Precision and cost-effectiveness of bioindicators to estimate nutrient regimes on coral reefs

Bioindicators are useful for determining nutrient regimes in marine environments, but their ability to evaluate corals reefs in different ecological states is poorly understood. The precision, availability and congruency of eight potential bioindicators (brown macroalgae, green macroalgae, turf algae, cyanobacteria, soft corals, zoanthids, sponges, and sediment) and their stable isotopic and elemental signatures (δ¹⁵N, δ¹³C, %N, %C, and C:N Ratio) were assessed across 21 reefs in the Inner Seychelles. The coefficient of variation (CoV) for δ¹⁵N showed that green and brown macroalgae were highly precise (2.47 ± 0.95, n = 11; 4.68 ± 1.33, n = 16, respectively), though were less common on coral-mortality reefs relative to macroalgal-dominated ones. Zoanthids were also highly precise for δ¹⁵N (2.98 ± 1.20), but were more readily available regardless of reef state (n = 18). Congruency was low among these indicators, suggesting that different physiological mechanisms for nutrient processing have a stronger influence on a bioindicator's effectiveness than reef state.

High Concentrations of Perfluoroalkyl Acids in Arctic Seawater Driven by Early Thawing Sea Ice

Poly- and perfluoroalkyl substances are synthetic chemicals that are widely present in the global environment including the Arctic. However, little is known about how these chemicals (particularly perfluoroalkyl acids, PFAA) enter the Arctic marine system and cycle between seawater and sea ice compartments. To evaluate this, we analyzed sea ice, snow, melt ponds, and near-surface seawater at two ice-covered stations located north of the Barents Sea (81 °N) with the aim of investigating PFAA dynamics in the late-season ice pack. Sea ice showed high concentrations of PFAA particularly at the surface with snow-ice (the uppermost sea ice layer strongly influenced by snow) comprising 26-62% of the total PFAA burden. Low salinities (<2.5 ppt) and low δ18OH20 values (<1‰ in snow and upper ice layers) in sea ice revealed the strong influence of meteoric water on sea ice, thus indicating a significant atmospheric source of PFAA with subsequent transfer down the sea ice column in meltwater. Importantly, the under-ice seawater (0.5 m depth) displayed some of the highest concentrations notably for the long-chain PFAA (e.g., PFOA 928 ± 617 pg L-1), which were ≈3-fold higher than those of deeper water (5 m depth) and ≈2-fold higher than those recently measured in surface waters of the North Sea infuenced by industrial inputs of PFAAs. The evidence provided here suggests that meltwater arising early in the melt season from snow and other surface ice floe components drives the higher PFAA concentrations observed in under-ice seawater, which could in turn influence the timing and extent of PFAA exposure for organisms at the base of the marine food web.

Investigating the Uptake and Fate of Poly- and Perfluoroalkylated Substances (PFAS) in Sea Ice Using an Experimental Sea Ice Chamber

Poly- and perfluoroalkyl substances (PFAS) are contaminants of emerging Arctic concern and are present in the marine environments of the polar regions. Their input to and fate within the marine cryosphere are poorly understood. We conducted a series of laboratory experiments to investigate the uptake, distribution, and release of 10 PFAS of varying carbon chain length (C₄-C₁₂) in young sea ice grown from artificial seawater (NaClsolution). We show that PFAS are incorporated into bulk sea ice during ice formation and regression analyses for individual PFAS concentrations in bulk sea ice were linearly related to salinity (r² = 0.30 to 0.88, n = 18, p < 0.05). This shows that their distribution is strongly governed by the presence and dynamics of brine (high salinity water) within the sea ice. Furthermore, long-chain PFAS (C₈-C₁₂), were enriched in bulk ice up to 3-fold more than short-chain PFAS (C₄-C₇) and NaCl. This suggests that chemical partitioning of PFAS between the different phases of sea ice also plays a role in their uptake during its formation. During sea ice melt, initial meltwater fractions were highly saline and predominantly contained short-chain PFAS, whereas the later, fresher meltwater fractions predominantly contained long-chain PFAS. Our results demonstrate that in highly saline parts of sea ice (near the upper and lower interfaces and in brine channels) significant chemical enrichment (ε) of PFAS can occur with concentrations in brine channels greatly exceeding those in seawater from which it forms (e.g., for PFOA, ε_brine = 10 ± 4). This observation has implications for biological exposure to PFAS present in brine channels, a common feature of first-year sea ice which is the dominant ice type in a warming Arctic.

