The Clarkia Palaeolake in northern Idaho formed when lava flows from the Colombia River Basalts blocked a steep-sided valley, damming the proto–Saint Maries River. The resultant lake contained a sedimentary environment in which were preserved an exquisite fossil biota, along with biomolecules, and isotope signals, which has been studied by scientists for almost five decades. Despite this attention, dating the Clarkia Palaeolake has proven elusive, due to a lack of direct radiometric, which means we neither fully understand the exact age of the deposits, nor the rates of sedimentation in the lake where they were laid down. Based upon the Plant remains found there, the Clarkia Palaeolake deposits are thought to be Early to Middle Miocene in age, with ash layers present that are thought to be correlatable to other ash layers in the Pacific Northwest for which dates have been established, making the deposits between 16 and 15.4 million years old. This correlates with the Miocene Climatic Optimum, a global warm period which is known to have had high atmospheric carbon dioxide levels, believed to be linked to outgassing from the Colombia River Basalts. The Clarkia Palaeolake sequence is a laminated sequence of beds, but it is unclear if these are the result of storm events or seasonal variation.
If it were possible to develop an accurate model of sedimentation rates in the Clarkia Palaeolake, combined with radiometric dating of the volcanic ash layers present, then this could be used to understand the wider environment of the Miocene Colombia Plateau, and the relationships between climate, volcanism, and sedimentation there, and potentially provide a model for understanding the climate and carbon cycle of the warmest part of the Neogene Period, as well as precise dating for, and a better understanding of the conditions that led to the formation of the Clarkia Palaeolake fossil Lagerstätte.
In a paper published in the journal Geology on 15 April 2021, Daianne Höfig and Yi Ge Zhang of the Department of Oceanography at Texas A&M University, Liviu Giosan of the Woods Hole Oceanographic Institution, Qin Leng, Jiaqi Liang, and Mengxiao Wu of the Laboratory for Terrestrial Environments at Bryant University, Brent Miller of the Department of Geology and Geophysics at Texas A&M University, and Hong Yang, also ot the Laboratory for Terrestrial Environments at Bryant University, present uranium-lead ages for ash layers present at P-33, the type locality for the Clarkia Palaeolake deposits, combined with micro–X-ray fluorescence and spectral analysis of elemental distribution of the laminated deposits, enabling them to calculate the annual- to centennial-scale sedimentation rates during the Miocene Climatic Optimum.
Höfig et al. were able to obtain uranium-lead dates from zircons from ash layers at the P-33 site. Zircons are minerals formed by the crystallisation of cooling igneous melts. Whenthey form they often contain trace amounts of uranium, which decays into (amongst other things) lead at a known rate. Since lead will not have been present in the original crystal, it is possible to calculate the age of a zircon crystal from the ratio between these elements. Sedimentation rates were then determined by detecting changes in the grain-size and elemental distribution along with the laminated beds.
Zircon dates obtained from the tephra (volcanic) layers at P-33 yielded dates ranging from 74.0 to 2533.5 million years (Unit 2C), 14.0 to 2704.5 million years (Unit 4), and 15.07 to 1914.7 million years (Unit 5B);the presence of much older zircons is not a surprise, as these crystals
can survive passing through subduction zones and then being re-erupted. Using only Miocene zircons produced mean ages of 15.42 and 15.65 for Units 4 and 5B. This appears to show that Unit 5B is older than Unit 4, which it overlies; however, the entire sequence is thought to have been deposited in less than 1000 years, so Höfig et al. are only looking for an average age for the entire sequence, which is calculated at 15.78 million years, in order to place it in context with the Colombia River Basalt Group.
Volcanic glass shards from Unit 4 have previously shown to have similar chemical signatures to material from the Cold Springs tuff in Nevada, which has been dated to between 15.85 and 15.50 million years old. There is also a possible connection between the Unit 2C and the Bully Creek Formation’s tuff in Oregon, which has been dated to 15.66 million years. An age of 15.78 million years is also consistent with the calculated ages of the palaeobotanical material from the Clarkia Palaeolake sequence.
