Geosites

The Holocene Great Salt Lake and Pleistocene Lake Bonneville System: Conserving our Geoheritage for Future Generations

Marjorie Chan — 2024-01-14

The modern (Holocene-age) Great Salt Lake (GSL) and Pleistocene Lake Bonneville of the Bonneville Basin (BB) together make a geosite (GSL-BB system) of exceptional scientific, cultural, aesthetic, and societal value. GSL is the largest saline lake in the Western Hemisphere and a sensitive recorder of climate. For millennia, this distinctive salty water body has been a dynamic and complex natural ecosystem, including an important waterway for birds and other wildlife and an archive of environmental change and history. Lake Bonneville is a seminal part of the history of science in the United States through the work of G.K. Gilbert, who in the 1870s and 1880s developed both critical scientific concepts (e.g., isostasy) and methods (e.g., multiple working hypotheses), which are still employed today. GSL is a major tourist attraction, an economic driver, and a place of scientific exploration. Yet today, the GSL is in grave danger of near total desiccation due to a combination of factors: human removal of waters that would normally replenish the lake, climate change, and other environmental pressures. Over the past few decades there has been a growing international movement to recognize and respect our geoheritage, by raising visibility and protection of high-priority geosites. The GSLBB system is a geoheritage site that urgently needs our protection.

Late Neogene and Quaternary Lacustrine History of the Great Salt Lake-Bonneville Basin

Charles Oviatt — 2024-01-14

The Great Salt Lake-Bonneville basin has contained lakes for many millions of years and has been hydrographically closed for most of its history. Lakes in the lacustrine system have ranged from saline to fresh, and from shallow to deep. Tectonics, specifically crustal extension, which began roughly 20 million years ago as part of the formation of the Basin and Range Province, is the cause of lake-basin formation. Much of the rock record of lakes from Miocene time is faulted and has been eroded and/or buried. Pliocene and Quaternary lakes are better known. For much of the past ~5 Ma the basin has probably appeared similar to today, with a shallow saline terminal lake in a dry desert surrounded by mountains. Freshwater marshes and fluvial systems existed on the basin floor during part of the past ~5 Ma, probably were caused by the lack of inflow from the upper Bear River during the Neogene Period and most of the Pleistocene Epoch (that river was diverted into the basin during the Late Pleistocene), combined with a warm and dry climate. The largest deep-lake cycles were caused by changes to a cold and wet climate, which affected the water budget of the lake system and were correlated with periods of global glaciation. Based on limited data, the total length of time deep lakes existed in the basin is thought to be less than 10% of the past ~773 ka. Lake Bonneville, the most-recent of the deep-lake cycles, was probably the deepest and largest manifestation of the lake system in the history of the basin. Named deep-lake cycles during the past ~773 ka, are Lava Creek (~620 ka), Pokes Point (~430 ka), Little Valley (~150 ka), Cutler Dam (~60 ka), and Bonneville (~30 -13 ka). Of the Quaternary deep-lake cycles, only Lake Bonneville is represented by lacustrine landforms, outcrops, and cores of offshore deposits; no landforms from older deep-lake cycles exist (some may be buried under Lake Bonneville deposits but are not visible at the surface), and pre-Bonneville lakes are represented by sediments in limited outcrops and drill holes (including a set of cores taken by A.J. Eardley in the mid 20th century). During the past ~773 ka, deep-lake cycles were correlated with changes in the total volume of global glacial ice; the available evidence indicates that prior to ~773 ka deep-lake cycles were rare.

