Editor's note: Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators of the Yellowstone Volcano Observatory. This week's contribution is from Lauren Harrison, assistant professor in the Department of Geosciences at Colorado State University.

Yellowstone National Park’s hydrothermal system has been powered 24 hours a day, 365 days a year for at least the last ~2 million years by the release of heat from the magmatic system in the subsurface. The presence of that heat source causes convection of meteoric water (from rain and snowmelt) that circulates between the magma reservoir and the surface. The activity and chemistry of the hydrothermal system is therefore controlled by inputs from both the surface (i.e., the amount of water and the temperature of shallow water-rock interactions) and from depth (the amount of heat, volatiles, or fluids supplied from crystallizing magma). You might imagine that as these variables change over time, the hydrothermal system at the surface also changes. But sorting out which surficial or deep input causes what change at the surface, especially when that change might have occurred hundreds to thousands of years ago, is difficult.

Hillside Springs group at the margin of the Summit Lake post-caldera rhyolite lava flow (the looming cliff) in Upper Geyser Basin, Yellowstone National Park. In the past few thousand years, the springs have occasionally deposited travertine, which can be found at the bottom of the hill below the active springs. This travertine, which is not being deposited today, is evidence that the climate was wetter off and on during the past 15,000 years, because travertine deposition within Yellowstone caldera correlates with periods of wetter climate.

Geologists addressed this challenge by investigating the puzzle of old travertine deposits located within Yellowstone caldera. Travertine, a hydrothermal deposit composed mostly of calcium carbonate (CaCO3) minerals, is found around Firehole Lake in Lower Geyser Basin and at Hillside Springs and near Morning Glory Spring in Upper Geyser Basin within Yellowstone caldera. The deposits are old—no travertine is being deposited today because most of the thermal waters in Upper and Lower Geyser Basin are saturated with silica (SiO2), not CaCO3. This means that the composition of thermal waters had to be different in the past to form travertine. Today, travertine actively forms north of the caldera at Mammoth Hot Springs, where waters saturated in CaCO3 have continuously been present for hundreds of thousands of years. But no travertine-forming waters have been observed within Upper and Lower Geyser basins today or for the last century.

Intrigued, scientists first investigated the age of the within-caldera travertines by measuring how much radioactive uranium, which was locked within the travertine when it formed, has decayed to thorium. Uranium-series decay is a useful method for dating some geological deposits. Most of the travertine found in the caldera formed during short periods within the last 15,000 years, during the Late Pleistocene to the late Holocene geologic epochs, with different deposits across the basins forming synchronously! Geologists also noticed that the chemistry of the deposits changed with time—from higher amounts of magnesium and a lower oxygen isotopic composition in the past, to the opposite in more recent samples. The isotopic composition of stable isotopes, like oxygen, are powerful tools used to trace the sources of waters. Low oxygen isotopic compositions are thought to be a result of cold climatic periods associated with major glaciations. So low oxygen isotopic compositions in the oldest travertine deposits indicates they formed from a glacial water source.

History of travertine deposition in Yellowstone caldera and correlation with past climate conditions. a) The age of travertine samples (based on the U-230Th geochronometer) from Old Hillside Springs, Hillside Springs, North Hillside Springs, and Morning Glory in Upper Geyser Basin and from Firehole Lake in Lower Geyser Basin. Travertine deposits formed during or shortly after cool and wet climatic periods. The Pinedale Glaciation was the last time ice covered Yellowstone Plateau, the Younger Dryas Cold Event was a global cold period thought to be caused by disruptions in ocean circulation, and the Yellowstone Lake Cool Period has been identified by climate and ecology scientists investigating Yellowstone Lake sediment cores. The gray histogram on the right side of the plot indicates the relative amount of travertine deposition that occurred over time. b,c,d) Photographs of sectioned and polished travertine under short-wave ultraviolet light, which causes the travertine to fluoresce different colors based on the trace metals that are found in different layers. Green is associated with high uranium concentrations, and pink is from manganese. Scale bars indicate 1 cm (0.4 in). b) Hillside Springs c) near Morning Glory Pool d) Firehole Lake. Photographs taken under research permit YELL-2021/2022-SCI-8192.

Second, scientists noticed that the timing of travertine deposits was coincident with known periods of cool, wet climate on the Yellowstone Plateau. The high amounts of precipitation during wet climatic times likely resulted in more chemical weathering of surficial sediments. Recharge to the hydrothermal system during these times is therefore more voluminous, colder, and carries more dissolved travertine-forming elements like calcium, magnesium, and bicarbonate. Mixing between these cool recharge waters with upflowing thermal water changes the thermal water chemistry for a short period of time in locations with the greatest mixing. This results in travertine deposits at the surface, creating a record of these cool climatic periods!

Schematic illustrating the conditions under which some travertine forms in Yellowstone caldera. Increased surficial weathering of glacial till and sediment during wet climate conditions causes greater infiltration of travertine ingredients (Ca2+, Mg2+, HCO3-) into thermal waters at the edges of Upper Geyser Basin (thin blue and yellow arrows) result in travertine deposition in those areas. Deeper sourced, hotter thermal water upflow continues in the center of Upper Geyser Basin and prevents travertine from forming in that area at any time (thick orange dashed arrows). Travertine deposits at Morning Glory and Hillside Springs are shown in pink.

Previously, investigation into Old Faithful and Steamboat geysers showed that their activity ceased during episodes of prolonged drought due to a lack of water supply to the hydrothermal system. The new study of the history of travertine deposition in Yellowstone caldera finds that the opposite condition can also impact geysers and hot springs: more water supply to the hydrothermal system results in changes to water chemistry and mineral deposits. The only constant in the Yellowstone hydrothermal system is change!