Glacial carbonate compensation in the Pacific Ocean constrained from paired oxygen and carbonate system reconstructions

editorial reviewedThe tendency of CaCO_3 dissolution/burial to minimise changes in the carbonate ion concentration of the deep ocean following perturbations to the carbon cycle (‘carbonate compensation’) is thought to act as a first order control on atmospheric CO_2 on timescales of ~10^3 to 10^5 years. Although carbonate compensation could account for up to ~half of the glacial drawdown of CO_2, quantitative estimates of changes in ocean alkalinity are lacking. As such, the role of carbonate compensation in driving glacial-interglacial CO_2 variations remains poorly understood. Here, we combine paired reconstructions of dissolved oxygen from the infaunal-epifaunal benthic foraminiferal δ^13C proxy (Δδ^13C) and the carbonate system from boron proxies (B/Ca, δ^11B) in benthic foraminifera; this approach allows us to quantify both changes in deep ocean respired CO_2 storage, and the response of the carbonate system to this addition/removal of respired CO_2, providing the first quantitative estimates on the amount and timing of alkalinity changes in the deep Pacific during the Last Glacial Maximum (LGM) and over deglaciation. Our results indicate an increase in deep ocean alkalinity during the LGM, and suggest the buffering of the deep ocean may occur substantially faster than the canonical timescale of ~5 kyr (Broecker and Peng, 1987). We present results from a series of sensitivity experiments and long-term simulations using the recently coupled iLOVECLIM-MEDUSA climate/carbon-cycle/sediment model, with implications for our understanding of carbonate compensation in both glacial times, and the long-term future

On the combination of the planktonic foraminiferal Mg/Ca, clumped (Δ₄₇) and conventional (δ¹⁸O) stable isotope paleothermometers in palaeoceanographic studies

International audienceAssuming that foraminiferal clumped isotope (Δ47) values are independent of seawater salinity and pH, the combination of Mg/Ca, δ18O and Δ47 values, may in theory allow us to disentangle the temperature, salinity/δ18Osw and pH signals. Here, we present a new Mg/Ca-Δ47 dataset for modern planktonic foraminifera, from various oceanographic basins and covering a large range of temperatures (from 0.2 to 25.4 °C). These measurements were performed on the same samples and species as the ones used for the foraminiferal Δ47 calibration of Peral et al. (2018), allowing comparison between both Mg/Ca and Δ47 paleothermometers (excluding the two benthic foraminiferal data points). There is a good agreement between these two paleothermometers when the Mg/Ca-temperature is corrected for seawater salinity and pH, suggesting that foraminiferal Δ47 may not be influenced by salinity or pH. However, our results show that Δ47 temperature uncertainties still limit our ability to reconstruct pH and δ18Osw from the combination of Mg/Ca, δ18O and Δ47 in a useful manner. We also find that disagreements between Mg/Ca and Δ47 values in G. bulloides persist after correction for vital, salinity and pH effects, suggesting that other process(es) may also influence Mg/Ca in this species.This study also provides an updated I-CDES version of the previously published planktonic and benthic foraminiferal Δ47 calibration of Peral et al. (2018), covering a range of temperature from −2 to 25.4 °C

Interlaboratory study for coral Sr/Ca and other element/Ca ratio measurements

The Sr/Ca ratio of coral aragonite is used to reconstruct past sea surface temperature (SST). Twenty-one laboratories took part in an interlaboratory study of coral Sr/Ca measurements. Results show interlaboratory bias can be significant, and in the extreme case could result in a range in SST estimates of 7°C. However, most of the data fall within a narrower range and the Porites coral reference material JCp-1 is now characterized well enough to have a certified Sr/Ca value of 8.838 mmol/mol with an expanded uncertainty of 0.089 mmol/mol following International Association of Geoanalysts (IAG) guidelines. This uncertainty, at the 95% confidence level, equates to 1.5°C for SST estimates using Porites, so is approaching fitness for purpose. The comparable median within laboratory error is \u3c0.5°C. This difference in uncertainties illustrates the interlaboratory bias component that should be reduced through the use of reference materials like the JCp-1. There are many potential sources contributing to biases in comparative methods but traces of Sr in Ca standards and uncertainties in reference solution composition can account for half of the combined uncertainty. Consensus values that fulfil the requirements to be certified values were also obtained for Mg/Ca in JCp-1 and for Sr/Ca and Mg/Ca ratios in the JCt-1 giant clam reference material. Reference values with variable fitness for purpose have also been obtained for Li/Ca, B/Ca, Ba/Ca, and U/Ca in both reference materials. In future, studies reporting coral element/Ca data should also report the average value obtained for a reference material such as the JCp-1

Interlaboratory comparison study of calibration standards for foraminiferal Mg/Ca thermometry

An interlaboratory study of Mg/Ca and Sr/Ca ratios in three commercially available carbonate reference materials (BAM RS3, CMSI 1767, and ECRM 752-1) was performed with the participation of 25 laboratories that determine foraminiferal Mg/Ca ratios worldwide. These reference materials containing Mg/Ca in the range of foraminiferal calcite (0.8 mmol/mol to 6 mmol/mol) were circulated with a dissolution protocol for analysis. Participants were asked to make replicate dissolutions of the powdered samples and to analyze them using the instruments and calibration standards routinely used in their laboratories. Statistical analysis was performed in accordance with the International Standardization Organization standard 5725, which is based on the analysis of variance (ANOVA) technique. Repeatability (RSDr%), an indicator of intralaboratory precision, for Mg/Ca determinations in solutions after centrifuging increased with decreasing Mg/Ca, ranging from 0.78% at Mg/Ca = 5.56 mmol/mol to 1.15% at Mg/Ca = 0.79 mmol/mol. Reproducibility (RSDR%), an indicator of the interlaboratory method precision, for Mg/Ca determinations in centrifuged solutions was noticeably worse than repeatability, ranging from 4.5% at Mg/Ca = 5.56 mmol/mol to 8.7% at Mg/Ca = 0.79 mmol/mol. Results of this study show that interlaboratory variability is dominated by inconsistencies among instrument calibrations and highlight the need to improve interlaboratory compatibility. Additionally, the study confirmed the suitability of these solid standards as reference materials for foraminiferal Mg/Ca (and Sr/Ca) determinations, provided that appropriate procedures are adopted to minimize and to monitor possible contamination from silicate mineral phases. <br/