Evolution of Great Salt Lake’s Exposed Lakebed (1984-2023)

Mark Radwin — 2024-01-14

The Great Salt Lake has been rapidly shrinking since the highstand of the mid-1980s, creating cause for concern in recent decades as the lake has reached historic lows. Many investigators have assessed the evolution of lake elevation, geochemistry, anthropogenic impacts, and links to climate and atmospheric processes; however, the use of remote sensing to study the evolution of the lake has been significantly limited. Harnessing recent advancements in cloud-processing, specifically Google Earth Engine cloud computing, this study utilizes over 600 Landsat TM/OLI and Sentinel MSI satellite images from 1984-2023 to present time-series analyses of remotely sensed Great Salt Lake water area, exposed lakebed area, surface cover types, and chlorophyll-a analyses paired with modelled estimates for water and exposed lakebed area. Results show that a analyses paired with modelled estimates for water and exposed lakebed area. Results show that area has increased to ~3,500 km2 from ~500 km2. The area of unconsolidated sediments not protected by vegetation or halite crusts has risen to ~2,400 km2. Significant halite crusts are observed in the North Arm, having a max extent of ~150 km2 between 2002 and 2003, while only small extents of halite crusts are observed for the South Arm. Vegetation is more prevalent in the Bear River Bay and South Arm, with surface area increases over 400% since 1990. Gypsum is widely observed independent of halite crusts. The results highlight multiple instances of land-use/water-management that led to observable changes in water/exposed lakebed area and halite crust extent. This study demonstrates the important benefits of maintaining a lake elevation above ~4,194 ft to maximize lake and halite crust area, which would help mitigate possible dust events and maintain a broad lake extent.

Record Low Water Surface Elevations at Great Salt Lake, Utah, 2021-2022

Ryan Rowland — 2024-01-14

The United States Geological Survey (USGS) operates two long-term water-surface elevation (WSE) gages on Great Salt Lake, Utah, one north of the Union Pacific Railroad causeway in the historic Little Valley Boat Harbor (Saline gage), and one south of the causeway in the harbor at Great Salt Lake State Park (Saltair gage). From September 28 to December 15, 2022, lake levels were too low in the harbor for the Saltair gage to operate and WSE data was measured at the South Causeway gage, a relatively new gaging station (installed in 2020) located immediately south of the causeway. Data collected at the South Causeway gage were used to estimate the daily mean WSE record for the Saltair gage for the period it was shut down, preserving the continuity of the 175-year WSE record that is associated with this gage. The long-standing historic low daily mean WSE measured at the Saltair gage on October 15, 1963 (4,191.35 feet, relative to the National Geodetic Vertical Datum of 1929 (NGVD29)) was broken on July 21, 2021. Seasonal lake-level declines Geodetic Vertical Datum of 1929 (NGVD29)) was broken on July 21, 2021. Seasonal lake-level declines from July 2021 to October 2021 and April 2022 to early November 2022 resulted in a new historic low daily mean WSE of 4,188.5 feet NGVD29, measured during several days during November 2022 at the South Causeway gage. The same value is also the new historic low daily mean WSE for the Saline gage and was measured during several days in November and December 2022 (the previous historic low of 4,188.98 feet NGVD29 was measured in September and October 2016 and was related to closure of two railroad causeway culverts). USGS also operates streamgages on major surface-water inflows including the Bear River, Weber River, Jordan River, and Surplus Canal. The combined annual discharge measured at these gages in water years 2021 and 2022 was 0.704 and 0.743 million acre-feet, respectively, which is less than half of the combined median annual discharge (1.57 million acre-feet) based on the period of record for each gage.

Use of remote imagery to map microbialite distribution at Great Salt Lake, Utah: Implications for microbialite exposure

