5:15 PM - 7:15 PM
[MIS14-P05] Reconstruction of terrestrial climate during the Eocene Hyperthermals based on the carbon isotope stratigraphy of the Green River Formation in Utah (Preliminary results)
Keywords:Hyperthermal, Eocene, Carbon isotopic stratigraphy
The early Eocene is known as a hothouse period during which rapid warming events, called "hyperthermals" occurred repeatedly. Possible causes of hyperthermal events include the rapid release of carbon dioxide from volcanic activity and the release of methane due to the thermal dissociation of submarine methane hydrates. However, the impact of this rapid warming on terrestrial environments remains largely unclear. In order to clarify the effect of rapid warming on the terrestrial environment during the early Eocene hyperthermal intervals , we investigated the lower-middle Eocene lacustrine strata of the Green River Formation in Utah, USA.
The Green River Formation is widely distributed in Utah, Wyoming, and Colorado in the central USA and represents a crucial record indicating the presence of a vast mid-latitude lake during the early-middle Eocene. This study focuses on the Indian Canyon section, located in the western part of the Uinta Basin in northern Utah. Field investigations were conducted over five years, starting in 2016, resulting in the construction of a detailed lithostratigraphic column covering approximately 1,100 meters of strata (Kuma et al., 2019, in prep.). Based on cyclic variations in lithology observed in Indian Canyon, orbital tuning has been applied, revealing that the sedimentary record corresponds to a time span of 52.8–43.7 Ma. Furthermore, paleomagnetic stratigraphy indicates that fluvial channel sandstone layers, which cyclically developed between 49 and 53 Ma, may correspond to hyperthermal events, consistent with suggestion from a previous study (Birgenheier et al., 2019).
However, confirming whether these fluvial channel sandstone layers truly correspond to hyperthermal events requires detailed correlation with marine records based on carbon isotope variations. When reconstructing carbon isotope fluctuations from terrestrial strata, it is also essential to consider variations in organic matter composition. Therefore, in this study, we first performed C/N ratio and palynofacies composition analysis to determine the organic matter composition of each of 118 samples collected from the Indian Canyon section . Then, we measured the carbon isotopes of organic carbon to identify hyperthermal intervals and reconstruct terrestrial environmental changes during these events.
The variations in C/N correspond well with few meter-scale lithological changes, showing lower values below 10 in shale layers (high lake level) and higher values ranging from 20 to 30 in calcareous mudstone to dolomitic marl (low lake level). Additionally, C/N ratio remains generally low in the interval between 300 and 500 meters, whereas it tends to be consistently higher above 500 meters stratigraphic level. This trend of C/N ratio can be interpreted as reflecting changes in lake levels: during periods of high lake levels, algal organic matter was dominant and showing lower C/N ratio, whereas during lower lake levels, an increased influx of terrestrial plant debris from the shoreline occurred and showing higher C/N ratio.
Furthermore, near the fluvial channel sandstone layers, which are potentially associated with hyperthermal events, C/N ratio exceeds 30, and the carbon isotope ratio shows an exceptionally low value of approximately -30‰. These results suggest either an excessive influx of plant debris or a direct reflection of the warming during the hyperthermal period. We will continue C/N ratio, palynofacies, and carbon isotopic analysis to refine the identification of hyperthermal intervals and reconstruct terrestrial environmental changes during these extreme warming events.
The Green River Formation is widely distributed in Utah, Wyoming, and Colorado in the central USA and represents a crucial record indicating the presence of a vast mid-latitude lake during the early-middle Eocene. This study focuses on the Indian Canyon section, located in the western part of the Uinta Basin in northern Utah. Field investigations were conducted over five years, starting in 2016, resulting in the construction of a detailed lithostratigraphic column covering approximately 1,100 meters of strata (Kuma et al., 2019, in prep.). Based on cyclic variations in lithology observed in Indian Canyon, orbital tuning has been applied, revealing that the sedimentary record corresponds to a time span of 52.8–43.7 Ma. Furthermore, paleomagnetic stratigraphy indicates that fluvial channel sandstone layers, which cyclically developed between 49 and 53 Ma, may correspond to hyperthermal events, consistent with suggestion from a previous study (Birgenheier et al., 2019).
However, confirming whether these fluvial channel sandstone layers truly correspond to hyperthermal events requires detailed correlation with marine records based on carbon isotope variations. When reconstructing carbon isotope fluctuations from terrestrial strata, it is also essential to consider variations in organic matter composition. Therefore, in this study, we first performed C/N ratio and palynofacies composition analysis to determine the organic matter composition of each of 118 samples collected from the Indian Canyon section . Then, we measured the carbon isotopes of organic carbon to identify hyperthermal intervals and reconstruct terrestrial environmental changes during these events.
The variations in C/N correspond well with few meter-scale lithological changes, showing lower values below 10 in shale layers (high lake level) and higher values ranging from 20 to 30 in calcareous mudstone to dolomitic marl (low lake level). Additionally, C/N ratio remains generally low in the interval between 300 and 500 meters, whereas it tends to be consistently higher above 500 meters stratigraphic level. This trend of C/N ratio can be interpreted as reflecting changes in lake levels: during periods of high lake levels, algal organic matter was dominant and showing lower C/N ratio, whereas during lower lake levels, an increased influx of terrestrial plant debris from the shoreline occurred and showing higher C/N ratio.
Furthermore, near the fluvial channel sandstone layers, which are potentially associated with hyperthermal events, C/N ratio exceeds 30, and the carbon isotope ratio shows an exceptionally low value of approximately -30‰. These results suggest either an excessive influx of plant debris or a direct reflection of the warming during the hyperthermal period. We will continue C/N ratio, palynofacies, and carbon isotopic analysis to refine the identification of hyperthermal intervals and reconstruct terrestrial environmental changes during these extreme warming events.