Carbon-14 in methane sources and in atmospheric methane: the contribution from fossil carbon

Measurements of carbon-14 in small samples of methane from major biogenic sources, from biomass burning, and in "clean air" samples from both the Northern and Southern hemispheres reveal that methane from ruminants contains contemporary carbon, whereas that from wetlands, pat bogs, rice fields, and tundra is somewhat, depleted in carbon-14. Atmospheric (14)GH(4) seems to have increased from 1986 to 1987, and levels at the end of 1987 were 123.3 +/- 0.8 percent modern carbon (pMC) in the Northern Hemisphere and 120.0 +/- 0.7 pMC in the Southern Hemisphere. Model calculations of source partitioning based on the carbon-14 data, CH(4) concentrations, and delta(13)C in CH(4) indicate that 21 +/- 3% of atmospheric CH(4) was derived from fossil carbon at the end of 1987. The data also indicate that pressurized water reactors are an increasingly important source of (14)CH(4).     

Application of carbon isotope stratigraphy to late miocene shallow marine sediments, new zealand

A distinct (0.5 per mil) carbon-13/carbon-12 isotopic shift in the light direction has been identified in a shallow marine sedimentary sequence of Late Miocene age at Blind River, New Zealand, and correlated with a similar shift in Late Miocene Deep Sea Drilling Project sequences throughout the Indo-Pacific. A dated piston core provides an age for the shift of 6.2 +/- 0.1 million years. Correlations based on the carbon isotopic change require a revision of the previously established magnetostratigraphy at Blind River. The carbon shift at Blind River occurs between 6.2 and 6.3 +/- 0.1 million years before present. A new chronology provides an age for the evolutionary first appearance datum of Globorotalia conomiozea at 6.1 +/- 0.1 million years, the beginning of a distinct latest Miocene cooling event associated with the Kapitean stage at 6.2 +/- 0.1 million years, and the beginning of a distinct shallowing of water depths at 6.1 +/- 0.1 million years. The Miocene-Pliocene boundary as recognized in New Zealand is now dated at 5.3 +/- 0.1 million years. Extension of carbon isotope stratigraphy to other shallow Late Miocene sequences should provide an important datum for international correlation of Late Miocene shallow and deep marine sequences.     

Middle holocene age of the sunnyvale human skeleton

A morphologically modern human skeleton from Sunnyvale, California, previously dated by aspartic acid racemization to be approximately 70,000 years old and by uranium series isotopic ratios to be 8300 and 9000 years old, appears to be younger when dated by the carbon-14 method. Four carbon-14 determinations made by both decay and direct counting on three organic fractions of postcranial bone support a middle Holocene age assignment for the skeleton, probably in the range of 3500 to 5000 carbon-14 years before the present. This dating evidence is consistent with the geologic, archeological, and anthropometric relationships of the burial as well as previously determined carbon-14 determinations on associated materials.     

Earthquake dating: an application of carbon-14 atom counting

Milligram-sized specimens of detrital charcoal from soil layers associated with prehistoric earthquakes on the Wasatch fault in Utah have been dated by direct atom counting of carbon-14 with a tandem Van de Graaff accelerator. The measured ratios of carbon-14 to carbon-12 correspond to ages of 7800, 8800, and 9000 years with uncertainties of +/- 600 years.     

Bacterioplankton: a sink for carbon in a coastal marine plankton community

Recent determinations of high production rates (up to 30 percent of primary production in surface waters) implicate free-living marine bacterioplankton as a link in a "microbial loop" that supplements phytoplankton as food for herbivores. An enclosed water column of 300 cubic meters was used to test the microbial loop hypothesis by following the fate of carbon-14-labeled bacterioplankton for over 50 days. Only 2 percent of the label initially fixed from carbon-14-labeled glucose by bacteria was present in larger organisms after 13 days, at which time about 20 percent of the total label added remained in the particulate fraction. Most of the label appeared to pass directly from particles smaller than 1 micrometer (heterotrophic bacterioplankton and some bacteriovores) to respired labeled carbon dioxide or to regenerated dissolved organic carbon-14. Secondary (and, by implication, primary) production by organisms smaller than 1 micrometer may not be an important food source in marine food chains. Bacterioplankton can be a sink for carbon in planktonic food webs and may serve principally as agents of nutrient regeneration rather than as food.     

