Radiocarbon, Volume 48 (2006): Recent submissions
Now showing items 21-40 of 46
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Paleoproductivity Variations in the Equatorial Arabian Sea: Implications for East African and Indian Summer Rainfalls and the El Niño FrequencyWe analyzed a sediment core from the equatorial Arabian Sea, chronologically constrained by accurate accelerator mass spectrometry (AMS) radiocarbon dates on selected planktonic foraminiferal species, for paleoproductivity variations corresponding to the variations in the Indian Ocean Equatorial Westerlies (IEW). The IEW in turn are positively correlated to the Southern Oscillation Index (SOI), which is a measure of El Nio, Southwest monsoon (SWM), and east African rainfall (EAR). The productivity data show that Indian and east African rainfalls declined from 35,000 calendar yr BP up to the last glacial maximum (LGM), with the maximum El Nio frequency during the last glacial period. From ~14,500 to ~2000 calendar yr BP (i.e. Core top), we find strengthening SWM and EAR along with declining El Nio frequency.
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Over 16,000 Years of Fire Frequency Determined from AMS Radiocarbon Dating of Soil Charcoal in an Alluvial Fan at Bear Flat, Northeastern British ColumbiaWe present results of radiocarbon dating of charcoal from paleosols and buried charcoal horizons in a unique sequence, which potentially records the last 36,000 yr, from a fan at Bear Flat, British Columbia (BC) (56 degrees 16'51"N, 121 degrees 13'39"W). Evidence for forest-fire charcoal is found over the last 13,500 +/110 14C yr before present (BP) or 16,250 +/700 cal BP. The study area is located east of the Rocky Mountains in an area that was ice-free at least 13,970 +/170 14C yr BP (17,450-16,150 cal BP) ago. The latest evidence of fire is during the Medieval Warm Period (MWP). The charcoal ages show a periodicity in large fires on a millennial scale through the Holocene—an average of 4 fires per thousand years. Higher fire frequencies are observed between 2200 to 2800 cal BP, ~5500 and ~6000 cal BP, ~7500 to 8200 cal BP, and 9000 to 10,000 cal BP. These intervals also appear to be times of above-average aggradation of the fan. We conclude that fire frequency is related to large-scale climatic events on a millennial time scale.
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New AMS 14C Dates from the Early Upper Paleolithic Sequence of Raqefet Cave, Mount Carmel, IsraelRaqefet Cave (35 degrees 04'21"N, 32 degrees 39'17"W) is situated in the southeastern side of Mount Carmel in Israel (Figure 1) on the left bank of wadi Raqefet (230 m asl), ~50 m above the wadi bed. It is 50 m long with an area of ~500 m2 (Figure 2). Eric Higgs of Cambridge University and Tamar Noy of the Israel Museum conducted excavations at the site between 1970 and 1972 (Higgs et al. 1975). New excavations at the cave began in 2004 (Lengyel et al. 2005). Studies on the lithic archaeological remains from the 1970-1972 stratigraphic units assign Late Mousterian or Middle to Upper Paleolithic transition (layers VIII-VI in squares B-G/18-23), indeterminate early Upper Paleolithic (layer IV in squares B-G/18-23), Levantine Aurignacian (layers IV, III, and II in squares B-G/18-23), indeterminate late Upper Paleolithic (layer II in area B-G/18-23), Late Kebaran (layer I in squares B-G/18-23), Geometric Kebaran (layer VII in squares J-M/24-28), Late Natufian (layers IV-VI in squares A-H/7-17), Neolithic (I-IV in squares J-M/24-28), and Bronze Age (pits in squares A-H/7-17 and B-G/18-23) occupations (Higgs et al.1975; Lengyel 2003, 2005; Lengyel and Bocquentin 2005; Lengyel et al. 2005; Noy and Higgs 1971; Sarel 2004; Ziffer 1978a,b).
