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The cause of the end-Cretaceous mass extinction is vigorously debated, owing to the occurrence of a very large bolide impact and flood basalt volcanism near the boundary. Disentangling their relative importance is complicated by uncertainty regarding kill mechanisms and the relative timing of volcanogenic outgassing, impact, and extinction. We used carbon cycle modeling and paleotemperature records to constrain the timing of volcanogenic outgassing. We found support for major outgassing beginning and ending distinctly before the impact, with only the impact coinciding with mass extinction and biologically amplified carbon cycle change. Our models show that these extinction-related carbon cycle changes would have allowed the ocean to absorb massive amounts of carbon dioxide, thus limiting the global warming otherwise expected from postextinction volcanism.

Mass extinction at the Cretaceous-Paleogene (K-Pg) boundary coincides with the Chicxulub bolide impact and also falls within the broader time frame of Deccan trap emplacement. Critically, though, empirical evidence as to how either of these factors could have driven observed extinction patterns and carbon cycle perturbations is still lacking. Here, using boron isotopes in foraminifera, we document a geologically rapid surface-ocean pH drop following the Chicxulub impact, supporting impact-induced ocean acidification as a mechanism for ecological collapse in the marine realm. Subsequently, surface water pH rebounded sharply with the extinction of marine calcifiers and the associated imbalance in the global carbon cycle. Our reconstructed water-column pH gradients, combined with Earth system modeling, indicate that a partial ∼50% reduction in global marine primary productivity is sufficient to explain observed marine carbon isotope patterns at the K-Pg, due to the underlying action of the solubility pump. While primary productivity recovered within a few tens of thousands of years, inefficiency in carbon export to the deep sea lasted much longer. This phased recovery scenario reconciles competing hypotheses previously put forward to explain the K-Pg carbon isotope records, and explains both spatially variable patterns of change in marine productivity across the event and a lack of extinction at the deep sea floor. In sum, we provide insights into the drivers of the last mass extinction, the recovery of marine carbon cycling in a postextinction world, and the way in which marine life imprints its isotopic signal onto the geological record.

The boron isotope ratio of seawater (δ11Bsw) is a parameter which must be known to reconstruct palaeo pH and CO2 from boron isotope measurements of marine carbonates. Beyond a few million years ago, δ11Bsw is likely to have been different to modern. Palaeo δ11Bsw can be estimated by simultaneously constraining the vertical gradients in foraminiferal δ11B (Δδ11B) and pH (ΔpH). A number of subtly different techniques have been used to estimate ΔpH in the past, all broadly based on assumptions about vertical gradients in oxygen, and/or carbon, or other carbonate system constraints. In this work we pull together existing data from previous studies, alongside a constraint on the rate of change of δ11Bsw from modeling. We combine this information in an overarching statistical framework called a Gaussian Process. The Gaussian Process technique allows us to bring together data and constraints on the rate of change in δ11Bsw to generate random plausible evolutions of δ11Bsw. We reconstruct δ11Bsw, and by extension palaeo pH, across the last 65Myr using this novel methodology. Reconstructed δ11Bsw is compared to other seawater isotope ratios, namely Sr87/86 ${}^{87/86}\mathrm{S}\mathrm{r}$, Os187/188 ${}^{187/188}\mathrm{O}\mathrm{s}$, and δ7Li, which we also reconstruct with Gaussian Processes. Our method provides a template for incorporation of future δ11Bsw constraints, and a mechanism for propagation of uncertainty in δ11Bsw into future studies. Plain Language Summary: Boron naturally exists in two forms—11B and 10B. Measuring the ratio of these two forms of boron within marine shells allows us to estimate how alkaline the ocean was in the past, which is related to how much carbon dioxide is in the atmosphere. Before we can do this calculation though, we need to know some other parameters, one of which is the relative abundance of the two forms of boron in the ocean at the time (which we call δ11Bsw). Preexisting studies have estimated δ11Bsw at particular times, and here we combine them to generate a full reconstruction across the last 65 million years, accounting for uncertainties. Our reconstruction is informed by limiting the rate at which δ11Bsw can change, based on model simulations. We provide a set of plausible evolutions of δ11Bsw which can be used in future work when calculating past ocean pH. Key Points: We reconstruct the temporal evolution of seawater isotope ratios of boron, strontium, lithium, and osmium over the last 65 million yearsThe evolution of seawater boron isotope ratio shows similarity to the evolution of strontium, lithium and osmium isotope ratiosRandomly drawn, smooth time series are provided for use in uncertainty propagation in calculation of palaeo pH [ABSTRACT FROM AUTHOR]

