L.J. Toolin

Preparation of small samples for 14C accelerator targets by catalytic reduction of CO.

Graphite in various forms has become the standard target for accelerator 14 C dating. Graphite has been made by catalytic graphitization of charcoals (Lowe, 1984). Thin films of graphite have also been produced by thermal cracking (Beukens & Lee, 1981), electric discharge (Andree et al , 1984; Wand, Gillespie & Hedges, 1984). Vogel et al (1984) pointed out the ease of graphite formation on iron from CO 2 and H 2 mixtures at ca 600°C. The deposition reactions of carbon from the CO, H 2 , and CO 2 equilibria are well known (Wagman et al , 1945) and well studied. Formation of graphite from CO 2 was discussed extensively by Boudouard (1902) and Schenck and Zimmerman (1903), and was known to chemists in France in 1851. We have used a related method, where graphite forms away from the iron, by using a higher temperature, and reduction of CO 2 to CO over Zn in the presence of H 2 (Jull et al , 1986) as an alternative to the use of Fe alone. The object of this paper is to point out an even simpler graphite preparation system, which eliminates hydrogen. The decomposition reaction of CO (Boudouard, 1902) takes place according to reaction (1).

First Direct AMS Dates on Early Maize From Tehuacan, Mexico

The Tehuacan region in Central Mexico is thought to be the locale of origin of Zea mays, or maize, a cultivated plant pivotal in the development of agriculture in the Americas (MacNeish, 1981, 1985). The age of the earliest maize, and its rate of dispersal are thus important components of cultural development in the New World. We have secured permission from the Federal Government of Mexico to date critical specimens from Tehuacan, which represent what are probably some of the earliest known stages of maize evolution. Twelve Zea mays samples have been dated, six from Cueva San Marcos and six from Cueva Coxcatlan. These were selected as having the best stratigraphic control and correlation with previously dated charcoal samples, and to represent the most ancient maize. Corn from Cueva San Marcos is oldest: four of the six specimens from this cave were within statistics of 4700 BP (uncalibrated). The oldest known domesticated corn is thus no older than 3600 cal BC (dendro-calibrated in calendric years).

Radiocarbon in dissolved organic and inorganic carbon from the central North Pacific

Author(s): Druffel, ER; Druffel, ER; Williams, PM; Robertson, KL; Griffin, S; Jull, AJ; Donahue, D | Abstract: Radiocarbon measurements are reported for dissolved organic carbon (DOC) and inorganic carbon (DIC) from seawater samples collected from the Alcyone-5 cruise in the central North Pacific Ocean in 1985. Differences between the UV-radiation techniques used here and those reported by Williams, Oeschger and Kinney (1969) to oxidize and recover the DOC from sea water are presented. UV unoxldizable DOC in these samples is discussed in a separate publication (Druffel, Williams a Suzuki,1989). We briefly discuss the penetration of the bomb 14C signal into the DOC and DIC pools. The temporal variability of z14C in DIC in surface samples taken every 2-3 days is presented. Concentrations of total dissolved free (FAA) plus combined (hydrolyzable) amino acids (THAA) and total dissolved carbohydrates TCHO) measured in the same water samples are also reported. Our main aim is to present the chemical and isotopic data from samples collected during the Alcyone-5 cruise. Detailed interpretation is published elsewhere.

Radiocarbon measurements at the University of Arizona AMS facility

Abstract The current status of 14C measurements at the University of Arizona accelerator mass spectrometry (AMS) facility are summarized. New developments in measurements of in situ produced 14C by the spallation of oxygen by cosmic-ray neutrons in terrestrial rocks are also discussed. These results indicate that spallogenic 14C can be measured in high-altitude rock samples, and that the measured production rates agree well with theoretical estimates.

Revised age of deglaciation of Lake Emma based on new radiocarbon and macrofossil analyses

Abstract Previous radiocarbon ages of detrital moss fragments in basal organic sediments of Lake Emma indicated that extensive deglaciation of the San Juan Mountains occurred prior to 14,900 yr B.P. (Carrara et al., 1984). Paleoecological analyses of insect and plant macrofossils from these basal sediments cast doubt on the reliability of the radiocarbon ages. Subsequent accelerator radiocarbon dates of insect fossils and wood fragments indicate an early Holocene age, rather than a late Pleistocene age, for the basal sediments of Lake Emma. These new radiocarbon ages suggest that by at least 10,000 yr B.P. deglaciation of the San Juan Mountains was complete. The insect and plant macrofossils from the basal organic sediments indicate a higher-than-present treeline during the early Holocene. The insect assemblages consisted of about 30% bark beetles, which contrasts markedly with the composition of insects from modern lake sediments and modern specimens collected in the Lake Emma cirque, in which bark beetles comprise only about 3% of the assemblages. In addition, in the fossil assemblages there were a number of flightless insect species (not subject to upslope transport by wind) indicative of coniferous forest environments. These insects were likewise absent in the modern assemblage.

