K.F. von Reden

Woce AMS radiocarbon. I: Pacific Ocean results (p6, p16 and p17) : 14C cycling and the oceans

AMS radiocarbon results from the World Ocean Circulation Experiment in the Pacific Ocean show dramatic changes in the inventory and distribution of bomb-produced 14 C since the time of the GEOSECS survey (8/73-6/74). Near-surface Δ 14 C values for the eastern portion of both the northern and southern subtropical gyres decreased by 25-50‰, with the change being greater in the north. Equatorial near-surface values have increased by ca. 25‰. Changes in the 250-750-m depth range are dramatically different between the northern and southern basins. The intermediate and mode waters of the southern basin have increased by as much as 75‰ since GEOSECS. Waters of similar density in the northern hemisphere are not exposed to the Southern Ocean circulation regime and are significantly less ventilated, showing maximum changes of ca. 50‰.

THE FIRST TRANS-ARCTIC 14C SECTION: COMPARISON OF THE MEAN AGES OF THE DEEP WATERS IN THE EURASIAN AND CANADIAN BASINS OF THE ARCTIC OCEAN

We present Δ14C data collected during three cruises to the Arctic Ocean that took place in the summers of 1987 (POLARSTERN cruise ARK IV/3), 1991 (ARCTIC 91 Expedition), and 1994 (Arctic Ocean Section 94). The cruise tracks of these three expeditions cover all major basins of the Arctic Ocean (Nansen, Amundsen, Makarov and Canada basins), and can be combined to a trans-Arctic section reaching from the Barents Sea slope to the southern Canada Basin just north of Bering Strait. The section is based on 17 stations covering the entire water column (about 250 data points). The combined Δ14C data set was produced from a mixture of large volume samples measured by low-level counting and small volume samples measured by Accelerator Mass Spectrometry (AMS). We use the Δ14C section, together with previously published Δ14C data from single stations located in several basins of the Arctic Ocean, to derive mean “ages” (isolation times) of the deep waters in the Arctic Ocean. We estimate these mean “ages” to be ≈ 250 years in the bottom waters of the Eurasian Basin and ≈ 450 years in the Canadian Basin Deep Water. A remarkable feature of the Δ14C section is the homogeneity in the 14C distribution observed in the deep Canadian Basin. Within the measurement precision of about ±2‰ (LV) to about ±5‰ (AMS), we cannot detect significant horizontal or vertical Δ14C gradients below 2000 m depth between the northern boundary of the Makarov Basin and the southern margin of the Canada Basin. There is no statistically significant difference between samples measured by AMS and by low-level counting.

A High-Performance 14C Accelerator Mass Spectrometry System

A new and unique radiocarbon accelerator mass spectrometry (AMS) facility has been constructed at the Woods Hole Oceanographic Institution. The defining characteristic of the new system is its large-gap optical elements that provide a larger-than-standard beam acceptance. Such a system is ideally suited for high-throughput, high-precision measurements of 14C. Details and performance of the new system are presented.

The National Ocean Sciences AMS facility at Woods Hole Oceanographic Institution

Abstract A new and highly automated AMS facility for producing and analyzing graphite samples from seawater and other marine sources is nearing completion. Simultaneous measurements of 12 C, 13 C and 14 C isotopes is planned with a throughput of up to 4500 samples per year.

Ten years after – The WOCE AMS radiocarbon program

The National Ocean Sciences Accelerator Mass Spectrometry (NOSAMS) Facility is measuring all of the samples collected as part of the US WOCE Program ‐ over 13,000 samples. We designed our extraction lines so that we also measure precise, oceanographically useful d 13 C-RCO2 values. We have completed the analysis of samples from the Pacific and Southern Oceans and are processing those from the Indian Ocean now. At present, this constitutes the world’s largest AMS data set. Reviews of the Pacific radiocarbon data are available and demonstrate the increased penetration of the ‘‘bomb signal’’ into the water column since the 1970s. Stable isotope data are being combined with those collected as part of NOAA’s Ocean-Atmosphere Carbon Exchange Study to study the ocean’s role in the anthropogenic CO2 cycle. The relationship of d 13 C to other chemical tracers, e.g., PO4 ,O 2 and chlorofluorocarbons, will further our understanding of basic oceanographic processes. We present preliminary results from these studies as well as investigate the relationship of 14 Ct o 13 C in the ocean. ” 2000 Elsevier Science B.V. All rights reserved.

Tests of positive ion beams from a microwave ion source for AMS

A test facility has been constructed to evaluate high-current positive ion beams from small gaseous samples for AMS applications. The major components include a compact permanent magnet microwave ion source built at the AECL Chalk River Laboratory and now on loan from the University of Toronto, and a double-focusing spectrometer magnet on loan from Argonne National Laboratory. Samples are introduced by means of a silica capillary injection system. Loop injection into a carrier gas provides a stable feed for the microwave driven plasma. The magnetic analysis system is utilized to isolate carbon ions derived from CO2 samples from other products of the plasma discharge, including argon ions of the carrier gas. With a smaller discharge chamber, we hope to exceed a conversion efficiency of 14% for carbon ions produced per atom, which we reported at AMS-7. The next step will be to construct an efficient charge-exchange cell, to produce negative ions for injection into the WHOI recombinator injector.

Test of negative ion beams from a microwave ion source with a charge exchange canal for accelerator mass spectrometry applications

A test facility has been constructed to evaluate negative ion beams from small gaseous samples for accelerator mass spectrometry applications. The positive ion beams from the microwave ion source are passed into a charge-exchange canal (CXC) where the ions exchange electrons with magnesium vapor and become negatively charged. Positive molecular ions were either neutralized or broken up into atomic ions and neutral atoms and molecules by collision processes. Most of the resulting particles were suspected to be neutrals. In studies with injections of CO2 pulses, the resulting positive and negative 12C current peaks gave a 0.09% yield of C− ions from CO2 molecules, which includes a charge-exchange efficiency of 10%. Since nitrogen does not form a stable negative ion, 14N background is virtually eliminated after the beam goes through the CXC, which is necessary for radiocarbon measurements.

Hold-up and memory effect for carbon in a compact microwave ion source

Abstract The production of C + in a microwave ion source injected with a gaseous CO 2 sample has been investigated as an alternative to sample graphitization. A continuous flow of argon gas maintained the discharge, and 20 μL of CO 2 gas at approximately 1 atm was pulsed into the source through a sample valve closely coupled to the plasma chamber. The C + component of the beam fell to 1% of its original intensity less than 10 s after the valve was operated. In a separate experiment, the microwave ion source was operated with pure CO 2 feed gas and the extracted beam was magnetically analyzed. Efficient breakup of the molecules was observed. These results are considered promising for such applications as accelerator mass spectrometry of C isotopes in gaseous samples.

Measurements of the Oxalic Acid II /Oxalic Acid I Ratio as a Quality Control Parameter at NOSAMS

Le rapport acide oxalique I/acide oxalique II est la reference la plus communement utilisee pour etalonner les spectrometres de masse et dans tous les cas, une relation lineaire doit etre observee entre le deplacement isotopique des 14 C et 13 C

The New National Ocean Sciences Accelerator Mass Spectrometer Facility at Woods Hole Oceanographic Institution: Progress and First Results

Start-up performance and first results of the new Woods Hole Accelerator Mass Spectrometer are discussed. Special attention is given to the hemispherical ionizer sputter source and the triple-isotope injector design.