We have developed techniques for accurately and precisely measuring samples containing less than a few hundred micrograms of carbon, using a compact AMS system (NEC 0.5 MV 1.5SDH-1). Detailed discussions of the sample preparation, measurement setup, data analysis and background corrections for a variety of standard samples ranging from 0.002 to 1 mgC are reported. Multiple aliquots of small amounts of CO 2 were reduced to graphite with H 2 over pre-baked iron powder catalyst. A reduction reaction temperature of 450°C was adopted for graphite samples below 0.05 mgC, rather than the usual 550°C used on samples of 0.1-1 mgC. In our regular reactors ($3.1 cm 3 ), this reduction in temperature improved the graphite yield from $60 to 90-100% for samples ranging from 0.006-0.02 mgC. The combination of lower reaction temperature with a reduced reactor volume ($1.6 cm 3 ) gave yields as high as 100% on graphite samples <0.006 mgC. High performance measurements on ultra-small samples are possible also due to a modified NEC MC-SNIC ion-source that generates C À currents of 1 lA per lg of carbon for samples in the 0.002 to 0.010 mgC range, combined with on-line measurement of 12 C and 13 C (AMS d 13 C) to correct machine-induced isotopic fractionation. Source efficiencies are in excess of 10%, which enables 4-5% of the radiocarbon atoms in 0.005-0.010 mgC samples to be measured. Examination of the background samples revealed two components: (a) 0.2-1 lg of modern carbon and (b) 0.1-0.5 lg of dead carbon. The latter component can be ignored when measuring unknown samples paired to small standards of precisely identical size (matching size normalizing standard method). Otherwise, one must make corrections for both background components. Ultra-small samples from 0.002 to 0.01 mgC can be measured with accuracy and precision of a few percent, based on scatter in results for multiple aliquots of a primary standard and deviations of secondary standards from their known values.

We present a brief discussion of sample preparation procedures at the Keck Carbon Cycle Accelerator Mass Spectrometer (KCCAMS), University of California, Irvine, and a systematic investigation of the use of Mg(ClO4)2 as an absorptive water trap, replacing the standard dry ice/ethanol cold finger in graphite sample preparation. We compare high-precision AMS measurement results from oxalic acid I and USGS coal samples using Mg(ClO4)2 under different conditions. The results obtained were also compared with those achieved using the conventional water removal technique. Final results demonstrate that the use of Mg(ClO4)2 as an alternative water trap seems very convenient and reliable, provided the Mg(ClO4)2 is replaced frequently.

