Impact of hydrothermalism on the ocean iron cycle

As the iron supplied from hydrothermalism is ultimately ventilated in the iron-limited Southern Ocean, it plays an important role in the ocean biological carbon pump. We deploy a set of focused sensitivity experiments with a state of the art global model of the ocean to examine the processes that regulate the lifetime of hydrothermal iron and the role of different ridge systems in governing the hydrothermal impact on the Southern Ocean biological carbon pump. Using GEOTRACES section data, we find that stabilization of hydrothermal iron is important in some, but not all regions. The impact on the Southern Ocean biological carbon pump is dominated by poorly explored southern ridge systems, highlighting the need for future exploration in this region. We find inter-basin differences in the isopycnal layer onto which hydrothermal Fe is supplied between the Atlantic and Pacific basins, which when combined with the inter-basin contrasts in oxidation kinetics suggests a muted influence of Atlantic ridges on the Southern Ocean biological carbon pump. Ultimately, we present a range of processes, operating at distinct scales, that must be better constrained to improve our understanding of how hydrothermalism affects the ocean cycling of iron and carbon

Simulation of a trust and reputation based mitigation protocol for a black hole style attack on VANETs

From a security standpoint, VANETs (Vehicular ad hoc Networks) are vulnerable to attacks by malicious users, due to the decentralized and open nature of the wireless system. For many of these kinds of attacks detection is unfeasible, thus making it hard to produce security. Despite their characterization as dynamically reconfigurable networks, it is nonetheless essential to identify topology and population properties that can optimise mitigation protocols’ deployment. In this paper, we provide an algorithmic definition and simulation of a trust and mitigation based protocol to contain a Black Hole style attack on a VANET. We experimentally show its optimal working conditions: total connectivity, followed by a random network; connection to external networks; early deployment of the protocol and ranking of the message. We compare results with those of existing protocols and future work shall focus on repeated broadcasting, opportunistic message forwarding and testing on real data

Impact of Chemically Specific Interactions between Anions and Weak Polyacids on Chain Ionization, Conformations, and Solution Energetics

The presence of salts in a solution containing weak polyelectrolytes is known to modify both their titration behavior and conformations due to electrostatic screening. Instead, little is currently known about the changes induced by chemically specific interactions (e.g., charged hydrogen bonds, c-H-bonds). To investigate this aspect, we simulated the titration of weak polyacids with a primitive model and Monte Carlo methods in the presence of monovalent salts whose anions are capable of forming c-H-bonds with associated acid groups. The interaction between anions and weak polyacids (e.g., poly(acrylic acid)) substantially hampers ionization at low pH despite the somewhat limited number of coordinated anions, whereas it has a limited impact once pH > pKa + 2 due to a progressive anion decoordination. Importantly, the suppression of ionization appears extremely local in nature, with different chain segments differing in pKa by up to 1.3 units. As for chain conformations, c-H-bonds reduce the average sizes of polyacids independently of their structure as a consequence of multidentate binding or multiarm coordination in starlike species. Analyzing the length of chain segments with all monomers coordinated or uncoordinated has also evidenced that anion binding is extremely local in nature. The energetic analysis of c-H-bond formation suggests that polyacid chemical potential may be strongly lowered (up to -0.7 kcal/mol per monomer), the impact of such results on a few phenomena relevant for the physical chemistry of polyacid-containing solutions being analyzed in some detail

Ocean biogeochemistry exhibits contrasting responses to a large scale reduction in dust deposition

Dust deposition of iron is thought to be an important control on ocean biogeochemistry and air-sea CO<sub>2</sub> exchange. In this study, we examine the impact of a large scale, yet climatically realistic, reduction in the aeolian Fe input during a 240 year transient simulation. In contrast to previous studies, we find that the ocean biogeochemical cycles of carbon and nitrogen are relatively insensitive (globally) to a 60% reduction in Fe input from dust. Net primary productivity (NPP) is reduced in the Fe limited regions, but the excess macronutrients that result are able to fuel additional NPP elsewhere. Overall, NPP and air-sea CO<sub>2</sub> exchange are only reduced by around 3% between 1860 and 2100. While the nitrogen cycle is perturbed more significantly (by ~15%), reduced N<sub>2</sub> fixation is balanced by a concomitant decline in denitrification. Feedbacks between N<sub>2</sub> fixation and denitrification are controlled by variability in surface utilization of inorganic nitrogen and subsurface oxygen consumption, as well as the direct influence of Fe on N<sub>2</sub> fixation. Overall, there is relatively little impact of reduced aeolian Fe input (<4%) on cumulative CO<sub>2</sub> fluxes over 240 years. The lower sensitivity of our model to changes in dust input is primarily due to the more detailed representation of the continental shelf Fe, which was absent in previous models

