TRAVERTINE RECORDS CLIMATE-INDUCED TRANSFORMATIONS OF THE YELLOWSTONE CALDERA HYDROTHERMAL SYSTEM FROM THE LATE PLEISTOCENE TO THE PRESENT

The evolution of hot springs, as recorded by historical chemical analyses of thermal waters and their mineral deposits, provides a rare window into the evolution of Yellowstone’s postglacial hydrothermal system. Today, most hydrothermal travertine forms outside the 631-ka Yellowstone Caldera (Mammoth Hot Springs, Upper Snake River) where subsurface sedimentary strata provide Ca²⁺ and HCO₃^- that allow for calcite saturation of those thermal waters. In contrast, low-Ca silicic volcanic rocks dominate the subsurface lithology within Yellowstone Caldera, resulting in low calcium concentrations in thermal waters that rarely deposit travertine. We dated five small travertine deposits located in the Upper and Lower Geyser Basins that record intra-Caldera travertine deposition since ~14 ka. ²³⁰Th-U ages indicate three main pulses of travertine deposition: the first occurred in the late Pleistocene soon after Pinedale glacier recession between 13.9-13.6 ka; the second followed between 12.2-9.5 ka; and the third in the late Holocene between 5.2-2.9 ka. Travertine groups from different locations and ages have similar strontium and oxygen isotopic compositions. The chemistry and stable isotopic composition of late-glacial travertines indicate a substantial influx of glacial meltwater pre-Holocene. Additionally, the leaching of fresh, exotically sourced glacial outwash, till, and loess likely provided critical sources of calcium, magnesium, and bicarbonate to the shallow hydrothermal system. Travertine deposition may have been locally buoyed by increases in CO₂ due to seismic activity, but the main driving mechanism for all three periods of travertine deposition was the influx of large volumes of cold meteoric water that decreased the temperature of groundwater-rock reactions and increased the rate of chemical weathering of surficial sediments. Therefore, episodes of intra-Caldera travertine deposition required periods of high precipitation and infiltration into the Yellowstone hydrothermal system, making travertine an opportune record of climatic changes in the region. The small volume of intra-Caldera travertine suggests there was not a massive postglacial surge of CO₂ within Yellowstone Caldera nor is magmatic CO₂ a significant control on travertine deposition.