Description: Neodymium (Nd) concentrations and isotopic signatures of living and fossil deep-sea coral species Lophelia pertusa, Desmophyllum dianthus and Madrepora oculata from the northeast Atlantic Ocean have been investigated in order to test the ability of deep-sea corals to reconstruct the seawater Nd isotopic signature and past changes of ocean circulation in the eastern North Atlantic. Small quantities of Nd—less than 45 ng/g—are incorporated into the aragonite skeleton of living deep-sea corals that dwell at upper intermediate depths throughout the Northeast Atlantic. Rigorous cleaning techniques are needed in order to avoid Nd contamination from manganese-oxide and iron hydroxide coatings. Moreover, Nd isotopic compositions have been measured using thermal ionization mass spectrometry (TIMS) by Nd-oxide method. Our data indicate that the isotopic signatures of modern corals are similar to those of adjacent water masses, implying that deep-sea corals can serve as an archive of the seawater Nd isotopic compositions in the past. The first results from few fully-cleaned fossils corals collected within the Porcupine Seabight and the southwest Rockall Bank reveal significantly higher εNd for corals dated between 150 ± 40 and 3060 ± 90 yrs than those of the living corals located in similar areas. This suggests rapid hydrological variations along the eastern margin of the North Atlantic Ocean at intermediate water depth with higher contribution of the Mediterranean Overflow Waters (MOW) or other temperate Atlantic mid-depth water masses (ENACW or NAC) in the past.

Description: We show that the Li/Mg systematics of a large suite of aragonitic coral skeletons, representing a wide range of species inhabiting disparate environments, provides a robust proxy for ambient seawater temperature. The corals encompass both zooxanthellate and azooxanthellate species (Acropora sp., Porites sp., Cladocora caespitosa, Lophelia pertusa, Madrepora oculata and Flabellum impensum) collected from shallow, intermediate, and deep-water habitats, as well as specimens cultured in tanks under temperature-controlled conditions. The Li/Mg ratios observed in corals from these diverse tropical, temperate, and deep-water environments are shown to be highly correlated with temperature, giving an exponential temperature relationship of: Li/Mg (mmol/mol) = 5.41 exp (−0.049 * T) (r2 = 0.975, n = 49). Based on the standard error of the Li/Mg versus temperature correlation, we obtain a typical precision of ±0.9 °C for the wide range of species analysed, similar or better than that of other less robust coral temperature proxies such as Sr/Ca ratios. The robustness and species independent character of the Li/Mg temperature proxy is shown to be the result of the normalization of Li to Mg, effectively eliminating the precipitation efficiency component such that temperature remains as the main controller of coral Li/Mg compositions. This is inferred from analysis of corresponding Li/Ca and Mg/Ca ratios with both ratios showing strong microstructure-related co-variations between the fibrous aragonite and centres of calcification, a characteristic that we attribute to varying physiological controls on growth rate. Furthermore, Li/Ca ratios show an offset between more rapidly growing zooxanthellate and azooxanthellate corals, and hence only an approximately inverse relationship to seawater temperature. Mg/Ca ratios show very strong physiological controls on growth rate but no significant dependence with temperature, except possibly for Acropora sp. and Porites sp. A strong positive correlation is nevertheless found between Li/Ca and Mg/Ca ratios at similar temperatures, indicating that both Li and Mg are subject to control by similar growth mechanisms, specifically the mass fraction of aragonite precipitated during calcification, which is shown to be consistent with a Rayleigh-based elemental fractionation model. The highly coherent array defined by Li/Mg versus temperature is thus largely independent of coral calcification mechanisms, with the strong temperature dependence reflecting the greater sensitivity of the KdLi/Ca partition coefficient relative to KdMg/Ca. Accordingly, Li/Mg ratios exhibit a highly coherent exponential correlation with temperature, thereby providing a more robust tool for reconstructing paleo-seawater temperatures.

Description: 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: (i) to assess the extent to which oxidative pre‐treatment, aimed at removing organic material from carbonate, can influence the resulting δ11B; (ii) to determine to what degree the chosen analytical approach may affect the resultant δ11B, and (iii) 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.

Description: 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.

Description: The Sr/Ca ratio of coral aragonite is used to reconstruct past sea surface temperature (SST). Twentyone 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 〈0.5°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.