Accelerator dating of a mixed assemblage of late Pleistocene insect fossils from the Lamb Spring site, Colorado

Abstract Fossil insects from the late-glacial deposits at the Lamb Spring archaeological site, near Denver, Colorado, are relatively abundant and diverse, providing considerable paleoecological data for the site. The late Pleistocene insect fauna from the site comprises 72 identified taxa, principally beetles. However, the fauna presented an interpretive problem because it contained a mixture of prairie and alpine tundra species. This was initially considered to be the result of a mixing of faunal elements during the climatic transition of late-glacial times, a “no-modern-analog” fauna. Accelerator dating of insect fossil specimens from the two ecological groups helped resolve the paleoecological problem. Fossil specimens of the prairie-associated species were dated at 17,850 ± 550 yr B.P., while specimens of the tundra-associated species yielded an age of 14,500 ± 500 yr B.P. These dates reveal that what appeared to be an ecological mixing was probably a taphonomic problem, wherein full-glacial-age fossils were probably reworked into latest Wisconsin sediments. While both faunal assemblages reflect climatic conditions substantially colder than present, initial results suggest that the full-glacial fauna represents a cold, dry grassland or steppe environment, while the younger fauna suggests moister and more tundra-like conditions.

Latest Pleistocene lake shorelines and glacial chronology in the Western Basin and Range Province, U.S.A.: insights from AMS radiocarbon dating of rock varnish and paleoclimatic implications

Accelerator mass spectrometry radiocarbon dating of organic matter in the basal layer of rock varnish provides a minimum surface exposure age for the underlying landform. Varnishes 14C ages indicate that the latest Pleistocene highstand of Lake Lahontan was abandoned before 12.7 ka, Searles Lake before 13.6 ka, Lake Manly before 13.0 ka, and Lake Mojave before 11.6 ka. The maxima of the Tioga glaciation at Pine Creek in the eastern Sierra Nevada, California, and the Middle Creek glaciation in the adjacent White Mountains of California-Nevada was just before 18–19 ka; major recessional moraines were deposited in these ranges before 12–13 ka. Penecontemporaneous glacial retreat and high levels in closed-basin lakes cannot be explained by glacial melt water, but probably reflects higher snowlines and more precipitation. A relatively minor glacial advance, perhaps related to the “Younger Dryas” in Europe, may be penecontemporaneous with a slight transgression in lake levels about 10–11 ka in the western Basin and Range, suggesting a temporary return to cooler or wetter conditions, or both at the termination of the Pleistocene.

Late Pleistocene-Recent atmospheric delta (super 13) C record in C (sub 4) grasses.

Samples of Setaria species from packrat middens, herbarium specimens and modern plants preserve a record of [delta][sup 13]C of atmospheric CO[sub 2] from 12,600 Bp to the present. No secular trend is detected between 12,600 and 1,800 Bp, when the mean value of [delta][sup 13]C during that period was [minus]6.5 [plus minus] 0.1[per thousand] (the error is the standard deviation of the mean). The value agrees with [delta][sup 13]C averages of pre-industrial CO[sub 2] from polar ice cores, and differs significantly from modern regional ([minus]8.2 [plus minus] 0.1[per thousand]) and global ([minus]7.7[per thousand]) values, which are higher because of fossil fuel burning.

Measurements of Cosmogenic 14C Produced by Spallation in High-Altitude Rocks

The production of radioisotopes at the Earths surface by cosmic-ray effects has been discussed for many years. Only in the past few years, with the higher sensitivity provided by accelerator mass spectrometry (AMS) in detecting 10Be, 26A1 and 36C1, have the radioisotopes produced in this way been measured. We report here our measurements of cosmogenic 14C in terrestrial rocks at high altitude, and comparisons to other exposure-dating methods.

STATUS OF THE NSF-ARIZONA AMS LABORATORY

Abstract The operation of the NSF-Arizona Laboratory is summarized. The methods used to determine accuracy and precision of radiocarbon measurements, and to make corrections for background contaminations are presented. An insulating support which has been installed, and a new heavy-ion beam line which is under construction, are described.