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During the Mid-Pleistocene Transition (MPT; 1,200-800 kya), Earth's orbitally paced ice age cycles intensified, lengthened from ∼40,000 (∼40 ky) to ∼100 ky, and became distinctly asymmetrical. Testing hypotheses that implicate changing atmospheric CO2 levels as a driver of the MPT has proven difficult with available observations. Here, we use orbitally resolved, boron isotope CO2 data to show that the glacial to interglacial CO2 difference increased from ∼43 to ∼75 μatm across the MPT, mainly because of lower glacial CO2 levels. Through carbon cycle modeling, we attribute this decline primarily to the initiation of substantive dust-borne iron fertilization of the Southern Ocean during peak glacial stages. We also observe a twofold steepening of the relationship between sea level and CO2-related climate forcing that is suggestive of a change in the dynamics that govern ice sheet stability, such as that expected from the removal of subglacial regolith or interhemispheric ice sheet phase-locking. We argue that neither ice sheet dynamics nor CO2 change in isolation can explain the MPT. Instead, we infer that the MPT was initiated by a change in ice sheet dynamics and that longer and deeper post-MPT ice ages were sustained by carbon cycle feedbacks related to dust fertilization of the Southern Ocean as a consequence of larger ice sheets.

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]

Boron isotopes in planktonic foraminifera are a widely used proxy to determine ancient surface seawater pH and by extension atmospheric CO 2 concentration and climate forcing on geological timescales. Yet, to reconstruct absolute values for pH and CO 2 , we require a δ11 B foram-borate to pH calibration and independent determinations of ocean temperature, salinity, a second carbonate parameter, and the boron isotope composition of seawater. Although δ11 B-derived records of atmospheric CO 2 have been shown to perform well against ice-core-based CO 2 reconstructions, these tests have been performed at only a few locations and with limited temporal resolution. Here we present two highly resolved CO 2 records for the late Pleistocene from Ocean Drilling Program (ODP) Sites 999 and 871. Our δ11 B-derived CO 2 record shows a very good agreement with the ice core CO 2 record with an average offset of 13±46 (2σ) and an RMSE of 26 ppm, with minor short-lived overestimations of CO 2 (of up to ∼50 ppm) occurring during some glacial onsets. We explore potential drivers of this disagreement and conclude that partial dissolution of foraminifera has a minimal effect on the CO 2 offset. We also observe that the general agreement between δ11 B-derived and ice core CO 2 is improved by optimising the δ11 B foram-borate calibration. Despite these minor issues, a strong linear relationship between relative change in climate forcing from CO 2 (from ice core data) and pH change (from δ11 B) exists over the late Pleistocene, confirming that pH change is a robust proxy of climate forcing over relatively short (<1 million year) intervals. Overall, these findings demonstrate that the boron isotope proxy is a reliable indicator of CO 2 beyond the reach of the ice cores and can help improve determinations of climate sensitivity for ancient time intervals. [ABSTRACT FROM AUTHOR]

• The δ11B of coralline algae skeleton offer a means to reconstruct coastal pH. • 2D maps of trace element content and δ11B were collected on Boreolithothamniom cf. soriferum. • These images were aligned using correlated multimodal imaging techniques. • We reconstruct an ocean acidification trend of −0.018 pH units yr-1 in Loch Sween. • Loch Sween switched from being a substantial sink of CO 2 to a source in ∼2008. Ocean Acidification (OA) arises from the increase in atmospheric carbon dioxide concentration following the industrial revolution. The ecological and socio-economic consequences of OA were first identified around 10–15 years ago but remain poorly understood. This is particularly true in coastal regions where local processes can have dramatic consequences on pH trends through time, obscuring and compounding the long-term effects from rising atmospheric CO 2. Here we explore the possibility of generating long records of coastal ocean pH using the skeletons of widely distributed coralline algae (CA). The skeletons of these slow growing (<1 mm/year) taxa often contain micron-scale heterogeneities, making sampling for high-resolution climate reconstructions using bulk sampling techniques difficult. Here we use laser ablation coupled to inductively coupled plasma mass spectrometers to generate high-resolution 2D images of the element/calcium ratios and boron isotope composition (δ11B) of a sample of Boreolithothamniom cf. soriferum from Loch Sween in Scotland, UK where we have been monitoring temperature since 2004 and pH during 2014. By carefully correlating the geochemical images with a scanning electron microscopy image we can segment them to remove the marginal portions of the skeleton, isolating the central growth axis to generate an age model and growth rate. The δ11B-pH is significantly elevated above the seawater pH in Loch Sween (8.4 to 8.9 vs. 7.9 to 8.1) consistent with other CA that show internal pH elevation. On a seasonal scale, internal pH is negatively correlated with temperature and also exhibits a long-term decline. By removing this temperature effect, internal pH can be correlated to seawater pH during the 2014 monitoring period allowing us to reconstruct a seawater acidification trend from 2004 to 2018 of -0.018 pH units per year, 10x higher than open ocean trends but consistent with contemporaneous monitoring efforts of UK coastal waters. Reconstructed aqueous CO 2 suggests that prior to ∼2008 Loch Sween was a sink of CO 2 but after this date, particularly during the early summer, it was a substantial CO 2 source. Comparison of reconstructed aqueous CO 2 with a record of calcification rate of our sample of Boreolithothamniom cf. soriferum suggests this acidification and associated rise in local seawater pCO 2 may have freed this sample from carbon limitation leading to a recent increase in calcification. [ABSTRACT FROM AUTHOR]

