J.S. Vogel

Performance of catalytically condensed carbon for use in accelerator mass spectrometry

Abstract Two catalytic processes have been explored for the preparation of suitable samples for use in 14C measurements on an accelerator mass spectrometer. A heavy hydrocarbon was condensed from C2H2 using AlBr3 as a catalyst. This process had low isotopic fractionation, and the carbon ion beam obtainable was 60–70% that from graphite. In the second process, iron powder was used to produce graphite directly from CO2 and H2 at 600 °C. A sample preparation system using this reaction has been built. The carbon product produces exceptionally intense, long-lived ion beams. The process introduces little 14C background, and has no observed memory effects.

super 14) C background levels in an accelerator mass spectrometry system.

The levels and sources of the measurement background in an AMS 14 C dating system have been studied in detail. The relative contributions to the total background from combustion, graphitization, storage, handling, and from the accelerator were determined by measuring the C concentrations in samples of anthracite coal ranging in size from 15μg to 20mg. The results show that, for the present system, the uncertainty in the background is greater than that due to measurement precision alone for very old or for very small samples. While samples containing 100μg of carbon can yield useful 14 C dates throughout the Holocene, 200 to 500μg are required for dating late Pleistocene materials. With the identification of the procedures that introduce contamination, the level and uncertainty of the total system background should both be reducible to the point that 100μg of carbon would be sufficient for dating most materials.

Catalyst and binder effects in the use of filamentous graphite for AMS

Filamentous graphite produced by the reduction of carbon dioxide over a transition metal catalyst has been used as a sample material in the AMS counting of 14C for several years. While iron has been the most common catalyst used, cobalt and nickel have also been investigated. We have compared the reactions total isotopic fractionation using these three metals in various powder forms. The graphites produced over these catalysts have been compared with respect to ion beam intensity and measured isotope ratio in our AMS system. The use of the reduction catalysts as graphite binders improved the very poor thermal conductivity of pure filamentous graphite. The relative advantages of the elements for modern contamination of the sample material were also determined.

Radiocarbon Dating of Pollen by Accelerator Mass Spectrometry

Radiocarbon dating of bulk sediments has been the standard method for establishing chronologies in the studies of lake sediment cores which have contributed significantly to our knowledge of late Quaternary paleo-environments. These bulk sediment dates are presumed to be direct ageindicators for the speciments (e.g., pollen or macrofossils) which are actually being studied. However, several recent studies have reinforced long-standing apprehensions concerning this presumption. In this study, we demonstrate for the first time the radiocarbon dating of pollen concentrate samples by accelerator mass spectrometry. The dates obtained by this method should provide more reliable radiocarbon chronologies for paleo-environmental studies than have been obtainable by bulk sediment dating.

Younger dryas climatic reversal in northeastern USA? AMS ages for an old problem

Abstract Late-glacial macrofossils from a 10-m core from Alpine Swamp, New Jersey, were radiocarbon dated using accelerator mass spectrometry (AMS). The arrival of the first trees to the area following deglaciation is indicated by maximum percentages of spruce pollen and a date of 12,290 ± 440 yr B.P. on a single spruce needle. Subsequent spread of deciduous hardwoods was followed by the expansion of boreal taxa, including spruce (Picea), fir (Abies), larch (Larix laricina), paper birch (Betula papyrifera), and alder (Alnus). Three AMS dates on paper birch seeds and a spruce needle during this boreal expansion indicate that it took place between 11,000 and 10,000 yr B.P. The timing of this vegetational shift and its correlation with late-glacial pollen stratigraphy from many sites in southern New England indicate that a climatic reversal correlative with the Younger Dryas characterized the North Atlantic seaboard of the United States.

Accelerator radiocarbon dating of human blood proteins in pigments from Late Pleistocene art sites in Australia

Absolute dating of rock paintings has always used an indirect means, generally by dating material in strata sealing or overlying the pictures. AMS dating of very small carbon samples now allows direct determination of the age of an organic portion in the matter of the picture itself.

Late-glacial pollen, macrofossils and fish remains in northeastern U.S.A. — The Younger Dryas oscillation: A contribution to the ‘North Atlantic seaboard programme’ of IGCP-253, ‘Termination of the Pleistocene’