Dissolved organic carbon (DOC), operationally defined as the carbon contained in that fraction of organic matter in water which passes through a 0.2-1 μm filter, is the largest pool (ca. 0.6 × 10 18 g C) of organic matter in the oceans and is approximately equal in size to atmospheric CO 2 (Hedges 1992). Despite its abundance in the global carbon cycle and nearly a century of research, the majority (> 60% -80%) of DOC has remained molecularly uncharacterized (Hedges 1992) and its biogeochemical fluxes incompletely resolved. Global riverine input of ~0.2 Gt DOC per year (Meybeck 1982) is sufficient to support its radiocarbon ( 14 C) based oceanic residence time, yet composition (Meyers-Schulte and Hedges 1986; Opsahl and Benner 1997) and stable carbon ( 13 C) isotopic signature (Williams and Gordon 1970) measurements suggest the majority of marine DOC is autochthonous, ultimately originating from primary production in the euphotic zone. Furthermore, while the 4000 -6000 y 14 C ages reported for deep marine DOC suggest that a major proportion cycles on very long time scales and ages in situ during deep water transit (Bauer et al. 1992;Williams and Druffel 1987), the source of apparent chemical resilience and the removal processes necessary to produce these ages remain unknown. The published body of Δ 14 C values has been critical to understanding marine DOC biogeochemistry, yet it contains relatively few measurements because of methodological difficulties associated with low DOC concentrations, an overwhelming proportion of salts, and high blanks.The marine DOC ultraviolet (UV) extraction method (Armstrong et al. 1966;Williams 1968; Williams and Gordon 1970; Williams et al. 1969), and subsequent refinements (Bauer et al. 1998a;Druffel et al. 1989) that co-evolved with smaller sample size requirements of accelerator mass spectrometry (AMS) 14 C measurement, has been particularly wellsuited for isotopic analysis of marine DOC. It is amenable to batch reactors capable of oxidizing the large seawater aliquots required to collect enough DOC for isotopic analysis. It is a minimally invasive technique, involving only acidification and sparging of dissolved inorganic carbon (DIC) from filtered seawater prior to photochemical oxidation of DOC to CO 2 , thus minimizing the risk of contamination. Lastly, replicate analyses by Bauer et al. (1998a) demonstrated that good reproducibility (with single standard deviations of ± 1 μM and ± 3‰ to 6‰ for concentration and Δ 14 C measurements, respectively) accompanies low blanks (< 1.5 μM). Based on these techniques, a modified method for UV extraction of bulk marine nonvolatile DOC for isotopic analysis has been developed using simple, novel devices to decrease the analytical blank to ~0.2 μM, and reduce the AbstractWe report the development of a modified, low blank, ultraviolet oxidation and vacuum line system to convert marine dissolved organic carbon into carbon dioxide for concentration, Δ 14 C, and δ 13 C analyses. The system performs quantitatively and precisely with preserv...

ABSTRACT. We present an overview of accelerator mass spectrometry (AMS) radiocarbon sample preparation and measurements, describing the technical upgrades that now allow us to routinely obtain 0.2-0.3% precision for 1-mg carbon samples. A precision of -1% on samples with 100 μg of carbon can also be achieved. We have also developed graphitization techniques and AMS procedures for ultra-small samples (down to 0.002 mg of carbon). Detailed time series are presented for large and small aliquots of standards such as NIST OX-I and OX-II; FIRI-C and -D; IAEA-C6, -C7 and -C8; and ,4 C-free samples.

Radiocarbon (A14C) and stable isotope (slSo and 513C) records are presented for biannual samples from a 323-year banded coral series collected from the southern Great Barrier Reef, Australia. The high-precision A14C record contains variations on an interannual timescale, that are particularly large between A.D. 1680 and 1730. By comparison with tree ring A14C records [$tuiver and Quay, 1980; M. Stuiver, personal communication, 1992), it is clear that these shifts were not caused by changes in the A14C of atmospheric COo.. Changes in vertical mixing and large scale advective changes involving source waters to the western Coral Sea region are likely processes that could account for these large A14C variations. Most low A14C values for the period A.D. 1635-1875 coincide with E1 Nifio/Southern Oscillation (ENSO) events as reported by Quinn et al. [1987] for the eastern tropical Pacific. However, ENSO does not explain all of the variations, especially during 1875-1920 when A14 C values remained high. Cross-spectral analysis of the early half of the A14C and 5180 records (A.D. 1635-1795) reveals that the 6-year period is coherent; this coherency is not present in the latter half (A.D. 1797-1957) of the isotope records. These data support the concept of century timescale changes in the nature of ENSO, as it is manifest in the southwestern Pacific. Our coral record shows no evidence of a Suess effect, the lowering of A14C from late 1800s through 1955 due mainly to COo. input from fossil fuel burning. This is coincident with the ctmnge we observe in the nature of ENSO and is further evidence that a long-term ctmnge in mixin• of upper waters occurred in this region. spheric A1 • C records. Most hermatypic, or reef-building, corals that grow in the surface ocean accrete aragonitic skeletons at rapid rates (0.2-2 cm/yr). Many of the corm species studied to date contain annum variations in skeletM density which are discernible by X-ray of a 1-cm-thick slab [Knutson et al., 1972]. The source of carbonate to the skeleton is primarily from dissolved inorganic carbon (DIC) in the surrounding seawa-1Now at Paper number 93JC02113. 0148-022 7] 93/93J C-02113505.00 ter. Thus, within the corm skeletons lie annum records of past seawater Al•C values [e.g., Nozaki et al., 1978; Druffel and Linick, 1978; Druffel, 1987]. The corm skeleton also holds a storehouse of other past chemicM and isotopic information, including trace element content [Shen and Boyle, 1988; Linnet al., 1990], stable carbon and oxygen isotope vMues [Weber et al., 1975; Dunbar et al., 1991] and fulvic acid concentrations [Isdale, 1984] in seawater. Coral Sea, July, 1968, Tech. Memo •/69, R. Aust. Nay. Res. Lab., Melbourne, 1969c. Shen, G. T., and E. A. Boyle, Determination of lead, cadmium, and other trace metals in armually-banded corals, Chem. Geol., 67, 47-62, 1988. Stuiver, M., and H. A. Polach, Discussion reporting of •4 C data, Radiocarbon, 19(3), 355-363, 1977. Stuiver, M., and P. D. Quay, Clxanges in atmospheric carbon-14 attributed to a variable Sun, Science,...

