Land use,soil erosion,and sediment yield at Pinto Lake,California: comparison of a simplified USLE model with the lake sediment record

Quantitative interpretation of past land use using palaeolimnological records of sediment yield requires an appropriate soil erosion model. This paper describes the application of a simplified USLE model, comparing the predicted sediment yield with the lake sediment record at Pinto Lake (Central Coast, California). Our principal finding is that simplified USLE prediction, without correction for sediment transport capacity, accurately predicts fine sediment yield. Because the fine component of the soil is delivered far more efficiently than the coarse component, this and related soil erosion models can more readily be applied to the interpretation of palaeolimnological records than to estimation of total sediment yield, for which reliable estimation of hillslope and fluvial sediment storage are more important. The focus on fine sediment also means that the model output is optimal for assessing past ecological impacts of soil erosion on stream water turbidity and particulate transport of pollutants and nutrients.     

Trace metals in harbour and slipway sediments from the island of Malta, central Mediterranean

Sediment samples collected from large harbours and public slipways on the island of Malta have been analysed for geochemically important metals (Al, Ca, Fe, Mn) and contaminant metals (As, Cd, Co, Cr, Cu, Ni, Pb, Sn, Zn) following fractionation (<63 μm) and digestion in aqua regia. Absolute and Al-normalised concentrations of contaminant metals exhibited relatively little dispersion both among different samples from the same location and between samples from different locations, notable exceptions including lower concentrations of Cr and Sn on the slipways than in the harbours. Sources of contaminant metals are attributed to diffuse and specific waste inputs from urban surroundings and boating and shipping activities. Overall, concentrations are similar to those reported for other large harbours in urban settings where equivalent sample fractionation-digestion has been performed. Relative to various sediment quality guidelines, Pb is predicted to exert the greatest threat to the marine environment of Malta.     

Mafic Plinian volcanism and ignimbrite emplacement at Tofua volcano,Tonga

Tofua Island is the largest emergent mafic volcano within the Tofua arc, Tonga, southwest Pacific. The volcano is dominated by a distinctive caldera averaging 4 km in diameter, containing a freshwater lake in the south and east. The latest paroxysmal (VEI 5–6) explosive volcanism includes two phases of activity, each emplacing a high-grade ignimbrite. The products are basaltic andesites with between 52 wt.% and 57 wt.% SiO₂. The first and largest eruption caused the inward collapse of a stratovolcano and produced the ‘Tofua’ ignimbrite and a sub-circular caldera located slightly northwest of the island’s centre. This ignimbrite was deposited in a radial fashion over the entire island, with associated Plinian fall deposits up to 0.5 m thick on islands >40 km away. Common sub-rounded and frequently cauliform scoria bombs throughout the ignimbrite attest to a small degree of marginal magma–water interaction. The common intense welding of the coarse-grained eruptive products, however, suggests that the majority of the erupted magma was hot, water-undersaturated and supplied at high rates with moderately low fragmentation efficiency and low levels of interaction with external water. We propose that the development of a water-saturated dacite body at shallow (<6 km) depth resulted in failure of the chamber roof to cause sudden evacuation of material, producing a Plinian eruption column. Following a brief period of quiescence, large-scale faulting in the southeast of the island produced a second explosive phase believed to result from recharge of a chemically distinct magma depleted in incompatible elements. This similar, but smaller eruption, emplaced the ‘Hokula’ Ignimbrite sheet in the northeast of the island. A maximum total volume of 8 km³ of juvenile material was erupted by these events. The main eruption column is estimated to have reached a height of ∼12 km, and to have produced a major atmospheric injection of gas, and tephra recorded in the widespread series of fall deposits found on coral islands 40–80 km to the east (in the direction of regional upper-tropospheric winds). Radiocarbon dating of charcoal below the Tofua ignimbrite and organic material below the related fall units imply this eruption sequence occurred post 1,000 years BP. We estimate an eruption magnitude of 2.24 × 10¹³ kg, sulphur release of 12 Tg and tentatively assign this eruption to the AD 1030 volcanic sulphate spike recorded in Antarctic ice sheet records.     

