Cold-Water Corals as Archives for Ocean Dynamics, Environmental Conditions and Glacial Reef Accumulation

Paleoceanographic reconstructions are fundamental for our understanding of past climate variabilities and have mainly focused on changes of circulation and environmental conditions in the surface or deep ocean. The thermocline (100 – 1000 m), acting as a link and buffer between the well-mixed warm surface and the slow and cold deep water, has been largely overlooked. In this study two cold-water coral (CWC) bearing cores from two depths in the thermocline of the southern Gulf of C´adiz (sGoC) were analysed in a mutli-proxy approach. U-series dated glacial CWCs were analysed for recorded Li/Mg temperatures, water mass 14C reservoir ages and radiogenic Nd isotope signatures (epsilonNd). For the first time, a CWC epsilonNd record was extended by two independent co-located hemipelagic sediment records. A seesaw pattern for the glacial sGoC intermediate waters alternating between predominant Eastern North Atlantic Central Water (ENACW) and Eastern Antarctic Intermediate Water (EAAIW) is proposed. Glacial ENACW and EAAIW both exhibited polar temperatures (~0°C) and more radiogenic epsilonNd signatures (~-9) than nowadays and were distinguishable by their reservoir ages, with better ventilated glacial ENACW than EAAIW. A compilation of existing CWC mound aggregation records allowed for a first estimate of initial CWC settlement in the ocean around 3.4Ma ago, coinciding with the mid-Pliocene warm period and the onset of Northern Hemisphere glaciation. Additionally, Ba isotopes (delta138/134Ba) recently introduced for seawater, was calibrated for the use in CWCs. This lays the foundation for the missing nutrient or surface biological productivity proxy in fossil CWCs

Extended calibration of cold-water coral Ba/Ca using multiple genera and co-located measurements of dissolved barium concentration

Biological productivity and ocean circulation are both important oceanographic variables that control the distribution of dissolved barium in the ocean interior ([Ba]sw). The ability to accurately reconstruct [Ba]swwill provide key constraints on these processes in the past. The geochemistry of cold-water corals has the potential to unlock paleoceanographic records at spatial and temporal resolutions not available using other sedimentary archives. Previous studies have suggested that the Ba/Ca ratio of coral skeletons is linearly related to [Ba]sw. However, these efforts have used a limited number of species, sparse global seawater databases, or have not explicitly measured the Ba/Ca ratio. Here we investigate the Ba/Ca ratio in a well-constrained set of cold-water scleractinian (aragonitic) corals as a proxy for [Ba]sw, using 58 specimens from 7 coral genera along with co-located measurements of [Ba]sw. We find that traditional chemical cleaning procedures do not significantly affect the Ba/Ca ratio of cold-water coral skeletons, allowing rapid sample throughput. We also determine that intra-sample variation in Ba/Ca ratios can be reduced by using larger sample sizes (e.g. 20 mg). By combining our results with existing data, we find that cold-water coral Ba/Ca is linearly related to [Ba]swaccording to the relationship: Ba/Ca μmol/mol = [0.15 ± 0.02] [Baswnmol/kg] + [2.5 ± 1.4], (R2= 0.7). We observe no species-specific ‘vital effects’ in cold-water coral Ba/Ca ratios, but site-specific effects could be a factor. Nevertheless, our results highlight the potential of Ba/Ca in cold-water corals to reconstruct biological and physical changes in the ocean interior

South Atlantic intermediate water advances into the North-east Atlantic with reduced Atlantic meridional overturning circulation during the last glacial period

The Nd isotopic composition (epsilon Nd) of seawater and cold-water coral (CWC) samples from the Gulf of Cadiz and the Alboran Sea, at a depth of 280-827 m were investigated in order to constrain middepth water mass dynamics within the Gulf of Cadiz over the past 40 ka. epsilon Nd of glacial and Holocene CWC from the Alboran Sea and the northern Gulf of Cadiz reveals relatively constant values (-8.6 to -9.0 and -9.5 to -10.4, respectively). Such values are similar to those of the surrounding present-day middepth waters from the Mediterranean Outflow Water (MOW; epsilon Nd approximate to -9.4) and Mediterranean Sea Water (MSW; epsilon Nd approximate to -9.9). In contrast, glacial epsilon Nd values for CWC collected at thermocline depth (550-827 m) in the southern Gulf of Cadiz display a higher average value (-8.90.4) compared to the present-day value (-11.70.3). This implies a higher relative contribution of water masses of Mediterranean (MSW) or South Atlantic origin (East Antarctic Intermediate Water, EAAIW). Our study has produced the first evidence of significant radiogenic epsilon Nd values (approximate to -8) at 19, 23-24, and 27 ka, which are coeval with increasing iceberg discharges and a weakening of Atlantic Meridional Overturning Circulation (AMOC). Since MOW epsilon Nd values remained stable during the last glacial period, it is suggested that these radiogenic epsilon Nd values most likely reflect an enhanced northward propagation of glacial EAAIW into the eastern Atlantic Basin

