ALBERT

Description: Determining factors that limit coseismic rupture is important to evaluate the hazard of powerful subduction zone earthquakes such as the 2011 Tohoku-Oki event (Mw=9.0). In 1960 (Mw=9.5) and 2010 (Mw=8.8) Chile was hit by such powerful earthquakes, the boundary of which was the site of a giant submarine slope failure with chaotic debris subducted to seismogenic zone depth. Here a continuous décollement is absent whereas away from the slope failure a continuous décollement is seismically imaged. We infer that underthrusting of inhomogeneous slide deposits prevents the development of a décollement, and thus the formation of a thin continuous slip zone necessary for earthquake rupture propagation. Thus coseismic rupture during the 1960 and 2010 earthquakes seems to be limited by underthrusted upper plate mass-wasting deposits. More generally our results suggest that upper plate dynamics and resulting surface processes can play a key role for determining rupture size of subduction zone earthquakes. This article is protected by copyright. All rights reserved.

Description: The age of the subducting Nazca Plate off Chile increases northwards from 0 Ma at the Chile Triple Junction (46°S) to 37 Ma at the latitude of Valparaíso (32°S). Age-related variations in the thermal state of the subducting plate impact on (a) the water influx to the subduction zone, as well as on (b) the volumes of water that are released under the continental forearc or, alternatively, carried beyond the arc. Southern Central Chile is an ideal setting to study this effect, because other factors for the subduction zone water budget appear constant. We determine the water influx by calculating the crustal water uptake and by modeling the upper mantle serpentinization at the outer rise of the Chile Trench. The water release under forearc and arc is determined by coupling FEM thermal models of the subducting plate with stability fields of water-releasing mineral reactions for upper and lower crust and hydrated mantle. Results show that both the influx of water stored in, and the outflux of water released from upper crust, lower crust and mantle vary drastically over segment boundaries. In particular, the oldest and coldest segments carry roughly twice as much water into the subduction zone as the youngest and hottest segments, but their release flux to the forearc is only about one fourth of the latter. This high variability over a subduction zone of 〈 1500 km length shows that it is insufficient to consider subduction zones as uniform entities in global estimates of subduction zone fluxes. This article is protected by copyright. All rights reserved.

Description: Numerical models of ocean biogeochemistry are relied upon to make projections about the impact of climate change on marine resources and test hypotheses regarding the drivers of past changes in climate and ecosystems. In large areas of the ocean, iron availability regulates the functioning of marine ecosystems and hence the ocean carbon cycle. Accordingly, our ability to quantify the drivers and impacts of fluctuations in ocean ecosystems and carbon cycling in space and time relies on first achieving an appropriate representation of the modern marine iron cycle in models. When the iron distributions from thirteen global ocean biogeochemistry models are compared against the latest oceanic sections from the GEOTRACES programme we find that all models struggle to reproduce many aspects of the observed spatial patterns. Models that reflect the emerging evidence for multiple iron sources or subtleties of its internal cycling perform much better in capturing observed features than their simpler contemporaries, particularly in the ocean interior. We show that the substantial uncertainty in the input fluxes of iron results in a very wide range of residence times across models, which has implications for the response of ecosystems and global carbon cycling to perturbations. Given this large uncertainty, iron-fertilisation experiments based on any single current generation model should be interpreted with caution. Improvements to how such models represent iron scavenging and also biological cycling are needed to raise confidence in their projections of global biogeochemical change in the ocean.

Description: In dry mixed‐conifer forests of central Oregon 100+ years of fire suppression has led to more crowded forests, increased competition and greater drought‐sensitivity despite repeated defoliation events by Pandora moth. Across this period, drought stress increased and precluded benefits of rising atmospheric CO2 concentrations where stand basal area at the initiation of fire suppression exceeded 25 m2/ha or leaf area index exceeded 2.3 m2/m−2. Therefore, greater resilience of dry mixed‐conifer could be obtained by managing landscapes to promote conditions that are more open compared to these threshold values. Abstract A century of fire suppression across the Western United States has led to more crowded forests and increased competition for resources. Studies of forest thinning or stand conditions after mortality events have provided indirect evidence for how competition can promote drought stress and predispose forests to severe fire and/or bark beetle outbreaks. Here, we demonstrate linkages between fire deficits and increasing drought stress through analyses of annually resolved tree‐ring growth, fire scars, and carbon isotope discrimination (Δ13C) across a dry mixed‐conifer forest landscape. Fire deficits across the study area have increased the sensitivity of leaf gas exchange to drought stress over the past 〉100 years. Since 1910, stand basal area in these forests has more than doubled and fire‐return intervals have increased from 25 to 140 years. Meanwhile, the portion of interannual variation in tree‐ring Δ13C explained by the Palmer Drought Severity Index has more than doubled in ca. 300–500‐year‐old Pinus ponderosa as well as in fire‐intolerant, ca. 90–190‐year‐old Abies grandis. Drought stress has increased in stands with a basal area of ≥25 m2/ha in 1910, as indicated by negative temporal Δ13C trends, whereas stands with basal area ≤25 m2/ha in 1910, due to frequent or intense wildfire activity in decades beforehand, were initially buffered from increased drought stress and have benefited more from rising ambient carbon dioxide concentrations, [CO2], as demonstrated by positive temporal Δ13C trends. Furthermore, the average Δ13C response across all P. ponderosa since 1830 indicates that photosynthetic assimilation rates and stomatal conductance have been reduced by ~10% and ~20%, respectively, compared to expected trends due to increasing [CO2]. Although disturbance legacies contribute to local‐scale intensity of drought stress, fire deficits have reduced drought resistance of mixed‐conifer forests and made them more susceptible to challenges by pests and pathogens and other disturbances.

