The Hg geochemistry of a geothermal stream, Steamboat Creek, Nevada: natural vs. anthropogenic influences

 A series of water samples from Steamboat Creek, Nevada, was analyzed for total mercury concentrations. Concentrations from these waters were 40 to 60 times higher than the pristine mountain streams entering the creek. The major source of the mercury entering Steamboat Creek is probably from gold/silver processing that took place in the 1860s. Received: 10 March 1997 · Accepted: 2 September 1997     

Architectural element analysis within the Kayenta Formation (Lower Jurassic) using ground-probing radar and sedimentological profiling, southwestern Colorado

A well exposed outcrop in the Kayenta Formation (Lower Jurassic) in southwestern Colorado was examined in order to delineate the stratigraphy in the subsurface and test the usefulness of ground-probing radar (GPR) in three-dimensional architectural studies.

Two fluvial styles are present within the Kayenta Formation. Sandbodies within the lower third of the outcrop are characterized by parallel laminations that can be followed in the cliff-face for well over 300 m. These sandbodies are sheet-like in appearance, and represent high-energy flood deposits that most likely resulted from episodic floods. The remainder of the outcrop is characterized by concave-up channel deposits with bank-attached and mid-channel macroforms. Their presence suggests a multiple channel river system.

The GPR data collected on the cliff-top, together with sedimentological data, provided a partial three-dimensional picture of the paleo-river system within the Kayenta Formation. The 3-D picture consists of stacked channel-bar lenses approximately 50 m in diameter.

The GPR technique offers a very effective means of delineating the subsurface stratigraphy. Its high resolution capabilities, easy mobility, and rapid rate of data collection make it a useful tool. Its shallow penetration depth and limitation to low-conductivity environments are its only drawbacks.     

Land Use Evolution in Hill Regions of the Dominican Republic*

This study combines data from a social survey and an air photo time series to gain both a temporal and spatial understanding of Dominican hill land use evolution. Research results indicate that disparities in present land use and productivity between two hill land study sites stem from their different stages of development along a common land use continuum. The authors emphasize the importance of understanding land use history when identifying key constraints to long-term sustainability of hill land farming systems.     

The source of Central American lavas: inferences from geochemical inverse modeling

The rate earth element chemistry of a large suite of samples from the Central American volcanic front has been determined to understand their petrogenesis. Different segments of the volcanic front are compared on the basis of their calculated source rare earth patterns as deduced from inverse modeling. The results yield a range in the extent of light rare earth enrichment of the source, as well as in source mineralogy. Moderateto-strong LREE enrichment and high modal garnet contents are observed for the sources of both Guatemala and central Costa Rica, whereas slight LREE depletion and little or no garnet occurs in the Nicaraguan source region. Although distinct source regions beneath each Central American segment are permitted by the modeling, it is more likely that the mantle wedge is broadly homogeneous but locally heterogeneous. Small volume, relatively enriched, garnet-bearing veins surrounded by a matrix of isotopically depleted mantle periodotite could exist throughout the mantle wedge. Apparently distinct sources occur due to the variation in partial melting beneath the different segments, controlled by the amount of subduction-generated flux per unit wedge volume, which in turn is a function of the dip of the subducted lithosphere.     

Vegetation and climate change on the Bolivian Altiplano between 108,000 and 18,000 yr ago

A 90,000-yr record of environmental change before 18,000 cal yr B.P. has been constructed using pollen analyses from a sediment core obtained from Salar de Uyuni (3653 m above sea level) on the Bolivian Altiplano. The sequence consists of alternating mud and salt, which reflect shifts between wet and dry periods. Low abundances of aquatic species between 108,000 and 50,000 yr ago (such as Myriophyllum and Isoëtes) and marked fluctuations in Pediastrum suggest generally dry conditions dominated by saltpans. Between 50,000 yr ago and 36,000 cal yr B.P., lacustrine sediments become increasingly dominant. The transition to the formation of paleolake “Minchin” begins with marked rises in Isoëtes and Myriophyllum, suggesting a lake of moderate depth. Similarly, between 36,000 and 26,000 cal yr B.P., the transition to paleolake Tauca is also initiated by rises in Isoëtes and Myriophyllum; the sustained presence of Isoëtes indicates the development of flooded littoral communities associated with a lake maintained at a higher water level. Polylepis tarapacana-dominated communities were probably an important component of the Altiplano terrestrial vegetation during much of the Last Glacial Maximum (LGM) and previous wet phases.