Paleohydrology of Desert Wetlands Completed
Springs and wetlands are among the most highly threatened ecosystems on Earth. Although geographically limited, they support more than 20% of all the threatened and endangered species in the United States. Scientists from the U.S. Geological Survey are examining the rock record to determine how springs and wetlands responded to abrupt climate change during prehistoric times and the recent geologic past. The results will inform conservation efforts aimed at protecting these fragile ecosystems in light of future climate warming.
What are desert wetlands? And how do they form?
Desert wetlands occur where groundwater reaches the surface in arid environments, usually in valley bottoms and on shallow hill slopes. They include point source discharge features, such as seeps and springs, as well as more dispersed discharge, such as marshes and wet meadows. The water that feeds the springs comes from precipitation that falls on surrounding mountains, which infiltrates the rocks and eventually makes its way to the surface by natural flow or by faulting.
Wetlands are common features in deserts around the world, and support a wide array of life. When active, they serve as important watering holes for humans and animals alike, support vegetation that depends on access to groundwater for survival, and act as catchments for eolian and alluvial sediments.
Why is this research important?
Desert wetlands are among the most biologically diverse ecosystems in the world. Thousands of endangered, threatened, and endemic species all call wetlands their home. They also play an integral role in migration corridors, providing oases for animals crossing the harsh desert landscape.
Despite their ecological importance, our knowledge and understanding of springs and wetlands is extremely limited. Only a handful of the nearly 15,000 springs that have been identified in the southwestern U.S. are currently being monitored. Even fewer (virtually none) have monitoring data going back beyond the past few years. In order to understand how springs responded to climate change in the past, we need to turn to the geologic record.
How do we study wetlands that no longer exist?
Traditionally, geologists have relied heavily on lake deposits to reconstruct the magnitude and timing of environmental changes that took place in the American Southwest. Wetland deposits contain complementary information to lakes and offer several advantages. First, they provide unequivocal evidence of past water-table levels. In many instances, the elevation of past discharge can be identified to within a few centimeters. Second, they are often exposed over fairly large areas, which affords researchers the opportunity to examine and interpret complex stratigraphic relations and sedimentological features that can be missed in sediment cores. Third, they can be dated by multiple techniques and, therefore, hydrologic and climatic conditions recorded by wetlands can be tied directly to other climate records, such as ice cores, marine and lake sediments, and speleothems (cave deposits). Fourth, we can recognize specific hydrologic flow regimes in the geologic record, including rheocrene (streams), limnocrene (ponds), and helocrene (marshes) flow. Finally, they are relatively common in the world’s arid lands and can be used to reconstruct past conditions on a variety of spatial and temporal scales.
Geologic deposits associated with springs and desert wetlands can be incredibly complex, exhibiting subtle variations in color, texture, and grain size that tell us about what the landscape was like at different times in the past.
The majority of our field work is done at the outcrop scale. Sedimentary features and contacts are described in the field, and units are traced across the landscape. This can be a tricky business as some of the units are similar in appearance and are often inset within or positioned adjacent to each another.
Many of the larger wetland systems in the southern Great Basin and Mojave Deserts dried up and disappeared during the early Holocene, roughly 8,000 years ago. To understand how desert wetlands have responded to subtle changes in climate since then, we drill sediment cores near extant (or active) springs and wetlands. During wetter times, water table rise, and wetlands expand. Conversely, when conditions become dry, the water table drops, and wetlands contract or collapse altogether. These cycles can be observed in the rock record and reveal how sensitive these systems are to climate change.
How do we date the deposits?
We use two different techniques, radiocarbon and luminescence, to determine the ages of the wetland units. Radiocarbon (or 14C) dating of charcoal, small terrestrial gastropod shells, and organic matter is used to date sediments that are as much as 40,000 years old. We use luminescence dating, specifically infrared-stimulated luminescence (or IRSL), to date sediments that are beyond this limit.
And then what?
Once the stratigraphy and chronology are firmly established for a given area, we use a number of "proxy" techniques to reconstruct various aspects of the paleoenvironment.
- Microfaunal assemblages (ostracodes, mollusks), tufa morphology and lineaments, and the types of sediments and bedforms present in the geologic record allow us to reconstruct past hydrologic flow regimes.
