Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators of the Yellowstone Volcano Observatory. This week's contribution is from Sabrina Brown, Assistant Professor of Environmental Science at Defiance College.

The Greater Yellowstone Ecosystem (GYE), with Yellowstone Lake at its heart, is composed of diverse environments that have been sensitive to climate changes since the end of the last ice age, about 14-15 thousand years ago.  In order to reconstruct past climate and environmental conditions of Yellowstone Lake and its surrounding catchment area since ice receded from the region, scientists collected an 11.82 m (38.8 ft) sediment core (designated YL16-2C) from the northern part of the lake in 2016.  From that core, diatoms frustules were extracted—these are the hard and porous cell walls of diatoms, which are a form of algae—and the oxygen isotope composition was measured to reconstruct climate changes.  Additionally, changes in vegetation and fire history were documented from pollen and charcoal data in the sediment core, as well as diatom assemblages and biologically generated silica.

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Data from core YL16-2C provide a continuous record of the climate history of Yellowstone Lake’s basin from 9,900 years ago to nearly the present.  The results indicate that most ecosystem changes were gradual and caused by slow changes in the seasonal cycle of solar energy input.  This led to warm, dry summer conditions early in the record (9,900-6,300 years ago) and progressively changed to cooler and wetter conditions during more recent times.

The record also underlined periods of rapid environmental changes, which were probably caused by sudden changes in climate (temperature and/or precipitation).  In particular, a succession of climate fluctuations occurred between 7,000 and 6,800 years ago, and distinct warming occurred between 4,500 and 3,000 years ago, as well as 1,000 to 700 years ago—a period known as the Medieval Warm Period. These rapid climate fluctuations caused short-lived changes in algae and vegetation. Other research in the park found that a megadrought during the Medieval Warm Period caused Old Faithful to run dry for nearly 150 years!

In the early portion of the Yellowstone Lake sediment record (9,880 to 6,300 years ago), climate conditions resulted in an open forest, small and frequent fires, high evaporation rates in summer, early spring snowmelt, generally low nutrient availability, and early melting of ice from the lake.  The middle portion of the record (6,300 to 3,000 years ago) reflects a cooling climate, resulting in denser forest establishment and larger fire episodes, as well as less summer evaporation and longer spring runoff.  Further cooling and increased moisture in the most recent portion of the record (3,000 years ago to recent times) resulted in the development of a closed forest with infrequent but large fire episodes, decreased summer evaporation, and high runoff into the lake.  The climate history recorded in the lake sediment clearly shows how climate conditions shaped the ecosystem of the Yellowstone region!

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The data plainly demonstrate that Yellowstone Lake and its catchment area had a climate history typical of this region of the GYE, where conditions were warmer and drier after the last ice age due to higher solar energy input compared to more recent times.  The climate became cooler and wetter throughout the record, reaching the relatively low temperatures and wet conditions of the pre-industrial present.  Additionally, the sediment record shows that climate has a minimal influence on lake level, suggesting that inflation and deflation of Yellowstone Caldera may be more important than climate on water depth and shoreline development of Yellowstone Lake.

Although this sediment-core record provides an excellent record of long-term, gradual climate change since the last ice age, it does not have the resolution to reconstruct more recent climate change during the last few centuries.  For readers interested in learning more about the recent climate history of Yellowstone, the Greater Yellowstone Climate Assessment is an excellent resource.  Analysis of the sediment core, however, can provide insight into how vegetation and environmental conditions in the GYE might respond to future climate changes.  An in-depth knowledge of the past is an important tool for understanding the potential of the future.   

The combination of data from the Yellowstone Lake core provides clear evidence that the sensitive ecosystems of Yellowstone National Park have been substantially influenced by climate at various times.  If you would like to read more about this research, please see the article at https://www.sciencedirect.com/science/article/abs/pii/S0277379121004820?dgcid=author for details.