We present Δ14C measurements of particulate organic carbon (POC) collected on four cruises at our time series site (station M) in the northeast Pacific Ocean. We observe a large gradient with depth in the suspended POC Δ14C values (124–160‰). These profiles display lower Δ14C values (by 20–30‰) in samples between 2500 m and the bottom during June 1992 and July 1993 than those during February and October 1992. Values of Δ14C in sinking POC from deep‐moored sediment trap collections suggest a semiannual trend that displays lower overall Δ14C in material collected during periods of high flux. A limited number of Δ14C measurements of small swimmers picked from the trap 650 m above bottom are similar to surface Δ14C measurements of dissolved inorganic carbon (DIC) and suspended POC, indicating a surface carbon source. Overall, we postulate that the major process causing lower Δ14C values of deep suspended and sinking POC is sorption (or biological incorporation) of “old” DOC onto particulate matter. There appears to be a higher ratio of DOC sorbed to sinking particulate matter at times of high flux (late spring and early fall) that can be thought of as a “stripping out” of DOC from the water column. The DIC Δ14C display a small seasonal variation in the surface waters and is not the sole source of the observed seasonality in the POC Δ14C signals.

Several lines of evidence indicate that microorganisms in the mesoand bathypelagic ocean are metabolically active and respiring carbon. In addition, growing evidence suggests that archaea are fixing inorganic carbon in this environment. However, direct quantification of the contribution from deep ocean carbon sources to community production in the dark ocean remains a challenge. In this study, carbon flow through the microbial community at 2 depths in the mesopelagic zone of the North Pacific Subtropical Gyre was examined by exploiting the unique radiocarbon signatures (⌬ 14 C) of the 3 major carbon sources in this environment. The radiocarbon content of nucleic acids, a biomarker for viable cells, isolated from size-fractionated particles (0.2-0.5 m and >0.5 m) showed the direct incorporation of carbon delivered by rapidly sinking particles. Most significantly, at the 2 mesopelagic depths examined (670 m and 915 m), carbon derived from in situ autotrophic fixation supported a significant fraction of the free-living microbial community (0.2-0.5 m size fraction), but the contribution of chemoautotrophy varied markedly between the 2 depths. Results further showed that utilization of the ocean's largest reduced carbon reservoir, 14 C-depleted, dissolved organic carbon, was negligible in this environment. This isotopic portrait of carbon assimilation by the in situ, free-living microbial community, integrated over >50,000 L of seawater, implies that recent, photosynthetic carbon is not always the major carbon source supporting microbial community production in the mesopelagic realm.microbial metabolism ͉ particle flux ͉ particulate organic carbon (POC) ͉ chemoautotrophy

No abstract