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A variety of proxies have been developed to reconstruct paleo‐CO2from fossil leaves. These proxies rely on some combination of stomatal morphology, leafδ13C, and leaf gas exchange. A common conceptual framework for evaluating these proxies is lacking, which has hampered efforts for inter‐comparison. Here we develop such a framework, based on the underlying physics and biochemistry. From this conceptual framework, we find that the more extensively parameterised proxies, such as the optimisation model, are likely to be the most robust. The simpler proxies, such as the stomatal ratio model, tend to under‐predict CO2, especially in warm (>15°C) and moist (>50%humidity) environments. This identification of a structural under‐prediction may help to explain the common observation that the simpler proxies often produce estimates of paleo‐CO2that are lower than those from the more complex proxies and other, non‐leaf‐based CO2proxies. The use of extensively parameterised models is not always possible, depending on the preservation state of the fossils and the state of knowledge about the fossil's nearest living relative. With this caveat in mind, our analysis highlights the value of using the most complex leaf‐based model as possible.
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Moderate to Elevated Atmospheric CO 2 During the Early Paleocene Recorded by Platanites Leaves of the San Juan Basin, New Mexico
Abstract Throughout the Phanerozoic, estimated CO2levels from CO2proxies generally correlate well with independent estimates of temperature. However, some proxy estimates of atmospheric CO2during the Late Cretaceous and early Paleocene are low (<400 ppm), seemingly at odds with elevated sea surface temperature. Here we evaluate early Paleocene CO2by applying a leaf gas‐exchange model toPlatanitesleaves of four early Paleocene localities from the San Juan Basin, New Mexico (65.66–64.59 Ma). We first calibrate the model on two modernPlatanusspecies,Platanus occidentalisandP. × acerifolia, where we find the leaf gas‐exchange model accurately predicts present‐day CO2, with a mean error rate between 5% and 14%. Applying the model to the early Paleocene, we find CO2varies between ∼660 and 1,140 ppm. These estimates are consistent with more recent CO2estimates from boron, leaf gas‐exchange, liverwort, and paleosol proxies that all suggest moderate to elevated levels of CO2during the Late Cretaceous and early Paleocene. These levels of atmospheric CO2are more in keeping with the elevated temperature during this period.
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- Award ID(s):
- 1805228
- PAR ID:
- 10446191
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Paleoceanography and Paleoclimatology
- Volume:
- 37
- Issue:
- 4
- ISSN:
- 2572-4517
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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