Eruptive history and magmatic evolution of the 1.9 kyr Plinian dacitic Chiltepe Tephra from Apoyeque volcano in west-central Nicaragua

The youngest dacitic Plinian eruption in west-central Nicaragua, forming the 18 km³ Chiltepe Tephra (CT), occurred about nineteen hundred years ago at Apoyeque stratovolcano, which dominates the Chiltepe volcanic complex 15 km north of the capital Managua, where the CT is 2 m thick. We have traced the CT from its proximal facies at the crater rim, through the medial facies in the lowlands around Apoyeque, and to the distal facies up to 550 km offshore in the Pacific. While medial and distal facies consist of widespread Plinian fall deposits, the proximal facies reveals the complexity of this eruption, which we divide into four phases (I–IV). Interaction of rising magma with a pre-existing crater lake generated the phreatomagmatic opening phase I of the eruption, which produced ash fall with accretionary lapilli. Phase II marked a rapid change to persistent magmatic activity that yielded several large Plinian eruptions, declining through a period of unstable eruption conditions, followed by a short hiatus. Phase III began with unstable conditions, probably as a result of eastward migration and widening of the vent, leading to a second period of Plinian eruptions with three major events reaching magma discharge rates five times larger than those of phase II. Phase III again declined through unstable eruption conditions before magmatic activity terminated. Numerous explosions in the shallow hydrothermal system during the final phase IV resulted in the formation of a phreatic tuff ring on the rim of Apoyeque crater. The white, highly-vesicular, dacitic CT pumice contains plagioclase (An_45–68), orthopyroxene, clinopyroxene, and minor hornblende, apatite and titanomagnetite phenocrysts. A very subordinate fraction of gray pumice has the highest crystal content, the least evolved bulk-rock, but the most evolved matrix-glass composition. The CT dacite has two unusual compositional features: (1) all white dacite has the same melt (matrix-glass) composition such that variations in bulk-rock compositions (64–68 wt% SiO₂) simply reflect different phenocryst contents of 10–35%, interpreted as the result of gradual phenocryst settling in the magma chamber. (2) Abundant olivine crystals with a bimodal distribution in Mg# (modes at Mg# = 0.75 and Mg# = 0.8) are dispersed throughout the erupted dacite. These are clearly out of equilibrium with the dacitic melt and are interpreted as xenocrysts derived from the basaltic Nejapa-Miraflores volcanic lineament that intersects the Chiltepe volcanic complex and was contemporaneously active. Thermobarometric estimates place the dacitic CT magma reservoir in the upper crust (<250 MPa), with a temperature of about 890°C and about 5 wt% water dissolved in the melt. Comparing water and chlorine contents with respective solubility models suggests that volatile degassing began in the magma reservoir and triggered the CT eruption. From the vertical compositional variation pattern of the CT we deduce that the conduit tapped the magma chamber not at the top but from the side, at some deeper level, and that subsequent magma withdrawal was governed by both variations in discharge rate and possible upward migration and/or widening of the conduit entrance.     

Improved GRACE kinematic orbit determination using GPS receiver clock modeling

Kinematic positions of Low Earth Orbiters based on GPS tracking are frequently used as pseudo-observations for single satellite gravity field determination. Unfortunately, the accuracy of the satellite trajectory is partly limited because the receiver synchronization error has to be estimated along with the kinematic coordinates at every observation epoch. We review the requirements for GPS receiver clock modeling in Precise Point Positioning (PPP) and analyze its impact on kinematic orbit determination for the two satellites of the Gravity Recovery and Climate Experiment (GRACE) mission using both simulated and real data. We demonstrate that a piecewise linear parameterization can be used to model the ultra-stable oscillators that drive the GPS receivers on board of the GRACE satellites. Using such a continuous clock model allows position estimation even if the number of usable GPS satellites drops to three and improves the robustness of the solution with respect to outliers. Furthermore, simulations indicate a potential accuracy improvement of the satellite trajectory of at least 40 % in the radial direction and up to 7 % in the along-track and cross-track directions when a 60-s piecewise linear clock model is estimated instead of epoch-wise independent receiver clock offsets. For PPP with real GRACE data, the accuracy evaluation is hampered by the lack of a reference orbit of significantly higher accuracy. However, comparisons with a smooth reduced-dynamic orbit indicate a significant reduction of the high-frequency noise in the radial component of the kinematic orbit.     

