Climate change and population history in the Pacific Lowlands of Southern Mesoamerica

Core MAN015 from Pacific coastal Guatemala contains sediments accumulated in a mangrove setting over the past 6500 yr. Chemical, pollen, and phytolith data, which indicate conditions of estuarine deposition and terrigenous inputs from adjacent dry land, document Holocene climate variability that parallels the Maya lowlands and other New World tropical locations. Human population history in this region may be driven partly by climate variation: sedentary human populations spread rapidly through the estuarine zone of the lower coast during a dry and variable 4th millennium B.P. Population growth and cultural florescence during a long, relatively moist period (2800–1200 B.P.) ended around 1200 B.P., a drying event that coincided with the Classic Maya collapse.     

Earth,Wind, and Fire: Wildfire Risk Perceptions in a Hurricane-Prone Environment

Wildfire is one of several potential disturbances that could have extraordinary impacts on individuals and communities in fire-prone areas. In this article we describe disturbance risk perceptions from interviews with residents in three Florida communities that face significant wildfire and hurricane risk. Although they live in areas characterized by emergency managers as having high wildfire risk and many participants have direct experience with wildfire, residents tended to share high hurricane and low wildfire risk perceptions. The different perceptions of risk seem linked to several factors: direct hurricane experience, different scales of impact, the local “hurricane culture,” effectiveness of local ordinances and development patterns, perceived predictability of the event, and perceived ability to control the event. This study shows that residents may perceive and act to reduce risk for one disturbance in relation to their perceptions, concern, and actions for another.     

Plagioclase nucleation and growth kinetics in a hydrous basaltic melt by decompression experiments

Isothermal single-step decompression experiments (at temperature of 1075 °C and pressure between 5 and 50 MPa) were used to study the crystallization kinetics of plagioclase in hydrous high-K basaltic melts as a function of pressure, effective undercooling (ΔT _eff) and time. Single-step decompression causes water exsolution and a consequent increase in the plagioclase liquidus, thus imposing an effective undercooling (?T _eff), accompanied by increased melt viscosity. Here, we show that the decompression process acts directly on viscosity and thermodynamic energy barriers (such as interfacial-free energy), controlling the nucleation process and favoring the formation of homogeneous nuclei also at high pressure (low effective undercoolings). In fact, this study shows that similar crystal number densities (N _a) can be obtained both at low and high pressure (between 5 and 50 MPa), whereas crystal growth processes are favored at low pressures (5–10 MPa). The main evidence of this study is that the crystallization of plagioclase in decompressed high-K basalts is more rapid than that in rhyolitic melts on similar timescales. The onset of the crystallization process during experiments was characterized by an initial nucleation event within the first hour of the experiment, which produced the largest amount of plagioclase. This nucleation event, at short experimental duration, can produce a dramatic change in crystal number density (N _a) and crystal fraction (?), triggering a significant textural evolution in only 1 h. In natural systems, this may affect the magma rheology and eruptive dynamics on very short time scales.     

Persistence of a Groundwater Contaminant Plume after Hydraulic Source Containment at a Chlorinated‐Solvent Contaminated Site

The objective of this study was to characterize the behavior of a groundwater contaminant (trichloroethene, TCE) plume after implementation of a source‐containment operation at a site in Arizona. The plume resides in a quasi‐three‐layer system comprising a sand/gravel unit bounded on the top and bottom by relatively thick silty clayey layers. The system was monitored for 60 months beginning at start‐up in 2007 to measure the change in contaminant concentrations within the plume, the change in plume area, the mass of the contaminant removed, and the integrated contaminant mass discharge (CMD). The concentrations of TCE in groundwater pumped from the plume extraction wells have declined significantly over the course of operation, as have concentrations for groundwater sampled from 40 monitoring wells located within the plume. The total CMD associated with operation of the plume extraction wells peaked at 0.23 kg/d, decreased significantly within 1 year, and thereafter began an asymptotic decline to a current value of approximately 0.03 kg/d. Despite an 87% reduction in contaminant mass and a comparable 87% reduction in CMD for the plume, the spatial area encompassed by the plume has decreased by only approximately 50%. This is much less than would be anticipated based on ideal flushing and mass‐removal behavior. Simulations produced with a simplified three‐dimensional (3D) numerical model matched reasonably well to the measured data. The results of the study suggest that permeability heterogeneity, back diffusion, hydraulic factors associated with the specific well field system, and residual discharge from the source zone are all contributing to the observed persistence of the plume, as well as the asymptotic behavior currently observed for mass removal and for the reduction in CMD.     

