Patterns of grain‐size dependent sediment transport in low‐ordered,ephemeral channels

Sediment data were analyzed to determine grain‐size dependant factors affecting sediment transport in a low‐ordered, ephemeral watershed. Sediment and flow samples were collected during 22 flow events at the outlet of a 4·53 ha sub‐watershed within the Walnut Gulch Experimental Watershed in south‐eastern Arizona. Measured concentrations ranged from 4191 to 115 045 mg l^?1 and included grain sizes up to 8·0 mm in diameter. Two grain‐size dependent transport patterns were observed, that of the finer grain‐size fraction (approximately < 0·25 mm) and that of a coarser grain‐size fraction (approximately ≥ 0·25 mm). The concentration of the fine fraction decreased with flow duration, peaking near the beginning of a flow event and declining thereafter. The concentration of the fine fraction showed slight trends with season and recovery period. The concentration of the coarse fraction displayed a slight negative trend with instantaneous discharge and was not correlated with event duration. These patterns typically produced a condition where the majority of the fine fraction of the sediment yield was evacuated out of the watershed before the hydrograph peak while the majority of the coarser sediment was evacuated during the falling limb of the hydrograph. Each grain‐size dependent transport pattern was likely influenced by the source of the associated sediment. At the flow event time scale, the fines were primarily wash load, supplied from the hillslopes and the coarser grains were entrained from the channel bed. Because transport patterns differ based on grain size, attempts to define the total sediment concentration and sediment yield by the behavior of a single grain‐size fraction may lead to erroneous results, especially when a large range of sediment grain sizes are present. Copyright © 2010 John Wiley & Sons, Ltd.     

Arbuscular mycorrhizal fungi on developing islands within a dynamic river floodplain: an investigation across successional gradients and soil depth

Most vascular plants form symbioses with mycorrhizal fungi that associate with roots and provide nutrients to hosts in exchange for carbohydrates, as well as serve a range of other functions. Mycorrhizal fungi have been studied extensively in upland ecosystems, but we know less about their ecology at aquatic-terrestrial interfaces, especially their distributions at depth. Our objectives were to determine whether abundances of arbuscular mycorrhizal fungi (AMF) change as floodplain islands develop and to describe vertical distributions of AMF from the forest floor to the water table along the freely flowing Tagliamento River in northeastern Italy. We sampled surface sediments (0–10 cm) from three stages of island development on the floodplain—fresh deposits, pioneer islands, and established islands. We also sampled sediments vertically (0–150 cm) from the ground surface to the water table on an established island. We characterized abundance of AMF propagules (colonized roots, spores, and hyphae) within sediments. Roots available to host fungi were absent on fresh deposits; however, some viable spores and hyphae were available at these sites. Pioneer and established islands each had similar hyphal lengths (~860 cm cm⁻³) and colonized root lengths (~3 cm cm⁻³). Abundance of spores increased from depositional (3.5 ± 0.9 (±SE) cm⁻³) to pioneer (17 ± 6.1 cm⁻³) to established (32 ± 6.3 cm⁻³) islands. On an established island, AMF propagules were present at all depths sampled, including at the water table, providing first documentation of these symbionts to such depths in a riparian setting. Mycorrhizal fungi likely link aquatic and terrestrial habitats by connecting plants, soil, and ground water and may influence nutrient transfers among these subsystems.     

The wave climate of Liverpool Bay—observations and modelling

Directional wave measurements have been made in Liverpool Bay by means of wave buoys and acoustic instruments within the footprint of a phased-array high frequency (HF) radar system, which measures currents and waves. Several years of data have now been collected and are supplemented by an 11-year wave model hindcast. Wave parameters have been derived from the various instruments and compared: the directional waverider buoy is taken to provide the ground truth, confirming the good observations obtained from the ADCP; the HF radar wave data have a positive bias, while the model data have a negative bias. The variation of wave climate over various time-scales from seasonal and inter-annual to inter-decadal is examined. Significant wave–current interactions may occur in this area of shallow water and high tidal range and the measurements provide a good test of coupled hydrodynamic-wave models. The waves are mainly fetch-limited: largest events are due to depressions, which track across the UK from SW, generating westerly and WNW winds in the right rear quadrant. Hence, the future extreme wave events will be closely related to future North Atlantic storm tracks. Projections of 50-year return period wave heights differ between different instruments and model datasets. The future wave climate of Liverpool Bay is not expected to change much from the present day; although a slight increase in the severity of the most extreme events is projected, the frequency of extreme wind and wave events in general is slightly reduced. There is evidence for variability on decadal time-scales, with some correlation with the North Atlantic oscillation.     

