Age Control (age + control)

Distribution by Scientific Domains

Selected Abstracts

Paleoindian geoarchaeology and paleoenvironments of the western Killpecker Dunes, Wyoming, U.S.A.

James H. Mayer
The Killpecker Dunes in southwestern Wyoming have long been known to contain evidence for Paleoindian occupation. This paper presents the results of geoarchaeological investigations in the western, dormant portion of the Killpecker Dunes. Five localities, including the Krmpotich Folsom and Finley Cody Complex sites, were examined in order to better understand the Paleoindian geochronology of the dune field, and to facilitate a late Quaternary paleoenvironmental reconstruction. Age control is provided by radiocarbon, optical, and artifact ages. Four late Quaternary eolian units, designated as strata 1,4, were recognized; strata 1 and 2 are relevant to the Paleoindian record. Aggradation of stratum 1 began by at least 14,690 cal yr (,12,550 14C yr B.P.), probably in the form of a sand sheet under cool, dry conditions. Redoximorphic features are ubiquitous in stratum 1, indicating a rise in the water table during the latest Pleistocene. A buried soil (Calcid) at the top of this unit indicates a period of stability between ca. 12,000 and 11,000 14C yr B.P. and has the potential to yield Clovis (11,200,10,900 14C yr B.P.) artifacts. Unconformably overlying stratum 1 is stratum 2, a latest Pleistocene to early Holocene sand containing Folsom (10,900,10,200 14C yr B.P.) through Cody Complex (9000,8500 14C yr B.P.) occupations. A buried soil characterized by the accumulation of illuvial clay and sodium (Natrargid) occurs at the top of stratum 2 and probably formed in and around interdunal ponds. The association of Paleoindian material with the stratum 2 soil suggests the use of interdunal areas as natural traps for hunting extinct bison. The accumulation of soluble salts indicates increased rates of evaporation during the early Holocene, probably from an increase in summer insolation. Although conditions in the dune field during the early Holocene became increasingly dry, they were probably moister than at present. © 2003 Wiley Periodicals, Inc. [source]

Advances in optically stimulated luminescence dating of individual grains of quartz from archeological deposits

Zenobia Jacobs
Abstract Paleoanthropologists and archeologists interested in occupation histories, faunal remains, and objects of material culture have become increasingly reliant on optically stimulated luminescence (OSL) dating to construct Quaternary chronologies. In part, the increased use of OSL dating reflects its capacity to date events beyond the range of radiocarbon dating and in contexts where suitable organic materials are absent. An earlier review in Evolutionary Anthropology by Feathers1 provides a general account of the principles of luminescence dating. Since then, however, important advances have been made in OSL dating of quartz, so that it is now possible to date individual sand-sized grains and thereby resolve issues of postdepositional mixing of archeological sediments. In this review, we discuss the most important of these advances and their implications with regard to improved age control of archeological sites. We cover aspects of instrumental and methodological development that have facilitated the widespread measurement of single grains related to archeological questions and illustrate our review with some examples of where archeological problems have been resolved using single-grain OSL dating. We do not propose single-grain dating as a panacea, because there are instances where it is not straightforward to use or the results may be difficult to interpret; dating in such contexts remains the subject of continuing research. [source]

Geoarchaeological insights gained from surficial geologic mapping, middle Moche Valley, Peru

Gary Huckleberry
We present the results of detailed surficial geologic mapping for a 10 km2 area of alluvial (quebrada) deposits located in the middle Moche Valley of Peru, where archaeological features and deposits provide cross-cutting relationships and numerical age control for late Holocene erosion and deposition associated with El Niño. Despite surfaces containing clasts that are heavily pitted and cracked due to salt weathering, archaeological and 14C dates indicate that most quebrada landforms in the study area are late Holocene in age and may reflect enhanced alluviation associated with increased El Niño activity beginning ca. 6000 cal yr B.P. Our study provides criteria for correlative dating of other Holocene landforms in the Moche Valley area but urges caution in long-distance (>100 km) correlation because of geographic variability in surface weathering. Surficial geologic mapping aided by archaeological age control allows improved understanding of the rates of landscape change important to the development of complex irrigation-based societies in the Peruvian Desert. © 2003 Wiley Periodicals, Inc. [source]

Depositional and tectonic evolution of a supradetachment basin: 40Ar/39Ar geochronology of the Nova Formation, Panamint Range, California