Geochemistry of a spring-dense karst watershed located in a complex structural setting, Appalachian Great Valley, West Virginia, USA

The distribution and chemistry of the springs in the Tuscarora Creek watershed is controlled by both geologic structure and karst dissolution. The watershed is located in eastern West Virginia in the structurally complex Great Valley of the Appalachian Valley and Ridge province. The upper portion of the stream parallels strike along a mapped fault zone and is bordered by clastic rocks that comprise North Mountain. The lower reaches of the stream flow cross-strike through Cambro-Ordovician carbonates. The controlling chemical signature in the spring water is carbonate dissolution. Little evidence was seen for the recharge from adjacent clastic rocks although differences in the Ca/Mg molar ratio between springs indicated the presence of localized spring basins in headwater reaches. Na, Cl and Ca generally increased from upstream to downstream in the cross-strike reaches. Comparison of stream and cumulative spring discharge was consistent with significant groundwater base-flow contribution directly to the creek, particularly in the strike-parallel region. The largest spring in the watershed (>162 L/s) was sampled during and after a large storm event along with the adjacent creek. The creek displayed a typical dilution response with each flood pulse, whereas the spring had only a limited or delayed response. The overall chemical and thermal stability of the spring, relative to the creek, indicated the lack of significant direct hydraulic connection between the two. The conceptual model for the area includes localized flowpaths in the headwater region where the stream flow is parallel to strike and a thrust fault. In addition to the shallow localized flowpaths, a deeper, more regional flowpath likely exists for a large spring further downstream.     

The Composite Nature of the Provo Level of Lake Bonneville, Great Basin, Western North America

Deposits of a transgressive-phase Lake Bonneville stillstand or oscillation are found just below the elevation of the regressive-phase Provo shoreline at numerous exposures throughout the Bonneville basin. Existence of these subProvo shoreline deposits provides a new explanation for the massive size of Provo depositional and erosional landforms, which can no longer be explained by a long stillstand at the Provo shoreline. Provo coastal landforms are large because they are superimposed on subProvo landforms. Results also help to clarify divergent interpretations regarding the relative age of the Provo shoreline and the number of times it was occupied by the water plane. Occupation of approximately the same level during both the transgressive and the regressive phase of Lake Bonneville may be coincidental, or it may indicate that a bedrock sill controlled outflow at subProvo as well as Provo time. Rise to the Bonneville level could have occurred after massive slope failure plugged the outlet pass.     

Freshwater organic deposits and stratified sediments between Early and Late Midlandian (Devensian) till sheets,at Aghnadarragh,County Antrim,Northern Ireland

The following events have been identified from a complex sequence of Midlandian (Devensian) sediments recently exposed at Aghnadarragh, County Antrim: (1) A lower till interpreted as Early Midlandian was deposited by a major lowland ice sheet which moved south-eastwards from central Ulster across the Lough Neagh depression. (2) Deglacial conditions were followed by a periglacial phase characterised by ice-wedge growth and sedimentation by gravelly debris flows. The latter contain the oldest known remains of Mammuthus primigenius in Ireland. (3) An Early Midlandian interstadial is represented by a woody detritus peat, with evidence of Betula, Pinuos and Picea woodlands and a rich beetle fauna. Wood from this horizon has been shown to be beyond the range of radiocarbon dating (>48 180 BP). (4) Interstadial conditions were succeeded by a cold, non-glacial phase dominated by in-channel gravelly flows and deposition of organic muds which contain plant and insect fossils. These horizons are older than 46 850 BP. (5) An upper, non-drumlinised till was deposited during the Late-Midlandian by a major lowland ice sheet which moved generally eastwards across the Lough Neagh Lowlands from central Ulster. This glaciation probably reached its maximum at ca 20–24 ka. (6) Drumlin formation occurred in the Lough Neagh Lowlands towards the end of the Late-Midlandian. The limiting moraines are dated to 17 ka. The lower till at Aghnadarragh is the first positive record of a major lowland ice sheet in Ireland during the early part of the last cold stage. The Early Midlandian interstadial peats have not been documented elsewhere in Ireland and correlate broadly with the Chelford Interstadial complex of the English Midlands. Related exposures elsewhere in Ulster confirm that the middle part of the last cold stage was free of major lowland ice masses but deposits of this period are absent from Aghnadarragh.     

