			Home About us Contact

Pleistocene Times (pleistocene + time)

Distribution by Scientific Domains

Earth and Environmental Science	40%

Selected Abstracts

Water Resource Implications of 18O and 2H Distributions in a Basalt Aquifer System

GROUND WATER, Issue 6 2000
Kathryn R. Larson
Ongoing decline of water levels in the confined basalt aquifers of the Pullman-Moscow Basin of Washington and Idaho has prompted study of the timing, amount and distribution of recharge to the system. Previous radiocarbon ages indicate residence times on the order of 103 years and greater and suggest a low rate of recharge to the lower basalt aquifer since the end of Pleistocene time. By contrast, more recent hydrodynamic flow modeling studies invoke a larger Holocene recharge rate through the unconfined loess unit to the upper and lower basalt aquifers, which implies relatively short residence times (102 years). Stable isotopes were used to independently assess contrasting recharge models by comparing 18O/16O and D/H ratios of late-Holocene shallow ground water and deep ground water. Linear regression of local precipitation ratios yields ,D = 6.9 ,18O ,18.5. There is no evidence of fractionation of ground water ratios by recharge processes or water-rock interactions. Deep basalt ground water ,18O values are depleted by 0.4 to 4.9 per mil relative to shallow, recently recharged ground waters and have ,18O values statistically distinct from waters sampled from other stratigraphic units. These findings suggest that the deep waters in the basin were not precipitated under current climate conditions and that aquifer recharge rates to the deep basalt aquifer are substantially lower than have been recently estimated. This in turn suggests that a sustainable ground water exploitation scheme must reduce reliance on the deep ground water resource. [source]

Holocene sedimentation in the Skagerrak interpreted from chirp sonar and core data,

JOURNAL OF QUATERNARY SCIENCE, Issue 1 2005
Richard Gyllencreutz
Abstract High-resolution chirp sonar profiling in the northeastern Skagerrak shows acoustically stratified sediments draping a rough-surfaced substratum. A 32 metre long sediment core retrieved from the survey area encompasses the entire Holocene and latest Pleistocene. The uppermost seismo-acoustic units in the chirp profiles represent Holocene marine sediments. The lowermost unit is interpreted as ice-proximal glacial-marine sediments rapidly deposited during the last deglaciation. The end of ice-proximal sedimentation is marked by a strong reflector, interpreted to have been formed during latest Pleistocene time as a consequence of rapid ice retreat and drastically lowered sedimentation rate. The subsequent distal glacial-marine sediments were deposited with initially high sedimentation rates caused by an isostatic rebound-associated sea-level fall. Based on correlation between the core and the chirp sonar profiles using measured sediment physical properties and AMS 14C dating, we propose a revised position for the Pleistocene/Holocene boundary in the seismo-acoustic stratigraphy of the investigated area. Copyright © 2005 John Wiley & Sons, Ltd. [source]

Sexual dimorphism in limb bones of ibex (Capra ibex L.): mixture analysis applied to modern and fossil data

INTERNATIONAL JOURNAL OF OSTEOARCHAEOLOGY, Issue 5 2007
H. Fernández
Abstract Estimating sex ratios of fossil bone assemblages is an important step in the determination of demographic profiles, which are essential for understanding the palaeobiology and palaeoethology of any particular species, as well as its exploitation patterns by humans. This is especially true for ibex (Capra ibex), which was a main source of food for hominids during Pleistocene times. Classical methods for determining sexual dimorphism and sex ratio, such as analyses using uni- and bivariate plots, are based on an arbitrary fixing of limits between sexes. Here we use a more robust statistical method termed mixture analysis (MA) to determine the sex of postcranial remains (long bones, metapodials and tarsals) from ibex. For the first time, we apply MA to both a modern and a fossil sample of one species, by using metric data taken from (i) a collection of present-day ibex skeletons and (ii) a Palaeolithic sample of the same species. Our results clearly show that the forelimb (humerus and radius) is more dimorphic than the hindlimb (femur and tibia) and is therefore better suited for sexing ibex. It also appears that metapodials should be used carefully for estimating sex ratios. On the basis of these results, we propose a classification of bone measurements that are more or less reliable for sexing ibex. The results of MA applied to the ibex fossil bones from the Upper Palaeolithic site of the Observatoire (Monaco) lead us to the conclusion that this assemblage consists of a majority of males. The quantitative estimations calculated by the MA make it possible to compare the size of Pleistocene and modern ibex for the whole set of variables used in this study. Copyright © 2006 John Wiley & Sons, Ltd. [source]

Late Cenozoic structural and stratigraphic evolution of the northern Chinese Tian Shan foreland

