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.
Radiocarbon measurements on a 175‐year (A.D. 1800 to 1974) growth of the coral Montastrea annularis from The Rocks reef off the Florida Keys reveal the rate of local uptake of fossil fuel CO2 and bomb 14C by surface ocean waters of the Gulf Stream. In the nineteenth century, the pre‐bomb, pre‐industrial Δ14C value of surface ocean waters as seen in these corals of the Gulf Stream in the Florida Straits was −51 ± 2‰. By 1955, uptake of industrial CO2 by these waters had lowered the Δ14C values to about −61‰. The results can be used to make predictions regarding anthropogenic CO2 that can be expected to enter the oceans in future decades. Bomb‐produced 14C is found to be present in the corals in comparable concentrations to that found in the dissolved inorganic carbon (DIOC) of the North Pacific and North Atlantic Oceans.
Because of increased interest in the marine and atmospheric sciences in elemental carbon (EC), or black carbon (BC) or soot carbon (SC), and because of the difficulties in analyzing or even defining this pervasive component of particulate carbon, it has become quite important to have appropriate reference materials for intercomparison and quality control. The NIST “urban dust” Standard Reference Material® SRM 1649a is useful in this respect, in part because it comprises a considerable array of inorganic and organic species, and because it exhibits a large degree of (14C) isotopic heterogeneity, with biomass carbon source contributions ranging from about 2 % (essentially fossil aliphatic fraction) to about 32 % (polar fraction).A primary purpose of this report is to provide documentation for the new isotopic and chemical particulate carbon data for the most recent (31 Jan. 2001) SRM 1649a Certificate of Analysis. Supporting this is a critical review of underlying international intercomparison data and methodologies, provided by 18 teams of analytical experts from 11 institutions. Key results of the intercomparison are: (1) a new, Certified Value for total carbon (TC) in SRM 1649a; (2) 14C Reference Values for total carbon and a number of organic species, including for the first time 8 individual PAHs; and (3) elemental carbon (EC) Information Values derived from 13 analytical methods applied to this component. Results for elemental carbon, which comprised a special focus of the intercomparison, were quite diverse, reflecting the confounding of methodological-matrix artifacts, and methods that tended to probe more or less refractory regions of this universal, but ill-defined product of incomplete combustion. Availability of both chemical and 14C speciation data for SRM 1649a holds great promise for improved analytical insight through comparative analysis (e.g., fossil/biomass partition in EC compared to PAH), and through application of the principle of isotopic mass balance.
We have made radiocarbon measurements of banded hermatypic corals from Florida, Belize, and the Galapagos Islands. Interpretation is presented here of these previously reported results. These measurements represent the 14C/12C ratios in dissolved inorganic carbon (DIOC) in the surface ocean waters of the Gulf Stream and the Peru Current at the time of coral ring formation. A depletion in radiocarbon concentration was observed in coral rings that grew from A.D. 1900–1952. It was caused by dilution of existing 14C levels with dead CO2 from fossil fuel burning (the Suess effect, or Se). The observed depletion of radiocarbon was greater in corals from the Gulf Stream (−11‰) than in corals from the Peru Current (−6‰). A similar trend was observed in the distribution of bomb‐produced 14C in corals that had grown during the years following A.D. 1952. The concentration of bomb‐produced radiocarbon was much higher in corals from temperate regions (Florida, Belize, Hawaiian Islands) than in corals from tropical regions (Galapagos Islands and Canton Island). A linear relationship appears to exist between the preanthropogenic Δ14C values and the Se values measured in the individual corals, because the 14C in corals is derived from two different carbon reservoirs: (1) the atmosphere and (2) the subsurface ocean. A linear relationship is also observed between the preanthropogenic Δ14C values and the concentration of bomb‐produced 14C in the individual corals during A.D. 1973. The apparent radiocarbon ages of the surface waters in temperate and tropical oceans during the preanthropogenic period range from about 280 to 520 years B.P. (−40 to −69‰). At all investigated locations, it is likely that waters at subsurface depths have the same apparent radiocarbon age of about 670 years B.P. From the change of oceanic Δ14C in the surface ocean during post‐bomb times, the approximate annual rate of net input of 14CO2 to the ocean waters is calculated to be about 8% of the prevailing 14C difference between atmosphere and ocean. From this input and from preanthropogenic Δ14C values found at each location, it can be seen that vertical mixing of water in the Peru Current is about 3 times greater than that in the Gulf Stream.
Sixty radiocarbon measurements were performed on aragonite from annually banded corals collected from three sites in the Galápagos Islands. Preanthropogenic Δ14C values of coral that grew around A.D. 1930 averaged −70‰. This is substantially lower than average values previously reported (−51‰) for corals from Florida and Belize in the western North Atlantic Ocean. A decrease of 6‰ was noticed in coral that grew from 1930 to 1954. This decrease could be interpreted as a Suess effect in surface ocean water. The 100‰ increase in Δ14C for coral that grew from 1954 to 1973 is the result of bomb‐produced 14C that was introduced to the surface ocean waters. The 14C levels in corals that grew during El Niño years were considerably higher than those for normal years. These higher values are attributed to the absence of up welling at the equator during El Niño events.
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