Showing papers on "Absolute dating published in 1998"

Cosmogenic Cl-36 dating of postglacial landsliding at The Storr, Isle of Skye, Scotland

Abstract: Major postglacial rock slope failures are a common feature of the Scottish Highlands and other mountainous areas that were deglaciated at the end of the Pleistocene, but evaluation of the causes and triggers of failure has been hindered by a lack of reliable dating evidence. We report the result of a pilot study designed to establish the absolute age of a large postglacial rotational rockslide at The Storr on the Isle of Skye, Scotland, using 36Cl surface exposure dating. Exposure ages of 6.3 ± 0.7 cal. ka BP and 6.6 ± 0.8 cal. ka BP were obtained for rock samples from two separate landslide blocks, giving an overall age estimate of 6.5 ± 0.5 cal. ka BP for rock slope failure at this site. This date is consistent with AMS radiocarbon dating of windblown sand derived from the failure scarp, and with previous inferences (based on relative dating evidence) concerning an early-Holocene age for most rock slope failures in the Scottish Highlands. The long time lag (. 7 ka) between deglaciation and failure sugge...

Long-distance correlations by sequence stratigraphy and cyclostratigraphy: examples and implications (Oxfordian from the Swiss Jura, Spain, and Normandy)

Abstract: The Oxfordian sedimentary successions studied in the Swiss Jura, in Normandy, and in the Soria and Cazorla regions of Spain display complex facies evolution and stacking patterns. Based on biostratigraphy and absolute age dating, it is suggested that the shallow-water depositional settings in the Jura, Normandy, and the Soria region as well as the deeper-water environments in the Cazorla region, recorded climatic and sea-level fluctuations in the Milankovitch frequency band. Beds and bedsets corresponding to 20-, 100-, and 400-ka cyclicities can be identified. Facies evolution inside such small-scale sequences and also in the larger sequences of million-year scale is interpreted in terms of sequence stratigraphy. Superposition of high-frequency cyclicity on a longer-term sea-level trend led to multiplication of diagnostic surfaces: sequence-boundary and maximum-flooding zones in the large-scale sequences can thus be defined. These zones are correlated between closely spaced sections, but also from the Swiss Jura to Normandy and to Spain. The narrow time lines given by Milankovitch cyclicity then allow comparison of facies evolution in the different regions on a scale of 100 ka or less. By filtering out local effects of differential subsidence and sediment supply, a long-term sea-level curve valid for the northwestern margin of the Tethys ocean can be reconstructed for the Middle to Late Oxfordian. Differential subsidence is implied from varying thicknesses of the sequences as well as from the distribution of siliciclastics which have been channelized through depressions. Tilted blocks, reduced sedimentation, or increased input of siliciclastics appearing at the same time in all study areas point to a widespread regional tectonic event. Distribution through the sequences of climate-dependent facies components such as corals, ooids, palynomorphs, and siliciclastics indicates that climate changes were dependent on atmospheric circulation patterns and thus on paleolatitude. Rainy periods and related increase of siliciclastics in the Swiss Jura were more abundant during low sea-level stands, whereas in the Soria region they coincided with sea-level highs. Through the combination of high-resolution sequence stratigraphy and cyclostratigraphy, and supported by biostratigraphy and absolute dating, it becomes possible to analyze paleoenvironmental changes in a very narrow time framework.

Revision and tentative extension of the tree-ring based 14C calibration, 9200-11,855 cal BP

Abstract: We report radiocarbon calibration data based on the revised German oak and pine series. The age range of the absolutely dated German oak series has been extended to 10,430 cal bp. The German pine series is tentatively linked to the oak series by 14C, and now reaches back to 11,871 cal bp (±20 yr). The revisions of the tree-ring time scale of the German oak chronology solved long-standing apparent discrepancies in the mid-Holocene 14C calibration data sets. The calibration data set based on the floating German pine is now in close agreement with the Preboreal part of 14C calibration series obtained from most varve chronologies and corals.