Identifying the causes and consequences of natural variations in ocean acidification and atmospheric CO2 due to complex earth processes has been a major challenge for climate scientists in the past few decades. Recent developments in the boron isotope (δ11B) based seawater pH and pCO2 (or pCO2sw) proxy have been pivotal in understanding the various oceanic processes involved in air-sea CO2 exchange. Here we present the first foraminifera-based δ11B record from the north-eastern Arabian Sea (NEAS) covering the mid-late Holocene (~ 8–1 ka). Our record suggests that the region was overall a moderate to strong CO2 sink during the last 7.7 kyr. The region behaved as a significant CO2 source during two short intervals around 5.5–4 ka and 2.8–2.5 ka. The decreased pH and increased CO2 outgassing during those abrupt episodes are associated with the increased upwelling in the area. The upwelled waters may have increased the nutrient content of the surface water through either increased supply or weaker export production. This new dataset from the coastal NEAS suggests that, as a potential result of changes in the strength of the El-Nino Southern Oscillation, the region experienced short episodes of high CO2 outgassing and pre-industrial ocean acidification comparable to or even greater than that experienced during the last ~ 200 years. [ABSTRACT FROM AUTHOR]

Physical parameters within boron isotope systematics form a complex interplay that determine the boron isotopic composition of rocks, minerals, and fluids, but to date, providing constraints on uncertainty within boron equilibrium isotope modelling remains elusive. This underlying uncertainty limits the potency of boron isotopes as a tool for detecting fluid-rock exchange. A new equilibrium boron mineral-fluid fractionation modelling approach, named EquiB, coupled with a Bayesian inversion engine is presented, providing robust and reproducible constraints on the uncertainty of physical parameters encoded into a boron isotopic composition of a rock in equilibrium with a fluid. We demonstrate the validity of our approach by applying the model to several basalt-fluid and peridotite-fluid exchange scenarios. The model output generates multi-dimensional posterior probability distributions that show temperature is the greatest control on mineral-fluid fractionation in all applied scenarios. At high temperatures (defined as >50 °C) pH-dependent fractionation is negligible, but at low temperatures (defined as <50 °C) pH-dependent fractionation is a control on boron isotopic compositions. At geologically reasonable conditions other parameters such as salinity, fluid density, and pressure have little effect on the extent of boron mineral-fluid fractionation. Model outputs agree with experimentally derived fractionation factors at typical hydrothermal conditions but diverge at low temperatures. This approach provides robust constraints of parameter uncertainty, enabling meaningful interpretation of boron isotope analyses and the ability to fingerprint isotopic compositions with greater confidence. [ABSTRACT FROM AUTHOR]

Throughout Earth's history, CO2 is thought to have exerted a fundamental control on environmental change. Here we review and revise CO2 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 CO2 and climate throughout the Cenozoic, with peak CO2 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 CO2 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 CO2 record highlights the truly geological scale of anthropogenic CO2 change: Current CO2 levels were last seen around 3 million years ago, and major cuts in emissions are required to prevent a return to the CO2 levels of the Miocene or Eocene in the coming century. CO2 reconstructions over the past 66 Myr from boron isotopes and alkenones are reviewed and re-evaluated. CO2 estimates from the different proxies show close agreement, yielding a consistent picture of the evolution of the ocean-atmosphere CO2 system over the Cenozoic. CO2 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 CO2 to levels not seen since the much warmer climates of Earth's distant past. [ABSTRACT FROM AUTHOR]

Boron isotopes in marine carbonates are increasingly used to reconstruct seawater pH and atmospheric pCO2 through Earth's history. While isotope ratio measurements from individual laboratories are often of high quality, it is important that records generated in different laboratories can equally be compared. Within this Boron Isotope Intercomparison Project (BIIP), we characterised the boron isotopic composition (commonly expressed in δ11B) of two marine carbonates: Geological Survey of Japan carbonate reference materials JCp‐1 (coral Porites) and JCt‐1 (giant clam Tridacna gigas). Our study has three foci: (a) to assess the extent to which oxidative pre‐treatment, aimed at removing organic material from carbonate, can influence the resulting δ11B; (b) to determine to what degree the chosen analytical approach may affect the resultant δ11B; and (c) to provide well‐constrained consensus δ11B values for JCp‐1 and JCt‐1. The resultant robust mean and associated robust standard deviation (s*) for un‐oxidised JCp‐1 is 24.36 ± 0.45‰ (2s*), compared with 24.25 ± 0.22‰ (2s*) for the same oxidised material. For un‐oxidised JCt‐1, respective compositions are 16.39 ± 0.60‰ (2s*; un‐oxidised) and 16.24 ± 0.38‰ (2s*; oxidised). The consistency between laboratories is generally better if carbonate powders were oxidatively cleaned prior to purification and measurement. [ABSTRACT FROM AUTHOR]

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]