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Marine Radiocarbon Reservoir Ages in Scottish Coastal and Fjordic WatersHigh freshwater inputs into Scottish sea lochs (fjords) combined with the restricted exchange between sea loch basin water and coastal Atlantic water masses are likely to result in reduced regional marine radiocarbon reservoir ages (R[t]) in these environments. To test this hypothesis, historical, museum-archived shells, collected live on known dates prior to AD 1950 from coastal locations in NW Scotland, were 14C dated to provide a means of determining R(t) and hence the regional deviation (R) from the modeled global surface ocean reservoir age (R). The sea loch data, when combined with 14C dates from the Scottish west coast (Harkness 1983), yield a regional Delta-R value of 26 +/14 yr. The R of sea loch (fjordic) and coastal waters of NW Scotland are statistically different (at a confidence level >95%) from the Delta-R value of 17 +/14 yr reported for UK coastal waters (Reimer 2005; data after Harkness 1983) and are in good agreement with the coastal Delta-R value of 33 +/93 yr reported by Reimer et al. (2002). Therefore, it is recommended that a regional Delta-R correction of 26 +/14 yr should be applied to modern (i.e. Pre-bomb but not prehistoric) marine 14C dates from the NW coast of Scotland.
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Intrashell Radiocarbon Variability in Marine MollusksWe demonstrate variable radiocarbon content within 2 historic (AD 1936) and 2 prehistoric (about 8200 BP and 3500 BP) Mytilus californianus shells from the Santa Barbara Channel region, California, USA. Historic specimens from the mainland coast exhibit a greater range of intrashell variability (i.e. 180-240 14C yr) than archaeological specimens from Daisy Cave on San Miguel Island (i.e. 120 14C yr in both shells). d13C and d18O profiles are in general agreement with the upwelling of deep ocean water depleted in 14C as a determinant of local marine reservoir correction (Delta-R) in the San Miguel Island samples. Upwelling cycles are difficult to identify in the mainland specimens, where intrashell variations in 14C content may be a complex product of oceanic mixing and periodic seasonal inputs of 14C-depeleted terrestrial runoff. Though the mechanisms controlling Delta-R at subannual to annual scales are not entirely clear, the fluctuations represent significant sources of random dating error in marine environments, particularly if a small section of shell is selected for accelerator mass spectrometry (AMS) dating. For maximum precision and accuracy in AMS dating of marine shells, we recommend that archaeologists, paleontologists, and 14C lab personnel average out these variations by sampling across multiple increments of growth.
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How to Convert Biological Carbon Into Graphite for AMSIsotope tracer studies, particularly radiocarbon measurements, play a key role in biological, nutritional, and environmental research. Accelerator mass spectrometry (AMS) is now the most sensitive detection method for 14C, but AMS is not widely used in kinetic studies of humans. Part of the reason is the expense, but costs would decrease if AMS were used more widely. One component in the cost is sample preparation for AMS. Biological and environmental samples are commonly reduced to graphite before they are analyzed by AMS. Improvements and mechanization of this multistep procedure is slowed by a lack of organized educational materials for AMS sample preparation that would allow new investigators to work with the technique without a substantial outlay of time and effort. We present a detailed sample preparation protocol for graphitizing biological samples for AMS and include examples of nutrition studies that have used this procedure.
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High-Accuracy 14C Measurements for Atmospheric CO2 Samples by AMSIn this paper, we investigate how to achieve high-accuracy radiocarbon measurements by accelerator mass spectrometry (AMS) and present measurement series (performed on archived CO2) of 14CO2 between 1985 and 1991 for Point Barrow (Alaska) and the South Pole. We report in detail the measurement plan, the error sources, and the calibration scheme that enabled us to reach a combined uncertainty of better than 3. The d13C correction and a suggestion for a span (or 2point) calibration for the 14C scale are discussed in detail. In addition, we report new, accurate values for the calibration and reference materials Ox2 and IAEA-C6 with respect to Ox1. The atmospheric 14CO2 records (1985-1991) are presented as well and are compared with other existing records for that period. The Point Barrow record agrees very well with the existing Fruholmen (northern Norway) record from the same latitude. The South Pole record shows a small seasonal cycle but with an extreme phase with a maximum on January 1st (+/13 days). Together with its generally elevated 14C level compared to the Neumayer record (coastal Antarctica), this makes our South Pole data set a valuable additional source of information for global carbon cycle modeling using 14CO2 as a constraint.
