skip to main content

Title: Atmospheric CO 2 over the Past 66 Million Years from Marine Archives
Throughout Earth's history, CO 2 is thought to have exerted a fundamental control on environmental change. Here we review and revise CO 2 reconstructions from boron isotopes in carbonates and carbon isotopes in organic matter over the Cenozoic—the past 66 million years. We find close coupling between CO 2 and climate throughout the Cenozoic, with peak CO 2 levels of ∼1,500 ppm in the Eocene greenhouse, decreasing to ∼500 ppm in the Miocene, and falling further into the ice age world of the Plio–Pleistocene. Around two-thirds of Cenozoic CO 2 drawdown is explained by an increase in the ratio of ocean alkalinity to dissolved inorganic carbon, likely linked to a change in the balance of weathering to outgassing, with the remaining one-third due to changing ocean temperature and major ion composition. Earth system climate sensitivity is explored and may vary between different time intervals. The Cenozoic CO 2 record highlights the truly geological scale of anthropogenic CO 2 change: Current CO 2 levels were last seen around 3 million years ago, and major cuts in emissions are required to prevent a return to the CO 2 levels of the Miocene or Eocene in the coming century. ▪  CO 2 reconstructions over the past 66 Myr from boron isotopes and alkenones are reviewed and re-evaluated. ▪  CO 2 estimates from the different proxies show close agreement, yielding a consistent picture of the evolution of the ocean-atmosphere CO 2 system over the Cenozoic. ▪  CO 2 and climate are coupled throughout the past 66 Myr, providing broad constraints on Earth system climate sensitivity. ▪  Twenty-first-century carbon emissions have the potential to return CO 2 to levels not seen since the much warmer climates of Earth's distant past.  more » « less
Award ID(s):
Author(s) / Creator(s):
; ; ; ; ;
Date Published:
Journal Name:
Annual Review of Earth and Planetary Sciences
Page Range / eLocation ID:
609 to 641
Medium: X
Sponsoring Org:
National Science Foundation
More Like this
  1. Abstract

    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.

    more » « less
  2. Alkenones are long-chain ketones produced by phytoplankton of the order Isochrysidales. They are widely used in reconstructing past sea surface temperatures, benefiting from their ubiquitous occurrence in the Cenozoic ocean. Carbon isotope fractionation (εp) between alkenones and dissolved inorganic carbon may also be used as a proxy for past atmospheric pCO2 and has provided continuous pCO2 estimates back to ca. 45 Ma. Here, an extended occurrence of alkenones from ca. 130 Ma is reported. We characterize the molecular structure and distribution of these Mesozoic alkenones and evaluate their potential phylogenetic relationship with Cenozoic alkenones. Using δ13C values of the C37 methyl alkenone (C37:2Me), the first alkenone-based pCO2 estimates for the Mesozoic are derived. These estimates suggest elevated pCO2 with a range of 548−4090 ppm (908 ppm median) during the super-greenhouse climate of the Early Cretaceous, in agreement with phytane-based pCO2 reconstructions. Finally, insights into the identity of the Cretaceous coccolithophores that possibly synthesized alkenones are also offered.