Particulate sulfur species in the water column of the Cariaco Basin

The biogeochemistry of iron sulfide minerals in the water column of the Cariaco Basin was investigated in November 2007 (non-upwelling season) and May 2008 (upwelling season) as part of the on-going CARIACO (CArbon Retention In A Colored Ocean) time series project. The concentrations of particulate sulfur species, specifically acid volatile sulfur (AVS), greigite, pyrite, and particulate elemental sulfur, were determined at high resolution near the O₂/H₂S interface. In November 2007, AVS was low throughout the water column, with the highest concentration at the depth where sulfide was first detected (260 m) and with a second peak at 500 m. Greigite, pyrite, and particulate elemental sulfur showed distinct concentration maxima near the interface. In May 2008, AVS was not detected in the water column. Maxima for greigite, pyrite, and particulate elemental sulfur were again observed near the interface. We also studied the iron sulfide flux using sediment trap materials collected at the Cariaco station. Pyrite comprised 0.2-0.4% of the total particulate flux in the anoxic water column, with a flux of 0.5-1.6 mg S m⁻² d⁻¹.Consistent with the water column concentration profiles for iron sulfide minerals, the sulfur isotope composition of particulate sulfur found in deep anoxic traps was similar to that of dissolved sulfide near the O₂/H₂S interface. We conclude that pyrite is formed mainly within the redoxcline where sulfur cycling imparts a distinct isotopic signature compared to dissolved sulfide in the deep anoxic water. This conclusion is consistent with our previous study of sulfur species and chemoautotrophic production, which suggests that reaction of sulfide with reactive iron is an important pathway for sulfide oxidation and sulfur intermediate formation near the interface. Pyrite and elemental sulfur distributions favor a pathway of pyrite formation via the reaction of FeS with polysulfides or particulate elemental sulfur near the interface. A comparison of thermodynamic predictions with actual concentration profiles for iron sulfides leads us to argue that microbes may mediate this precipitation.     

Exposure of developing countries to sea-level rise and storm surges

An increase in sea surface temperature is strongly evident at all latitudes and in all oceans. The scientific evidence to date suggests that increased sea surface temperature will intensify cyclone activity and heighten storm surges. The paper assesses the exposure of (coastal) developing countries to sea-level rise and the intensification of storm surges. Geographic Information System (GIS) software is used to overlay the best available, spatially-disaggregated global data on critical exposed elements (land, population, GDP, agricultural extent and wetlands) with the inundation zones projected with heightened storm surges and a 1 m sea-level rise. Country-level results indicate a significant increase in exposure of developing countries to these climate-induced changes.     

Precession‐scale cyclicity in the fluvial lower Eocene Willwood Formation of the Bighorn Basin,Wyoming (USA)

Little is known about controls on river avulsion at geological time scales longer than 10⁴ years, primarily because it is difficult to link observed changes in alluvial architecture to well‐defined allogenic mechanisms and to disentangle allogenic from autogenic processes. Recognition of Milankovitch‐sale orbital forcing in alluvial stratigraphy would provide unprecedented age control in terrestrial deposits, and also exploit models of allogenic forcing enabling more rigorous testing of allocyclic and autocyclic controls. The Willwood Formation of the Bighorn Basin is a lower Eocene fluvial unit distinctive for its thick sequence of laterally extensive lithological cycles on a scale of 4 to 10 m. Intervals of red palaeosols that formed on overbank mudstones are related to periods of relative channel stability when gradients between channel belts and floodplains were low. The intervening drab, heterolithic intervals with weak palaeosol development are attributed to episodes of channel avulsion that occurred when channels became super‐elevated above the floodplain. In the Deer Creek Amphitheater section in the McCullough Peaks area, these overbank and avulsion deposits alternate with a dominant cycle thickness of ca 7·1 m. Using integrated stratigraphic age constraints, this cyclicity has an estimated period of ca 21·6 kyr, which is in the range of the period of precession climate cycles in the early Eocene. Previous analyses of three older and younger sections in the Bighorn Basin showed a similar 7 to 8 m spacing of red palaeosol clusters with an estimated duration close to the precession period. Intervals of floodplain stability alternating with episodes of large‐scale reorganization of the fluvial system could be entirely autogenic; however, the remarkable regularity and the match in time scales documented here indicate that these alternations were probably paced by allogenic, astronomically forced climate change.     

An automatic HyLoggerTM mineral mapping method using a machine-learning-based computer vision technique

Abstract

HyLogger profile scanning is commonly utilised for drill-core logging but the limited scanning area may not detect all important geological features. The study presented in this paper aims to develop a mineral mapping solution for this core-logging process by leveraging the colour image captured during the scanning process. A machine-learning-based computer vision program was developed by implementing a k-means clustering and a global colour profiling algorithm. A suite of drill-core images was used to validate the developed program. Results indicate that there is a direct correlation between the mineral assemblage of a rock type and its colour specifications. The identified mineral type and relative abundance were comparable with HyLogger scan results.     

Historical Comparison of Fish Community Structure in Lower Chesapeake Bay Seagrass Habitats

Seagrass beds provide important habitat for fishes and invertebrates in many regions around the world. Accordingly, changes in seagrass coverage may affect fish communities and/or populations, given that many species utilize these habitats during vulnerable early life history stages. In lower Chesapeake Bay, seagrass distribution has contracted appreciably over recent decades due to decreased water clarity and increased water temperature; however, effects of changing vegetated habitat on fish community structure have not been well documented. We compared fish community composition data collected at similar seagrass sites from 1976–1977 and 2009–2011 to investigate potential changes in species richness, community composition, and relative abundance within these habitats. While seagrass coverage at the specific study sites did not vary considerably between time periods, contemporary species richness was lower and multivariate analysis showed that assemblages differed between the two datasets. The majority of sampled species were common to both datasets but several species were exclusive to only one dataset. For some species, relative abundances were similar between the two datasets, while for others, there were notable differences without directional uniformity. Spot (Leiostomus xanthurus) and northern pipefish (Syngnathus fuscus) were considerably less abundant in the contemporary dataset, while dusky pipefish (Syngnathus floridae) was more abundant. Observed changes in community structure may be more attributable to higher overall bay water temperature in recent years and other anthropogenic influences than to changes in seagrass coverage at our study sites.