Strong Chemical Gradients Research Articles

Revealing H2O dissociation in WASP-76 b through combined high- and low-resolution transmission spectroscopy

ABSTRACT Numerous chemical constraints have been possible for exoplanetary atmospheres thanks to high-resolution spectroscopy (HRS) from ground-based facilities as well as low-resolution spectroscopy (LRS) from space. These two techniques have complementary strengths, and hence combined HRS and LRS analyses have the potential for more accurate abundance constraints and increased sensitivity to trace species. In this work, we retrieve the atmosphere of the ultra-hot Jupiter WASP-76 b, using high-resolution CARMENES/CAHA (Calar Alto high-Resolution search for M dwarfs with Exoearths with Near-infrared and optical Échelle Spectrographs) and low-resolution Hubble Space Telescope’s (HST) Wide Field Camera 3 (WFC3) and Spitzer observations of the primary eclipse. As such, hot planets are expected to have a substantial fraction of H2O dissociated, we conduct retrievals including both H2O and OH. We explore two retrieval models, one with self-consistent treatment of H2O dissociation and another where H2O and OH are vertically homogeneous. Both models constrain H2O and OH, with H2O primarily detected by LRS and OH through HRS, highlighting the strengths of each technique and demonstrating the need for combined retrievals to fully constrain chemical compositions. We see only a slight preference for the H2O-dissociation model given that the photospheric constraints for both are very similar, indicating $\log (\mathrm{OH/H_2O}) = 0.7^{+0.3}_{-0.3}$ at 1.5 mbar, showing that the majority of the H2O in the photosphere is dissociated. However, the bulk O/H and C/O ratios inferred from the models differs significantly, and highlights the challenge of constraining bulk compositions from photospheric abundances with strong vertical chemical gradients. Further observations with JWST and ground-based facilities may help shed more light on these processes.

Time to anoxia: Observations and predictions of oxygen drawdown following coastal flood events

The coastal terrestrial-aquatic interface (TAI) is a highly dynamic system characterized by strong physical, chemical, and biological gradients. In particular, shifting soil redox conditions and consumption of terminal electron acceptors, due in part to dynamic hydrologic conditions, is a strong driver of carbon availability and transformations across TAIs. However, while redox dynamics are well described, our ability to quantitatively forecast rates ofoxic to anoxic shifts in soils with different characteristics and inundation regimes is limited. We integrated field measurements, laboratory incubations, and model simulations to improve mechanistic understanding of oxygen consumption dynamics in coastal soils. Continuous in situ monitoring unexpectedly revealed that flooding caused temporary spikes in subsurface dissolved oxygen followed by rapid consumption in the wetlands. To further investigate these mechanisms in a controlled setting, we performed laboratory incubations using surface and subsurface soils from a TAI gradient (defined here as upland forest to transitional forest to wetland) in Western Lake Erie to measure oxygen consumption rates in TAI soils during flood events. In our experiments, wetland soils reached anoxia the fastest, in ∼ 9 h on average, whereas upland soils turned anoxic in ∼ 18 h. Subsurface upland soils did not turn anoxic even after two weeks of saturation in the lab, and their oxygen consumption patterns suggested carbon and/or nutrient limitation. These results are consistent with in-situ groundwater redox and oxygen measurements in the field, where wetland soils exhibited the highest rates of oxygen consumption along the TAI. Model simulations of oxygen consumption suggested that oxygen consumption had stronger abiotic controls in wetland soils but stronger biotic controls in upland soils, providing a useful framework for future incubation experiments. Microbial activity is a strong driver of oxygen consumption in TAI soils, although it is constrained by the availability of dissolved carbon in subsurface soils.

Nano-scale solution of the Poisson-Nernst-Planck (PNP) equations in a fraction of two neighboring cells reveals the magnitude of intercellular electrochemical waves.

