Potential climatic transitions with profound impact on Europe

We discuss potential transitions of six climatic subsystems with large-scale impact on Europe, sometimes denoted as tipping elements. These are the ice sheets on Greenland and West Antarctica, the Atlantic thermohaline circulation, Arctic sea ice, Alpine glaciers and northern hemisphere stratospheric ozone. Each system is represented by co-authors actively publishing in the corresponding field. For each subsystem we summarize the mechanism of a potential transition in a warmer climate along with its impact on Europe and assess the likelihood for such a transition based on published scientific literature. As a summary, the ‘tipping’ potential for each system is provided as a function of global mean temperature increase which required some subjective interpretation of scientific facts by the authors and should be considered as a snapshot of our current understanding.     

Climate change impacts on dryland cropping systems in the Central Great Plains,USA

Agricultural systems models are essential tools to assess potential climate change (CC) impacts on crop production and help guide policy decisions. In this study, impacts of projected CC on dryland crop rotations of wheat-fallow (WF), wheat-corn-fallow (WCF), and wheat-corn-millet (WCM) in the U.S. Central Great Plains (Akron, Colorado) were simulated using the CERES V4.0 crop modules in RZWQM2. The CC scenarios for CO₂, temperature and precipitation were based on a synthesis of Intergovernmental Panel on Climate Change (IPCC 2007) projections for Colorado. The CC for years 2025, 2050, 2075, and 2100 (CC projection years) were super-imposed on measured baseline climate data for 15–17 years collected during the long-term WF and WCF (1992–2008), and WCM (1994–2008) experiments at the location to provide inter-annual variability. For all the CC projection years, a decline in simulated wheat yield and an increase in actual transpiration were observed, but compared to the baseline these changes were not significant (p > 0.05) in all cases but one. However, corn and proso millet yields in all rotations and projection years declined significantly (p < 0.05), which resulted in decreased transpiration. Overall, the projected negative effects of rising temperatures on crop production dominated over any positive impacts of atmospheric CO₂ increases in these dryland cropping systems. Simulated adaptation via changes in planting dates did not mitigate the yield losses of the crops significantly. However, the no-tillage maintained higher wheat yields than the conventional tillage in the WF rotation to year 2075. Possible effects of historical CO₂ increases during the past century (from 300 to 380 ppm) on crop yields were also simulated using 96 years of measured climate data (1912–2008) at the location. On average the CO₂ increase enhanced wheat yields by about 30%, and millet yields by about 17%, with no significant changes in corn yields.     

AuPdFe ternary solution model and applications to understanding the fO2 of hydrous, high-pressure experiments

This study provides an experimental calibration of the equilibrium constant for AuPdFe alloys with Fe-bearing silicate melts. The ideal metal capsules for H₂O-bearing experiments are pure Au, because of its slow hydrogen diffusivity. However, above the melting point of Au, other materials must be used. The solution to this problem is to use AuPd alloy capsules. However, under most relevant fO₂ conditions, this alloy absorbs Fe from the coexisting silicate melt, thus changing the bulk composition of the experimental charge. This study combines previous work on the Au–Pd, Pd–Fe, and Au–Fe binary systems to develop a ternary thermodynamic solution model for AuPdFe. This solution model is used with experiments to calculate an equilibrium reaction coefficient for the FeO_melt → Fe_alloy + 1/2O₂ exchange reaction. Using a non-ideal ternary solution model, the fO₂ conditions of hydrous, piston cylinder experiments can be estimated by analyzing the sample capsule alloy and the coexisting liquid composition.     

The radiocarbon age of organic carbon in marine surface sediments

Long-term carbon cycling and climate change are strongly dependent on organic carbon (OC) burial in marine sediments. Radiocarbon (¹⁴C) has been widely used to constrain the sources, sinks, and processing of sedimentary OC. To elucidate the dominant controls on the radiocarbon content of total organic carbon (¹⁴C_TOC) accumulating in surface sediments we construct a box model that predicts ¹⁴C_TOC in the sediment mixed layer (measured as fraction modern, Fm). Our model defines three distinct OC pools (“degradable,” “semi-labile,” and “refractory”) and assumes that ¹⁴C_TOC flux to sediments is exclusively derived from surface ocean primary productivity, and hence follows a “generic” surface ocean dissolved inorganic carbon (DIC) bomb curve. Model predictions are compared to a set of 75 surface sediment samples, which span a wide geographic range and reflect diverse water column and depositional conditions, and for which sedimentation rate and mixed-layer depth are well characterized. Our model overestimates the Fm value for a majority (65%) of these sites, especially at shallow water depths and for sites characterized by depleted δ¹³C_TOC values. The model is most sensitive to sedimentation rate and mixed-layer depth. Therefore, slight changes to these parameters can lead to a match between modeled and measured Fm values at many sites. Yet, in some cases, we find that measured Fm values cannot be simulated without large and unrealistic changes to sedimentation rate and mixed-layer depth. These results point to sources of pre-aged OC to surface sediments and implicate soil-derived terrestrial OC, reworked marine OC, and/or anthropogenic carbon as important components of the organic matter present in surface sediments. This approach provides a valuable framework within which to explore controls on sedimentary organic matter composition and carbon burial over a range of spatial and temporal scales.     

