Microgranitoid enclave swarms in granitic plutons, central Sierra Nevada, California

Microgranitoid enclaves are common in granitic plutons worldwide, occurring individually and in homogeneous or heterogeneous swarms. Three plutons in the central Sierra Nevada batholith contain swarms with mostly heterogeneous suites of enclaves in the intermediate composition range, and occur in a number of two-dimensional shapes, specifically as dikes, small rafts, lenses, pipe/vortices and large massive shapes. Swarms are characterized by various features, including the nature of their boundary with the host, their planar or non-planar character, internal geometry, density of enclave packing, presence or absence of schlieren and crystal aggregates, and axial ratios and degree of preferred alignment of enclaves. We propose that heterogeneous enclave swarms form by one, or some combination of, the following mechanisms: (1) velocity-gradient sorting parallel or normal to the flow, (2) gravitational sorting or (3) break-up of heterogeneous dikes. Common sites where enclave swarms form include pluton margins or internal viscosity walls, within fractures, and near the pluton roof.     

Growth rates of American alligators in estuarine and palustrine wetlands in Louisiana

A comparative study of American alligator (Alligator mississippiensis) growth rates was made in estuarine and palustrine wetlands in southwestern Louisiana. In the estuarine wetlands, where characteristic salinity levels were ≤5%, alligators grew faster and therefore reached sexual maturity earlier than did those in palustrine wetlands, which are characterized by shallow, freshwater marsh vegetation. Slower growth rates in palustrine wetlands appeared to be related to prey density, indicated by previous studies to be lower than in estuarine wetlands. Males grew faster than females and therefore reached sexual maturity at an earlier age in both habitats. This study revealed a major limitation in using total lengths as an index upon which population age structure can be based even when alligators are in the same geographic region.     

Revision of lunar Rb-Sr ages

Becalibration of ‘spikes’ using stoichiometric Rb and Sr salts recently issued by the National Bureau of Standards shows that our determinations of the ratio ${}^{87}\text{Rb}{}^{86}\text{Sr}$ in lunar samples have been low by 1.8 per cent, which we attribute to non-stoichiometry of the salts previously used for calibration, particularly RbCl. As a result, our lunar age-determinations must be decreased by this amount.     

241Pu and241Am in sediments from coastal basins off California and Mexico

In sediments deposited in the coastal basins off the western North American continent since the early 1960's, there is more²⁴¹Am activity than one would predict if its presence was solely due to the decay of²⁴¹Pu that was produced during the testing of weapons in 1961–1962 (taken as July 1, 1962 for calculations). This excess can be accounted for by the decay of²⁴¹Pu, if pre-1962 fallout (fallout from the pre-moratorium testing) has been continuously introduced into the recent sediments along with fallout from post-moratorium testing. This hypothesis is supported by the plutonium depth profiles which do not reflect direct fallout from the atmosphere, as characterized by two peaks corresponding to the two major weapons testing periods, but continuously increasing plutonium and americium concentrations from 1954 to 1974. Thus, the nuclides may be coming from a large reservoir that has accumulated and mixed fallout since 1952. Such a concentration gradient for all of the nuclides requires their immobilization in the sediment after deposition.     

Microstructural evidence of time-relationships between metamorphism and deformation in the metasedimentary sequence of the Northern Hill end trough, New South Wales, Australia

Microstructural evidence of time-relationships between regional metamorphism and deformation has been obtained in the higher-grade, biotite-bearing parts of the metasedimentary sequence of the northern Hill End Tough, New South Wales. The rocks have only one prominent foliation (slaty cleavage) and one mineral-streaking lineation, which are of consistent orientation through the area. Therefore, they should be suitable for obtaining clear evidence of time-relationships. Microstructural criteria in many of the rocks are ambiguous, but relatively clear evidence is shown by porphyroblastic biotite phyllites (locally containing garnet), in which some of the porphyroblasts show microboudinage, new biotite occurring with quartz in the interboudin areas. These rocks indicate that biotite was stable during the later stages of deformation. The study shows that careful microstructural studies can be diagnostic of time-relationships in low-grade metamorphic areas, where critical rocks are available.     

A mathematical model to predict the inelastic response of a steel frame: Formulation of the model

The purpose of this research is to use data from experiments to formulate a mathematical model that will predict the non-linear response of a single-storey steel frame to an earthquake input. The process used in this formulation is system identification. The form of the model is a second-order non-linear differential equation with linear viscous damping and Ramberg—Osgood type hysteresis. The damping coefficient and the three parameters in the hysteretic model are to be established. An integral weighted mean squared error function is used to evaluate the [goodness of fit] between the model's response and the structure's response when both are subjected to the same excitation. The function includes errors in displacement and acceleration and is integrated from zero to a time T, which may be the full duration of the recorded response or only a portion of it. The parameters are adjusted using a modified Gauss-Newton method until the error function is minimized. The computer program incorporating these steps in the system identification process is verified with simulated data. Results given in the paper show that in every case the program converges in few iterations to the assigned set of parameters.     