BASIN RESEARCH, Issue 3 2010
Honghua Lu
ABSTRACT Three successive zones of fault-related folds disrupt the proximal part of the northern Tian Shan foreland in NW China. A new magnetostratigraphy of the Taxi He section on the north limb of the Tugulu anticline in the middle deformed zone clarifies the chronology of both tectonic deformation and depositional evolution of this collisional mountain belt. Our ,1200-m-thick section encompasses the upper Cenozoic terrigenous sequence within which ,300 sampling horizons yield an age span of ,8,2 Ma. Although the basal age in the Taxi He section of the Xiyu conglomerate (often cited as an indicator of initial deformation) is ,2.1 Ma, much earlier growth of the Tugulu anticline is inferred from growth strata dated at ,6.0 Ma. Folding of Neogene strata and angular unconformities in anticlines in the more proximal and distal deformed zones indicate deformation during Miocene and Early Pleistocene times, respectively. In the Taxi He area, sediment-accumulation rates significantly accelerate at ,4 Ma, apparently in response to encroaching thrust loads. Together, growth strata, angular unconformities, and sediment-accumulation rates document the northward migration of tectonic deformation into the northern Tian Shan foreland basin during the late Cenozoic. A progradational alluvial,lacustrine system associated with this northward progression is subdivided into two facies associations at Tugulu: a shallow lacustrine environment before ,5.9 Ma and an alluvial fan environment subsequently. The lithofacies progradation encompasses the time-transgressive Xiyu conglomerate deposits, which should only be recognized as a lithostratigraphic unit. Along the length of the foreland, the locus of maximum shortening shifts between the medial and proximal zones of folding, whereas the total shortening across the foreland remains quite homogeneous along strike, suggesting spatially steady tectonic forcing since late Miocene times. [source]

Mangrove Distribution during the Holocene in Tribugí Gulf, Colombia,

BIOTROPICA, Issue 1 2000
Carlos Jaramillo
ABSTRACT Neotropical mangrove ecosystems have undergone drastic changes in terms of extension and floristic composition during Plio,Pleistocene times. It is unclear if the northern Pacific coast of Colombia has been occupied continuously by mangroves during the last 5000 years, or if their presence is a recent phenomenon. Two basic questions were asked: (1) is the establishment of mangroves recent?; and (2) what were the dominant floral taxa during the late Holocene? In the Gulf of Tribugí, northern Colombian Pacific, 22 sediment cores were drilled to a maximum depth of 2 m in a diverse suite of mangrove substrates and positions relative to the shoreline. Data were gathered from sedimentological descriptions, palynological analyses, and radiocarbon dating of these cores. During the last 4500 years, mangroves in the southern area of the Tribugí Gulf have remained floristically stable and dominated by Rhizophora. The abundant presence of Acrostichum aureum is recent, probably related to human activities. In contrast, two areas in the northern part of the Gulf show a different history. In the first area, the establishment of mangrove has been relatively recent (ca 2600 yr), probably a result of local subsidence due to tectonic faulting; this mangrove forest was and is dominated by Pelliciem rhizophorae. In the second area, mangrove pollen was not found in sediments younger than 2500 years, suggesting that the scarce presence of mangrove in the area is a result of recent colonization, and not due to logging as previously thought. RESUMEN Ecosistemas Neotropicales de manglar han sufrido cambios dramáticos tanto en su extensión como en su composición florística asociados con cambios climáticos y variaciories del nivel del mar durante el Plio-Pleistoceno. No es claro si la costa norte del Pacífico Colombiano ha sido ocupada por manglares durante los últimos 5000 años. Quisimos responder dos preguncas básicas: Es el establecimiento del manglar un fenómeno reciente? y Cuales fueron las especies que dominaron el mangle durante el Holoceno? Para responder estas preguntas extrajimos 22 corazones de sedimento cn el Golfo de Tribugá, costa norte del Pacífico Colombiano. Los corazones, de máximo dos metros de profundidad, fueron analizados con méttodos palinológicos, sedimentológicos y dataciones radiométricas. El tipo de sedimento más común fué lodo arenoso verde altamente bioperturbado por actividad infaunal. Los manglares mostraron ser muy estables y doniinados por Rhizophora en la mayoría de los sitios estudiados, con excepción de los manglares en la Chunga (Utría) y El Valle. Allí 10s manglares se establecieron recientemente. En la Chunga, un ascenso relativo del nivel del mar hace aproximadamente 2600 años a.p., probablemente local y asociado con la falla de Utría, posibilitó la colonización del sustrato por Pelliciera rhizophorae; es incierto determinar si este ascenso aún se esta produciendo. En el Valle, el escaso establecimiento del manglar es un proceso reciente de colonización y no una reducción en la cobertura del manglar producto de actividades humanas como previamente se asumía. [source]