40Ar/39Ar dating in paleoanthropology and archeology

Abstract: The potassium-argon (K-Ar) dating method has been widely used over the past 40 years to provide radioisotopic age control of hominid/hominoid evolutionary time scales. The wide appeal of the technique to paleoanthropology and archeology has been, in part, a result of its broad time range of applicability, from materials as young as a few thousand years old to an essentially unbounded upper age limit. Another reason for its appeal is the many geological circumstances in which datable materials are found. Beginning about two decades ago and accelerating into this decade, however, the conventional K-Ar technique has given way to 40Ar/39Ar dating as the method of preference. This technique is not only more precise and accurate when dating ideal materials, but also permits excellent ages to be obtained from situations that often stymie the conventional K-Ar technique, such as dating of contaminated tuffs and altered rocks. © 1998 Wiley-Liss, Inc.

Absolute Dating of the Last 7000 Years of the Vostok Ice Core Using 10Be

Abstract: Ice cores have proven to be very valuable sources of palaeoclimatic information. Complete exploitation of this information requires reliable dating of the ice. In regions of relatively high ice accumulation, seasonal variations in several parameters (stable isotopes, conductivity, trace impurities, dust etc) have been used to obtain continuous year by year dating, at least in the upper portion of the core where these signals are still identifiable. Over large areas of the Antarctic plateau however, the accumulation rates are too low to record such yearly signals. In these cases chronologies have largely relied on modelling and/ or stratigraphic correlation with parameters assumed to be globally synchronous (for example volcanic horizons or methane trapped in bubbles in the ice). As has been previously pointed out, variation in cosmogenie 1~ can also been used for stratigraphie correlation (Raisbeck and Yiou, 1989). The cosmogenie production rate of l~ depends on (i) the primary cosmic ray intensity, (ii) the strength of the geomagnetic field, (iii) the level of solar magnetic activity. In the Antarctic, and over the time scales of interest here, this last parameter is likely the most important. However, independent of their origin, variations of 1~ production offer a globally synchronous and potentially very high resolution signal for stratigraphic correlation. In addition, if these variations can be correlated with those in an absolutely dated reservoir, they offer a way of obtaining absolute dating for the ice. We discuss such an application in the present paper. Using the Gif sur Yvette accelerator mass spectrometry facility, a nearly continuous profile of l~ concentrations with 0.5 m resolution has been measured in a 178 m ice core at Vostok station. On the basis of the EGT time scale (Jouzel et al, 1993), this depth corresponds to ~ 6700 years BP. This profile shows significant secular variations which are believed to be mainly the result of variable solar modulation. Assuming that the cosmogenic a4C production rate was proportionnal to the l~ concentrations on the EGT time scale, the l~ data have been used as input data in a carbon cycle model to simulate an atmospheric 14C record (Bard et al, 1997). This 14C record was then compared to the observed 14C profile in dendrochronologically dated tree rings. Differences between the simulated and observed 14C curves were then used to modify the assumed ice ages, and the process repeated iteratively to obtain a maximum in the correlation coefficient between the simulated and observed 14C curves. The resulting age for the Vostok core is older than the originally assumed EGT time scale by a gradually increasing amount which eventually reaches -400 years at 7000 BP.

The Unitary Association Method of Relative Dating and Its Application to Archaeological Data

Abstract: The primary objective of relative dating techniques is to determine a reliable sequence of archaeological deposits. This task becomes more difficult whenever our research steps beyond individual sites to the study of intercommunity relationships because we need to develop some means of associating unconnected deposits in time. Radiocarbon dating, as a stand-alone method, cannot always be used to draw reliable correlations between sites. The relevance of archaeological dates, including absolute dates, relies ultimately on the determination of artifact or sample associations and their respective superpositional relationships. The Unitary Association Method of Relative Dating is an alternative to seriation methods that is less susceptible to spatial variation and offers analytical strengths needed for regional chronological analyses.

Dating rock images in the tropical monsoon region of northern Australia

Abstract: One of the most exciting research frontiers in Australian prehistory over the past five or so years has been the emerging possibility of applying advanced absolute dating methods - in particular, accelerator mass spectrometry (AMS) radiocarbon dating - to measuring the antiquity of Aboriginal rock images (paintings and engravings). It is a field of international interest, with the leading research in recent years having been carried out in Australia and France. In both areas, credible direct dates from the art itself have been obtained extending back to some 25-30,000 years ago. These research results can capture the public imagination, as evidenced by newspaper reports (Dayton 1993; Hammond 1993; Leech 1998).