Coastal and marine ecosystem services valuation for policy and management: Managed realignment case studies in England

The recognition of the economic value of nature and the services it provides to humanity has become increasingly essential in the context of coastal zone management. In this paper we review the progress to date on both the necessary conceptual framework and empirical valuation studies required to bolster decision support systems (DSS) targeted at integrated coastal zone management goals. We highlight the importance of using an ecosystem services approach and recommend some basic sequential, although overlapping, analytical steps for appropriate environmental valuation and policy assessment: spatial explicitness; marginal changes; double counting; non-linearities; and threshold effects. We illustrate the practical use of the DSS by reviewing an existing UK case study on managed coastal realignment along the Eastern coast of England, and a new UK case study on managed realignment implemented in a southern area on the same coast. Comparing the two studies, the importance of spatial explicitness and the need for a sequential decision support system when dealing with ecosystem services valuation and project policy appraisal become particularly evident.     

Alongshore fluid motions in the swash zone of a sandy and gravel beach

Field experiments were conducted on a low-gradient, high-energy sandy beach (Truc Vert, France) and a steep, low-energy gravel beach (Slapton, UK) to examine alongshore-directed currents within the swash zone. At Truc Vert, data were collected over 33 tidal cycles with offshore significant wave heights of 1–4 m and periods of 5–12 s. At Slapton data were collected during 12 tides with wave heights of 0.3–1 m and periods of 4–9 s. The swash motion was predominantly at infragravity frequencies at Truc Vert and incident frequencies at Slapton.     

Tides in the English Channel and Southern North Sea. A frequency domain analysis using model TEA-NL

The frequency-time finite element model TEA-NL is used to investigate tidal propagation and circulation in the English Channel and the southern North Sea, in the context of the Tidal Flow Forum benchmark. Quality of TEA-NL simulations, relative to both field data and simulations by other numerical models is discussed. Sensitivity analyses are performed with regard to the effect on accuracy of bottom friction, and the effect on computational cost of the strategy used to harmonically decompose and iterate non-linear loading terms in the shallow water equations.     

Respiration rates and hypoxia on the Louisiana shelf

The spatial and temporal variation in water-column respiration, estimated from enzymatic respiratory electron-transport-system activity, was measured monthly on a cross-shelf transect on the Louisiana shelf from May through October 1991. In July 1991, water-column respiration was also determined on an alongshore transect, and in situ benthic respiration and photosynthesis rates were determined at jour stations on the cross-shelf transect. Bottom waters were persistently hypoxic (O₂<2 mg 1^?1) at most stations in July and August and sporadically hypoxic at other times. Water-column respiration rates were in the same range as earlier, less extensive studies and not unusually high for coastal and estuarine waters. They were highest in summer, decreased with distance offshore and depth, and increased with temperature. Their variation with pigment and oxygen concentrations were complex functions of season and depth. Oxygen depletion below the oxycline could occur within days to months, depending on the season and location. In July, benthic respiration rates were also not unusually high in comparison with other shallow sediments, although the ratio of benthic: total (water column+benthic) respiration was high. Combined water-column and benthic respiration could deplete the bottom water oxygen in approximately 1 mo. Because the system rarely goes anoxic (defined as observing sulfide), some mechanism(s) must exist to reaerate bottom waters. Most physical mechanisms are unlikely to provide significant reaeration at this time of year. Measured benthic and conservatively estimated bottom-water photosynthesis could resupply 23% of the oxygen lost daily by respiration. Although this is too limited a dataset from which to draw conclusions about the relative importance of bottom-water and benthic respiration and photosynthesis in determining bottom-water oxygen concentrations, it does suggest that all these processes must be considered.     

