Compositional in situ analysis of dusty material stemming from Saturn’s main rings

<p>During the final mission phase, the Cassini spacecraft travelled through the gap between Saturn and its innermost D ring. One goal of these highly inclined orbits was sampling the dust population, mostly made of impact ejecta from the main rings, in the vicinity of the planet. These in situ measurements were primarily carried out by the Cosmic Dust Analyzer (CDA) onboard the spacecraft, which provided time-of-flight mass spectra of individual ice and dust grains, mostly between about 10 and 50 nm in size. Here we present an update on the composition of the silicate dust fraction stemming from Saturn’s main rings, which makes up about 30 % of the observed particles with water ice being the remaining fraction [1].</p> <p>Elemental analysis of the silicate spectra was performed using an updated deconvolution method, based on a technique originally applied to the interpretation of CDA interstellar dust measurements [2]. Neighboring spectral peaks due to mineral-forming ions such as Mg<sup>+</sup>, Al<sup>+</sup> and Si<sup>+</sup> are often unresolvable, because of CDA’s relatively low (m/dm = 20–50) mass resolution [3]. Therefore, application of a deconvolution technique is required to disentangle the peak interferences and derive valuable compositional information. The robustness of the applied method has been tested and optimized through comparison with an independent automated fit algorithm. In order to calculate elemental abundances within the particles, the derived ion abundances were combined with experimentally-determined relative sensitivity factors (RSFs) [4]. To provide context to the measured element ratios, we compared them with a variety of space-relevant materials. We find an overlap with chondritic material for Mg/Si and Fe/Mg ratios. The observed range within the element ratios, however, indicates the contribution of a variety of minerals such as olivine, plagioclase or pyroxenes. Although our results agree with realistic mineral compositions, the calculated abundances of Al<sup>+</sup> ions are still relatively uncertain and can be seen as an upper limit.</p> <p>Additionally, we present the results of a dynamical model, which allow us to derive the likely source region within the main rings of individually detected silicate grains. We find the C and B rings to be the most likely sources of the vast majority of grains with the D ring being only a minor source. Currently an analysis of compositional diversity between the different ring segments is under way.</p> <p> </p> <p><strong>References</strong></p> <p>[1] H.-W. Hsu et al. (2018) In situ collection of dust grains falling from Saturn’s rings into its atmosphere. Science 362.</p> <p>[2] N. Altobelli et al. (2016) Flux and composition of interstellar dust at Saturn from Cassini’s Cosmic Dust Analyzer. Science 352, 312–318.</p> <p>[3] R. Srama et al. (2004) The Cassini Cosmic Dust Analyzer. Space Science Reviews 114, 465–518.</p> <p>[4] K. Fiege et al. (2014) Calibration of relative sensitivity factors for impact ionization detectors with high-velocity silicate microparticles. Icarus 241, 336–345.</p>

Middle-Late Paleozoic conodont ecogeochemistry: an overview

The ecogeochemistry is the application of geochemical features to study animal ecology. This approach is promising for using in reconstructions of ancient pelagic ecosystems. Among other Paleozoic fossils of pelagic animals, remains of conodonts are the most suitable for ecogeochemical investigations. The article reviews ecogeochemical applications of Middle to Late Paleozoic conodont elements. The following features are considered as the most informative: calcium isotopic composition and element ratios (e.g. Sr/Ca) of conodont apatite, and isotopic composition of carbon of conodont elements. These parameters allow us supposing ecological specialization of conodont species, and temporal and spatial dynamics of the ecogeochemistry of conodonts can be used to reconstruct transformations of ancient pelagic ecosystems.

Contrasting conifer species productivity in relation to soil carbon, nitrogen and phosphorus stoichiometry of British Columbia perhumid rainforests