Barium isotopes in cold-water corals

Recent studies have introduced stable Ba isotopes (δ138/134δ138/134Ba) as a novel tracer for ocean processes. Ba isotopes could potentially provide insight into the oceanic Ba cycle, the ocean's biological pump, water-mass provenance in the deep ocean, changes in activity of hydrothermal vents, and land-sea interactions including tracing riverine inputs. Here, we show that aragonite skeletons of various colonial and solitary cold-water coral (CWC) taxa record the seawater (SW) Ba isotope composition. Thirty-six corals of eight different taxa from three oceanic regions were analysed and compared to δ138/134δ138/134Ba measurements of co-located seawater samples. Sites were chosen to cover a wide range of temperature, salinity, Ba concentrations and Ba isotope compositions. Seawater samples at the three sites exhibit the well-established anti-correlation between Ba concentration and δ138/134δ138/134Ba. Furthermore, our data set suggests that Ba/Ca values in CWCs are linearly correlated with dissolved [Ba] in ambient seawater, with an average partition coefficient of DCWC/SW = 1.8 ± 0.4 (2SD). The mean isotope fractionation of Ba between seawater and CWCs Δ138/134BaCWC–SW is −0.21 ± 0.08‰ (2SD), indicating that CWC aragonite preferentially incorporates the lighter isotopes. This fractionation likely does not depend on temperature or other environmental variables, suggesting that aragonite CWCs could be used to trace the Ba isotope composition in ambient seawater. Coupled [Ba] and δ138/134δ138/134Ba analysis on fossil CWCs has the potential to provide new information about past changes in the local and global relationship between [Ba] and δ138/134δ138/134Ba and hence about the operation of the past global oceanic Ba cycle in different climate regimes

Barium isotopes in cold-water corals

Recent studies have introduced stable Ba isotopes (delta(138)/Ba-134) as a novel tracer for ocean processes. Ba isotopes could potentially provide insight into the oceanic Ba cycle, the ocean's biological pump, water-mass provenance in the deep ocean, changes in activity of hydrothermal vents, and land-sea interactions including tracing riverine inputs. Here, we show that aragonite skeletons of various colonial and solitary cold-water coral (CWC) taxa record the seawater (SW) Ba isotope composition. Thirty-six corals of eight different taxa from three oceanic regions were analysed and compared to delta(138)/Ba-134 measurements of co-located seawater samples. Sites were chosen to cover a wide range of temperature, salinity, Ba concentrations and Ba isotope compositions. Seawater samples at the three sites exhibit the well-established anti-correlation between Ba concentration and delta(138)/Ba-134. Furthermore, our data set suggests that Ba/Ca values in CWCs are linearly correlated with dissolved Ba] in ambient seawater, with an average partition coefficient of D-CWC/SW = 1.8 +/- 0.4 (2SD). The mean isotope fractionation of Ba between seawater and CWCs Delta(138)/Ba-134(CWC-SW) is -0.21 +/- 0.08 parts per thousand (2SD), indicating that CWC aragonite preferentially incorporates the lighter isotopes. This fractionation likely does not depend on temperature or other environmental variables, suggesting that aragonite CWCs could be used to trace the Ba isotope composition in ambient seawater. Coupled Ba] and delta(138)/Ba-134 analysis on fossil CWCs has the potential to provide new information about past changes in the local and global relationship between Ba] and delta(138)/Ba-134 and hence about the operation of the past global oceanic Ba cycle in different climate regimes. (C) 2018 Elsevier B.V. All rights reserved

NIST RM 8301 Boron Isotopes in Marine Carbonate (Simulated Coral and Foraminifera Solutions): Inter‐laboratory δ 11 B and Trace Element Ratio Value Assignment

International audienc

NIST RM 8301 boron isotopes in marine carbonate (simulated coral and foraminifera solutions):inter-laboratory δ¹¹B and trace element ratio value assignment

The boron isotopic (δ11BSRM951) and trace element composition of marine carbonates are key proxies for understanding carbon cycling (pH) and palaeoceanographic change. However, 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 marine carbonate reference materials (RM), NIST RM 8301 (Coral) and NIST RM 8301 (Foram) and to assess analytical variability 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 values among laboratories (e.g., Mg/Ca ± 5%) was often greater than analytical precision (% RSD) 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

NIST RM 8301 boron isotopes in marine carbonate (simulated coral and foraminifera solutions): inter‐laboratory δ11B and trace element ratio value assignment

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