BASIN RESEARCH, Issue 1 2000
N. P. Snyder
The Nova Basin contains an upper Miocene to Pliocene supradetachment sedimentary succession that records the unroofing of the Panamint metamorphic core complex, west of Death Valley, California. Basin stratigraphy reflects the evolution of sedimentation processes from landslide emplacement during basin initiation to the development of alluvial fans composed of reworked, uplifted sections of the basin fill. 40Ar/39Ar geochronology of volcanic units in middle and lower parts of the sequence provide age control on the tectonic and depositional evolution of the basin and, more generally, insights regarding the rate of change of depositional environments in supradetachment basins. Our work, along with earlier research, indicate basin deposition from 11.38 Ma to 3.35 Ma. The data imply sedimentation rates, uncorrected for compaction, of ~100 m Myr,1 in the lower, high-energy part to ~1000 m Myr,1 in the middle part characterized by debris-flow fan deposition. The observed variation in sediment flux rate during basin evolution suggests that supradetachment basins have complex depositional histories involving rapid transitions in both the style and rate of sedimentation. [source]

Response of the Rhine,Meuse fluvial system to Saalian ice-sheet dynamics

BOREAS, Issue 3 2008
A new reconstruction of the interaction between the Saalian Drente glaciation ice margin and the Rhine,Meuse fluvial system is presented based on a sedimentary analysis of continuous core material, archived data and a section in an ice-pushed ridge. Optically Stimulated Luminescence (OSL) was applied to obtain independent age control on these sediments and to establish a first absolute chronology for palaeogeographical events prior to and during the glaciation. We identified several Rhine and Meuse river courses that were active before the Drente glaciation (MIS 11-7). The Drente glaciation ice advance into The Netherlands (OSL-dated to fall within MIS 6) led to major re-arrangement of this drainage network. The invading ice sheet overrode existing fluvial morphology and forced the Rhine,Meuse system into a proglacial position. During deglaciation, the Rhine shifted into a basin in the formerly glaciated area, while the Meuse remained south of the former ice limit, a configuration that persisted throughout most of the Eemian and Weichselian periods. An enigmatic high position of proglacial fluvial units and their subsequent dissection during deglaciation by the Meuse may partially be explained by glacio-isostatic rebound of the area, but primarily reflects a phase of high base level related to a temporary proglacial lake in the southern North Sea area, with lake levels approximating modern sea levels. Our reconstruction indicates that full ,opening' of the Dover Strait and lowering of the Southern Bight, enabling interglacial marine exchange between the English Channel and the North Sea, is to be attributed to events during the end of MIS 6. [source]

Luminescence dating of Würmian (Weichselian) proglacial sediments from Switzerland: methodological aspects and stratigraphical conclusions

BOREAS, Issue 2 2007
Proglacial deposits from three independently dated sites in the Swiss lowlands were investigated by optically stimulated luminescence (OSL) to test the reliability of the applied dating approach. Using the single-aliquot regenerative-dose technique applied to small aliquots of quartz revealed that in all samples the OSL signal was differentially bleached prior to deposition. A statistical approach is introduced to extract the fraction of aliquots in which the OSL was apparently set to zero at deposition. The statistical treatment provided OSL ages in good consistency with independent age control. However, some uncertainties remain with this approach, since it is difficult to assess the natural scatter caused by dose-rate inhomogeneity exactly. The available data set implies that glaciers reached the Swiss lowlands prior to about 25 000 yr ago. Previously published data from other sites demonstrate that glaciers were present in lowland Switzerland up to about 20000 yr ago. Furthermore, one OSL date of about 70 000 yr for proglacial outwash sediments points to the presence of the Rhône Glacier in the Lake Neuchâtel area during the middle part of the Late Pleistocene. [source]

