Sequence Of Strata Research Articles

Bayesian Refinement of a Stratified Sequence of Radiometric Dates from Punta De Chimino, Guatemala

Bayesian analysis of 6 radiocarbon and 2 luminescence determinations from Punta de Chimino's acropolis provides subcentury chronometric accuracy for a Protoclassic hiatus and a more decisive, incipient Early Classic abandonment. For the latter event, sensitivity tests and a redundant modal value pattern reduce the period of historical interest from a few centuries to several decades. The findings aid in selecting between 2 historical scenarios and demonstrate that improved chronological accuracy is attainable for sites and contexts lacking calendrical dates.

First biostratigraphic (palynological) dating of Middle and Late Cambrian strata in the subsurface of northwestern Algeria, North Africa: Implications for regional stratigraphy

The first biostratigraphic (palynological) evidence of Middle to Late Cambrian sedimentation in the Algerian subsurface is presented and discussed in this paper, resulting from the palynological analysis of pre-Ordovician clastic sequences penetrated by borehole AMG-1 in northwestern Algeria. Well preserved and moderately diversified acritarchs occur in a transgressive sequence comprising the upper third of the Sebkhet el Mellah Formation and most part of the Aïn en Nechea Formation. This acritarch suite corresponds to the Cristallinium cambriense — Eliasum/ Timofeevia acritarch Superzone established in Morocco as well as to the Adara alea acritarch Zone of eastern Newfondland, both directly correlated to the Paradoxides paradoxissimus Trilobite Zone of Middle Cambrian age. This stratigraphic interval correlates clearly with the mixed carbonate–siliciclastic successions outcropping in the Tacheddirt Valley of the Anti Atlas, Morocco. The uppermost part of the Aïn en Nechea Formation is possibly attributed to the Late Cambrian, based on the absence of the typical Middle Cambrian species, and the presence of taxa of Timofeevia and Cristallinium, associated to an increase in abundance and diversity of diacromorphs acritarchs, suggesting that the Late Cambrian stratal sequences must be considerably reduced and/or partly eroded in the study area. These results provide the basis for improved correlation with the acritarch-bearing Middle Cambrian sediments of the Anti Atlas in Morocco, and with the outcrops of the Ougarta Ranges further south in Algeria, facilitating future sequence stratigraphic studies of the pre-Ordovician clastic successions of the North Saharan Platform, with a higher resolution than previuosly available.

Human Impacts on Nearshore Shellfish Taxa: A 7,000 Year Record from Santa Rosa Island, California

Within the broad framework of historical and behavioral ecology, we analyzed faunal remains from a large habitation site (CA-SRI-147) on Santa Rosa Island to explore a 7,000 year record of coastal subsistence, nearshore ecological dynamics, and human impacts on shellfish populations. This long, stratified sequence provides a rare opportunity to study the effects of prolonged human predation on local intertidal and nearshore habitats. During the past 7,000 years, the Island Chumash and their predecessors had significant impacts on nearshore ecosystems, caused by growing human populations and depletion of marine and terrestrial ecosystems. At CA-SRI-147, local depletion of higher ranked shellfish species stimulated dietary expansion and a heavier reliance on lower-ranked shellfish taxa and more intensive exploitation of nearshore and pelagic fishes. In the Late Holocene, as local ecosystems were increasingly depleted, the Island Chumash relied increasingly on craft specialization and trade to meet their subsistence needs. Native peoples clearly impacted Channel Island ecosystems, but data from CA-SRI-147 suggest that they adjusted their subsistence strategies toward productive fisheries that sustained the high population densities and sociopolitical complexity recorded by early Spanish chroniclers at European contact.

