The Li/Mg, Sr/Ca and oxygen isotopic (δ 18 O) compositions of many marine biogenic carbonates are sensitive to seawater temperature. Corals, as cosmopolitan marine taxa with carbonate skeletons that can be precisely dated, represent ideal hosts for these geochemical proxies. However, efforts to calibrate and refine temperature proxies in cold-water corals (<20 • C) remain limited. Here we present skeletal Li/Mg, Sr/Ca, δ 18 O and carbon isotope (δ 13 C) data from live-collected specimens of aragonitic scleractinian corals (Balanophyllia, Caryophyllia, Desmophyllum, Enallopsammia, Flabellum, Lophelia, and Vaughanella), both aragonitic and high-Mg calcitic stylasterid genera (Stylaster and Errina), and shallow-water high-Mg calcite crustose coralline algae (Lithophyllum, Hydrolithon, and Neogoniolithon). We interpret these data in conjunction with results from previously explored taxa including aragonitic zooxanthellate scleractinia and foraminifera, and high-Mg calcite octocorals. We show that Li/Mg ratios covary most strongly with seawater temperature, both for aragonitic and high-Mg calcitic taxa, making for reliable and universal seawater temperature proxies. Combining all of our biogenic aragonitic Li/Mg data with previous calibration efforts we report a refined relationship to temperature: Li/Mg All Aragonite = 5.42 exp(−0.050 × T (• C)) (R 2 = 0.97). This calibration now permits paleo-temperature reconstruction to better than ±3.4 • C (95% prediction intervals) across biogenic aragonites, regardless of taxon, from 0 to 30 • C. For taxa in this study, aragonitic stylasterid Li/Mg offers the most robust temperature proxy (Li/Mg Stylasterid (Arag) = 5.64 exp(−0.046 × T (• C)) (R 2 = 0.95)) with a reproducibility of ±2.3 • C. For the first time, we show that high-Mg calcites have a similar exponential relationship with temperature, but with a lower intercept value (Li/Mg = 0.63 exp(−0.050 × T (• C) (R 2 = 0.92)). This calibration opens the possibility of temperature reconstruction using high-Mg calcite corals and coralline algae. The commonality in the relationship between Li/Mg and temperature transcends phylogeny and suggests a similar abiogenic trace metal incorporation mechanism.
Ice core records of carbon dioxide (CO2) throughout the last 2000 years provide context for the unprecedented anthropogenic rise in atmospheric CO2 and insights into global carbon cycle dynamics on centennial and multidecadal timescales. Yet the atmospheric history of CO2 remains uncertain in some time intervals. A particular source of debate is the exact timing and magnitude of the decrease in atmospheric CO2 after 1550 CE. Here we present new ice core measurements of CO2 and methane (CH4) in the Skytrain Ice Rise ice core from 1450 to 1700 CE. The measurements, alongside analysis of the effects of gas record smoothing, suggest that a sudden decrease in ice core CO2 around 1610 CE in one widely used record is most likely an artefact of a small number of anomalously low values. Instead, our analysis suggests a more gradual decrease in CO2 of 0.5 ppm per decade between 1516 and 1670 CE, with an inferred land carbon sink of 2.7 PgC per decade. Furthermore, a rapid decrease in CO2 at 1610 CE is incompatible with even the most extreme modelled scenarios for land-use change, whereas our data support scenarios of large-scale reorganization of land use in the Americas following New World-Old World contact.
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>
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