Seismic Inference using Genetic Algorithms

A flood of reliable seismic data will soon arrive. The migration to largertelescopes on the ground may free up 4-m class instruments for multi-sitecampaigns, and several forthcoming satellite missions promise to yieldnearly uninterrupted long-term coverage of many pulsating stars. We willthen face the challenge of determining the fundamental properties of thesestars from the data, by trying to match them with the output of ourcomputer models. The traditional approach to this task is to make informedguesses for each of the model parameters, and then adjust them iterativelyuntil an adequate match is found. The trouble is: how do we know that oursolution is unique, or that some other combination of parameters will notdo even better? Computers are now sufficiently powerful and inexpensivethat we can produce large grids of models and simply compare all ofthem to the observations. The question then becomes: what range ofparameters do we want to consider, and how many models do we want tocalculate? This can minimize the subjective nature of the process, but itmay not be the most efficient approach and it may give us a false sense ofsecurity that the final result is correct, when it is really justoptimal. I discuss these issues in the context of recent advances inthe asteroseismological analysis of white dwarf stars.     

The Citation Impact of Digital Preprint Archives for Solar Physics Papers

Papers that are posted to a digital preprint archive are typically cited twice as often as papers that are not posted. This has been demonstrated for papers published in a wide variety of journals, and in many different subfields of astronomy. Most astronomers now use the arXiv.org server (astro-ph) to distribute preprints, but the solar physics community has an independent archive hosted at Montana State University. For several samples of solar physics papers published in 2003, I quantify the boost in citation rates for preprints posted to each of these servers. I show that papers on the MSU archive typically have citation rates 1.7 times higher than the average of similar papers that are not posted as preprints, while those posted to astro-ph get 2.6 times the average. A comparable boost is found for papers published in conference proceedings, suggesting that the higher citation rates are not the result of self-selection of above-average papers. Editors’ Note: This paper lies outside the normal purview of Solar Physics papers, however the editors feel that the content is of sufficient importance for all Solar Physics authors and readers to merit its publication.     

Magmatism in the eastern Aleutian Arc: temporal characteristic of igneous activity on Akutan Island

Lavas from Akutan Island, located in the eastern Aleutian arc at the transition between continental and oceanic crust, show a gradual change in their petrologic and chemical characteristics over the last 4 million years. The oldest lavas exposed on the island, the Hot Springs Bay Volcanics (HSBV), range from magnesian basalt to dacite (45%–62% SiO₂). The most mafic basalts contain salitic clinopyroxene, Cr- and Al-rich spinel, and pargasitic amphibole suggesting that they were derived from relatively hydrous magmas at greater pressures than lavas from the younger Akutan Volcanics (AKV) and the modern volcano (MOD). AKV lavas also range between basalt and dacite (46%–63% SiO₂), but contain no hydrous phenocrysts and seem to have fractionated within a shallow level magma chamber. Lavas from the modern volcano are andesitic (52%–57% SiO₂) and have a mineral assemblage similar to that of AKV lavas of similar composition. With the exception of clinopyroxene and spinel in the most mafic lavas, the compositions of plagioclase (An_92?45), olivine (Fo_88?51), orthopyroxene (En_69?56), and titanomagnetite (15%–21% TiO₂) phenocrysts found in these lavas are within the range observed in lavas from other Aleutian volcanoes. Variations in the major element chemistry of the older lavas can be reproduced by fractional crystallization of the observed mineral assemblages, however closed system crystal fractionation models are inadequate to explain the trace element variations. During the last 4 million years, La/Yb ratios have decreased (6.5–3.3 for HSBV lavas and 2.9–1.9 for MOD lavas) whereas Ba/La ratios appear to have increased slightly (37–43 for HSBV and AKV, and 41–45 of MOD). The lower La/Yb ratios of MOD lavas correspond with lower total abundances of the REE and slightly higher Sr and Pb isotopic ratios. The increased⁸⁷Sr/⁸⁶Sr ratios and Pb isotopic ratios in the MOD lavas, the less enriched LREE, and the higher Ba/La ratios may result from partial melting of an arc source which has experienced previous melting events but has continued to be contaminated by a component from the subducting slab. It may also indicate a change in the degree of partial melting of the underlying mantle, which corresponds to a different percentage of a slab derived component being incorporated into the overlying mantle.     

Spatial and temporal constraints on regional-scale groundwater flow in the Pampa del Tamarugal Basin,Atacama Desert,Chile

Aquifers within the Pampa del Tamarugal Basin (Atacama Desert, northern Chile) are the sole source of water for the coastal city of Iquique and the economically important mining industry. Despite this, the regional groundwater system remains poorly understood. Although it is widely accepted that aquifer recharge originates as precipitation in the Altiplano and Andean Cordillera to the east, there remains debate on whether recharge is driven primarily by near-surface groundwater flow in response to periodic flood events or by basal groundwater flux through deep-seated basin fractures. In addressing this debate, the present study quantifies spatial and temporal variability in regional-scale groundwater flow paths at 20.5°S latitude by combining a two-dimensional model of groundwater and heat flow with field observations and δ¹⁸O isotope values in surface water and groundwater. Results suggest that both previously proposed aquifer recharge mechanisms are likely influencing aquifers within the Pampa del Tamarugal Basin; however, each mechanism is operating on different spatial and temporal scales. Storm-driven flood events in the Altiplano readily transmit groundwater to the eastern Pampa del Tamarugal Basin through near-surface groundwater flow on short time scales, e.g., 10⁰–10¹ years, but these effects are likely isolated to aquifers in the eastern third of the basin. In addition, this study illustrates a physical mechanism for groundwater originating in the eastern highlands to recharge aquifers and salars in the western Pampa del Tamarugal Basin over timescales of 10⁴–10⁵ years.     