- Stable and clumped isotopes tell us about temperature, relative humidity, and the isotopic content of groundwater in the past.
- Pollen, plant macrofossils, and charred wood provide direct evidence of the types of plants that once inhabited the area.
- Fossilized remains of extinct megafauna (mammoth, sloth, bison, horse, camel, etc.) provide opportunities to understand how faunal communities responded to past climate change in arid environments, as well as to investigate the potential ties between Pleistocene extinctions, the arrival of humans, and changes in paleoenvironmental conditions.
Results
A new way of looking at past hydrologic changes
Often thought of as stagnant and unchanging, new research suggests that springs and wetlands in the American Southwest responded dynamically to past episodes of abrupt climate change. Water tables rose and fell in response to climatic perturbations, affecting the growth and collapse of the wetlands. In the Las Vegas Valley, for example, our research group has found that wetland deposits provide a detailed and nearly complete record of dynamic hydrologic changes for the past 35,000 years, including cycles of wetland expansion, contraction, and even total ecosystem collapse when conditions became too hot/dry.
Study Locations
Recent geologic mapping by the U.S. Geological Survey has identified hundreds of localities in the southern Great Basin and Mojave Deserts that exhibit evidence of ancient springs and wetlands. Dozens, if not hundreds, of additional localities are scattered throughout the Sonoran and Chihuahuan Deserts. Scientific examination of wetland sites throughout the American Southwest will reveal important information on the sensitivity of these systems to climate change, the sources of moisture that fed the extinct wetlands, and allow us to ground truth (and thus improve) climate models.
1. Las Vegas Valley
Las Vegas is the linchpin for paleowetland studies in the southwestern U.S. The nearby mountains are exceptionally high (Mt. Charleston to the west reaches 3632 meters above sea level), which captures abundant precipitation, supplying groundwater to the valley during both glacial and interglacial periods.
Geologic research has gone on in Las Vegas for more than half a century, beginning in earnest with the "Big Dig" of 1962-63. Dr. C. Vance Haynes, Jr., directed the geologic work at that time, using large earth moving equipment to cut deep trenches to view and map the complex stratigraphic relations of the deposits without the interference of naturally eroded topography.
Geologic research in the valley resumed in the 1990s when scientists from the San Bernardino County Museum (SBCM) began working in the upper Las Vegas Wash. These efforts, which continued into the 2000’s, were initially related to paleontologic mitigation associated with Bureau of Land Management (BLM) land transfers and construction activities. Subsequent field investigations between 2008 and 2014 by the SBCM and USGS combined geologic mapping and detailed stratigraphic analyses with expanded and better-defined hydrologic interpretations and more refined chronology, resulting in the formal designations of the Las Vegas Formation and Tule Springs Local Fauna.
Ultimately, the combination of the initial protection of these fossil-rich lands by the BLM, the excitement by the general public and political powers over the fossil discoveries and ongoing scientific studies, and an enthusiastic and steadfast advocacy group led to the designation of Tule Springs Fossil Beds National Monument (or TUSK) in 2014. The designation protects these lands in perpetuity.
2. Casting the net wider—southern Great Basin and Mojave Deserts
We are able to recognize many units within the Las Vegas Formation in wetland deposits elsewhere in the southern Great Basin and Mojave Deserts, including Valley Wells, Piute Valley, Mesquite Spring, Lathrop Wells, and Dove Spring. Ongoing studies are aimed at determining if local watersheds responded in unison to changes in climate over regional spatial scales.
3. And wider still—Sonoran and Chihuahuan Deserts
Wetlands in southeastern Arizona, near the border between the Sonoran and Chihuahuan Deserts, exhibit many of the same features as those seen at sites far to the west. By comparing the magnitude and timing of changes in the local water tables to those in the southern Great Basin and Mojave Deserts, we may be able to determine the relative contributions of westerly and southerly moisture sources, a key unknown in modeling hydrologic conditions in the past.
Key unanswered questions
We are now studying similar deposits at sites throughout the southwestern U.S. to determine how local watersheds responded to past changes in climate on both local and regional scales. Our goal is to be able to provide answers to several outstanding questions:
- How sensitive are desert wetlands to climate change and at what temporal resolution can we detect this change?
- What made wetland ecosystems collapse entirely at times during the late Pleistocene? And how long were they absent from the landscape before they were reactivated?