Incentive-elicited striatal activation in adolescent children of alcoholics

ADDICTION, Issue 8 2008
James M. Bjork
ABSTRACT Aims Deficient recruitment of motivational circuitry by non-drug rewards has been postulated as a pre-morbid risk factor for substance dependence (SD). We tested whether parental alcoholism, which confers risk of SD, is correlated with altered recruitment of ventral striatum (VS) by non-drug rewards in adolescence. Design During functional magnetic resonance imaging, adolescent children of alcoholics (COA; age 12,16 years) with no psychiatric disorders (including substance abuse) and similarly aged children with no risk factors responded to targets to win or avoid losing $0, $0.20, $1, $5 or a variable amount (ranging from $0.20 to $5). Results In general, brain activation by either reward anticipation or outcome notification did not differ between COA and age/gender-matched controls. Cue-elicited reward anticipation activated portions of VS in both COA and controls. In nucleus accumbens (NAcc), signal change increased with anticipated reward magnitude (with intermediate recruitment by variable incentives) but not with loss magnitudes. Reward deliveries activated the NAcc and mesofrontal cortex in both COA and controls. Losses activated anterior insula bilaterally in both groups, with more extensive right anterior insula activation by losses in controls. NAcc signal change during anticipation of maximum rewards (relative to non-reward) correlated positively with both Brief Sensation-Seeking Scale scores and with self-reported excitement in response to maximum reward cues (relative to cues for non-reward). Conclusions Among adolescents with no psychiatric disorders, incentive-elicited VS activation may relate more to individual differences in sensation-seeking personality than to presence of parental alcoholism alone. Future research could focus on adolescents with behavior disorders or additional risk factors. [source]

Incentive-elicited mesolimbic activation and externalizing symptomatology in adolescents

James M. Bjork
Background:, Opponent-process theories of externalizing disorders (ExD) attribute them to some combination of overactive reward processing systems and/or underactive behavior inhibition systems. Reward processing has been indexed by recruitment of incentive-motivational neurocircuitry of the ventral striatum (VS), including nucleus accumbens (NAcc). Methods:, We used functional magnetic resonance imaging (fMRI) with an incentive task to determine whether externalizing symptomatology in adolescence is correlated with an enhanced VS recruitment by cues for rewards, or by deliveries of rewards. Twelve community-recruited adolescents with externalizing disorders (AED) and 12 age/gender-matched controls responded to targets to win or avoid losing $0, $0.20, $1, $5, or an unknown amount (ranging from $0.20 to $5). Results:, Cues to respond for rewards activated the NAcc (relative to cues for no incentive), in both subject groups similarly, with greatest NAcc recruitment by cues for the largest reward. Loss-anticipatory NAcc signal increase was detected in a volume-of-interest analysis , but this increase occurred only in trials when subjects hit the target. Relative to controls, AED showed significantly elevated NAcc activation by a linear contrast between reward notification versus notification of failure to win reward. In a post hoc reanalysis, VS and pregenual anterior cingulate activation by the reward versus non-reward outcome contrast also directly correlated with Child Behavior Checklist (CBCL) Externalizing total scores (across all subjects) in lieu of a binary diagnosis. Finally, both groups showed right insula activation by loss notifications (contrasted with avoided losses). Conclusions:, Externalizing behavior, whether assessed dimensionally with a questionnaire, or in the form of a diagnostic categorization, is associated with an exaggerated limbic response to outcomes of reward-directed behavior. This could be a neurobiological signature of the behavioral sensitivity to laboratory reward delivery that is characteristic of children with externalizing symptomatology. Of interest is future research on incentive-motivational processing in more severe, clinically referred AED. [source]

Temporal alterations in brain water diffusivity in neonatal meningitis

Gyanendra K Malik
Abstract Aim:, To compare changes in apparent diffusion coefficient (ADC) in neonatal meningitis using serial diffusion-weighted imaging (DWI). Method:, Thirty neonates with meningitis and 12 age/sex-matched controls were studied using DWI. ADC was quantified by placing region of interest(s) on periventricular white matter during acute illness and again at 21 days. Three groups of patients were studied: those with normal findings on both conventional MRI and DWI, those with abnormal DWI only and those with abnormal conventional MRI as well as DWI. Neurodevelopment assessment was performed in controls and patients at 3 months using Indian adaptation of Bayley scales of infant development (BSID) kit. Results:, Patients with neonatal meningitis with normal imaging (n = 8) showed no significant difference in ADC compared to controls. Patients showing abnormality only on DWI (n = 10) and on both conventional magnetic resonance imaging (MRI) as well as DWI (n = 12) had significantly reduced ADC (p = 0.001) than controls at baseline study. Follow-up study showed no significant differences in ADC in controls compared to any patient group. Significantly reduced neurodevelopmental scores were observed in patient groups compared to controls. Conclusion:, We conclude that quantitative ADC may detect meningitis-induced hypoxia early in brain parenchyma, which may be associated with abnormal motor and mental development. [source]