Differentiation of asteroids in the early solar system from Hf-W systematics

The first major differentiation in primitive planetesimals is the formation of metal- or metal-sulfide cores. To constrain the time of asteroidal differentiation, meteoritic metals and silicates coexisting with metals were analyzed for Wisotopes and Hf and W concentrations. The study focused on silicate inclusions in non-magmatic IAB iron meteorites and the corresponding metal phases as well as on winonaites and acapulcoites were analyzed. IAB iron meteorites formed by crystallization of metal ponds under low pressure conditions at or near the surface of their asteroidal parent body. The presence of abundant silicate inclusions reflects either incomplete metal-silicate separation or mixing of metal and silicates by impacts. Hafnium-W measurements were performed on magnetic and non-magnetic separates from seven IAB iron meteorites to constrain the exact timing of metal separation, silicate differentiation and metamorphism. IAB metals have a deficit in 182W of - 3.1 ± 0.2 ε-units (relative to terrestrial standard materials). The silicate fractions in IAB iron meteorites have εW values ranging from -3 to +35, indicating that the exchange of W between metals and silicates ceased within the lifetime of 182Hf. Based on a combined IAB silicate isochron defined by three different bulk inclusions, silicate differentiation in the mantle-like reservoir of the IAB parent body must have occurred 2.9 ± 2.2 Myr after the formation of Ca,Al-rich inclusions (CAIs), the oldest objects known from the solar nebula. This event therefore occurred contemporaneously with the main metal segregation event on the IAB parent body at 3.5 ± 2.3 Myr after CAI formation, which is obtained from the W isotope composition of the metal phase (assuming evolution in a chondritic Hf/W ratio before metal separation). Hence, metal segregation and silicate differentiation occurred early enough, so that 26Al was responsible for parent body heating causing differentiation. On the other hand, internal isochrones for silicates of the two IAB-iron meteorites El Taco and Lueders yield ages 11.3 ± 2.3 Myr and ~12 Myr after CAI formation, reflecting impact induced heating and redistribution of radiogenic W. Additional later metal-silicate equilibration on the IAB parent body is also found in Mundrabilla (-2.6 ± 0.5 ε- units). Therefore, a prolonged epoch of most likely impact-controlled W reequilibration on the IAB parent body can be inferred. In contrast to the combined IAB silicate isochron, most of the analyzed Winonaites define a combined isochron with an age that postdates CAI formation by 14.5 ± 2.8 Myr. This age is apparently too young for heating and melting of the parent body by an internal heat-source and indistinguishable within uncertainty from the internal El Taco and Lueders isochrones. Assuming a common origin for IABs and winonaites these ages could be related to a single impact-controlled W re-equilibration on the Winonaite/IAB parent body. A combined isochron defined by different Acapulcoite separates yields an age of 4.6 ± 1.3 Myr after CAI formation. This is within the range of Hf-W ages of ordinary chondrites, although Acapulcoites were heated to significantly higher temperatures than ordinary chondrites, implying a higher level of 26Al. Other chronometers indicate fast cooling of acapulcoites, which is difficult to reconcile with similar cooling rates as those of ordinary chondrites. A comparison with Hf-W literature data shows that asteroidal differentiation on the IAB parent body occurred after the segregation of most magmatic iron meteorites and contemporaneously with the accretion of chondrite parent bodies, lending further support for a revised solar system chronology where chondrites cannot be the precursor of most differentiated asteroids. Chondritic meteorites with higher equilibration temperatures, such as winonaites and acapulcoites have even younger ages. They are certainly no precursor material for iron meteorites and their comparatively younger ages probably reflect an impact origin after accretion of chondrite parent bodies

Labellings for assumption-based and abstract argumentation

The semantics of Assumption-Based Argumentation (ABA) frameworks are traditionally characterised as assumption extensions, i.e. sets of accepted assumptions. Assumption labellings are an alternative way to express the semantics of flat ABA frameworks, where one of the labels in, out, or undec is assigned to each assumption. They are beneficial for applications where it is important to distinguish not only between accepted and non-accepted assumptions, but further divide the non-accepted assumptions into those which are clearly rejected and those which are neither accepted nor rejected and thus undecided. We prove one-to-one correspondences between assumption labellings and extensions for the admissible, grounded, complete, preferred, ideal, semi-stable and stable semantics. We also show how the definition of assumption labellings for flat ABA frameworks can be extended to assumption labellings for any (flat and non-flat) ABA framework, enabling reasoning with a wider range of scenarios. Since flat ABA frameworks are structured instances of Abstract Argumentation (AA) frameworks, we furthermore investigate the relation between assumption labellings for flat ABA frameworks and argument labellings for AA frameworks. Building upon prior work on complete assumption and argument labellings, we prove one-to-one correspondences between grounded, preferred, ideal, and stable assumption and argument labellings, and a one-to-many correspondence between admissible assumption and argument labellings. Inspired by the notion of admissible assumption labellings we introduce committed admissible argument labellings for AA frameworks, which correspond more closely to admissible assumption labellings of ABA frameworks than admissible argument labellings do