The basic building blocks of the electrophysiology of cardiomyocytes are ion channels integrated in the cell membranes. Close to the ion channels there are very strong electrical and chemical gradients. However, these gradients extend for only a few nano-meters and are therefore commonly ignored in mathematical models. The full complexity of the dynamics is modelled by the Poisson-Nernst-Planck (PNP) equations but these equations must be solved using temporal and spatial scales of nano-seconds and nano-meters. Here we report solutions of the PNP equations in a fraction of two abuttal cells separated by a tiny extracellular space. We show that when only the potassium channels of the two cells are open, a stationary solution is reached with the well-known Debye layer close to the membranes. When the sodium channels of one of the cells are opened, a very strong and brief electrochemical wave emanates from the channels. If the extracellular space is sufficiently small and the number of sodium channels is sufficiently high, the wave extends all the way over to the neighboring cell and may therefore explain cardiac conduction even at very low levels of gap junctional coupling.

Prevalence and ecological features of deep chlorophyll layers in Lake of the Woods, a complex hydrological system with strong trophic, physical, and chemical gradients

We characterized deep chlorophyll layers (DCLs), previously unknown, in Lake of the Woods (Canada), a complex hydrological system with strong trophic, physical, and chemical gradients. Of 42 sites, five (12 %) contained at least one dense, thin metalimnetic peak – often more, overlapping or vertically stratified. In spring, highest biomass (>4 mg/L) was found at 3 m in Bigstone Bay, composed of a mixed phytoplankton community. Other, deeper, spring DCLs were dominated by the diatom Cyclotella (∼1 mg/L, 21 m), dinoflagellate Gymnodinium (>1 mg/L, 17 m), cyanobacterium Dolichospermum (>3 mg/L, 10 m), or cyanobacterium Woronichinia (>1 mg/L, ∼8 m). In summer, the highest biomass of a DCL was in Yellow Girl Bay (>21 mg/L), over five times higher than spring and higher than large surface blooms from the shallow, eutrophic south (Sabaskong Bay ∼19 mg/L). Summer DCLs (∼7 m) were not as deep as spring, owing to reduced light penetration (2.5-fold lower Zeu:Zmix). Most summer DCLs were dominated by Dolichospermum (max. >16 mg/L) but other species were also high (∼1 mg/L biomass), including cyanobacterium Aphanizomenon, diatom Aulacoseira, and cryptophyte Cryptomonas. Euphotic zone in these embayments intersected with a nutrient-enriched (PO4, Fe, Mn, Si) hypoxic hypolimnion, conditions conducive to DCLs. Other drivers of composition and activity of DCLs remain to be elucidated. These high levels of DCL biomass, up to fivefold higher than near-surface, have routinely gone unreported. Their inclusion in establishing baselines and tracking change would inform lake management decisions and qualify expectations for ecosystem response.

Spatial and diel patterns of volatile organic compounds, DMSP-derived compounds, and planktonic microorganisms around a tropical scleractinian coral colony

Volatile organic compounds (VOCs) are constituents of marine ecosystems including coral reefs, where they are sources of atmospheric reactivity, indicators of ecosystem state, components of defense strategies, and infochemicals. Most VOCs result from sunlight-related processes; however, their light-driven dynamics are still poorly understood. We studied the spatial variability of a suite of VOCs, including dimethylsulfide (DMS), and the other dimethylsulfoniopropionate-derived compounds (DMSPCs), namely, DMSP, acrylate, and dimethylsulfoxide (DMSO), in waters around colonies of two scleractinian corals (Acropora pulchra and Pocillopora sp.) and the brown seaweed Turbinaria ornata in Mo’orean reefs, French Polynesia. Concentration gradients indicated that the corals were sources of DMSPCs, but less or null sources of VOCs other than DMS, while the seaweed was a source of DMSPCs, carbonyl sulfide (COS), and poly-halomethanes. A focused study was conducted around an A. pulchra colony where VOC and DMSPC concentrations and free-living microorganism abundances were monitored every 6 h over 30 h. DMSPC concentrations near the polyps paralleled sunlight intensity, with large diurnal increases and nocturnal decrease. rDNA metabarcoding and metagenomics allowed the determination of microbial diversity and the relative abundance of target functional genes. Seawater near coral polyps was enriched in DMS as the only VOC, plus DMSP, acrylate, and DMSO, with a large increase during the day, coinciding with high abundances of symbiodiniacean sequences. Only 10 cm below, near the coral skeleton colonized by a turf alga, DMSPC concentrations were much lower and the microbial community was significantly different. Two meters down current from the coral, DMSPCs decreased further and the microbial community was more similar to that near the polyps than that near the turf alga. Several DMSP cycling genes were enriched in near-polyp with respect to down-current waters, namely, the eukaryotic DMS production and DMS oxidation encoding genes, attributed to the coral and the algal symbiont, and the prokaryotic DMS production gene dddD, harbored by coral-associated Gammaproteobacteria. Our results suggest that solar radiation-induced oxidative stress caused the release of DMSPCs by the coral holobiont, either directly or through symbiont expulsion. Strong chemical and biological gradients occurred in the water between the coral branches, which we attribute to layered hydrodynamics.