Possible links between extreme oxygen perturbations and the Cambrian radiation of animals

The role of oxygen as a driver for early animal evolution is widely debated. During the Cambrian explosion, episodic radiations of major animal phyla occurred coincident with repeated carbon isotope fluctuations. However, the driver of these isotope fluctuations and potential links to environmental oxygenation are unclear. Here, we report high-resolution carbon and sulphur isotope data for marine carbonates from the southeastern Siberian Platform that document the canonical explosive phase of the Cambrian radiation from ~524 to ~514 Myr ago. These analyses demonstrate a strong positive covariation between carbonate δ¹³C and carbonate-associated sulphate δ³⁴S through five isotope cycles. Biogeochemical modelling suggests that this isotopic coupling reflects periodic oscillations in atmospheric O₂ and the extent of shallow ocean oxygenation. Episodic maxima in the biodiversity of animal phyla directly coincided with these extreme oxygen perturbations. Conversely, the subsequent Botoman-Toyonian animal extinction events (~514 to ~512 Myr ago) coincided with decoupled isotope records that suggest a shrinking marine sulphate reservoir and expanded shallow marine anoxia. We suggest that fluctuations in oxygen availability in the shallow marine realm exerted a primary control on the timing and tempo of biodiversity radiations at a crucial phase in the early history of animal life.

Interpretation and application of carbon isotope ratios in freshwater diatom silica

Carbon incorporated into diatom frustule walls is protected from degradation enabling analysis for carbon isotope composition (δ¹³C_diatom). This presents potential for tracing carbon cycles via a single photosynthetic host with well-constrained ecophysiology. Improved understanding of environmental processes controlling carbon delivery and assimilation is essential to interpret changes in freshwater δ¹³C_diatom. Here relationships between water chemistry and δ¹³C_diatom from contemporary regional data sets are investigated. Modern diatom and water samples were collected from river catchments within England and lake sediments from across Europe. The data suggest dissolved, biogenically produced carbon supplied proportionately to catchment productivity was critical in the rivers and soft water lakes. However, dissolved carbon from calcareous geology overwhelmed the carbon signature in hard water catchments. Both results demonstrate carbon source characteristics were the most important control on δ¹³C_diatom, with a greater impact than productivity. Application of these principles was made to a sediment record from Lake Tanganyika. δ¹³C_diatom co-varied with δ¹³C_bulk through the last glacial and Holocene. This suggests carbon supply was again dominant and exceeded authigenic demand. This first systematic evaluation of contemporary δ¹³C_diatom controls demonstrates that diatoms have the potential to supply a record of carbon cycling through lake catchments from sediment records over millennial timescales.

Drip water electrical conductivity as an indicator of cave ventilation at the event scale

The use of speleothems to reconstruct past climatic and environmental change through chemical proxies is becoming increasingly common. Speleothem chemistry is controlled by hydrological and atmospheric processes which vary over seasonal time scales. However, as many reconstructions using speleothem carbonate are now endeavouring to acquire information about precipitation and temperature dynamics at a scale that can capture short term hydrological events, our understanding of within cave processes must match this resolution. Monitoring within Cueva de Asiul (N. Spain) has identified rapid (hourly resolution) changes in drip water electrical conductivity (EC), which is regulated by the pCO2 in the cave air. Drip water EC is therefore controlled by different modes of cave ventilation. In Cueva de Asiul a combination of density differences, and external pressure changes control ventilation patterns. Density driven changes in cave ventilation occur on a diurnal scale at this site irrespective of season, driven by fluctuations in external temperature across the cave internal temperature threshold. As external temperatures drop below those within the cave low pCO2 external air enters the void, facilitating the deposition of speleothem carbonate and causing a reduction in measured drip water EC. Additionally, decreases in external pressure related to storm activity act as a secondary ventilation mechanism. Reductions in external air pressure cause a drop in cave air pressure, enhancing karst air draw down, increasing the pCO2 of the cave and therefore the EC measured within drip waters. EC thereby serves as a first order indicator of cave ventilation, regardless of changes in speleothem drip rates and karst hydrological conditions. High resolution monitoring of cave drip water electrical conductivity reveals the highly sensitive nature of ventilation dynamics within cave environments, and highlights the importance of this for understanding trace element incorporation into speleothem carbonate at the event scale.