On the responsibility for undecisiveness in preferred and stable labellings in abstract argumentation

Different semantics of abstract Argumentation Frameworks (AFs) provide different levels of decisiveness for reasoning about the acceptability of conflicting arguments. The stable semantics is useful for applications requiring a high level of decisiveness, as it assigns to each argument the label “accepted” or the label “rejected”. Unfortunately, stable labellings are not guaranteed to exist, thus raising the question as to which parts of AFs are responsible for the non-existence. In this paper, we address this question by investigating a more general question concerning preferred labellings (which may be less decisive than stable labellings but are always guaranteed to exist), namely why a given preferred labelling may not be stable and thus undecided on some arguments. In particular, (1) we give various characterisations of parts of an AF, based on the given preferred labelling, and (2) we show that these parts are indeed responsible for the undecisiveness if the preferred labelling is not stable. We then use these characterisations to explain the non-existence of stable labellings. We present two types of characterisations, based on labellings that are more (or equally) committed than the given preferred labelling on the one hand, and based on the structure of the given AF on the other, and compare the respective AF parts deemed responsible. To prove that our characterisations indeed yield responsible parts, we use a notion of enforcement of labels through structural revision, by means of which the preferred labelling of the given AF can be turned into a stable labelling of the structurally revised AF. Rather than prescribing how this structural revision is carried out, we focus on the enforcement of labels and leave the engineering of the revision open to fulfil differing requirements of applications and information available to users

Logic programming in assumption-based argumentation revisited — semantics and graphical representation

Logic Programming and Argumentation Theory have been existing side by side as two separate, yet related, techniques in the field of Knowledge Representation and Reasoning for many years. When Assumption-Based Argumentation (ABA) was first introduced in the nineties, the authors showed how a logic program can be encoded in an ABA framework and proved that the stable semantics of a logic program corresponds to the stable extension semantics of the ABA framework encoding this logic program. We revisit this initial work by proving that the 3-valued stable semantics of a logic program coincides with the complete semantics of the encoding ABA framework, and that the L-stable semantics of this logic program coincides with the semi-stable semantics of the encoding ABA framework. Furthermore, we show how to graphically represent the structure of a logic program encoded in an ABA framework and that not only logic programming and ABA semantics but also Abstract Argumentation semantics can be easily applied to a logic program using these graphical representations

Justifying Answer Sets using Argumentation

An answer set is a plain set of literals which has no further structure that would explain why certain literals are part of it and why others are not. We show how argumentation theory can help to explain why a literal is or is not contained in a given answer set by defining two justification methods, both of which make use of the correspondence between answer sets of a logic program and stable extensions of the Assumption-Based Argumentation (ABA) framework constructed from the same logic program. Attack Trees justify a literal in argumentation-theoretic terms, i.e. using arguments and attacks between them, whereas ABA-Based Answer Set Justifications express the same justification structure in logic programming terms, that is using literals and their relationships. Interestingly, an ABA-Based Answer Set Justification corresponds to an admissible fragment of the answer set in question, and an Attack Tree corresponds to an admissible fragment of the stable extension corresponding to this answer set.Comment: This article has been accepted for publication in Theory and Practice of Logic Programmin

Spatial and dietary sources of elevated mercury exposure in white-tailed eagle nestlings in an Arctic freshwater environment

Human-induced mercury (Hg) contamination is of global concern and its effects on wildlife remain of high concern, especially in environmental hotspots such as inland aquatic ecosystems. Mercury biomagnifies through the food web resulting in high exposure in apex predators, such as the white-tailed eagle (Haliaeetus albicilla), making them excellent sentinel species for environmental Hg contamination. An expanding population of white-tailed eagles is inhabiting a sparsely populated inland area in Lapland, northern Finland, mainly around two large reservoirs flooded 50 years ago. As previous preliminary work revealed elevated Hg levels in this population, we measured Hg exposure along with dietary proxies (delta C-13 and delta N-15) in body feathers collected from white-tailed eagle nestlings in this area between 2007 and 2018. Mercury concentrations were investigated in relation to territory characteristics, proximity to the reservoirs and dietary ecology as potential driving factors of Hg contamination. Mercury concentrations in the nestlings (4.97-31.02 mu g g(-1) dw) were elevated, compared to earlier reported values in nestlings from the Finnish Baltic coast, and exceeded normal background levels (40.00 mu g g(-1)). The main drivers of Hg contamination were trophic position (proxied by delta N-15), the dietary proportion of the predatory fish pike (Esox Lucius), and the vicinity to the Porttipahta reservoir. We also identified a potential evolutionary trap, as increased intake of the preferred prey, pike, increases exposure. All in all, we present results for poorly understood freshwater lake environments and show that more efforts should be dedicated to further unravel potentially complex pathways of Hg exposure to wildlife.Peer reviewe

