Establishing boundary classes for the classification of UK marine waters using phytoplankton communities   总被引：2，自引：0，他引：2  

Devlin M  Best M  Coates D  Bresnan E  O'Boyle S  Park R  Silke J  Cusack C  Skeats J 《Marine pollution bulletin》2007,55(1-6):91-103

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Climate, agriculture, and cycles of human occupation over the last 4000 yr in southern Zacatecas, Mexico     

Michelle Elliott  Christopher T. Fisher  Roberto S. Molina Garza  Deborah M. Pearsall 《Quaternary Research》2010,74(1):26-35

Scholars attribute the growth and decline of Classic period (AD 200-900) settlements in the semi-arid northern frontier zone of Mesoamerica to rainfall cycles that controlled the extent of arable land. However, there is little empirical evidence to support this claim. We present phytolith, organic carbon, and magnetic susceptibility analyses of a 4000-yr alluvial record of climate and human land use from the Malpaso Valley, the site of one such Classic frontier community. The earliest farming occupation is detected around 500 BC and appears related to a slight increase of aridity, similar to the level of the modern day valley. By AD 500, the valley's Classic period Mesoamerican settlements were founded under these same dry conditions, which continued into the Postclassic period. This indicates that the La Quemada occupation did not develop during a period of increased rainfall, but rather an arid phase. The most dramatic changes detected in the valley resulted from the erosion associated with Spanish Colonial grazing and deforestation that began in the 16th century. The landscape of the modern Malpaso Valley is thus primarily the product of a series of intense and rapid transformations that were concentrated within the last 400 yr.  相似文献   

Spatio-temporal variability in debris-flow activity: a tree-ring study at Geisstriftbach (Swiss Alps) extending back to AD 1736     

Markus Stoffel  Michelle Bollschweiler  Susanne Widmer  Annina Sorg 《Swiss Journal of Geoscience》2010,103(2):283-292

Past debris-flow activity on the forested cone of the Geisstriftbach torrent (St. Niklaus, Valais, Swiss Alps) was assessed from growth disturbances in old conifer trees, providing a much improved record of past events. The study of 633 tree-ring sequences sampled from 252 European larch (Larix decidua Mill.), Norway spruce (Picea abies (L.) Karst.) and Silver birch (Betula pendula Roth.) trees allowed reconstruction of 53 debris-flow events since AD 1736. The spatial analysis of trees affected during particular events on the geomorphic map allowed for a spatial representation of individual events and a reconstruction of four flow patterns. Based on our results and Siegfried maps, we believe that before the formation of a dogleg near the cone apex in the late 1890s, debris flows preferentially used the channels located in the west-southwestern part of the Geisstriftbach cone. This study contributes to our understanding of debris-flow processes on cones and provides an example of how dendrogeomorphic techniques may help in the reconstruction and understanding of debris flows in Alpine areas.  相似文献   

Magma storage and mixing conditions for the 1953–1974 eruptions of Southwest Trident volcano, Katmai National Park, Alaska     

Michelle L. Coombs  John C. Eichelberger  Malcolm J. Rutherford 《Contributions to Mineralogy and Petrology》2000,140(1):99-118

Between 1953 and 1974, approximately 0.5 km³ of andesite and dacite erupted from a new vent on the southwest flank of Trident volcano in Katmai National Park, Alaska, forming an edifice now known as Southwest (or New) Trident. Field, analytical, and experimental evidence shows that the eruption commenced soon after mixing of dacite and andesite magmas at shallow crustal levels. Four lava flows (58.3–65.5 wt% SiO₂) are the dominant products of the eruption; these contain discrete andesitic enclaves (55.8–58.9 wt% SiO₂) as well as micro- and macro-scale compositional banding. Tephra from the eruption spans the same compositional range as lava flows; however, andesite scoria (56–58.1 wt% SiO₂) is more abundant relative to dacite tephra, and is the explosively erupted counterpart to andesite enclaves. Fe–Ti oxide pairs from andesite scoria show a limited temperature range, clustered around 1000 °C. Temperatures from grains found in dacite lavas possess a wider range; however, cores from large (>100 μm) magnetite and coexisting ilmenite give temperatures of ∼890 °C, taken to represent a pre-mixing temperature for the dacite. Water contents from dacite phenocryst melt inclusions and phase equilibria experiments on the andesite imply that the two magmas last resided at a water pressure of 90 MPa, and contained ∼3.5 wt% H₂O, equivalent to 3 km depth if saturated. Unzoned pyroxene and sodic plagioclase in the dacite suggest that it likely underwent significant crystallization at this depth; highly resorbed anorthitic plagioclase from the andesite suggests that it originated at greater depths and underwent relatively rapid ascent until it reached 3 km, mixed with dacite, and erupted. Diffusion profiles in phenocrysts suggest that mixing preceded eruption of earliest lava by approximately one month. The lack of a compositional gap in the erupted rock suite indicates that thorough mixing of the andesite and dacite occurred quickly, via disaggregation of enclaves, phenocryst transfer from one magma to another, and direct mixing of compositionally distinct melt phases. Received: 22 September 1999 / Accepted: 4 April 2000  相似文献   