Discovery of a Color-selected Quasar at z = 5.50

I show how the existing observational data on Local Group dwarf galaxies can be used to estimate the average star formation law during the first 3 Gyr of the history of the universe. I find that the observational data are consistent with the orthodox Schmidt law with a star formation efficiency of about 4% if the star formation is continuous (during the first 3 Gyr). The efficiency is proportionally higher if most of the gas in the dwarfs was consumed (and never replenished) in a short time interval well before the universe turned 3 Gyr.     

Spatial interpolation methods for nonstationary plume data

Plume interpolation consists of estimating contaminant concentrations at unsampled locations using the available contaminant data surrounding those locations. The goal of ground water plume interpolation is to maximize the accuracy in estimating the spatial distribution of the contaminant plume given the data limitations associated with sparse monitoring networks with irregular geometries. Beyond data limitations, contaminant plume interpolation is a difficult task because contaminant concentration fields are highly heterogeneous, anisotropic, and nonstationary phenomena. This study provides a comprehensive performance analysis of six interpolation methods for scatter-point concentration data, ranging in complexity from intrinsic kriging based on intrinsic random function theory to a traditional implementation of inverse-distance weighting. High resolution simulation data of perchloroethylene (PCE) contamination in a highly heterogeneous alluvial aquifer were used to generate three test cases, which vary in the size and complexity of their contaminant plumes as well as the number of data available to support interpolation. Overall, the variability of PCE samples and preferential sampling controlled how well each of the interpolation schemes performed. Quantile kriging was the most robust of the interpolation methods, showing the least bias from both of these factors. This study provides guidance to practitioners balancing opposing theoretical perspectives, ease-of-implementation, and effectiveness when choosing a plume interpolation method.     

A modular injection system, multilevel sampler, and manifold for tracer tests

Ground water injection and sampling systems were developed for bacterial transport experiments in both homogenous and heterogeneous unconsolidated, surficial aquifers. Two types of injection systems, a large single tank and a dynamic mixing tank, were designed to deliver more than 800 L of amended ground water to the aquifer over 12 hours, without altering the ground water temperature, pH, Eh, or dissolved gas composition. Two types of multilevel samplers (MLSs) were designed and installed. Permanent MLSs performed well for the homogenous surficial aquifer, but their installation procedure promoted vertical mixing, which could obfuscate experimental data obtained from vertically stratified, heterogeneous aquifers. A novel, removable MLS was designed to fit in 2- and 4-inch wells. Expandable O-rings between each sampling port hydraulically isolated each port for sample collection when a nut was tightened at the land surface. A low-cost vacuum manifold system designed to work with both MLS designs used 50 mL centrifuge tubes to efficiently sample 12 MLS ports with one peristaltic pump head. The integrated system was developed and used during four field campaigns over a period of three years. During each campaign, more than 3000 ground water samples were collected in less than one week. This system should prove particularly useful for ground water tracer, injection, and push-pull experiments that require high-frequency and/or high-density sampling.     

Fractional crystallization and mantle-melting controls on calc-alkaline differentiation trends

The phase relations of primitive magnesian andesites and basaltic andesites from the Mt. Shasta region, N California have been determined over a range of pressure and temperature conditions and H₂O contents. The experimental results are used to explore the influence of H₂O and pressure on fractional crystallization and mantle melting behavior in subduction zone environments. At 200-MPa H₂O-saturated conditions the experimentally determined liquid line of descent reproduces the compositional variation found in the Mt. Shasta region lavas. This calc-alkaline differentiation trend begins at the lowest values of FeO*/MgO and the highest SiO₂ contents found in any arc magma system and exhibits only a modest increase in FeO*/MgO with increasing SiO₂. We propose a two-stage process for the origin of these lavas. (1) Extensive hydrous mantle melting produces H₂O-rich (>4.5--6 wt% H₂O) melts that are in equilibrium with a refractory harzburgite (olivine + orthopyroxene) residue. Trace elements and H₂O are contributed from a slab-derived fluid and/or melt. (2) This mantle melt ascends into the overlying crust and undergoes fractional crystallization. Crustal-level differentiation occurs under near-H₂O saturated conditions producing the distinctive high SiO₂ and low FeO*/MgO characteristics of these calc-alkaline andesite and dacite lavas. In a subset of Mt. Shasta region lavas, magnesian pargasitic amphibole provides evidence of high pre-eruptive H₂O contents (>10 wt% H₂O) and lower crustal crystallization pressures (800 MPa). Igneous rocks that possess major and trace element characteristics similar to those of the Mt. Shasta region lavas are found at Adak, Aleutians, Setouchi Belt, Japan, the Mexican Volcanic Belt, Cook Island, Andes and in Archean trondhjemite--tonalite--granodiorite suites (TTG suites). We propose that these magmas also form by hydrous mantle melting.Editorial responsibility: J. Hoefs