D.J. Donahue

A Large Drop in Atmospheric 14C/12C and Reduced Melting in the Younger Dryas, Documented with 230Th Ages of Corals.

Paired carbon-14 (14C) and thorium-230(230Th) ages were determined on fossil corals from the Huon Peninsula, Papua New Guinea. The ages were used to calibrate part of the 14C time scale and to estimate rates of sea-level rise during the last deglaciation. An abrupt offset between the 14C and 230Th ages suggests that the atmospheric 14C/12C ratio dropped by 15 percent during the latter part of and after the Younger Dryas (YD). This prominent drop coincides with greatly reduced rates of sea-level rise. Reduction of melting because of cooler conditions during the YD may have caused an increase in the rate of ocean ventilation, which caused the atmospheric 14C/12C ratio to fall. The record of sea-level rise also shows that globally averaged rates of melting were relatively high at the beginning of the YD. Thus, these measurements satisfy one of the conditions required by the hypothesis that the diversion of meltwater from the Mississippi to the St. Lawrence River triggered the YD event.

Isotope-Ratio and Background Corrections for Accelerator Mass Spectrometry Radiocarbon Measurements

We present here the method we use to convert to radiocarbon ages (14C/13C) ratios measured in the Arizona Accelerator Mass Spectrometer facility. We describe the procedures we use to convert sample and standard isotope ratios to values appropriate for calculation of radiocarbon ages. We also discuss, in some detail, corrections to account for sample contamination.

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. Bones 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. AMS 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 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.

Enhanced cosmic‐ray production of 10Be coincident with the Mono Lake and Laschamp Geomagnetic Excursions

The cosmogenic isotope 10 Be , total Be, and Al were measured in partly varved sediments from the upper 50 m of core 480, leg 64 (DSDP), Gulf of California. The concentration of 10 Be from 1 to 50 kyr is in general agreement with estimates of the geomagnetic dipole moment obtained from archaeomagnetic and marine core research. 10 Be anomalies were also found at 32 kyr and 43 kyr, contemporaneous with the Mono Lake and Laschamp excursions, respectively. The production of 10 Be required to explain these anomalies is too high, particularly for the Mono Lake excursion, to be produced by a combination of decreased geomagnetic field and unprecedented long-term solar activity. We conclude that the cause is a change in the galactic cosmic-ray flux consistent with a supernova event. The coincidence with the two excursions remains a paradox.

Teleconnection of climatic events between East Asia and polar, high latitude areas during the last deglaciation

Abstract From observations of continuous aeolian and swamp sediment sequences, augmented by a detailed 14 C chronology, δ 13 C and organic C analyses, we can demonstrate century- to millennium-scale fluctuations in East Asian monsoon palaeoclimatic events. We also infer significant precipitation variability within the last deglaciation. The major climatic zones that have been recognised in Europe, and find counterparts in East Asia are, the Bolling–Allerod (14,750–12,800 cal yr B.P.), and the Younger Dryas (12,800–11,600 cal yr B.P.). The last deglaciation sediment sequences are characterised by frequent facies changes reflecting climate instability. These frequent, abrupt climatic events correlate well with fluctuations recorded in high latitude and polar areas, as represented by the Greenland ice core GISP 2 and a core from the North Sea. This indicates a palaeoclimate teleconnection between the high latitude and polar areas and the East Asian monsoon areas through cold air mass activity, and the related atmospheric pressure system.

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.

A high-resolution radiocarbon calibration between 11,700 and 12,400 calendar years BP derived from 230Th ages of corals from Espiritu Santo Island, Vanuatu

This paper presents radiocarbon results from a single Diploastrea heliopora coral from Vanuatu that lived during the Younger Dryas climatic episode, between ca. 11,700 and 12,400 calendar yr BP. The specimen has been independently dated with multiple 230Th measurements to permit calibration of the 14C time scale. Growth bands in the coral were used to identify individual years of growth. 14C measurements were made on each year. These values were averaged to achieve decadal resolution for the 14C calibration. The relative uncertainty of the decadal 14C data was below 1% (2σ). The data are in good agreement with the existing dendrochronology and allow for high-resolution calibration for most years. Variations in the fine structure of the 14C time series preserved in this specimen demonstrate sporadic rapid increases in the Δ14C content of the surface ocean and atmosphere. Certain sharp rises in Δ14C are coincident with gaps in coral growth evidenced by several hiatuses. These may be related to rapid climatic changes that occurred during the Younger Dryas. This is the first coral calibration with decadal resolution and the only such data set to extend beyond the dendrochronology-based 14C calibration.