Quaternary Hydroclimate Records of Spring Ecosystems

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Desert springs and wetlands are among the most biologically productive, diverse, and fragile ecosystems on Earth. They are home to thousands of rare, endemic, and endangered plants and animals and reflect the availability and health of emergent groundwater. Despite the ecological importance of these wetlands, our knowledge of how they might respond to predicted future climate change is limited. This project seeks to understand how springs and wetlands in the Mojave Desert responded to past abrupt warming events, which may be analogs for future conditions. To accomplish this, USGS scientists will reconstruct the hydrologic history of ancient spring ecosystems and establish patterns and rates of change in groundwater availability by investigating paleowetland deposits dating to the latest Pleistocene and Holocene. The results will provide critical information for developing strategies aimed at managing these fragile ecosystems and mitigating the effects of climate change in the arid lands of the American Southwest.

Statement of Problem: The southwestern U.S. is the hottest and driest region in North America and is especially susceptible to drought. Climate models predict this region will become progressively hotter and drier throughout the remainder of the 21st century, and future droughts will be more frequent, intense, and longer lasting than what has been recorded thus far in the historical record. Higher temperatures and changes in the amount and spatial patterns of rainfall and snowpack will affect the lives and economies of more than 50 million people currently living in the region, forcing increased competition for limited water supplies between agriculture and energy sectors, urban populations, and plant and animal life. This presents a tremendous challenge for managing water resources for human consumption while anticipating and minimizing the potential impact on fragile desert ecosystems.

Why this Research is Important: Desert springs and wetlands provide windows into the health of aquifers and groundwater-fed ecosystems in arid environments. They are exceptionally sensitive to changes in climate and can act as harbingers for future conditions, but only if we quantitatively understand how they responded to abrupt climate events in the past. This study will establish the first regional-scale, multi-proxy hydroclimate record for the Mojave Desert that spans the time interval from the latest Pleistocene to Holocene and historical times (approximately the past 15,000 years). The results will provide critical input for federal, state, and local resource managers and other stakeholders for developing strategies aimed at protecting spring ecosystems in light of projected future conditions. In addition, the results will provide robust, ground-truthed data that can be used to test climate models and improve constraints on regional climate projections.

paleowetland deposits

Late Quaternary paleowetland deposits in the Rogers basin of Death Valley National Park, California. Photograph taken by Kathleen Springer in October 2018. Data from this site have not been published.

(Credit: Kathleen Springer, USGS. Public domain.)

Objective(s): The primary objective of this project is to understand how spring ecosystems responded to subtle and short-lived episodes of abrupt climate change during the recent geologic past. To accomplish this, we will document the response of desert spring ecosystems to climate variability and land-use changes during the late Quaternary. The resulting data will be synthesized to create the first regional-scale hydroclimate record for the southwestern U.S. that spans this time period. These efforts will contribute to the USGS national-scale effort to quantify Holocene hydroclimate conditions and provide critical context to inform current and future management and conservation of extant desert springs and wetlands.
 
Methods: We will reconstruct past hydroclimate conditions at multiple locations throughout the Mojave Desert by taking a novel approach that combines investigations of paleowetland deposits exposed in geological outcrops and sediment cores taken near active springs. The opportunity to combine these methodologies at sites that are in geographic proximity to one another is rare and will result in more complete and detailed high-resolution hydroclimate records than what could be obtained from either method alone. Sediments exposed in outcrops and in the cores will be described and measured, sampled for geophysical (grain size, density, magnetic susceptibility), geochemical (major and trace elements, stable isotopes), and biological (ostracodes, diatoms, and gastropods) analyses, and dated using a combination of radiocarbon, luminescence, and tephrochronologic techniques. The proxy data will be combined on common temporal scales and compared to existing high-resolution paleowetland records elsewhere in the region, as well as other sources of past climate information such as ice cores, lakes, speleothems, and marine records, to improve our understanding of ecosystem response to abrupt warming events.
 
scientists near Saratoga Springs, Death Valley

USGS scientists collecting sediment cores near Saratoga Springs of Death Valley National Park, California. Photograph taken by Jeff Pigati in February 2017. Data from this site have not been published.

(Credit: Jeff Pigati, USGS. Public domain.)