Excluding the ash layers, the sediments at P-33 are made up of laminations of fining-upward couplets, with chemical and mineral compositions, grain-size structures, and fossil contents which strongly suggest an annual cycle. These laminations show alternating light, coarse-grained, fossil-barren layers, interpreted as having been laid down in spring and summer, when high rainfall carried much sedimentary material into the lake, and dark, fine-grained, fossil-rich layers, interpreted being laid down in autumn and winter, when finer particles had time to settle out of the water column, and plants were shedding leaves abundantly; fossil leaves are only found in these finer, darker layers. The uppermost layers of the sequence are more oxidised than those below, and produce less fossils.
A spectral analysis of element distributions within these bedding planes revealed a similar pattern, with rhythmic variations which match the grain-size distribution; the laminae dominated by sand-sized particles are enriched in (heavy) titanium and zirconium minerals, while the other layers are dominated by potassium and rubidium minerals, typical of clays and micas. The fine-grained, fossil-rich layers were also found to be rich in organic material. This pattern of changes occurs throughout the sequence, typically happening every 10 mm in the oxidised zone at the top of the sequence, and every 5 mm throughout the rest of the sequence.
The Clarkia Palaeolake fossil assemblage is dominated by the leaves of deciduous Plants, thought to be indicative of a warm temperate environment with strong seasonality and moderate rainfall. This again ties in with a climatic regime where there were strong seasonal variations in sediment deposition, with the presence of leaf fossils in a laminated environment without bioturbation would appear to indicate an anoxic lake-bottom environment.
An understanding of these laminated beds is essential for the reconstruction of depositional environment at Clarkia Palaeolake. Assuming that depositional rates are constant through transitional Unit 3, and that the ash-beds were laid down more-or-less instantly, Höfig et al. calculate that the roughly 7.5 m section at site P-33 was laid down over approximately 840 years, at the end of the primary phase of Columbia River Basalt Group eruptions. The uranium-lead dates obtained from the volcanic ash layers are not quite accurate enough to calibrate this model, but do help to place the palaeolake within the overall temporal framework for the Colombia River Basalt eruptions. The age model developed also helps to understand the lake's evolutionary history, and the exceptional preservation seen in these deposits.
Höfig et al. favour a seasonally fluctuating climate as an origin of the laminated beds over a storm-induced origin due to the tight coupling between changes in grain size and fossil abundance. The presence of numerous leaf fossils in the lamellae interpreted as having been laid down in autumn is consistent with a seasonal deposition pattern. Storm events can sometimes cause high sedimentation rates, but it is unlikely that repeated events could create laminar stratification over such a long time period without any storm-induced turbulence affecting the regular pattern of deposition.
The rapid deposition and burial of ancient organisms in laminated beds in ancient lakes is an important pathway for the deposition of organic fossils. The sedimentation rate seen in the Clarkia Palaeolake during the time period when the deposits were being laid down in an anoxic environment, the sedimentation rate was about 12 mm per year, exceptionally high for this type of setting, something which prevented the decay of organic material, and allowed the preservation of tissue structures and biomolecules. At the upper end of the sequence, a shift in the deposition regime caused the lake to shallow rapidly, ending the stratification regime, as the waters became polymictic (i.e. active water circulation reached the bottom at all times of year), resulting in a much lower sedimentation rate (about 6 mm per year), with deposits laid down in fully oxygenated waters. This still enabled the preservation of leaf-impressions, but led to a disappearance of biomolecules in the sequence.
Höfig et al.'s uranium-lead zircon ages suggest that the Clarkia Palaeolake deposits were being laid down at the same time as the Priest Rapids Member from the Wanapum Basalt of the Columbia River Basalt Group (about 15.895 million years ago), which was formed after the peak of the eruption. The Columbia River Basalt Group produced about 12 175 km³ of basalt and released about 240-280 million kilotonnes of carbon into the atmosphere. This event it thought to be closely related to the fluctuations in atmospheric carbon dioxide known to have occurred during the Miocene, and a better constrained date for the Clarkia Palaeolake sequence has the potential to help us understand the atmospheric and climatic changes of this time.
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Online courses in Palaeontology.
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