Geology topics

Sabrina R. Brown

Publications and source records attributed to Sabrina R. Brown.

5 recordsLinked to original sources

USGS2026

Postglacial ecosystem development of a hydrothermal landscape in Yellowstone National Park

The Yellowstone geo-ecosystem has been the subject of much research, but the ecological history of the Yellowstone Plateau volcanic field and its iconic geyser basins is less known. In this investigation, paleoenvironmental analyses of sediment cores from lakes in Lower Geyser Basin were compared with regional records and paleoclimate model simulations to reconstruct the vegetation, wildfire, limnology, hydrothermal dynamics, and climate drivers since deglaciation, 15,000 to 14,000 years ago. Pollen data from Lower Geyser Basin lakes reveal the strong influence of infertile rhyolitic soils on vegetation history: an initial late-glacial steppe was replaced by lodgepole pine forest from 12.8 to 11.0 ka, with little change in forest composition or cover thereafter despite changing climate. This stability contrasts with the more dynamic vegetation response on nonrhyolite substrates in the Yellowstone region where nutrient and moisture availability is greater. Highest wildfire activity and low lake nutrient levels in Lower Geyser Basin occurred from 12 to 4 ka, when summers were substantially warmer and drier and fire-inducing vapor pressure deficits were 29 to 56% higher. The hydrothermal history, inferred from sedimentary arsenic and cesium abundances, was spatially and temporally variable but lake-forming hydrothermal events align with periods of abundant moisture. Thus, long-term changes in wildfire, limnology, and, to some extent, hydrothermal activity were governed by insolation-driven climate variations, whereas the vegetation response was muted and constrained by geologic processes. These findings suggest that warmer, drier conditions in the future could result in less hydrothermal activity yet little change in forest cover across the Yellowstone Plateau volcanic field despite more wildfires.

Idaho, Montana, Wyoming↗

USGS2022

The dynamic floor of Yellowstone Lake, Wyoming, USA: The last 14 k.y. of hydrothermal explosions, venting, doming, and faulting

Hydrothermal explosions are significant potential hazards in Yellowstone National Park, Wyoming, USA. The northern Yellowstone Lake area hosts the three largest hydrothermal explosion craters known on Earth empowered by the highest heat flow values in Yellowstone and active seismicity and deformation. Geological and geochemical studies of eighteen sublacustrine cores provide the first detailed synthesis of the age, sedimentary facies, and origin of multiple hydrothermal explosion deposits. New tephrochronology and radiocarbon results provide a four-dimensional view of recent geologic activity since recession at ca. 15–14.5 ka of the >1-km-thick Pinedale ice sheet. The sedimentary record in Yellowstone Lake contains multiple hydrothermal explosion deposits ranging in age from ca. 13 ka to ~1860 CE. Hydrothermal explosions require a sudden drop in pressure resulting in rapid expansion of high-temperature fluids causing fragmentation, ejection, and crater formation; explosions may be initiated by seismicity, faulting, deformation, or rapid lake-level changes. Fallout and transport of ejecta produces distinct facies of subaqueous hydrothermal explosion deposits. Yellowstone hydrothermal systems are characterized by alkaline-Cl and/or vapor-dominated fluids that, respectively, produce alteration dominated by silica-smectite-chlorite or by kaolinite. Alkaline-Cl liquids flash to steam during hydrothermal explosions, producing much more energetic events than simple vapor expansion in vapor-dominated systems. Two enormous explosion events in Yellowstone Lake were triggered quite differently: Elliott’s Crater explosion resulted from a major seismic event (8 ka) that ruptured an impervious hydrothermal dome, whereas the Mary Bay explosion (13 ka) was triggered by a sudden drop in lake level stimulated by a seismic event, tsunami, and outlet channel erosion.

