Geochemistry of Radiocarbon in Organic Materials

Transcription

1 Geochemistry of Radiocarbon in Organic Materials Suggested Reading: Eglinton T.I., Benitez-Nelson B.C., Pearson A., McNichol A.P., Bauer J.E. and Druffel E.R.M. (1997) Variability in radiocarbon ages of individual organic compounds from marine sediments. Science 277, Pearson A., Eglinton T.I. and McNichol A.P. (2000) An organic tracer for surface ocean radiocarbon. Paleoceanog. 15, Wang X.-C., Druffel E.R.M., Griffin S., Lee C. and Kashgarian M. (1998) Radiocarbon studies of organic compound classes in plankton and sediment of the northeastern Pacific Ocean. Geochim. Cosmochim. Acta 62, Raymond P.A. and Bauer J.E. (2001) Riverine export of aged terrestrial organic matter to the North Atlantic Ocean. Nature 409, Blair N.E. et al. (2003) The persistence of memory: The fate of ancient sedimentary organic carbon in a modern sedimentary system. Geochim. Cosmochim. Acta 67, MEASURING THE AGE OF REALLY OLD STUFF 1. The Sun sends blasts of 2. This Produces radioactive Carbon-14 Radiation toward Earth in the Upper atmosphere. (It's called carbon-14 because there are 6 Protons and 8 Neutrons in the nucleus) 3. The cells in all living plants and animals are continually being replaced. In that process, old carbon is replaced by new. Most of the new stuff is non-radioactive carbon-12, but some of it is the radioactive CARBON Plant fix carbon through 5. Animal absorb carbon by eating 6. photosynthesis the plants (or by eating other animals that have done so.) Carbon 14 Carbon 12 When the animals and plants die, the carbon-14 in them \ starts to decay, But the Carbon-12 does not. 7. The Rate of decay (called a half-life") After years Half of the Carbon-14 is left. (After years a quarter of it is left, and so on) 8. Since carbon-14 starts to decay as soon as something dies, and it decays at a steadily decreasing rate, scientists can estimate when death occurred by measuring the speed of decay. Same, give or take 100 years I'd say 10,000 years ago, what about you?

2 Geochemistry of Radiocarbon in Organic Materials Natural abundance of stable and radio isotopes of carbon 12 C - 99% 13 C - 1% 14 C - 1 part per trillion (10-12 ) in modern carbon 14 C is a cosmogenic nuclide - continually formed in the upper atmosphere (lower stratosphere/upper troposphere) by interaction of neutrons (produced by cosmic rays) and nitrogen atoms. 14 N + neutron 14 C + proton After formation, 14 C atoms rapidly combine with oxygen to form CO 2 which mixes throughout atmosphere, dissolves in the oceans, and enters the biosphere via photosynthetic carbon fixation. Geochemistry of Radiocarbon in Organic Materials There is a dynamic equilibrium between 14 C formation and decay leading to an approximately constant level in the atmosphere. Current best estimate for half-life, T½, of 14 C = 5730 yr Conventional (Libby) half-life adopted for reporting 14 C ages = 5568 yr (3% smaller than true half-life). The half-life is related to the meanlife,τ, by: T½ = (ln2)τ (or T½ = 0.693τ) Corresponding meanlife, τ, for Libby half-life is 8033yr. The radiocarbon age, t, can be determined from: t = -τ ln(a/a 0 ) (or t = ln(a/a 0 )) where A is the number of atoms left after time t and A 0 is the initial number of atoms. Image removed due to copyright restrictions. Key attribute: 5730 yr half life of 14 C is suitable for studying processes and dating carbonaceous materials over yr time-scales.