Biogeochemical Niche of Magnetotactic Cocci Capable of Sequestering Large Polyphosphate Inclusions in the Anoxic Layer of the Lake Pavin Water Column.

Magnetotactic bacteria (MTB) are microorganisms thriving mostly at oxic–anoxic boundaries of aquatic habitats. MTB are efficient in biomineralising or sequestering diverse elements intracellularly, which makes them potentially important actors in biogeochemical cycles. Lake Pavin is a unique aqueous system populated by a wide diversity of MTB with two communities harbouring the capability to sequester not only iron under the form of magnetosomes but also phosphorus and magnesium under the form of polyphosphates, or calcium carbonates, respectively. MTB thrive in the water column of Lake Pavin over a few metres along strong redox and chemical gradients representing a series of different microenvironments. In this study, we investigate the relative abundance and the vertical stratification of the diverse populations of MTB in relation to environmental parameters, by using a new method coupling a precise sampling for geochemical analyses, MTB morphotype description, and in situ measurement of the physicochemical parameters. We assess the ultrastructure of MTB as a function of depth using light and electron microscopy. We evidence the biogeochemical niche of magnetotactic cocci, capable of sequestering large PolyP inclusions below the oxic–anoxic transition zone. Our results suggest a tight link between the S and P metabolisms of these bacteria and pave the way to better understand the implication of MTB for the P cycle in stratified environmental conditions.

Lithium distribution and isotopic composition in zircon megacrysts as constraints for their mantle origin and volcanic transfer timescales

Lithium in zircon has attracted considerable attention for identifying magma sources and as a diffusion chronometer, despite the exact diffusion mechanisms remaining controversial. Zircon megacrysts sourced from Quaternary intraplate basaltic volcanoes in the Ratanakiri Volcanic Province of northeastern Cambodia were analyzed here for Li isotopic and elemental concentration profiles to constrain diffusion behavior of Li in zircon megacrysts entrained in high temperature melts. This is complemented by high-spatial resolution U-Pb geochronology, O-Hf isotope compositions and trace element abundances to refine the long-term pre-eruptive history of these zircon megacrysts. Ratanakiri zircon megacrysts yield a Tera–Wasserburg regression U-Pb age of 0.98 ± 0.02 Ma (n = 615 spot analyses) that is indistinguishable from eruption ages reported for the host basalts, although zircon was unstable in the enclosing magma as evidenced by marginal corrosion and abundant tubular channels within the crystals. Extreme Li-isotopic variations (within −30 to +96‰ on the δ7LiLSVEC scale) of the zircon megacrysts require kinetic Li fractionation due to diffusive mobilization, which is further supported by offsets in intracrystalline domain boundaries with different Li and Y abundances detected in scanning ion images. Complex abundance and isotopic ratio patterns for Li in zircon megacrysts point to multi-mode Li diffusion that is influenced by variable trace element inventories within individual crystals, which correlate with minor differences in the O-Hf isotopic compositions. The overall tendency of decreasing Li abundances along with increasing δ7Li values toward crystal rims indicates diffusion-driven equilibration between zircon and Li-depleted basaltic melt. Diffusion modeling of Li and δ7Li on domain-boundaries within crystals show pre-eruptive heating timescales of about 18 days, which is consistent with the magma transfer duration estimated from zircon megacryst settling rate calculations, implying the zircon megacrysts should be transported from their source region to the surface by ascending alkali basaltic magma within few weeks. Nearly one to two orders of magnitude longer Li diffusion model timescales were computed for crystal rim domains. This apparent faster diffusion of Li is attributed to H+ loss into a volatile phase during eruption, providing an additional driving force for Li+ diffusion, which might become more pronounced at the crystal boundary where zircon is exposed to strong chemical gradients in volatile elements due to decompression and concomitant degassing. Lithium isotope compositions for the innermost domains of zircon megacrysts are relatively heavy with δ7Li values of +12 to +22‰. These values are considered to be less influenced by Li-loss after entrainment by the host basalts than rim compositions. These values in combination with O (δ18O = +3.5 to +5.3‰) and Hf (εHf(0.98 Ma) = +6.5 to +7.9) isotopic compositions as well as inclusions and internal structures suggest that zircon megacrysts were originally derived from partial melting of a metasomatized lithospheric mantle source altered by fluid released from dissolved carbonates during subduction. Lithium isotopes in Ratanakiri zircon megacrysts provide novel insights into using Li isotopes as a tool to constrain the durations of magma transfer from the mantle to the surface, although Li diffusion mechanisms in zircon need to be further scrutinized.

