Oxygen isotope composition of annually banded modern and mid-Holocene travertine and evidence of paleomonsoon floods, Grand Canyon, Arizona, USA

Holocene and modern travertine formed in spring-fed Havasu Creek of the Grand Canyon, Arizona, was studied to determine the factors governing its oxygen-isotope composition. Analysis of substrate-grown travertine indicates that calculated calcite-formation temperatures compare favorably with measured water temperatures, and include silt-rich laminae deposited by monsoon-driven floods. Ancient spring-pool travertine is dated by U-series at 7380 ± 110 yr and consists of 14 travertine-silt couplets of probable annual deposition. One hundred eighty high-resolution δ18O analyses of this mid-Holocene sample average −11.0‰ PDB. The average value for modern travertine is ∼0.5‰ lower, perhaps because mid-Holocene temperature was higher or there was proportionally greater summer recharge. δ18O cyclicity in the mid-Holocene travertine has average amplitude of 1.9 ± 0.5‰ PDB, slightly less than the inferred modern-day annual temperature range of Havasu Creek. The annual temperature range might have been reduced during the 14-yr interval compared to present, although other non-temperature factors could account for the muted annual variation. Silt-rich laminae within isotopically lower calcite in the modern and mid-Holocene travertine verifies the seasonal resolution of both samples, and suggests that similar temperature-precipitation conditions, as well as monsoon-generated summer floods, prevailed in the mid-Holocene as they do throughout the Grand Canyon region today.