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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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CONTINENTAL RESPONSES TO THE EARLY EOCENE CLIMATIC OPTIMUM: A HIGH-RESOLUTION PERSPECTIVE FROM THE GREEN RIVER FORMATION USING XRF CORE SCANNING
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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- Award ID(s):
- 1812741
- PAR ID:
- 10554277
- Publisher / Repository:
- Geological Society of America
- Date Published:
- Volume:
- 51
- Issue:
- 5
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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null (Ed.)ABSTRACT The Green River Formation preserves an extraordinary archive of terrestrial paleoclimate during the Early Eocene Climatic Optimum (EECO; ∼ 53–50 Ma), expressing multiple scales of sedimentary cyclicity previously interpreted to reflect annual to Milankovitch-scale forcing. Here we utilize X-ray fluorescence (XRF) core scanning and micro X-ray fluorescence (micro-XRF) scanning in combination with radioisotopic age data to evaluate a rock core record of laminated oil shale and carbonate mudstone from Utah's Uinta Basin, with the parallel objectives of elucidating the paleo-environmental significance of the sedimentary rhythms, testing a range of forcing hypotheses, and evaluating potential linkages between high- and low-frequency forcing. This new assessment reveals that the ∼ 100-μm-scale laminae—the most fundamental rhythm of the Green River Formation—are most strongly expressed by variations in abundance of iron and sulfur. We propose that these variations reflect changes in redox state, consistent with annual stratification of the lake. In contrast to previous studies, no support was found for ENSO or sunspot cycles. However, millimeter- to centimeter-scale rhythms—temporally constrained to the decadal to centennial scale—are strongly expressed as alternations in the abundance of silicate- versus carbonate-associated elements (e.g., Al and Si vs. Ca), suggesting changes in precipitation and sediment delivery to the paleo-lake. Variations also occur at the meter scale, defining an approximate 4 m cycle interpreted to reflect precession. We also identify punctuated intervals, associated principally with one phase of the proposed precession cycle, where Si disconnects from the silicate input. We propose an alternative authigenic or biogenic Si source for these intervals, which reflects periods of enhanced productivity. This result reveals how long-term astronomical forcings can influence short-term processes, yielding insight into decadal- to millennial-scale terrestrial climate change in the Eocene greenhouse earth.more » « less
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The Green River Formation preserves an extraordinary archive of terrestrial paleoclimate during the Early Eocene Climate Optimum (EECO; ~53-50 Ma), expressing multiple scales of sedimentary cyclicity previously interpreted to reflect annual to Milankovitch-scale forcing. Here we utilize X-ray fluorescence (XRF) core scanning and micro X-ray fluorescence (micro-XRF) scanning in combination with radioisotopic age data to evaluate a rock core record of laminated oil shale and carbonate mudstone from Utah’s Uinta Basin, with the parallel objectives of elucidating the paleo-environmental significance of the sedimentary rhythms, testing a range of forcing hypotheses, and evaluating potential linkages between high- and low-frequency forcing. This new assessment reveals that the ~100 μm-scale laminae – the most fundamental rhythm of the Green River Formation –are most strongly expressed by variations in iron and sulfur abundance. We propose that these variations reflect changes in redox state, consistent with annual stratification of the lake. In contrast to previous studies, no support was found for ENSO or sunspot cycles. However, millimeter to centimeter-scale rhythms—temporally constrained to the decadal to centennial scale—are strongly expressed as alternations in the abundance of silicate- versus carbonate-associated elements (e.g., Al and Si vs. Ca), suggesting changes in precipitation and sediment delivery to the paleo-lake. Variations also occur at the meter-scale, defining a ~4 m cycle interpreted to reflect precession. We also identify punctuated intervals, primarily associated with one phase of the proposed precession cycle, where Si disconnects from the silicate input. We propose an alternate authigenic or biogenic Si source for these intervals, which reflects periods of enhanced productivity. This result reveals how long-term astronomical forcings can govern the response of the system to shorter-term processes, yielding insight into decadal to millennial scale terrestrial climate change in the Eocene greenhouse earth.more » « less
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Accepted Manuscript1.0
- Conference Paper:
- https://doi.org/10.1130/abs/2019AM-339584