Biomarker and compound-specific δ<Superscript>13</Superscript>C evidence for changing environmental conditions and carbon limitation at Lake Koucha,eastern Tibetan Plateau

A sediment core from Lake Koucha (eastern Tibetan Plateau) was investigated using organic biomarkers and their stable carbon isotope signatures. The correlation between TOC content, total amount of aquatic macrophyte-derived n-alkanes (e.g. nC₂₃) and δ¹³C values of TOC and nC₂₃ indicates that Lake Koucha was macrophyte-dominated before 8 cal ka BP. Shortly after the lake turned from a saline to a freshwater system at 7.2 cal ka BP, a variety of algal and bacterial markers such as hopanoids and isoprenoids emerged, of which phytane, pentamethylicosene (PMI), moretene and diploptene are particularly abundant. Phytane and PMI show different isotopic signals (≈−18 and ≈−28‰, respectively), which indicates that they originated from different sources. Phytane may have been derived from cyanobacteria, while methanogenic archaea may be the source of PMI. The isotopic depletion of diploptene and moretene (≈−60‰) indicates the presence of methanotrophs. After 6.1 cal ka BP, the saturated C₂₀ highly branched isoprenoid (HBI) became the dominant constituent of the aliphatic hydrocarbon fraction. Such dominance has rarely been reported in lacustrine environments, and indicates a strong presence of algae (most likely diatoms) or cyanobacteria. At 4.7 cal ka BP, the appearance of an unsaturated C₂₅ HBI, which is a specific biomarker for diatoms, was noted. Furthermore, the level of nC₁₇-alkane was observed to increase in abundance in the uppermost two samples. These results suggest that the lake was phytoplankton-dominated during the last 6.1 ka. Relatively low biomarker concentrations and δ¹³C values at 6.0, 3.1 and 1.8 cal ka BP indicate the occurrence of cool periods, which is in agreement with inferences from other locations on the Tibetan Plateau. The δ¹³C values of nC₂₃ range from −23.5 to −12.6‰, with high values at the peak of macrophyte abundance at ca. 11 cal ka BP and at the phytoplankton maximum between ca 6.1 and 2.8 cal ka BP. Thus, aquatic macrophyte-derived mid-chain n-alkanes have been found to be excellent indicators of carbon-limiting conditions, which lead to the assimilation of isotopically-enriched carbon species. The limitation of carbon sources could be a localized phenomenon occurring in dense plant stands (as in the older section of the core), or it may be induced by high primary productivity (as in the younger section). Since the δ¹³C value of the inorganic carbon source may vary, the offset between the δ¹³C values of nC₂₃ and TIC could serve as a more precise proxy for carbon-limiting conditions in lacustrine environments, which could in turn be interpreted with respect to lacustrine paleo-productivity.     

Practical Use of Computationally Frugal Model Analysis Methods

Three challenges compromise the utility of mathematical models of groundwater and other environmental systems: (1) a dizzying array of model analysis methods and metrics make it difficult to compare evaluations of model adequacy, sensitivity, and uncertainty; (2) the high computational demands of many popular model analysis methods (requiring 1000's, 10,000 s, or more model runs) make them difficult to apply to complex models; and (3) many models are plagued by unrealistic nonlinearities arising from the numerical model formulation and implementation. This study proposes a strategy to address these challenges through a careful combination of model analysis and implementation methods. In this strategy, computationally frugal model analysis methods (often requiring a few dozen parallelizable model runs) play a major role, and computationally demanding methods are used for problems where (relatively) inexpensive diagnostics suggest the frugal methods are unreliable. We also argue in favor of detecting and, where possible, eliminating unrealistic model nonlinearities—this increases the realism of the model itself and facilitates the application of frugal methods. Literature examples are used to demonstrate the use of frugal methods and associated diagnostics. We suggest that the strategy proposed in this paper would allow the environmental sciences community to achieve greater transparency and falsifiability of environmental models, and obtain greater scientific insight from ongoing and future modeling efforts.     

