Responses of interrill runoff and erosion rates to vegetation change in southern Arizona

Vegetation change, fi-om grassland to shrubland, has occurred over much of the Sonoran and Chihuahuan Deserts during the past century. The effect of this vegetation change on interrill runoff and erosion was examined by conducting rainfall-simulation experiments on large runoff plots on contemporary grassland and shrubland hillslopes. These experiments show that, compared to the grassland, the interrill portions of shrubland hillslopes (1) have higher runoff rates, (2) experience equilibrium runoff conditions much more frequently, (3) exhibit higher overland flow velocities, and (4) are subject to greater rates of erosion. The environmental change that has led to the vegetation change has been relatively minor, but its geomorphic impact has been substantial.     

The abundance of ammonium in the granites of central Spain,and the behaviour of the ammonium ion during anatexis and fractional crystallization

Summary The granites of the Sistema Central Espanol are richer in ammonium than those of most other regions, and have a mean NH₄ ⁺ content of 84 ppm (range = 1–243 ppm). Among the possible causes for the high level of ammonium, a high proportion of organic-rich pelitic protolith and reducing conditions during anatexis are considered to be the most significant. The behaviour of the ammonium ion during magmatic differentiation is discussed by reference to its distribution in the Pedrobernardo layered intrusion: ammonium is depleted in the final liquid fraction, but there is no relative fractionation of NH₄ ⁺ and K⁺. The depletion of the melt in NH₄ ⁺ during crystallization is attributed to its removal by biotite and to a lesser extent by K-feldspar. The behaviour of the ammonium ion during anatexis is discussed with reference to the Peña Negra migmatite complex. It is shown that large amounts of NH₄ ⁺ are present in these high grade metamorphic rocks, and that NH₄ ⁺ is preferentially partitioned into the restite fraction during partial melting. These relationships are attributed to the preferential incorporation of NH₄ ⁺ into potassic host minerals in the order: biotite > muscovite > K-feldspar.

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Ammonium in Zentralspanischen Graniten, und das Verhalten des Ammonium-Ions während Anatexis und fraktionierter Kristallisation

Zusammenfassung Die Granite des Sistema Central Espanol sind reicher an Ammonium als die der meisten anderen Regionen, und haben einen durchschnittlichen NH₄ ⁺ Gehalt von 84 ppm (von 1-243 ppm). Der hohe Ammoniumgehalt könme auf einen hohen Anteil peiitischer Ausgangsgesteine, die reich an organischen Material sind, and auf reduzierende Bedingungen während der Anatexis zurückgehen. Das Verhalten des Ammonium-Ions während magmatischer Differentiation wind in Hinblick seiner Verteilung in der geschichteten Intrusion von Pedrobernardo diskutiert: Ammonium ist in der finalen Schmelzfraktion angereichert, aber es gibt keine relative Fraktionierung von NH₄ ⁺ and K⁺. Die Verarmung der Schmelze an NH₄ ⁺ wahrend der Kristallisation geht darauf zurück, daß NH₄ ⁺ von Biotit and in einem geringen Ausmaß von K-Feldspat aufgenommen wird. Das Verhalten des Ammonium-Ions während der Anatexis wird am Peña Negra Migmatit-Komplex diskutiert. Es zeigt rich, daß große Mengen von NH₄ ⁺ in diesen hochgradig metamorphen Gesteinen vorkommen, and das NH₄ ⁺ während teilweiser Aufschmelzung vorzugsweise in der Restit-Fraktion angereichert wird. Diese Beziehungen gehen auf die vorzugsweise Aufnahme von NH₄ ⁺ in Kali-führenden Gastmineralen zurück, and zwar in folgender Ordnung: Biotit > Muskovit > K-Feldspat.