Detrital record of Mesozoic shortening in the Yanshan belt, NE China: testing structural interpretations with basin analysis

ABSTRACTThe Yanshan fold‐thrust belt is an exposed portion of a major Mesozoic orogenic system that lies north of Beijing in northeast China. Structures and strata within the Yanshan record a complex history of thrust faulting characterized by multiple deformational events. Initially, Triassic thrusting led to the erosion of a thick sequence of Proterozoic and Palaeozoic sedimentary strata from northern reaches of the thrust belt; Triassic–Lower Jurassic strata that record this episode are deposited in a thin belt south of this zone of erosion. This was followed by postulated Late Jurassic emplacement of a major allochthon (the Chengde thrust plate), which is thought to have overridden structures and strata associated with the Triassic event and is cut by two younger thrusts (the Gubeikou and Chengde County thrusts). The Chengde allochthon is now expressed as a major east–west trending, thrust‐bounded synform (the Chengde synform), which has been interpreted as a folded klippe 20 km wide underlain by a single, north‐vergent thrust fault. Two sedimentary basins, defined on the basis of provenance, geochronology and palaeodispersal trends, developed within the Yanshan belt during Late Jurassic–Early Cretaceous time and are closely associated with the Chengde thrust and allied structures. Shouwangfen basin developed in the footwall of the Gubeikou thrust and records syntectonic unroofing of the hanging wall of that fault. Chengde basin developed in part atop Proterozoic strata interpreted as the upper plate of the Chengde allochthon and records unroofing of the adjacent Chengde County thrust. Both the Chengde County thrust and the Gubeikou thrust are younger than emplacement of the postulated Chengde allochthon, and structurally underlie it, yet neither Shouwangfen basin nor Chengde basin contain a detrital record of the erosion of this overlying structure. In addition, facies, palaeodispersal patterns and geochronology of Upper Jurassic strata that are cut by the Chengde thrust suggest only limited (ca. 5 km) displacement along this fault. We suggest that the units forming the Chengde synform are autochthonous, and that the synform is bounded by two limited‐displacement faults of opposing north and south vergence, rather than a single large north‐directed thrust. This conclusion implies that the Yanshan belt experienced far less Late Jurassic shortening than was previously thought, and has major implications for the Mesozoic evolution of the region. Specifically, we argue that the bulk of shortening and uplift in the Yanshan belt was accomplished during Triassic–Early Jurassic time, and that Late Jurassic structures modified and locally ponded sediments from a well‐developed southward drainage system developed atop this older orogen. Although Upper Jurassic strata are widespread throughout the Yanshan belt, it is clear that these strata developed within several discrete intermontane basins that are not correlable across the belt as a single entity. Thus, the Yanshan has no obvious associated foreland basin, and determining where the Mesozoic erosional products of this orogen ultimately lie is one of the more intriguing unresolved questions surrounding the palaeogeography of North China.

3-D physical model in strong ground motion numerical simulation: A case study of Kunming basin

Seismic hazard assessment based on urban active faults can provide scientific bases for city planning and project construction, while numerical simulation of strong ground motion is an important method for seismic hazard prediction and assessment. A 3-D physical model in conformity with real strata configuration of (mainly) the Quaternary is a prerequisite to ensure the reliability of the simulation results. In this paper, we give a detailed account of the technical scheme and process for creating a 3-D physical model in Kunming basin. The data used are synthesized from seismogeological data, borehole data, topographic data, digital elevation mode (DEM) data, seismic exploration results and wave velocity measurements. Stratigraphic division is based mainly on shear wave velocity, with strata sequence taken into consideration. The model construction is finally accomplished with ArcGIS and many relevant programming techniques via layer-by-layer stacking (in depth direction) of the adjacent medium interfaces (meshes). Meanwhile, a database of 3-D physical models is set up, which provides model data and parameters for strong ground motion simulation. Some processing methods and significant issues are also addressed in the paper in accordance with different types of exploration and experimental data.