Biometric characteristics of Orbulina universa (d'Orbigny) were used to differentiate two morphotypes present in sediment trap samples collected from the Cariaco Basin, Venezuela. Specifically, wall thickness and weight-area relationships were used to separate shells into thin (Mthin) and thick (Mthick) morphotypes. Mthick (mean thickness=19-41μm) comprises 75% of the total O. universa in these samples and has morphometric characteristics similar to that of the previously described Type I Caribbean genotype, whereas Mthin (mean thickness=6-22μm) is comparable to the Type III Mediterranean genotype. The flux of Mthick increases during periods of upwelling, whereas Mthin flux shows no systematic relationship with changing hydrographic regimes in the basin. The δ18O and δ13C of Mthick are on average 0.34% higher and 0.38% lower, respectively, than those of Mthin, suggesting that they calcify their final spherical chamber at different depths in the water column and/or differ in their vital effects on shell geochemistry. Additionally, the absolute offset in the stable isotopic compositions of the two morphotypes varies as a function of surface ocean stratification. During periods of upwelling, the δ18O and δ13C absolute offsets between Mthin and Mthick are on average -0.15 and 0.25%, whereas average δ18O and δ13C absolute offsets between these two morphotypes increase to -0.54 and 0.41% during non-upwelling periods. We attribute the higher δ18O and δ13C offsets during non-upwelling periods to a difference in final chamber calcification depth and the combined effects of temperature (δ18O), δ13CDIC (δ13C), irradiance (δ13C) and [CO32-] (δ18O and δ13C). These data provide field evidence that thin and thick morphotypes of O. universa likely experience different environmental conditions during the formation of their final chamber and, therefore, should not be combined in geochemical analyses for reconstructing past surface ocean conditions. Finally, we introduce a new proxy for reconstructing past surface ocean stratification changes via the use of Mthin and Mthick δ18O and δ13C offsets.

The boron isotopic ratio of 11B/10B (δ11BSRM951) and trace element composition of marine carbonates are key proxies for understanding carbon cycling (pH) and palaeoceanographic change. However, method validation and comparability of results between laboratories requires carbonate reference materials. Here, we report results of an inter‐laboratory comparison study to both assign δ11BSRM951 and trace element compositions to new synthetic marine carbonate reference materials (RMs), NIST RM 8301 (Coral) and NIST RM 8301 (Foram) and to assess the variance of data among laboratories. Non‐certified reference values and expanded 95% uncertainties for δ11BSRM951 in NIST RM 8301 (Coral) (+24.17‰ ± 0.18‰) and NIST RM 8301 (Foram) (+14.51‰ ± 0.17‰) solutions were assigned by consensus approach using inter‐laboratory data. Differences reported among laboratories were considerably smaller than some previous inter‐laboratory comparisons, yet discrepancies could still lead to large differences in calculated seawater pH. Similarly, variability in reported trace element information among laboratories (e.g., Mg/Ca ± 5% RSD) was often greater than within a single laboratory (e.g., Mg/Ca < 2%). Such differences potentially alter proxy‐reconstructed seawater temperature by more than 2 °C. These now well‐characterised solutions are useful reference materials to help the palaeoceanographic community build a comprehensive view of past ocean changes. [ABSTRACT FROM AUTHOR]