Temperate rainforest soils of the Pacific Northwest are often carbon (C) rich and encompass a wide range of fertility, reflecting varying nitrogen (N) and phosphorus (P) availability. Soil resource stoichiometry (C : N : P) may provide an effective measure of site nutrient status and help refine species-dependent patterns in forest productivity across edaphic gradients. We determined mineral soil and forest floor nutrient concentrations across very wet (perhumid) rainforest sites of southwestern Vancouver Island (Canada) and employed soil element ratios as covariates in a long-term planting density trial to test their utility in defining basal area growth response of four conifer species. There were strong positive correlations in mineral soil C, N, and organic P (Po) concentrations and close alignment in C : N and C : Po both among and between substrates. Stand basal area after 5 decades was best reflected by mineral soil and forest floor C : N, but in either case included a significant species–soil interaction. The conifers with ectomycorrhizal fungi had diverging growth responses displaying either competitive (Picea sitchensis) or stress-tolerant (Tsuga heterophylla, Pseudotsuga menziesii) attributes, in contrast to a more generalist response by an arbuscular mycorrhizal tree (Thuja plicata). Despite the consistent patterns in organic matter quality, we found no evidence for increased foliar P concentrations with declining element ratios (C : Po or C : Ptotal) as we did for N. The often high C : Po ratios (as much as 3000) of these soils may reflect a stronger immobilization sink for P than N, which, along with ongoing sorption of PO4-, could limit the utility of C : Po or N : Po to adequately reflect P supply. The dynamics and availability of soil P to trees, particularly as Po, deserves greater attention, as many perhumid rainforests were co-limited by N and P, or, in some stands, possibly P alone.

Inconsistency between records of δ18O and trace element ratios from stalagmites: Evidence for increasing mid–late Holocene moisture in arid central Asia

The interpretation of trace element/calcium ratios of speleothems as indicators of local hydroclimatic variability in the vicinity of caves has led to controversy in reconstructing the evolution of moisture conditions in arid central Asia (ACA) during the Holocene. Here we present records of Mg/Ca, Sr/Ca, Ba/Ca, and U/Ca from precisely dated stalagmites from Baluk cave in Xinjiang (northwest China), spanning the past 9370 years. The co-variations of the trace element ratios, together with the slopes of the regression lines of the corresponding logarithmically transformed data, suggest that they are dominated by prior calcite precipitation (PCP) and thus can be used as reliable proxies of changes in moisture/precipitation. The trace element ratios are relatively high during ~9 to 5 ka and lower from 5 ka to the present, indicating a trend of increasing mid–late Holocene moisture in ACA. The long-term trend of variation of the trace element ratios is correlative with two other records of speleothem trace element ratios from caves in ACA: Kesang cave (western Xinjiang) and Ton cave (Uzbekistan). This spatial coherency of the trend of inferred moisture conditions from three caves that are separated by hundreds of kilometers demonstrates that speleothem trace element ratios are indicative of large spatial scale rather than local hydroclimatic variability in ACA during the Holocene. However, the long-term trend of variation of the trace element ratios is the inverse of the corresponding oxygen isotope (δ18O) records from the same cave sites, which implies that Holocene speleothem δ18O records do not represent changes in the precipitation amount in ACA; rather, they most likely reflect moisture sources and related water vapor transport controlled by Northern Hemisphere summer insolation (NHSI). Our findings provide new evidence for a ‘westerlies-dominated climatic regime’, which influenced hydroclimatic changes in ACA during the Holocene.

Soil carbon, nitrogen and phosphorus stoichiometry (C : N : P) in relation to conifer species productivity and nutrition across British Columbia perhumid rainforests

Temperate rainforest soils of the Pacific Northwest are often carbon (C) rich and encompass a wide range in fertility reflecting varying nitrogen (N) and phosphorus (P) availability. Soil resource stoichiometry (C : N : P) may provide an effective measure of site nutrient status and help refine species-dependent patterns in forest productivity across edaphic gradients. We described the nature of soil organic matter for mineral soil and forest floor substrates across very wet (perhumid) rainforest sites of southwestern Vancouver Island (Canada), and employed soil element ratios as covariates in a long-term planting density trial to test their utility in defining basal area growth response of four conifer species. There were strong positive correlations in mineral soil C, N and organic P (Po) concentrations, and close alignment in C : N and C : Po both among and between substrates. Stand basal area after five decades was best reflected by soil C : N but included a significant species-soil interaction. The conifers with ectomycorrhizal fungi had diverging growth responses displaying either competitive (Picea sitchensis) or stress-tolerant (Tsuga heterophylla, Pseudotsuga menziesii) attributes, in contrast to a more generalist response by an arbuscular mycorrhizal tree (Thuja plicata). Despite the consistent patterns in organic matter quality we found no evidence via foliar nutrition for increased P availability with declining element ratios as we did for N. The often high C : Po ratios (as much as 3000) of these soils may reflect a stronger immobilization sink for P than N, which, along with ongoing sorption of PO4−, could limit the utility of C : Po or N : Po to adequately reflect P supply. The dynamics and availability of soil P to trees, particularly as Po, deserves greater attention as many perhumid rainforests were co-limited by N and P, or, in some stands, possibly P alone.