Functional interdependence of the actin regulators CAP1 and cofilin1 in control of dendritic spine morphology

The vast majority of excitatory synapses are formed on small dendritic protrusions termed dendritic spines. Dendritic spines vary in size and density that are crucial determinants of excitatory synaptic transmission. Aberrations in spine morphogenesis can compromise brain function and have been associated with neuropsychiatric disorders. Actin filaments (F-actin) are the major structural component of dendritic spines, and therefore, actin-binding proteins (ABP) that control F-actin dis-/assembly moved into the focus as critical regulators of brain function. Studies of the past decade identified the ABP cofilin1 as a key regulator of spine morphology, synaptic transmission, and behavior, and they emphasized the necessity for a tight control of cofilin1 to ensure proper brain function. Here, we report spine enrichment of cyclase-associated protein 1 (CAP1), a conserved multidomain protein with largely unknown physiological functions. Super-resolution microscopy and live cell imaging of CAP1-deficient hippocampal neurons revealed impaired synaptic F-actin organization and dynamics associated with alterations in spine morphology. Mechanistically, we found that CAP1 cooperates with cofilin1 in spines and that its helical folded domain is relevant for this interaction. Moreover, our data proved functional interdependence of CAP1 and cofilin1 in control of spine morphology. In summary, we identified CAP1 as a novel regulator of the postsynaptic actin cytoskeleton that is essential for synaptic cofilin1 activity

Spatial and dietary sources of elevated mercury exposure in white-tailed eagle nestlings in an Arctic freshwater environment

Human-induced mercury (Hg) contamination is of global concern and its effects on wildlife remain of high concern, especially in environmental hotspots such as inland aquatic ecosystems. Mercury biomagnifies through the food web resulting in high exposure in apex predators, such as the white-tailed eagle (Haliaeetus albicilla), making them excellent sentinel species for environmental Hg contamination. An expanding population of white-tailed eagles is inhabiting a sparsely populated inland area in Lapland, northern Finland, mainly around two large reservoirs flooded 50 years ago. As previous preliminary work revealed elevated Hg levels in this population, we measured Hg exposure along with dietary proxies (δ13C and δ15N) in body feathers collected from white-tailed eagle nestlings in this area between 2007 and 2018. Mercury concentrations were investigated in relation to territory characteristics, proximity to the reservoirs and dietary ecology as potential driving factors of Hg contamination. Mercury concentrations in the nestlings (4.97–31.02 μg g−1 dw) were elevated, compared to earlier reported values in nestlings from the Finnish Baltic coast, and exceeded normal background levels (≤5.00 μg g−1) while remaining below the tentative threshold of elevated risk for Hg exposure mediated health effect (>40.00 μg g−1). The main drivers of Hg contamination were trophic position (proxied by δ15N), the dietary proportion of the predatory fish pike (Esox lucius), and the vicinity to the Porttipahta reservoir. We also identified a potential evolutionary trap, as increased intake of the preferred prey, pike, increases exposure. All in all, we present results for poorly understood freshwater lake environments and show that more efforts should be dedicated to further unravel potentially complex pathways of Hg exposure to wildlife.</p

Trophic Dynamics of Mercury in the Baltic Archipelago Sea Food Web: The Impact of Ecological and Ecophysiological Traits

We investigated trophic dynamics of Hg in the polluted Baltic Archipelago Sea using established trophic magnification (TMFs) and biomagnification factors (BMFs) on a comprehensive set of bird, fish, and invertebrate species. As different ecological and ecophysiological species traits may affect trophic dynamics, we explored the effect of food chain (benthic, pelagic, benthopelagic) and thermoregulatory strategy on trophic total Hg (THg) dynamics, using different approaches to accommodate benthopelagic species and normalize for trophic position (TP). We observed TMFs and most BMFs greater than 1, indicating overall THg biomagnification. We found significantly higher pelagic TMFs (3.58-4.02) compared to benthic ones (2.11-2.34) when the homeotherm bird species were excluded from models, but not when included. This difference between the benthic and pelagic TMFs remained regardless of how the TP of benthopelagic species was modeled, or whether TMFs were normalized for TP or not. TP-corrected BMFs showed a larger range (0.44-508) compared to BMFs representing predator-prey concentration ratios (0.05-82.2). Overall, the present study shows the importance of including and evaluating the effect of ecological and ecophysiological traits when investigating trophic contaminant dynamics