An official website of the United States government
As atmospheric carbon dioxide (CO2) and temperatures increase with modern climate change, ancient hothouse periods become a focal point for understanding ecosystem function under similar conditions. The early Eocene exhibited high temperatures, high CO2 levels, and similar tectonic plate configuration as today, so it has been invoked as an analog to modern climate change. During the early Eocene, the greater Green River Basin (GGRB) of southwestern Wyoming was covered by an ancient hypersaline lake (Lake Gosiute; Green River Formation) and associated fluvial and floodplain systems (Wasatch and Bridger formations). The volcaniclastic Bridger Formation was deposited by an inland delta that drained from the northwest into freshwater Lake Gosiute and is known for its vast paleontological assemblages. Using this well-preserved basin deposited during a period of tectonic and paleoclimatic interest, we employ multiple proxies to study trends in provenance, parent material, weathering, and climate throughout 1 million years. The Blue Rim escarpment exposes approximately 100 m of the lower Bridger Formation, which includes plant and mammal fossils, solitary paleosol profiles, and organic remains suitable for geochemical analyses, as well as ash beds and volcaniclastic sandstone beds suitable for radioisotopic dating. New 40Ar/39Ar ages from the middle and top of the Blue Rim escarpment constrain the age of its strata to ∼ 49.5–48.5 Myr ago during the “falling limb” of the early Eocene Climatic Optimum. We used several geochemical tools to study provenance and parent material in both the paleosols and the associated sediments and found no change in sediment input source despite significant variation in sedimentary facies and organic carbon burial. We also reconstructed environmental conditions, including temperature, precipitation (both from paleosols), and the isotopic composition of atmospheric CO2 from plants found in the floral assemblages. Results from paleosol-based reconstructions were compared to semi-co-temporal reconstructions made using leaf physiognomic techniques and marine proxies. The paleosol-based reconstructions (near the base of the section) of precipitation (608–1167 mm yr−1) and temperature (10.4 to 12.0 ∘C) were within error of, although lower than, those based on floral assemblages, which were stratigraphically higher in the section and represented a highly preserved event later in time. Geochemistry and detrital feldspar geochronology indicate a consistent provenance for Blue Rim sediments, sourcing predominantly from the Idaho paleoriver, which drained the active Challis volcanic field. Thus, because there was neither significant climatic change nor significant provenance change, variation in sedimentary facies and organic carbon burial likely reflected localized geomorphic controls and the relative height of the water table. The ecosystem can be characterized as a wet, subtropical-like forest (i.e., paratropical) throughout the interval based upon the floral humidity province and Holdridge life zone schemes. Given the mid-paleolatitude position of the Blue Rim escarpment, those results are consistent with marine proxies that indicate that globally warm climatic conditions continued beyond the peak warm conditions of the early Eocene Climatic Optimum. The reconstructed atmospheric δ13C value (−5.3 ‰ to −5.8 ‰) closely matches the independently reconstructed value from marine microfossils (−5.4 ‰), which provides confidence in this reconstruction. Likewise, the isotopic composition reconstructed matches the mantle most closely (−5.4 ‰), agreeing with other postulations that warming was maintained by volcanic outgassing rather than a much more isotopically depleted source, such as methane hydrates.
