The Cocos and Carnegie Aseismic Ridges: a Trace Element Record of Long-term Plume-Spreading Center Interaction

The aseismic Cocos and Carnegie Ridges, two prominent bathymetricfeatures in the eastern Pacific, record 20 Myr of interactionbetween the Galápagos hotspot and the adjacent GalápagosSpreading Center. Trace element data determined by inductivelycoupled plasma-mass spectrometry in >90 dredged seamountlavas are used to estimate melt generation conditions and mantlesource compositions along the ridges. Lavas from seamount provinceson the Cocos Ridge are alkalic and more enriched in incompatibletrace elements than any in the Galápagos archipelagotoday. The seamount lavas are effectively modeled as small degreemelts of a Galápagos plume source. Their eruption immediatelyfollows the failure of a rift zone at each seamount province'slocation. Thus the anomalously young alkalic lavas of the CocosRidge, including Cocos Island, are probably caused by post-abandonmentvolcanism following either a ridge jump or rift failure, andnot the direct activity of the Galápagos plume. The seamountshave plume-like signatures because they tap underlying mantlepreviously infused with Galápagos plume material. Whereasplume heterogeneities appear to be long-lived, tectonic rearrangementsof the ridge plate boundary may be the dominant factor in controllingregional eruptive behavior and compositional variations. KEY WORDS: mantle plume; mid-ocean ridge; Galápagos; abandoned rift; partial melting of the mantle     

FLUVIAL ADJUSTMENTS TO SOIL EROSION AND PLANT COVER CHANGES IN THE CENTRAL SPANISH PYRENEES

ABSTRACT. Until the middle of the 20th century, Pyrenean rivers were characterized by braided channels, unstable sedimentary structures and an almost complete lack of plant cover in the alluvial plain, due to the high sediment yield in hillslopes and the occurrence of frequent and intense flooding. This was probably related to strong demographic pressures, including the cultivation of steep slopes, frequent fires, deforestation and overgrazing. Depopulation and farmland abandonment resulted in plant recolonization in formerly cultivated areas, causing a decrease in runoff and sediment yield. As a consequence, most Pyrenean rivers tend to reduce the width of the alluvial plain and to replace the braided pattern with an incised, somewhat meandering pattern, involving the construction of new terrace levels and the stabilization of fluvial bars.     

Varve-counting by the annual pattern of diatoms accumulated in the sediment of Big Watab Lake, Minnesota, AD 1837–1990

The goal of this study was to develop a method for extracting quantitative annual records of diatoms and other microfossils from sediments that are resistant to existing varve-counting methods because of their variable, complex, or indistinct laminations. Very linely laminated organic sediments from a deep diamictic lake in central Minnesota, USA were collected by freeze-core, dehydrated by solvent exchange, embedded with epoxy, and petrographically thin-sectioned. Diatom frustules and pollen grains were counted in contiguous 400 pm microscope fields-of-view and used to construct a sediment chronology. The concentration of diatoms in the sediment was cyclic and lowest in sediment deposited during summer. The concentration of pollen in the sediment was also cyclic and sediment deposited during the summer contained the highest concentrations of pollen and the highest percentages of ragweed ( Ambrosia spp.) pollen. The sediment chronology that was produced accurately records the onset of regional (c. 1850) and watershed ( c . 1880) development, a historic tornadic storm complex (1894), and individual drought years (1932, 1934 & 1935) within the 1930's drought.     

Variability in nutrient concentrations around Elephant Island, Antarctica, during 1991-1993

The nutrient status of the various water mass structures within a large sampling grid around Elephant Island are reported and the nutrient concentrations relative to jata from the physical and biological components of the Antarctic Marine Living Resources Programme are discussed. Concentrations of silicic acid, nitrate and phosphate (Si/N/P) were measured in the upper water column during January-March of three successive years. Samples were taken from eleven depths at 17 stations in 1991, and at four depths at 144 stations in 1992 and 182 stations in 1993. There was considerable variability in the concentrations of all three nutrients within the study area, but silicic acid showed the greatest variance among the water masses present in the sampling grid. The ratios (Si/N/P) of the nutrient deficits (difference in winter and summer values) in the upper 100 m differed considerably in Drake Passage waters as compared to Bransfield Strait waters, with both nitrate and silicic acid showing the greatest variance. Nutrient deficits did not increase from January to February, indicating that rates of replenishment of nutrients to the euphotic zone by physical processes and/or biological regeneration were approximately equal to the rate of uptake and assimilation by phytoplankton during that time period. The seasonal deficits, however, were substantial. Estimates of daily rates of primary production based on these nutrient deficits were comparable to the rates as measured by radiocarbon for Drake Passage waters, but much smaller for Bransfield Strait waters.