Carbon-13/carbon-12 ratio is relatively uniform among honeys

The variability of the carbon-13/carbon-12 ratio in honeys was evaluated preliminary to use of the ratio to detect the addition of high-fructose corn syrup to honey. Eighty-four honey samples representing 34 states and including 37 floral types from 17 plant families were analyzed. The mean value of the per mil increment in carbon-13 (delta13C) for all samples is -25.2 per mil, and the coefficient of variation is 3.7 percent. This is the smallest variation yet encountered for a honey constituent or physical property. The range and magnitude of the values suggest that the floral sources are C(3) plants.     

A molecular organic carbon isotope record of miocene climate changes

The difference in carbon-13 ((13)C) contents of hopane and sterane biomarkers in the Monterey formation (Naples Beach, California) parallels the Miocene inorganic record of the change in (18)O (delta(18)O), reflecting the Miocene evolution from a well-mixed to a highly stratified photic zone (upper 100 meters) in the Pacific. Steranes (delta(13)C = 25.4 +/- 0.7 per mil versus the Pee Dee belemnite standard) from shallow photic-zone organisms do not change isotopically throughout the Miocene. In contrast, sulfur-bound C(35) hopanes (likely derived from bacterial plankton living at the base of the photic zone) have systematically decreasing (13)C concentrations in Middle and Late Miocene samples (delta(13)C = -29.5 to -31.5 per mil), consistent with the Middle Miocene formation of a carbon dioxide-rich cold water mass at the base of the photic zone.     

Glucose naturally labeled with carbon-13: use for metabolic studies in man

The ratio of carbon-13 to carbon-12 is much higher in most commerical preparations of glucose used for oral glucose tolerance tests than it is in carbon dioxide in expired air. This recent discovery provided a novel and potentially significant means of studying glucose metabolism. The changes in the ratio of carbon-13 to carbon-12 in carbon dioxide expired after oral glucose administration were determined by mass spectrometry. In six healthy male volunteers, the administration of glucose resulted in a marked, reproducible rise in the isotopic ratio in expired carbon dioxide; the ratio reached its maximum at 4 hours and then declined progressively.     

Carbon isotopic evidence for different feeding patterns in two hyrax species occupying the same habitat

The carbon-13/carbon-12 ratios of the carbonate and collagen fractions of bone of the sympatric hyrax species Procavia johnstoni and Heterohyrax brucei indicate that the former obtains most of its diet by grazing while the latter is primarily a browser. The carbon-13/carbon-12 ratios of these fractions in fossil bone will record information about diet if they have not been altered during diagenesis.     

Interstellar carbon in meteorites

The Murchison and Allende chondrites contain up to 5 parts per million carbon that is enriched in carbon-13 by up to + 1100 per mil (the ratio of carbon-12 to carbon-13 is approximately 42, compared to 88 to 93 for terrestrial carbon). This "heavy" carbon is associated with neon-22 and with anomalous krypton and xenon showing the signature of the s-process (neutron capture on a slow time scale). It apparently represents interstellar grains ejected from late-type stars. A second anomalous xenon component ("CCFXe") is associated with a distinctive, light carbon (depleted in carbon-13 by 38 per mil), which, however, falls within the terrestrial range and hence may be of either local or exotic origin.     

Carbon-14 dating: a comparison of Beta and ion counting

Accelerator ion counting compares favorably with conventional beta counting. The major advantage of ion counting is that milligram samples can be analyzed. When sample size is not limiting, ion counting complements beta counting for the more routine carbon-14 determinations. Further development is needed before ion counting can achieve the same high precision as beta counting for large samples (+/- 2 per mil). A solution to the background variability has to be found before ion counting can be used to date samples back to the 75,000-year limit of beta counting.     

Carbonate sediments: oriented lithified samples from the north atlantic

Indurated carbonate samples, obtained from the North Atlantic sea floor with a deep-sea drill coring apparatus, suggest that the phenomenon of deep-sea carbonate lithification is more complex than had been thought previously. Lithified-nonlithified couplets can now be related in age and orientation. Age determinations based on the method of carbon-14 dating show that adjacent nonlithified-lithified layers may differ in age by more than 30,000 years.     