Strategy for groundwater management in developing countries: A case study in northern Costa Rica

Summary On the basis of a case study in northern Costa Rica, in an area of one of the country’s most important volcanic aquifers, we developed a GIS-based groundwater management system suitable for the limited financial, technical and data resources of developing countries. Input and processing of attribute data-like information about groundwater wells or geological outcrops are realized with the help of an ACCESS database. The GIS-platform ArcView is used for input and analysis of all types of spatial data, e.g. maps of geology, hydrogeology, land use, or locations of groundwater wells. A three-dimensional model of the subsurface geology is constructed with the help of the interactive 3D-Modeller RHINOCEROS (R) – NURBS modelling for Windows. The GIS-platform ArcView is used for the final integrated spatial analysis of the three-dimensional model and the generated spatial and attribute databases. The basic concept for data analysis is a five-step numerical index as a simple but efficient matter of standardization in order to take into account a wide spectrum of input data of varying quality and precision. As an example of application, the assessment of the vulnerability of aquifers is presented, which can be calculated on the basis of the hydrogeological profile (sequence of different strata with varying permeability), the infiltration potential of the soil layer and the distance to fault zones for each location of the study area.

A Characterization of the MSA Stone Artefact Assemblage from the 1984 Excavations at Klein Kliphuis, Western Cape

INTRODUCTION The primary objective of this paper is to present a characterization of the Middle Stone Age assemblage from the site of Klein Kliphuis (KKH) in the Western Cape (Fig. 1). KKH was excavated in 1984 by a team from the South African Museum (now part of Iziko Museums of Cape Town). The excavation was to form part of a larger regional project looking at the nature and distribution of Later Stone Age (LSA) huntergatherer and herder sites. During the course of the excavation a single 1 m × 1 m square was excavated below the perceived end of the LSA, into what were presumed to be Middle Stone Age (MSA) artefact-bearing layers. The excavator’s original division between the LSA and MSA layers of Klein Kliphuis is retained here as a useful distinction, though it is the contention of this paper that some of the artefacts excavated from below this division are of LSA association. Information pertaining to the LSA component of the site was published (van Rijssen 1992), but although the MSA component was sorted, it was neither analysed in depth nor published. Analysis conducted in 2005 and 2006 suggests the presence of a stratified sequence at Klein Kliphuis which appears to parallel part of the sequence at Diepkloof Rock Shelter some 50 km to the southwest. This paper discusses the layers at Klein Kliphuis in terms of their culture–historical affiliation, based on comparison with published data from better resolved MSA excavations elsewhere.

The geochronological potential of isoleucine epimerisation in cassowary and megapode eggshells from archaeological sites

Our research demonstrates that the extent of isoleucine epimerisation ( A/ I) in fragments of avian eggshells provides geochronological information in archaeological contexts. In the archaeological sequence of Hay Cave, northern Queensland, Australia, there is an excellent correspondence between the A/ I values of Australian brush-turkey ( Alectura lathami) eggshells ( n = 99) and independent geochronological control ( n = 16 radiocarbon ages including 4 on eggshell calcite). The A/ I values identify three phases of deposition during the Holocene at Hay Cave. In contrast to the Alectura eggshell A/ I values, a poor correspondence was observed between the A/ I values of cassowary ( Casuarius) eggshells from Toé ( n = 35) and Kria caves ( n = 23) (Ayamaru Plateau, Papua) and the depths from which the specimens were recovered in these stratified sequences. Given coherent archaeofauna trends and radiocarbon chronologies ( n = 8 and 2 eggshell calcite radiocarbon ages at Toé and Kria, respectively) with respect to depth, the variable A/ I values are not explicable in terms of mixing. Rather, the variability is most likely due to exposure of the eggshells to the high temperatures of campfires. Despite the variability, eggshells with relatively low A/ I values amongst specimens recovered from similar depths (and therefore presumably least influenced by high temperatures) exhibit a gradual increase in A/ I with respect to depth, as expected in a stratified deposit. From this observation it is suggested that the identification of heated eggshells will increase confidence in geochronological information provided by A/ I. These studies illustrate the complications that arise from campfire-induced acceleration of amino acid racemisation and emphasise that although this phenomenon is common, it is not universally encountered in archaeological contexts.