Reactive Transport Simulation of Low-pH Cement Interacting with Opalinus Clay Using a Dual Porosity Electrostatic Model

Strong chemical gradients between clay and concrete porewater lead to diffusive transport across the interface and subsequent mineral reactions in both materials. These reactions may influence clay properties such as swelling behaviour, permeability or radionuclide retention, which are relevant for the safety of a radioactive waste repository. Different cement types lead to different interactions with Opalinus Clay (OPA), which must be understood to choose the most suitable material. The consideration of anion-depleted porosity due to electrostatic repulsion in clay modelling substantially influences overall diffusive transport and pore clogging at interfaces. The identical dual porosity model approach previously used to predict interaction between Portland cement and OPA is now applied to low-alkali cement—OPA interaction. The predictions are compared with corresponding samples from the cement-clay interaction (CI) experiment in the Mont Terri underground rock laboratory (Switzerland). Predicted decalcification of the cement at the interface (depletion of C–S–H and absence of ettringite within 1 mm from the interface), the Mg enrichment in clay and cement close to the interface (neoformation of up to 17 vol% Mg hydroxides in concrete, and up to 6 vol% in OPA within 0.6 mm at the interface), and the slightly increased S content in the cement 3–4 mm away from the interface qualitatively match the sample characterisation. Simulations of Portland cement—OPA interaction indicate a weaker chemical disturbance over a larger distance compared with low-pH cement—OPA. In the latter case, local changes in porosity are stronger and lead to predicted pore clogging.

Calcite Precipitation at Cement–Bentonite Interface. Part 1: Effect of Carbonate Admixture in Bentonite

Calcium leaching from cementitious materials into bentonite is a key process for the long-term alteration of cement–clay interfaces of engineered barrier systems. Strong chemical gradients between cement and clay drive the precipitation of minerals such as calcium silicate hydrate (C–S–H) and calcite. To analyze the mineralogical and porosity evolution at the cement–clay interface, composite specimens consisting of cement paste and bentonite mixed with various amounts of sodium carbonate were subjected to immersion and chloride migrations tests and were investigated by electron probe micro-analysis (EPMA), thermogravimetry/differential thermal analysis (TG-DTA), and X-ray diffraction (XRD) after 4–20 months of immersion. The results show that adding sodium carbonate to the bentonite enhanced the formation of calcite in the form of a surface layer on the cement paste. This suggests pore clogging at the interface and implies the existence of a threshold amount of carbonate addition above which pore clogging occurs. This is the first of two papers; the accelerated evolution of the samples in the presence of an electrical field is discussed in the second paper.

