Chalcogen‐Transfer Rearrangement: Exploring Inter‐ versus Intramolecular P−P Bond Activation

Gefördert im Rahmen des Projekts DEA

Little engagement of attention by salient distractors defined in a different dimension or modality to the visual search target

Singleton distractors may inadvertently capture attention, interfering with the task at hand. The underlying neural mechanisms of how we prevent or handle distractor interference remain elusive. Here, we varied the type of salient distractor introduced in a visual search task: the distractor could be defined in the same (shape) dimension as the target, a different (color) dimension, or a different (tactile) modality (intra-dimensional, cross-dimensional, and, respectively, cross-modal distractor, all matched for physical salience); and besides behavioral interference, we measured lateralized electrophysiological indicators of attentional selectivity (the N2pc, Ppc, PD, CCN/CCP, CDA, and cCDA). The results revealed the intra-dimensional distractor to produce the strongest reaction-time interference, associated with the smallest target-elicited N2pc. In contrast, the cross-dimensional and cross-modal distractors did not engender any significant interference, and the target-elicited N2pc was comparable to the condition in which the search display contained only the target singleton, thus ruling out early attentional capture. Moreover, the cross-modal distractor elicited a significant early CCN/CCP, but did not influence the target-elicited N2pc, suggesting that the tactile distractor is registered by the somatosensory system (rather than being proactively suppressed), without, however, engaging attention. Together, our findings indicate that, in contrast to distractors defined in the same dimension as the target, distractors singled out in a different dimension or modality can be effectively prevented to engage attention, consistent with dimension- or modality-weighting accounts of attentional priority computation

At the limits of bisphosphonio-substituted stannylenes

Gefördert im Rahmen des Projekts DEALGerman Science Fund (DFG) for financial support (CRC 1319 ELCH); János Bolyai Research Scholarship and Project UNKP-23-5 BME-41

Long‐term (statistically learnt) and short‐term (inter‐trial) distractor‐location effects arise at different pre‐ and post‐selective processing stages

A salient distractor interferes less with visual search if it appears at a location where it is likely to occur, referred to as distractor-location probability cueing. Conversely, if the current target appears at the same location as a distractor on the preceding trial, search is impeded. While these two location-specific “suppression” effects reflect long-term, statistically learnt and short-term, inter-trial adaptations of the system to distractors, it is unclear at what stage(s) of processing they arise. Here, we adopted the additional-singleton paradigm and examined lateralized event-related potentials (L-ERPs) and lateralized alpha (8–12 Hz) power to track the temporal dynamics of these effects. Behaviorally, we confirmed both effects: reaction times (RTs) interference was reduced for distractors at frequent versus rare (distractor) locations, and RTs were delayed for targets that appeared at previous distractor versus non-distractor locations. Electrophysiologically, the statistical-learning effect was not associated with lateralized alpha power during the pre-stimulus period. Rather, it was seen in an early N1pc referenced to the frequent distractor location (whether or not a distractor or a target occurred there), indicative of a learnt top-down prioritization of this location. This early top-down influence was systematically modulated by (competing) target- and distractor-generated bottom-up saliency signals in the display. In contrast, the inter-trial effect was reflected in an enhanced SPCN when the target was preceded by a distractor at its location. This suggests that establishing that an attentionally selected item is a task-relevant target, rather than an irrelevant distractor, is more demanding at a previously “rejected” distractor location

Chromium stable isotope distributions in the southwest Pacific Ocean and constraints on hydrothermal input from the Kermadec Arc

Special attention has been given to chromium (Cr) as a paleoproxy tracing redox cycling throughout Earth’s history, due to differences in the solubility of its primary redox species at Earth’s surface (Cr (III) and Cr(VI)) and isotope fractionation associated with their interconversion. In turn, chromium’s pale- oproxy potential has motivated studies of the modern ocean to better understand which processes drive its cycling and to constrain their impact on the Cr isotope composition (d53Cr) of seawater. Here, we pre- sent total dissolved seawater Cr concentrations and d53Cr along the GEOTRACES GP13 section. This sec- tion is a zonal transect extending from Australia in the subtropical southwest Pacific Ocean. Surface signals of local biological Cr cycling are minimal, in agreement with distributions of dissolved major nutrients as well as biologically-controlled trace metals in this low productivity, oligotrophic environ- ment. Depth profiles have Cr concentration minima in surface waters and maxima at depth, and are lar- gely shaped by the advection of nutrient- and Cr-rich subsurface waters rather than vertically-driven processes. Samples close to the sediment–water interface indicate important benthic Cr fluxes across the section. The GP13 transect crosses the hydrothermally-active Kermadec Arc. Hydrothermal fluids (consisting of <15% background seawater) were collected from three venting sites at the Brothers Volcano (along the Kermadec Arc). These fluids yielded near-crustal d53Cr values (!0.17 to +0.08‰) and elevated [Cr] (7.5–23 nmol kg!1, hydrothermal endmember [Cr] % 8–27 nmol kg!1), indicating that the Kermadec Arc may be an isotopically light Cr source. Dissolved [Fe] enrichments have been reported previously in deep waters ($1600–3000 m) along the GP13 transect, east of the Kermadec Arc. These same waters show ele- vated [Cr] compared to Circumpolar Deep Water ([Cr] = 3.88 ± 0.11, d53Cr = 0.89 ± 0.08, n = 32), with an aver- age [Cr] accumulation of 0.71 ± 0.11 nmol kg!1 (1 SD), and an estimated d53Cr of +0.46 ± 0.30‰ (2 SD, n = 9) for the accumulated Cr. Comparing high-temperature vent and neutrally buoyant plume data, hydrothermal-sourced Cr is likely negligable compared to Cr contributions from other processes (benthic fluxes, release from particles), and the advection of more Cr-rich Pacific Deep Water. It is unlikely that hydrothermal vents would be a major contributor within the regional or global biogeochemical Cr cycle, even if hydrothermal fluxes change by orders of magnitude, and therefore d53Cr trends in the paleorecord may be attributable, at least in part, to major changes in other controls on Cr (e.g. widespread anoxia)