Coral resilience to ocean acidification is largely determined by the degree of physiological control corals can exert on their calcifying fluid carbonate chemistry. In this study, the boron isotopic composition (δ11B) of a Porites colony growing on a reef flat on Kiritimati Island in the equatorial central Pacific is examined to quantify the sensitivity of calcifying fluid pH (pHcf) to ambient environmental conditions. Skeletal δ11B along the growth axis of one annual growth band was determined with bulk analysis and by laser ablation (LA) MC‐ICP‐MS. Furthermore, the oxygen and carbon isotopic composition, trace element ratios, and skeletal density were quantified. Sclerochronological data were interpreted in the context of simultaneous recordings of reef flat seawater pH (pHsw), temperature, salinity, and water depth, and by measurements of these parameters on the fore‐reef. A recent model of pHcf upregulation, after optimization with seasonally resolved data, was used to simulate pHcf variability on a diurnal scale. Results showed that on a seasonal scale, Porites pHcf is upregulated compared to ambient seawater: both bulk and LA‐MC‐ICP‐MS derived δ11B resulted in a mean pHcf of 8.35 pH units. Calcifying fluid pH upregulation primarily followed variations in seawater temperatures, that is likely related to the control of temperature on calcification rate. On the reef flat, the diurnal range in pHsw was substantially higher (0.29 pH units) than on the fore‐reef (0.07 pH units). However, model results suggest that the high diurnal variability in reef flat pHsw resulted only in a limited variability in Porites pHcf. Plain Language Summary: Ocean acidification and the associated decline in seawater pH has impacted the ability of many marine organisms to calcify. However, tropical corals do not precipitate their aragonite skeletons from seawater, but instead from a calcifying fluid located between skeleton and living tissue. The chemistry of this calcifying fluid is modified by physiological processes. This study examines the sensitivity of the calcifying fluid pH to variations in environmental conditions. Seawater pH, temperature, salinity, and water depth on a reef flat at Kiritimati Island in the central Pacific were monitored for 1 year. The calcifying fluid pH of a synchronously precipitated annual growth band of a Porites colony was determined using boron isotope analysis with dissolution of samples and novel laser ablation at enhanced resolution. Furthermore, a numerical model predicted calcifying fluid pH variability at a diurnal scale based on the environmental and geochemical data collected. Results indicate that on a seasonal scale, seawater temperatures rather than seawater pH primarily control the calcifying fluid pH. Further geochemical analysis suggests that this is related to the temperature dependency of the calcification rate. On a diurnal scale, model results suggest no elevated variability in calcifying fluid pH despite highly variable reef flat seawater conditions. Key Points: Porites calcifying fluid pH was investigated using conventional and laser ablation boron isotope analysis, and a calcification modelOn a seasonal scale, Porites calcifying fluid pH followed mainly seawater temperatures that control calcification ratesOn a diurnal scale, model estimates suggest limited calcifying fluid pH variability despite highly variable reef flat seawater conditions [ABSTRACT FROM AUTHOR]

Rationale: To detect the small changes in past pH, the boron isotope ratio of coral carbonates, expressed as the δ11B value, needs to be both precise and accurate (2sd <<1‰). Boron measurements by Multi-Collector Inductively Coupled Plasma Mass Spectrometry (MC-ICPMS) require the boron to be carefully purified before analysis, which is time consuming, and requires specialist training. Here, we use the prepFAST-MC method that enables the automatic extraction of B (up to 25 ng load) from a CaCO3 matrix. Methods: Samples were purified using the prepFAST-MC automated system with a ~25-μL column of Amberlite IRA743 resin. Boron isotope measurements were performed by MC-ICPMS. The effects of matrix load, speed of sample loading onto the column, and blank contamination were tested to evaluate the effects on the purification process. The optimised protocol was tested on various standards and samples of aragonite corals. Results: The blank contribution for the approach is ~60 pg and is negligible given our sample size (<0.2% sample size). Efficiency of matrix removal is demonstrated with the addition of up to 1.6 mg of dissolved low-B calcium carbonate to NIST SRM 951 with no impact on the accuracy of δ11B values. The Japanese Geological Survey Porites reference material JCp-1, boric acid standard NIST SRM 951, and seawater, all processed on the prepFAST-MC system, give δ11B values within error of literature values (δ11BJCp-1 = 24.31 ± 0.20‰ (2sd, n = 20); δ11BNIST 951 = -0.02 ± 0.15‰ (2sd, n = 13) and δ11BSeawater = 39.50 ± 0.06‰(2sd, n = 2)). Results obtained from the coral Siderastrea siderea purified with the prepFAST-MC system show an average offset from the manual ion-exchange protocols of Δδ11B = 0.01 ± 0.28‰(2sd, n = 12). Conclusions: Our study demonstrates the capacity of the prepFAST-MC method to generate accurate and reproducible δ11B values for a range of materials, without fractionation, with efficient matrix removal and with negligible blank contribution. [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]

The high-latitude oceans are key areas of carbon and heat exchange between the atmosphere and the ocean. As such, they are a focus of both modern oceanographic and palaeoclimate research. However, most palaeoclimate proxies that could provide a long-term perspective are based on calcareous organisms, such as foraminifera, that are scarce or entirely absent in deep-sea sediments south of 50° S in the Southern Ocean and north of 40° N in the North Pacific. As a result, proxies need to be developed for the opal-based organisms (e.g. diatoms) found at these high latitudes, which dominate the biogenic sediments recovered from these regions. Here we present a method for the analysis of the boron (B) content and isotopic composition (δ[sup 11]B) of diatom opal. We apply it for the first time to evaluate the relationship between seawater pH, δ[sup 11]B and B concentration ([B]) in the frustules of the diatom Thalassiosira weissflogii, cultured across a range of carbon dioxide partial pressure (pCO2) and pH values. In agreement with existing data, we find that the [B] of the cultured diatom frustules increases with increasing pH (Mejía et al., 2013). δ[sup 11]B shows a relatively well defined negative trend with increasing pH, completely distinct from any other biomineral previously measured. This relationship not only has implications for the magnitude of the isotopic fractionation that occurs during boron incorporation into opal, but also allows us to explore the potential of the boron-based proxies for palaeo-pH and palaeo-CO2 reconstruction in high-latitude marine sediments that have, up until now, eluded study due to the lack of suitable carbonate material. [ABSTRACT FROM AUTHOR]