Laura Wilcock — 2024-01-14

The elevation of Great Salt Lake has fallen to historic lows in recent years, exposing once submerged microbialites along the lake’s shores. Although prior studies have attempted to map microbialite locations, this has proved challenging, with mapped microbialite areas limited to accessible shoreline locations or via indirect sonographic evidence. Meanwhile, the importance of Great Salt Lake’s microbialites to the lake’s food chain has made quantifying the extent of microbialites exposed versus submerged at different lake elevations critical to lake management decisions. Low lake levels combined with seasonal high-water clarity have enabled microbialite reefs to be spotted in aerial and satellite imagery, even in deeper areas of the lake. In this study, satellite images were used to identify and map microbialite reef areas in Great Salt Lake and along its dry shores. In the south arm, submerged microbialites were easily recognized as dark green reefs against a light-colored benthic background (primarily ooid sand). Stationary microbialite mounds were distinguished from rip-up clasts or other dark-colored mobile material by comparing potential microbialite regions across several high-visibility timepoints. In this way, we identified 649 km2 (251 mi2) of putative microbialite reef area: 288 km2 (111 mi2) in the north arm, 360 km2 (139 mi2) in the south arm, of which 375 km2 (145 mi2) was mapped at a high degree of confidence. We also produced geospatial shapefiles of these areas. This map, combined with currently available lake bathymetric data, permits the estimation of the extent of microbialite reef exposed vs. submerged in various parts of the lake at different lake elevations. At the end of fall 2022, when lake level dipped to 1276.7 masl (4188.5 ft-asl) in elevation, we estimate that ~40% of the south arm microbialite reef area was exposed.

Radiocarbon Chronology/Growth Rates of Ooids from Great Salt Lake, Utah

Olivia Paradis — 2024-01-14

Ooids (calcium carbonate coated grains) are common in carbonate environments throughout geologic time, but the mechanism by which they form remains unclear. In particular, the rate of ooid growth remains elusive in all but a few modern marine environments. In order to investigate the rate of ooid growth in a non-marine setting, we used 14C to date ooids from Great Salt Lake, Utah, a well-known site of aragonitic ooids. Bulk ooids obtained from the northern shore of Antelope Island and the northeast shore of Great Salt Lake near Spiral Jetty were sieved into different size fractions and produced mean ages ranging between 2728±15 and 4373±20 14C yr BP. Larger ooids were older than smaller ooids, implying that larger ooids grew in the environment for a longer duration, with the caveat that bulk age dating integrates the growth history of an ooid. To better resolve growth history, ooids from the coarse fraction were sequentially dissolved, and 14C ages were obtained for each dissolution step to create a time series of ooid growth. The results of the sequential dating indicate that the coarse Great Salt Lake ooid growth began between 5800-6600 ± 60 14C yr BP while their outer cortices are nearly modern. Sequentially dated ooids from the South Arm of Great Salt Lake at Antelope Island record a nearly linear growth history (~ 10-15 µm/kyr), whereas ooids from Spiral Jetty record somewhat faster growth between ~6000 and 4000 years ago (0.03 – 0.06 µm/yr) followed by a 10x slower growth history for the remainder of their lifespan (0.003 – 0.008 µm/yr). The lifespan of Great Salt Lake aragonitic ooids is two to six times longer than those from modern marine environments, and thus provides a unique end member for understanding the mechanisms behind ooid formation. The ooid age range indicates that geochemical parameters measured from bulk ooid dissolution integrates over ~6000 years and thus does not represent a geochemical snapshot in time, as some previous studies have suggested.

Shoreline superelevation, clues to coastal processes of Great Salt Lake

Genevieve Atwood — 2024-01-14

Coastal processes create the shoreline evidence of Great Salt Lake. Shoreline superelevation is the difference in elevation between still water lake level and the shoreline evidence produced by the lake at that level. Processes of formation include effects of wind strength, fetch, beach attributes, coastline aspect, and coast morphology. A series of field studies from 1986 through 2000 concluded strong storm winds from the northwest contribute to the patterns and magnitude of shoreline superelevation. Weather data for 2020-2023 for Gunnison Island and Hat Island document strong storm winds from the north and northwest for Gunnison Bay and with more complexity for Gilbert Bay. The strongest wind patterns are consistent with the geologic evidence of shoreline superelevation produced by the high lake stands of 1986-1987. Wind strength, fetch, and storm duration cause Great Salt Lake wave regimes. The wave-regimes of Great Salt Lake are fetch-limited due to the size and morphology of the water body. In contrast, the long fetch of large lakes such as Lake Bonneville (the enlarged manifestation of the Great Salt Lake lacustrine system), determines the magnitude and patterns of their shoreline superelevation. Geologic evidence of shoreline superelevation of modern- and paleo- fetch-limited lakes similar to Great Salt Lake may be durable evidence of storm wind direction.