The late-glacial environmental histories of Allamuchy Pond, New Jersey and Linsley Pond, Connecticut are reconstructed from pollen, macrofossil and fish scale remains. Accelerator mass spectrometry (AMS) 14C dating of seeds and needles indicates that the first organic deposition, evidenced by fossil Picea (spruce) needles, occurred approximately 12,400 BP. A major regional warming began in the northeastern United States at this time, correlative with the Bolling/Allerod warming of Europe and Greenland. The increase in Quercus (oak) pollen and presence of Pinus strobus (white pine) needles demonstrates the magnitude of warming reached at about 11,000 BP. The subsequent decline of thermophilous species and increase in boreal Picea, Abies (fir), Larix (larch), Betula papyrifera (paper birch) and Alnus (alder) from 10,800-10,000 BP was a regional vegetational reversal. Thus we find a North American expression of the Younger Dryas with a mean annual temperature depression of 3–4° C. The subsequent classical southern New England pine pollen zone ‘B’ and Pinus strobus macrofossils signalled a return to warmer conditions at approximately 10,000 BP, regionally, within approximately 50–100 years. A large increase in Quercus follows. This study is unique in documenting a continuous late-glacial record of fish remains from Allamuchy Pond, New Jersey sediments, indicating that members of the families Centrarchidae (sunfish), Salmonidae (trout), Percidae (perch) and Cyprinidae (minnow) were regionally present.

Beryllium isotope distribution in the western North Atlantic : a comparison to the Pacific

Abstract The surface depletion/deep enrichment of the 10 Be and 9 Be distributions observed in the Pacific is not as conspicuous in the western North Atlantic Ocean between 33° and 42°N. In the case of 9 Be, a surface excess sometimes even exists. While 10 Be concentrations in the surface waters of the two oceans are comparable, 9 Be concentrations in the surface North Atlantic are about five times those in the surface Pacific. Deep waters show an increase of concentrations from the Atlantic (via Antarctic) to the Pacific, but the degree of increase for the two isotopes is different: about 10% for 9 Be and 2.5-fold for 10 Be. Thus there is a systematic increasing trend in the ration 10 Be/ 9 Be ( atom / atom × 10 −7 ) along the advective flow lines: surface North Atlantic (0.4)→ deep North Atlantic (0.6) → Circumpolar (1.0) → deep Pacific (1.2) → surface Pacific (1–3). These distributional patterns and contrasting North Atlantic-Pacific features can be explained in terms of: (a) a strong fluvial and continental dust input of 9 Be to the North Atlantic and an ocean-wide, more-or-less uniform pluvial input of cosmogenically produced 10 Be; (b) active participation of Be in the particle recycling involving surface uptake and deep water regeneration; and (c) mean removal time of Be from the ocean, which, based on the 10 Be/ 230 Th ratios in surface sediments of two deep-sea cores, are estimated as ∼1200 y in the open Pacific and ∼500 a in the open Atlantic, values that are close to water mixing times of the individual ocean basins.

Distribution of10Be and9Be in the Pacific Ocean

Abstract The vertical distributions of10Be and9Be at three locations in the Pacific (25°N, 170°E; 17°N, 118°W; 3°S, 117°W) are presented. The results show that both isotopes exhibit nutrient-like profiles. From the surface to the bottom, the increase for10Be is two- to threefold and that for9Be is about fivefold. While the inter-station variations in surface water concentrations may reach a factor of two, deep-water values tend to be much more uniform averaging about 2000 atoms/g for10Be and 30 pM for9Be. A similar situation applies to the10Be/9Be ratio; it varies approximately from 1 to 3 × 10−7 (atom/atom) at shallow depths but tends toward a value close to 1.1 × 10−7 in the deep ocean. The variation of10Be/9Be can be viewed as resulting from the fact that10Be in a given parcel of water consists of two components: recycled and primary. The recycled component is that part of10Be which has reached tracer equilibrium with9Be, as opposed to the primary component which, upon entering the sea from the atmosphere, has yet to equilibrate with9Be through particle cycling and mixing processes. It is estimated that 70% to nearly 100% of10Be at the three stations are being recycled, and the recycled beryllium bears an atomic ratio of10Be/9Be close to 1 × 10−7. The oceanic residence time of Be is of the order of 1000–4000 years, comparable to or slightly longer than the ocean mixing time.

10Be in a deep-sea core: implications regarding10Be production changes over the past 420 ka

Abstract The influx of 10 Be into a globigerinid ooze core (CH72-02) from the eastern North Atlantic has been studied. This core contains a depositional record of the first 11 δ 18 O stages covering the last 423 ka. It is shown that the marine deposition of 10 Be is strongly influenced by the sedimentation of clays. Clay particles appear 10 times more efficient than the carbonate component as a carrier in bringing 10 Be to the bottom sediments. In core CH72-02, the deposition rates of 10 Be averaged over each oxygen-isotope stage for the past 11 stages show a scatter of ±40% about the mean value of 6.6 × 10 8 atoms cm −2 ka −1 . However, after correction for changes in lithology, the data show that the production rate of 10 Be over the same period has varied no more than ±25%, and the variations are not systematic in that high or low 10 Be production appear to be associated with either cold or warm climates. On the time scale of this investigation (intervals of ca. 50 ka over the last 420 ka, with resolutions as fine as 10 ka for portions of the record), it is unlikely that the shielding effect of the solar wind has deviated by more than ±25% or the geomagnetic field intensity has deviated by more than a factor of 1.6 from their long-term averages.