A. J. T. Jull

Accelerator Radiocarbon Dating at the Molecular-Level

Molecular level 14C dating is the isolation of specific classes of molecules for their 14C dating by accelerator mass spectrometry (AMS). Complex matrices such as fossil bone are difficult to date due to their extreme chemical heterogeneity. By isolating individual amino acids, contaminants (humates) are removed and crystalline amino acids result. n nBones with ≥0·1–0·2% N and collagenous compositions can be dated accurately because structural collagen is present; contaminants are removable with XAD resin. Bones with ⩽0·1% N and non-collagenous compositions yield dates hundreds to thousands of years too young because most of the preserved organic matter is exogenous. Accelerator 14C dates on collagenous and non-collagenous bones are not comparable due to intrinsic dating inaccuracies. n nAMS 14C dating of amino acids demonstrated that (1) post 10,800 year ages for North American megafauna are due to sample contamination, not Holocene ages on extinct fauna, (2) a Clovis age (10,900 years) was established for a human fossil from the Anzick site, Montana, (3) Holocene ages cannot be established absolutely for many North American human fossils because the bones were non-collagenous, (4) accurate ages are attainable on vertebrate fossils as small as passerine birds from Pacific Island localities, (5) well preserved bones are datable without their destruction by extracting protein with water at high temperatures, and (6) stratigraphic anomalies to 45,000 years in European Upper Paleolithic rock shelters are recognizable by dating bone directly.

Study of bone radiocarbon dating accuracy at the University of Arizona NSF accelerator facility for radioisotope analysis.

Bone would seem to be an ideal material for 14 C dating because this calcified tissue contains 20 weight per cent protein. Fossil bone, however, can lose most of its original organic matter and frequently contains contaminants having different 14 C ages. Numerous 14 C dates on bone have been available to archaeologists and geologists but many age determinations have been inaccurate despite over 30 years of research in the field following the first 14 C age determinations on bone (Arnold & Libby, 1951). This situation remained unchanged until simple pretreatments were abandoned and more bone-specific fractions were isolated. The ideal solution is to use accelerator mass spectrometer 14 C dating, which facilitates the use of milligram-sized amounts of highly purified compounds—an approach impossible to pursue using conventional 14 C decay-counting methods.

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 calendricnyears).

Application of tandem accelerator mass-spectrometer dating to late pleistocene-holocene sediments of the East Antarctic continental shelf

Abstract Glacial recession from the Antarctic continental shelf is recorded by glacial-marine diamictons, sands, and overlying siliceous oozes. In order to clarify the chronology for this sequence, use was made of the University of Arizona tandem accelerator mass-spectrometer (TAMS) for 14 C dating. Small samples of benthic and planktonic foraminifera were selectively removed from diamictons, graded sands, and surface sediments which were recovered from the Wilkes Land continental shelf and slope, East Antarctica. Organic carbon was also utilized as a source for TAMS dating of the siliceous oozes and muds. Uncorrected ages varied from 14,260 ± 140 to 3230 ± 200 yr B.P. Carbon fixed by phytoplankton and foraminifera is strongly influenced by old, glacial-derived CO 2 . Thus, reservoir corrections of up to 5500 yr are needed for the 14 C dates. Iceberg turbation reworks foraminifera so that dates from resulting deposits (diamictons) are interpreted as maximum ages. The consistency of corrected ages from the shelf, along with the sedimentologic interpretation, suggests a rather recent recession, perhaps mid-Holocene for this portion of the East Antarctic ice sheet. Further application of the TAMS method should help clarify other problems concerning the late Quaternary glacial history of Antarctica.

Chronology of Holocene vertebrate extinction in the Galápagos Islands

Abstract Eleven conventional and 15 AMS (accelerator mass spectrometer) radiocarbon dates establish chronologies for nine Holocene vertebrate fossil sites in the Galapagos Islands, Ecuador. A Holocene fossil record approaching 500,000 bones, more than 90% of which predate the arrival of people, shows that most or all cases of extinction or extirpation in the Galapagos occurred after first human contact in AD 1535. This is especially significant for two species of large rodents (Megaoryzomys spp.) that are known only from bones found in lava tubes. The highest rate of background (prehuman) extinction interpreted from the Galapagos fossil record is probably hundreds of times less than the modern rate of human-related extinction.

Radiocarbon AMS dating of pollen concentrated from eolian sediments : Implications for monsoon climate change since the Late Quaternary

Dating pollen concentrated from eolian sediments provides a new way to establish a chronological framework on the Loess Plateau of China. We show that pollen deposited simultaneously with sediment in a stable environment can provide reliable ages. We suggest that the reliability of pollen dating can be evaluated by comparison with wood cellulose or charcoal ages from the same stratigraphic level. Dating pollen concentrates from the various profiles indicates paleomonsoon precipitation variability at the loess/desert transitional belt from the late Pleistocene to the early Holocene.

Accuracy of North American human skeleton ages

Accelerator mass spectrometry (AMS) radiocarbon dates fail to provide conclusive evidence that all New World human fossils are younger than approximately 11,000 yr. Because fossil bones vary widely in preservation, their radiocarbon dates are not equally accurate. Molecular-level radiocarbon dating, which used individual amino acids to assess fossil diagenesis, revealed that dates on known-age, noncollagenous bone were underestimated by at least 2000 to 9000 yr. The significance is that >11,000-yr-old fossil bones with poor preservation would yield Holocene and not Pleistocene radiocarbon ages, regardless of what chemical pretreatment or 14C counting method was used. Irreplaceable evidence for Pleistocene-age fossils in the New World could be lost if the diagenesis of fossil bones is not evaluated before the bones are radiocarbon dated. In contrast, radiocarbon ages for collagenous fossils can be determined more accurately if 14C is measured in several individual amino acids that are isolated from collagenous bone protein. Molecular-level radiocarbon dating will greatly improve not only the accuracy of chronologies for human migrations and animal extinctions, but of all late Quaternary chronologies that are based upon the 14C dating of fossil proteins.

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.

The reliability of AMS radiocarbon dating of shells from China

We tested the feasibility of dating freshwater and terrestrial molluscs from the semiarid and arid zone in China, since these types of shell material deposit only aragonite to form their shell structure, and shell integrity can be easily observed using X-ray diffraction. We also tested the possibility of estimating microenvironmental changes from shell delta (super 13) C values, but variations within shell populations preclude the use of these values as a reliable indicator. Reservoir ages were calculated for living shells of the same species as fossil shells by using their measured (super 14) C ages, which were recalculated using an average value of atmospheric (super 14) C activity for the years spanning their time of collection as the modern standard. The results indicate that freshwater and terrestrial shells are potentially useful as dating material, provided extreme care is taken in their collection and other datable material (in this case wood and pollen) is within the profile to act as a comparison.

Reappraisal of Chinese Loess Plateau stratigraphic sequences over the last 30,000 years : Precursors of an important Holocene Monsoon climatic event

Through the establishment of radiocarbon chronozones relating common geological events within lacustrine and eolian sediments from five profiles representative of loess yuan (tablelands), river valley and northwest margin features of the Loess Plateau, we propose a series of stratigraphic divisions within the last 30 ka. The focus of this detailed study involves stratigraphic relationships contributing to evidence of Younger Dryas events, with the recognition of cold-dry, cool-wet and cold-dry periods represented within the Heiheze silt, Midiwan peat and Liushuwan eolian sand. The stratigraphic profiles reflect century-scale fluctuations of the East Asian monsoons. The precursor events enable us to place the Pleistocene/Holocene boundary at 10,000 BP.