U-series dating of stylasterid coral skeletons

Abstract

<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 – 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.  </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> </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, 1306–1321</p>