Spatial variability and temporal trends in water-use efficiency of European forests

The increasing carbon dioxide (CO2 ) concentration in the atmosphere in combination with climatic changes throughout the last century are likely to have had a profound effect on the physiology of trees: altering the carbon and water fluxes passing through the stomatal pores. However, the magnitude and spatial patterns of such changes in natural forests remain highly uncertain. Here, stable carbon isotope ratios from a network of 35 tree-ring sites located across Europe are investigated to determine the intrinsic water-use efficiency (iWUE), the ratio of photosynthesis to stomatal conductance from 1901 to 2000. The results were compared with simulations of a dynamic vegetation model (LPX-Bern 1.0) that integrates numerous ecosystem and land-atmosphere exchange processes in a theoretical framework. The spatial pattern of tree-ring derived iWUE of the investigated coniferous and deciduous species and the model results agreed significantly with a clear south-to-north gradient, as well as a general increase in iWUE over the 20th century. The magnitude of the iWUE increase was not spatially uniform, with the strongest increase observed and modelled for temperate forests in Central Europe, a region where summer soil-water availability decreased over the last century. We were able to demonstrate that the combined effects of increasing CO2 and climate change leading to soil drying have resulted in an accelerated increase in iWUE. These findings will help to reduce uncertainties in the land surface schemes of global climate models, where vegetation-climate feedbacks are currently still poorly constrained by observational data.

A framework for the extraction and interpretation of organic molecules in speleothem carbonate

RATIONALE

The organic content of speleothem calcite is a well-recognized component of their chemical composition. To date, the techniques for interpretation of this material include UV fluorescence, FTIR spectroscopy and biomarker analysis using gas chromatography/mass spectroscopy (GC/MS). However, investigation of the minute concentrations of molecules in speleothems demands careful sampling and laboratory controls.

METHODS

To be certain extracted molecules were encapsulated at the time of speleothem growth and do not represent contamination, we submitted three pieces of speleothem calcite to a rigorous extraction procedure. Based on sequential digestion and analysis by GC/MS, we measured concentration profiles of individual compounds with increasing distance from sample surfaces.

RESULTS

Declining concentrations toward interior extracts identified cholesterol, phthalates, and n-alkanes as surface contaminants. In contrast, iodo organic compounds had homogeneous concentration profiles and were also significantly above laboratory background levels, consistent with an indigenous origin. However, further laboratory testing demonstrated that iodo organics were produced by the reaction of iodine derived from the speleothem with solvent additives and other impurities of the extraction procedure. Sitosterol and some fatty acids demonstrated distributions which were probably indigenous to the speleothem archive, thus recording environmental conditions commensurate with time of growth.

CONCLUSIONS

We do not aim to provide an environmental interpretation of extracted molecules, but highlight the caution necessary before doing so. We ultimately establish a framework for differentiating between organic constituents that are introduced to the speleothems during storage, handling and as artifacts of extraction, and those encapsulated in situ at the time of growth.

