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Abstract The rate and consequences of future high latitude ice sheet retreat remain a major concern given ongoing anthropogenic warming. Here, new precisely dated stalagmite data from NW Iberia provide the first direct, high-resolution records of periods of rapid melting of Northern Hemisphere ice sheets during the penultimate deglaciation. These records reveal the penultimate deglaciation initiated with rapid century-scale meltwater pulses which subsequently trigger abrupt coolings of air temperature in NW Iberia consistent with freshwater-induced AMOC slowdowns. The first of these AMOC slowdowns, 600-year duration, was shorter than Heinrich 1 of the last deglaciation. Although similar insolation forcing initiated the last two deglaciations, the more rapid and sustained rate of freshening in the eastern North Atlantic penultimate deglaciation likely reflects a larger volume of ice stored in the marine-based Eurasian Ice sheet during the penultimate glacial in contrast to the land-based ice sheet on North America as during the last glacial. 

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. 

Carbon isotope records from alkenone biomarkers (ε_p37:2) produced by haptophyte algae are frequently used for atmospheric CO₂paleobarometry, but this method has yielded inconsistent results during periods where CO₂variations are known independently. Recent syntheses of algal cultures have quantitatively demonstrated that ε_p37:2indeed records CO₂information: ε_p37:2increases as aqueous CO₂concentrations increase relative to carbon demand. However, interpretations of ε_p37:2are complicated by irradiance, where higher irradiance yields higher ε_p37:2. Here we examine the roles of physiology and environment in setting ε_p37:2in the ocean. We compile water‐column and sediment core‐top ε_p37:2data and add new core‐top measurements, including estimates of cell sizes and growth rates of the alkenone‐producing population. In support of culture studies, we find irradiance to be a key control on ε_p37:2in the modern ocean. We test a culture‐derived model of ε_p37:2and find that the quantitative relationships calibrated in culture experiments can be used to predict ε_p37:2in sediment samples. In water‐column samples, the model substantially overestimates ε_p37:2, largely resulting from higher irradiance at the depth of sample collection than the integrated light conditions under which the sampled biomass was produced and vertically mixed to the collection depth. We argue that the theory underpinning the conventional diffusive alkenone carbon isotope fractionation model, including the “b” parameter, is not supported by field data and should not be used to reconstruct past CO₂changes. Future estimates of CO₂from ε_p37:2should use empirical or mechanistic models to quantitatively account for irradiance and cell size variations.

 

Membrane permeabilities to CO₂and HCO₃⁻constrain the function of CO₂concentrating mechanisms that algae use to supply inorganic carbon for photosynthesis. In diatoms and green algae, plasma membranes are moderately to highly permeable to CO₂but effectively impermeable to HCO₃⁻. Here, CO₂and HCO₃⁻membrane permeabilities were measured using an¹⁸O‐exchange technique on two species of haptophyte algae,Emiliania huxleyiandCalcidiscus leptoporus, which showed that the plasma membranes of these species are also highly permeable to CO₂(0.006–0.02 cm · s⁻¹) but minimally permeable to HCO₃⁻. Increased temperature and CO₂generally increased CO₂membrane permeabilities in both species, possibly due to changes in lipid composition or CO₂channel proteins. Changes in CO₂membrane permeabilities showed no association with the density of calcium carbonate coccoliths surrounding the cell, which could potentially impede passage of compounds. Haptophyte plasma‐membrane permeabilities to CO₂were somewhat lower than those of diatoms but generally higher than membrane permeabilities of green algae. One caveat of these measurements is that the model used to interpret¹⁸O‐exchange data assumes that carbonic anhydrase, which catalyzes¹⁸O‐exchange, is homogeneously distributed in the cell. The implications of this assumption were tested using a two‐compartment model with an inhomogeneous distribution of carbonic anhydrase to simulate¹⁸O‐exchange data and then inferring plasma‐membrane CO₂permeabilities from the simulated data. This analysis showed that the inferred plasma‐membrane CO₂permeabilities are minimal estimates but should be quite accurate under most conditions.