Wave dynamics and sediment transport in Great Salt Lake

Ben Smith — 2024-01-14

Great Salt Lake is a natural laboratory to test and refine ideas about the relationship between sediment transport by waves and the characteristics of shoreline carbonate sediments, in particular ooid sands and microbialite mounds. In this chapter, we present a year-long series of wave data collected from July 2021 through June 2022 and use these wave data to assess the performance of a US Army Corps of Engineers wave model previously used to estimate bed shear velocity and intermittency of sediment transport in Great Salt Lake (Smith and others, 2020). We use this model-data comparison to identify the strengths and weaknesses of the existing model for both geological and ecological applications, and areas of improvement for future model development. We also use shallow sediment cores and Unmanned Aerial Vehicle (UAV)-based orthomosaics collected from shorelines near each buoy to assess how the wave climate along two parts of the lake shore influences the stratigraphic record and the surface morphology of the lakebed.

Great Salt Lake wetland vegetation and what it tells us about environmental gradients, drought, and disturbance

Becka Downard — 2024-01-14

Great Salt Lake (GSL) wetlands support more than 300 species of migratory birds and provide many ecosystem functions, including flood and drought attenuation, dust mitigation, and water quality improvement. Wetland vegetation is a key factor in providing those services and can also tell us about how healthy a wetland is. From 2103 to 2022, 135 GSL wetlands were surveyed to develop a multi-metric index of GSL wetland condition. That wetland condition data, along with environmental variables like soil and water chemistry and physical disturbance, are summarized here as 1) an ecological characterization of the three main types of GSL wetlands, 2) a description of how the plant community differs across environmental and anthropogenic disturbance gradients, and 3) assessment of the major risks to GSL wetland health. GSL wetland plant species are generally resistant to environmental disturbance because of the anatomical and physical adaptations that allow them to survive in dynamic wetland environments. However, land use conversion and the rapid expansion of invasive species, the major threats to GSL wetland health, have seriously degraded wetland condition around GSL. In addition to being useful in wetland monitoring and assessment, the results presented here can also identify wetlands in need of enhanced protection or those with restoration potential as well as setting realistic wetland restoration goals for the region.

Estimate of groundwater flow and salinity contribution to the Great Salt Lake using groundwater levels and spatial analysis

Hector Zamora — 2024-01-14

Groundwater discharge to Great Salt Lake (GSL) is difficult to quantify but represents a potentially significant source of water and salinity to the lake’s overall water budget and chemistry, respectively. Understanding groundwater and its role in the overall health of GSL is critical due to the current and historically low lake levels. We compiled existing groundwater level data in basin-fill wells around GSL and used spatial analysis methods to 1) create potentiometric-surface maps in the areas adjoining GSL, 2) calculate groundwater contributions to GSL, and 3) estimate salinity inputs from groundwater to GSL. We observed groundwater-level declines in most of the basin-fill wells from the 1980s to 2010s. These declines are consistent with historical groundwater-level trends in the Salt Lake, Tooele, Curlew, and Weber Valleys and are a consequence of aquifer overdraft associated with less than average precipitation in the basin and increased groundwater withdrawals in the GSL watershed. Using the Darcy flux equation, we calculated a groundwater flux to GSL of 313,500 acre-feet per year, substantially greater than previous estimates derived from water balance studies but consistent with estimates derived from geochemical modeling of GSL water chemistry. We calculated a salt contribution from groundwater to GSL of 1.18 million metric tons per year, which represents about 10% of the solutes derived from surface flows to GSL in 2013.