Description: Total alkalinity (AT) and dissolved inorganic carbon (CT) in the oceans are important properties with respect to understanding the ocean carbon cycle and its link to global change (ocean carbon sinks and sources, ocean acidification) and ultimately finding carbon-based solutions or mitigation procedures (marine carbon removal). We present a database of more than 44 400 AT and CT observations along with basic ancillary data (spatiotemporal location, depth, temperature and salinity) from various ocean regions obtained, mainly in the framework of French projects, since 1993. This includes both surface and water column data acquired in the open ocean, coastal zones and in the Mediterranean Sea and either from time series or dedicated one-off cruises. Most AT and CT data in this synthesis were measured from discrete samples using the same closed-cell potentiometric titration calibrated with Certified Reference Material, with an overall accuracy of ±4 µmol kg−1 for both AT and CT. The data are provided in two separate datasets – for the Global Ocean and the Mediterranean Sea (https://doi.org/10.17882/95414, Metzl et al., 2023), respectively – that offer a direct use for regional or global purposes, e.g., AT–salinity relationships, long-term CT estimates, and constraint and validation of diagnostic CT and AT reconstructed fields or ocean carbon and coupled climate–carbon models simulations as well as data derived from Biogeochemical-Argo (BGC-Argo) floats. When associated with other properties, these data can also be used to calculate pH, the fugacity of CO2 (fCO2) and other carbon system properties to derive ocean acidification rates or air–sea CO2 fluxes.

Description: U-series age patterns obtained on reef framework-forming cold-water corals collected over a nearly 6000-km-long continental margin sector, extending from off Mauritania (17{degrees}N; northwest Africa) to the southwestern Barents Sea (70{degrees}N; northeastern Europe), reveal strong climate influences on the geographical distribution and sustained development of these ecosystems. Over the past three glacial-interglacial cycles, framework-forming cold-water corals (Lophelia pertusa and Madrepora oculata) seem to have predominantly populated reefs, canyons, and patches in the temperate East Atlantic and the Mediterranean Sea. Above 50{degrees}N corals colonize reefs in the northern East Atlantic primarily during warm climate periods with the biogeographic limit advancing from [~]50{degrees}N to [~]70{degrees}N. We propose that north-south oscillations of the biogeographic limit of reef developments are paced by ice ages and may occur synchronously with north-south displacement of cold nutrient-rich intermediate waters and surface productivity related to changes of the polar front.

Description: We present the neodymium isotopic composition (epsilon-Nd) of mixed planktonic foraminifera species from a sediment core collected at 622 m water depth in the Balearic Sea, as well as epsilon-Nd of scleractinian cold-water corals (CWC; Madrepora oculata, Lophelia pertusa) retrieved between 280 and 442 m water depth in the Alboran Sea and at 414 m depth in the southern Sardinian continental margin. The aim is to constrain hydrological variations at intermediate depths in the western Mediterranean Sea during the last 20 kyr. Planktonic (Globigerina bulloides) and benthic (Cibicidoides pachyderma) foraminifera from the Balearic Sea were also analyzed for stable oxygen (d18O) and carbon (d13C) isotopes. The foraminiferal and coral epsilon-Nd values from the Balearic and Alboran seas are comparable over the last ~13 kyr, with mean values of -8.94 ± 0.26 (1 Sigma; n = 24) and -8.91 ± 0.18 (1 Sigma; n = 25), respectively. Before 13 ka BP, the foraminiferal epsilon-Nd values are slightly lower (-9.28 ± 0.15) and tend to reflect higher mixing between intermediate and deep waters, which are characterized by more unradiogenic epsilon-Nd values. The slight epsilon-Nd increase after 13 ka BP is associated with a decoupling in the benthic foraminiferal d13C composition between intermediate and deeper depths, which started at ~16 ka BP. This suggests an earlier stratification of the water masses and a subsequent reduced contribution of unradiogenic epsilon-Nd from deep waters. The CWC from the Sardinia Channel show a much larger scatter of epsilon-Nd values, from -8.66 ± 0.30 to 5.99 ± 0.50, and a lower average (-7.31 ± 0.73; n = 19) compared to the CWC and foraminifera from the Alboran and Balearic seas, indicative of intermediate waters sourced from the Levantine basin. At the time of sapropel S1 deposition (10.2 to 6.4 ka), the epsilon-Nd values of the Sardinian CWC become more unradiogenic (-8.38 ± 0.47; n = 3 at ~8.7 ka BP), suggesting a significant contribution of intermediate waters originated from the western basin. We propose that western Mediterranean intermediate waters replaced the Levantine Intermediate Water (LIW), and thus there was a strong reduction of the LIW during the mid-sapropel (~8.7 ka BP). This observation supports a notable change of Mediterranean circulation pattern centered on sapropel S1 that needs further investigation to be confirmed.