Interrogating trees for isotopic archives of atmospheric sulphur deposition and comparison to speleothem records

Palaeorecords which depict changes in sulphur dynamics form an invaluable resource for recording atmospheric pollution. Tree rings constitute an archive that are ubiquitously available and can be absolutely dated, providing the potential to explore local- to regional-scale trends in sulphur availability. Rapid isotopic analysis by a novel "on-line" method using elemental analyser isotope ratio mass spectrometry (EA-IRMS) is developed, achieving sample precision of <0.4‰ using sample sizes of 40 mg wood powder. Tree cores from NE Italy show trends in pollution, evidenced through increasing concentrations of sulphur towards the youngest growth, and inverse trends in sulphur isotopes differentiating modern growth with light sulphur isotopes (+0.7‰) from pre-industrial growth (+7.5‰) influenced by bedrock composition. Comparison with speleothem records from the same location demonstrate replication, albeit offset in isotopic value due to groundwater storage. Using EA-IRMS, tree ring archives form a valuable resource for understanding local- to regional-scale sulphur pollution dynamics.

Petrology and geochemistry of annually laminated stalagmites from an Alpine cave (Obir, Austria): seasonal cave physiology

Seasonality is encoded in palaeoproxies of secondary cave mineral deposits (speleothems) and the code is becoming cracked. The petrology of calcite stalagmites from Obir, an Alpine (1100 m altitude), perennially wet cave, was characterized by optical and electron backscatter diffraction, and their chemistry by bulk ICP-MS analysis, ion microprobe and synchrotron-based micro-X-ray fluorescence. Vadose water penetrates 70 m through Triassic limestones (with some Pb–Zn mineralization) to the chamber Säulenhalle where the stalagmites were collected. Strong seasonal ventilation in the cave leads to low PCO2 in winter associated with falls in speleothem sulphate S and increase in δ13C values. All samples display autumnal event lamination defined by a narrow, optically visible zone with increases in trace element concentrations, within which synchrotron studies have resolved μm-scale enrichments of Pb and Zn. Small-scale (10 µm) lateral trace element variations reflect alternate flat faces and rough crystal edges, influenced by high Zn content. The elemental covariations are consistent with the transport of Pb, Zn, P, F, Br and I adsorbed onto organic colloids in dripwater, but the final deposition may have been from aerosols and we propose this as a new mechanism requiring further investigation. This study represents the most complete demonstration of how chemical variations are powerful expressions of seasonal cave physiology in humid temperate caves, including the contrast between summer and winter conditions, and the preservation of sub-weekly events during the autumn season.

Sulfur fixation in wood mapped by synchrotron X-ray studies: implications for environmental archives

There is a shortage of archives of sulfur that can be used to investigate industrial orvolcanic pollution in terrestrial catchments, but the role of S as a nutrient, coupled with sparse published evidence, suggests that trees are promising targets. We focused on two conifer species (Picea abies (L.) Karst and Abies alba Miller) from an Alpine site in NE Italy. Bulk analyses of Abies demonstrate that S concentrations were higher in the second half of the 20th century but with some high outliers possibly reflecting particulate impurities. X-ray synchrotron analyses confirmed the observed time trend, which is similar to that of a nearby stalagmite, and reflects an atmospheric pollution record mediated by storage in the soil and ecosystem. S and P were found to be localized in the inner cell wall (ca. 2 microm wide), local thickenings of which probably account for some outlying high values of S in synchrotron studies. S occurs as a mixture of oxidation states (0 to +0.5, +2, +5, and +6) which are consistent in space and time. The results indicate that wood older than a few years contains archive-quality S but that robust conclusions require multiple replicate analyses.

Stretching the envelope of past surface environments: Neoproterozoic glacial lakes from Svalbard

The oxygen isotope composition of terrestrial sulfate is affected measurably by many Earth-surface processes. During the Neoproterozoic, severe "snowball" glaciations would have had an extreme impact on the biosphere and the atmosphere. Here, we report that sulfate extracted from carbonate lenses within a Neoproterozoic glacial diamictite suite from Svalbard, with an age of approximately 635 million years ago, falls well outside the currently known natural range of triple oxygen isotope compositions and indicates that the atmosphere had either an exceptionally high atmospheric carbon dioxide concentration or an utterly unfamiliar oxygen cycle during deposition of the diamictites.