The null hypothesis: globally steady rates of erosion,weathering fluxes and shelf sediment accumulation during Late Cenozoic mountain uplift and glaciation

At the largest time and space scales, the pace of erosion and chemical weathering is determined by tectonic uplift rates. Deviations from equilibrium arise from the transient response of landscape denudation to climatic and tectonic perturbations. We posit that the constraint of mass balance, however, makes it unlikely that such disequilibrium persists at the global scale over millions of years, as has been proposed for late Cenozoic erosion. We synthesize weathering fluxes, global sedimentation rates, sediment yields and tectonic motions to show a remarkable constancy in the pace of Earth‐surface evolution over the last 10 Ma and support the null hypothesis – that global rates of landscape change have remained constant over this time period, despite global climate change and mountain building events. This work undermines the hypothesis that increased weathering due to mountain building or climate change was the primary agent for a decrease in global temperatures.     

Carbon loss by water erosion in drylands: implications from a study of vegetation change in the south‐west USA

Soil organic carbon (SOC) is an important component of the global carbon cycle yet is rarely quantified adequately in terms of its spatial variability resulting from losses of SOC due to erosion by water. Furthermore, in drylands, little is known about the effect of widespread vegetation change on changes in SOC stores and the potential for water erosion to redistribute SOC around the landscape especially during high‐magnitude run‐off events (flash floods). This study assesses the change in SOC stores across a shrub‐encroachment gradient in the Chihuahuan Desert of the south‐west USA. A robust estimate of SOC storage in surface soils is presented, indicating that more SOC is stored beneath vegetation than in bare soil areas. In addition, the change in SOC storage over a shrub‐encroachment gradient is shown to be nonlinear and highly variable within each vegetation type. Over the gradient of vegetation change, the heterogeneity of SOC increases, and newer carbon from C₃ plants becomes dominant. This increase in the heterogeneity of SOC is related to an increase in water erosion and SOC loss from inter‐shrub areas, which is self‐reinforcing. Shrub‐dominated drylands lose more than three times as much SOC as their grass counterparts. The implications of this study are twofold: (1) quantifying the effects of vegetation change on carbon loss via water erosion and the highly variable effects of land degradation on soil carbon stocks is critical. (2) If landscape‐scale understanding of carbon loss by water erosion in drylands is required, studies must characterize the heterogeneity of ecosystem structure and its effects on ecosystem function across ecotones subject to vegetation change. Copyright © 2013 John Wiley & Sons, Ltd.     

Subglacial till behaviour derived from in situ wireless multi-sensor subglacial probes: Rheology,hydro-mechanical interactions and till formation

The rheology and hydro-mechanical interactions at the ice–bed interface form an important component of the glacier system, influencing glacier dynamics and the formation of till. We demonstrate that the sand-rich till at Briksdalsbreen in Norway, undergoes deformation throughout the year. On the bulk rheology scale, till deformation exhibits elastic behaviour during the winter, when water pressures are low; and linear viscous behaviour after a critical yield stress of 35 kPa, when water pressures are high during the spring and summer. On the clast and matrix scale, low water pressures, correspond with high case stress variability and till temperatures. Meltwater driven, stick-slip, glacier velocity increases were transmitted through a relatively strong till grain network, causing brittle deformation. Intermediate water pressures, during late summer were linked to intermediate case stress variability and high till temperatures associated with the heat generated from stick-slip motion. High water pressures in the till were associated with low case stress variability and low, meltwater controlled, till temperatures, and occurred in the spring and autumn. Once the till was saturated, the ductile till absorbed any stick-slip velocity increases. We discuss, with examples, the different till forming processes associated with these changing conditions, demonstrating that the resultant till will represent a complex amalgamation of all of these processes.     

Nitrogen and phosphorus dynamics during runoff events over a transition from grassland to shrubland in the south‐western United States

During the last 150 years, land degradation across the semi‐arid grasslands of the south‐western United States has been associated with an increase in runoff and erosion. Concurrent with this increase in runoff and erosion is a loss of nitrogen (N) and phosphorus (P), which are plant‐essential nutrients. This study investigates the runoff‐driven redistribution and loss of dissolved and particulate‐bound N and P that occurs during natural runoff events over a trajectory of degradation, from grassland to degraded shrubland, in central New Mexico. Runoff‐driven nutrient dynamics were monitored at four stages over a transition from grassland to shrubland, for naturally occurring rainfall events over 10 × 30 m bounded runoff plots. Results show that particulate‐bound forms of N and P are responsible for most of N and P lost from the plots due to erosion occurring during runoff events. Results suggest that for high‐magnitude rainfall events, the output of N and P from the plots may greatly exceed the amount input into the plots, particularly over shrub‐dominated plots where erosion rates are higher. As these results only become apparent when monitoring these processes over larger hillslope plots, it is important to recognize that processes of nutrient cycling related to the islands of fertility hypothesis may have previously been overstated when observed only at smaller spatial scales. Thus, the progressive degradation of semi‐arid grassland ecosystems across the south‐western United States and other semi‐arid ecosystems worldwide has the potential to affect N and P cycling significantly through an increase in nutrient redistribution and loss in runoff. Copyright © 2010 John Wiley & Sons, Ltd.     

