Comparing levoglucosan and mannosan ratios in sediments and corresponding aerosols from recent Australian fires.

The monosaccharide anhydrides levoglucosan, mannosan, and galactosan are known as 'fire sugars' as they are powerful proxies used to trace fire events. Despite their increasing use, their application is not completely understood, especially in the context of tracing past fire events using sediment samples. There are many uncertainties about fire sugar formation, partitioning, transport, complexation, and stability along all stages of the source-to-sink pathway. While these uncertainties exist, the efficacy of fire sugars as fire tracers remains limited. This study compared high-resolution fire sugar fluxes in freshwater sediment cores to known fire records in Tasmania, Australia. Past fire events correlated with fire sugar flux increases down-core, with the magnitude of the flux inversely proportional to the distance of the fires from the study site. For the first time, fire sugar ratios (levoglucosan/mannosan, L/M) in aerosols were compared with those in sediments from the same time-period. The L/M ratio in surface sediments (1.42-2.58) were significantly lower than in corresponding aerosols (5.08-15.62). We propose two hypotheses that may explain the lower average L/M of sediments. Firstly, the degradation rate of levoglucosan is higher than mannosan in the water column, sediment-water interface, and/or sediment. Secondly, the L/M ratio of non-atmospheric emissions during fires may be lower than that of atmospheric emissions from the same fire. Due to the uncertainties about transport partitioning (atmospheric versus non-atmospheric emissions) and fire sugar degradation along all stages of the source-to-sink pathway, we advise caution when inferring vegetation type (e.g. softwood, hardwood, or grasses) based purely on fire sugar ratios in sediments (e.g. L/M ratio). Future investigations are required to increase the efficacy of fire sugars as a complimentary, or standalone, fire tracer in sediments.

One-third of Southern Ocean productivity is supported by dust deposition.

Natural iron fertilization of the Southern Ocean by windblown dust has been suggested to enhance biological productivity and modulate the climate1-3. Yet, this process has never been quantified across the Southern Ocean and at annual timescales4,5. Here we combined 11 years of nitrate observations from autonomous biogeochemical ocean profiling floats with a Southern Hemisphere dust simulation to empirically derive the relationship between dust-iron deposition and annual net community production (ANCP) in the iron-limited Southern Ocean. Using this relationship, we determined the biological response to dust-iron in the pelagic perennially ice-free Southern Ocean at present and during the last glacial maximum (LGM). We estimate that dust-iron now supports 33% ± 15% of Southern Ocean ANCP. During the LGM, when dust deposition was 5-40-fold higher than today, the contribution of dust to Southern Ocean ANCP was much greater, estimated at 64% ± 13%. We provide quantitative evidence of basin-wide dust-iron fertilization of the Southern Ocean and the potential magnitude of its impact on glacial-interglacial timescales, supporting the idea of the important role of dust in the global carbon cycle and climate6-8.

A fast and simple extraction method for analysing levoglucosan and its isomers in sediments by ion chromatography tandem mass spectrometry.

The ability to trace current and past biomass burning events is important for understanding the links between human activity, fire frequency, and climate. One method of tracing biomass burning is to measure the concentrations of certain monosaccharides anhydrides (MAs), specifically levoglucosan (LEV) and its isomers, mannosan (MAN) and galactosan (GAL), which are products of cellulose and hemicellulose pyrolysis. This work presents a simple extraction method allowing for the rapid, sensitive, and selective determination of MAs in sediments. MAs detection was performed using suppressed ion chromatography with electrospray - triple-stage quadrupole tandem mass spectrometry (IC-TSQ-MS). The extraction method involves ultrasound probe sonication using water as the solvent. Extraction time, amplitude, and sonication mode were optimised. Recoveries higher than 86% for all MAs tested were achieved by applying 70% amplitude in continuous mode for 60 s. Analytical performance of the method included instrumental LODs of 0.10, 0.12 and 0.50 µg L-1 for LEV, MAN and GAL, respectively. No carryover issues, no matrix effect and no co-elution of targeted MAs with other sugars likely present in sediments samples were observed. The developed extraction method was further validated by the analysis of LEV and MAN in NIST® 1649b urban dust reference material and the resulting concentrations were in excellent agreement with previously reported values. MAs quantification in 70 lake sediment samples were carried out with concentrations found to range from 0.009 to 0.390 µg g-1 for LEV and from 0.009 to 0.194 µg g-1 for MAN. Plotting MAs concentrations versus approximate sediment age allowed the reconstruction of recent fire events impacting two locations in the Central Highlands of Tasmania, Australia.