Geographic signatures of North American West Coast estuaries     

Robert Emmett  Roberto Llansó  Jan Newton  Ron Thom  Michelle Hornberger  Cheryl Morgan  Colin Levings  Andrea Copping  Paul Fishman 《Estuaries and Coasts》2000,23(6):765-792

West Coast estuaries are geologically young and composed of a variety of geomorphological types. These estuaries range from large fjords to shallow lagoons; from large to low freshwater flows. Natural hazards include E1 Niños, strong Pacific storms, and active tectonic activity. West Coast estuaries support a wide range of living resources: five salmon species, harvestable shellfish, waterfowl and marine birds, marine mammals, and a variety of algae and plants. Although populations of many of these living resources have declined (salmonids), others have increased (marine mammals). West Coast estuaries are also centers of commerce and increasingly large shipping traffic. The West Coast human population is rising faster than most other areas of the U.S. and Canada, and is distributed heavily in southern California, the San Francisco Bay area, around Puget Sound, and the Fraser River estuary. While water pollution is a problem in many of the urbanized estuaries, most estuaries do not suffer from poor water quality. Primary estuarine problems include habitat alterations, degradation, and loss; diverted freshwater flows; marine sediment contamination; and exotic species introductions. The growing West Coast economy and population are in part related to the quality of life, which is dependent on the use and enjoyment of abundant coastal natural resources.  相似文献   

The Politics of AIDS Surveillance     

Michelle Cochrane 《The Professional geographer》2000,52(2):205-218

This paper examines the social construction of official HIV/AIDS surveillance data in San Francisco and the implications of this research for understanding the past and future trajectories of the epidemic in the United States. I employ a methodology of social constructivism in an analysis of the historical evolution of the epidemic and a critique of its quantitative representation by public health officials. I argue that medical geographers have erroneously interpreted AIDS surveillance data as evidence for an exponential growth in AIDS cases during the past 17 years, neglecting to critically explore the multiple and extensive revisions in the definition of the disease (in 1982, 1985, 1987, 1993) by which patients have been reported over time. In contrast, social constructivism exemplifies a method for elucidating the sociopolitical interests of public health institutions and community activists which underlie the constant metamorphosis of the subjects (e.g., patients) and object (e.g., ‘AIDS’) comprising this epidemic.  相似文献   

A Massive Coccolithophorid Bloom Observed in Mikawa Bay, Japan     

Masanobu Kai  Tamotsu Hara  Hiroaki Aoyama  Nobuo Kuroda 《Journal of Oceanography》1999,55(3):395-406

In April 1996, a massive algal bloom of the coccolithophorid Gephyrocapsa oceanica developed in both Chita Bay and Atsumi Bay which comprise the bay known as Mikawa Bay of Japan. It was the first record of such a bloom in this area. In Chita Bay, the bloom persisted until the middle of May, however in Atsumi Bay, it remained until early June. From the analysis of salinity, water temperature, and current velocity and direction data, it is considered that the following mechanism accounts for the occurrence and maintenance of the bloom: Before the bloom, the standing crop of phytoplankton was poor, resulting in relatively rich nutrients throughout the bay. Thereafter, with the influx of oceanic water into Mikawa Bay, high salinity occurred firstly in Chita Bay. Under these hydrographic conditions, the bloom occurred first in Chita Bay, and extended throughout the bay with the clockwise circulation of water into Atsumi Bay. In Chita Bay, the bloom was influenced by rainfall and G. oceanica flowed out from this area. Whereas, in Atsumi Bay, the bloom persisted for longer due to the clockwise circulation and another influx of oceanic water.  相似文献   

Correction to: Synoptic conditions and potential causes of the extreme heavy rainfall event of January 2009 over Mindanao Island,Philippines     

Olaguera  Lyndon Mark Payanay  Caballar  Michelle Español  De Mata  Joseph Cabacungan  Dagami  Loida Ann Torres  Matsumoto  Jun  Kubota  Hisayuki 《Natural Hazards》2022,110(3):2405-2406

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