Rationale: Boron isotope analysis of marine carbonates by laser ablation multi‐collector inductively coupled plasma mass spectrometry (LA‐MC‐ICP‐MS) offers the potential for rapid sample throughput, and the means to examine micron‐scale variations in the δ11B signatures of fossil skeletons and shells/tests of marine organisms. Existing studies demonstrate an acceptable level of reproducibility is achievable, but also typically show a level of accuracy outside the limits required by most applications. Here we investigate matrix interference effects as a cause of inaccuracy and imprecision. Methods: Analyses were performed on a standard format Thermo Scientific Neptune Plus MC‐ICP mass spectrometer coupled to a New Wave Research 193 nm ArF laser ablation system. The effects of matrix interference on δ11B analysis were investigated through analyses of a set of reference materials with differing B/Ca ratios. Three approaches to correct for matrix‐induced effects were trialled: (1) use of matrix‐matched standards, (2) utilisation of the relationship between δ11B inaccuracy and11B/43Ca, 11B/40ArCa4+ or 11B/Cainterference from three reference materials with known δ11B values and varying B/Ca ratios, and (3) direct characterisation of the (sloping) interference itself. Results: Matrix interference from scattered Ca ions on 10B can impede both the accuracy and the reproducibility of δ11B analysis by LA‐MC‐ICP‐MS. Based on analyses of two in‐house reference materials, deep sea coral PS69/3181 and inorganic calcite UWC‐1, we find approach 2, following the 11B/Cainterference relationship, gives the best mean accuracies (within 0.4‰ of solution values) and external reproducibilities (± 0.5‰ 2 SD for PS69/3181). This approach has been applied to analyses of an annual growth cycle of a Siderastrea siderea coral and eight Cibicidoides wuellerstorfi benthic foraminifera. Both coral and foraminifera data match solution MC‐ICP‐MS analyses within reported uncertainties. Conclusions: LA‐MC‐ICP‐MS can produce accurate and precise δ11B data to a 0.5‰ (2σ) level on <0.3 ng B after correction for Ca interference effects. [ABSTRACT FROM AUTHOR]

The high latitude oceans are key areas of carbon and heat exchange between the atmosphere and the ocean. As such, they are a focus of both modern oceanographic and palaeoclimate research. However, most palaeoclimate proxies that could provide a long-term perspective are based on calcareous organisms, such as foraminifera, that are scarce or entirely absent in deep-sea sediments south of 50° latitude in the Southern Ocean and north of 40° in the North Pacific. As a result, proxies need to be developed for the opal-based organisms (e.g. diatoms) that are found at these high latitudes, and which dominate the biogenic sediments that are recovered from these regions. Here we present a method for the analysis of the boron (B) content and isotopic composition (δ11B) of diatom opal. We also apply it for the first time to evaluate the relationship between seawater pH and δ11B and B concentration ([B]) in the frustules of the diatom Thalassiosira weissflogii, cultured at a range of pCO2/pH. In agreement with existing data, we find that the [B] of the cultured diatom frustules increases with increasing pH (Mejia et al., 2013). δ11B shows a relatively well-defined negative trend with increasing pH; a completely distinct relationship from any other biomineral previously measured. This relationship not only has implications for the magnitude of the isotopic fractionation that occurs during boron incorporation into opal, but also allows us to explore the potential of the boron-based proxies for palaeo-pH and palaeo-CO2 reconstruction in high latitude marine sediments that have, up until now, eluded study due to the lack of suitable carbonate material. [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]

Atmospheric pCO2 is a critical component of the global carbon system and is considered to be the major control of Earth's past, present, and future climate. Accurate and precise reconstructions of its concentration through geological time are therefore crucial to our understanding of the Earth system. Ice core records document pCO2 for the past 800 kyr, but at no point during this interval were CO2 levels higher than today. Interpretation of older pCO2 has been hampered by discrepancies during some time intervals between two of the main ocean-based proxy methods used to reconstruct pCO2 : the carbon isotope fractionation that occurs during photosynthesis as recorded by haptophyte biomarkers (alkenones) and the boron isotope composition (δ11B) of foraminifer shells. Here, we present alkenone and δ11B -based pCO2 reconstructions generated from the same samples from the Pliocene and across a Pleistocene glacial–interglacial cycle at Ocean Drilling Program (ODP) Site 999. We find a muted response to pCO2 in the alkenone record compared to contemporaneous ice core and δ11B records, suggesting caution in the interpretation of alkenone-based records at low pCO2 levels. This is possibly caused by the physiology of CO2 uptake in the haptophytes. Our new understanding resolves some of the inconsistencies between the proxies and highlights that caution may be required when interpreting alkenone-based reconstructions of pCO2. [ABSTRACT FROM AUTHOR]

The boron isotope composition (δ11B) of foraminiferal calcite reflects the pH and the boron isotope composition of the seawater the foraminifer grew in. For pH reconstructions, the δ11B of seawater must therefore be known, but information on this parameter is limited. Here we reconstruct Neogene seawater δ11B based on the δ11B difference between paired measurements of planktic and benthic foraminifera and an estimate of the coeval water column pH gradient from their δ13C values. Carbon cycle model simulations underscore that the 1pH-1δ13C relationship is relatively insensitive to ocean and carbon cycle changes, validating our approach. Our reconstructions suggest that δ11Bsw was ~37.5‰ during the early and middle Miocene (roughly 23-12 Ma) and rapidly increased during the late Miocene (between 12 and 5 Ma) towards the modern value of 39.61‰. Strikingly, this pattern is similar to the evolution of the seawater isotope composition of Mg, Li and Ca, suggesting a common forcing mechanism. Based on the observed direction of change, we hypothesize that an increase in secondary mineral formation during continental weathering affected the isotope composition of riverine input to the ocean since 14 Ma. [ABSTRACT FROM AUTHOR]