- What were conditions like when they became active again?
- What types of hydrologic conditions and subsurface structures dictate spring development?
- Were springs and desert wetlands still around during the Holocene?
- How did wetlands respond to recent climatic events, such as the Little Ice Age?
- How might active springs respond to future climate change?
- What does this tell us about arid land springs and wetlands in other parts of the world?
Below are publications associated with this project.
The Great Acceleration and the disappearing surficial geologic record
The Tule Springs local fauna: Rancholabrean vertebrates from the Las Vegas Formation, Nevada
Activation of a small ephemeral lake in southern Jordan during the last full glacial period and its paleoclimatic implications
Pliocene-Pleistocene water bodies and associated geologic deposits in Southern Israel and Southern Jordan
Vertebrate paleontology, stratigraphy, and paleohydrology of Tule Springs Fossil Beds National Monument, Nevada (USA)
Geology and vertebrate paleontology of Tule Springs Fossil Beds National Monument, Nevada, USA
First records of Canis dirus and Smilodon fatalis from the late Pleistocene Tule Springs local fauna, upper Las Vegas Wash, Nevada
Hydrologic response of desert wetlands to Holocene climate change: preliminary results from the Soda Springs area, Mojave National Preserve, California
The Bear River's history and diversion: Constraints, unsolved problems, and implications for the Lake Bonneville record: Chapter 2
Desert wetlands—Archives of a wetter past
Dynamic response of desert wetlands to abrupt climate change
Directly dated MIS 3 lake-level record from Lake Manix, Mojave Desert, California, USA
Below are partners associated with this project.
- Overview
Springs and wetlands are among the most highly threatened ecosystems on Earth. Although geographically limited, they support more than 20% of all the threatened and endangered species in the United States. Scientists from the U.S. Geological Survey are examining the rock record to determine how springs and wetlands responded to abrupt climate change during prehistoric times and the recent geologic past. The results will inform conservation efforts aimed at protecting these fragile ecosystems in light of future climate warming.
What are desert wetlands? And how do they form?
Desert wetlands occur where groundwater reaches the surface in arid environments, usually in valley bottoms and on shallow hill slopes. They include point source discharge features, such as seeps and springs, as well as more dispersed discharge, such as marshes and wet meadows. The water that feeds the springs comes from precipitation that falls on surrounding mountains, which infiltrates the rocks and eventually makes its way to the surface by natural flow or by faulting.
Wetlands are common features in deserts around the world, and support a wide array of life. When active, they serve as important watering holes for humans and animals alike, support vegetation that depends on access to groundwater for survival, and act as catchments for eolian and alluvial sediments.
Why is this research important?
Desert wetlands are among the most biologically diverse ecosystems in the world. Thousands of endangered, threatened, and endemic species all call wetlands their home. They also play an integral role in migration corridors, providing oases for animals crossing the harsh desert landscape.
Despite their ecological importance, our knowledge and understanding of springs and wetlands is extremely limited. Only a handful of the nearly 15,000 springs that have been identified in the southwestern U.S. are currently being monitored. Even fewer (virtually none) have monitoring data going back beyond the past few years. In order to understand how springs responded to climate change in the past, we need to turn to the geologic record.
How do we study wetlands that no longer exist?
Traditionally, geologists have relied heavily on lake deposits to reconstruct the magnitude and timing of environmental changes that took place in the American Southwest. Wetland deposits contain complementary information to lakes and offer several advantages. First, they provide unequivocal evidence of past water-table levels. In many instances, the elevation of past discharge can be identified to within a few centimeters. Second, they are often exposed over fairly large areas, which affords researchers the opportunity to examine and interpret complex stratigraphic relations and sedimentological features that can be missed in sediment cores. Third, they can be dated by multiple techniques and, therefore, hydrologic and climatic conditions recorded by wetlands can be tied directly to other climate records, such as ice cores, marine and lake sediments, and speleothems (cave deposits). Fourth, we can recognize specific hydrologic flow regimes in the geologic record, including rheocrene (streams), limnocrene (ponds), and helocrene (marshes) flow. Finally, they are relatively common in the world’s arid lands and can be used to reconstruct past conditions on a variety of spatial and temporal scales.