The Middle Eocene Climatic Optimum (MECO) was a gradual warming event and carbon cycle perturbation that occurred between 40.5 and 40.1 Ma. A number of characteristics, including greater‐than‐expected deep‐sea carbonate dissolution, a lack of globally coherent negative δ13C excursion in marine carbonates, a duration longer than the characteristic timescale of carbon cycle recovery, and the absence of a clear trigger mechanism, challenge our current understanding of the Earth system and its regulatory feedbacks. This makes the MECO one of the most enigmatic events in the Cenozoic, dubbed a middle Eocene "carbon cycle conundrum." Here we use boron isotopes in planktic foraminifera to better constrain pCO2 changes over the event. Over the MECO itself, we find that pCO2 rose by only 0.55–0.75 doublings, thus requiring a much more modest carbon injection than previously indicated by the alkenone δ13C‐pCO2 proxy. In addition, this rise in pCO2 was focused around the peak of the 400 kyr warming trend. Before this, considerable global carbonate δ18O change was asynchronous with any coherent ocean pH (and hence pCO2) excursion. This finding suggests that middle Eocene climate (and perhaps a nascent cryosphere) was highly sensitive to small changes in radiative forcing. Plain Language Summary: Geoscientists often look to periods of global warming in the geological past to understand how the Earth responds to input of atmospheric CO2. However, during the Middle Eocene Climatic Optimum (or MECO) 40 million years ago, the Earth did not respond in the way one would expect, given what we know from these earlier warming events. The MECO poses a number of puzzles for geoscientists relating to what caused it and why the Earth system responded in the way it did. Before we can hope to answer these questions, however, we need to know what atmospheric CO2 levels were in the middle Eocene and how much they changed over the MECO event. Here we use boron isotope ratios in fossil plankton shells to tell us how ocean pH (which predominantly reflects CO2 levels) changed over the MECO. We show that relatively little change in CO2 at this time were associated with large‐scale changes in climate. This suggests that during the Eocene, when CO2 levels were similar to those likely to be reached by the end of this century, the Earth's climate (and possibly ice sheets) was very sensitive to minor disturbances. Key Points: We present a new record of pCO2 across the MECO, from boron isotopes in foraminifera from multiple ocean drilling sitesIncorporating carbon cycle modeling, our data indicate pCO2 rise of about two thirds of a doubling across the eventpCO2 change during the MECO onset warming was limited, indicating heightened climate sensitivity or a nonthermal component to δ18O change [ABSTRACT FROM AUTHOR]

Paleoclimate records suggest that a rapid major transient Antarctic glaciation occurred across the Oligocene‐Miocene transition (OMT; ca. 23 Ma; ~50‐m sea level equivalent in 200–300 kyr). Orbital forcing has long been cited as an important factor determining the timing of the OMT glacial event. A similar orbital configuration occurred 1.2 Myr prior to the OMT, however, and was not associated with a major climate event, suggesting that additional mechanisms play an important role in ice sheet growth and decay. To improve our understanding of the OMT, we present a boron isotope‐based CO2 record between 22 and 24 Ma. This new record shows that δ11B/CO2 was comparatively stable in the million years prior to the OMT glaciation and decreased by 0.7‰ (equivalent to a CO2 increase of ~65 ppm) over ~300 kyr during the subsequent deglaciation. More data are needed, but we propose that the OMT glaciation was triggered by the same forces that initiated sustained Antarctic glaciation at the Eocene‐Oligocene transition: long‐term decline in CO2 to a critical threshold and a superimposed orbital configuration favorable to glaciation (an eccentricity minimum and low‐amplitude obliquity change). When comparing the reconstructed CO2 increase with estimates of δ18Osw during the deglaciation phase of the OMT, we find that the sensitivity of the cryosphere to CO2 forcing is consistent with recent ice sheet modeling studies that incorporate retreat into subglacial basins via ice cliff collapse with modest CO2 increase, with clear implications for future sea level rise. Key Points: CO2 levels were relatively low (~265 ppm; 2σ−111+166 ppm) and comparatively stable in the 500 kyr prior to and during the glaciationCO2 increased by ~65 ppm during the OMT deglaciation consistent with the latest generation of ice sheet modelsThe timing of the OMT glaciation is most likely controlled by both changes in CO2 and favorable orbital forcing [ABSTRACT FROM AUTHOR]