Plastic and natural inorganic microparticles do not differ in their effects on adult mussels (Mytilidae) from different geographic regions.

Microplastics are ubiquitous in the marine environment and studies on their effects on benthic filter feeders at least partly revealed a negative influence. However, it is still unclear whether the effects of microplastics differ from those of natural suspended microparticles, which constitute a common stressor in many coastal environments. We present a series of experiments that compared the effects of six-week exposures of marine mussels to two types of natural particles (red clay and diatom shells) to two types of plastic particles (Polymethyl Methacrylate and Polyvinyl Chloride). Mussels of the family Mytilidae from temperate regions (Japan, Chile, Tasmania) through subtropical (Israel) to tropical environments (Cabo Verde) were exposed to concentrations of 1.5 mg/L, 15 mg/L and 150 mg/L of the respective microparticles. At the end of this period, we found significant effects of suspended particles on respiration rate, byssus production and condition index of the animals. There was no significant effect on clearance rate and survival. Surprisingly, we observed only small differences between the effects of the different types of particles, which suggests that the mussels were generally equally robust towards exposure to variable concentrations of suspended solids regardless of whether they were natural or plastic. We conclude, that microplastics and suspended solids elicit similar effects on the tested response variables, and that both types of microparticles mainly cause acute responses rather than more persistent carry-over effects.

A comparison with natural particles reveals a small specific effect of PVC microplastics on mussel performance.

Effects of microplastics on marine taxa have become a focal point in marine experimental biology. Almost all studies so far, however, assessed the influence of microplastics on animals only in relation to a zero-particle group. Documented microplastic impacts may thus be overestimated, since many marine species also experience natural suspended solids as a stressor. Here, we compared the effects of polyvinyl chloride (PVC) and red clay (mean for both particles: ~12-14 µm) on the Mediterranean mussel Mytilus galloprovincialis across three particle concentrations (1.5, 15, 150 mg l-1). Exposure to PVC for 35 days lowered mussel body condition index by 14% in relation to clay, but no difference in byssus production, respiration and survival rates emerged between the two particle types. This suggests that the effects of synthetic particles on filter feeders may emulate those of natural suspended solids, and highlights the importance of including natural particles in microplastic exposure studies.

Marine nitrogen fixers mediate a low latitude pathway for atmospheric CO2 drawdown.

Roughly a third (~30 ppm) of the carbon dioxide (CO2) that entered the ocean during ice ages is attributed to biological mechanisms. A leading hypothesis for the biological drawdown of CO2 is iron (Fe) fertilisation of the high latitudes, but modelling efforts attribute at most 10 ppm to this mechanism, leaving ~20 ppm unexplained. We show that an Fe-induced stimulation of dinitrogen (N2) fixation can induce a low latitude drawdown of 7-16 ppm CO2. This mechanism involves a closer coupling between N2 fixers and denitrifiers that alleviates widespread nitrate limitation. Consequently, phosphate utilisation and carbon export increase near upwelling zones, causing deoxygenation and deeper carbon injection. Furthermore, this low latitude mechanism reproduces the regional patterns of organic δ15N deposited in glacial sediments. The positive response of marine N2 fixation to dusty ice age conditions, first proposed twenty years ago, therefore compliments high latitude changes to amplify CO2 drawdown.

Pre-concentration of thorium and neodymium isotopes using Nobias chelating resin: Method development and application to chromatographic separation.