In order to better understand the effect of CO2 on the Earth system in the future, geologists may look to CO2-induced environmental change in Earth's past. Here we describe how CO2 can be reconstructed using the boron isotopic composition (δ11B) of marine calcium carbonate. We review the chemical principles that underlie the proxy, summarize the available calibration data, and detail how boron isotopes can be used to estimate ocean pH and ultimately atmospheric CO2 in the past. δ11B in a variety of marine carbonates shows a coherent relationship with seawater pH, in broad agreement with simple models for this proxy. Offsets between measured and predicted δ11B may in part be explained by physiological influences, though the exact mechanisms of boron incorporation into carbonate remain unknown. Despite these uncertainties, we demonstrate that δ11B may provide crucial constraints on past ocean acidification and atmospheric CO2. [ABSTRACT FROM AUTHOR]

Abstract: In this study we make the first attempt to inter-calibrate boron isotope (δ11B) measurements on marine biogenic carbonates measured by four different laboratories, each using a different analytical technique. The importance of such calibrations lies in the major implications of relatively small changes in δ11B (<2‰), which are typical of palaeoclimate applications of the δ11B-pH proxy. Despite the variety of mass spectrometric techniques used by the different laboratories in this study (two variants of negative ion thermal ionisation mass spectrometry and two variants of multicollector inductively coupled plasma mass spectrometry), for samples without a complex sample matrix, such as boric acid, there are no significant interlaboratory biases: the pooled 2sd of these samples is 0.39‰, which is within the measurement uncertainties reported by each laboratory (which range between 0.18 and 0.80‰). For seawater, a common in-house reference material, we find a similarly good agreement (δ11B=39.65±0.41‰, 2sd) despite four different sample preparation procedures. Reported δ11B for calcium carbonate samples have a pooled 2sd of 1.46‰, which is larger than the measurement uncertainty reported by each laboratory. We have attempted to identify the source of this interlaboratory variability, and find that overall sample size (in terms of available B) and B/Ca ratio (i.e. amount of boron relative to matrix) may play a role, but the exact mechanisms remain uncertain. Observed variations in reported δ11B for the CaCO3 samples are, however, relatively systematic for each laboratory for a given sample matrix (i.e. similar B/Ca and amount of boron for analysis). This implies that relative differences in δ11B in a sample set of a given matrix can be reconstructed by the four laboratories involved in this study more accurately than the absolute boron isotope ratios. [Copyright &y& Elsevier]

Abstract: The middle Miocene Climatic Optimum (17–15Ma; MCO) is a period of global warmth and relatively high CO2 and is thought to be associated with a significant retreat of the Antarctic Ice Sheet (AIS). We present here a new planktic foraminiferal δ11B record from 16.6 to 11.8Ma from two deep ocean sites currently in equilibrium with the atmosphere with respect to CO2. These new data demonstrate that the evolution of global climate during the middle Miocene (as reflected by changes in the cyrosphere) was well correlated to variations in the concentration of atmospheric CO2. What is more, within our sampling resolution (∼1 sample per 300kyr) there is no evidence of hysteresis in the response of ice volume to CO2 forcing during the middle Miocene, contrary to what is understood about the Antarctic Ice Sheet from ice sheet modelling studies. In agreement with previous data, we show that absolute levels of CO2 during the MCO were relatively modest (350–400ppm) and levels either side of the MCO are similar or lower than the pre-industrial (200–260ppm). These new data imply the presence of either a very dynamic AIS at relatively low CO2 during the middle Miocene or the advance and retreat of significant northern hemisphere ice. Recent drilling on the Antarctic margin and shore based studies indicate significant retreat and advance beyond the modern limits of the AIS did occur during the middle Miocene, but the complete loss of the AIS was unlikely. Consequently, it seems that ice volume and climate variations during the middle Miocene probably involved a more dynamic AIS than the modern but also some component of land-based ice in the northern hemisphere. [Copyright &y& Elsevier]

Abstract: Accurate records of the state of the ocean carbonate system are critical for understanding past changes in pCO2, ocean acidification and climate. The chemical principles underlying the proxy of oceanic pH provided by the boron isotope ratio of foraminiferal carbonate are relatively well understood, but the proxy''s reliability has been questioned. We present 76 new Multi-Collector Inductively-Coupled Plasma Mass Spectrometry (MC-ICPMS) δ 11B measurements on a range of benthic foraminifera from 23 late-Holocene samples from the Atlantic that reaffirm the utility of the δ 11B-pH proxy. Our boron isotope measurements on ~10 benthic foraminifera tests typically yield a precision of ~±0.25‰ at 2 s.d. (equivalent to ~±0.03 pH units). δ 11B values of epifaunal species are within analytical uncertainty of those predicted from a simple model assuming sole incorporation of B(OH)4 − from seawater and no vital effects, using the independently determined fractionation factor of 1.0272 between 11B/10B of aqueous boron species. Infaunal foraminifera are consistent with this model, but record the combined effects of lower pore-water δ 11B and pH. No influence of partial dissolution or shell size on δ 11B is observed. We have also measured the B/Ca ratios of the same samples. For individual Cibicidoides species, B/Ca shows a good correlation with Δ[CO3 2−], but the B/Ca of different co-occurring species morphotypes varies considerably. These effects are not seen in δ 11B, which may therefore provide a more robust proxy of the ocean carbonate system. Whilst in theory δ 11B and B/Ca can be combined to provide a quantitative reconstruction of alkalinity and dissolved inorganic carbonate (DIC), in practice this is precluded by propagated uncertainties. δ 11B data give significant constraints on foraminifera calcification mechanisms, and seem most simply explained by incorporation of B(OH)4 − into a HCO3 − pool, which is then completely incorporated in foraminiferal CaCO3. Our demonstration of the predictable variation of δ 11B with pH, across a wide range of species and locations, provides confidence in the application of MC-ICPMS measurements of foraminiferal δ 11B to reconstruct past changes in the ocean carbonate system. [Copyright &y& Elsevier]

