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CARA ROURE JOHNSON GAIL M. ASHLEY† CAROL B. DE WET‡ RACHEL DVORETSKY‡ LISA PARK§ VICTORIA C. HOVER¶ R. Bernhart Owen SALLY MCBREARTY 《Sedimentology》2009,56(4):1115-1137
Lithological and biological features of a fossiliferous tufa in the Kapthurin Formation, Baringo, Kenya, reveal the presence of a lush wetland in a semi-arid environment during the Middle Pleistocene ( ca 500 ka) in this portion of the East African Rift Valley. Four geological sections, each between 3 m and 8 m in thickness, exposed over a distance of 0·5 km, reveal a 1 to 2 m thick paludal tufa which is composed of three carbonate beds, two dark grey silty claystones and a reddish-brown silty palaeosol. High resolution stratigraphic analysis, carbonate petrography, stable isotope and elemental geochemistry, clay mineralogy and fossil remains (molluscs, ostracods, diatoms and charophytes) reveal a ground water-fed system that fluctuated in depth and periodically disappeared altogether. Oxygen isotope ratios (δ18 O) of tufa matrix range from −4·5‰ to −8·0‰ (Vienna Pee Dee Belemnite) and become more positive up section, indicating the decreasing influence of fault-related fluids and increasing residence time or freshness of wetland water, rather than evaporative enrichment. This spring was situated on a lake margin during low lake levels, thrived during periods of increased ground water input and was ultimately replaced by an alkaline lake. The wetland would appear to have existed during a cool interval within the generally warm Marine Isotope Stage 13 or perhaps during the warm second half of Marine Isotope Stage 13. The ground water source of this wetland arose through a fault system. Thus, the position of the tufa deposit is controlled structurally but the timing and duration of the wetland system may have been influenced by both climatic and tectonic factors. 相似文献
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ABSTRACT. Group identity serves as a mechanism for claiming rights of control and access to land in the United States. Public land managers face myriad identity‐based claims to land in their care. Identity shapes claims that must appear valid within the strictures of a legal system created by a dominant culture to serve its interests. The very form of those systems—of which public lands are a large part—makes possible the expression of particular forms of identity. The story of the Coast Miwok community and the Point Reyes National Seashore suggests that geographical links among identity, landscape, and history are actively constructed through political work and rarely are as obvious as they first appear. Both the formal legal process of federal tribal recognition and restoration and the far less formal Coast Miwok claims to land at Point Reyes National Seashore teach important lessons about neotraditional identity‐based claims to public land. 相似文献
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Noble gas concentrations and isotopic compositions have been measured in eight samples of pillow basalt glasses collected from seven different localities along 250 km of the Mariana Trough spreading and rifting axis. The samples have uniform and mid-ocean ridge basalt (MORB)-like 3 He/4 He values of 9–12 × 10–6 (6.4–8.6 times atmospheric) despite large variations in 4 He. Concentrations of the noble gases Ne, Ar, Kr, and Xe show much smaller variations between samples, but larger variations in isotopic compositions of Ne, Ar, and Xe. Excess radiogenic 21 Ne is observed in some samples. 40 Ar/36 Ar varies widely (atmospheric to 1880). Kr is atmospheric in composition for all samples. Some samples show a clear excess 129 Xe, which is a well-known MORB signature. Isotopic compositions of the heavier noble gases (Ar, Kr, and Xe) in some samples, however, show more atmospheric components. These data reflect the interaction of a MORB-like magma with an atmospheric component such as seawater or of a depleted mantle source with a water-rich component that was probably derived from the subducting slab. 相似文献
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ABSTRACT. Farmers in Marin and Sonoma Counties, located north of San Francisco, are experimenting with numerous alternatives to California's widely known industrial dairy style. Many analysts suggest that consumer politics, food scares, and globalization explain such shifts to organic and other types of “quality” food production. While acknowledging the importance of these factors, we argue that the alternatives in this region are best understood as an outcome of broad‐based land‐conservation efforts developed through historical and ongoing struggles over urban growth, rising concerns about environmental values, and deep regional interests in dairy preservation. Over time, preservation of this agricultural landscape has contributed to the emergence of a quality food industry historically rooted in the region's politics of place. 相似文献
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