Geologic deposits associated with springs and desert wetlands can be incredibly complex, exhibiting subtle variations in color, texture, and grain size that tell us about what the landscape was like at different times in the past.
The majority of our field work is done at the outcrop scale. Sedimentary features and contacts are described in the field, and units are traced across the landscape. This can be a tricky business as some of the units are similar in appearance and are often inset within or positioned adjacent to each another.
Many of the larger wetland systems in the southern Great Basin and Mojave Deserts dried up and disappeared during the early Holocene, roughly 8,000 years ago. To understand how desert wetlands have responded to subtle changes in climate since then, we drill sediment cores near extant (or active) springs and wetlands. During wetter times, water table rise, and wetlands expand. Conversely, when conditions become dry, the water table drops, and wetlands contract or collapse altogether. These cycles can be observed in the rock record and reveal how sensitive these systems are to climate change.
How do we date the deposits?
We use two different techniques, radiocarbon and luminescence, to determine the ages of the wetland units. Radiocarbon (or 14C) dating of charcoal, small terrestrial gastropod shells, and organic matter is used to date sediments that are as much as 40,000 years old. We use luminescence dating, specifically infrared-stimulated luminescence (or IRSL), to date sediments that are beyond this limit.
And then what?
Once the stratigraphy and chronology are firmly established for a given area, we use a number of "proxy" techniques to reconstruct various aspects of the paleoenvironment.
- Microfaunal assemblages (ostracodes, mollusks), tufa morphology and lineaments, and the types of sediments and bedforms present in the geologic record allow us to reconstruct past hydrologic flow regimes.
- Stable and clumped isotopes tell us about temperature, relative humidity, and the isotopic content of groundwater in the past.
- Pollen, plant macrofossils, and charred wood provide direct evidence of the types of plants that once inhabited the area.
- Fossilized remains of extinct megafauna (mammoth, sloth, bison, horse, camel, etc.) provide opportunities to understand how faunal communities responded to past climate change in arid environments, as well as to investigate the potential ties between Pleistocene extinctions, the arrival of humans, and changes in paleoenvironmental conditions.
Results
A new way of looking at past hydrologic changes
Often thought of as stagnant and unchanging, new research suggests that springs and wetlands in the American Southwest responded dynamically to past episodes of abrupt climate change. Water tables rose and fell in response to climatic perturbations, affecting the growth and collapse of the wetlands. In the Las Vegas Valley, for example, our research group has found that wetland deposits provide a detailed and nearly complete record of dynamic hydrologic changes for the past 35,000 years, including cycles of wetland expansion, contraction, and even total ecosystem collapse when conditions became too hot/dry.
Study Locations
Recent geologic mapping by the U.S. Geological Survey has identified hundreds of localities in the southern Great Basin and Mojave Deserts that exhibit evidence of ancient springs and wetlands. Dozens, if not hundreds, of additional localities are scattered throughout the Sonoran and Chihuahuan Deserts. Scientific examination of wetland sites throughout the American Southwest will reveal important information on the sensitivity of these systems to climate change, the sources of moisture that fed the extinct wetlands, and allow us to ground truth (and thus improve) climate models.
1. Las Vegas Valley
Las Vegas is the linchpin for paleowetland studies in the southwestern U.S. The nearby mountains are exceptionally high (Mt. Charleston to the west reaches 3632 meters above sea level), which captures abundant precipitation, supplying groundwater to the valley during both glacial and interglacial periods.
Geologic research has gone on in Las Vegas for more than half a century, beginning in earnest with the "Big Dig" of 1962-63. Dr. C. Vance Haynes, Jr., directed the geologic work at that time, using large earth moving equipment to cut deep trenches to view and map the complex stratigraphic relations of the deposits without the interference of naturally eroded topography.
Geologic research in the valley resumed in the 1990s when scientists from the San Bernardino County Museum (SBCM) began working in the upper Las Vegas Wash. These efforts, which continued into the 2000’s, were initially related to paleontologic mitigation associated with Bureau of Land Management (BLM) land transfers and construction activities. Subsequent field investigations between 2008 and 2014 by the SBCM and USGS combined geologic mapping and detailed stratigraphic analyses with expanded and better-defined hydrologic interpretations and more refined chronology, resulting in the formal designations of the Las Vegas Formation and Tule Springs Local Fauna.