Oxidation state and metasomatism of the lithospheric mantle beneath the Rae Craton, Canada: strong gradients reflect craton formation and evolution

We present the first oxidation state measurements for the subcontinental lithospheric mantle (SCLM) beneath the Rae craton, northern Canada, one of the largest components of the Canadian shield. In combination with major and trace element compositions for garnet and clinopyroxene, we assess the relationship between oxidation state and metasomatic overprinting. The sample suite comprises peridotite xenoliths from the central part (Pelly Bay) and the craton margin (Somerset Island) providing insights into lateral and vertical variations in lithospheric character. Our suite contains spinel, garnet-spinel and garnet peridotites, with most samples originating from 100 to 140 km depth. Within this narrow depth range we observe strong chemical gradients, including variations in oxygen fugacity (ƒO2) of over 4 log units. Both Pelly Bay and Somerset Island peridotites reveal a change in metasomatic type with depth. Observed geochemical systematics and textural evidence support the notion that Rae SCLM developed through amalgamation of different local domains, establishing chemical gradients from the start. These gradients were subsequently modified by migrating melts that drove further development of different types of metasomatic overprinting and variable oxidation at a range of length scales. This oxidation already apparent at ~ 100 km depth could have locally destabilised any pre-existing diamond or graphite.

Occurrence and Spatiotemporal Dynamics of Pharmaceuticals in a Temperate-Region Wastewater Effluent-Dominated Stream: Variable Inputs and Differential Attenuation Yield Evolving Complex Exposure Mixtures.

Effluent-dominated streams are becoming increasingly common in temperate regions and generate complex pharmaceutical mixture exposure conditions that may impact aquatic organisms via drug-drug interactions. Here, we quantified spatiotemporal pharmaceutical exposure concentrations and composition mixture dynamics during baseflow conditions at four sites in a temperate-region effluent-dominated stream (upstream, at, and progressively downstream from effluent discharge). Samples were analyzed monthly for 1 year for 109 pharmaceuticals/degradates using a comprehensive U.S. Geological Survey analytical method and biweekly for 2 years focused on 14 most common pharmaceuticals/degradates. We observed a strong chemical gradient with pharmaceuticals only sporadically detected upstream from the effluent. Seventy-four individual pharmaceuticals/degradates were detected, spanning 5 orders of magnitude from 0.28 to 13 500 ng/L, with 38 compounds detected in >50% of samples. "Biweekly" compounds represented 77 ± 8% of the overall pharmaceutical concentration. The antidiabetic drug metformin consistently had the highest concentration with limited in-stream attenuation. The antihistamine drug fexofenadine inputs were greater during warm- than cool-season conditions but also attenuated faster. Differential attenuation of individual pharmaceuticals (i.e., high = citalopram; low = metformin) contributed to complex mixture evolution along the stream reach. This research demonstrates that variable inputs over multiple years and differential in-stream attenuation of individual compounds generate evolving complex mixture exposure conditions for biota, with implications for interactive effects.

Abundant trace fossil Polykampton in Palaeogene deep-sea flysch deposits of the Lesser Caucasus in Georgia: Palaeoecological and palaeoenvironmental implications

The new ichnospecies of Polykampton georgianum occurs abundantly in a part of the Borjomi Flysch (Upper Paleocene – Lower Eocene) in Georgia, in particular in the 45.3 m thick Ardagani-1 section that is dominated by grey silt- and mudstones having sandstones intercalated. The trace fossil occurs in the middle and upper part of the section. The trace fossil is composed of a median tunnel and side lobes that are filled with material differing from the host sediment. It is interpreted as a sequestrichnion, wherein the tracemaker stored organic-rich mud in the lobes as a food resource for times of food deficiency. The today ferruginization of the median tunnel resulting from the former presence of pyrite suggests strong chemical gradient between its lumen and the surrounding. Chemichnial behaviour is not excluded as nutritional strategy of the worm-like tracemaker (a polychaete?). It occupies a middle tier and is commonly crosscut by Trichichnus, Chondrites and Scolicia. Rare or absent other trace fossils typical of deep-sea fine-grained deposits suggest slight dysoxia. Significant impoverishment of the tiering pattern (only Trichichnus present) and absence of trace fossils in the lower part of the section point to strong dysoxia or episodic anoxia. The trace fossil assemblage is atypical, but in general it resembles the Nereites ichnosubfacies of the Nereites ichnofacies, which characterizes distal parts of the turbiditic depositional systems. The contrast between the dark grey to grey silt- and mudstones deposited during the waning phase of gravitational flows and greenish pelagic mud suggests that the former originated from an organic-rich depositional setting, presumably within the oxygen minimum zone in the upper-slope to outer-shelf domain. Sequestration of the organic-rich mud was a successful adaptation of the Polykampton georgianum producers in this specific environmental setting.

