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.
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Spatiotemporal quantification of organic matter accumulation in the Eocene Green River Formation, Bridger Basin, Wyoming
It has long been recognized that lakes can bury large amounts of organic carbon (CORG) in their sediment, with important consequences for conventional and unconventional petroleum resources and potentially for the global carbon cycle. The detailed distribution of lacustrine organic carbon through space and time is important to understanding its commercial and climatic implications, but has seldom been documented in detail. The Green River Formation offers a unique opportunity to improve this understanding, due to extensive Fischer assay analyses of its oil generative potential and to recently published radioisotopic age analyses of intercalat ed volcanic tuffs. Fischer assay analyses reveal distinctly different patterns of organic matter enrichment that correlate with different lacustrine facies associations. Histograms of oil generative potential for evaporative facies of the Wilkins Peak Member exhibit an approximately exponential distribution. This pattern is interpret ed to result from episodic expansion and contraction of Eocene Lake Gosiute across a low-gradient basin floor that experienced frequent desiccation. In contrast, histograms for fluctuating profundal facies of the upper Rife Bed of the Tipton Member and the lower LaClede Bed of the Laney Member exhibit an approximately normal or log normal distribution, with modes as high as 16–18 gallons per ton. This pattern is interpreted to reflect generally deeper conditions when the lake often intersected basin-bounding uplifts. Within the Bridger basin, burial of CORG was greatest in the south during initial Wilkins Peak Member deposition, reflecting greater rates of accommodation near the Uinta uplift. The locus of CORG burial shifted north during upper Wilkins Peak Member deposition, coincident with a decrease in differential accommodation. CORG burial during deposition of the upper Rife and lower LaClede Beds was greatest in the southeast, due either to greater accommodation or localized influx of river-borne nutrients. Average CORG burial fluxes are consistently ~4-5 g/m2 yr for each interval, which is an order of magnitude less than fluxes reported for small Holocene lakes in the northern hemisphere. Maximum rates of CORG burial during deposition of organic-rich mudstone beds (oil shale) were likely similar to Holocene lakes however. Deposition of carbonate minerals in the Bridger basin resulted in additional, inorganic carbon burial. Overall it appears that carbon burial by Eocene lakes could have influenced the global carbon cycle, but only if synchronized across multiple lake systems.
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- Award ID(s):
- 1813278
- NSF-PAR ID:
- 10438619
- Date Published:
- Journal Name:
- Geosites
- Volume:
- 50
- ISSN:
- 0375-8176
- Page Range / eLocation ID:
- 1 to 20
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract Modern and ancient lacustrine carbonate build‐ups provide uniquely sensitive sedimentary and geochemical records for understanding the interaction between tectonics, past climates, and local and regional scale basin hydrology. Large (metre to decametre), well‐developed carbonate mounds in the Green River Formation have long been recognized along the margins of an Eocene lake, known as Lake Gosiute. However, their mode of origin and significance with respect to palaeohydrology remain controversial. Here, new sedimentological, Sr isotope data and structural evidence show that significant spring discharge led to the formation of a decametre size complex of shoreline carbonate mounds in the upper Wilkins Peak Member of the Green River Formation at Little Mesa and adjacent areas in the Bridger Basin, Wyoming, USA. Sedimentological evidence indicates that spring discharge was predominantly subaqueous but was, at times, also subaerial, which produced tufa cascades and micro‐rimstone dam structures. The87Sr/86Sr ratios measured from these subaerial spring deposits are less radiogenic (87Sr/86Sr = 0.71040 to 0.71101) than contemporaneous palaeolake carbonates (87Sr/86Sr = 0.71195 to 0.71561) because their parent groundwaters likely interacted with less‐radiogenic Palaeozoic carbonate. Calcite‐cemented sandstone cones and spires (87Sr/86Sr = 0.71037 to 0.71057) in the Wasatch Formation directly below the spring deposits suggest that groundwaters derived from Palaeozoic carbonates preferentially flowed along thrust faults. These results imply that high spring discharge coincided with lake high stands of the upper Wilkins Peak Member, suggesting that recharge at the north‐west margin of the Bridger Basin contributed to Lake Gosiute’s water budget and lowered the salinity of an underfilled, evaporative lake basin. The findings of this study provide criteria for the recognition of groundwater discharge in palaeolake systems which may lead to the discovery of palaeospring systems in other ancient lake deposits.
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Mineralogy, petrographic textures, and sedimentary structures from the world’s largest trona deposit, the Wilkins Peak Member (WPM) of the early Eocene Green River Formation (GRF), Bridger subbasin, Wyoming, provide key data about depositional conditions and paleoenvironments. The 250 m-long WPM interval in the Solvay S-34-1 drill core analyzed in this study contains a detailed record of sedimentation in the Bridger subbasin at the deepest area of a hydrologically-closed basin during peak Cenozoic atmospheric CO2 concentrations. Large accumulations of trona (Na3(HCO3)(CO3)·2H2O), shortite (Na2Ca2(CO3)3), northupite (Na3Mg (CO3)2Cl), and halite (NaCl; now replaced by trona), occur in the lower half of the WPM. Modern saline lake environments such as Lake Magadi, Kenya, and the Dead Sea, Israel-Jordan, are useful analogues for interpreting paleolake conditions associated with evaporite deposition in the Solvay S-34-1 core. Solvay saline lake deposits are organized into meter-scale shallowing-upward successions, beginning with (1) oil shale overlain by (2) trona, in places interbedded with oil shale, followed by (3) peloidal dolomite grainstone and/or silty dolomitic mudstone, and (4) massive mudstone with disruption features or desiccation cracks, and/or siliciclastic sandstone with ripple cross-stratification. Based on observations of modern hypersaline lake environments, WPM evaporite deposition at the basin depocenter is interpreted to be controlled by inflow water composition and volume, evaporative concentration, and seasonally-driven lake temperature fluctuations, resulting in recurrent patterns in evaporite mineralogies and textures.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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The Wilkins Peak Member (WPM) of the Green River Formation in Wyoming, USA, comprises alternating lacustrine and alluvial strata that preserve a record of terrestrial climate during the early Eocene climatic optimum. We use a Bayesian framework to develop age-depth models for three sites, based on new 40Ar/39Ar sanidine and 206Pb/238U zircon ages from seven tuffs. The new models provide two- to ten-fold increases in temporal resolution compared to previous radioisotopic age models, confirming eccentricity-scale pacing of WPM facies, and permitting their direct comparison to astronomical solutions. Starting at ca. 51 Ma, the median ages for basin-wide flooding surfaces atop six successive alluvial marker beds coincide with short eccentricity maxima in the astronomical solutions. These eccentricity maxima have been associated with hyperthermal events recorded in marine strata during the early Eocene. WPM strata older than ca. 51 Ma do not exhibit a clear relationship to the eccentricity solutions, but accumulated 31%−35% more rapidly, suggesting that the influence of astronomical forcing on sedimentation was modulated by basin tectonics. Additional high-precision radioisotopic ages are needed to reduce the uncertainty of the Bayesian model, but this approach shows promise for unambiguous evaluation of the phase relationship between alluvial marker beds and theoretical eccentricity solutions.more » « less
- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.31711/ugap.v50i.117
