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The Early Eocene Climatic Optimum (~53-50 Ma) represents the most recent episode of sustained greenhouse climate, during which the deep oceans were as much as 12°C warmer than today. The lacustrine Wilkins Peak Member of the Green River Formation (Wyoming, USA) is one of the premier locations to study this period of global warmth due to its rich terrestrial archive of climate dynamics, biology, and geomorphology. Using radioisotopic geochronology, cyclostratigraphy, sedimentology, and geochemistry, previous studies have leveraged this extensive record to evaluate the ancient lake system’s temporal evolution and response to climate. Much prior work on Green River Formation cyclostratigraphy, including that of Alfred G. Fischer, has focused on the evaluation of oil yield, a measure of organic richness. In this study, X-Ray fluorescence (XRF) core scanning of a basin center core, Solvay S-34-1, is used to produce a high resolution (5mm), continuous, multi-proxy elemental record of the complete Wilkins Peak Member, spanning 240 meters. This new geochemical assessment is a component of a larger multidisciplinary investigation that that is underway, including new magnetostratigraphic and radioisotopic geochronology. Elemental abundances for a range of measured elements, such as Si, S, Cl, K, Ca, Ti, Fe, Zn, Br, and Rb, are interpreted in terms of evaporitic, siliciclastic, and redox-sensitive sedimentation, and show variable responses at specific Milankovitch (eccentricity, obliquity, precession) and sub-Milankovitch time scales. Using this long high-resolution geochemical dataset of the Early Eocene Climatic Optimum, we consider potential linkages between Milankovitch forcing and sub-Milankovitch forcing, and plausible non-linear transfer functions that translate the astronomical insolation signal into the stratigraphic archive.
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Influence of lake-basin morphology on climate-sediment transfer functions: Early Eocene Wilkins Peak Member, Green River Formation, Wyoming
Lacustrine strata are often among the highest-resolution terrestrial paleoclimate archives available. The manner in which climate signals are registered into lacustrine deposits varies, however, as a function of complex sedimentologic and diagenetic processes. The retrieval of reliable records of climatic forcing therefore requires a means of evaluating the potential influence of changing sedimentary transfer functions. Here, we use high-resolution X-ray fluorescence core scanning of the Wilkins Peak Member of the Green River Formation to characterize the long-term evolution of transfer functions in an ancient lacustrine record. Our analysis identifies a shift in the frequency distribution of Milankovitch-band variance between the lower and middle Wilkins Peak Member across a range of temporally calibrated elemental intensity records. Spectral analysis of the lower Wilkins Peak Member shows strong short eccentricity, obliquity, precession, and sub-Milankovitch−scale variability, while the middle Wilkins Peak Member shows strong eccentricity variability and reduced power at higher frequencies. This transition coincides with a dramatic decline in the number and volume of evaporite beds. We attribute this shift to a change in the Wilkins Peak Member depositional transfer function caused by evolving basin morphology, which directly influenced the preservation of bedded evaporite as the paleolake developed from a deeper, meromictic lake to a shallower, holomictic lake. The loss of bedded evaporite, combined with secondary evaporite growth, results in reduced obliquity- and precession-band power and enhanced eccentricity-band power in the stratigraphic record. These results underscore the need for careful integration of basin and depositional system history with cyclostratigraphic interpretation of the dominant astronomical signals preserved in the stratigraphic archive.
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- PAR ID:
- 10438603
- Date Published:
- Journal Name:
- GSA Bulletin
- ISSN:
- 0016-7606
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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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
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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
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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
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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
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1130/B36566.1