Implications and hydrographs for two Pre-Bonneville pluvial lakes and double geosols from 14 OSL-IRSL ages in Cache Valley, NE Bonneville Basin

Robert Oaks — 2024-01-14

In the northeastern Great Basin, USA, thirteen new optically stimulated luminescence (OSL) ages and one infrared stimulated luminescence (IRSL) age show that two deep pluvial lakes preceded the Bonneville lake cycle in Cache Valley during marine oxygen-isotope stages (MIS) 6 (123-191 ka) and 4 (56-71 ka), respectively. Our new data define quantitative hydrographs of the Little Valley and Cutler Dam lake cycles in both Cache Valley and the main Bonneville basin. In western Cache Valley, excavation of a faulted, east-plunging spit has sequentially exposed these deposits and overlying MIS 3 Fielding humid-over-arid double geosols that end westward at a strand of the east-dipping Dayton-Oxford normal-fault zone. Lithologically identical double paleosols in eastern Cache Valley overlie a variety of deposits, including dated Little Valley lake beds, and persist above the Bonneville shoreline. Six new ages show that the Little Valley lake cycle in Cache Valley began before 169 ka and ended after 143 ka, and its highest shoreline was above 1493 m. The >25 kyr duration of this pluvial lake cycle rivals the combined durations of the two subsequent lake cycles, during MIS 4 and MIS 2. The Cutler Dam lake rose at least to ~1450 m by ~67 ka in Cache Valley. In the type area in the main Bonneville basin, west of Cutler Narrows, four averaged IRSL dates from Cutler Dam lake beds show that the lake level there had dropped to ~1340 m by ~59 ka. The Little Valley lake rose at least 40 to 50 m above the local Provo shoreline whereas the Cutler Dam lake missed reaching the Provo shoreline by ~13 m. Beneath central Cache Valley, southeast of the study area, there are two laterally extensive, confining layers of silty clay with an intervening sandy gravel layer, all overlying thick gravelly sediment. Both confining layers enclose additional thin and discontinuous gravel layers with adjacent oxidized clays. These alternating coarse and fine sediments are probably correlative with the exposed MIS 6 to MIS 1 deposits and, possibly, older lake cycles.

Observations of Decadal-Scale Brine Geochemical Change at the Bonneville Salt Flats

Jeremiah Bernau — 2024-01-14

Over the past century, the Bonneville Salt Flats, which lies on the western edge of the Great Salt Lake watershed, has experienced changing environmental conditions and a unique history of land use, including resource extraction and recreation. The perennial halite salt crust has decreased in thickness since at least 1960. An experimental restoration project to return mined solutes began in 1997, but it has not resulted in anticipated salt crust growth. Here, primary observations of the Bonneville Salt Flats surface and subsurface brine chemistry and water levels collected from 2013 to 2023 are reported. Spatial and temporal patterns in chemistry, focused on density and water stable isotopes, are evaluated and compared with observations across seven periods of research spanning from 1925 to 2023. Declining salinity in the areas to the east of extraction ditches and south of Interstate 80 were observed. Brine extracted for potash production decreased in salinity as extraction rates increased. Between the years 1964 and 1997, the salinity of the shallow aquifer brine located beneath and to the east of the crust experienced a decrease. However, following this period, the salinity stabilized and subsequently increased. Salinity recovery was concurrent with declines in brine extraction and the salt restoration project, with the largest decrease in brine extraction being concurrent with the largest recovery in salinity. The specific impact of the restoration project on the brine salinity increase remains unclear. To the west, the shallow aquifer in the area between the Silver Island Mountains and the salt crust has increased in salinity. This increase is accompanied by a decline in groundwater levels, which enables the underground movement of solutes from east to west, following a salinity gradient away from the saline pan. Over the past 25 years, the alluvial-fan aquifer along the Silver Island Mountains has markedly declined, leading to increasingly more saline and isotopically heavier basinal waters to be extracted for industrial use. This change is concurrent with the onset of the salt restoration project, which relies on alluvial-fan aquifer waters. This compilation of changes in groundwater chemistry provides an important resource for stakeholders working to understand and manage this dynamic and ephemeral evaporite system. It also offers an example of decadal-scale change in a highly managed Great Salt Lake watershed saline system.

