The effects of aggressive mitigation on steric sea level rise and sea ice changes

With an increasing political focus on limiting global warming to less than 2 °C above pre-industrial levels it is vital to understand the consequences of these targets on key parts of the climate system. Here, we focus on changes in sea level and sea ice, comparing twenty-first century projections with increased greenhouse gas concentrations (using the mid-range IPCC A1B emissions scenario) with those under a mitigation scenario with large reductions in emissions (the E1 scenario). At the end of the twenty-first century, the global mean steric sea level rise is reduced by about a third in the mitigation scenario compared with the A1B scenario. Changes in surface air temperature are found to be poorly correlated with steric sea level changes. While the projected decreases in sea ice extent during the first half of the twenty-first century are independent of the season or scenario, especially in the Arctic, the seasonal cycle of sea ice extent is amplified. By the end of the century the Arctic becomes sea ice free in September in the A1B scenario in most models. In the mitigation scenario the ice does not disappear in the majority of models, but is reduced by 42 % of the present September extent. Results for Antarctic sea ice changes reveal large initial biases in the models and a significant correlation between projected changes and the initial extent. This latter result highlights the necessity for further refinements in Antarctic sea ice modelling for more reliable projections of future sea ice.     

Impact of the LMDZ atmospheric grid configuration on the climate and sensitivity of the IPSL-CM5A coupled model

The IPSL-CM5A climate model was used to perform a large number of control, historical and climate change simulations in the frame of CMIP5. The refined horizontal and vertical grid of the atmospheric component, LMDZ, constitutes a major difference compared to the previous IPSL-CM4 version used for CMIP3. From imposed-SST (Sea Surface Temperature) and coupled numerical experiments, we systematically analyze the impact of the horizontal and vertical grid resolution on the simulated climate. The refinement of the horizontal grid results in a systematic reduction of major biases in the mean tropospheric structures and SST. The mid-latitude jets, located too close to the equator with the coarsest grids, move poleward. This robust feature, is accompanied by a drying at mid-latitudes and a reduction of cold biases in mid-latitudes relative to the equator. The model was also extended to the stratosphere by increasing the number of layers on the vertical from 19 to 39 (15 in the stratosphere) and adding relevant parameterizations. The 39-layer version captures the dominant modes of the stratospheric variability and exhibits stratospheric sudden warmings. Changing either the vertical or horizontal resolution modifies the global energy balance in imposed-SST simulations by typically several W/m² which translates in the coupled atmosphere-ocean simulations into a different global-mean SST. The sensitivity is of about 1.2 K per 1 W/m² when varying the horizontal grid. A re-tuning of model parameters was thus required to restore this energy balance in the imposed-SST simulations and reduce the biases in the simulated mean surface temperature and, to some extent, latitudinal SST variations in the coupled experiments for the modern climate. The tuning hardly compensates, however, for robust biases of the coupled model. Despite the wide range of grid configurations explored and their significant impact on the present-day climate, the climate sensitivity remains essentially unchanged.     

Platinum-group element distribution in base-metal sulfides of the UG2 chromitite,Bushveld Complex,South Africa—a reconnaissance study

