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1. Atmospheric CO 2 Estimates for the Late Oligocene and Early Miocene Using Multi‐Species Cross‐Calibrations of Boron Isotopes

   https://doi.org/10.1029/2022PA004569  

   Anderson, Lloyd B. ; Hönisch, Bärbel ; Coxall, Helen K. ; Bolge, Louise ( January 2024 , Paleoceanography and Paleoclimatology)

   The boron isotope (δ¹¹B) proxy for seawater pH is a tried and tested means to reconstruct atmospheric CO₂in the geologic past, but uncertainty remains over how to treat species‐specific calibrations that link foraminiferal δ¹¹B to pH estimates prior to 22 My. In addition, no δ¹¹B‐based reconstructions of atmospheric CO₂exist for wide swaths of the Oligocene (33–23 Ma), and large variability in CO₂reconstructions during this epoch based on other proxy evidence leaves climate evolution during this period relatively unconstrained. To add to our understanding of Oligocene and early Miocene climate, we generated new atmospheric CO₂estimates from new δ¹¹B data from fossil shells of surface‐dwelling planktic foraminifera from the mid‐Oligocene to early Miocene (∼28–18 Ma). We estimate atmospheric CO₂of ∼680 ppm for the mid‐Oligocene, which then evolves to fluctuate between ∼500–570 ppm during the late Oligocene and between ∼420–700 ppm in the early Miocene. These estimates tend to trend higher than Oligo‐Miocene CO₂estimates from other proxies, although we observe good proxy agreement in the late Oligocene. Reconstructions of CO₂fall lower than estimates from paleoclimate model simulations in the early Miocene and mid Oligocene, which indicates that more proxy and/or model refinement is needed for these periods. Our species cross‐calibrations, assessing δ¹¹B, Mg/Ca, δ¹⁸O, and δ¹³C, are able to pinpoint and evaluate small differences in the geochemistry of surface‐dwelling planktic foraminifera, lending confidence to paleoceanographers applying this approach even further back in time.

    

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2. 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)

   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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3. Triple oxygen isotope constraints on atmospheric O 2 and biological productivity during the mid-Proterozoic

   https://doi.org/10.1073/pnas.2105074118  

   Liu, Peng ; Liu, Jingjun ; Ji, Aoshuang ; Reinhard, Christopher T. ; Planavsky, Noah J. ; Babikov, Dmitri ; Najjar, Raymond G. ; Kasting, James F. ( December 2021 , Proceedings of the National Academy of Sciences)

   Reconstructing the history of biological productivity and atmospheric oxygen partial pressure ( p O 2 ) is a fundamental goal of geobiology. Recently, the mass-independent fractionation of oxygen isotopes (O-MIF) has been used as a tool for estimating p O 2 and productivity during the Proterozoic. O-MIF, reported as Δ′ 17 O, is produced during the formation of ozone and destroyed by isotopic exchange with water by biological and chemical processes. Atmospheric O-MIF can be preserved in the geologic record when pyrite (FeS 2 ) is oxidized during weathering, and the sulfur is redeposited as sulfate. Here, sedimentary sulfates from the ∼1.4-Ga Sibley Formation are reanalyzed using a detailed one-dimensional photochemical model that includes physical constraints on air–sea gas exchange. Previous analyses of these data concluded that p O 2 at that time was <1% PAL (times the present atmospheric level). Our model shows that the upper limit on p O 2 is essentially unconstrained by these data. Indeed, p O 2 levels below 0.8% PAL are possible only if atmospheric methane was more abundant than today (so that p CO 2 could have been lower) or if the Sibley O-MIF data were diluted by reprocessing before the sulfates were deposited. Our model also shows that, contrary to previous assertions, marine productivity cannot be reliably constrained by the O-MIF data because the exchange of molecular oxygen (O 2 ) between the atmosphere and surface ocean is controlled more by air–sea gas transfer rates than by biological productivity. Improved estimates of p CO 2 and/or improved proxies for Δ′ 17 O of atmospheric O 2 would allow tighter constraints to be placed on mid-Proterozoic p O 2 . 

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4. 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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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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