Foreign-Area Expertise in U.S. Geography: An Assessment of Capacity based on Foreign-Area Dissertations, 1977–1991*

Interpretations of a decline in foreign-area specialization have raised concerns about the capacity of geography in the United States to serve the expected increase in demands for foreign-area expertise. An assessment of foreign-area dissertations awarded by U.S. geography programs over a 15-year period (1977–1991) fails to support the concerns raised. No significant declines, if any at all, are found for early career interest and research in foreign areas. Several caveats are warranted, however. Geography has lost three doctoral programs with strong foreign-area dissertation records for the period in question. Also, non-U.S. citizens, many of whom do not remain in the United States after completion of their doctorate work, account for a significant proportion of foreign-area dissertations. Various questions that must be addressed to analyze the capacity question more broadly are identified.     

Petrology,geochemistry, and age of the Spurr volcanic complex,eastern Aleutian arc

The Spurr volcanic complex (SVC) is a calc-alkaline, medium-K, sequence of andesites erupted over the last 250000 years by the eastern-most currently active volcanic center in the Aleutian arc. The ancestral Mt. Spurr was built mostly of andesites of uniform composition (58%–60% SiO₂), although andesite production was episodically interrupted by the introduction of new batches of more mafic magma. Near the end of the Pleistocene the ancestral Mt. Spurr underwent avalanche caldera formation, resulting in the production of a volcanic debris avalanche with overlying ashflows. Immediately afterward, a large dome (the present Mt. Spurr) formed in the caldera. Both the ash flows and dome are made of acid andesite more silicic (60%–63% SiO₂) than any analyzed lavas from the ancestral Mt. Spurr, yet contain olivine and amphibole xenocrysts derived from more mafic magma. The mafic magma (53%–57% SiO₂) erupted during and after dome emplacement from a separate vent only 3 km away. Hybrid block-and-ash flows and lavas were also produced. The vents for the silicic and mafic lavas are in the center and in the breach of the 5-by-6-km horseshoe-shaped caldera, respectively, and are less than 4 km apart. Late Holocene eruptive activity is restricted to Crater Peak, and magmas continue to be relatively mafic. SVC lavas are plag ±ol+cpx±opx+mt bearing. All postcaldera units contain small amounts of high-Al₂O₃, high-alkali amphibole, and proto-Crater Peak and Crater Peak lavas contain abundant pyroxenite and anorthosite clots presumably derived from an immediately preexisting magma chamber. Ranges of mineral chemistries within individual samples are often nearly as large as ranges of mineral chemistries throughout the SVC suite, suggesting that magma mixing is common. Elevated Sr, Pb, and O isotope ratios and trace-element systematics incompatible with fractional crystallization suggest that a significant amount of continental crust from the upper plate has been assimilated by SVC magmas during their evolution.     

Viscous entrainment by sinking plumes

Geochemical models invoking several distinct reservoirs in the mantle, with different time histories, raise important questions about the exchange of mass between them. If two of these reservoirs are the upper and lower mantle, above and below about 700 km, then sinking of cold slabs through this level is one of a number of possible ways in which mixing can occur. In addition, if slabs do penetrate the transition zone, surrounding upper layer material will be dragged downwards. We have examined the interaction of very viscous plumes, or slabs, with density and viscosity interfaces in a series of laboratory experiments using fluids of different viscosities and densities and have documented several mechanisms which can lead to significant entrainment and mixing. If a slab remains planar as it passes through a density interface, a boundary layer of lighter fluid is pulled into the lower layer and we predict the consequent mass flux. When a near-vertical slab becomes unstable to folding (as it does if it has a sufficient viscosity contrast with its surroundings and its length is greater than about five times its thickness), there is another more efficient entrainment mechanism: upper layer fluid is trapped between the folds in the slab. The effective entrainment increases as the density difference between the upper and lower layers decreases. An increase in viscosity with depth also leads to buckling instability and folding of the surrounding material into the slab material. On the other hand, when there is substantial density difference between the layers a dense slab can cease to sink through the interface but spread out along the interface because it is unstable and incorporates enough upper layer fluid between its folds to become neutrally buoyant. The range of slab behaviour occurring in the mantle is not known but we draw attention to the various possibilities and to the implications for mass flux between layers.