Wyoming↗

USGS2021

High elevation ice patch documents Holocene climate variability in the northern Rocky Mountains

Paleoclimate records from ice cores generally are considered to be the most direct indicators of environmental change, but are rare from mid-latitude, continental regions such as the western United States. High-elevation ice patches are known to be important archaeological archives in alpine regions and potentially could provide records important for Earth System Model evaluation and to understand linkages between climate and early human activities, but this potential largely is unexplored. Here we use a well-dated ice-core record from a shallow ice patch to investigate Rocky Mountain winter-season climate during the Holocene. Our records indicate that this ice patch consistently accumulated ice over the past 10 kyr, preserving a regionally representative climate record of stable water isotopes and ice accretion rates that documented generally cooler and wetter conditions during the early Holocene and 500 years of anomalous winter season warmth centered at 4100 cal yr BP followed by a rapid cooling and 1500 years of cooler and wetter winters.

Montana, Colorado, Utah, Wyoming↗

USGS2021

Multi-proxy record of Holocene paleoenvironmental conditions from Yellowstone Lake, Wyoming, USA

A composite 11.82 m-long (9876–-67 cal yr BP) sediment record from Yellowstone Lake, Wyoming was analyzed using a robust set of biological and geochemical proxies to investigate the paleoenvironmental evolution of the lake and its catchment in response to long-term climate forcing. Oxygen isotopes from diatom frustules were analyzed to reconstruct Holocene climate changes, and pollen, charcoal, diatom assemblages, and biogenic silica provided information on terrestrial and limnological responses. The long-term trends recorded in the terrestrial and limnic ecosystems over the last 9800 years reflect the influence of changes in the amplification of the seasonal cycle of insolation on regional climate. The early Holocene (9880–6700 cal yr BP) summer insolation maximum and strengthening of the northeastern Pacific subtropical high-pressure system created warm dry conditions and decreasing summer insolation in the middle (6700–3000 cal yr BP) and late (3000–-67 cal yr BP) Holocene resulted in progressively cooler, wetter conditions. Submillenial climate variation is also apparent, with a wetter/cooler interval between 7000 and 6800 cal yr BP and warmer and/or drier conditions from 4500 to 3000 cal yr BP and at ca. 1100 cal yr BP. These data show that the Yellowstone Lake basin had a climate history typical of a summer-dry region, which helps to better define the spatial variability of Holocene climate in the Greater Yellowstone Ecosystem.

Wyoming↗

USGS2020

Fire history across forest types in the southern Beartooth Mountains, Wyoming

Fire is a critical ecosystem process that has played a key role in shaping forests throughout the Beartooth Mountains in northwestern Wyoming. The highly variable topography of the area provides ideal conditions to compare fire regimes across contiguous forest types, yet pyro-dendrochronological research in this area is limited. We reconstructed fire frequency, tree age structure, and post-fire tree growth response in the Clarks Fork Ranger District of the Shoshone National Forest to infer variations in historical fire behavior and stand effects. We collected fire-scarred trees and plot-based tree ages on plots ranging 0.5-5 km 2 in size across two forest types separated by 2 km: a lower-elevation forest of mixed Douglas-fir and lodgepole pine and a higher elevation treeline forest dominated by whitebark pine. Fires occurred in the lower-elevation forest in 1664, 1706, 1785, 1804, 1846, and 1900 with a mean fire return interval of 47 years. The fires in 1804 and 1900 were also recorded in the higher elevation forest, with significant tree mortality at high elevation in the 1900 fire. Both forests were multi-aged with little evidence of tree cohorts in response to severe, stand-replacing events. On average, tree growth increased after fires, with mean ring widths after fire 39% wider in Douglas-fir and 40% wider in lodgepole pine than pre-fire averages, suggesting that some tree mortality likely occurred in association with lower-elevation forest fires. Burns were more frequent in the lower-elevation forest and were occasionally able to spread into the upper-elevation whitebark stand. Although we suspect the transition of fires from low-to high-elevation occurred during drier years, we did not find any relationship between fire years and available climatic reconstructions via superposed epoch analysis. Regeneration during the 20 th Century in the whitebark forest documents recovery of this forest after the 1900 moderate-severity fire event. Finally, especially in the lower-elevation Douglas-fir forest, the period since the last recorded fire (1900) appears to be longer than any fire-free period in the historical record, suggesting that fire exclusion may be creating changes in landscape and patch-scale stand structures, which will likely impact future fire behavior, especially the extent of crown-replacing fire, in these forests.

Wyoming↗