3 Methods of 14 C measurement 1. Conventional method - Determination of 14 C activity of a weighed sample by counting the number of electrons (beta particles) emitted from nucleus per unit time by the decay of 14 C. Beta-counting can be performed by samples combusted to CO 2 (gas proportional beta counting) or on samples converted to benzene and measured photometrically after addition of a scintillator (liquid scintillation counting). Sample size requirements: > 1 g C and long counting times (days). 2. Direct measurement of the proportion of 14 C atoms (relative to 13 C or 12 C) by accelerator mass spectrometry (AMS). Measurements are typically made on graphite (CO 2 also possible). Graphite is formed by combustion of sample to CO 2 and then reduction of CO 2 to graphite. Measurement times as short as 20 min. Key attribute of AMS - Isobar rejection: Negative ions (Cs sputter source) remove ( 14 N + ) Electron stripping (accelerator) to remove hydrides ( 13 CH - ) Sensitivity of AMS = ( 60,000 yr; 10 half-lives). Sample size requirements: Standard targets < 1 milligram C* (as little as 300 µg C) for full precision (+/- 4 ) As low as 25 µg C possible at reduced precision (+/ ). Standards: Oxalic acid (HOxI, HoxII) *Small sample size has opened up many new applications for 14 C. Accelerator Mass Spectrometer Courtesy of Lawrence Livermore National Laboritories. Used with permission.

4 14 C systematics The absolute international standard of 14 C activity (A abs ) is defined as 95% of the 14 C activity of the original oxalic acid standard (HOxI), in the year This is equivalent to the activity of 19th century wood, and represents the 14 C concentration of the atmosphere prior to anthropogenic influence (fossil fuel combustion, atomic weapons testing). The measured activity of HOxI (A ox ) is corrected from fractionation effects using a defined δ 13 C 13 ox 2(19 +δ C) A value of 19 to yield the fractionation- ON = 0.95A ox normalized activity (A ON ): This is corrected to account for radioactive decay between 1950 and the year of measurement: λ( y 1950) A abs = A ON e 14 C systematics The measured 14 C activity of a sample (A s ) is normalized (A sn ) to a constant δ 13 C value of 25 2(25 +δ 13 C sample ) to remove the influence of isotopic A sn = A s 1 fractionation on the reported concentration: 1000 To a first approximation, the above equation treats the 14 C fractionation as twice the 13 C fractionation (to account for the greater mass difference). This is based on physical-chemical 2 derivations that suggest the 14 C fractionation is approximately equal to the square of the 13 C 1000 fractionation. The mean age correction is about A sn = A s 2 16 years for every 1 difference from -25. δ 13 C sample This may be simplified to:

5 14 C systematics The above equations were first developed for 14 C measurements from decay counting techniques. AMS yields absolute ratios of 14 C/ 12 C in a sample, rather than the rate of decay. The above equations are still applicable, as activity and R 14/12 are proportional via the decay constant, λ. AMS data are reported as fraction modern (f m ) values, rather than activities: A sn R sn f m = = 14 A ON R 12 ON When a radiocarbon age (year date) is not desired, data are reported as 14 C values in one of two forms. For samples with no age correction, where y is the year of measurement: C = A 1 λ ( y 1950) *1000 = ( f m e A ONe 14 sn λ ( y 1950) 1)*1000 For samples of known geochronological age, where y is the year of measurement, and x is the year of sample formation: A e λ ( y x) 14 sn λ (1950 x) C = 1 λ ( y 1950) *1000 = ( f me 1)*1000 A ONe The radiocarbon age of a sample is strictly defined as the age calculated using the Libby half-life (5568 y) for radiocarbon. - In classical radiocarbon dating applications, the calculated radiocarbon ages are converted to calendar ages using calibration curves Reporting of Radiocarbon Data Percent Modern Carbon (pmc) Fraction Modern (F m ) C (permil) 14C age (yr BP)

6 14 C is useful for isotopic mass balance calculations Radiocarbon age versus 14 C Image removed due to copyright restrictions. 14 C originally defined by Broecker and Olson (1959). Am. J. Sci. Radiocarbon Suppl. 1, Factors influencing radiocarbon abundances: 1. Atmospheric 14 C variations Variations in solar (cosmic ray flux) activity (long-term, ~ 10 3 yr variations in production rate). Variations in Earth s geomagnetic field strength (short-term, < 10 2 yr variation in production rate) Climate induced variations - solubility of CO 2 in water a function of temperature. Volcanic activity Anthropogenic activity. - Fossil fuel burning ( Suess effect ). - Nuclear weapons testing ( Bomb spike ). 2. Fractionation effects The fractionation effect for 14 C is assumed to be double that for 13 C (reflecting mass difference relative to 12 C). Conventional radiocarbon ages are corrected to a single δ 13 C value (-25 = approximate value for wood). 3. Source or reservoir effects. There is rapid global mixing between the atmosphere and the terrestrial biosphere. However, mixing rates in deep ocean are slow. Mixing between surface mixed layer (high 14 C) and deeper layers (lower 14 C) gives rise to an offset between mixed layer and atmosphere. This offset ( reservoir effect ) for the pre-bomb era is on average ca. 400 yr, but varies spatially and temporally. Thus organic matter synthesized in the oceans will have an apparent age which is 400 yr older than terrestrial biomass synthesized at the same time.

