Sclerochronological and geochemical records from Porites spp. in Fiji: Assessing massive coral response to environmental changes

Coral reefs are one of the most biologically diverse systems on Earth and the ecosystem services they provide are of high importance for coastal communities and beyond. Yet, they are becoming increasingly threatened by global (climate change) and local stressors (e.g., land-derived pollution, overfishing). Many studies have focused on understanding how scleractinian (reef-building) corals in inshore reefs are responding to these environmental stressors and their potential compounding effects. However, this is complicated due to the lack of high-resolution records of both environmental parameters and coral growth. Records of coral growth and geochemistry from the aragonitic skeletons of massive scleractinian corals can be used as effective archives of modern and past changes in reef water quality, coral calcification rate and physiology, improving our understanding of reef-specific and species-specific responses of corals to multiple stressors. In this thesis, I aim to disentangle the relationship between coral growth, skeletal deposition, geochemical composition, and environmental conditions in Fiji. I acquire sclerochronological and geochemical records from massive Porites spp. corals from four inshore reefs adjacent to different catchment environments in Viti Levu, Fiji, alongside remotely sensed environmental data. I reconstruct the sensitivity of Porites spp. to environmental changes, explore the fundamental physiological controls on the incorporation of SST proxies into skeletal aragonite, and explore site-specific environmental and hydrological controls on reef water quality and the use of proposed proxies for terrigenous input. Sclerochronological records of coral growth (linear extension, density, and calcification) are obtained from Porites spp. cores using Computed Tomography. Geochemical records include measurements of a suite of trace and minor elemental concentrations in the coral skeletons using Laser Ablation – Inductive Coupled Plasma Mass Spectroscopy to assess a range of SST proxies (Sr/Ca, Mg/Ca, Li/Ca, and U/Ca) and terrigenous input proxies (Ba/Ca, Mn/Ca, Y/Ca, La/Ca, and Ce/Ca). Coral growth records from inshore reefs show that although water quality plays a significant role in driving mean linear extension and calcification rate between 1998 and 2016, persistent thermal stress has the capacity to reduce coral growth across all locations, regardless of local conditions. This is important because under a scenario of amplified global warming and persistent thermal stress local management might not be enough to palliate the effects of climate change. Traditional coral-derived SST proxies in inshore reefs in Fiji reflect SST variability to some degree and are applicable for paleotemperature reconstructions. However, multiple core replicates across locations are needed to build a composite record that accurately reflects SST. Results show the existence of biological controls ('vital effects') on the elemental composition of the corals as a response to reef-specific environmental factors, complicating the extraction of an SST signal. In addition, application of terrigenous input proxies, although successfully recorded by the corals of this study, need to be considered on a site-by-site basis. This is because variable environmental and hydrological mechanisms lead to reef-specific changes in water quality as a response to climatic events (e.g., rainfall seasonality, tropical cyclones). As such, future interpretations of terrigenous input proxies will require of a deep understanding of hydrological and climatic catchment-coastal linkages. Overall, this work showcases the use of coupled sclerochronological and geochemical methods for providing robust growth and environmental records and a better understanding of how environmental conditions have affected coral growth in vulnerable inshore reefs in the past

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

Iberian Neanderthals in forests and savannahs

This article aims to delve into the reality of glacial refuges of forests and tree species (including conifers, mesothermophilous angiosperms and xerothermic scrub) during the cold dry phases of the Iberian Pleistocene in which there is evidence of occupation of Middle Palaeolithic people. The research framework focuses on the eastern sector of the Iberian Peninsula due to the physiographic, palaeobotanical and archaeological peculiarities, substantiated by recent studies. We contend that some Neanderthal occupations developed in the context of high geobiological complexity, high biological diversity and highly structured forest ecosystems. We highlight the importance of glacial refuges as local anomalies that, however, would be contingent on vegetational development, and on the survival of Palaeolithic groups in areas with a broad diversity of natural resources.Peer reviewe

Reinterpreting radiocarbon records in bamboo corals ? New insights from the tropical North Atlantic

Deep-sea bamboo corals (family Isididae) have been used as archives for reconstructing changes in the past ocean. However, uncertainties remain regarding the interpretation of geochemical signals from their organic nodes, specifically the water depth of the signals recorded by the coral. Here we explore this question by measuring radiocarbon (14C) and nitrogen (15N) isotopic compositions of the organic nodes in six bamboo corals collected from the central and eastern tropical Atlantic between 700 m and 2000 m water depth. By comparing coral 14C to measured seawater data, regional shallow-water coral records and climate-model outputs, we find contrasting results between the two regions. Our bamboo coral 14C results from the eastern tropical Atlantic support previous studies that suggest organic node carbon is sourced primarily from the mixed layer of the ocean. By contrast, the 14C of bamboo coral organic nodes from the oligotrophic central Atlantic better correlates with the 14C content of the subsurface deep chlorophyll maximum layer rather than the surface mixed layer. Combined with nitrogen isotope data, this observation suggests that sinking and/or ambient zooplankton feeding on phytoplankton from the deep chlorophyll maximum layer can contribute a significant proportion of the diet of bamboo corals. These results suggest that the carbon source for bamboo corals organic nodes may not always reside in the mixed layer, especially in oligotrophic regions, which has implications for 14C-based age model development in bamboo corals

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

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

The biogeochemical impact of glacial meltwater from Southwest Greenland

Biogeochemical cycling in high-latitude regions has a disproportionate impact on global nutrient budgets. Here, we introduce a holistic, multi-disciplinary framework for elucidating the influence of glacial meltwaters, shelf currents, and biological production on biogeochemical cycling in high-latitude continental margins, with a focus on the silica cycle. Our findings highlight the impact of significant glacial discharge on nutrient supply to shelf and slope waters, as well as surface and benthic production in these regions, over a range of timescales from days to thousands of years. Whilst biological uptake in fjords and strong diatom activity in coastal waters maintains low dissolved silicon concentrations in surface waters, we find important but spatially heterogeneous additions of particulates into the system, which are transported rapidly away from the shore. We expect the glacially-derived particles – together with biogenic silica tests – to be cycled rapidly through shallow sediments, resulting in a strong benthic flux of dissolved silicon. Entrainment of this benthic silicon into boundary currents may supply an important source of this key nutrient into the Labrador Sea, and is also likely to recirculate back into the deep fjords inshore. This study illustrates how geochemical and oceanographic analyses can be used together to probe further into modern nutrient cycling in this region, as well as the palaeoclimatological approaches to investigating changes in glacial meltwater discharge through time, especially during periods of rapid climatic change in the Late Quaternary

Comparison of seven prognostic tools to identify low-risk pulmonary embolism in patients aged <50 years

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Rate and duration of hospitalisation for acute pulmonary embolism in the real-world clinical practice of different countries : Analysis from the RIETE registry

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