Stylasterid corals: a new paleotemperature archive

Stylasterids are a ubiquitous deep-sea coral taxon that build their skeletons from either calcite, aragonite, or both. Yet, robust geochemical proxy data from these corals are limited. In this study, 95 modern stylasterids, spanning a wide range of depths (63 to 2894 m) and ambient seawater temperatures (0 to 17 °C), were tested for their potential use as paleoceanographic archives. Stable oxygen and carbon isotopic composition (O and C) were measured from the main trunk of all specimens and five specimens were further sub-sampled to assess internal chemical variability. The isotope data show non-equilibrium precipitation from seawater for both O and C, with the growing tips of colonies yielding the isotopically lowest values. Overall, the calcitic corals showed lower isotope values for O and C than aragonitic specimens. Within the aragonite corals, we present a O:temperature calibration that exhibits a significant linear relationship with the equation Ocoral-seawater = −0.22(°C) + 3.33(±0.06) across a temperature range of 0 to 30 °C, using samples from this study and published data. This work highlights the potential application of stylasterid coral O data to reconstruct paleo seawater temperature

Refining trace metal temperature proxies in cold-water scleractinian and stylasterid corals

The Li/Mg, Sr/Ca and oxygen isotopic (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, O and carbon isotope (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/MgAll Aragonite = (). 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/MgStylasterid (Arag) = ()) 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 = ()). 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 abiogenic trace metal incorporation mechanism

Reconciling ice core CO2 and land use change following New World-Old World contact

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. Yet the atmospheric history of CO2 remains uncertain in some time intervals. Here we present measurements of CO2 and methane (CH4) in the Skytrain ice core from 1450 to 1700 CE. Results suggest a sudden decrease in CO2 around 1610 CE in one widely used record may be an artefact of a small number of anomalously low values. Our analysis supports a more gradual decrease in CO2 of 0.5 ppm per decade from 1516 to 1670 CE, with an inferred land carbon sink of 2.6 PgC per decade. This corroborates modelled scenarios of large-scale reorganisation of land use in the Americas following New World-Old World contact, whereas a rapid decrease in CO2 at 1610 CE is incompatible with even the most extreme land-use change scenarios

Ba/Ca of stylasterid coral skeletons records dissolved seawater barium concentrations

The concentration of dissolved barium in seawater ([Ba]SW) is influenced by both primary productivity and ocean circulation patterns. Reconstructing past subsurface [Ba]SW can therefore provide important information on processes which regulate global climate. Previous Ba/Ca measurements of scleractinian and bamboo deep-sea coral skeletons exhibit linear relationships with [Ba]SW, acting as archives for past Ba cycling. However, skeletal Ba/Ca ratios of the Stylasteridae – a group of widely distributed, azooxanthellate, hydrozoan coral – have not been previously studied. Here, we present Ba/Ca ratios of modern stylasterid (aragonitic, calcitic and mixed mineralogy) and azooxanthellate scleractinian skeletons, paired with published proximal hydrographic data. We find that [Ba]SW and sample mineralogy are the primary controls on stylasterid Ba/Ca, while seawater temperature exerts a weak secondary control. [Ba]SW also exerts a strong control on azooxanthellate scleractinian Ba/Ca. However, Ba-incorporation into scleractinian skeletons varies between locations and across depth gradients, and we find a more sensitive relationship between scleractinian Ba/Ca and [Ba]SW than previously reported. Paired Sr/Ca measurements suggest that this variability in scleractinian Ba/Ca may result from the influence of varying degrees of Rayleigh fractionation during calcification. We find that these processes exert a smaller influence on Ba-incorporation into stylasterid coral skeletons, a result consistent with other aspects of their skeletal geochemistry. Stylasterid Ba/Ca ratios are therefore a powerful, novel archive of past changes in [Ba]SW, particularly when measured in combination with temperature sensitive tracers such as Li/Mg or Sr/Ca. Indeed, with robust [Ba]SW and temperature proxies now established, stylasterids have the potential to be an important new archive for palaeoceanographic studies

Antarctic ice core carbon monoxide records and composite spline 1821-1995 CE (Strawson et al., 2024)

Carbon monoxide mixing ratio of air trapped in ice core bubbles. Datasets from three Antarctic ice cores are presented: Jurassic, Bryan Coast and Dyer Plateau. These datasets contain a quasi-annual signal thought to result from the in situ production of carbon monoxide in ice. The composite spline should be used as a reference for the composition of past atmosphere. Ice cores were drilled by the British Antarctic Survey in 2011/12 (Jurassic and Bryan Coast) and Dyer Plateau in 1989. Data were generated via Continuous Flow Analysis (CFA) of 3.4 x 3.4 cm stick of ice core using an Optical Feedback Cavity Enhanced Spectroscopy (OF-CEAS) optical spectrometer across three analytical campaigns between 11/2021 and 01/2023. Data are 10 second averages from an original acquisition rate of 4 Hz. Data are corrected for minor blank and solubility effects and reported on the NOAA WMOX-2014 scale. The composite spline is the average of the 5th percentile records from the three cores. Uncertainty is given as 2 sigma (units of parts per billion (ppb)) and includes uncertainty on blank quantification and long-term reproducibility. Uncertainty on the composite spline is given as 2 sigma (ppb).Isaac Newton Trust at the University of Cambridge (LBZG /080.01.G101121

Pre-industrial Southern Hemisphere biomass burning variability inferred from ice core carbon monoxide records

Biomass burning plays an important role in climate-forcing and atmospheric chemistry. The drivers of fire activity over the past two centuries, however, are hotly debated and fuelled by poor constraints on the magnitude and trends of pre-industrial fire regimes. As a powerful tracer of biomass burning, reconstructions of paleo-atmospheric carbon monoxide (CO) can provide valuable information on the evolution of fire activity across the pre-industrial to industrial transition. Here too, however, significant disagreements between existing CO records currently allow for opposing fire histories. In this study, we reconstruct a continuous record of Antarctic ice core CO between 1822 and 1995 CE to overlap with direct atmospheric observations. Our record indicates that the Southern Hemisphere CO burden ([CO]) increased by 50% from a pre-industrial mixing ratio of ca. 35 ppb to ca. 53 ppb by 1995 CE but with a greater level of variability than allowed for by state-of-the-art chemistry-climate models, suggesting that historic CO dynamics have been not fully accounted for. Using a 6-troposphere box model, a 40-50% decrease in Southern Hemisphere biomass burning, coincident with unprecedented rates of early 20th century anthropogenic land-use change, is identified as a strong candidate for this mismatch

Stylasterid corals build aragonite skeletons in undersaturated water despite low pH at the site of calcification.

Funder: Antarctic Science BursaryAnthropogenic 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