HR: 09:30h
AN: PP21D-07    [Abstracts]
TI: Seasonal and inter-annual climate responses revealed in an ultra-high resolution isotope record in a speleothem from Gibraltar
AU: * Mattey, D
EM: mattey@gl.rhul.ac.uk
AF: Geology Dept, Royal Holloway College, Egham Hill, Egham, TW20 0EX United Kingdom
AU: Lowry, D
EM: d.lowry@gl.rhul.ac.uk
AF: Geology Dept, Royal Holloway College, Egham Hill, Egham, TW20 0EX United Kingdom
AU: Fisher, R
EM: r.fisher@gl.rhul.ac.uk
AF: Geology Dept, Royal Holloway College, Egham Hill, Egham, TW20 0EX United Kingdom
AU: Duffet, J
EM: j.duffet@gl.rhul.ac.uk
AF: Geology Dept, Royal Holloway College, Egham Hill, Egham, TW20 0EX United Kingdom
AB: An ultra-high resolution micromill study of a 45mm long active and fast growing speleothem from New St Michaels Cave, Gibraltar has revealed clear seasonal variations in δ¹³C and δ¹⁸O that are correlated to fabric and trace element patterns. Regular oscillations in δ¹³C related to progressive drying of the cave during the summer drought provide a precise chronology in calendar years from 1945 to 2004 enabling an inter-annual comparison with the Gibraltar instrumental record, which includes GNIP data for precipitation isotopes since 1961. Ongoing monitoring in New St Michaels Cave suggests that cave air pCO₂ is highest in the late winter when water seepage flows are at a maximum with growth of dark compact calcite with high P and low Mg & Sr. Summer drying of the cave under declining pCO₂ conditions results in deposition of light dendritic calcite with low P, rising Mg and Sr, and rising δ¹³C. Seasonal changes in cave air and dripwater DIC δ¹³C measured since June 2004 seem consistent with increasing calcite deposition as roof straws and higher degrees of kinetic fractionation associated with degassing as the summer progresses. The oxygen isotope record is complex, with a combination of seasonal change (lowest δ¹⁸O in the winter, and generally rising δ¹⁸O in the summer) superimposed on longer term (decadal) variations. The winter δ¹⁸O signal, located at δ¹³C minima, provides an interannual oxygen isotope record least affected by evapotranspiration processes in the aquifer and summer drying in the cave. We show that the winter δ¹⁸O signal extracted from the speleothem record closely tracks changes in isotopic composition of incoming precipitation at interannual resolution. The calculated δ¹⁸O of precipitation derived from the winter speleothem record shows a significant interannual correlation with observed winter precipitation (r² = 0.53). The decadal trends in the 60 year winter δ¹⁸O extracted from the speleothem record has been compared with the local instrumental record for precipitation amount, MAT and NAO index. Speleothem winter δ¹⁸O is strongly correlated with decadal trends in MAT (r²=0.73) from 1947-2005. Relationships with precipitation amount are more complex. Decadal trends in rainfall amount show only weak correlations with speleothem δ¹⁸O suggesting that the amount effect is not a strong controlling process on the decadal scale but the data show evidence of short term shifts of speleothem δ¹⁸O during unusually wet years. Detection of an NAO signal in the speleothem record is of considerable interest and the Gibraltar climate record shows evidence of higher rainfall and lower temperatures during -ve NAO phases. Overall, the response in the speleothem δ¹⁸O record, interpreted as being dominated by temperature effect with minor influence of the amount effect, preserves a net decadal covariance with the NAO index. The precipitation isotope records in Faro (Portugal) and Fez (Morrocco) are consistent with the Gibraltar data indicating that these interpretations have regional significance.
DE: 1616 Climate variability (1635, 3305, 3309, 4215, 4513)
DE: 3344 Paleoclimatology (0473, 4900)
DE: 4958 Speleothems
SC: Paleoceanography and Paleoclimatology [PP]
MN: Fall Meeting 2005