Endogenous carbon in carbonaceous meteorites

Seven carbonaceous chondrites have been analyzed for soluble organic compounds, carbonate, and residual carbon. Carbon-13/carbon-12 isotopic mneasurements on these fractions gave the following values relative to a marine carbonate standard: carbonate, +40 to +70 per mil; residual carbon, -15 to -17 per mil; soluble organic material, -17 to -27 per mil, with one value of -5.5 per mil. These values are interpreted to indicate that carbonate, residual carbon, and part of the extractable organic material are endogenous to these meteorites.     

Rates of Submergence of Coastal New England and Acadia

Altitudinal and carbon-14 age determinations of in-place Pinus strobus stumps of drowned forests at Odiorne Point, N.H., and Grand Pré and Fort Lawrence, Nova Scotia, yield apparent average rates of submergence of 3.1, 14.5, and 20.3 feet per 1000 C(14) years, respectively. Rate differences are assessed in terms of eustatic rise of sea level, crustal movements, and tidal effects.     

Natural abundance carbon-13 nuclear magnetic resonance spectra of human serum lipoproteins

Human serum lipoproteins have been studied by Fourier transform nuclear magnetic resonance of carbon-13 in natural abundance. Spectra of highdensity, low-density, and very-low-density lipoproteins were recorded and partly assigned. The prominent features of these spectra reflect the qualitative and quantitative composition of the lipid moiety of these complexes. The results suggest that carbon-13 nuclear magnetic resonance will be a useful technique for studies of the structural and dynamic parameters of lipoproteins.     

In vivo determination of the pyridine nucleotide reduction charge by carbon-13 nuclear magnetic resonance spectroscopy

An intracellular coenzyme has been observed by carbon-13 nuclear magnetic resonance spectroscopy. The pyridine nucleotides in Escherichia coli were specifically labeled with carbon-13 from the biosynthetic precursor, nicotinic acid. The intracellular redox status and metabolic transformations of the pyridine nucleotides were examined under a variety of conditions. A highly reduced nicotinamide adenine dinucleotide pool was observed under anaerobic conditions only in cells that were cultured aerobically on glycerol.     

Indirect observation by 13C NMR spectroscopy of a novel CO2 fixation pathway in methanogens

High-field carbon-13 nuclear magnetic resonance (NMR) spectroscopy has been used to monitor the isotopic dilution of doubly carbon-13-labeled precursors for 2,3-cyclopyrophosphoglycerate, a novel primary metabolite that occurs in certain methanogens. A unique carbon dioxide fixation pathway that gives rise to asymmetric labeling of acetyl coenzyme A has been demonstrated in Methanobacterium thermoautotrophicum. The effect of selected metabolic inhibitors on the labeled species in the pathway has been examined by NMR. These techniques establish a general, sensitive method for the delineation of convergent biosynthetic pathways.     

Elucidation of the chain conformation in a glassy polyester, PET, by two-dimensional NMR

The chain conformation of glassy poly(ethylene terephthalate) (PET) was characterized by two-dimensional double-quantum nuclear magnetic resonance (NMR). In amorphous carbon-13-labeled PET, the statistics of the O-13CH2-13CH2-O torsion angle were determined, on the basis of the distinct shapes of the two-dimensional NMR patterns of trans and gauche conformations. In crystalline PET, the trans content is 100 percent, but in the amorphous PET it is only 14 percent (+/-5 percent). An average gauche torsion angle of 70 degrees (+/-9 degrees) was obtained. Implications for materials properties of polyesters are discussed.     

Single molecule electron paramagnetic resonance spectroscopy: hyperfine splitting owing to a single nucleus

Individual pentacene-d(14) molecules doped into a p-terphenyl-d(14) host crystal have been studied by optically detected electron paramagnetic resonance spectroscopy. The magnetic resonance transitions between the triplet sublevels of the pentacene molecule and the splitting of the resonance lines for a molecule that contains a carbon-13 nucleus have been observed in an external magnetic field. This splitting is caused by the hyperfine interaction of the triplet electron spin with the single carbon-13 nuclear spin.