Submarine landslides: processes, triggers and hazard prediction

Huge landslides, mobilizing hundreds to thousands of km(3) of sediment and rock are ubiquitous in submarine settings ranging from the steepest volcanic island slopes to the gentlest muddy slopes of submarine deltas. Here, we summarize current knowledge of such landslides and the problems of assessing their hazard potential. The major hazards related to submarine landslides include destruction of seabed infrastructure, collapse of coastal areas into the sea and landslide-generated tsunamis. Most submarine slopes are inherently stable. Elevated pore pressures (leading to decreased frictional resistance to sliding) and specific weak layers within stratified sequences appear to be the key factors influencing landslide occurrence. Elevated pore pressures can result from normal depositional processes or from transient processes such as earthquake shaking; historical evidence suggests that the majority of large submarine landslides are triggered by earthquakes. Because of their tsunamigenic potential, ocean-island flank collapses and rockslides in fjords have been identified as the most dangerous of all landslide related hazards. Published models of ocean-island landslides mainly examine 'worst-case scenarios' that have a low probability of occurrence. Areas prone to submarine landsliding are relatively easy to identify, but we are still some way from being able to forecast individual events with precision. Monitoring of critical areas where landslides might be imminent and modelling landslide consequences so that appropriate mitigation strategies can be developed would appear to be areas where advances on current practice are possible.

Proterozoic intracontinental basin: The Vindhyan example

The features of the Vindhyan succession clearly indicate a vast intracratonic basin that remained within tens of meters of sea level throughout its lifetime. Apparently, shallow water condition was maintained over a large area for a long period of time suggesting that the sub-Vindhyan lithosphere suffered subsidence over a larger area producing a wide shallow ramp type basin. Hundreds of meters thick accumulation of peritidal strata in sequence 5 of the Vindhyan succession indicates that the subsidence rate was in perfect concert with the rate of sediment supply for a considerably long period of time during the end phase of Vindhyan basin evolution — the hallmark of cratonic basins Sloss (1988a, b). It is inferred that during the terminal period of the Vindhyan sedimentation a self-regulating system of uplift, erosion, sedimentation and subsidence controlled the accumulation of strata.

Timing of Cenozoic volcanism and Basin and Range extension in northwestern Nevada: New constraints from the northern Pine Forest Range

Eocene–middle Miocene volcanic rocks in the northern Pine Forest Range, Nevada, are ideally situated for reconstructing the timing and style of volcanism and extensional faulting in the northwesternmost part of the Basin and Range province. A conformable sequence of Cenozoic volcanic and sedimentary strata in the northern Pine Forest Range dips ~30°W, and 11 new 40 Ar/ 39 Ar ages from this sequence defi ne 3 major episodes of volcanic activity. Pre-Tertiary basement and older (ca. 38 Ma) Tertiary intrusive rocks are overlain unconformably by Oligocene (ca. 30–23 Ma) basalt fl ows and dacitic to rhyolitic ash-fl ow tuffs interbedded with fi ne-grained tuffaceous sedimentary rocks. Oligocene rocks are overlain by ~550 m of ca. 17–16 Ma basalt fl ows equivalent to the Steens Basalt in southern Oregon, and basalt fl ows are capped by a thin 16.3 Ma ignimbrite that likely is correlative with either the Idaho Canyon Tuff or the Tuff of Oregon Canyon. The northern Pine Forest Range is bounded to the east by a major down-to-the-east normal fault that dips ~40°E with well-developed fault striations indicative of dip-slip motion. This fault initiated at an angle of ~70° and was rotated ~30° during uplift of the range. A suite of 17 apatite fi ssion-track ages from the Pine Forest footwall block demonstrates that exhumation, uplift, and slip on the range-bounding fault began ca. 12–11 Ma and continued until at least 7 Ma, with moderate slip since then. The Pine Forest Range did not undergo signifi cant extension before or during peak Oligocene and Miocene volcanism, and similar geologic relationships in nearby ranges suggest that a larger region of northwestern Nevada was also little extended during this interval. Basin and Range faulting in northwestern Nevada appears to have begun no earlier than 12 Ma, making it distinctly younger than deformation in much of central and southern Nevada, where peak extension occurred in the middle Miocene or earlier.