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With 5 Figures     

Calculating surface water PCO2 from foraminiferal organic δC

The δ¹³C of organic matter bound within the crystal lattice of foraminiferal calcite tests may provide a potential tracer of the isotopic composition of the surface water primary photosynthate. Using δ¹³C of the organic matter extracted from the crystal lattice and the calcite test, it is theoretically possible to estimate the paleo-surface water pCO₂. We have tailored this technique initially for the subpolar planktonic foraminifera species Globigerina bulloides. Initial surface water pCO₂ estimates from deep-sea core BOFS 5K (50°41.3′N, 21°51.9′W, water depth 3547 m) indicate that the northeast Atlantic Ocean may have been a greater sink for CO₂ during the last glacial than during the Holocene. Greatly reduced benthic foraminifera abundances, especially phytodetritus feeders, in BOFS 5K during the last glacial indicates low surface productivity. This rules out a productivity-driven CO₂ sink. The enhanced glacial CO₂ sink must, therefore, have results from a southwards shift of the centre of deep water formation.     

Evidence for freeze–thaw events and their implications for rock weathering in northern Canada

Discussions regarding weathering in cold environments generally centre on mechanical processes and on the freeze–thaw mechanism in particular. Despite the almost ubiquitous assumption of freeze–thaw weathering, unequivocal proof of interstitial rock water actually freezing and thawing is singularly lacking. Equally, many studies have used the crossing of 0 °C, or values close to that, as the basis for determining the number of ‘freeze–thaw events’. In order to assess the weathering regime at a site in northern Canada, temperatures were collected at the surface, 1 cm and 3 cm depth for sets of paving bricks, with exposures both vertical and at 45°, orientated to the four cardinal directions. Temperature data were collected at 1 min intervals for 1 year. These data provide unequivocal proof for the occurrence of the freezing and thawing of water on and within the rock (freeze–thaw events). The freeze event is evidenced by the exotherm associated with the release of latent heat as the water actually freezes. This is thought to be the ?rst record of such events from a ?eld situation. More signi?cantly, it was found that the temperature at which freezing occurred varied signi?cantly through the year and that on occasion the 1 cm depth froze prior to the rock surface. The change in freeze temperature is thought to be due to the chemical weathering of the material (coupled with on‐going salt inputs via the melting of snowfall), which, it is shown, could occur throughout the winter despite air temperatures down to ?30 °C. This ?nding regarding chemical weathering is also considered to be highly signi?cant. A number of thermal stress events were also recorded, suggesting that rock weathering in cold regions is a synergistic combination of various chemical and mechanical weathering mechanisms. Copyright © 2003 John Wiley & Sons, Ltd.     

Li/Ca in multiple species of benthic and planktonic foraminifera: thermocline, latitudinal, and glacial-interglacial variation

Li/Ca ratios were measured in planktonic and benthic foraminifera from a variety of hydrographic settings to investigate the factors influencing lithium incorporation into foraminiferal tests including temperature, dissolution, pressure, and interspecies differences. Down-core measurements of planktonic (Orbulina universa, Globigerinoides ruber, and Globigerinoides sacculifer) and benthic foraminifera (calcitic Cibicides wuellerstorfi and aragonitic Hoeglandina elegans) show a systematic variation in Li/Ca with δ¹⁸O through the last glacial-interglacial transition. All species examined exhibit an increase in Li/Ca between 14 to 50% from the Holocene to the last glacial maximum. Li/Ca generally increases with decreasing temperature as seen in a latitudinal transect of planktonic O. universa and down-slope benthic species along the Bahama Bank margins. Postdepositional dissolution possibly causes a decrease in planktonic foraminiferal Li/Ca along the Sierra Leone Rise, and increased water depth causes a decrease in benthic foraminiferal Li/Ca in the deep Caribbean. However, none of these effects are sufficient to account for the observed glacial-interglacial changes. Physiological factors such as calcification rate may affect the Li/Ca content of foraminiferal calcite. The calcification rate in turn may be a function of carbonate ion concentration of ambient ocean water. This work shows that incorporation of lithium by foraminifera appears to be influenced by factors other than seawater composition and does not appear to be dominated by changes in temperature, dissolution, or pressure. We hypothesize that the consistent increase in foraminiferal Li/Ca during the last glacial maximum may be linked to changes in seawater carbonate ion concentration. Important parameters to be tested include calcification rate and foraminiferal test size and weight. If foraminiferal Li/Ca is dominantly controlled by calcification rate as a function of seawater carbonate ion concentration, then Li/Ca may act as a proxy of past atmospheric CO₂.     