more »
« less
ASSESSMENT OF BASIN EVOLUTION FROM INTEGRATED CHEMICAL MODELS AND LITHOSTRATIGRAPHY OF THE LOWER WILKINS PEAK MEMBER OF THE GREEN RIVER FORMATION, SOUTHWEST WYOMING, USA
Chemical sediments from the Early Eocene Green River Formation can be used for assessing hydroclimate and basin evolution during their deposition. The Wilkins Peak Member (WPM) of the Green River Formation contains a relatively continuous record of perennial closed-basin saline lake deposition in the Bridger Basin, southwest Wyoming, from approximately 51.6 to 49.8 Ma. The volumes and paragenesis of authigenic chemical sediments in the WPM are intrinsically related to the chemical evolution of basin brines. The geographic distribution of those chemical sediments across the Bridger Basin relates to the syn- and post-depositional tectonic history of the basin. In this study, we integrated thermodynamic modeling of chemical evolution of lake brines with chemostratigraphic and lithostratigraphic interpretations of the basin-center Solvay S-34-1 core to evaluate physical and chemical changes to and within ancient Lake Gosiute during the Early Eocene. Fine-scale X-ray fluorescence (XRF) scanning along the length of the core provides a high-resolution chemical stratigraphy of the WPM. Thermodynamic modeling of the evaporation of hypothetical inflow waters and lake brines yield predicted sequences of evaporite minerals, allowing estimation of the salinities and evaporated volumes of water required to reach saturation with respect to observed mineral deposits from the basin. The spatial distributions of bedded evaporites allow us to investigate tectonic changes to the basin during and after the deposition of the WPM. Here, we integrate these data to interpret changes in lake-level, salinity, and hydroclimate of ancient Lake Gosiute during the Early Eocene.
more »
« less
- Award ID(s):
- 1812741
- PAR ID:
- 10554267
- Publisher / Repository:
- Geological Society of America
- Date Published:
- Volume:
- 55
- Issue:
- 6
- Format(s):
- Medium: X
- Location:
- Pittsburgh, PA
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
The Green River Formation of Wyoming, USA, is host to the world’s largest known lacustrine sodium carbonate deposits, which accumulated in a closed basin during the early Eocene greenhouse. Alkaline brines are hypothesized to have been delivered to ancient Gosiute Lake by the Aspen paleoriver that flowed from the Colorado Mineral Belt. To precisely trace fluvial provenance in the resulting deposits, we conducted X-ray fluorescence analyses and petrographic studies across a suite of well-dated sandstone marker beds of the Wilkins Peak Member of the Green River Formation. Principal component analysis reveals strong correlation among elemental abundances, grain composition, and sedimentary lithofacies. To isolate a detrital signal, elements least affected by authigenic minerals, weathering, and other processes were included in a principal component analysis, the results of which are consistent with petrographic sandstone modes and detrital zircon chronofacies of the basin. Sandstone marker beds formed during eccentricity-paced lacustrine lowstands and record the migration of fluvial distributary channel networks from multiple catchments around a migrating depocenter, including two major paleorivers. The depositional topography of these convergent fluvial fans would have inversely defined bathymetric lows during subsequent phases of lacustrine inundation, locations where trona could accumulate below a thermocline. Provenance mapping verifies fluvial connectivity to the Aspen paleoriver and to sources of alkalinity in the Colorado Mineral Belt across Wilkins Peak Member deposition, and shows that the greatest volumes of sediment were delivered from the Aspen paleoriver during deposition of marker beds A, B, D, and I, each of which were deposited coincident with prominent “hyperthermal” isotopic excursions documented in oceanic cores.more » « less
-
None (Ed.)Abstract The Green River Formation of Wyoming, USA, is host to the world’s largest known lacustrine sodium carbonate deposits, which accumulated in a closed basin during the early Eocene greenhouse. Alkaline brines are hypothesized to have been delivered to ancient Gosiute Lake by the Aspen paleoriver that flowed from the Colorado Mineral Belt. To precisely trace fluvial provenance in the resulting deposits, we conducted X-ray fluorescence analyses and petrographic studies across a suite of well-dated sandstone marker beds of the Wilkins Peak Member of the Green River Formation. Principal component analysis reveals strong correlation among elemental abundances, grain composition, and sedimentary lithofacies. To isolate a detrital signal, elements least affected by authigenic minerals, weathering, and other processes were included in a principal