Stages of Cenozoic Tectonic Movement in Hexi Corridor,Gansu Province

Abstract: Hexi corridor,located on the northern margin of the Qinghai Xizang plateau,is one of the most typical regions of compressional tectonic basins in northwest China.Accompanying the uplift of the Qinghai Xizang plateau since the late Paleogene,the four basins of the corridor began subsiding and accepting deposits.Since then,at least seven tectonic movement stages can be identified.According to the results of absolute dating(includes ESR,TL and IRSL dates) and relative dating(includes rock weathering rinds and palaeomagnetic stratigraphy) of Cenozoic strata and landforms,the seven movements occurred at about 37.7,30.3～29.5,24.5～22.2,4.66,2.47～2.23,1.95～1.48 and 080Ma～present,each of them represented by uncomformity and deformation of strata,active folding and faulting,and geomorphic evolution. The first tectonic movement stage took place at the beginning of Oligocene after a long period of erosion and denudation since the late Cretaceous.The forth stage,characterized by the preliminary accumulation of gloomy conglomerate (Yumen formation),is the turning point of Cenozoic tectonic movement as the intensity of tectonic activity became obviously stronger.It is in the latest stage that the main part of modern landform evolved.Some important morphologic units,including piedmont platform,transverse uplifts and northern mountains,generated or strongly deformed and uplifted in the stage.On the other hand,less deform tracks can be found in strata. Further divisions are made in order to avoid that the interval time between two neighbouring movements became shorter and shorter as time passed because latelyformed records preserved better.For example,mainly according to the luminescence ages of river terraces,at least five tectonic substages can be identified during the latest stage.They started at about 800,350,250,150 and 40ka respectively.Furthermore,in the tectonic substage started from 150ka,four tectonic events,which took place at 145,110,90 and 70ka respectively,can be identified,and in the last substage,three tectonic events(40,30 and 10ka) can be identified.

U-Th Isochron Dating of the Marine Oxygen-Isotope Record

Abstract: Dating marine records of climatic and oceanographic change has proved problematic. Current age models for events beyond the 14C age range are constructed by tuning the marine oxygen-isotope record to northern-hemisphere insolation (e.g. Imbrie et al. 1984). These models rely on an assumed northernhemisphere mechanism for climate change and have an absolute precision limited to about half the precession cycle (10,000 years). Recent work (Slowey et al. 1996) has demonstrated that aragonite-rich sediments from the Bahamas are amenable to U-Th dating and may provide a highprecision chronology for the last 350 kyr which is independent of any assumed mechanism. Early work successfully dated interglacials, during which

Chronometric Dating in Archaeology, vol. 2

Abstract: Chronometric Dating in Archaeology. vol. 2. R. E. Taylor and Martin J. Aitken. eds. New York: Plenum, 1997.396 pp.

Potassium-argon dating method

Abstract: Potassium Argon Dating of Iron Meteorites This article summarised meteorite dating in 1967. 7 Even 40 years later things are no better. In the opening paragraph he states that the iron meteorite from Weekeroo Station is date at ten billion years old. He then continues: “The formation or solidification ages of iron meteorites have never been well determined.” 8 He then cites earlier dating which produced an age of seven billion years. 9 The author concludes with the following remark: “The ages found by us are typical of the great ages found for most iron meteorites. From these, in conjunction with the Strontium: Rubidium data of Wasserburg et al. on silicate inclusions in this meteorite, we conclude that the potassium: argon dating technique as applied to iron meteorites gives unreliable results.” 10

Geochronology in the Late Quaternary and Relating Problems

Abstract: To reconstruct environmental changes in the late Quaternary, accurate dating is the most important. The framework of the late Quaternary chronology is based on magnetostratigraphy due to the reversal of geomagnetic polarity and oxygen isotope stratigraphy of foraminifera in marine sediments, which has been considered to be driven by astronomical forcing of solar energy reaching to the Earth, that is, the Milancovitch cycle. For dating during the past several tens of thouthands of years including the Last Glacial Period and the Holocene, radiocarbon dating is the most useful method. Recent developement of accelerator mass spectrometry (AMS) has realized precise dating by using mg amounts of carbon and AMS systems have been introduced widely. Basic concepts and methods of radiocarbon dating, especially on AMS radiocarbon dating, are reviewed and problems relating calibration of measured radiocarbon ages to calender ages are discussed.