Characterization of hydrothermal sources of iron in the oceans - Constraints from iron stable isotopes

In recent years, iron (Fe) has been recognized as one of the key elements in Earth’s biogeochemical cycles, acting as an important micronutrient for photosynthetic organisms, and a limiting factor in regulating primary productivity. Consequently, the oceanic biogeochemical cycle of Fe is tightly linked to the global carbon cycle and hence global climate processes. This perception has triggered intense scientific interest in understanding the marine biogeochemistry of Fe and quantifying its spatial distribution and transport in the oceans, as well as the technological advancements necessary to facilitate high-precision analyses of Fe concentrations and Fe isotopic compositions of marine samples, including seawater. Indentifying the sinks, sources and transformations of Fe species in the global ocean, as well as lateral and vertical modes of transportation and ocean mixing, supplying dissolved Fe to areas away from Fe sources, are the main aims of current research campaigns. Multiple Fe sources, such as aerosol dust, contribute to the marine dissolved Fe budget and novel tools utilizing isotopic characterization of these sources and mass balance calculations are anticipated to facilitate resolution of the complex interplay of the various Fe fluxes. Hydrothermal discharge has received growing attention as a major source of dissolved Fe to the deep ocean, particularly in areas with little dust deposition. However, studies of Fe isotope fractionation in hydrothermal plumes have resulted in controversial findings with implications for constraining the isotopic signature/s of hydrothermally derived Fe and the role of hydrothermal Fe in the global oceanic Fe inventory. In order to constrain Fe isotope fractionation within a submarine hydrothermal vent field and hydrothermal plume, this study aided in the development of new seawater preconcentration and separation protocols. These new methods facilitate direct Fe isotope analysis of dissolved Fe in plume samples, for the first time, utilizing double spiking techniques and multiple-collector ICPMS (MC-ICPMS) instrument settings specifically adapted to meet the requirements for analysis of low concentration samples. In order to provide a better understanding the Fe isotope systematics in submarine hydrothermal systems, the present study investigates the Fe isotopic compositions of hydrothermal fluids and precipitates from the newly discovered Nifonea vent field in Vanuatu. Located in a young back-arc basin, Nifonea provides a unique opportunity to (1) study processes affecting Fe isotope fractionation, represented by the 56Fe/54Fe signature (reformulated as δ56Fe), in environments characterized by short-lived heat pulses and relatively low water depths, and (2) better constrain Fe isotope effects resulting from subsurface sulfide precipitation and phase separation. The results indicate the complex interplay of sulfide formation and phase separation producing large spatial variability of fluid Fe isotopic compositions with generally low δ56Fe. High temperatures from recent volcanic events are interpreted to facilitate slow precipitation of chalcopyrite with systematically higher δ56Fe compared to hydrothermal fluids causing considerable Fe isotope effects. In addition, phase separation at relatively low pressure conditions produces low-Cl vapor phases and appears to strongly partition the Fe isotopes into vapor and liquid phases. For the first time, we demonstrate fractionation of Fe isotopes during phase separation with similar isotope effects as suggested from recent experimental studies (Syverson et al., 2014). The processes controlling Fe isotope fractionation and Fe speciation during plume formation above Nifonea are approached by coupled Fe isotope analysis of dissolved and particulate Fe and the chemical composition (major and trace elements) of plume particles. Plume processes largely regulate the transport of dissolved Fe to the open ocean through mineral precipitation and redox processes. Removal and transformation of hydrothermally derived Fe from, or within, the plume also strongly fractionate the original Fe isotopic composition of dissolved hydrothermal Fe towards heavier isotopic compositions, resulting in δ56Fe as low as -0.74‰ and up to -0.19‰ in more distal parts of the plume, as suggested in the presented study. These results further support predictions from previous research (Bennett et al., 2009) however, they also reveal the very complex interplay of various processes affecting the Fe isotope systematics in hydrothermal plumes and preclude a generalized isotopic signature for hydrothermally derived Fe for input into mass balance models. Results of this thesis emphasize the need for further research investigating transformation processes of Fe within hydrothermal plumes, from near-field to far-field, thereby integrating dissolved and particulate Fe isotopic composition, and speciation, as well as particle geochemistry