[1] Measurements of the d 13 C of dissolved inorganic carbon primarily during World Ocean Circulation Experiment and the Ocean Atmosphere Carbon Exchange Study cruises in the 1990s are used to determine ocean-wide changes in the d 13 C that have occurred due to uptake of anthropogenic CO 2 . This new ocean-wide d 13 C data set ($25,000 measurements) substantially improves the usefulness of d 13 C as a tracer of the anthropogenic CO 2 perturbation. The global mean d 13 C change in the surface ocean is estimated at À0.16 ± 0.02% per decade between the 1970s and 1990s with the greatest changes observed in the subtropics and the smallest changes in the polar and southern oceans. The global mean air-sea d 13 C disequilibrium in 1995 is estimated at 0.60 ± 0.10% with basin-wide disequilibrium values of 0.73, 0.63, and 0.23% for the Pacific, Atlantic, and Indian oceans, respectively. The global mean depth-integrated anthropogenic change in d 13 C between the 1970s and 1990s was estimated at À65 ± 33% m per decade. These new estimates of air-sea d 13 C disequilibrium and depth-integrated d 13 C changes yield an oceanic CO 2 uptake rate of 1.5 ± 0.6 Gt C yr À1 between 1970 and 1990 based on the atmospheric CO 2 and 13 CO 2 budget approaches of Quay et al. [1992] and Tans et al.[1993] and the dynamic method of Heimann and Maier-Reimer [1996]. Box-diffusion model simulations of the oceanic uptake of anthropogenic CO 2 and its d 13 C perturbation indicate that a CO 2 uptake rate of 1.9 ± 0.4 Gt C yr À1 (1970-1990) explains both the observed surface ocean and depth-integrated d 13 C changes. Constraining a box diffusion ocean model to match both the observed d 13 C and bomb 14 C changes yields an oceanic CO 2 uptake rate of 1.7 ± 0.2 Gt C yr À1 (1970-1990

This paper describes the application of a novel, practical approach for isolation of individual compounds from complex organic matrices for natural abundance radiocarbon measurement. This is achieved through the use of automated preparative capillary gas chromatography (PCGC) to separate and recover sufficient quantities of individual target compounds for 14 C analysis by accelerator mass spectrometry (AMS). We developed and tested this approach using a suite of samples (plant lipids, petroleums) whose ages spanned the 14 C time scale and which contained a variety of compound types (fatty acids, sterols, hydrocarbons). Comparison of individual compound and bulk radiocarbon signatures for the isotopically homogeneous samples studied revealed that ∆ 14 C values generally agreed well ((10%). Background contamination was assessed at each stage of the isolation procedure, and incomplete solvent removal prior to combustion was the only significant source of additional carbon. Isotope fractionation was addressed through compound-specific stable carbon isotopic analyses. Fractionation of isotopes during isolation of individual compounds was minimal (<5‰ for δ 13 C), provided the entire peak was collected during PCGC. Trapping of partially coeluting peaks did cause errors, and these results highlight the importance of conducting stable carbon isotopic measurements of each trapped compound in concert with AMS for reliable radiocarbon measurements. The addition of carbon accompanying derivatization of functionalized compounds (e.g., fatty acids and sterols) prior to chromatographic separation represents a further source of potential error. This contribution can be removed using a simple isotopic mass balance approach. Based on these preliminary results, the PCGC-based approach holds promise for accurately determining 14 C ages on compounds specific to a given source within complex, heterogeneous samples.

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Techniques for making precise and accurate radiocarbon accelerator mass spectrometry (AMS) measurements on samples containing less than a few hundred micrograms of carbon are being developed at the NOSAMS facility. A detailed examination of all aspects of the sample preparation and data analysis process shows encouraging results. Small quantities of CO2 are reduced to graphite over cobalt catalyst at an optimal temperature of 605°C. Measured 14C/12C ratios of the resulting targets are affected by machine-induced isotopic fractionation, which appears directly related to the decrease in ion current generated by the smaller sample sizes. It is possible to compensate effectively for this fractionation by measuring samples relative to small standards of identical size. Examination of the various potential sources of background 14C contamination indicates that the sample combustion process is the largest contributor, adding ca. 1 µg of carbon with a less-than-modern 14C concentration. c

To determine the relative inputs of polycyclic aromatic hydrocarbons (PAHs) and black carbon (BC) in environmental samples from the combustion of fossil fuels and biomass, we have developed two independent analytical methods for determining the 14C abundance of PAHs and BC. The 5730 yr half-life of 14C makes it an ideal tracer for identifying combustion products derived from fossil fuels (14C-free) versus those stemming from modern biomass (contemporary 14C). The 14C abundance of PAHs in several environmental Standard Reference Materials was measured by accelerator mass spectrometry after extraction and then purification by high-performance liquid chromatography and preparative capillary gas chromatography. This method yields pure compounds that allow for a high degree of confidence in the 14C results. The PAHs data were then used to compare and evaluate results from an operationally defined thermal oxidation method used to isolate a BC fraction. The 14C compositions of PAHs and BC were very similar and suggest that the thermal oxidation method employed for isolating BC is robust and free from interferences by non-BC components. In addition, these data indicate that both the PAHs and the BC species derive mostly from fossil fuels and/or their combustion products.

Organic carbon (OC) from multiple sources can be delivered contemporaneously to aquatic sediments. The influence of different OC inputs on carbon-14–based sediment chronologies is illustrated in the carbon-14 ages of purified, source-specific (biomarker) organic compounds from near-surface sediments underlying two contrasting marine systems, the Black Sea and the Arabian Sea. In the Black Sea, isotopic heterogeneity of n -alkanes indicated that OC was contributed from both fossil and contemporary sources. Compounds reflecting different source inputs to the Arabian Sea exhibit a 10,000-year range in conventional carbon-14 ages. Radiocarbon measurements of biomarkers of marine photoautotrophy enable sediment chronologies to be constructed independent of detrital OC influences.

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ABSTRACT. We have established a laboratory for extracting ECO2 from seawater samples for AMS analysis of the radiocarbon content. The seawater samples are collected at sea, poisoned and stored until analysis in the laboratory. Each sample is acidified; the inorganic carbon is stripped out as CO2 with an inert carrier gas and then converted to graphite. We present results for Buzzards Bay surface H2O and Na2CO3 standards that demonstrate we strip >98% of inorganic carbon from seawater. Stable isotope analyses are performed to better than 0.2%o, and the reproducibility of 14C measurements on Buzzards Bay seawater is better than 13%o. Finally, we compare data from samples collected in 1991 to those collected in the 1970s and to large volume samples.