Hillaire-Marcel et al. question our interpretation of U-series data of Costa Rican stalagmite V1 (Lachniet et al., 2004a), and suggest growth during two short events at 8.8 ka (subsamples 0–87 mm) and 4.9 ka (samples 208–346 mm), possibly separated by a 4 k.y.-long hiatus. They also note the moderate δ18O and δ13C covariance (r2 = 0.48) and express concern about our climatic interpretations. We take this opportunity to re-emphasize the following points: (1) no hiatus is present in V1, (2) the U-series data are consistent with the calculated ages and chronology of our subsamples, and (3) the stable isotope values indeed record an environmental signal.
First, we analyzed thin sections of V1 under magnification to evaluate crystal fabrics and stratigraphy. The thin sections show overlapping, uninterrupted columnar calcite crystals extending from the base to the tip of the stalagmite, with no evidence of a hiatus along the drip axis. We infer continuous deposition of calcite between ~8.8 and 4.9 ka.
Second, the U-series data replotted as three-dimensional isochrons support our base and tip ages (Figure 1). In contrast to the classical Rosholt two-dimensional plot of Hillaire-Marcel et al., the subsamples from 0–87 mm clearly do not form a good isochron, and likely reflect an age spread of 1.5–2.0 k.y. Because of the low 230Th/232Th and 234U/232Th ratios of the 12–87 mm nonisochron subsamples, the data plot close to the origin in a Rosholt plot and their fit to isochrons of multiple ages is not surprising. In this case a classical Rosholt isochron is not sufficient to constrain the ages of nonisochron subsamples and initial 230Th/232Th ratios except along a single layer. Therefore, given no stratigraphic evidence for a hiatus, the stratigraphically acceptable decrease in ages with distance from the base (Fig. 1 in Lachniet et al., 2004a; with the exception of sample 84, which likely has a high initial 230Th/232Th value), the U-series data of the 12–87 mm subsamples are best interpreted as consistent with the ages and chronology reported in our paper. Hillaire-Marcel et al.'s interpretation of two rapid growth intervals separated by a 4-k.y.-long hiatus is not supported by the data.
Finally, results of three Hendy tests on V1 do not show a systematic increase in δ18O away from the drip-axis, and we conclude that our climatic interpretations are robust. More generally, in the thirty years since Hendy suggested that δ18O and δ13C covariation indicates nonclimatic kinetic isotopic effects (Hendy, 1971), studies have demonstrated that environmental factors may affect both δ13C and δ18O values and therefore lead to moderate covariation (Burns et al., 2002; Frappier et al., 2002; Lachniet et al., 2004b; Richards and Dorale, 2003). Stalagmite δ13C values are controlled in part by soil respiration rate. In the tropics, rainfall amount controls δ18Orainfall and soil moisture content and respiration rate, so it is not surprising to see some oxygen/carbon covariance. A Hendy test may only indicate kinetic effects with distance away from the growth axis, that part of the stalagmite not sampled for stable isotopes, whereas calcite along the growth axis may have been deposited in isotopic equilibrium.