Cosmogenic nuclide methods for measuring long-term rates of physical erosion and chemical weathering

Understanding the evolution of geochemical and geomorphic systems requires measurements of long-term rates of physical erosion and chemical weathering. Erosion and weathering rates have traditionally been estimated from measurements of sediment and solute fluxes in streams. However, modern sediment and solute fluxes are often decoupled from long-term rates of erosion and weathering, due to storage or re-mobilization of sediment and solutes upstream from the sampling point. Recently, cosmogenic nuclides such as ¹⁰Be and ²⁶Al have become important new tools for measuring long-term rates of physical erosion and chemical weathering. Cosmogenic nuclides can be used to infer the total denudation flux (the sum of the rates of physical erosion and chemical weathering) in actively eroding terrain. Here we review recent work showing how this total denudation flux can be partitioned into its physical and chemical components, using the enrichment of insoluble tracers (such as Zr) in regolith relative to parent rock. By combining cosmogenic nuclide measurements with the bulk elemental composition of rock and soil, geochemists can measure rates of physical erosion and chemical weathering over 1000- to 10,000-year time scales.     

Phytoliths as indicators of pedogenesis and paleoenvironmental changes in the Brazilian cerrado

This paper presents the preliminary results of phytolith analyses of a peat located in the cerrado of the Uberaba municipality, State of Minas Gerais, Brazil. The phytolith soil sequence is discussed by comparison with phytolith assemblages extracted from dominant plants (Cyperaceae and Poaceae). Increasing mean age of phytolith assemblages with depth is assumed. Poaceae Cuneiform bulliform cell, Parallelepiped bulliform cell and Elongate smooth long cell types dominate up to 80 cm, rapidly drop to 42% at 80 cm and regularly decrease from 42% to 2% upwards. Cyperaceae Rondel concave type shows the inverse trend, being dominant in the upper part of the profile. This pattern can be assigned to increasing selective dissolution of the Cyperaceae phytolith type with depth, or/and to a decrease of water stress suffered by the grasses, leading to a decrease of bulliform cell silicification. Soil processes and paleo-environmental changes hypotheses are discussed.     

Effects of irrigation on geochemical processes in a paddy soil and in ground waters in Camargue (France)

In waterlogged soils, dynamics of water influence the redox conditions and thus the mobility of elements. Irrigation of rice in Camargue (South eastern France) induces yearly dynamics of water. In order to determine the impact of irrigation on the geochemical properties of ground waters, a continuously in situ record of physico-chemical parameters (pH, Eh, temperature and electric conductivity) is performed during 1 year in an irrigated rice field. Seasonal dynamics show large Eh and pH variations. An annual irrigation cycle generates fast precipitations of Ca–Mg carbonates and Fe oxides between 50 and 110 cm depth when the soil is waterlogged. The dissolution of these minerals is initiated during a year without irrigation.     

Neoproterozoic alkaline magmatism at Sivamalai, southern India

The Sivamalai alkaline complex lies at the southern margin of the Cauvery Shear System that separates the Archaean and Proterozoic domains of the Southern Granulite Terrain in India. U–Pb TIMS dating of zircon from a pegmatitic syenite sample in the complex yields a concordant age of 590.2 ± 1.3 (2σ) Ma which is interpreted to date the intrusion of the alkaline rocks. A lower concordia intercept at 168 ± 210 Ma defined by two grains with high common lead may indicate post-magmatic disturbances due to recrystallisation which is also evident in the CL images of the zircons. EPMA dating of monazite from a post-kinematic pegmatite which intrudes the crystalline basement hosting the alkaline rocks yields an age of 478 ± 29 (2σ) Ma and provides a lower bracket for the main phase of tectonism in this part of the Southern Granulite Terrain. The Pan-African high-grade metamorphism and ductile deformation has thus most likely affected the alkaline rocks. This is supported by the presence of a metamorphic foliation and extensive recrystallisation textures seen in the rocks. The major and trace element concentrations measured on selected samples reveals the presence of both enriched and depleted rock types. The enriched group includes ferrosyenite and nepheline syenite while the depleted group has only nepheline syenites. The trace element depletion of some nepheline syenites is interpreted to be a result of fractional crystallization involving the removal of accessory phases like zircon, titanite, apatite and allanite.     

Responses of resident marsh fishes to stages ofPhragmites australis invasion in three mid Atlantic estuaries

Modification of brackish marshes by nonindigenousPhragmites australis has occurred across a broad geographical area in eastern North America. Among its effects on marsh processes,Phragmites may be increasingly unfavorable to marsh surface fishes as its invasion progresses within an estuary. We assessed the effect of thePhragmites invasion on resident marsh surface fishes by examining the population response ofFundulus heteroclitus (mummichog, 5–48 mm TL) andF. luciae (spotfin killifish, 5–41 mm TL) to four distinct invasion stages in three estuaries of the U.S. mid Atlantic region (New Jersey, Delaware, and Maryland). We documented precipitous declines in mean catch per unit effort ofF. heteroclitus in pit traps from natural marsh (51.6), through initial (33.8), early (12.3), and late invasion stages (2.4) across all sites. A similar pattern was documented forF. luciae, with mean catch per unit effort in pit traps declining from natural marsh (48.9), through initial (39.1), early (9.3), and late invasion stages (2.7). Population structure of both species also changed somewhat across invasion stages such that we collected a narrower size range of individuals of both species from late invasion stages. Patterns suggest that as thePhragmites invasion progresses, there is a decline in habitat function for larval and juvenileF. heteroclitus and an increased risk of extirpation ofF. luciae from brackish marshes along the east coast of the U.S.