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.
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 to
- Award ID(s):
- 1805228
- NSF-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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Abstract Despite recent empirical and modeling advances, atmospheric CO2concentrations during the early Pleistocene remain uncertain. Using a recently‐developed Bayesian paleoclimate model, an ensemble of seven different CO2records is inferred conditional on reconstructions of past sea level. Five ensemble members give a consensus prediction that CO2concentrations averaged 241 ppm (238–245 ppm 95% CI) between 2 and 0.8 Ma. Uncertainty estimates account for contributions from orbital forcing, age uncertainties, and other factors. Our consensus prediction aligns well with a compilation of previously published
δ 11B‐based CO2reconstructions after calibration to late‐Pleistocene ice‐core CO2values, as well as with 60 early‐Pleistocene CO2measurements from the Allan Hills in East Antarctica. Our consensus prediction can be definitively tested by obtaining continuous ice‐core atmospheric CO2records that extend into the early Pleistocene. -
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<bold>SUMMARY</bold> While the adverse effects of elevated salinity levels on leaf gas exchange in many crops are not in dispute, representing such effects on leaf photosynthetic rates (
A ) continues to draw research attention. Here, an optimization model for stomatal conductance (g c) that maximizesA while accounting for mesophyll conductance (g m) was used to interpret new leaf gas exchange measurements collected for five irrigation water salinity levels. A function between chloroplastic CO2concentration (c c) and intercellular CO2concentration (c i) modified by salinity stress to estimateg mwas proposed. Results showed that with increased salinity, the estimatedg mand maximum photosynthetic capacity were both reduced, whereas the marginal water use efficiencyλ increased linearly. Adjustments ofg m,λ and photosynthetic capacity were shown to be consistent with a large corpus of drought‐stress experiments. The inferred model parameters were then used to evaluate the combined effects of elevated salinity and atmospheric CO2concentration (c a) on leaf gas exchange. For a given salinity level, increasingc aincreasedA linearly, but these increases were accompanied by mild reductions ing cand transpiration. Thec alevel needed to ameliorateA reductions due to increased salinity is also discussed using the aforementioned model calculations.
