e80 COMMENT C. Hillaire-Marcel B. Ghaleb Centre de Recherche en Géochimie Isotopique et Géodynamique (GEOTOP)–Université du Québec à Montréal (UQAM) & McGill, BP 8888, Succursale Centre-Ville, Montreal, Quebec H3C 3P8, Canada D. Genty Unité Mixte de Recherche 1572, Commissariat à l’Energie Atomique– Centre National de la Recherche Scientifique, L’Orme des Merisiers, 91191 Gif-sur-Yvette, France Lachniet et al. (2004) intend to illustrate in their paper a climate shift at low latitude, marked by a change in the monsoon regime recorded by oxygen isotopes of a stalagmite, that would correlate with the so-called “8.2 ka event” (de Vernal et al., 1997; Alley et al., 1997). Although the title includes a question mark, the reader of the paper is still led to the conclusion that, despite some uncertainties, the studied time series does provide a record of the critical period (i.e., ca 8.2 ka, assigned here to a layer ~40 mm above the base of the stalagmite, as suggested by Figure 1 in Lachniet et al., 2004). Unfortunately, the U-series data used to establish a chronology may be interpreted in a different way. Although relatively low, the 232 Th-con- tents of the carbonate layers, especially between 12 and 87 mm, still indi- cate significant mixing between a detrital phase and an authigenic phase. When plotted as a classical Rosholt isochron (e.g., Rosholt, 1976; Luo and Ku, 1991; Ludwig and Titterington, 1994), the corresponding data fit quite well on the isochron defined by subsamples A–C (Table 1 in Lachniet et al., 2004), from the base of the stalagmite, as illustrated in Figure 1. This suggests that most of the stalagmite was deposited during two short phases of precipitation dated 8.8 and 4.9 ka, respectively, possibly separated by a ~4 k.y. gap. In this scenario, the stable isotope excursion observed at a distance of ~40 mm above the base of the stalagmite (this is unclear in the paper) and attributed to a dry event would date from the early phase of precipitation (i.e., ~8.8 ka). Therefore, it cannot be linked with certainty to any more recent climate excursions that would correlate with the 8.2 ka event. Furthermore, the δ 13 C and δ 18 O records of the studied stalagmite depict a relatively high correlation coefficient (r 2 = 0.48), which raises concerns about the possibility that at least part of the data sets would not pass a Hendy test (Hendy, 1971) and would thus be more difficult to inter- pret in terms of a general change in climate. This also needs to be further documented by the authors to permit an unequivocal interpretation of the isotopic shift in their record. REFERENCES CITED Alley, R.B., Mayewski, P.A., Sowers, T., Stuiver, M., Taylor, K.C., and Clark, P.U., 1997, Holocene climatic instability: A prominent widespread event 8200 yr ago: Geology, v. 25, p. 483–486, doi: 10.1130/0091-7613(1997)025<0483: HCIAPW>2.3.CO;2. de Vernal, A., Hillaire-Marcel, C., von Grafenstein, U., and Barber, D., 1997, Researchers look for links among paleoclimate events: EOS (Transactions, American Geophysical Union), v. 78, p. 247–249. Hendy, C.H., 1971, The isotopic geochemistry of speleothems: The calculations of the effects of different modes of formation on the isotopic composition of speleothems and their applicability as paleoclimate indicators: Geochimica et Cosmochimica Acta, v. 35, no. 8, p. 801–824, doi: 10.1016/0016- 7037(71)90127-X. Lachniet, M.S., Asmerom, Y., Burns, S.J., Patterson, W.P., Polyak, V.J., and Seltzer, G.O., 2004, Tropical response to the 8200 yr B.P. cold event? Speleothem isotopes indicate a weakened early Holocene monsoon in Costa Rica: Geology, v. 32, p. 957–960, doi: 10.1130/G20797.1. Ludwig, K.R., and Titterington, D.M., 1994, Calculation of 230 Th/U isochrons, ages, and errors: Geochimica et Cosmochimica Acta, v. 58, p. 5031–5042, doi: 10.1016/0016-7037(94)90229-1. Luo, S., and Ku, T.L., 1991, U-series dating: A generalized method employing total sample dissolution: Geochimica et Cosmochimica Acta, v. 55, p. 555– 564, doi: 10.1016/0016-7037(91)90012-T. Rosholt, N., 1976, 230 Th/ 234 U dating of travertines and caliche rinds: Geological Society of America Abstracts with Programs, v. 8, p. 1079. REPLY Matthew S. Lachniet Department of Geosciences, MS-4010, University of Nevada, Las Vegas, Nevada 89154, USA Yemane Asmerom Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131, USA Tropical response to the 8200 yr B.P. cold event? Speleothem isotopes indicate a weakened early Holocene monsoon in Costa Rica: Comment and Reply 0 200 400 600 800 1000 0 2000 4000 6000 8000 10000 12000 230 Th/ 232 Th 234 U/ 232 Th y=(0.4±4.9) + [(0.0778±0.0011)x] MSWD = 22 8.79±0.13 ka 5.0±0.3 ka MSWD = 6.9 y=(4.4±5.1 + [(0.0445±0.0023)x] 10 100 1000 10 4 100 1000 10000 10 5 230 Th/ 232 Th 234 U/ 232 Th SAMPLES FROM 12 to 87 mm ABOVE THE BASE OF THE STALAGMITE ISOCHRON FROM SUBSAMPLES A,B,C AT BOTTOM OF THE STALAGMITE y=(0.4±4.9) + [(0.0778±0.0011)x] A B C Figure 1. Left: Isochron plots of two sets of samples from studied stalagmite (red—samples from bottom to 87 mm; blue—samples from 208 to 346 mm). Isochrons suggest two short phases of precipitation (ca 8.8 and 4.9 ka, respectively) for two sets of samples, possibly separated by a gap of close to 4 k.y. Right: Blow-up of the lower end of red isochron, defined by bottom subsamples A–C, showing that all samples between 12 and 87 mm fit on this isochron.