The isotopes of thorium (Th) and neodymium (Nd) are used as tracers in oceanography, and are key parameters in the international GEOTRACES program. The very low concentrations of Th and Nd as well as the reactive nature of Th isotopes makes the analysis of seawater samples a complex process. Analysis requires time-consuming pre-concentration from over 5 L of seawater. We describe a method to simultaneously pre-concentrate dissolved Th and Nd from acidified seawater samples using the Nobias® PA1L chelating resin. Prior to pre-concentration, hydrofluoric acid is added to the sample to stabilise Th, ammonium acetate buffer added (0.05 M), pH adjusted to 4.75, and then finally the prepared sample is pumped through the Nobias resin at a rate of 15 ml min-1. Up to 6 samples can be processed simultaneously. Following elution in 3 M HNO3, both elements are chromatographically separated and determined using Inductively Coupled Plasma Mass Spectrometry. Oxidation of the sample between all column separation steps, including after the initial Nobias resin, is important for obtaining maximum elemental recoveries. The method has >90% recovery with blank levels typically <10 pg for 232Th and <70 pg for Nd. Accuracy is excellent, as our reported values generally agree within 1% of the GEOTRACES intercalibration standards. The long-term analysis of these materials also indicates excellent reproducibility. The pre-concentration of Th and Nd using the Nobias resin is a time saving option compared to the widely used iron co-precipitation technique. Sample handling is also reduced, decreasing the risk of sample contamination. The simplicity of our suggested pre-concentration procedure makes it possible to be applied at sea.

Dissolution of fluoride complexes following microwave-assisted hydrofluoric acid digestion of marine sediments.

Microwave-assisted, hydrofluoric acid digestion is an increasingly common tool for the preparation of marine sediment samples for analysis by a variety of spectrometric techniques. Here we report that analysis of terrigenous-dominated sediment samples occasionally results in anomalously low values for several elements, including Al, Ba, Ca, Mg, and Sr. Measured concentrations of these elements increased with time between sample preparation and sample analysis, reaching stable values after 8-29 days. This lag is explained by the formation and subsequent dissolution of poorly soluble fluoride phases during digestion. Other elements, such as Fe, Mn, and Ti, showed little or no lag and were quickly measurable at a stable value. Full re-dissolution of the least soluble fluorides, which incorporate Al and Mg, requires up to four weeks at room temperature, and this duration can vary among sedimentary matrices. This waiting time can be reduced to 6 days (or shorter) if the samples are heated to ≈ 60°C for 24h.

Microplate-reader method for the rapid analysis of copper in natural waters with chemiluminescence detection.

We have developed a method for the determination of copper in natural waters at nanomolar levels. The use of a microplate-reader minimizes sample processing time (~25 s per sample), reagent consumption (~120 µL per sample), and sample volume (~700 µL). Copper is detected by chemiluminescence. This technique is based on the formation of a complex between copper and 1,10-phenanthroline and the subsequent emission of light during the oxidation of the complex by hydrogen peroxide. Samples are acidified to pH 1.7 and then introduced directly into a 24-well plate. Reagents are added during data acquisition via two reagent injectors. When trace metal clean protocols are employed, the reproducibility is generally less than 7% on blanks and the detection limit is 0.7 nM for seawater and 0.4 nM for freshwater. More than 100 samples per hour can be analyzed with this technique, which is simple, robust, and amenable to at-sea analysis. Seawater samples from Storm Bay in Tasmania illustrate the utility of the method for environmental science. Indeed other trace metals for which optical detection methods exist (e.g., chemiluminescence, fluorescence, and absorbance) could be adapted to the microplate-reader.

Southern Ocean iron enrichment experiment: carbon cycling in high- and low-Si waters.

The availability of iron is known to exert a controlling influence on biological productivity in surface waters over large areas of the ocean and may have been an important factor in the variation of the concentration of atmospheric carbon dioxide over glacial cycles. The effect of iron in the Southern Ocean is particularly important because of its large area and abundant nitrate, yet iron-enhanced growth of phytoplankton may be differentially expressed between waters with high silicic acid in the south and low silicic acid in the north, where diatom growth may be limited by both silicic acid and iron. Two mesoscale experiments, designed to investigate the effects of iron enrichment in regions with high and low concentrations of silicic acid, were performed in the Southern Ocean. These experiments demonstrate iron's pivotal role in controlling carbon uptake and regulating atmospheric partial pressure of carbon dioxide.