Latitudinal constraints on the abundance and activity of the cyanobacterium UCYN‐A and other marine diazotrophs in the North Pacific

AbstractThe number of marine environments known to harbor dinitrogen (N2)‐fixing (diazotrophic) microorganisms is increasing, prompting a reassessment of the biogeography of marine diazotrophs and N2fixation rates (NFRs). Here, we investigate the diversity, abundance, and activity of diazotrophic microorganisms in the North Pacific Subtropical Gyre (NPSG), a diazotrophic habitat, and the North Pacific Transition Zone (NPTZ), a region characterized by strong physical, chemical, and biological gradients. Samples were collected on two springtime meridional cruises during 2016 and 2017, spanning from 23.5°N to 41.4°N along 158°W. We observed an abrupt decrease in diazotrophic abundances near the southern edge of the NPTZ, which coincided with a salinity front and with a ∼10‐fold increase inSynechococcusabundance, but without a concomitant change in phosphate or nitrate concentrations. In NPSG waters south of this diazotrophic boundary,nifHgenes and NFRs were consistently detected and diazotrophic communities were dominated by UCYN‐A, an uncultivated, symbiotic cyanobacterium (2.8 × 103to 1.0 × 106nifHgene copies L−1). There was a significant positive relationship between quantitative polymerase chain reaction‐derived UCYN‐AnifHgene abundances and community NFRs in the NPSG, suggesting a large contribution of UCYN‐A to community NFRs. In the NPTZ waters to the north, NFRs were low or undetected andnifHgenes were rare, with the few detected sequences represented by UCYN‐A and noncyanobacterial diazotrophs. The patterns we observed in UCYN‐A abundance in the context of local biogeochemistry suggest that the environmental controls of this organism may differ from those of cultivated marine cyanobacterial diazotrophs.

Analytical modelling of period spacings across the HR diagram

ABSTRACT The characterization of stellar cores may be accomplished through the modelling of asteroseismic data from stars exhibiting either gravity-mode or mixed-mode pulsations, potentially shedding light on the physical processes responsible for the production, mixing, and segregation of chemical elements. In this work, we validate against model data an analytical expression for the period spacing that will facilitate the inference of the properties of stellar cores, including the detection and characterization of buoyancy glitches (strong chemical gradients). This asymptotically based analytical expression is tested both in models with and without buoyancy glitches. It does not assume that glitches are small and, consequently, predicts non-sinusoidal glitch-induced period-spacing variations, as often seen in model and real data. We show that the glitch position and width inferred from the fitting of the analytical expression to model data consisting of pure gravity modes are in close agreement (typically better than 7 ${{\ \rm per\ cent}}$ relative difference) with the properties measured directly from the stellar models. In the case of fitting mixed-mode model data, the same expression is shown to reproduce well the numerical results, when the glitch properties are known a priori. In addition, the fits performed to mixed-mode model data reveal a frequency dependence of the coupling coefficient, q, for a moderate-luminosity red-giant-branch model star. Finally, we find that fitting the analytical expression to the mixed-mode period spacings may provide a way to infer the frequencies of the pure acoustic dipole modes that would exist if no coupling took place between acoustic and gravity waves.

Rotation rate of the solar core as a key constraint to magnetic angular momentum transport in stellar interiors

