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<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>

<p>The Arctic region is noted to be sensitive in its response to anthropogenic warming. The Greenland Ice Sheet is experiencing accelerated mass loss via surface melting and ice discharge. This freshwater input is likely to influence global heat distribution via the Atlantic Meridional Overturning Circulation (AMOC). To better understand past natural variations in this system, proxy reconstructions are required to give a longer-term perspective. Previous proxy studies have suggested that human-induced AMOC slowdown began as early as the nineteenth century. However, the lack of high temporal resolution data from the last millennium means that the role of meltwater discharge on the evolution of North Atlantic intermediate waters, especially during the Little Ice Age (LIA), remains unclear.</p><p>Here, we present both weathering and temperature records from deep-sea scleractinian corals collected from Southwest Greenland (Nuuk Trough). We analysed <sup>234</sup>U/<sup>238</sup>U, rare earth elements with yttrium (REEY) and trace elements (Li/Mg temperature proxy) along with precise U-Th dating of corals. Samples were from 750 m and 1200 m water depth with ages spanning the last 1000 years. The study site is influenced by surface meltwater from the West Greenland Ice Sheet. It is also at the convergence point of shallow cold Arctic-sourced water and deeper warm Atlantic-sourced water, providing an ideal location for tracing AMOC variations.</p><p>Our coral data show West Greenland seawater δ<sup>234</sup>U has increased ~2‰ toward modern seawater value since the end of the LIA (1700 C.E.), suggesting an increase in subglacial physical weathering input. This is supported by our terrestrial discharge record from REEY data that indicates an increase in meltwater discharge since the end of the LIA. The temperature record shows a gradual cooling trend from 1600 to 1900 C.E. at 1200 m depth, followed by warming at 750m. We suggest that the temperature drop at intermediate depth is linked to a change in water mass structure, as the thermocline shallowed and colder, deeper waters expanded. Cooling at this depth is consistent with a weakened AMOC, with less penetration of warm Atlantic waters. Our findings highlight the complex interactions between glacial meltwater and intermediate water circulation in the last millennium.</p>

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