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Underneath the Roots of the Sequoias: Paleoenvironmental Records within the Paleosols of the Florissant Fossil Beds National Monument, central Colorado
Miranda, Ariana
Miranda, Ariana
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2025-08
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Geology
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https://doi.org/10.34944/akzd-vn27
Abstract
The Eocene-Oligocene Transition (EOT) records the rapid change, between 33.9 to 33.4 Ma, from global hothouse conditions to a global icehouse that persists into the modern. The Florissant Formation, dated to 34.1 Ma, records paleoclimatic conditions prior to the EOT. The Florissant Fm., located in central Colorado, is composed of lacustrine tuffaceous shales and mudstones with minor channel conglomerates/sandstones and floodplain mudstones. It is a significant fossil locality with a preserved petrified forest of Sequoia affinis stumps. Paleosols, lithified soils, preserve signatures of ancient climates as a function of the amount of weathering, chemistry and structure of a soil profile. Eight paleosols in Florissant Fossil Beds National Monument were recorded and sampled to determine paleoclimate and classify paleosols along the ancient forest surface using both field and laboratory techniques in summer 2023.Fieldwork observations revealed paleosols had thin, simple profiles with ubiquitous redoximorphic features and carbonized plant material. X-ray diffraction analysis detected smectite as the dominant mineralogy in all soil profiles, a common byproduct of hydrolyzed volcanic ash and glass. Illite, sanidine, albite and quartz were detected in smaller concentrations in the profiles. Petrographic thin sections displayed weak pedogenic features including some clay illuviation with intact, euhedral to subeuhedral crystals of volcanogenic material.
Geochemical analysis using x-ray fluorescence of paleosols, suggests that paleosols underwent minimal pedogenesis by using molecular weathering ratios. Molecular weathering ratios from B horizons of paleosols were used to calculate mean annual temperature (MAT) and mean annual precipitation (MAP). An average of 11.4°C for MAT, and an average of 811 mm/yr for MAP were calculated using climofunctions. MAT values are consistent with previous paleobotanical estimations of MAT, but bias towards lower temperatures of the range. MAP was slightly higher than previous estimates of MAP via paleobotanical methods.
Pedogenic derived MAT and MAP indicate Eocene Florissant experienced mesic to thermic temperatures, with a seasonally aquic moisture regime. Paleosols had weak pedogenesis, were poorly drained (seasonally), acidic and had abundant carbonized organic material. The bowl-shaped topography of the sequoia forest promoted a high water table leading to seasonal aquic conditions. Translocated clay and wedge shaped peds indicate drainage improved for a portion of the year. Based on pedogenic data, Eocene Florissant is interpreted as forested wetlands with cyclical wet and dry periods. This contrasts to modern day Sequoia sempervirens forests whose soils are well drained and receive ~600 mm of precipitation a year. My interpretation is in agreement with the paleobotany and paleoecology of Eocene Florissant.
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