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ENEA Radiocarbon Measurements IThis paper includes determinations of archeological and geological samples from different sites in central Italy performed at the Ente per le Nuove Tecnologie l'Energia e l'Ambiente (ENEA) Radiocarbon Laboratory. This laboratory has been in operation since 1985 at the ENEA Bologna Research Center.
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ErratumDepartment of Geosciences, The University of Arizona, 2006-01-01There is an error in the previous issue of Radiocarbon.
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ENEA Radiocarbon Measurements IIThis paper includes determinations of geological samples coming from the Emilia Romagna region (northern Italy) performed at the ENEA Radiocarbon Laboratory. These analyses were executed as part of the Geological Cartography (CARG) project aimed to realize a new Italian Geological Map.
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Determination of 90Sr/90Y in Wheat Grains, Soil, and Deposition Samples by TBP Extraction and Cerenkov CountingWithin the framework of radioecological studies, 90Sr was determined in wheat grains, soil, and deposition samples. The radiochemical purification of 90Y consisted of liquid-liquid extraction by tributyl phosphate (TBP), followed by hydroxide and oxalate precipitations and, if necessary, the removal of thorium by anion exchange chromatography. The procedure proved to be very robust and reliable, having yttrium yields of 92.7 4.6% for 1-kg wheat samples, 90.9 4.2% for 50-g soil samples, and 90.6 3.2% for wet and dry deposition samples. 90Y was determined by Cerenkov counting and proportional counting. By optimizing the Cerenkov counting window, a figure of merit (FOM) of 4750 could be reached using a Quantulus 1220 system. Minimum detectable activities were in the range of 10 mBq.
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Dilemma of Dating on Lacustrine Deposits in an Hyperarid Inland Basin of NW ChinaConventional and accelerator mass spectrometry (AMS) radiocarbon, TL, OSL, and IRSL dating results on samples from the cores D100 and I70 from Ejina Basin, one of the most important inland basins in arid-hyperarid NW China, show that it is difficult to determine the ages of sediments at different depths. AMS ages of core D100 samples demonstrate that the sediments at depths from 10 to 90 m were formed between 14 to 30 kyr BP. The inverted ages from both the D100 and I70 cores imply that there was a strong reworking of the sediments during and after deposition processes. The inverted ages also indicate drastic fluctuations of groundwater bearing soluble organic matters, which might be related to neotectonic activities and climate changes during the period. Consequently, it is impossible to establish an accurate and reliable chronology for the cores based only on these dates. All AMS ages, if they are reliable and acceptable, indicate a high deposition rate (5~8 mm/yr), and since all TL, OSL, and IRSL ages are much older than those given by AMS, it makes these methods questionable for determining the ages of lacustrine-fluvial-alluvial deposits.
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Dating Paleosol and Animal Remains in Loess DepositsAccurate and reliable dating of paleosols, animal remains, and artifacts is of crucial importance in reconstructing environmental change and understanding the interrelationship between human activities and natural environments. Dating different materials in the same sample can help resolve problems such as soil carbon sources and carbon storage state. Conventional radiocarbon dating of soil (inorganic and organic matter) and accelerator mass spectrometry (AMS) dating of animal remains (fossil bones and teeth) result in different ages for materials from the same sample position in a typical loess section at Xinglong Mountain, Yuzhong County, Gansu Province in NW China. Inorganic matter is ~3400 yr older than organic matter, 4175 +/175 cal BP to 3808 +/90 cal BP. A 1610-yr difference between the 14C ages of fossils (animal bones and teeth) and soil organic matter suggests that a depositional hiatus exists in the studied profile. The varying 14C ages of fossils and soil organic and inorganic matter have important implications for paleoclimate reconstructions from loess sections. It is critical to consider the meaning of the variable 14C ages from different material components from the same sample position in terms of soil organic and inorganic carbon storage, vegetation history reconstruction, archaeology, and the study of ancient civilizations.