Effects of irradiance on benthic and water column processes in a Gulf of Mexico estuary: Pensacola Bay, Florida, USA

We examined the effect of light on water column and benthic fluxes in the Pensacola Bay estuary, a river-dominated system in the northeastern Gulf of Mexico. Measurements were made during the summers of 2003 and 2004 on 16 dates distributed along depth and salinity gradients. Dissolved oxygen fluxes were measured on replicate sediment and water column samples exposed to a gradient of photosynthetically active radiation. Sediment inorganic nutrient (NH₄⁺, NO₃⁻, PO₄³⁻) fluxes were measured. The response of dissolved oxygen fluxes to variation in light was fit to a photosynthesis–irradiance model and the parameter estimates were used to calculate daily integrated production in the water column and the benthos. The results suggest that shoal environments supported substantial benthic productivity, averaging 13.6 ± 4.7 mmol O₂ m⁻² d⁻¹, whereas channel environments supported low benthic productivity, averaging 0.5 ± 0.3 mmol O₂ m⁻² d⁻¹ (±SE). Estimates of baywide microphytobenthic productivity ranged from 8.1 to 16.5 mmol O₂ m⁻² d⁻¹, comprising about 16–32% of total system productivity. Benthic and water column dark respiration averaged 15.2 ± 3.2 and 33.6 ± 3.7 mmol O₂ m⁻² d⁻¹, respectively Inorganic nutrient fluxes were generally low compared to relevant estuarine literature values, and responded minimally to light exposure. Across all stations, nutrient fluxes from sediments to the water column averaged 1.11 ± 0.98 mmol m⁻² d⁻¹ for NH₄⁺, 0.58 ± 1.08 mmol m⁻² d⁻¹ for NO₃⁻, 0.01 ± 0.09 mmol m⁻² d⁻¹ for PO₄³⁻. The results of this study illustrate how light reaching the sediments is an important modulator of benthic nutrient and oxygen dynamics in shallow estuarine systems.     

Spatial variations of DMS, DMSP and phytoplankton in the Bay of Bengal during the summer monsoon 2001

Data on the distribution of dimethylsulphide (DMS) and dimethylsulphoniopropionate (DMSP) in relation to phytoplankton abundance in different oceanic environments is important to understand the biogeochemistry of DMS, which plays an important role in the radiation balance of the earth. During the summer monsoon of 2001 measurements were made for DMS and DMSPt (total DMSP) together with related biological parameters in the Bay of Bengal. Both DMS and DMSPt were restricted to the upper 40 m of the water column. Diatoms accounted for more than 95% of the phytoplankton and were the major contributors to the DMS and DMSPt pool. The mean concentration of DMS in the upper 40 m was observed to be around 1.8+/-1.9 nM in the study area, while DMSPt concentrations varied between 0.7 nM and 40.2 nM with a mean of 10.4+/-8.2 nM. The observed lower DMSPt in the northern Bay in spite of higher mean primary productivity, chlorophyll a and phytoplankton cell counts seemed to result from grazing. Though salinity divides the Bay into different biogeochemical provinces there is no relation between salinity and DMS or DMSPt. On the other hand DMS was linearly related to chlorophyll a:phaeopigments ratio. The results suggest the need for deeper insight into the role of diatoms in the biogeochemical cycling of DMS.     

Groundwater resources and quality in northeastern Jordan: safe yield and sustainability

Over-abstraction of groundwater is one concern of the Badia Research and Development Programme. The upper aquifer of the Azraq basin forms the largest resource of good-quality water, but current abstraction exceeds both average recharge and the safe yield of the aquifer, which is over-exploited. Although there has been no deterioration in water quality and only minor drawdown, the springs at Azraq have dried up, with severe, undesirable environmental impacts. Total abstraction of 20 × 10⁶ m³ yr⁻¹ appears to be sustainable and would allow some springflow, but this leaves a shortfall of 40 × 10⁶ m³ yr⁻¹ for domestic supply and agriculture.     

Suspended sediment transport,sedimentation, and resuspension in Lake Houston,Texas: Implications for water quality

Lake Houston is a man-made reservoir located northeast of Houston, Texas. The purpose of this investigation was to document suspended sediment transport, sedimentation, and resuspension in the lake with a view towards estimating the influence of sedimentation on water quality. Sediment traps were placed in strategic locations in the lake to collect suspended sediments. Samples were analyzed for bulk density, grain size, organic carbon, and a number of trace elements. These data were analyzed along with meteorological data to examine those factors which regulate suspended sediment input and dispersal, and the role of suspended sediments in controlling water quality within the lake. Sediment input to the lake depends primarily on the intensity of rainfall in the watershed. Sediment movement within the lake is strongly influenced by wave activity, which resuspends sediments from shallow areas, and by wind-driven circulation. The increased residence time of suspended sediments due to resuspension allows greater decomposition of organic matter and the release of several trace elements from sediments to the water column. Virtually all samples from sediment traps suspended between 1 and 5 m above the lake bottom contain medium to coarse silt, and even some very fine sand-sized material. This implies that circulation in Lake Houston is periodically intense enough to transport this size material in suspension. During winter, northerly winds with sustained velocities of greater than 5 m/sec provide the most suitable condition for rapid (<1 d) transport of suspended sediment down the length of the lake. Fluctuations in current velocities and the subsequent suspension/deposition of particles may explain variations in the abundance of coliform bacteria in Lake Houston.