    more » « less
  3. null (Ed.)
    Abstract. Cenozoic stable carbon (δ13C) and oxygen (δ18O)isotope ratios of deep-sea foraminiferal calcite co-vary with the 405 kyreccentricity cycle, suggesting a link between orbital forcing, the climatesystem, and the carbon cycle. Variations in δ18O are partlyforced by ice-volume changes that have mostly occurred since the Oligocene.The cyclic δ13C–δ18O co-variation is found inboth ice-free and glaciated climate states, however. Consequently, thereshould be a mechanism that forces the δ13C cyclesindependently of ice dynamics. In search of this mechanism, we simulate theresponse of several key components of the carbon cycle to orbital forcing inthe Long-term Ocean-atmosphere-Sediment CArbon cycle Reservoir model(LOSCAR). We force the model by changing the burial of organic carbon in theocean with various astronomical solutions and noise and study the responseof the main carbon cycle tracers. Consistent with previous work, thesimulations reveal that low-frequency oscillations in the forcing arepreferentially amplified relative to higher frequencies. However, whileoceanic δ13C mainly varies with a 405 kyr period in themodel, the dynamics of dissolved inorganic carbon in the oceans and ofatmospheric CO2 are dominated by the 2.4 Myr cycle of eccentricity.This implies that the total ocean and atmosphere carbon inventory is stronglyinfluenced by carbon cycle variability that exceeds the timescale of the405 kyr period (such as silicate weathering). To test the applicability ofthe model results, we assemble a long (∼22 Myr) δ13C andδ18O composite record spanning the Eocene to Miocene(34–12 Ma) and perform spectral analysis to assess the presence of the2.4 Myr cycle. We find that, while the 2.4 Myr cycle appears to beovershadowed by long-term changes in the composite record, it is present asan amplitude modulator of the 405 and 100 kyr eccentricity cycles. 
    more » « less
  4. Abstract

    Biological productivity in the ocean directly influences the partitioning of carbon between the atmosphere and ocean interior. Through this carbon cycle feedback, changing ocean productivity has long been hypothesized as a key pathway for modulating past atmospheric carbon dioxide levels and hence global climate. Because phytoplankton preferentially assimilate the light isotopes of carbon and the major nutrients nitrate and silicic acid, stable isotopes of carbon (C), nitrogen (N), and silicon (Si) in seawater and marine sediments can inform on ocean carbon and nutrient cycling, and by extension the relationship with biological productivity and global climate. Here, we compile water column C, N, and Si stable isotopes from GEOTRACES‐era data in four key ocean regions to review geochemical proxies of oceanic carbon and nutrient cycling based on the C, N, and Si isotopic composition of marine sediments. External sources and sinks as well as internal cycling (including assimilation, particulate matter export, and regeneration) are discussed as likely drivers of observed C, N, and Si isotope distributions in the ocean. The potential for C, N, and Si isotope measurements in sedimentary archives to record aspects of past ocean C and nutrient cycling is evaluated, along with key uncertainties and limitations associated with each proxy. Constraints on ocean C and nutrient cycling during late Quaternary glacial‐interglacial cycles and over the Cenozoic are examined. This review highlights opportunities for future research using multielement stable isotope proxy applications and emphasizes the importance of such applications to reconstructing past changes in the oceans and climate system.

    more » « less
  5. Abstract

    The Miocene epoch, spanning 23.03–5.33 Ma, was a dynamic climate of sustained, polar amplified warmth. Miocene atmospheric CO2concentrations are typically reconstructed between 300 and 600 ppm and were potentially higher during the Miocene Climatic Optimum (16.75–14.5 Ma). With surface temperature reconstructions pointing to substantial midlatitude and polar warmth, it is unclear what processes maintained the much weaker‐than‐modern equator‐to‐pole temperature difference. Here, we synthesize several Miocene climate modeling efforts together with available terrestrial and ocean surface temperature reconstructions. We evaluate the range of model‐data agreement, highlight robust mechanisms operating across Miocene modeling efforts and regions where differences across experiments result in a large spread in warming responses. Prescribed CO2is the primary factor controlling global warming across the ensemble. On average, elements other than CO2, such as Miocene paleogeography and ice sheets, raise global mean temperature by ∼2°C, with the spread in warming under a given CO2concentration (due to a combination of the spread in imposed boundary conditions and climate feedback strengths) equivalent to ∼1.2 times a CO2doubling. This study uses an ensemble of opportunity: models, boundary conditions, and reference data sets represent the state‐of‐art for the Miocene, but are inhomogeneous and not ideal for a formal intermodel comparison effort. Acknowledging this caveat, this study is nevertheless the first Miocene multi‐model, multi‐proxy comparison attempted so far. This study serves to take stock of the current progress toward simulating Miocene warmth while isolating remaining challenges that may be well served by community‐led efforts to coordinate modeling and data activities within a common analytical framework.

    more » « less