SEASONAL δ18O RECORD FROM ANNUALLY BANDED, MODERN AND MID HOLOCENE TRAVERTINE—THERMOMETRY AND EVIDENCE OF PALEOMONSOON FLOODS, GRAND CANYON, USA

Modern and mid-Holocene travertine deposited in spring-fed Havasu Creek, Grand Canyon, USA, was studied to determine its suitability for O-isotope thermometry. δ¹⁸O_water of spring water and δ¹⁸O_calcite from fluvial-dam travertine grown on substrates yielded calcite precipitation temperatures within 1-3°C of measured water temperatures, which reflect seasonal air temperatures. Mid-Holocene travertine from a former spring-fed pool was dated by U-series at 7380 ± 110 yr and is physically and isotopically similar to the substrate-grown travertine. Both contain silty laminae deposited during monsoon-related floods, which are associated with calcite that is isotopically depleted relative to adjacent, visibly pure calcite laminae. The Holocene travertine contains up to four such silty laminae per year in each of 14 annual couplets. δ¹⁸O_calcite varies cyclically through each couplet with averaged maxima (-10.2 ± 0.4‰) and minima (-12.0 ± 0.5‰) that correspond to pure and silt-rich calcite, respectively. Averaged ranges for all couplets (δ¹⁸O_calcite = 1.9 ± 0.5‰) represents a seasonal water temperature range (9°C) that falls within the expected modern water temperature range of Havasu Creek, but is ~5°C lower than the average seasonal range measured between 1990 and 1993. Despite the short instrumental record and annual weather-dependent variability, the discrepancy could be caused by surface runoff or diagenetic effects, or may indicate that the seasonal temperature range during the mid-Holocene was smaller than today. All analyses (n = 180) average -11.0‰, which is slightly enriched relative to the modern substrate-grown travertine (-11.6‰). This could indicate that atmospheric condensation temperature was higher at 7.4 ka, or δ¹⁸O_water was higher, perhaps due to proportionally higher summer recharge. Our study demonstrates the potential of using fluvial travertine to provide paleoclimate records in the Grand Canyon region, and shows that summer monsoon-driven floods like those of today are likely to have occurred during the mid-Holocene.