Abstract: Values of δ 11B reported in the literature for Holocene samples of the same foraminiferal species show large variability (6‰) relative to cited precision (<1‰). This is indicative of significant inter-laboratory biases and raises concerns about the accuracy of foraminiferal proxy pH records. To investigate this problem we have measured the δ 11B of modern ocean carbonates using several different analytical procedures. We have used total evaporation negative thermal ionisation mass spectrometry (TE-NTIMS), with various sample preparation and loading procedures and multi-collector inductively coupled plasma mass spectrometry (MC-ICPMS). The δ 11B of pure boric acid solutions measured by TE-NTIMS and MC-ICPMS agree well, demonstrating no fundamental biases between the techniques. Yet the δ 11B values measured by TE-NTIMS for non-foraminiferal carbonates are about 2‰ lighter than those by MC-ICPMS whereas foraminifers measured by the same standard TE-NTIMS procedure are 2 to 6‰ heavier than those by MC-ICPMS. Significantly, we found that foraminiferal samples kept in acidic solution over several months yielded lower δ 11B values (up to 5‰ lower) and better reproducibility when re-measured by TE-NTIMS. We infer that organic material, released on foraminiferal dissolution, causes biases in NTIMS measurements. No residual organics should be present in the MC-ICPMS measurements as matrix is separated from sample before analysis. Moreover we demonstrate by standard addition the absence of any matrix influence in the MC-ICPMS procedure beyond the inherent uncertainties in the standard addition approach (∼±0.35‰ 2sigma). Degradation of the inferred organic residue, either by long term storage in acidic solution or by loading samples in 30% H2O2, reduces but does not totally remove the (variable) bias between TE-NTIMS and MC-ICPMS δ 11B measurements. Despite these problems, careful analysis of similar samples by NTIMS may permit data to be obtained with consistent relative differences that still yield valuable proxy records, but there is clearly considerable potential for inaccuracies to result in such an approach from hard to detect changes in matrix. [Copyright &y& Elsevier]

Abstract: The Pliocene period is the most recent time when the Earth was globally significantly (∼3°C) warmer than today. However, the existing pCO2 data for the Pliocene are sparse and there is little agreement between the various techniques used to reconstruct palaeo-pCO2. This disagreement, coupled with the general low temporal resolution of the published records, does not allow a robust assessment of the role of declining pCO2 in the intensification of the Northern Hemisphere Glaciation (INHG) and a direct comparison to other proxy records are lacking. For the first time, we use a combination of foraminiferal (δ 11B) and organic biomarker (alkenone-derived carbon isotopes) proxies to determine the concentration of atmospheric CO2 over the past 5Ma. Both proxy records show that during the warm Pliocene pCO2 was between 330 and 400ppm, i.e. similar to today. The decrease to values similar to pre-industrial times (275–285ppm) occurred between 3.2Ma and 2.8Ma — coincident with the INHG and affirming the link between global climate, the cryosphere and pCO2. [Copyright &y& Elsevier]

Geological and geochemical evidence indicates that the Antarctic ice sheet formed during the Eocene–Oligocene transition, 33.5–34.0 million years ago. Modelling studies suggest that such ice-sheet formation might have been triggered when atmospheric carbon dioxide levels () fell below a critical threshold of ∼750 p.p.m.v., but the timing and magnitude of relative to the evolution of the ice sheet has remained unclear. Here we use the boron isotope pH proxy on exceptionally well-preserved carbonate microfossils from a recently discovered geological section in Tanzania to estimate before, during and after the climate transition. Our data suggest that a reduction in occurred before the main phase of ice growth, followed by a sharp recovery to pre-transition values and then a more gradual decline. During maximum ice-sheet growth, was between ∼450 and ∼1,500 p.p.m.v., with a central estimate of ∼760 p.p.m.v. The ice cap survived the period of recovery, although possibly with some reduction in its volume, implying (as models predict) a nonlinear response to climate forcing during melting. Overall, our results confirm the central role of declining in the development of the Antarctic ice sheet (in broad agreement with carbon cycle modelling) and help to constrain mechanisms and feedbacks associated with the Earth’s biggest climate switch of the past 65 Myr. [ABSTRACT FROM AUTHOR]