Optical Excitation of a Nanoparticle Cu/p-NiO Photocathode Improves Reaction Selectivity for CO₂ Reduction in Aqueous Electrolytes

We report the light-induced modification of catalytic selectivity for photoelectrochemical CO₂ reduction in aqueous media using copper (Cu) nanoparticles dispersed onto p-type nickel oxide (p-NiO) photocathodes. Optical excitation of Cu nanoparticles generates hot electrons available for driving CO₂ reduction on the Cu surface, while charge separation is accomplished by hot-hole injection from the Cu nanoparticles into the underlying p-NiO support. Photoelectrochemical studies demonstrate that optical excitation of plasmonic Cu/p-NiO photocathodes imparts increased selectivity for CO₂ reduction over hydrogen evolution in aqueous electrolytes. Specifically, we observed that plasmon-driven CO₂ reduction increased the production of carbon monoxide and formate, while simultaneously reducing the evolution of hydrogen. Our results demonstrate an optical route toward steering the selectivity of artificial photosynthetic systems with plasmon-driven photocathodes for photoelectrochemical CO₂ reduction in aqueous media

Tunable Knot Segregation in Copolyelectrolyte Rings Carrying a Neutral Segment

We use Langevin dynamics simulations to study the knotting properties of copolyelectrolyte rings carrying neutral segments. We show that by solely tuning the relative length of the neutral and charged blocks, one can achieve different combinations of knot contour position and size. Strikingly, the latter is shown to vary nonmonotonically with the length of the neutral segment; at the same time, the knot switches from being pinned at the block's edge to becoming trapped inside it. Model calculations relate both effects to the competition between two adversarial mechanisms: the energy gain of localizing one or more of the knot's essential crossings on the neutral segment and the entropic cost of such localization. Tuning the length of the neutral segment sets the balance between the two mechanisms and hence the number of localized essential crossings, which in turn modulates the knot's size. This general principle ought to be useful in more complex systems, such as multiblock copolyelectrolytes, to achieve a more granular control of topological constraints

Tuning Knotted Copolyelectrolyte Conformations via Solution Properties

We used Langevin dynamics simulations to study coarse-grained knotted copolyelectrolytes, composed by a neutral and a charged segment, in solutions of different salt concentrations, valency, and solvent screening power. We show that the facile variation of these parameters allows for tuning the length and position of the knotted region, which in turn controls the overall metric properties. Specifically, adding either monovalent or divalent ions causes the knot to swell at the expense of the copolyelectrolyte overall size. However, the knot typically straddles the charged-neutral interface in the presence of monovalent counterions, whereas it is attracted on the charged segment with divalent ones. Notably, similar modulations of knot size and position can also be achieved by varying the dielectric constant of the solvent. Our results demonstrate the feasibility of harnessing the solution-mediated balance of electrostatics and conformational entropy toward a facile external tuning of the conformational properties of knotted polymers

High Photovoltaic Quantum Efficiency in Ultrathin van der Waals Heterostructures

We report experimental measurements for ultrathin (< 15 nm) van der Waals heterostructures exhibiting external quantum efficiencies exceeding 50%, and show that these structures can achieve experimental absorbance > 90%. By coupling electromagnetic simulations and experimental measurements, we show that pn WSe2/MoS2 heterojunctions with vertical carrier collection can have internal photocarrier collection efficiencies exceeding 70%.Comment: ACS Nano, 2017. Manuscript (25 pages, 7 figures) plus supporting information (7 pages, 4 figures