Sedimentation rates of the middle Miocene Clarkia Lake deposit, Nothern Idaho, USA

A global warming phase related to the onset of the Columbia River volcanism in the USA is recorded in the middle Miocene Clarkia Lake deposit, which yields abundant fossil leaves of subtropical to warm-termperate species preserved in extraordinary conditions [1]. These leaf fossils are found in varve-like laminated successions that presumably represent seasonal phases interleaved with volcanic-ash layers [2]. Despite being studied for over four decades, this paleolake deposit remains poorly constrained in its time-scale. Defining its sedimentation rate is pivotal for reconstructing the paleoclimatic conditions during the middle Miocene. X-Ray Fluorescence (XRF) scanning of key intervals offered insights about the elemental ratio distribution in the Clarkia Lake deposit, which might hold the answer to the sedimentation rate question. Accelerating voltages of 10, 30, and 50 kV detected counts of Mg, Al, Si, P, S, Cl, Ar, K, Ca, Ti, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, As, Br, Rb, Sr, Y, Zr, Nb, Mo, and Ba. Plots of ratios using 702 element-combinations show a strong, positive correlation between the observed varve-like structures and S/Rb and Zr/Rb ratios. The former ratio is interpreted as a tracer of fluvial dilution of the presumably constant rate of reduced sulfur deposition, and the latter denotes variation in the grain-size distribution. For both ratios, low countings represent light-colored, coarse-grained, and quartz-rich layers while high countings correspond to dark-colored, fine-grained, and organic-rich layers. Volcanic ash-layers are distinguishable by enhanced signals of Si, Al, Ti, Zn, and Rb as well as low counts of Fe and Mn. Ratios of Zn and trace elements remarkably detect the extension of these layers along the profiles. Preliminary statistic treatment of this XRF data, employing spectral analysis, suggests depositional cycles at every 1.

Variations of stomatal frequency in Taxodium and Metasequoia populations at the mid-Miocene Clarkia Lake deposits: Implications for atmospheric CO2 reconstruction

Stomatal frequency (such as stomatal index-SI and stomatal density-SD) has been widely used to reconstruct atmospheric CO2 levels in geological history as it is one of the most reliable proxies of paleo-CO2 that predate the oldest icecore records. However, living plants show large variations on stomatal frequency within the same species, potentially generating large error margins for estimated paleo-CO2 levels using limited fossil specimen(s). The extraordinarily wellpreserved and abundant fossil leaves from the mid-Miocene (~15Ma) Clarkia Lake deposits in northern Idaho, the USA, allow us to test variations within a population of a fossil species and to compare that cross different contemporary species. Our preliminary results from the SD of 15 cuticular membranes of Taxodium revealed a range of variation leading to CO2 levels of 345-445 parts per million (ppm). The SI of eight cuticular membranes of Metasequoia from the same fossiliferous layers reconstructed CO2 levels of 290-345 ppm. These wide and discrepant ranges imply that randomly selected fossil leaves with limited sample numbers may give a large range of CO2 reconstructions and different methods (such as SD or SI) and different plant taxa (such as Taxodium and Metasequoia) may result in different CO2 results. A better understanding of stomatal frequency variations within populations and consistent sampling method will reduce errors in paleo-CO2 reconstruction

Early Miocene Redwood Fossils from Inner Mongolia: CO2 Reconstructions and Paleoclimate Effects of a Low Mongolian Plateau

The early Miocene (~16–23 Ma) marks a critical transition in the Earth climate history from an Oligocene (~23–34 Ma) cooling trend towards the well-documented warm middle Miocene Climate Optimum at ~ 15 Ma. In eastern Asia, this transition links changes of key topographic features, such as the Tibetan plateau and the Mongolian plateau, and their impact on the reorganization of climate systems, such as the Eastern Asian summer monsoon. Yet the dynamics of the interplay among these factors remain poorly understood, precluding our understanding of future climate changes. Global temperatures during the early Miocene were warmer than the present day by 3–4 °C, which are seemingly incompatible with both the low (\u3c300 ppm) and high (\u3e800 ppm) ends of presently available reconstructions of atmospheric carbon dioxide (CO2). Here we report a rare co-occurrence of two redwoods, Metasequoia and Sequoia, from sediments of the early Miocene Hannuoba Formation in Zhuozi County in China’s Inner Mongolia at the southeastern margin of the current Mongolian plateau. The Zhuozi Metasequoia fossils possess uneven type cuticles, which dominate its living population but have rarely been reported throughout the abundant fossil record of this genus. By applying wellconstrained Franks model parameters obtained from these redwood fossils using a cleared leaf epidermis method, we estimated the early Miocene CO2 level at ~400 ppm, putting it at the lower end of the model requirement for sustaining a relatively warm global temperature during this period. Our Franks model estimates are ~100 ppm higher than that obtained using stomatal index method based on Metasequoia material, further confirming a systematic underestimation of ancient CO2 using the Metasequoia stomatal index method reported in previous analyses. We recommend a re-examination of previous CO2 reconstructions solely based upon Metasequoia’s inverse relationship between stomatal index and CO2 concentrations. Ultimately, the occurrence of these redwood fossils in Inner Mongolia is consistent with a weak or muted Eastern Asian summer monsoon in the region with the absence of an elevated Mongolian plateau during the early Miocene. A shift of moisture sources for the region accompanying the change from a Westerlies-dominated climate to the present-day monsoondominated climate system occurred after the early Miocene time

Annually-resolved Sedimentation of the Middle Miocene Clarkia Lake Deposit (USA)

Lacustrine varved sediments are high-resolution paleoenvironmental archives, in which their layers function as temporal markers of seasonality. The temporal framework of such records is required to improve benchmark climate models because they offer snapshots of past climate systems within annual to millennial timescales. The middle Miocene Clarkia Lake Deposit (Idado, USA) is a plant fossil Lagerstätte containing varve-like sediments deposited during the Miocene Climate Optimum, a phase of global warming and carbon cycle perturbation at ca. 16 million years ago. Despite being extensively studied in the past decades, its absolute age and temporal relationship with the Columbia River Flood Basalt, and the causes of the varves remained elusive. This study presents the first tephra- derived U-Pb zircon ages for the Clarkia Lake Deposit and an age model based on spectral analysis of elemental and color distribution of the sediments obtained via micro X-Ray fluorescence. The results show that the Clarkia Lake Deposit (1) is dated at 15.78 ± 0.03 Ma, after the most intense eruption phases of the Columbia River Flood Basalt Group; and (2) represents ~840 varve-years. These stratigraphic relationships highlight the Clarkia Lake Deposit as a high-resolution archive of the Miocene Climate Optimum, in a greenhouse world that may be employed as an analog for modeling future Earth’s climate