Karl F. von Reden

The Keck Carbon Cycle AMS Laboratory, University of California, Irvine : status report

We present a status report of the accelerator mass spectrometry (AMS) facility at the University of California, Irvine, USA. Recent spectrometer upgrades and repairs are discussed. Modifications to preparation laboratory procedures designed to improve sample throughput efficiency while maintaining precision of 2-3‰ for 1-mg samples (Santos et al. 2007c) are presented.

The NOSAMS sample preparation laboratory in the next millenium: Progress after the WOCE program

Since 1991, the primary charge of the National Ocean Sciences AMS (NOSAMS) facility at the Woods Hole Oceanographic Institution has been to supply high throughput, high precision AMS 14C analyses for seawater samples collected as part of the World Ocean Circulation Experiment (WOCE). Approximately 13,000 samples taken as part of WOCE should be fully analyzed by the end of Y2K. Additional sample sources and techniques must be identified and incorporated if NOSAMS is to continue in its present operation mode. A trend in AMS today is the ability to routinely process and analyze radiocarbon samples that contain tiny amounts (<100 μg) of carbon. The capability to mass-produce small samples for 14C analysis has been recognized as a major facility goal. The installation of a new 134-position MC-SNICS ion source, which utilizes a smaller graphite target cartridge than presently used, is one step towards realizing this goal. New preparation systems constructed in the sample preparation laboratory (SPL) include an automated bank of 10 small-volume graphite reactors, an automated system to process organic carbon samples, and a multi-dimensional preparative capillary gas chromatograph (PCGC).

Performance characteristics of the 3 MV Tandetron AMS system at the National Ocean Sciences AMS facility

Abstract Operational and machine performance parameters are discussed for the National Ocean Sciences AMS system. The system now routinely measures between 50 and 100 carbon samples per week in largely unattended mode using one of two functional high-current ion sources. System development and procedures are described that enable us to reach and maintain the high precision level required for the measurement of deep sea water dissolved inorganic carbon samples.

Comparative study of Fe-C bead and graphite target performance with the National Ocean Science AMS (NOSAMS) facility recombinator ion source

Abstract An accelerator mass spectrometry (AMS) experiment was designed to investigate 14 C target performance for two target types over a range of isotopic concentrations and sample sizes, with a special focus on the ability to measure 14 C in environmental samples having only microgram amounts of carbon. The findings were positive, showing that precision, accuracy, and stability were adequate to determine 14 C to 1% or better in samples containing as little as 25 μg carbon. Satisfactory Poisson uncertainty and target stability were demonstrated down to a level of 7 μg carbon, but experimental data showed that accurate measurements at that level require detailed knowledge of blank variability and mass dependence of the modern carbon calibration factor.

High-precision AMS radiocarbon measurements of central Arctic Ocean sea waters

Abstract We report on the first sub-5%. precision radiocarbon dataset measured on single targets using accelerator mass spectrometry (AMS). A 13 sample water column profile collected in the Canada Basin (74°N, 150°W, 3850 m water depth) of the central Arctic Ocean in September 1992 has been analyzed in duplicate and the average total precision achieved for the 26 targets is ± 3.2%.. The reproducibility of the 13 paired analyses averages +- 4.8%. as determined by a chi-square fit minimization for a quality factor of unity, and ± 7.8%. using ANOVA. Eliminating two of the 13 paired analyses because of apparent outlier behavior in one of the two analyses comprising the pair results in a total precison of ± 3.2%., a chi-square fit of ± 3.5%., and ANOVA precision of ± 3.5%.. Comparison with a recently published AMS 14 C profile from the same basin suggests these data are accurate as well. Results show that the deep waters of the Canada Basin have a renewal rate of 430 years, in comparison with 250 years estimated for the deep waters of the Eurasian Basin. Although the major requirement of the World Ocean Circulation Experiment (WOCE) for a radiocarbon analysis precision of ± 3 to 4%. for deep water samples can now be met with the AMS technology available at the National Ocean Sciences AMS Facility at the Woods Hole Oceanographic Institution for single-target analyses, a careful program of duplicate analyses should be included to insure the highest quality in the WOCE Δ 14 C dataset.