Abstract: We report a total evaporation negative ion thermal mass spectrometry (TE-NTIMS) technique that enables precise and accurate (±0.7‰; 2 s.d.) measurements of boron isotope ratios. The fundamental advantage of TE-NTIMS is that the effect of instrumental mass fractionation is minimised and sample signal maximised by analysing samples to exhaustion. We can analyse as little as 300 pg of B, which enables repeat analyses of dissolutions of small numbers of foraminifera (as little as 0.1 mg or ∼10 individual foraminfera). This represents a several fold reduction in the number of tests required compared to previous NTIMS studies and brings the amount of sample into line with other commonly used paleo-proxies. Standard addition experiments indicate that the 11B/ 10B ratio of the NIST SRM 951 standard is not biased by differing amounts of seawater or carbonate matrix and yield an 11B/ 10B within error of the certified value. We also show that our sample preparation induces no additional variations (e.g. blank contribution) beyond our analytical uncertainty. We obtain 11B/ 10B ratios for seawater within error of values obtained using plasma ionisation, positive and negative thermal ionisation mass-spectrometry. Our measurements of core-top G. sacculifer from three ocean basins yield δ 11B within analytical error (23.3–24.3‰) and fall within the range of published values. This study, however, further highlights significant interlaboratory biases in isotopic compositions of core-top foraminifera. Significantly, we show that our approach is not influenced by processing blank nor systematic differences in mass bias between measurements of sample and standard, which has yet to be documented for some other laboratories. [Copyright &y& Elsevier]

The isotopic composition of boron (δ 11 B) in marine carbonates is well established as a proxy for past ocean pH, however, its robust application to palaeo-environments relies on the generation of species-specific calibrations. Existing calibrations utilising the deep-sea coral (DSC) Desmophyllum dianthus highlight the potential application of this pervasive species to pH reconstructions of intermediate depth waters. Nevertheless, considerable uncertainty remains regarding the estimation of seawater pH from these bulk skeletal δ 11 B measurements, likely resulting from microstructural heterogeneities in δ 11 B of D. dianthus . To circumvent this problem, thus improving the reliability of the D. dianthus δ 11 B-pH calibration, we present a new δ 11 B calibration of micro-sampled fibrous aragonite from this species. Modern coral specimens recovered from the Atlantic, Pacific, and Southern Oceans, micro-sampled using microdrilling, micromilling, and laser cutting extraction, were analysed for trace element (B/Ca, Mg/Ca, Sr/Ca, and U/Ca) and boron isotopic composition. We find the best calibration against the δ 11 B of borate in local ambient seawater (a function of pH and taken from hydrographic data sets; pH range 7.57 to 8.05) utilises δ 11 B measurements of fibres with likely slow growth rates and minimal contamination from adjacent microstructures (identified by low Mg/Ca) for each coral specimen. This new calibration exhibits a stronger, and better-defined dependence on ambient seawater pH compared to bulk coral δ 11 B; δ 11 B fibre = ( 0.93 ± 0.17) × δ 11 B borate + ( 12.02 ± 2.63). We suggest that the majority of the variability in measured δ 11 B between replicate bands of fibrous aragonite from a D. dianthus specimen can be explained by small incorporation of non-fibrous aragonite and surface impurities during microsampling and growth rate effects. This study confirms the utility of D. dianthus as an archive of precise palaeo-pH (± 0.07 pH units), provided that suitable sampling strategies are applied. [ABSTRACT FROM AUTHOR]

Abstract: The boron isotope-pH proxy, applied to mixed-layer planktic foraminifera, has great potential for estimating past CO2 levels, which in turn is crucial to advance our understanding of how this greenhouse gas influences Earth''s climate. Previous culture experiments have shown that, although the boron isotopic compositions of various planktic foraminifera are pH dependent, they do not agree with the aqueous geochemical basis of the proxy. Here we outline the results of culture experiments on Globigerinoides ruber (white) across a range of pH (∼7.5–8.2) and analysed via multicollector inductively-coupled plasma mass spectrometry (MC-ICPMS), and compare these data to core-top and sediment-trap samples to derive a robust new species-specific boron isotope-pH calibration. Consistent with earlier culture studies, we show a reduced pH dependency of the boron isotopic composition of symbiont-bearing planktonic foraminifera compared to borate ion in seawater. We also present evidence for a size fraction effect in the δ 11B of G. ruber. Finally, we reconstruct atmospheric CO2 concentrations over the last deglacial using our new calibration at two equatorial sites, ODP Site 999A and Site GeoB1523-1. These data provide further grounding for the application of the boron isotope-pH proxy in reconstructions of past atmospheric CO2 levels. [Copyright &y& Elsevier]

Boron isotope ratios – as measured in marine calcium carbonate – are an established tracer of past seawater and calcifying fluid pH, and thus a powerful tool for probing marine calcifier physiology and reconstructing past atmospheric CO 2 levels. For such applications, understanding the inorganic baseline upon which foraminiferal vital effects or coral pH upregulation are superimposed should be an important prerequisite. Yet, investigations into boron isotope fractionation in synthetic CaCO 3 polymorphs have often reported variable and even conflicting results, implying our understanding of the pathways of boron incorporation into calcium carbonate is incomplete. Here we address this topic with experimental data from synthetic calcite and aragonite precipitated across a range of pH in the presence of both Mg and Ca. We observe coherent patterns in B/Ca and Na/Ca ratios that, we suggest, point to paired substitution of Na and B into the carbonate lattice to achieve local charge balance. In addition, we confirm the results of previous studies that the boron isotope composition of inorganic aragonite precipitates closely reflects that of aqueous borate ion, but that inorganic calcites display a higher degree of scatter, and diverge from the boron isotope composition of aqueous borate ion at low pH. With reference to the simultaneous incorporation of other trace and minor elements, we put forward possible explanations for the observed variability in the concentration and isotopic composition of boron in synthetic CaCO 3. In particular, we highlight the potential importance of interface electrostatics in driving variability in our own and published synthetic carbonate datasets. Importantly for palaeo-reconstruction, however, these electrostatic effects are unlikely to play as important a role during natural precipitation of biogenic carbonates. [ABSTRACT FROM AUTHOR]

