Rapid Isotope Profiling of Speleothem by He Flow-through Laser Ablation: Insight Into Abrupt Climate Fluctuations From A new Exceptionally High Resolution Holocene \delta18O Record From SW Ireland

Stable isotope analysis of carbonates at high spatial resolution is normally achieved either by microdrilling or by laser sampling, although the latter has not been widely employed, despite early promise, due to higher analytical errors particularly for oxygen isotopes. Laser ablation using a sample chamber continuously swept by He can be shown to significantly improve the accuracy and precision of carbon and oxygen isotope data, with the advantage of the rapid analysis times inherent with He flow-through technology. An automated He flow through laser-ablation system has been used to obtain continuous high resolution isotope profiles along growth axes of speleothem at a resolution of 250 microns or better. The system uses a 25W CO₂ laser heat source with a continuous helium flow sample chamber on line to a conventional gas chromatograph-isotope ratio monitoring system (Micromass Optima - Isochrome). In-situ analysis of Carrara Marble yield \delta¹³C and \delta¹⁸O values reproducible to better than +/-0.1\permil. A new high resolution isotope record comprising 1640 analyses has been obtained on a well dated 465 mm Holocene Stalagmite (CC3). For the latter part of the Holocene each analysis represents 10-22 years, but for the period prior to 5,300 years where speleothem growth rates were higher, resolution is sub-decadal. The new data accurately reproduce the first-order variation obtained by drill-sampling with \delta¹⁸O varying from -11.65 to -0.82\permil, but with most data varying by +/-1.75% around a mean value of -3.26\permil. Historically cooler periods are associated with lower \delta¹⁸O but the new profile also reveals short lived high amplitude (>3\permil) isotope shifts. One of these events exhibits an extremely large shift of c.8\permil over 1mm of calcite dated to between 8445 and 8400 calendar years BP. This high amplitude event is superimposed on a 350 year old episode of lowered \delta¹⁸O that commenced at 8470 years BP but is too large to attribute solely to cooling and may reflect change in the isotopic composition of N. Atlantic surface waters consistent with catastrophic release of Laurentide ice sheet melt waters.