7 14 C variations and radiocarbon calibration Image removed due to copyright restrictions. High precision 14C calibration cure for the past 7000 yr (from Irish Oak). Straight line is the 1:1 correspondence between 14C age and dendrochronological age. Short-term (10 2 yr) variations are due to geomagnetic field variations. Long-term (10 3 yr) variations are due to variations in cosmic ray flux. Recent variations of 14 C in atmospheric CO Fraction modern (f M ) Maunder Minimum The Bomb Spike C ( ) 1 The Suess Effect Calender Year 2000

8 Wiggle-matching, dendrochronologies and varve chronologies 14 C variations vs varve age for Cariaco basin sediments, compared to those from tree rings Images removed due to copyright restrictions. Hughen et al., 1998 Nature, v391 Potential limitations in assigning Calendar ages from 14 C data Radio-carbon years BP t+σ t-σ t+σ t-σ t+σ t-σ cal BC cal BC cal BC

9 Geochemical Applications of Radiocarbon Development of sediment chronologies Tracer studies (e.g., bomb-spike) Isotopic mass balance The Global Organic Carbon Cycle (ca. 1950) fossil fuel utilization SEDIMENTARY ROCKS (KEROGEN) (15,000,000) -20 to -35 t = infinite uplift physi cal wea theri ng (0.2) ATMOSPHERIC CO 2 (750) LAND PLANTS (570) -25 to -30 (C3) -14 to -20 (C4) t = 0 yr humification (50) SOIL HUMUS (1,600) -25 to -30 (C3) -14 to -20 (C4) t = 10-10,000 yr net terrestrial primary production (50) river or eolian transport (POC=0.15) net marine primary production (50) OCEAN (BIOTA) (3) -20 to -30 (C3) t = 400 yr OC preservation sediment (0.15) redistribution (units=10 15 gc) Modified after Hedges (1992) burial RECENT SEDIMENTS (100) -20 to -25 t =?? yr

10 Radiocarbon age of bulk OC in Long Island Sound sediments 14 C age (yr BP) Depth (cm) (Benoit et al., 1979) Variations in stable carbon isotopic and radiocarbon composition of organic matter as a function of particle size 20µm sieve Fraction 1 "total sample <2000µm 20µm sieve 0-20 µm µm 0-63 µm µm Fraction Fraction 2 3 Fraction Fraction 4 5 Schematic representation of the sieving and settling procedure. 1. Total 70.9% Percent modern carbon % δ 13 C µm µm 80.1 % 51.7% % -23.1% µm µm 74.2% 50.1% % % Particle size (µm) Megens et al. (2002) Org. Geochem

11 Variations in stable carbon isotopic and radiocarbon composition of organic matter as a function of chemical compound class R δ 13 C 12 δ 14 a % H2O, 100 C 1 M H2SO4, 100 C Percent Modem Carbon 80 p MC CHCL3:MeOH E Saponification R E M HCL, 100 C R E CHCL 3 extraction R E Extract Megens et al. R M KOH 100 C 1h Residue E Scheme of the extraction procedure. R = residue; E = extract. See text for chemical identification Variations in stable carbon isotopic and radiocarbon composition of organic matter as a function of chemical compound class Height above bottom (m) TOC THAA TCHO Lipid Acid-insol BC alkenone aggregates flocc plankton zooplankton sinking POM Depth in sediment (cm) Station M Wang, Druffel et al a δ 13 C ( ) C ( ) b

12 Plankton 105 m POC Flux,mg/m 2 d Percent of Organic Carbon Weight Percent Plankton 105 m Sediment Traps 1000 m > 3500 m Surface Sediment a Amino acid Uncharacterized Lipid Carbohydrate b Lipid Amino acid Carbohydrate c 1000 m > 3500 m Surface Sediment Isotopic ( 13 C, 14 C) evidence for a lipid-like source for acid insoluble macromolecular organic matter in sinking POM Image removed due to copyright restrictions. Image removed due to copyright restrictions. Image removed due to copyright restrictions. Hwang & Druffel 2003, Science 299,882