A mathematical model to predict the inelastic response of a steel frame: Establishment of parameters from shaking table experiments

The purpose of this research is to use data from experiments to formulate a mathematical model that will predict the non-linear response of a single-storey steel frame to an earthquake input. The process used in this formulation is system identification. In experiments performed on a shaking table, the frame was subjected to two earthquake motions at several intensities. In each case the frame underwent severe inelastic deformation. A computer program which incorporates the concepts of system identification makes use of the recorded data to establish four parameters in a non-linear mathematical model. When different amounts of data are used in the program, parameter sets are established which give the best model response for that amount of test data. The resulting sets of parameters reflect the way in which the properties of the structure change during the excitation. However, when the full durations of the different excitations are used, the sets of parameters are almost identical. For each of these sets of parameters, the correlation of the computed accelerations with the measured is excellent, and the shape of the computed displacement response compares very well with the measured response, although the permanent offset of the displacements is not computed exactly. Suggestions are given on how to overcome this deficiency in the mathematical model.     

Effects of dam operation and land use on stream channel morphology and riparian vegetation

Dams are well known for influencing channel and vegetation dynamics downstream, but little work has focused on distinguishing effects of land use and channel responses to the impoundment. In this paper, we examined interacting effects of a dam and land use on downstream changes in channel morphology and riparian vegetation along an agricultural stream system in northern California. Measurements of planform channel morphology, vegetation area, and land use were mapped along multiple stream segments based on a chronological sequence of historical aerial photographs over a 34-yr period prior to operation of the dam in 1983 and over a 17-yr period after dam operation, and compared to a nearby, undammed reference stream. A two-factor analysis of covariance (ANCOVA) was used to examine the effect of the dam on changes in bankfull area, stream length, and riparian vegetation area while accounting for the effect of land use and distance downstream. The dammed stream's bankfull area contracted 94% after dam operation. Prior to dam operation, bankfull area decreased when land use area increased, but not after operation of the dam. Stream length varied 64% less after dam operation as a consequence of less frequent episodic channel migration and entrenchment. The area of riparian vegetation was decreasing during the pre-dam period, but then increased 72% after operation of the dam. Across time periods, decreases in the area of riparian vegetation were also associated with increases in land use area in both the dammed and reference stream. After operation of the dam, reduced peak discharges and sediment reduction likely lead to channel incision and constrained channel migration, which allowed vegetation to increase 50% on less accessible, abandoned banks. Rating curve and hydraulic exponent analyses based on stream gauge measurements corroborate statistical analyses of the mapped changes. In conclusion, we found that operation of the dam and land use patterns together influenced spatial and temporal changes in channel morphology and riparian vegetation. Use of a nearby undammed reference stream in conjunction with multivariable analysis of spatially and temporally replicated observations provided an effective framework for unraveling interacting effects of dams and land use activities on stream channel and vegetation dynamics.     

CO2 Mitigation by Agriculture: An Overview

Agriculture currently contributes significantly to the increase of CO2 in the atmosphere, primarily through the conversion of native ecosystems to agricultural uses in the tropics. Yet there are major opportunities for mitigation of CO2 and other greenhouse gas emissions through changes in the use and management of agricultural lands. Agricultural mitigation options can be broadly divided into two categories: (I) strategies to maintain and increase stocks of organic C in soils (and biomass), and (ii) reductions in fossil C consumption, including reduced emissions by the agricultural sector itself and through agricultural production of biofuels to substitute for fossil fuels.Reducing the conversion of new land to agriculture in the tropics could substantially reduce CO2 emissions, but this option faces several difficult issues including population increase, land tenure and other socio-political factors in developing countries. The most significant opportunities for reducing tropical land conversions are in the humid tropics and in tropical wetlands. An important linkage is to improve the productivity and sustainability of existing agricultural lands in these regions.Globally, we estimate potential agricultural CO2 mitigation through soil C sequestration to be 0.4-0.9 Pg C y-1, through better management of existing agricultural soils, restoration of degraded lands, permanent "set-asides" of surplus agricultural lands in temperate developed countries and restoration of 10-20% of former wetlands now being used for agriculture. However, soils have a finite capacity to store additional C and therefore any increases in C stocks following changes in management would be largely realized within 50-100 years.Mitigation potential through reducing direct agricultural emissions is modest, 0.01-0.05 Pg C y-1. However, the potential to offset fossil C consumption through the use of biofuels produced by agriculture is substantial, 0.5-1.6 Pg C y-1, mainly through the production of dedicated biofuel crops with a smaller contribution (0.2-0.3 Pg C y-1) from crop residues.Many agricultural mitigation options represent "win-win" situations, in that there are important side benefits, in addition to CO2 mitigation, that could be achieved, e.g. improved soil fertility with higher soil organic matter, protection of lands poorly suited for permanent agriculture, cost saving for fossil fuel inputs and diversification of agricultural production (e.g. biofuels). However, the needs for global food production and farmer/societal acceptability suggest that mitigation technologies should conform to: (I) the enhancement of agricultural production levels in parts of the world where food production and population demand are in delicate balance and (ii) the accrual of additional benefits to the farmer (e.g., reduced labor, reduced or more efficient use of inputs) and society at large.