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1. Moderate to Elevated Atmospheric CO ₂ During the Early Paleocene Recorded by Platanites Leaves of the San Juan Basin, New Mexico

   https://doi.org/10.1029/2021PA004408  

   Milligan, Joseph N.; Flynn, Andrew G.; Kowalczyk, Jennifer B.; Barclay, Richard S.; Geng, Jie; Royer, Dana L.; Peppe, Daniel J. (April 2022, Paleoceanography and Paleoclimatology)

   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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2. Reconciling early Deccan Traps CO ₂ outgassing and pre-KPB global climate

   https://doi.org/10.1073/pnas.2007797118  

   Hernandez Nava, Andres; Black, Benjamin A.; Gibson, Sally A.; Bodnar, Robert J.; Renne, Paul R.; Vanderkluysen, Loÿc (March 2021, Proceedings of the National Academy of Sciences)

   A 2 to 4 °C warming episode, known as the Latest Maastrichtian warming event (LMWE), preceded the Cretaceous–Paleogene boundary (KPB) mass extinction at 66.05 ± 0.08 Ma and has been linked with the onset of voluminous Deccan Traps volcanism. Here, we use direct measurements of melt-inclusion CO₂concentrations and trace-element proxies for CO₂to test the hypothesis that early Deccan magmatism triggered this warming interval. We report CO₂concentrations from NanoSIMS and Raman spectroscopic analyses of melt-inclusion glass and vapor bubbles hosted in magnesian olivines from pre-KPB Deccan primitive basalts. Reconstructed melt-inclusion CO₂concentrations range up to 0.23 to 1.2 wt% CO₂for lavas from the Saurashtra Peninsula and the Thakurvadi Formation in the Western Ghats region. Trace-element proxies for CO₂concentration (Ba and Nb) yield estimates of initial melt concentrations of 0.4 to 1.3 wt% CO₂prior to degassing. Our data imply carbon saturation and degassing of Deccan magmas initiated at high pressures near the Moho or in the lower crust. Furthermore, we find that the earliest Deccan magmas were more CO₂rich, which we hypothesize facilitated more efficient flushing and outgassing from intrusive magmas. Based on carbon cycle modeling and estimates of preserved lava volumes for pre-KPB lavas, we find that volcanic CO₂outgassing alone remains insufficient to account for the magnitude of the observed latest Maastrichtian warming. However, accounting for intrusive outgassing can reconcile early carbon-rich Deccan Traps outgassing with observed changes in climate and atmospheric pCO₂. 

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3. Quantitative critique of leaf‐based paleo‐CO ₂ proxies: Consequences for their reliability and applicability

   https://doi.org/10.1002/gj.3807  

   Konrad, Wilfried; Royer, Dana L.; Franks, Peter J.; Roth‐Nebelsick, Anita (March 2020, Geological Journal)

   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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4. Solar‐Induced Fluorescence Helps Constrain Global Patterns in Net Biosphere Exchange, as Estimated Using Atmospheric CO ₂ Observations

   https://doi.org/10.1029/2023JG007703  

   Zhang, Mingyang; Berry, Joseph A.; Shiga, Yoichi P.; Doughty, Russell B.; Madani, Nima; Li, Xing; Xiao, Jingfeng; Wen, Jiaming; Sun, Ying; Miller, Scot M. (December 2023, Journal of Geophysical Research: Biogeosciences)

   Abstract Solar‐induced chlorophyll fluorescence (SIF) shows enormous promise as a proxy for photosynthesis and as a tool for modeling variability in gross primary productivity and net biosphere exchange (NBE). In this study, we explore the skill of SIF and other vegetation indicators in predicting variability in global atmospheric CO₂observations, and thus global variability in NBE. We do so using a 4‐year record of CO₂observations from NASA's Orbiting Carbon Observatory 2 satellite and using a geostatistical inverse model. We find that existing SIF products closely correlate with space‐time variability in atmospheric CO₂observations, particularly in the extratropics. In the extratropics, all SIF products exhibit greater skill in explaining variability in atmospheric CO₂observations compared to an ensemble of process‐based CO₂flux models and other vegetation indicators. With that said, other vegetation indicators, when multiplied by photosynthetically active radiation, yield similar results as SIF and may therefore be an effective structural SIF proxy at regional to global spatial scales. Furthermore, we find that using SIF as a predictor variable in the geostatistical inverse model shifts the seasonal cycle of estimated NBE and yields an earlier end to the growing season relative to other vegetation indicators. These results highlight how SIF can help constrain global‐scale variability in NBE. 

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5. Ice core evidence for atmospheric oxygen decline since the Mid-Pleistocene transition

   https://doi.org/10.1126/sciadv.abj9341  

   Yan, Yuzhen; Brook, Edward J.; Kurbatov, Andrei V.; Severinghaus, Jeffrey P.; Higgins, John A. (December 2021, Science Advances)

   The history of atmospheric oxygen ( P O 2 ) and the processes that act to regulate it remain enigmatic because of difficulties in quantitative reconstructions using indirect proxies. Here, we extend the ice-core record of P O 2 using 1.5-million-year-old (Ma) discontinuous ice samples drilled from Allan Hills Blue Ice Area, East Antarctica. No statistically significant difference exists in P O 2 between samples at 1.5 Ma and 810 thousand years (ka), suggesting that the Late-Pleistocene imbalance in O 2 sources and sinks began around the time of the transition from 40- to 100-ka glacial cycles in the Mid-Pleistocene between ~1.2 Ma and 700 ka. The absence of a coeval secular increase in atmospheric CO 2 over the past ~1 Ma requires negative feedback mechanisms such as P co 2 -dependent silicate weathering. Fast processes must also act to suppress the immediate P co 2 increase because of the imbalance in O 2 sinks over sources beginning in the Mid-Pleistocene. 

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