Short‐term intertidal bar mobility on a ridge‐and‐runnel beach,Merlimont, northern France

Digital elevation models and topographic pro?les of a beach with intertidal bar and trough (ridge‐and‐runnel) morphology in Merlimont, northern France, were analysed in order to assess patterns of cross‐shore and longshore intertidal bar mobility. The beach exhibited a pronounced dual bar–trough system that showed cross‐shore stationarity. The bars and troughs were, however, characterized by signi?cant longshore advection of sand under the in?uence of suspension by waves and transport by strong tide‐ and wind‐driven longshore currents. Pro?le changes were due in part to the longshore migration of medium‐sized bedforms. The potential for cross‐shore bar migration appears to be mitigated by the large size of the two bars relative to incident wave energy, which is modulated by high vertical tidal excursion rates on this beach due to the large tidal range (mean spring tidal range = 8·3 m). Cross‐shore bar migration is also probably hindered by the well‐entrenched troughs which are maintained by channelled high‐energy intertidal ?ows generated by swash bores and by tidal discharge and drainage. The longshore migration of intertidal bars affecting Merlimont beach is embedded in a regional coastal sand transport pathway involving tidal and wind‐forced northward residual ?ows affecting the rectilinear northern French coast in the eastern English Channel. Copyright © 2004 John Wiley & Sons, Ltd.     

Rock albedo and monitoring of thermal conditions in respect of weathering: some expected and some unexpected results

Broadly speaking, there is, at least within geomorphic circles, a general acceptance that rocks with low albedos will warm both faster and to higher temperatures than rocks with high albedos, reflectivity influencing radiative warming. Upon this foundation are built notions of weathering in respect of the resulting thermal differences, both at the grain scale and at the scale of rock masses. Here, a series of paving bricks painted in 20 per cent reflectivity intervals from black through to white were used to monitor albedo‐influenced temperatures at a site in northern Canada in an attempt to test this premise. Temperatures were collected, for five months, for the rock surface and the base of the rock, the blocks being set within a mass of local sediment. Resulting thermal data did indeed show that the dark bricks were warmer than the white but only when their temperatures were equal to or cooler than the air temperature. As brick temperature exceeded that of the air, so the dark and light bricks moved to parity; indeed, the white bricks frequently became warmer than the dark. It is argued that this ‘negating’ of the albedo influence on heating is a result of the necessity of the bricks, both white and black, to convect heat away to the surrounding cooler air; the darker brick, being hotter, initially convects faster than the white as a product of the temperature difference between the two media. Thus, where the bricks become significantly hotter than the air, they lose energy to that air and so their respective temperatures become closer, the albedo influence being superceded by the requirement to equilibrate with the surrounding air. It is argued that this finding will have importance to our understanding of weathering in general and to our perceptions of weathering differences between different lithologies. Copyright © 2005 John Wiley & Sons, Ltd.     

Counting Beans: Agrobiodiversity,Indigeneity, and Agrarian Reform

In‐field conservation of locally domesticated crop varieties ameliorates agrobiodiversity losses, but the interaction among nationally regulated socioeconomic factors at the local scale tends to discourage this. Analyses of household surveys conducted in Ecuador demonstrate that state and nongovernmental institutions interact to discourage cultivation of locally domesticated varieties of beans (Phaseolus spp.). Land privatization, agricultural extension, and credit programs favor market production of introduced modern bean varieties, and locally domesticated varieties are noted for favorable nutrition, culinary, and agroecological qualities. Resolving disconnections between the market and social values for landrace beans may provide agrobiodiversity conservation opportunities.