T111 Cenozoic Lakes Across Western North America – from the Varved Beds of the Green River Formation to the Ice-Covered Alpine Lakes of the Sierra Nevada (Posters)

Monday, 23 September 2024
8:00 AM - 5:30 PM
Hall D

PALEO-CO2 RECONSTRUCTIONS USING FOSSIL PLANTS FROM THE MIDDLE MIOCENE CLARKIA LAKE DEPOSIT

Paper number: T111-12
Program: Technical Sessions
Time: 8:00 AM – 5:30 PM

Authors

Abstract

To better understand the current climate change and its sensitivities caused by anthropogenic CO2 emissions, precise reconstructions of ancient atmospheric CO2 levels are necessary for building a more reliable relationship between global temperatures and CO2 levels. Among various paleo-CO2 estimate methods, those based on plant fossils have been focused on morphological and/or physiological features of higher plants that are sensitive to CO2 levels through photosynthesis.

The ancient Clarkia Lake was created due to the Columbia River Basalt damming the proto–St. Maries River in northern Idaho during the middle Miocene (15.78 Ma). The lacustrine deposit provides a thousand years of high-resolution record with finely laminated depositional layers yielding abundant, diversified, and well-preserved fossils for the reconstruction of paleoclimate, paleoenvironments, and paleoecosystem during the Miocene warming period. In the past decade, our lab has used the empirical stomatal frequency—SF (including stomatal density—SD, stomatal index—SI, and stomatal number per leaf needle length) methods, the C3 plant carbon isotope fractionation (Δ13C) method, and the mechanistic gas exchange modeling method such as the Franks Model to both gymnosperm and angiosperm fossils (mainly Metasequoia, Taxodium, and Lithocarpus) from this world-renowned ancient lake deposit.

We show that two deciduous conifers (Metasequoia and Taxodium) that are most commonly found from Clarkia fossil sites provide more consistent and reliable paleo-CO2 reconstruction than most angiosperm fossils. Recent results confirmed that the SF methods generally underestimate paleo-CO2. For example, the mean values of Metasequoia and Taxodium using SF methods are 282ppm and 371ppm respectively. The C3 plant Δ13C method yields wide range of estimates, from 255ppm to 787ppm, averaged 388ppm which is also lower than expected. Compared with these two methods, the Franks Model’s results using gymnosperms are more consistent, around 500ppm, with most of the previously reported middle Miocene CO2 levels. We also tested the reliability of these methods by using their nearest living relatives (NLRs) grown under known CO2 levels and compared our results with CO2 reconstructions for the middle Miocene warming using a variety of methods.

Geological Society of America Abstracts with Programs. Vol. 56, No. 5

doi: 10.1130/abs/2024AM-405295

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