Two drill cores of the UG2 chromitite from the eastern and western Bushveld Complex were studied by whole-rock analysis, ore microscopy, SEM/Mineral Liberation Analysis (MLA), and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) analysis. The top and base of the UG2 main seam have the highest bulk-rock Pd and Pt concentrations. Sulfides mostly occur as aggregates of pentlandite, chalcopyrite, and rare pyrrhotite and pyrite or as individual grains associated mostly with chromite grains. In situ LA-ICP-MS analyses reveal that pentlandite carries distinctly elevated platinum-group element (PGE) contents. In contrast, pyrrhotite and chalcopyrite contain very low PGE concentrations. Pentlandite shows average maximum values of 350–1,000 ppm Pd, 200 ppm Rh, 130–175 ppm Ru, 20 ppm Os, and 150 ppm Ir, and is the principal host of Pd and Rh in the studied ores of the UG2. Mass balance calculations were conducted for samples representing the UG2 main seam of the drill core DT46, eastern Bushveld. Pentlandite consistently hosts elevated contents of the whole-rock Pd (up to 55 %) and Rh (up to 46 %), and erratic contents of Os (up to 50 %), Ir (2 to 17 %), and Ru (1–39 %). Platinum-group mineral (PGM) investigations support these mass balance results; most of the PGM are Pt-dominant such as braggite/cooperite and Pt-Fe alloys or laurite (carrying elevated concentrations of Os and Ir). Palladium and Rh-bearing PGM are rare. Both PGE concentrations and their distribution in base-metal sulfides (BMS) in the UG2 largely resemble that of the Merensky Reef, as most of the Pd and Rh are incorporated in pentlandite, whereas pyrrhotite, chalcopyrite, and pyrite are almost devoid of PGE.     

Be exposure dating of river terraces at the southern mountain front of the Dzungarian Alatau (SE Kazakhstan) reveals rate of thrust faulting over the past ~ 400 ka

The mountain belts of the Dzungarian Alatau (SE Kazakhstan) and the Tien Shan are part of the actively deforming India–Asia collision zone but how the strain is partitioned on individual faults remains poorly known. Here we use terrace mapping, topographic profiling, and ¹⁰Be exposure dating to constrain the slip rate of the 160-km-long Usek thrust fault, which defines the southern front of the Dzungarian Alatau. In the eastern part of the fault, where the Usek River has formed five terraces (T₁–T₅), the Usek thrust fault has vertically displaced terrace T₄ by 132 ± 10 m. At two sites on T₄, exposure dating of boulders, amalgamated quartz pebbles, and sand from a depth profile yielded ¹⁰Be ages of 366 ± 60 ka and 360 ^+ 77/_− 48 ka (both calculated for an erosion rate of 0.5 mm/ka). Combined with the vertical offset and a 45–70° dip of the Usek fault, these age constraints result in vertical and horizontal slip rates of ~ 0.4 and ~ 0.25 mm/a, respectively. These rates are below the current resolution of GPS measurements and highlight the importance of determining slip rates for individual faults by dating deformed landforms to resolve the pattern of strain distribution across intracontinental mountain belts.     

Process dependence of grain size distributions in rock avalanche deposits

Rock avalanches are a form of hazardous long-runout landslide and leave fragmented deposits of complex sedimentology that, if studied in detail, can provide insight into their emplacement processes. Complexity arises due to the myriad overlapping factors known to contribute to the final deposit fabric, such as source structures, lithology (i.e. material properties), topographic feedback, substrate interaction and emplacement processes (i.e. internal factors), as well as our reliance on (un)suitable exposures. Herein, we present sedimentological data from two carbonate rock avalanche deposits (Tschirgant in Austria and Flims in Switzerland), where changes in lithology can be eliminated from the causal equation due to their largely mono-mineralic composition. We further eliminated the effects of external influences such as topography or substrate interactions by detailed facies mapping of the deposit interior. Since sedimentary properties locally vary within less than 1-m² outcrop area, emplacement processes are the only causes that remain to explain the different fabrics. Characteristic (fractal) grain size distributions of three distinctive sub-facies in the interior of these, and other, rock avalanche deposits—jigsaw-fractured, fragmented, and shear zone facies—can be linked to specific processes acting during emplacement. We suggest that a heterogeneously distributed and progressively increasing particle breakage in the moving granular mass best explains the ranges of fractal dimensions and associated features for the respective sub-facies, from simple breakage along pre-existing planes, through dynamic fragmentation which locally minimises coordination number, to zones of shear concentration. No exotic emplacement mechanisms (such as air-layer lubrication or fluidised substrates) are required to produce these features; continued, heterogeneous degrees of fragmentation of an initially intact source rock best explains the sedimentary record of rock avalanches.