Radiocarbon carbon sediments, sources

All of the above particulate investigations were based on mini-radiocarbon measurement techniques, with sample masses typically in the range of 5-10 mg-carbon. This constituted a major advantage, because it was practicable to select special samples (given region, source impact, sediment depth) and to further subject such samples to physical (size) or chemical separation before 14C measurement. This type of "serial selectivity" provides maximum information content about the samples and in fact it is essential when information is sought for the sources or atmospheric distributions of pure chemical species, such as methane or elemental carbon. 

Figure 16 Bulk and molecular-level radiocarbon variations (expressed as %o, and age, yr BP) in surface (0-3 cm) sediment from the northeastern Bermuda Rise in the subtropical North Atlantic (33° 4TN, 57° 36 W, —4,500 m) planktonic foraminiferal calcite (diamonds), total OC (circles), C37-C39 alkenones (squares), fatty acids (down triangles), and hydrocarbons (up triangles). Carbon numbers of fatty acids and hydrocarbons are adjacent to the symbols (UCM = unresolved complex mixture of hydrocarbons). The sedimentation rate at this site during the Holocene averages 10-20 cmkyr (source Ohkouchi and Eglinton, unpublished).

Most of the allochthonous, or foreign, sources represent carbon with lower concentrations ( older radiocarbon ages) than the fraction of TOC originating from phytoplanktonic production. The only exception is the rapid transport and sedimentation of recently synthesized terrestrial plant material, which is in equilibrium with the " C concentration of atmospheric CO2. Other sources of nonmarine carbon typically are of intermediate (lO lO" years) or infinite A C (beyond the detection limit of 50-60 000 years) radiocarbon age, depending on the amount of time spent in other reservoirs such as soils, fluvial deposits, or carbon-rich rocks. 

Stable carbon isotopes data—The data from stable isotope and radiocarbon measurements are summarized in Table 1. Analytical precision is 0.l%o for C. Stable carbon isotope ratios of the different fractions span from — 20.14 to — 27.82%o with the unextracted and extracted sediments exhibiting values, as expected, in the midrange around — 22%o (Fig. 1). The only exception is the fatty acid fraction which is anomalously depleted in Stable carbon isotope numbers of the different compound classes from the SMB samples, in general, suggest a mixing of terrestrial and marine components with humic and fidvic acids implying major contribution from marine sources (Sweeney and Kaplan, 1980 Stuermer et al., 1978). 

In radiocarbon dating, a reasonable assumption is that the ratio of carbon-14 to carbon-12 in the atmosphere has been relatively constant for the past 50,000 yr. However, because variations in solar activity control the amount of carbon-14 produced in the atmosphere, that ratio can fluctuate. We can correct for this effect by using other kinds of data. Recently scientists have compared carbon-14 data with data from tree rings, corals, lake sediments, ice cores, and other natural sources to correct variations in the carbon-14 clock back to 26,000 yr. 

---