Description: The South Chilean marine fore arc (35°S–40°S) is separated into four tectonic segments, Concepción North, Concepción South, Nahuelbuta, and Tolten (from north to south). These are each characterized by their individual tectonic geomorphology and reflect different ways of mechanical and kinematic interaction of the convergent Nazca and South American plates. Splay faults that cut through continental framework rock are seismically imaged in both Concepción segments and the Tolten Segment. Additionally, the Concepción South Segment exhibits prominent upper plate normal faults. Normal faults apparently relate to uplift caused by sediment underthrusting at depth. This has led to oversteepening and gravitational collapse of the marine fore arc. There is also evidence for sediment underthrusting and basal accretion to the overriding plate in the Tolten Segment. There, uplift of the continental slope has created a landward inclined seafloor over a latitudinal distance of 50 km. In the Nahuelbuta Segment transpressive upper plate faults, aligned oblique to the direction of plate motion, control the seafloor morphology. Based on a unique acoustic data set including 〉90% of bathymetric coverage of the continental slope we are able to reveal an along-strike heterogeneity of a complexly deformed marine fore arc which had escaped attention in previous studies that only considered the structure along transects normal to the plate margin.

Description: Stem cell therapy is a promising new option for patients suffering from heart failure. Though many clinical studies show encouraging results, little is known about the signals which cause stem cells to home to diseased but not to healthy hearts. We hypothesized that aldosterone as one of the main players of heart failure functions as an attractant for progenitor cells and stimulates their migration. Stem cell antigen-1 (Sca-1) positive cells were isolated from the hearts of wild type FVB mice via magnetic cell sorting. The migration rate of the cells was determined using aldosterone as an attractant in a modified Boyden chamber (n = 5). Aldosterone led to a dose dependent increase in migration rate and this effect could be prevented by adding its blocker eplerenone. The mineralocorticoid receptor could be detected on Sca-1+ cells via western blot and immunofluorescence. Therefore, aldosterone seems to play a role in stem cell migration and there the effect is most likely mediated by the mineralocorticoid receptor. This article is protected by copyright. All rights reserved

Description: The Southern Ocean is a key region for global carbon uptake and is characterised by a strong seasonality with the annual CO 2 uptake being mediated by biological carbon draw-down in summer. Here, we show that the contribution of biology to CO 2 uptake will become even more important until 2100. This is the case even if biological production remains unaltered and can be explained by the decreasing buffer capacity of the ocean as its carbon content increases. The same amount of biological carbon draw-down leads to a more than twice as large reduction in CO 2(aq) concentration and hence to a larger CO 2 gradient between ocean and atmosphere that drives the gas-exchange. While the winter uptake south of 44°S changes little, the summer uptake increases largely and is responsible for the annual mean response. The combination of decreasing buffer capacity and strong seasonality of biological carbon draw-down introduces a strong and increasing seasonality in the anthropogenic carbon uptake.

Description: Rising atmospheric [CO 2 ], c a , is expected to affect stomatal regulation of leaf gas-exchange of woody plants, thus influencing energy fluxes as well as carbon (C), water and nutrient cycling of forests. Researchers have proposed various strategies for stomatal regulation of leaf gas-exchange that include maintaining a constant leaf internal [CO 2 ], c i , a constant drawdown in CO 2 ( c a - c i ), and a constant c i / c a . These strategies can result in drastically different consequences for leaf gas-exchange. The accuracy of Earth systems models depends in part on assumptions about generalizable patterns in leaf gas-exchange responses to varying c a . The concept of optimal stomatal behavior, exemplified by woody plants shifting along a continuum of these strategies, provides a unifying framework for understanding leaf gas-exchange responses to c a . To assess leaf gas-exchange regulation strategies, we analyzed patterns in c i inferred from studies reporting C stable isotope ratios ( δ 13 C) or photosynthetic discrimination (∆) in woody angiosperms and gymnosperms that grew across a range of c a spanning at least 100 ppm. Our results suggest that much of the c a -induced changes in c i / c a occurred across c a spanning 200 to 400 ppm. These patterns imply that c a - c i will eventually approach a constant level at high c a because assimilation rates will reach a maximum and stomatal conductance of each species should be constrained to some minimum level. These analyses are not consistent with canalization towards any single strategy, particularly maintaining a constant c i . Rather, the results are consistent with the existence of a broadly conserved pattern of stomatal optimization in woody angiosperms and gymnosperms. This results in trees being profligate water users at low c a , when additional water loss is small for each unit of C gain, and increasingly water-conservative at high c a , when photosystems are saturated and water loss is large for each unit C gain. This article is protected by copyright. All rights reserved.

Description: The δ 30 Si of biogenic silica ( δ 30 Si BSi ) in marine sediments is a promising proxy for the reconstruction of silicic acid utilization by diatoms in the geological past. The application of this proxy, however, requires an understanding of the modern δ 30 Si distributions and their controlling mechanisms. Here we present results from a modern climate simulation with a coupled ocean-sediment model that includes a prognostic formulation of biogenic silica production with concurrent silicon isotopic fractionation. In agreement with previous studies, biological fractionation combined with physical transport and mixing determine the oceanic distribution of simulated δ 30 Si. A new finding is a distinct seasonal cycle of δ 30 Si in the surface ocean, which is inversely related to that of silicic acid concentration and mixed layer depth. We also provide the first simulation results of sedimentary δ 30 Si, which reveal that (1) the δ 30 Si BSi distribution in the surface sediment reflects the exported δ 30 Si BSi signal from the euphotic zone and (2) the dissolution of biogenic silica in the sediment acts as a source of relatively light δ 30 Si into the bottom waters of the polar oceans, while it is a source of heavier δ 30 Si to the subtropical South Atlantic and South Pacific.