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Comments on Sveinbjörnsdóttir et al. (2004) and the Settlement of IcelandRadiocarbon dates on samples aimed to date the settlement of Iceland are given together with comments by the laboratory, since many of the results and descriptions given by Sveinbjörnsdóttir et al. (2004) in Radiocarbon, together with new results, are in error. The intention of this paper is to present correct dates and further relevant information regarding samples used earlier and to discuss possible complications inherent in the method of Sveinbjörnsdóttir et al. (2004). Examples are given of how critical the collection, treatment, and interpretation of samples may be. An age difference between birch charcoal and grains for a site is expected due to various reasons. If the difference amounts up to ~100 yr, as reported by Sveinbjörnsdóttir et al. (2004), it must only to a small degree be due to biological age. Reference to an excavation report, details regarding stratigraphy, and discussions of the risk for displacement and contamination are missing in their paper. A final evaluation of the time for settlement should not be done until more research is completed and other possible or earlier suggested or even dated sites are discussed. A summary is given of the research on the island and volcanic effects on the 14C activity of the atmospheric CO2, especially over Iceland.
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Climate in the Great Lakes Region Between 14,000 and 4000 Years Ago from Isotopic Composition of Conifer WoodThe isotopic composition of ancient wood has the potential to provide information about past environments. We analyzed the d13C, d18O, and d2H of cellulose of conifer trees from several cross-sections at each of 9 sites around the Great Lakes region ranging from ~4000 to 14,000 cal BP. Isotopic values of Picea, Pinus, and Thuja species seem interchangeable for d18O and d2H comparisons, but Thuja appears distinctly different from the other 2 in its d13C composition. Isotopic results suggest that the 2 sites of near-Younger Dryas age experienced the coldest conditions, although the Gribben Basin site near the Laurentide ice sheet was relatively dry, whereas the Liverpool site 500 km south was moister. The spatial isotopic variability of 3 of the 4 sites of Two Creeks age shows evidence of an elevation effect, perhaps related to sites farther inland from the Lake Michigan shoreline experiencing warmer daytime growing season temperatures. Thus, despite floristic similarity across sites (wood samples at 7 of the sites being Picea), the isotopes appear to reflect environmental differences that might not be readily evident from a purely floristic interpretation of macrofossil or pollen identification.
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Coastal Upwelling and Radiocarbon—Evidence for Temporal Fluctuations in Ocean Reservoir Effect off Portugal During the HoloceneThis paper focuses on the use of the radiocarbon content of marine shells collected along the Portuguese coast as a proxy for the intensity of coastal upwelling off of Portugal. Differences in the 14C ages of closely associated marine mollusk shells and terrestrial material (charcoal or bones) from several Portuguese archaeological contexts seem to be significant throughout the Holocene. Delta-R values range from 940 +/50 to 160 +/40 14C yr. Five of these values are significantly higher than the modern value (250 +/25 14C yr), while the remaining values are lower. The modern value was calculated by measuring the 14C content of live-collected, pre-bomb marine mollusk shells. This value is in accordance with an active upwelling of strong intensity that currently occurs off of Portugal. Some primary observations based on data presented here can be made: i) during the Holocene important changes have occurred in the ocean reservoir effect off the Portuguese coast; ii) these fluctuations may be correlated with regional oceanographic changes, namely with changes in the strength of coastal upwelling; and iii) these changes suggest some sort of variability of the climatic factors forcing coastal upwelling off of Portugal.
