Seawater nutrient and carbonate ion concentrations recorded as P/Ca, Ba/Ca, and U/Ca in the deep-sea coral Desmophyllum dianthus

Abstract

As paleoceanographic archives, deep sea coral skeletons offer the potential for high temporal resolution and precise absolute dating, but have not been fully investigated for geochemical reconstructions of past ocean conditions. Here we assess the utility of skeletal P/Ca, Ba/Ca and U/Ca in the deep sea coral D. dianthus as proxies of dissolved phosphate (remineralized at shallow depths), dissolved barium (trace element with silicate-type distribution) and carbonate ion concentrations, respectively. Measurements of these proxies in globally distributed D. dianthus specimens show clear dependence on corresponding seawater properties. Linear regression fits of mean coral Element/Ca ratios against seawater properties yield the equations: P/Ca coral (lmol/ mol) = (0.6 ± 0.1) P/Ca sw (lmol/mol) -(23 ± 18), R 2 = 0.6, n = 16 and Ba/Ca coral (lmol/mol) = (1.4 ± 0.3) Ba/Ca sw (lmol/ mol) + (0 ± 2), R 2 = 0.6, n = 17; no significant relationship is observed between the residuals of each regression and seawater temperature, salinity, pressure, pH or carbonate ion concentrations, suggesting that these variables were not significant secondary dependencies of these proxies. Four D. dianthus specimens growing at locations with O arag 6 0.6 displayed markedly depleted P/Ca compared to the regression based on the remaining samples, a behavior attributed to an undersaturation effect. These corals were excluded from the calibration. Coral U/Ca correlates with seawater carbonate ion: U/Ca coral (lmol/ mol) = (À0.016 ± 0.003) ½CO 2À 3 (lmol/kg) + (3.2 ± 0.3), R 2 = 0.6, n = 17. The residuals of the U/Ca calibration are not significantly related to temperature, salinity, or pressure. Scatter about the linear calibration lines is attributed to imperfect spatialtemporal matches between the selected globally distributed specimens and available water column chemical data, and potentially to unresolved additional effects. The uncertainties of these initial proxy calibration regressions predict that dissolved phosphate could be reconstructed to ±0.4 lmol/kg (for 1.3-1.9 lmol/kg phosphate), and dissolved Ba to ±19 nmol/kg (for 41-82 nmol/ kg Ba sw ). Carbonate ion concentration derived from U/Ca has an uncertainty of ±31lmol/kg (for 60-120 lmol=kg CO 2À 3 ). The effect of microskeletal variability on P/Ca, Ba/Ca, and U/Ca was also assessed, with emphasis on centers of calcification, Fe-Mn phases, and external contaminants. Overall, the results show strong potential for reconstructing aspects of water mass mixing and biogeochemical processes in intermediate and deep waters using fossil deep-sea corals.

The influence of seawater pH on U / Ca ratios in the scleractinian cold-water coral <i>Lophelia pertusa</i>

“…Case et al, 2010;Cohen et al, 2006;Gagnon et al, 2007;Lutringer et al, 2005;Montagna et al, 2006;Raddatz et al, 2013;RollionBard et al, 2009;Rüggeberg et al, 2008;Smith et al, 2000). To date only a few studies have focused on the reconstruction of the carbonate system using CWC skeletons (Anagnostou et al, 2011(Anagnostou et al, , 2012Blamart et al, 2007;McCulloch et al, 2012;Rollion-Bard et al, 2011a, b;Thresher et al, 2011). Coral uranium to calcium (U / Ca) ratios are known to depend on the carbonate system parameters (Anagnostou et al, 2011;Inoue et al, 2011;Min et al, 1995;Shen and Dunbar, 1995;Swart and Hubbard, 1982).…”

“…To date only a few studies have focused on the reconstruction of the carbonate system using CWC skeletons (Anagnostou et al, 2011(Anagnostou et al, , 2012Blamart et al, 2007;McCulloch et al, 2012;Rollion-Bard et al, 2011a, b;Thresher et al, 2011). Coral uranium to calcium (U / Ca) ratios are known to depend on the carbonate system parameters (Anagnostou et al, 2011;Inoue et al, 2011;Min et al, 1995;Shen and Dunbar, 1995;Swart and Hubbard, 1982). Initial studies showed that coral U / Ca ratios are related to seawater temperature (Min et al, 1995;Shen and Dunbar, 1995), but also suggested that seawater pH may also play an important role in the incorporation of uranium into the skeleton of tropical corals.…”

Low Florida coral calcification rates in the Plio-Pleistocene

“…One coral specimen (452K1) was analysed geochemically using LA-ICP-MS (Böcker, 2014) with regard to concentrations of environmentally sensitive elements (e.g. Sr / Ca, U / Ca, B / Ca) and following recommendations for evaluating the diagenetic status of corals from strongly lithified and altered limestone (Anagnostou et al, 2011;Gothmann et al, 2015). LA-ICP-MS analyses were performed at the Max Planck Institut für Chemie (Mainz, Germany) using a NewWave UP 213 laser ablation system coupled to a ThermoFisher Element 2 ICP-MS.…”

“…According to our conviction, all these data provide no critical evidence for the alteration of the original skeleton. Because of this line of reasoning and low overall calcite content evident from XRD analysis (calcite below detection limits), we refrained from measuring element ratios sensitive to the redox conditions of calcite precipitating freshwaters or burial fluids (Fe / Ca, Mn / Ca) and other more sophisticated geochemical methods as potential measures of alteration (Anagnostou et al, 2011;Gothmann et al, 2015).…”

“…While the petrographic aspect of the calcite documents the type of freshwater or burial alteration environment, cathodoluminescence analysis and geochemical data may provide further information as to the redox character of the diagenetic fluids (Flügel, 2004). Alteration of aragonite is commonly a rapid process, but in the rare event of low porewater circulation rates, corals do escape diagenetic alteration (Anagnostou et al, 2011;Brachert et al, 2006aBrachert et al, , 2016Denniston et al, 2008a;Gothmann et al, 2015;Griffiths et al, 2013;Mertz-Kraus et al, 2008).…”