MID-HOLOCENE CLIMATE IN SOUTHERN ARIZONA INFERRED FROM SPELEOTHEM STABLE ISOTOPES

We have collected stalagmites from Cave of the Bells (elevation 1700 m) located ~75 km southeast of Tucson, Arizona on the northeast side of the Santa Rita Mountains. High resolution (<10 years) δ¹⁸O data from a Holocene stalagmite (~4.5-8.2 ky BP, U-series chronology) exhibit enriched values relative to modern. Speleothem δ¹⁸O at this site should reflect a combination of temperature of formation and δ¹⁸O of the cave waters, controlled by temperature, amount, and seasonality of precipitation. Studies of modern cave water and precipitation at this site and regional groundwater studies indicate that most recharge is from winter moisture despite a summer monsoon that contributes ~50% of annual rainfall. We thus interpret higher stalagmite δ¹⁸O in the mid-Holocene to reflect warmer/drier conditions during the winter season, and we make no inferences regarding summer conditions. Previous work indicates contradictory paleohydrologic and paleoclimatic interpretations of the mid-Holocene and is often complicated by unclear seasonal dependences. Our results agree with the most common inference that the mid-Holocene was warmer and drier than present, and we tie these interpretations to winter (non-monsoon) conditions. Spectral analysis of the δ¹⁸O record reveals substantial multidecadal-multicentury variability, with a shift at ~5.5 ka from strength at a period of ~280 years in the early part of the record to a broad range at a higher frequency, around 120-70 years, in the more recent part. Evidence from in and around the tropical Pacific suggests an increase in the frequency of warm ENSO events after 5-7 ky BP. Today, warm ENSO events are associated with wetter winters in the southwest. Although our record does not resolve interannual variability, a shift to relatively higher frequency variability at ca. 5500 yr BP may be caused by shifts in ENSO. In the southwestern US, the mid-Holocene ENSO shift appears to be more closely tied to the frequency of climate variance than to substantial mean-state changes.