Context. The internal rotation of the Sun constitutes a fundamental constraint when modelling angular momentum transport in stellar interiors. In addition to the more external regions of the solar radiative zone probed by pressure modes, measurements of rotational splittings of gravity modes would offer an invaluable constraint on the rotation of the solar core. Aims. We study the constraints that a measurement of the core rotation rate of the Sun could bring on magnetic angular momentum transport in stellar radiative zones. Methods. Solar models accounting for angular momentum transport by hydrodynamic and magnetic instabilities were computed for different initial velocities and disc lifetimes on the pre-main sequence to reproduce the surface rotation velocities observed for solar-type stars in open clusters. The internal rotation of these solar models was then compared to helioseismic measurements. Results. We first show that models computed with angular momentum transport by magnetic instabilities and a recent prescription for the braking of the stellar surface by magnetized winds can reproduce the observations of surface velocities of stars in open clusters. These solar models predict both a flat rotation profile in the external part of the solar radiative zone probed by pressure modes and an increase in the rotation rate in the solar core, where the stabilizing effect of chemical gradients plays a key role. A rapid rotation of the core of the Sun, as suggested by reported detections of gravity modes, is thus found to be compatible with angular momentum transport by magnetic instabilities. Moreover, we show that the efficiency of magnetic angular momentum transport in regions of strong chemical gradients can be calibrated by the solar core rotation rate independently from the unknown rotational history of the Sun. In particular, we find that a recent revised prescription for the transport of angular momentum by the Tayler instability can be easily distinguished from the original Tayler–Spruit dynamo, with a faster rotating solar core supporting the original prescription. Conclusions. By calibrating the efficiency of magnetic angular momentum transport in regions of strong chemical gradients, a determination of the solar core rotation rate through gravity modes is of prime relevance not only for the Sun, but for stars in general, since radial differential rotation precisely develops in these regions during the more advanced stages of evolution.

Larval fish assemblages from channels and fjords of south Pacific Patagonia: effects of environmental conditions

Chilean Patagonian fjords are characterized by strong stratification and chemical gradients which influence the ichthyoplankton distribution and abundance. Plankton samples were collected through bongo net oblique tows in 40 stations from onboard a bio-oceanographic cruise took place in inner fjords of southern Chile (47° to 51°S). Water column physical data were obtained with a conductivity-temperature-depth profiler (CTD) and turbidity was measured with a portable turbidimeter from 0 to 100 m depth. Stations were distributed by three zones: oceanic, channels and continental waters. A total of 1424 larvae were collected, representing 21 families. Dominant taxa were Maurolicus parvipinnis (31%), Sebastes oculatus (12%), Merluccius australis (11%), Lampanyctodes hectoris (10%), and Bathylagichthys parini (8%). Most of the environmental variability was determined by salinity, mainly in the area where continental waters are discharged. The water column in the oceanic zone presented mixed waters dominated by the myctophid L. hectoris and the sternoptychid M. parvipinnis. No significant effect of turbidity gradients on larval fish assemblages was evident during spring. The channel zone was both more saline, and density stratified with less turbidity, and lower abundance of species. Two ichthyoplanktonic assemblages were evident, one living in oceanic waters, and the other from channels and inner zones.

Upscaling Strategies of Porosity-Permeability Correlations in Reacting Environments from Pore-Scale Simulations

In geochemically reacting environments, the mineral dissolution and precipitation alters the structural and transport properties of the media of interest. The chemical and structural heterogeneities of the porous media affect the temporal evolution of the permeability with respect to porosity. Such correlations follow a nonlinear trend, which is difficult to estimate a priori and without knowledge of the microstructure itself, especially under the presence of strong chemical gradients. Macroscopic field-scale codes require such an input, and in the absence of exact descriptions, simplified correlations are used. After highlighting the diversity of microstructural evolution paths, due to dissolution, we discuss possible upscaling strategies.

Diamond brecciation and annealing accompanying major metasomatism in eclogite xenoliths from the Sask Craton, Canada

We studied eclogite xenoliths (diamond-free n = 28; diamondiferous n = 22) from the Cretaceous Fort a la Corne Kimberlite Field in Western Canada for their major element, trace element and oxygen isotope compositions to assess their origin and metasomatic history, and possible relationships between metasomatism and diamond formation. All eclogites have major element and oxygen isotope compositions consistent with a derivation from different levels of subducted, seawater altered oceanic crust. While barren xenoliths are more likely to be of gabbroic origin, diamond-bearing samples commonly have signatures consistent with shallow basaltic protoliths. The mineral chemistry in bimineralic diamond-free eclogites spans a wide compositional range, yet it is typically homogenous within individual xenoliths. Temperatures calculated from Mg-Fe exchange between garnet and clinopyroxene range widely for these eclogites, from 740 to 1300 °C, indicating the presence of eclogite through most of the lithospheric mantle. Diamondiferous samples are restricted to high temperatures (1180–1390 °C), consistent with derivation from the zone of diamond stability. Compositionally, diamond-bearing eclogites span a broad range similar to their barren counterparts, but there is also heterogeneity in mineral chemistry on the intra-sample level and in particular garnets are characterised by strong internal chemical gradients. This intra-sample heterogeneity is interpreted as the result of intense melt metasomatism, which occurred in temporal proximity to host kimberlite magmatism, strongly affected major, trace and even oxygen isotope values and resulted in diamond brecciation and annealing.