Characteristics of chlorine isotope distribution and analysis on sylvinite deposit formation based on ancient salt rock in the western Tarim Basin

During formation of the Tarim Basin, there repetitively happened seawater invasion for it was ever connected with wide Paleo-mediterranean Sea. At the same time, for the application of tectonic and paleoenvironmental condition, the basin deposits many sequences of thick strata of evaporites, especially in its two largest subbasins of Kuqa in the north and Shaqa in the southwest. Therefore, the western Tarim Basin has been considered to be a favorable location to prospect ancient sylvinite deposit. But it is unfortunate that the Br×103/Cl ratio, as a common geochemical index to uncover clues of sylvinite deposit, is almost invalid to indicate the stage of ancient brine condensation for very low content of bromine in salt rock samples (less than 10−6 commonly) in the western Tarim Basin evaporites. Fortunately, we noticed recently that the values of chlorine isotope (δ 37Cl value) for salt rock samples follow some clear laws from place to place, which in the Shaqa subbasin, all δ 37Cl values of salt rock samples behaved negatively and on the contrary, they all surprisingly showed positive values in the Kuqa subbasin. Based on conclusions of previous research and referring to the ratios of Br×103/Cl for salt rock or brine samples, we proved that the chlorine isotope (δ 37Cl) could be used as an effective index to find ancient sylvinite deposit in those evaporites rock basin with low content of Br element background, such as the Tarim Basin. Thus according to the new index of chlorine isotope, we conclude that the concentration of ancient brines formed ancient salt rocks is much more concentrated in the Shaqa subbasin than that in the Kuqa subbasin. Particularly, the δ 37Cl value of salt rock in Kashi depression of the western Shaqa subbasin is greatly negative (<-1.00‰), which may predicate that the ancient brine has been concentrated to the terminal stage of salt deposit or even the later stage of sylvinite deposit. Then it is possible to find sylvinite-bearing deposits around the western region (periphery of Kashi depression) of the Shaqa subbasin if we carry out more careful exploration in later works.

KOMÉ TO EBOMÉ: ARCHAEOLOGICAL RESEARCH FOR THE CHAD EXPORT PROJECT, 1999-2003

Over the past five years, archaeological survey and excavation has been undertaken in southern Chad and Cameroon, as the result of a cultural heritage management agreement between the governments of the two countries, the World Bank and a consortium of international oil companies. These initiatives were undertaken as part of the Chad Export Project, which involved the construction of an oil pipeline from Komé in southern Chad to the Atlantic coast of Cameroon near Ebomé. Research by archaeologists associated with the project has resulted in location and excavation of cultural remains along a 1070-kilometre transect, in part through regions of Africa where little research had previously been undertaken. This preliminary report examines the results of this research. Major results include: (1) the discovery of 470 sites in Chad and Cameroon; (2) the excavation of a mid- to late-Holocene stratified sequence in southern Cameroon; (3) the discovery of sites containing pit features in the forests of southern Cameroon between the Atlantic coast and Nanga Eboko, a larger area than had been indicated by previous research, and (4) the discovery of evidence for significant ironworking activity in the wooded savanna environments of northeastern Cameroon and southern Chad.

Upper Paleozoic petroleum system, Ordos Basin, China

The Ordos Basin is a typical lapped basin, including three sequences of strata: early Paleozoic, late Paleozoic and Mesozoic, with a total thickness of 4000–6000 m. Impermeable sealing beds are well developed at the top and base of the upper Paleozoic sequence, separating it from the Mesozoic and the lower Paleozoic strata to form an independent petroleum system. In this petroleum system, the source rocks are widely distributed coals and dark mudstones occurring in the Carboniferous–lower Permian coal measures, with a thickness of 10–15 and 40–60 m, respectively. The reservoirs are mainly early Permian tight sandstones, mostly with a porosity of 4–8% and a permeability of 0.1–1.0×10 −3 μm 2. The regional cap rock is a 100–150 m thick mudstone in the upper Permian strata. The structural framework of the basin is a huge asymmetric syncline, dipping gently toward the east and north, and steeply toward the south and west. Well data show that gas-saturated, gas–water transition and water-saturated zones are developed from the depositional center to the basin edges. The gas-saturated zone mainly lies in the gently dipping slope area of the Shanbei Slope. Toward eastern and northern up-dip directions the water–gas transitional zone occurs, and finally the water-saturated zone, presenting a reverse relation of water on top of gas. An abnormal negative strata pressure is developed in the gas-bearing area, with a pressure coefficient ( C p) ranging from 0.83 to 0.95. Fluid inclusion data indicate that the upper Paleozoic gas pool began to develop around the Wuqi area at about 150 Ma, and it extended toward the north and was largely formed at about 120 Ma, showing there was a regional migration of the gas-water interface from south to north during the gas pool formation. These characteristics appear to show that the northern and eastern margins of the petroleum system are defined by a regional hydrodynamic regime. The critical moment of the petroleum system occurred during the early Cretaceous. The basin experienced a rapid subsidence and a significant geothermal event, resulting in the source rocks being matured into the main stage of gas generation to form a huge amount of gas. The gas migrated into the tight sandstone reservoirs and displaced the rock pore water to form the gas pools and developing the regional hydrodynamic trap.

