Nd,Sr, and Pb isotopic characteristics of Cretaceous intrusive rocks from deep levels of the Sierra Nevada batholith,Tehachapi Mountains,California

The Cretaceous gabbroic to granitic intrusive rocks of the Tehachapi Mountains were emplaced at depths of 25–30 km and thus afford a view of deep processes in the Sierra Nevada batholith. They consist of the 115 Ma Tehachapi suite and the 100 Ma Bear Valley suite; new zircon U-Pb age data reveal the presence of the latter as far west as Grapevine Canyon. The Nd, Sr, Pb, and O isotopic whole-rock data and zircon Pb inheritance patterns for the bulk of the suites suggest an origin by mixing between depleted mantlederived magmas and metasedimentary material with a substantial component of old continental material. However, this mixing is not evident in variations between isotopic ratios and chemical and lithologic parameters. This implies that isotopic hybridization of magmas took place deeper than 30 km, and that fractionation processes are likely responsible for the bulk of the chemical variation in this part of the Sierra Nevada batholith. Consideration of the isotopic data in the context of the Sierra Nevada batholith as a whole suggests that the well-known east-to-west isotopic gradients in the batholith may reflect a change in the average isotopic character of the preintrusive frame-work rather than a change in amount of crustal component. On the other hand, the lack of areal gradients in Sr and Nd isotopic ratios in the main study area may indicate a lack of pronounced gradation at deep levels, at least within the western batholith.     

Portable autonomous vertical profiler for estuarine applications

The design and implementation of a portable autonomous vertical profiler are documented and example data sets from a mesotidal estuary and a microtidal, wind-driven estuary are presented. The profiler sampling range dynamically adjusts to changes in water column depth, and a typical vertical sampling resolution of 4 cm is attained. Example data show the onset and vertical and temporal extent of stratification in the water column. The profiler is ideal for water quality monitoring programs in shallow estuaries that require frequent profiles of hydrographic, chemical, and biological parameters that can be measured in situ.     

Induced Microearthquake Patterns in Hydrocarbon and Geothermal Reservoirs: Six Case Studies

—?The injection or production of fluids can induce microseismic events in hydrocarbon and geothermal reservoirs. By deploying sensors downhole, data sets have been collected that consist of a few hundred to well over 10,000 induced events. We find that most induced events cluster into well-defined geometrical patterns. In many cases, we must apply high-precision, relative location techniques to observe these patterns. At three sedimentary sites, thin horizontal strands of activity are commonly found within the location patterns. We believe this reflects fracture containment between stratigraphic layers of differing mechanical properties or states of stress. At a massive carbonate and two crystalline sites, combinations of linear and planar features indicate networks of intersecting fractures and allow us to infer positions of aseismic fractures through their influence on the location patterns. In addition, the fine-scale seismicity patterns often evolve systematically with time. At sedimentary sites, migration of seismicity toward the injection point has been observed and may result from slip-induced stress along fractures that initially have little resolved shear. In such cases, triggering events may be critical to generate high levels of seismic activity. At one crystalline site, the early occurrence of linear features that traverse planes of activity indicate permeable zones and possible flow paths within fractures. We hope the continued development of microseismic techniques and refinement of conceptual models will further increase our understanding of fluid behavior and lead to improved resource management in fractured reservoirs.     

Discrimination between climate and human-induced dryland degradation

In this study we present a technique to discriminate between climate or human-induced dryland degradation, based on evaluations of AVHRR NDVI data and rainfall data. Since dryland areas typically have high inter-annual rainfall variations and rainfall has a dominant role in determining vegetation growth, minor biomass trends imposed by human influences are difficult to verify. By performing many linear regression calculations between different periods of accumulated precipitation and the annual NDVI_max, we identify the rainfall period that is best related to the NDVI_max and by this the proportion of biomass triggered by rainfall. Positive or negative deviations in biomass from this relationship, expressed in the residuals, are interpreted as human-induced. We discuss several approaches that use either a temporally fixed NDVI peaking time or an absolute one, a best mean rainfall period for the entire drylands or the best rainfall period for each individual pixel. Advantages and disadvantages of either approach or one of its combinations for discriminating between climate and human-induced degradation are discussed. Depending on the particular land-use either method has advantages. To locate areas with a high likelihood of human-induced degradation we therefore recommend combining results from each approach.     

Channel adjustments of the lower Trinity River,Texas, downstream of Livingston Dam

Channel cross‐sectional changes since construction of Livingston Dam and Lake Livingston in 1968 were studied in the lower Trinity River, Texas, to test theoretical models of channel adjustment, and to determine controls on the spatial extent of channel response. High and average flows were not significantly modified by the dam, but sediment transport is greatly reduced. The study is treated as an opportunistic experiment to examine the effects of a reduction in sediment supply when discharge regime is unchanged. Channel scour is evident for about 60 km downstream, and the general phenomena of incision, widening, coarsening of channel sediment and a decrease in channel slope are successfully predicted, in a qualitative sense, by standard models of channel response. However, there is no consistent channel response within this reach, as various qualitatively different combinations of increases, decreases or no change in width, depth, slope and roughness occur. These multiple modes of adjustment are predicted by the unstable hydraulic geometry model. Between about 60 km and the Trinity delta 175 km downstream of the dam, no morphological response to the dam is observed. Rather than a diminution of the dam's effects on fluvial processes, this is due to a fundamental change in controls of the fluvial system. The downstream end of the scour zone corresponds to the upstream extent of channel response to Holocene sea level rise. Beyond 60 km downstream, the Trinity River is characterized by extensive sediment storage and reduced conveyance capacity, so that even after dam construction sediment supply still exceeds transport capacity. The channel bed of much of this reach is near or below sea level, so that sea level rise and backwater effects from the estuary are more important controls on the fluvial system than upstream inputs. Copyright © 2005 John Wiley & Sons, Ltd.