Ultimately, the combination of the initial protection of these fossil-rich lands by the BLM, the excitement by the general public and political powers over the fossil discoveries and ongoing scientific studies, and an enthusiastic and steadfast advocacy group led to the designation of Tule Springs Fossil Beds National Monument (or TUSK) in 2014. The designation protects these lands in perpetuity.
2. Casting the net wider—southern Great Basin and Mojave Deserts
We are able to recognize many units within the Las Vegas Formation in wetland deposits elsewhere in the southern Great Basin and Mojave Deserts, including Valley Wells, Piute Valley, Mesquite Spring, Lathrop Wells, and Dove Spring. Ongoing studies are aimed at determining if local watersheds responded in unison to changes in climate over regional spatial scales.
3. And wider still—Sonoran and Chihuahuan Deserts
Wetlands in southeastern Arizona, near the border between the Sonoran and Chihuahuan Deserts, exhibit many of the same features as those seen at sites far to the west. By comparing the magnitude and timing of changes in the local water tables to those in the southern Great Basin and Mojave Deserts, we may be able to determine the relative contributions of westerly and southerly moisture sources, a key unknown in modeling hydrologic conditions in the past.
Key unanswered questions
We are now studying similar deposits at sites throughout the southwestern U.S. to determine how local watersheds responded to past changes in climate on both local and regional scales. Our goal is to be able to provide answers to several outstanding questions:
- How sensitive are desert wetlands to climate change and at what temporal resolution can we detect this change?
- What made wetland ecosystems collapse entirely at times during the late Pleistocene? And how long were they absent from the landscape before they were reactivated?
- What were conditions like when they became active again?
- What types of hydrologic conditions and subsurface structures dictate spring development?
- Were springs and desert wetlands still around during the Holocene?
- How did wetlands respond to recent climatic events, such as the Little Ice Age?
- How might active springs respond to future climate change?
- What does this tell us about arid land springs and wetlands in other parts of the world?
- Publications
Below are publications associated with this project.
Filter Total Items: 37The Great Acceleration and the disappearing surficial geologic record
The surficial geologic record is the relatively thin veneer of young (<~1 Ma) and mostly unconsolidated sediments that cover portions of Earth’s terrestrial surface (Fig. 1). Once largely ignored as “overburden” by geologists, surficial deposits are now studied to address a wide range of issues related to the sustainability of human societies. Geologists use surficial deposits to determine the freAuthorsJason A. Rech, Kathleen B. Springer, Jeffrey S. PigatiThe Tule Springs local fauna: Rancholabrean vertebrates from the Las Vegas Formation, Nevada
A middle to late Pleistocene sedimentary sequence in the upper Las Vegas Wash, north of Las Vegas, Nevada, has yielded the largest open-site Rancholabrean vertebrate fossil assemblage in the southern Great Basin and Mojave Deserts. Recent paleontologic field studies have led to the discovery of hundreds of fossil localities and specimens, greatly extending the geographic and temporal footprint ofAuthorsEric Scott, Kathleen B. Springer, James C. SagebielActivation of a small ephemeral lake in southern Jordan during the last full glacial period and its paleoclimatic implications
Playas, or ephemeral lakes, are one of the most common depositional environments in arid and semiarid lands worldwide. Playa deposits, however, have mostly been avoided as paleoclimatic archives because they typically contain exceptionally low concentrations of organic material, making 14C dating difficult. Here, we describe a technique for concentrating organic matter in sediments for radiocarbonAuthorsGentry A. Catlett, Jason A. Rech, Jeffrey S. Pigati, Mustafa Al Kuisi, Shanying Li, Jeffrey S. HonkePliocene-Pleistocene water bodies and associated geologic deposits in Southern Israel and Southern Jordan
No abstract available.AuthorsJason A. Rech, Hanan Ginat, Gentry Catlett, Steffen Mischke, Emily Winer-Tully, Jeffrey S. PigatiVertebrate paleontology, stratigraphy, and paleohydrology of Tule Springs Fossil Beds National Monument, Nevada (USA)
