Anthropogenic carbon emissions are causing seawater pH to decline, yet the impact on marine calcifiers is uncertain. Scleractinian corals and coralline algae strongly elevate the pH of their calcifying fluid (CF) to promote calcification. Other organisms adopt less energetically demanding calcification approaches but restrict their habitat. Stylasterid corals occur widely (extending well below the carbonate saturation horizon) and precipitate both aragonite and high-Mg calcite, however, their mode of biocalcification and resilience to ocean acidification are unknown. Here we measure skeletal boron isotopes (δ11B), B/Ca, and U/Ca to provide the first assessment of pH and rate of seawater flushing of stylasterid CF. Remarkably, both aragonitic and high-Mg calcitic stylasterids have low δ11B values implying little modification of internal pH. Collectively, our results suggest stylasterids have low seawater exchange rates into the calcifying space or rely on organic molecule templating to facilitate calcification. Thus, despite occupying similar niches to Scleractinia, Stylasteridae exhibit highly contrasting biocalcification, calling into question their resilience to ocean acidification.
<p>The geochemistry of deep-sea coral (DSC) skeletons has been widely used to reconstruct past changes in ocean conditions [1]. Previous work has focused predominantly on anthozoan coral groups (e.g., Scleractinia), while hydrozoan corals &#8211; such as stylasterids - have received far less attention. However, it has recently been demonstrated that stylasterid skeletal geochemistry reliably records seawater conditions (e.g., temperature [2, 3]). The application of stylasterid geochemistry in palaeoceanographic contexts is now contingent on further developing tools for dating stylasterid skeletons over a range of timescales. Growth chronologies have been successfully constructed for modern stylasterids using radiocarbon methods [4], however, the application of U-series techniques to stylasterid corals is yet to be fully explored. &#160;</p><p>Here, we present U/Ca ratios of modern stylasterid and scleractinian DSCs, in addition to U-series isotope data from sub-fossil stylasterid skeletons. Stylasterids build skeletons from aragonite, high-Mg calcite, or a mixture of both polymorphs, and we observe a mineralogical control on U-incorporation into stylasterid carbonate. However, both aragonitic and high-Mg calcitic stylasterids have significantly lower U/Ca than Scleractinia. This result likely stems from the differing calcification mechanisms of these two coral groups; an interpretation supported by other aspects of their skeletal geochemistry [2, 3].</p><p>Low uranium concentrations complicate the application of traditional U-series dating techniques to stylasterids. We show that the low abundance of parent nuclei (<sup>238</sup>U) leads to small amounts of radiogenic <sup>230</sup>Th production, resulting in significantly larger chronological uncertainties than those achievable for Scleractinia. Additionally, the highly porous structure of some stylasterid skeletons means they are particularly prone to diagenetic alteration and contamination. Despite this, stylasterids dated by U-series techniques may be informative where high precision is not required, while isochron methods can be applied to larger samples, reducing chronological uncertainties.</p><p>Although stylasterids dated by U-series techniques may be useful in certain contexts, our data suggest that their palaeoceanographic utility lies elsewhere. Where possible, growth chronologies for individual stylasterids should be constructed using radiocarbon techniques (e.g. [4]) and/or radial growth-band counting. When combined with robust temperature proxies [2, 3], stylasterids dated in this manner may have special utility as high-resolution archives of recent (i.e. decadal to centennial [4]) changes in ocean conditions.</p><p>&#160;</p><p>1) Robinson et al. [2014] Deep Sea Research Part II: Topical Studies in Oceanography. 99, 184 - 198</p><p>2) Stewart et al. [2020] EPSL. 545, 116412</p><p>3) Samperiz et al. [2020] EPSL. 545, 116407</p><p>4) King et al. [2018] Paleoceanography and Paleoclimatology. 33, 1306&#8211;1321</p>
<p>Geochemical evidence suggests that Atlantic circulation during the Last Glacial Maximum (LGM) was considerably different from modern and promoted carbon accumulation in the deep. During the last deglaciation, atmospheric CO<sub>2</sub> concentration and temperature rose significantly, while the radiocarbon (<sup>14</sup>C) content dropped. Marine records indicate that ocean circulation may have influenced these atmospheric parameters, for instance via outgassing of the carbon-rich and <sup>14</sup>C-depleted glacial oceanic reservoir. Temperature is also suggested to impact ocean circulation through the redistribution of heat, particularly during climate swings of the last deglaciation. Despite the crucial role that intermediate waters play in linking the deep sea to the atmosphere they remain understudied. Here, we use precisely dated (U/Th) cold-water corals to reconstruct the seawater radiocarbon, temperature, and barium concentration ([Ba]<sub>sw</sub>) of the intermediate depths at Tropic Seamount (tropical Northeast Atlantic). We analysed <sup>14</sup>C, Li/Mg and Ba/Ca ratios of corals distributed from 970 m to 1800 m and dating from 32.7 thousand years (ka) to 0.2 ka. Our results highlight the dynamic behaviour of the intermediate ocean and suggest climate driven variability with distinct features during Heinrich Event 3 (HS3, ~30 ka), Last Glacial Maximum (~22 to 18 ka) and the deglaciation (~18 to 11 ka). Overall, seawater radiocarbon values were higher and temperatures lower in the LGM compared to the deglaciation. We observe a rapid (~500 yr) decrease in intermediate water radiocarbon and temperature during the mid-LGM. This variation and an increase in [Ba]<sub>sw</sub> support a change on the water column configuration, and likely a shift on the boundary between intermediate and deep waters. During the deglaciation, we find larger radiocarbon and temperature changes, suggesting that a warmer and well-ventilated intermediate water was established in parallel with Southern Hemisphere warming, but prior to Northern hemisphere temperature spike at 14.5 ka. These findings reinforce the importance of the intermediate ocean as a vector of climate change and emphasize the advantage of multi-proxy approaches in cold-water corals to investigate environmental conditions.</p>
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