13 Molecular-level Radiocarbon Analysis The Problem: Many samples contain heterogeneous mixtures of organic compounds of diverse origin (and age). Age variability can be a source of interference, or information. The Approach: Structurally diverse organic compounds are preserved in sediments and carry a wealth of biogeochemical information. Measure the stable- and radio- carbon isotopic composition of individual organic compounds in order to constrain the origin of OC buried in sediments. Isotopic mass balance using both 14 C and 13 C allows for three OC source inputs (phytoplankton, vascular plant, relict organic matter) to be defined. Select compounds for 14 C and 13 C analysis using biochemical criteria, rather than characterizing OC pools based on operational definitions. Molecular 14 C contents also provide apparent ages for assessment of the residence times and cycling rates within (and between) carbon reservoirs. The Challenge: To measure the natural abundance of 14 C in individual organic compounds in complex mixtures. Greater than 25 µg C required for reliable 14 C measurement (by AMS). Isolation of target analytes in very high purity. Conventional capillary GC resolves < 500ng compound. The Approach: Automated Preparative Capillary Gas Chromatography (PCGC). Typical isolation scheme for CSRA Sediment Solvent extraction FID Intensity C 26 5,22 + C µgc C µgc 5α-C µgc C 28 5, µgc C µgc 5α-C 29 (sample lost) Extract SiO 2 chromatography Polarity fraction Derivatisation? Target compound mixture PCGC irm-gc-ms/gc-ms Trap 1 Trap 2 Trap 3 Trap 4 Trap 5 Trap 6 Trap 0 Total Combustion irm-gc-ms/gc-ms Retention Time (min.) Carbon dioxide Reduction Graphite AMS

14 Correction for derivative carbon Figure removed due to copyright restrictions. N.B. Derivatives are typically derived from petrochemicals (i.e., 14 C = ) Alkenone purification methodology O Sediment Solvent extraction Total Lipid Extract Saponification Neutral fraction SiO 2 chromatography Ketone fraction Urea adduction Straight-chain ketones O Ag + SiO 2 chromatography Unsaturated ketones Al 2 O 3 chromatography Purified alkenones Combustion Image removed due to copyright restrictions. Carbon dioxide Graphite Reduction AMS irms

15 Bulk vs molecular 14 C - isotopically homogeneous samples C compound (% ) Green River (hydrocarbons) Egypt Oil (fatty acids)* A. americana (fatty acids)* C. argentea (hydrocarbons) Juniper (phenols)* Two Creeks (phenols)* White Oak (phenols)* Natural Flavors (phenols)* Synthetic Flavors (phenols)* Nat./Syn Flavor mix (phenols)* Authentic std (fatty acid)* C bulk (% ) Pre-Anthropogenic 14C Distribution, Santa Monica Basin D 14C (% 0 ) Soil Riverine FOC DOC Photoautotrophs DOC CO Chemoautotrophs Surface Sediment Heterotrophs Mixed-layer DIC Deep Basin DIC Petroleum and Gas Seeps

16 Pre-Anthropogenic 14C Distribution, Santa Monica Basin Soil Riverine FOC DOC Photoautotrophs DOC CO Chemoautotrophs Surface Sediment Heterotrophs Mixed-layer DIC Deep Basin DIC D 14C (% 0 ) Petroleum and Gas Seeps Atmospheric CO 2 Mixed-Layer DIC Surface Sediment TOC C (% ) year

17 TOC and foram 14 C in Santa Monica Basin sediments Depth in sediment core z (cm) C Benthic forams Post-Bomb interval DIC TOC Planktonic forams Pre-Bomb interval C contents of algal sterols in CBB sediments Post-bomb DIC ± C ± 19 SMB, TOC SMB 0-1 SBB 0-1 SMB Pre-bomb DIC -150 SMB, TOC δ 13 C Data from Pearson et al. (2000)

18 14 C contents of lipid biomarkers in CBB sediments Surface DIC, 1996 Land Plants, Fossil Carbon Phytoplankton, Zooplankton, Bacteria Surface DIC, Pre-1950 Archaea -400 Fames n-alcohols C30-15-one-1-ol C30-1, 15-diol Sterols Hypanols C40 isoprenoids n-alkanes Data from Pearson et al. (2001)