Cyclic-facies analysis of geological sections at massive sulfide deposits of the Urals

Cyclic-facies analysis of stratified volcanic sequences in the ore-controlling depressions makes it possible to recognize the recurrent and genetically related sets of rock layers that make up micro-, meso-, and mega-scale eruptive cycles (elementary cycle, mesocycle, and megacycle). Massive sulfide ores occupy a specific position in geological sections. They are confined to the upper portions of elementary eruptive cycles and hosted in volcanosedimentary units, indicating their formation during the periods of waning volcanic activity. The elementary cycles are not all accompanied by ore mineralization. The mineralization is most complete in the upper elementary cycles of each eruptive meso- and megacycles.

Strain patterns within part of the Willard thrust sheet, Idaho–Utah–Wyoming thrust belt

The Willard thrust sheet, located in northern Utah, USA, contains a 10–15-km-thick sequence of strata that were internally deformed and translated about 50 km ESE. Upper levels of the sheet are relatively little deformed, lower levels have minor folds and cleavage that increase in intensity downward and rotate into parallelism with the thrust, and the basal fault zone contains mylonite and ultracataclasite. Finite strain displays systematic changes with structural position and lithology. Basal levels have strain ratios ( R≈ R XZ ) of 3–5, and angles ( θ) between stretch directions and the thrust of <10° in the profile plane. Diamictite, slate, and greywacke at lower levels have R of 1.5–3 and θ of 10–35°, with overall correlations between increasing R, decreasing θ, and greater mica content. Overlying micaceous quartzite has relatively low strain, with R of 1.3–2 and θ>50°. Principal stretching ( X) directions trend perpendicular to oblique to minor fold axes, which are locally oblique to the overall transport direction, reflecting partial decoupling of internal deformation from thrust slip. Principal axial ratios have R YZ > R XY , but microfabrics indicate only limited extension in the intermediate stretching direction ( Y), corresponding to volume losses of about 10–40%. Strain is factored into components of thrust-perpendicular thinning and thrust-parallel shear that increase downward and in mica-rich layers, minor thrust-parallel shortening to extension, minor longitudinal extension, and minor longitudinal (wrench) shear. These patterns reflect litho-mechanical layering of the thrust sheet, which includes a very weak basal fault zone that formed at elevated fluid pressures, a weak mica-rich lower level with widespread crystal plastic and mass transfer deformation, and a stronger upper part with limited internal deformation.

Geoarchaeological context of late Pleistocene and early Holocene occupation at the Cooper's Ferry site, western Idaho, USA