Storm pulse chemographs of saturation index and carbon dioxide pressure: implications for shifting recharge sources during storm events in the karst aquifer at Fort Campbell,Kentucky/Tennessee,USA

Continuous records of discharge, specific conductance, and temperature were collected through a series of storm pulses on two limestone springs at Fort Campbell, western Kentucky/Tennessee, USA. Water samples, collected at short time intervals across the same storm pulses, were analyzed for calcium, magnesium, bicarbonate, total organic carbon, and pH. Chemographs of calcium, calcite saturation index, and carbon dioxide partial pressure were superimposed on the storm hydrographs. Calcium concentration and specific conductance track together and dip to a minimum either coincident with the peak of the hydrograph or lag slightly behind it. The CO₂ pressure continues to rise on the recession limb of the hydrograph and, as a result, the saturation index decreases on the recession limb of the hydrograph. These results are interpreted as being due to dispersed infiltration through CO₂-rich soils lagging the arrival of quickflow from sinkhole recharge in the transport of storm flow to the springs. Karst spring hydrographs reflect not only the changing mix of base flow and storm flow but also a shift in source of recharge water over the course of the storm.

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Resumen Se ha registrado en continuo la descarga, conductancia específica y temperatura de una serie de episodios de tormenta en dos manantiales en calizas ubicados en Fort Campbell, en el oeste de Kentucky/Tennessee (Estados Unidos de América). Se ha analizado muestras de agua recogidas en breves intervalos de tiempo durante los episodios de tormenta, determinando el calcio, magnesio, bicarbonato, carbono orgánico total y pH. Se ha superpuesto quimiogramas de calcio, índice de saturación en calcita y presión parcial de dióxido de carbono en los hidrogramas de las tormentas. La concentración de calcio y la conductancia específica se comportan de forma similar y presentan un mínimo que coincide también con un pico del hidrograma o que se retrasa ligeramente con respecto a él. La presión de dióxido de carbono sigue aumentando en la rama de recesión del hidrograma y, como consecuencia, disminuye el índice de saturación de la rama de recesión del hidrograma. Se interpreta que estos resultados son debidos a la infiltración dispersa a través de suelos enriquecidos en dióxido de carbono que retrasan el flujo rápido desde la recarga en los sumideros hasta su afloramiento en los manantiales. Los hidrogramas en manantiales kársticos reflejan no sólo la mezcla cambiante del flujo de base y el de tormenta, sino también el cambio en el origen del agua de recarga durante el curso de la tormenta.

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Résumé Lenregistrement en continu du débit, de la conductivité et de la température de leau a été réalisé au cours dune série de crues à deux sources émergeant de calcaires, à Fort Campbell (Kentucky occidental, Tennessee, États-Unis). Des échantillons deau, prélevés à de courts pas de temps lors de ces crues, ont été analysés pour le calcium, le magnésium, les bicarbonates, le carbone organique total et le pH. Les chimiogrammes de calcium, dindice de saturation de la calcite et de la pression partielle en CO₂ ont été superposés aux hydrogrammes de crue. La concentration en calcium et la conductivité de leau se suivent bien et passent par un minimum correspondant au pic de lhydrogramme ou légèrement retardé. La pression partielle en CO₂ continue de croître au cours de la récession de lhydrogramme de même que lindice de saturation de la calcite décroît. Ces résultats sont interprétés comme étant dus à linfiltration dispersée au travers de sols riches en CO₂, décalée par rapport à larrivée de lécoulement rapide provenant de la recharge, à partir dune perte, de lécoulement de crue vers les sources. Les hydrogrammes de sources karstiques ne reflètent pas seulement le mélange variable de lécoulement de base et de lécoulement de crue, mais également un changement dorigine de leau de la recharge au cours de lépisode de crue.