Bonneville basin critical zones

Jory Lerback — 2024-01-14

Playa margin wetlands in the Bonneville basin are sustained by groundwater-fed brackish springs, which transport salts and other solutes into the playa basin. These wetlands are sensitive to changing water availability and quality, which are impacted by changing climate and land use, and whose sediments also provide important records of changing environmental conditions. Gastropods building their shells in these springs provide important recorders of water chemistry and may reflect changing aqueous conditions. In this paper, we analyze spring water chemistry, gastropod ecology and gastropod shell chemistry of Blue Lake (BL) and Horseshoe Springs (HRS), two groundwater-fed wetlands in the Great Salt Lake watershed. We report the physical parameters including pH, temperature, and specific conductivity across the spring pond at Horseshoe springs. There was a slight but statistically significant variation in these physical characteristics between the deeper and shallower parts of the pool, providing evidence that there are different subsite microclimates, which may impact the populations and the isotopic composition of gastropod shells. We measured gastropod population diversity amongst nearly 12,000 shells sampled at Horseshoe springs, finding low population diversity (Shannon’s Diversity Index of 0.432), although the populations of shallow and deep snails are slightly different. The dominant snail at HRS is the Pyrgulopsis which is imperiled, and we also note that we did not find living snails here. We evaluated the bulk shell variation of stable carbonate isotopes (δ13C, and δ18O) across sites and genera. We show that there were no significant subsite-level differences in gastropod δ13C compositions, suggesting that water depth and productivity were not impacting the isotopic signal. We found subsite- and genera-specific differences in snail δ18O compositions, which we interpret to be more dependent on the geography and microclimate of where the snail lived rather than the genera’s physiology (pulmonate versus gil-breathing). We report concentrations of alkali metals (Li, Na, K, Rb, Cs), alkali earth metals (Be, Mg, Ca, Sr, Ba), and metals and metalloids (Al, Sc, Mn, Fe, Cu, Ni, Zn, As) at spring site waters and in bulk shells as potential baseline data for interpreting future or past environmental changes as recorded in shell material. We found trace element concentration and certain elemental ratio differences between genera at the same site (particularly of note were Li, Zn, Mn and Al) that will be important to constrain if these shells are to be applied as a paleoenvironmental proxy and are sometimes attributed to land use change.

Great Salt Lake desert landscape change over multiple temporal scales

Jeremiah Bernau — 2024-01-14

This one-day (~260-mile) field trip guide provides an overview of the late Pleistocene to Holocene history of the Great Salt Lake Desert. Stops include Knolls Sand Dunes and areas on or surrounding the Bonneville Salt Flats, such as Juke Box trench, the Bonneville Salt Flats International Speedway, and the saline pan center and edge (Figure 1). We cover the post-Lake Bonneville geomorphic evolution of the Great Salt Lake Desert including changes in land cover over the past century. The Great Salt Lake Desert area provides unique access to saline landscape features including gypsum dunes and a perennial saline pan. We discuss the origin of these features and how they fit within the area’s broader geologic context. The accessibility of sites discussed here depends on surface conditions. In general, late summer to early fall is the most opportune time to visit this area. Vehicular travel to any of the off-road sites is discouraged when there is standing water or high near-surface moisture (wet mud with little traction). Surface conditions can change rapidly, and we recommend researching current conditions before initiating this trip. This desert is hot and dry during the summer and there is no shade and limited access to water; please plan accordingly. Past and current Great Salt Lake Desert depositional changes provide an analog for the modern Great Salt Lake with changing water availability, potential dust production, competing priorities, and rapidly changing land cover. The information presented here impacts understanding natural and geologic heritage, changing management strategies, and landscape dynamism over multiple spatial and temporal scales.