Abstract
The world-renowned Miocene Clarkia paleolake in northern Idaho (USA) is closely associated with Columbia River Basalt Group volcanism. The flood basalt dammed a local drainage system to form the paleolake, which preserved a plant fossil Lagerstätte in its deposits. However, the precise age and temporal duration of the lake remain unsettled. We present the first unequivocal U-Pb zircon ages from interbedded volcanic ashes at the P-33 type location, constraining the deposition to 15.78 ± 0.039 Ma. Using micro–X-ray fluorescence and petrographic and spectral analyses, we establish the annual characteristics of laminations throughout the stratigraphic profile using the distribution of elemental ratios, mineral assemblages, and grain-size structures, as well as organic and fossil contents. Consequently, the ~7.5-m-thick varved deposit at the type location P-33 represents ~840 yr of deposition, coincident with the end of the main phase of Columbia River Basalt Group eruptions during the Miocene Climate Optimum. The timing and temporal resolution of the deposit offer a unique opportunity to study climate change in unprecedented detail during global warming associated with carbon-cycle perturbations.
Highlights
INTRODUCTIONGroup dammed the proto–Saint Maries River in present-day northern Idaho, USA (Fig. 1A), creating the Clarkia paleolake in a steep-sided narrow valley (Fig. 1B; Smiley and Rember, 1985)
Lava flows from the Columbia River BasaltGroup dammed the proto–Saint Maries River in present-day northern Idaho, USA (Fig. 1A), creating the Clarkia paleolake in a steep-sided narrow valley (Fig. 1B; Smiley and Rember, 1985)
Despite the Clarkia deposit being studied for almost 50 years, for its exquisitely preserved fossil biota, biomolecules, and isotope signals (Yang and Huang, 2003, and references therein), a precise temporal framework of this classic Cenozoic lacustrine deposit remains elusive due to the lack of direct radiometric ages and unresolved sedimentation rates
Summary
Group dammed the proto–Saint Maries River in present-day northern Idaho, USA (Fig. 1A), creating the Clarkia paleolake in a steep-sided narrow valley (Fig. 1B; Smiley and Rember, 1985). Despite the Clarkia deposit being studied for almost 50 years, for its exquisitely preserved fossil biota, biomolecules, and isotope signals (Yang and Huang, 2003, and references therein), a precise temporal framework of this classic Cenozoic lacustrine deposit remains elusive due to the lack of direct radiometric ages and unresolved sedimentation rates. Precise sedimentation rates for Clarkia deposits coupled with radiometric dating on their volcanic ashes can constrain the sedimentaryvolcanic-climatic relations within the Columbia Plateau. They offer a chronological framework for investigating global climate and carbonatecycle perturbations during the warmest phase of the entire Neogene. Our data contextualize the Clarkia paleolake in a millennial time frame at the end of the primary phase of Columbia River Basalt Group eruptions (Kasbohm and Schoene, 2018) with elevated greenhouse gas levels and global warming (Hodell and Woodruff, 1994; Armstrong McKay et al, 2014; Kasbohm and Schoene, 2018; Sosdian et al, 2020)
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