component analysis, the results of which are consistent with petrographic sandstone modes and detrital zircon chronofacies of the basin. Sandstone marker beds formed during eccentricity-paced lacustrine lowstands and record the migration of fluvial distributary channel networks from multiple catchments around a migrating depocenter, including two major paleorivers. The depositional topography of these convergent fluvial fans would have inversely defined bathymetric lows during subsequent phases of lacustrine inundation, locations where trona could accumulate below a thermocline. Provenance mapping verifies fluvial connectivity to the Aspen paleoriver and to sources of alkalinity in the Colorado Mineral Belt across Wilkins Peak Member deposition, and shows that the greatest volumes of sediment were delivered from the Aspen paleoriver during deposition of marker beds A, B, D, and I, each of which were deposited coincident with prominent “hyperthermal” isotopic excursions documented in oceanic cores.more » « less
-
Lacustrine chemical sediments of the Wilkins Peak Member, Eocene Green River Formation potentially preserve paleoclimate information relating to the conditions of their formation and preservation within the lake basin during the Early Eocene Climatic Optimum. The Green River Formation comprises the world’s largest sodium-carbonate evaporite deposit in the form of trona (Na2CO3⋅NaHCO3⋅2H2O) in the Bridger sub-basin and nahcolite (NaHCO3) in the neighboring Piceance Creek basin. Modern analogues suggest that these minerals necessitate the existence of an alkaline source water. Detrital provenance geochronometers suggest that the most likely source for volcanic waters to the Greater Green River Basin is the Colorado Mineral Belt, connected to the basin via the Aspen paleo-river. We tested the hypothesis that magmatic waters from the Colorado Mineral Belt could have supplied the Greater Green River Basin with the alkalinity needed to precipitate sodium-carbonate evaporites that are preserved in the Wilkins Peak Member by numerically simulating the evaporation of modern soda spring waters from northwestern Colorado at various temperature and atmospheric pCO2 conditions. We compare the resulting simulated evaporite sequences of the modern soda spring waters to the mineralogy preserved within the Wilkins Peak Member. Simulated evaporation of Steamboat Springs water produces the closest match to core observations and mineralogy. These simulations provide constraints on the salinities at which various minerals precipitated in the Wilkins Peak Member as well as insights into the regional temperature (>15ºC for gaylussite and trona; >27º for pirssonite and trona) and pCO2 conditions (<1200ppm for gaylussite and trona) during the EECO.more » « less
-
Lacustrine evaporites have potential to document ancient terrestrial climates, including temperatures and their seasonal variations, and atmospheric pCO2. The sodium carbonate mineral nahcolite (NaHCO3) in the early Eocene Parachute Creek Member, Green River Formation, Piceance subbasin, indicates elevated pCO2 concentrations (> 680 ppm) in the water column and in the atmosphere if in contact with brine. These data support a causal connection between elevated atmospheric pCO2 and global warmth during the early Eocene Climatic Optimum. Trona (Na2CO3⋅NaHCO3⋅2H2O), not nahcolite, is the dominant sodium carbonate mineral in the coeval Wilkins Peak Member in the Bridger subbasin, which may be explained by interbasin variations in (1) brine chemistry, (2) temperature, and (3) pCO2. These interpretations are based on equilibrium thermodynamics and simulations that evaporate lake water, but they ignore seasonal changes in water column temperature and pCO2. Winter cooling, rather than evaporative concentration, best explains the fine-scale alternations of nahcolite, halite (NaCl), and nahcolite + halite in the Parachute Creek Member. Simulated evaporation of alkaline source waters from the paleo Aspen River at temperatures between 15⁰ and 27⁰ C and pCO2 at or below 1200 ppm produces the observed mineral sequence in the Wilkins Peak Member: gaylussite (Na2CO3⋅CaCO3⋅5H2O) at temperatures < 27⁰ C and pirssonite (Na2CO3⋅CaCO3⋅2H2O) > 27⁰ C (both now replaced by shortite Na2CO3·2CaCO3), then northupite (Na3Mg(CO3)2Cl), trona, and halite. The challenge of determining paleo-lake temperatures in the Bridger and Piceance subbasins using microthermometry has now been solved using femtosecond lasers that promote nucleation of vapor bubbles in brine inclusions without deforming the halite host crystal. This method shows general agreement with thermodynamic-based calculations and will be used to document mean annual temperatures in the Greater Green River Basin.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Conference Paper:
- The DOI is not currently available.
