35: 802; Smith, I, et al. 2000. Inl. J. Climat. 20: 1913.
DE: 1040 Isotopic composition/chemistry
DE: 1045 Low-temperature geochemistry
DE: 1065 Trace elements (3670)
SC: Global Climate Change [GC]
MN: 2003 Fall Meeting