AbstractArchaeological excavations at the Cooper's Ferry site (10IH73), located in the lower Salmon River canyon of western Idaho, revealed a stratified sequence of cultural occupations that included a pit feature containing stemmed points. However, radiocarbon ages determined on charcoal and bone in the pit feature range between ca. 12,000 yr B.P. and 7300 yr B.P. By considering the effects of postdepositional processes on dated samples, and by comparing the lithostratigraphy, pedostratigraphy, and stable isotope geochemistry of pedogenic carbonates from Cooper's Ferry with other well‐dated stratigraphic sections in the canyon, site geochronology is clarified. Based on the presence of key radiocarbon ages and distinctive stratigraphic criteria, we argue that the initial occupation and interment of lithic artifacts in a pit feature at Cooper's Ferry occurred during the late Pleistocene, between ca. 11,410 and 11,370 yr B.P., and not during the early Holocene. Records of geomorphic change and paleoenvironmental proxy data from the site reveal that early occupation in the lower Salmon River canyon corresponds with evolving riparian ecosystems, which must be considered as a contextual aspect of local prehistoric cultural ecology. © 2004 Wiley Periodicals, Inc.

Ounjougou (Mali): A history of holocene settlement at the southern edge of the Sahara

The area of Ounjougou consists of a series of gullies cut through Upper Pleistocene and Holocene formations on the Dogon Plateau in the Sahel at the south edge of the Sahara Desert. Here the authors have chronicled a stratified sequence of human occupation from the tenth to the second millennium BC, recording natural and anthropogenic strata containing artefacts and micro- and macro- palaeoecological remains, mostly in an excellent state of preservation. They present a first synthesis of the archaeological and environmental sequence for the Holocene period, define five main occupation phases for Ounjougou, and attempt to place them within the context of West African prehistory.

Geochemistry, radiocarbon ages, and paleorecharge conditions along a transect in the central High Plains aquifer, southwestern Kansas, USA

Water samples from short-screen monitoring wells installed along a 90-km transect in southwestern Kansas were analyzed for major ions, trace elements, isotopes (H, B, C, N, O, S, Sr), and dissolved gases (He, Ne, N 2, Ar, O 2, CH 4) to evaluate the geochemistry, radiocarbon ages, and paleorecharge conditions in the unconfined central High Plains aquifer. The primary reactions controlling water chemistry were dedolomitization, cation exchange, feldspar weathering, and O 2 reduction and denitrification. Radiocarbon ages adjusted for C mass transfers ranged from <2.6 ka ( 14C) B.P. near the water table to 12.8 ± 0.9 ka ( 14C) B.P. at the base of the aquifer, indicating the unconfined central High Plains aquifer contained a stratified sequence of ground water spanning Holocene time. A cross-sectional model of steady-state ground-water flow, calibrated using radiocarbon ages, is consistent with recharge rates ranging from 0.8 mm/a in areas overlain by loess to 8 mm/a in areas overlain by dune sand. Paleorecharge temperatures ranged from an average of 15.2 ± 0.7 °C for the most recently recharged waters to 11.6 ± 0.4 °C for the oldest waters. The temperature difference between Early and Late Holocene recharge was estimated to be 2.4 ± 0.7 °C, after taking into account variable recharge elevations. Nitrogen isotope data indicate NO 3 in paleorecharge (average concentration=193 μM) was derived from a relatively uniform source such as soil N, whereas NO 3 in recent recharge (average concentration=885 μM) contained N from varying proportions of fertilizer, manure, and soil N. Deep water samples contained components of N 2 derived from atmospheric, denitrification, and deep natural gas sources. Denitrification rates in the aquifer were slow (5 ± 2× 10 −3 μmol N L −1 a −1), indicating this process would require >10 ka to reduce the average NO 3 concentration in recent recharge to the Holocene background concentration.

Processing of metallic glass-forming liquids under ultra-high gravity

Abstract A high-temperature centrifugation experiment is presented where a metallic liquid of composition Mg 56 Al 30 Li 7 Cu 7 is processed at an inertial acceleration of 60,000 g and temperature of up to 530 °C. Slow cooling across the melting temperature during continuous centrifugation results in a pronounced stratification of crystalline phases. Primary crystals, which form first upon cooling, sediment to the sample ends until finally eutectic microstructures solidify from the remaining liquid in the central region of the sample. The stratification sequence of the primary crystals and the isolation of multi-phase eutectic compositions provide valuable information towards the development of new bulk metallic glasses. Theoretical considerations for the sedimentation of crystals in viscous metallic liquids are outlined and the setup of the high-temperature centrifugation experiment is presented.