Tule Springs Fossil Beds National Monument (TUSK) preserves 22,650 acres of the upper Las Vegas Wash in the northern Las Vegas Valley (Nevada, USA). TUSK is home to extensive and stratigraphically complex groundwater discharge (GWD) deposits, called the Las Vegas Formation, which represent springs and desert wetlands that covered much of the valley during the late Quaternary. The GWD deposits recoAuthorsKathleen B. Springer, Jeffery S. Pigati, Eric ScottGeology and vertebrate paleontology of Tule Springs Fossil Beds National Monument, Nevada, USA
Tule Springs Fossil Beds National Monument (TUSK) preserves 22,650 acres of the upper Las Vegas Wash in the northern Las Vegas Valley, Nevada, USA. TUSK is home to extensive and stratigraphically complex groundwater discharge (GWD) deposits, called the Las Vegas Formation, which represent springs and desert wetlands that covered much of the valley during the late Quaternary. The GWD deposits recorAuthorsKathleen B. Springer, Jeffrey S. Pigati, Eric ScottFirst records of Canis dirus and Smilodon fatalis from the late Pleistocene Tule Springs local fauna, upper Las Vegas Wash, Nevada
Late Pleistocene groundwater discharge deposits (paleowetlands) in the upper Las Vegas Wash north of Las Vegas, Nevada, have yielded an abundant and diverse vertebrate fossil assemblage, the Tule Springs local fauna (TSLF). The TSLF is the largest open-site vertebrate fossil assemblage dating to the Rancholabrean North American Land Mammal Age in the southern Great Basin and Mojave Desert. Over 60AuthorsEric Scott, Kathleen B. SpringerHydrologic response of desert wetlands to Holocene climate change: preliminary results from the Soda Springs area, Mojave National Preserve, California
Desert wetlands are common features in arid environments and include a variety of hydrologic facies, including seeps, springs, marshes, wet meadows, ponds, and spring pools. Wet ground conditions and dense stands of vegetation in these settings combine to trap eolian, alluvial, and fluvial sediments that accumulate over time. The resulting deposits are collectively called ground-water dischargeAuthorsJeffrey S. Pigati, Marith C. Reheis, John P. McGeehin, Jeffrey S. Honke, J. BrightThe Bear River's history and diversion: Constraints, unsolved problems, and implications for the Lake Bonneville record: Chapter 2
The shifting course of the Bear River has influenced the hydrologic balance of the Bonneville basin through time, including the magnitude of Lake Bonneville. This was first recognized by G.K. Gilbert and addressed in the early work of Robert Bright, who focused on the southeastern Idaho region of Gem Valley and Oneida Narrows. In this chapter, we summarize and evaluate existing knowledge from thisAuthorsJoel L. Pederson, Susanne U. Janecke, Marith C. Reheis, Darrell S. Kaufmann, Robert Q. OaksDesert wetlands—Archives of a wetter past
Scientists from the U.S. Geological Survey (USGS) are finding evidence of a much wetter past in the deserts of the American Southwest using a most unlikely source—wetlands. Wetlands form in arid environments where water tables approach or breach the ground surface. Often thought of as stagnant and unchanging, new evidence suggests that springs and wetlands responded dynamically to past episodes ofAuthorsJeffery S. Pigati, Kathleen B. Springer, Craig R. MankerDynamic response of desert wetlands to abrupt climate change
Desert wetlands are keystone ecosystems in arid environments and are preserved in the geologic record as groundwater discharge (GWD) deposits. GWD deposits are inherently discontinuous and stratigraphically complex, which has limited our understanding of how desert wetlands responded to past episodes of rapid climate change. Previous studies have shown that wetlands responded to climate change onAuthorsKathleen B. Springer, Craig R. Manker, Jeffrey S. PigatiDirectly dated MIS 3 lake-level record from Lake Manix, Mojave Desert, California, USA
An outcrop-based lake-level curve, constrained by ~ 70 calibrated 14C ages on Anodonta shells, indicates at least 8 highstands between 45 and 25 cal ka BP within 10 m of the 543-m upper threshold of Lake Manix in the Mojave Desert of southern California. Correlations of Manix highstands with ice, marine, and speleothem records suggest that at least the youngest three highstands coincide with DansgAuthorsMarith C. Reheis, David M. Miller, John P. McGeehin, Joanna R. Redwine, Charles G. Oviatt, Jordon E. Bright - Partners
Below are partners associated with this project.