U.S. Geological Survey scientists are finding evidence of a much wetter past in the deserts of the American Southwest in an unlikely place: wetlands. Wetlands form in dry country where the water table reaches or breaks the surface, and the deposits they leave behind record how water and climate changed.
Wetlands in the geologic record
As the continental ice sheets grew and shrank in the late Pleistocene, worldwide changes in the atmosphere, living world and water cycle transformed the Southwest's deserts. Plant communities often shifted 1,000 meters or more downslope, lakes stood where only dry playas lie today, and large animals (megafauna) lived in places now too hot and dry for them. Cooler, wetter conditions also raised groundwater, feeding networks of marshes, wet meadows, seeps and springs in many low valleys and basins.
In a wetland, wind-blown, stream-borne and slope sediments are trapped by wet ground and dense plants. Over time they build up into paleowetland deposits, a mix of clastic sediment, chemical precipitates and organic matter that records when and how far water tables rose and fell.
Geologists have long relied on lake deposits to reconstruct the Southwest's past. Wetland deposits add to them, with several advantages:
- they give unambiguous evidence of past water tables, often placing the level of past discharge to within a few centimeters;
- they are often exposed over large areas, showing complex features a sediment core can miss;
- they can be dated by several methods, tying them to other climate records such as ice cores, marine and lake sediments, and cave deposits (speleothems);
- they are fairly common in the world's drylands, so past conditions can be rebuilt at many scales.
They are also archeological and fossil archives. Wetlands were dependable sources of fresh water in dry land, so people and animals gathered at them. The bones of large animals often survive in their deposits, letting researchers see how animal communities responded to past climate change and probe the links between megafauna extinctions, environmental change and the arrival of humans.
Las Vegas Valley: an ecosystem and climate change
Las Vegas, now ringed by harsh desert, was founded in the mid-19th century beside a few springs and marshes that gave early residents steady water. In prehistoric times the marshes, the "meadows" the city is named for, covered much of the valley. They were home to some of North America's earliest people and to Pleistocene animals including mammoth, sloth, bison, camel, horse, and even American lion and sabre-toothed cat.
Early science in the valley focused on its many archeological sites. In 1962, C. Vance Haynes, Jr., known for pioneering work on how people first reached North America and a founder of desert wetland research in the Southwest, began studying wetland deposits at the Tule Springs site in the Upper Las Vegas Wash: what drew ancient people there, how they survived southern Nevada's dry climate, and whether they lived alongside Pleistocene megafauna. Nearly half a century later, USGS scientists are still refining his geologic and chronological framework.
More recently the USGS and the San Bernardino County Museum, funded by the Bureau of Land Management, have studied paleowetland deposits in the Upper Las Vegas Wash, just outside the city. Museum scientists found and excavated hundreds of fossil sites in what is now Tule Springs Fossil Beds National Monument, and USGS scientists refined the layering and dating of the deposits to place the fossils in firm geologic context and see how the water system responded to past climate change.
Figure 2: Groundwater discharge in the Las Vegas Valley against the Greenland ice core record (higher values, to the right, are warmer). Wetlands grew and shrank in step with short northern-hemisphere climate swings: Dansgaard-Oeschger (D-O) cycles, the Bølling (B) and Allerød (A) warm periods and the Younger Dryas cold event (YD). USGS.
The results show that the valley's wetlands expanded and contracted many times as climate changed during the late Pleistocene. Rapid warming events stopped groundwater discharge and made whole wetland systems collapse, repeatedly. Drought-like spells, marked by widespread erosion and desert soils, usually lasted a few centuries, showing how vulnerable these fragile ecosystems are to climate change.
What comes next
USGS scientists are now studying wetland deposits elsewhere in the southern Great Basin and Mojave Desert to see how water tables responded to past climate across the region. The answers could help predict how these ecosystems will behave in future, and matter to those managing today's springs and wetlands, which are often home to threatened and endangered desert species.
Sources
- Jeffery S. Pigati, Kathleen B. Springer and Craig R. Manker, Desert wetlands—Archives of a wetter past, U.S. Geological Survey Fact Sheet 2015–3077. https://pubs.usgs.gov/publication/fs20153077
- Two photographs in the original, from Eric Scott and the San Bernardino County Museum, are not reproduced here.
Licence: CC0 1.0 (public domain) · Adapted from pubs.usgs.gov
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