Macrobenthic infaunal communities associated with deep‐sea hydrocarbon seeps in the northern Gulf of Mexico

There are thousands of seeps in the deep ocean worldwide; however, many questions remain about their contributions to global biodiversity and the surrounding deep‐sea environment. In addition to being globally distributed, seeps provide several benefits to humans such as unique habitats, organisms with novel genes, and carbon regulation. The purpose of this study is to determine whether there are unique seep macrobenthic assemblages, by comparing seep and nonseep environments, different seep habitats, and seeps at different depths and locations. Infaunal community composition, diversity, and abundance were examined between seep and nonseep background environments and among three seep habitats (i.e., microbial mats, tubeworms, and soft‐bottom seeps). Abundances were higher at seep sites compared to background areas. Abundance and diversity also differed among microbial mat, tubeworm, and soft‐bottom seep habitats. Although seeps contained different macrobenthic assemblages than nonseep areas, infaunal communities were also generally unique for each seep. Variability was 75% greater within communities near seeps compared to communities in background areas. Thus, high variability in community structure characterized seep communities rather than specific taxa. The lack of similarity among seep sites supports the idea that there are no specific infauna that can be used as indicators of seepage throughout the northern Gulf of Mexico, at least at higher taxonomic levels.     

Observations of a lee wave and rotor episode in central California

Acoustic sounders and in-situ meteorological sensors were used to record the boundary-layer flow structure along the eastern slopes of the Altamont Hills in central California during a wind energy field program. Oscillating speeds in this area suggest the development of quasi-stationary lee waves and rotors under certain atmospheric conditions. Data averaging of 15 minutes or less is needed to study these phenomena.     

Some Biases in Sampling Multilevel Piezometers for Volatile Organics

Multilevel piezometers are cost-effective monitoring devices for determining the three-dimensional distribution of solutes in ground water. Construction includes flexible tubing (plastic or Teflon®). Their sampling is subject to a number of'potential biases, particularly: (1) losses of volatile organic solutes via volatilization, (2) sorption onto the flexible tubing of the piezometers, (3) leaching of organics from this tubing, and (4) collection of unrepresentative samples due to inadequate piezometer flushing. It is shown that these biases are minimal or are easily controlled in most situations.
Another source of bias has been recognized. Organic solutes present in ground water above the screened level can penetrate the flexible plastic or Teflon tubing and contaminate the sampled water being drawn through this tubing. Laboratory tests and field results indicate this transmission causes low organic contaminant concentrations to be erroneously attributed to ground water which is free of such contaminants. The transmitted organics apparently desorb from the plastic tubing during flushing of even 40 piezometer volumes.
Recognition of this transmission problem provides for a better interpretation of existing organic contaminant distribution data. Caution is advised when considering the use of these monitoring devices in organic solute contaminant studies.     

Effects of large organic material on channel form and fluvial processes

Stream channel development in forested areas is profoundly influenced by large organic debris (logs, limbs and rootwads greater than 10 cm in diameter) in the channels. In low gradient meandering streams large organic debris enters the channel through bank erosion, mass wasting, blowdown, and collapse of trees due to ice loading. In small streams large organic debris may locally influence channel morphology and sediment transport processes because the stream may not have the competency to redistribute the debris. In larger streams flowing water may move large organic debris, concentrating it into distinct accumulations (debris jams). Organic debris may greatly affect channel form and process by: increasing or decreasing stability of stream banks; influencing development of midchannel bars and short braided reaches; and facilitating, with other favourable circumstances, development of meander cutoffs. In steep gradient mountain streams organic debris may enter the channel by all the processes mentioned for low gradient streams. In addition, considerable debris may also enter the channel by way of debris avalanches or debris torrents. In small to intermediate size mountain streams with steep valley walls and little or no floodplain or flat valley floor, the effects of large organic debris on the fluvial processes and channel form may be very significant. Debris jams may locally accelerate or retard channel bed and bank erosion and/or deposition; create sites for significant sediment storage; and produce a stepped channel profile, herein referred to as ‘organic stepping’, which provides for variable channel morphology and flow conditions. The effect of live or dead trees anchored by rootwads into the stream bank may not only greatly retard bank erosion but also influence channel width and the development of small scour holes along the channel beneath tree roots. Once trees fall into the stream, their influence on the channel form and process may be quite different than when they were defending the banks, and, depending on the size of the debris, size of the stream, and many other factors, their effects range from insignificant to very important.