Centennial-Scale Holocene Climate Variability Revealed by a High-Resolution Speleothem d 18 O Record from SW Ireland

the 50% melt contour would completely penetrate a 3-km-thick layer of ice overlying liquid water at 270 K. Complete melt-through of a Europan ice shell during cratering would preclude central peak formation. The crater Man-annán [Web fig. 2 (24)], which is comparable in diameter to Pwyll (Fig. 2F), does not have a well-defined central peak (23). This could be the result of an impact into thinner or warmer ice that hindered formation or preservation of a central peak, suggesting that local variations in temperature gradient or ice thickness may exist. Because several, widely distributed Europan craters exhibit central peaks, our simulations demonstrate that at the times and locations these craters formed, a cold ice layer could not have been as thin as 3 to 4 km. Although complete melt-through of the ice layer does not occur in the simulations with slightly thicker ice, our transient crater sizes are lower limits. Moreover, impacts disrupt target material well beyond the zone of partial melting (36), so our simulations put a lower limit on the thickness of the ice. Therefore, we conclude that an ice shell must have been more than 3 to 4 km thick at the times and locations of complex crater formation. for helpful discussions and for reviewing the manuscript. We are also grateful to two anonymous reviewers who provided helpful comments. Evaluating the significance of Holocene submillennial ␦ 18 O variability in the Greenland ice cores is crucial for understanding how natural climate oscillations may modulate future anthropogenic warming. A high-resolution oxygen isotope record from a speleothem in southwestern Ireland provides evidence for centennial-scale ␦ 18 O variations that correlate with subtle ␦ 18 O changes in the Greenland ice cores, indicating regionally coherent variability in the early Holocene. Evidence for previously undetected early Holocene cooling events is presented, but mid-to late-Holocene ice rafting in the North Atlantic appears to have had little impact on ␦ 18 O at this ocean margin site. It is widely accepted that climate variability on time scales of 10 3 to 10 5 years is driven primarily by orbital, or so-called Milankovitch, forcing. Less well understood is the cause of the centennial-to millennial-scale variability that characterizes the ␦ 18 O records of both the gla-cial and interglacial intervals of the GRIP and GISP2 ice cores (1, 2), yet this higher frequency variability may be important for predicting future climate change. Unlike the last …