Trace metal contamination as a toxic and structuring factor impacting ultraphytoplankton communities in a multicontaminated Mediterranean coastal area

Toulon Bay is a Mediterranean coastal marine ecosystem facing important anthropogenic pressures resulting in strong and multiple chemical contamination gradients. In this study, we examined the spatial variations of both abundance and structure of ultraphytoplankton in relation with resources and potentially toxic trace metal concentrations in surface water in Toulon Bay. Sampling and on-field measurements were conducted in February 2014 and February 2015 throughout the entire bay. Strong Cd, Cu, Pb, and Zn contamination gradients in surface water were observed, with the highest levels being recorded in the most anthropized areas, while the open part of Toulon Bay presented levels close to those observed in the open sea. The abundance of Synechococcus-like cells was strongly negatively correlated to the concentrations of Cd, Cu, Pb, and Zn, whereas the abundance of picoeukaryote-like cells was positively correlated to these four trace metals. Trace metal contamination appeared to be one of the most significant contributors to ultraphytoplankton structure by statistically explaining up to 58.4% of its spatial variability. Incubation experiments were conducted to assess the specific impact of trace metal contamination on ultraphytoplankton of Toulon Bay. During around one month, ultraphytoplanktonic communities isolated from an uncontaminated site (representative from offshore communities) were exposed to an artificial cocktail of trace metals representative of the most anthropized part of Toulon Bay (in concentrations and proportions). A 90–99% decrease of total ultraphytoplankton abundance was observed within six days, followed by its recovery before the end of the experiment. This exposure to the trace metal cocktail additionally induced a shift from a Synechococcus-like dominated community, representative of the open sea, to a photosynthetic picoeukaryotes dominated one, mimicking the ultraphytoplanktonic community encountered in the most contaminated sites of the bay. Exposure to either natural waters representing the contaminated sites of the bay or elutriate of sediment resuspension did not promote a noticeable toxic effect on the total abundance of ultraphytoplankton, but affected its community structure, suggesting complex interplays.

Delving deeper: Metabolic processes in the metalimnion of stratified lakes

AbstractMany lakes exhibit seasonal stratification, during which they develop strong thermal and chemical gradients. An expansion of depth‐integrated monitoring programs has provided insight into the importance of organic carbon processing that occurs below the upper mixed layer. However, the chemical and physical drivers of metabolism and metabolic coupling remain unresolved, especially in the metalimnion. In this depth zone, sharp gradients in key resources such as light and temperature co‐occur with dynamic physical conditions that influence metabolic processes directly and simultaneously hamper the accurate tracing of biological activity. We evaluated the drivers of metalimnetic metabolism and its associated uncertainty across 10 stratified lakes in Europe and North America. We hypothesized that the metalimnion would contribute highly to whole‐lake functioning in clear oligotrophic lakes, and that metabolic rates would be highly variable in unstable polymictic lakes. Depth‐integrated rates of gross primary production (GPP) and ecosystem respiration (ER) were modelled from diel dissolved oxygen curves using a Bayesian approach. Metabolic estimates were more uncertain below the epilimnion, but uncertainty was not consistently related to lake morphology or mixing regime. Metalimnetic rates exhibited high day‐to‐day variability in all trophic states, with the metalimnetic contribution to daily whole‐lake GPP and ER ranging from 0% to 87% and < 1% to 92%, respectively. Nonetheless, the metalimnion of low‐nutrient lakes contributed strongly to whole‐lake metabolism on average, driven by a collinear combination of highlight, low surface‐water phosphorous concentration and high metalimnetic volume. Consequently, a single‐sensor approach does not necessarily reflect whole‐ecosystem carbon dynamics in stratified lakes.