The increase in atmospheric carbon dioxide (CO 2 ) observed since the industrial revolution has reduced surface ocean pH by ∼0.1 pH units, with further change in the oceanic system predicted in the coming decades. Calcareous organisms can be negatively affected by extreme changes in seawater pH (pH sw ) such as this due to the associated changes in the oceanic carbonate system. The boron isotopic composition (δ 11 B) of biogenic carbonates has been previously used to monitor pH at the calcification site (pH cf ) in scleractinian corals, providing mechanistic insights into coral biomineralisation and the impact of variable pH sw on this process. Motivated by these investigations, this study examines the δ 11 B of the high-Mg calcite skeleton of the coralline red alga Neogoniolithon sp. to constrain pH cf , and investigates how this taxon’s pH cf is impacted by ocean acidification. δ 11 B was measured in multiple algal replicates ( n = 4–5) cultured at four different p CO 2 scenarios – averaging (±1σ) 409 (±6), 606 (±7), 903 (±12) and 2856 (±54) μatm, corresponding to average pH sw (±1σ) of 8.19 (±0.03), 8.05 (±0.06), 7.91 (±0.03) and 7.49 (±0.02) respectively. Results show that skeletal δ 11 B is elevated relative to the δ 11 B of seawater borate at all pH sw treatments by up to 18‰. Although substantial variability in δ 11 B exists between replicate samples cultured at a given pH sw (smallest range = 2.32‰ at pH sw 8.19, largest range = 6.08‰ at pH sw 7.91), strong correlations are identified between δ 11 B and pH sw ( R 2 = 0.72, p < 0.0001, n = 16) and between δ 11 B and B/Ca ( R 2 = 0.72, p < 0.0001, n = 16). Assuming that skeletal δ 11 B reflects pH cf as previously observed for scleractinian corals, the average pH cf across all experiments was 1.20 pH units (0.79 to 1.56) higher than pH sw , with the magnitude of this offset varying parabolically with decreasing pH sw , with a maximum difference between pH sw and pH cf at a pH sw of 7.91. Observed relationships between pH sw and calcification rate, and between pH sw and pH cf , suggest that coralline algae exhibit some resilience to moderate ocean acidification via increase of pH cf relative to pH sw in a similar manner to scleractinian corals. However, these results also indicate that pH cf cannot be sufficiently increased by algae exposed to a larger reduction in pH sw , adversely impacting calcification rates of coralline red algae. [ABSTRACT FROM AUTHOR]

Boron is a quintessential crustal element but its conspicuous presence in diamond – a mantle mineral – raises questions about potential subduction pathways for boron and other volatiles. It has been a long-standing goal to characterize the isotopic composition of boron in blue, boron-bearing (Type IIb) diamonds to reveal its origin. Mineral inclusions indicate that Type IIb diamonds crystallize at transition zone to lower mantle depths, meaning that if the boron is subducted it would trace a pathway of volatile elements into the deep mantle. Here, using off-line laser ablation sampling, we present the first boron isotope compositions, along with trace element contents and carbon isotope compositions, of a suite of blue diamonds mainly sourced from the Cullinan diamond mine in South Africa. The ten analyzed blue diamonds have a wide range in δ 11 B, between −9.2 ± 2‰ and −0.5 ± 2‰, compared to the more restricted range for mid-ocean ridge basalts (−7.1 ± 0.9‰). Carbon isotope values for the blue diamonds range between −20.6 and −1.8‰, with a mode at −17‰, significantly more negative than the main mantle mode at −5‰. Combined, the boron and carbon isotope compositions require fluid input from subducted oceanic lithosphere down into deep mantle source regions of blue diamonds. This finding highlights a deep subduction pathway for volatiles to stoke the source regions of deeply-derived magmas. • Boron-bearing blue diamonds from Cullinan formed at transition zone to lower mantle depths. • Cullinan blue diamonds have δ 11 B between −9.2 ± 2‰ and −0.5 ± 2‰. • Boron and carbon isotope compositions require fluid input from subducted serpentinized oceanic lithosphere. • Only subducting slabs with the coolest PT paths should retain boron, water and carbon to lower mantle depths. • Re-evaluation is needed of the mechanisms through which boron is transported to the transition zone and lower mantle. [ABSTRACT FROM AUTHOR]

Abstract: The boron isotope composition of marine carbonates such as foraminiferal tests and coral skeletons is increasingly being used to reconstruct seawater pH values and atmospheric pCO2 concentrations spanning hundreds of thousands or even millions of years. However, inter specific and small scale (microns) intra specific isotopic variation, either due to life processes of an organism or patchy recrystallisation and dissolution in fossilised material, can limit the use of the boron isotope ratio as palaeo-pH recorder. One approach to address this problem is the combined use of in situ and high spatial resolution mass spectrometry and high precision bulk analytical techniques. While much effort has been invested to establish reference material for silicates little attention has been devoted towards characterizing and using carbonate material allowing direct isotopic comparisons between B isotope data produced on carbonates by different techniques and laboratories. Hence, here we present data on the boron isotope composition of carbonate material determined by multicollector inductively coupled plasma mass spectrometry (MC-ICP-MS), thermal ionization mass spectrometry (TIMS), and secondary ionization mass spectrometry (SIMS). The new reference material permits the B isotope analysis of single foraminiferal chambers using SIMS and to empirically relate between seawater pH, the B isotope composition of O. universa and the predicted foraminiferal vital effect. [Copyright &y& Elsevier]