African baobabs with false inner cavities : the radiocarbon investigation of the Lebombo Eco Trail Baobab

The article reports the radiocarbon investigation results of the Lebombo Eco Trail tree, a representative African baobab from Mozambique. Several wood samples collected from the large inner cavity and from the outer part of the tree were investigated by AMS radiocarbon dating. According to dating results, the age values of all samples increase from the sampling point with the distance into the wood. For samples collected from the cavity walls, the increase of age values with the distance into the wood (up to a point of maximum age) represents a major anomaly. The only realistic explanation for this anomaly is that such inner cavities are, in fact, natural empty spaces between several fused stems disposed in a ring-shaped structure. We named them false cavities. Several important differences between normal cavities and false cavities are presented. Eventually, we dated other African baobabs with false inner cavities. We found that this new architecture enables baobabs to reach large sizes and old ages. The radiocarbon date of the oldest sample was 1425 ± 24 BP, which corresponds to a calibrated age of 1355 ± 15 yr. The dating results also show that the Lebombo baobab consists of five fused stems, with ages between 900 and 1400 years; these five stems build the complete ring. The ring and the false cavity closed 800–900 years ago. The results also indicate that the stems stopped growing toward the false cavity over the past 500 years.

Fire history of a giant African baobab evinced by radiocarbon dating

The article reports the first radiocarbon dating of a live African baobab (Adansonia digitata L.), by investigating wood samples collected from 2 inner cavities of the very large 2-stemmed Platland tree of South Africa. Some 16 segments extracted from determined positions of the samples, which correspond to a depth of up to 15-20 cm in the wood, were processed and analyzed by accelerator mass spectrometry (AMS). Calibrated ages of segments are not correlated with their positions in the stems of the tree. Dating results indicate that the segments originate from new growth layers, with a thickness of several centimeters, which cover the original old wood. Four new growth layers were dated before the reference year AD 1950 and 2 layers were dated post-AD 1950, in the post-bomb period. Formation of these layers was triggered by major damage inside the cavities. Fire episodes are the only possible explanation for such successive major wounds over large areas or over the entire area of the inner cavities of the Platland tree, able to trigger regrowth.

Optimized data analysis for AMS radiocarbon dating

Abstract Because efficiencies of detection of 14 C and 12 C vary with time during AMS data acquisition, it is desirable to have a data analysis technique which recognizes time variations and uses all available data to extract the maximum possible information content from a data set while providing meaningful statistical information about measurement errors. Toward this end, a new method of data reduction and error analysis is being developed for the determination of 14 C to 12 C ratios for radiocarbon dating from data sets taken at the National Ocean Sciences AMS facility.

Age and Growth Rate Dynamics of an Old African Baobab Determined by Radiocarbon Dating

In 2008, a large African baobab (Adansonia digitata L.) from Makulu Makete, South Africa, split vertically into 2 sections, revealing a large enclosed cavity. Several wood samples collected from the cavity were processed and radiocarbon dated by accelerator mass spectrometry (AMS) for determining the age and growth rate dynamics of the tree. The 14C date of the oldest sample was found to be of 1016 ± 22 BP, which corresponds to a calibrated age of 1000 ± 15 yr. Thus, the Makulu Makete tree, which eventually collapsed to the ground and died, becomes the second oldest African baobab dated accurately to at least 1000 yr. The conventional growth rate of the trunk, estimated by the radial increase, declined gradually over its life cycle. However, the growth rate expressed more adequately by the cross-sectional area increase and by the volume increase accelerated up to the age of 650 yr and remained almost constant over the past 450 yr.

Methods for data screening, flagging and error analysis at the National Ocean Sciences AMS Facility

Abstract All data collection, from sample submittal through processing into targets and AMS analysis, is integrated within a large relational database (Sybase). Over 50 tables are linked through key fields. Through structured queries, the information is analysed and presented for a wide variety of applications. Benefits include enhanced quality control, more complete reports to users and more accurate transfer of data among the several laboratories on the network.