Constraints on the angular distribution of satellite galaxies about spiral hosts

We present, using a novel technique, a study of the angular distribution of satellite galaxies around a sample of isolated, blue host galaxies selected from the sixth data release of the Sloan Digital Sky Survey. As a complement to previous studies, we subdivide the sample of galaxies into bins of differing inclination and use the systematic differences that would exist between the different bins as the basis for our approach. We parametrize the cumulative distribution function of satellite galaxies and apply a maximum likelihood, Monte Carlo technique to determine allowable distributions, which we show as an exclusion plot. We find that the allowed distributions of the satellites of spiral hosts are very nearly isotropic. We outline our formalism and our analysis and discuss how this technique may be refined for future studies and future surveys.     

A spatially explicit assessment of current and future hotspots of hunger in Sub-Saharan Africa in the context of global change

Hunger knows no boundaries or borders. While much research has focused on undernutrition on a national scale, this report evaluates it at subnational levels for Sub-Saharan Africa (SSA) to pinpoint hotspots where the greatest challenges exist. Undernutrition is assessed with a spatial resolution of 30 arc-minutes by investigating anthropometric data on weight and length of individuals. The impact of climate change on production of six major crops (cassava, maize, wheat, sorghum, rice and millet) is analyzed with a GIS-based Environmental Policy Integrated Climate (GEPIC) model with the same spatial resolution. Future hotspots of hunger are projected in the context of the anticipated climate, social, economic, and bio-physical changes. The results show that some regions in northern and southwestern Nigeria, Sudan and Angola with a currently high number of people with undernutrition might be able to improve their food security situation mainly through increasing purchasing power. In the near future, regions located in Ethiopia, Uganda, Rwanda and Burundi, southwestern Niger, and Madagascar are likely to remain hotspots of food insecurity, while regions located in Tanzania, Mozambique and the Democratic Republic of Congo might face more serious undernutrition. It is likely that both the groups of regions will suffer from lower capacity of importing food as well as lower per capita calorie availability, while the latter group will probably have sharper reduction in per capita calorie availability. Special attention must be paid to the hotspot areas in order to meet the hunger alleviation goals in SSA.     

Nonlinear Flow Process: A New Package to Compute Nonlinear Flow in MODFLOW

A new MODFLOW package (Nonlinear Flow Process; NLFP) simulating nonlinear flow following the Forchheimer equation was developed and implemented in MODLFOW‐2005. The method is based on an iterative modification of the conductance calculated and used by MODFLOW to obtain an effective Forchheimer conductance. The package is compatible with the different layer types, boundary conditions, and solvers as well as the wetting capability of MODFLOW. The correct implementation is demonstrated using four different benchmark scenarios for which analytical solutions are available. A scenario considering transient flow in a more realistic setting and a larger model domain with a higher number of cells demonstrates that NLFP performs well under more complex conditions, although it converges moderately slower than the standard MODFLOW depending on the nonlinearity of flow. Thus, this new tool opens a field of opportunities to groundwater flow simulation with MODFLOW, especially for core sample simulation or vuggy karstified aquifers as well as for nonlinear flow in the vicinity of pumping wells.     

The influence of leaf litter diversity and soil fauna on initial soil erosion in subtropical forests

Although the protective role of leaf litter cover against soil erosion is known for a long time, little research has been conducted on the processes involved. Moreover, the impact of soil meso‐ and macrofauna within the litter layer on erosion control is not clear. To investigate how leaf litter cover and diversity as well as meso‐ and macrofauna influence sediment discharge in subtropical forest ecosystems, a field experiment has been carried out in Southeast China. A full‐factorial random design with 96 micro‐scale runoff plots and 7 domestic leaf species was established and erosion was triggered by a rainfall simulator. Our results demonstrate that leaf litter cover protects soil from erosion (?82 % sediment discharge on leaf covered plots) by rainfall and this protection is removed as litter decomposes. The protective effect is influenced by the presence or absence of soil meso‐ and macrofauna. Fauna presence increases soil erosion rates significantly by 58 %, while leaf species diversity shows a non‐significant negative trend. We assume that the faunal effect arises from arthropods slackening and processing the soil surface as well as fragmenting and decomposing the protecting leaf litter covers. Even though the diversity level did not show a significant influence, single leaf species in monocultures show rather different impacts on sediment discharge and thus, erosion control. In our experiment, runoff plots with leaf litter from Machilus thunbergii showed the highest sediment discharge (68.0 g m^?2) whereas plots with Cyclobalanopsis glauca showed the smallest rates (7.9 g m^?2). Copyright © 2015 John Wiley & Sons, Ltd.