D.E. Nelson

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.

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.

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.

Be isotopes in rivers/estuaries and their oceanic budgets

10Be and 9Be have been determined in several North American rivers and in the estuaries of San Francisco Bay and the Pearl River (China). The average fluvial concentration of dissolved 10Be in the rivers is 3220 ± 1960 (1σ) atoms/g, almost an order of magnitude higher than the observed estuarine values and slightly higher than the 10Be concentration in the ocean ( ∼ 1700 atoms/g). For dissolved 9Be, a similar order-of-magnitude drop in concentration also occurs in going from rivers (750 ± 740 pM) to estuaries (80–140 pM). However, in contrast to the 10Be situation, there appears another order-of-magnitude drop from estuaries to the ocean, which has 9Be concentrations of 5–30 pM. In spite of the wide range of concentrations for both isotopes in river and estuarine waters, 10Be/9Be ratios range mostly from 2 to 12 × 10−9, with a median value of 7 × 10−9 (atom/atom), and are much lower than the seawater ratio of ∼ 10−7. In the rivers studied, a given volume of water contains roughly equal amounts of particulate and dissolved 10Be. The 10Be concentration in particulate matter ranges from 2 × 106 to 1 × 109 atoms/g-particulate with a median value of about 5 × 107 atoms/g-particulate. A two-box model calculation shows that coastal regions play an important role in the removal of Be isotopes from the ocean and that eolian dusts may be the chief source of oceanic 9Be. Because of the marginal removal effect, the overall oceanic residence time of Be should be shorter than the residence time of 500–1200 yr estimated for the open ocean.

Growth of a manganese nodule from Peru Basin: A radiochemical anatomy

Abstract Attempts have been made to study the entire growth history of a manganese nodule from the northern part of Peru Basin in the Pacific using radiochemical profiles of 230 Th 232 Th , 227 Th 230 Th , and 10 Be 9 Be . Combined with the observations on Fe-Mn contents and textural variation, the radiochemical data indicate that the nodule grew more or less concentrically throughout most of its existence since it formed 1.5 my ago, receiving Mn from both bottom water and pore water. This condition appeared to have changed about 180 ky ago when the growth became asymmetric in that the top and bottom sides became fixed in their relative positions on the sea floor. Since then, the bottom side accreted with a fast rate of close to 200 mm/my, apparently fueled by the supply of diagenetically remobilized Mn in pore water from the sediment substrate. In the meantime, the top side accumulated at about 6 mm/my, a value which is in the normal range for deep-sea nodules having their Mn supplied from the hydrogenous source.

The distribution of 10Be and 9Be in ocean water

Abstract The vertical distributions of 10Be and 9Be have been measured at several locations in the Pacific and the North Atlantic. The results from the Pacific show that both isotopes exhibit nutrientlike profiles resulting from participation in the cycling of marine particulates. The 10 Be 9 Be ratios from the different stations vary from 1 to 3 × 10−7 in the mixed layer but tend towards a value close to 1.1 × 10−7 in the deep ocean. Profiles from North Atlantic stations show a lower deep-water 10 Be 9 Be ratio of about 6 × 10−8.

The measurement of 10Be with an accelerator at 3 MV

Abstract We have detected and measured 10 Be at natural concentrations with a tandem accelerator at 3 MV. Details of the measurement technique are discussed.

Progress in 14C and 10Be dating at SFU

Abstract Since early 1982, a substantial upgrading of the Simon Fraser University radioisotope dating system at the McMaster FN accelerator has taken place. The accelerator itself was equipped with a new charging system and beam tubes, resulting in excellent stability and beam transmission. A new isotope filtering system of two magnets and a Wien filter has removed previously troublesome backgrounds: our carbon background from graphite is now ∼ 47 ka BP, and is due to real 14C rather than scattered 13C. Measurement of 10Be in natural samples is now routine, with several hundred samples processed. Other work on 10Be has included studies which showed for the first time that 10Be could be measured at low energies (3 MV). We have also started work on 26Al detection as a preliminary to the investigation of 26Al/10Be dating. Finally, we present examples of our applications of this facility to studies in oceanography and archaeology.