Marine Organic Geochemistry - Radiocarbon in Organic Materials

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, 796-799. Pearson A., Eglinton T.I. and McNichol A.P. (2000) An organic tracer for surface ocean radiocarbon. Paleoceanog. 15, 541-550. 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, 1365­ 1378. Raymond P.A. and Bauer J.E. (2001) Riverine export of aged terrestrial organic matter to the North Atlantic Ocean. Nature 409, 497-500. 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, 63-73. MEASURING THE AGE OF REALLY OLD STUFF 1. The Sun sends blasts of Radiation toward Earth 2. This Produces radioactive Carbon-14 in the Upper atmosphere. 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-14 (It's called carbon-14 because there are 6 Protons and 8 Neutrons in the nucleus) 4. Plant fix carbon through photosynthesis Carbon 14 Carbon 12 5. Animal absorb carbon by eating the plants (or by eating other animals that have done so.) 6. When the animals and plants die, the carbon-14 in them \ starts to decay, But the Carbon-12 does not. 8. Since carbon-14 starts to decay as soon as something dies, and it decays at a steadily decreasing I'd say 10,000 years 7. The Rate of decay rate, scientists ago, what about you? (called a half-life") can estimate when death After 5.730 years occurred by Same, give or Half of the Carbon-14 measuring the take 100 years is left. speed of decay. (After 11.460 years a quarter of it is left, and so on) 1 Geochemistry of Radiocarbon in Organic Materials • Natural abundance of stable and radio isotopes of carbon – – – • 12C - 99% - 1% 14C - 1 part per trillion (10-12) in modern carbon 13C 14C 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. 14N • + neutron Œ 14C + proton After formation, 14C atoms rapidly combine with oxygen to form CO2 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 14C formation and decay leading to an approximately constant level in the atmosphere. Current best estimate for half-life, T½, of 14C = 5730 yr Conventional (Libby) half-life adopted for reporting 14C ages = 5568 yr (3% smaller than true half-life). The half-life is related to the meanlife,τ, by: T½ = (ln2)τ • • Corresponding meanlife, τ, for Libby half-life is 8033yr. The radiocarbon age, t, can be determined from: t = -τ ln(A/A0) • (or T½ = 0.693τ) (or t = -8033 ln(A/A0)) where A is the number of atoms left after time t and A0 is the initial number of atoms. Key attribute: 5730 yr half life of 14C is suitable for studying processes and dating carbonaceous materials over 102 – 103 yr time-scales. 2 Methods of 14C measurement 1. Conventional method - Determination of 14C activity of a weighed sample by counting the number of electrons (beta particles) emitted from nucleus per unit time by the decay of 14C. • Beta-counting can be performed by samples combusted to CO2 (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 14C atoms (relative to 13C or 12C) by accelerator mass spectrometry (AMS). • Measurements are typically made on graphite (CO2 also possible). Graphite is formed by combustion of sample to CO2 and then reduction of CO2 to graphite. • Measurement times as short as 20 min. • Key attribute of AMS - Isobar rejection: – – • • Negative ions (Cs sputter source) remove (14N+) Electron stripping (accelerator) to remove hydrides (13CH-) Sensitivity of AMS = 6 × 10-16 (≡ 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 (+/- 15-20 ‰). • Standards: • *Small sample size has opened up many new applications for 14C. – Oxalic acid (HOxI, HoxII) Accelerator Mass Spectrometer Courtesy of Lawrence Livermore National Laboritories. Used with permission. 3 14C systematics • The absolute international standard of 14C activity (Aabs) is defined as 95% of the 14C activity of the original oxalic acid standard (HOxI), in the year 1950. This is equivalent to the activity of 19th century wood, and represents the 14C concentration of the atmosphere prior to anthropogenic influence (fossil fuel combustion, atomic weapons testing). • The measured activity of HOxI (Aox) is corrected from fractionation effects using a defined δ13Cox value of –19 ‰ to yield the fractionationnormalized activity (AON): • This is corrected to account for radioactive decay between 1950 and the year of measurement: 14C (  2 19 +δ 13C AON = 0.95Aox 1− 1000  )    Aabs = AONeλ( y−1950) systematics • The measured 14C activity of a sample (As) is normalized (Asn) to a constant δ13C value of –25 ‰ to remove the influence of isotopic fractionation on the reported concentration: • To a first approximation, the above equation treats the 14C fractionation as twice the 13C fractionation (to account for the greater mass difference). This is based on physical-chemical derivations that suggest the 14C fractionation is approximately equal to the square of the 13C fractionation. The mean age correction is about 16 years for every 1 ‰ difference from -25 ‰. This may be simplified to:  2(25 + δ 13Csample )   Asn = As 1−   1000     2  1+ − 25    1000   Asn = As   2 13   δ Csample   1+  1000    4 14C systematics The above equations were first developed for 14C measurements from decay counting techniques. AMS yields absolute ratios of 14C/12C in a sample, rather than the rate of decay. The above equations are still applicable, as activity and R14/12 are proportional via the decay constant, λ. AMS data are reported as fraction modern (fm) values, rather than activities: • • 14 A R 12 sn f m = sn = 14 AON R 12 ON When a radiocarbon age (year date) is not desired, data are reported as ∆14C values in one of two forms. For samples with no age correction , where y is the year of measurement: • •   Asn ∆14C =  −1 *1000 = f m e −λ ( y−1950) −1 *1000 λ ( y−1950)  AON e  ( 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)  ∆14C =  sn λ ( y−1950) −1 *1000 = f m e −λ (1950− x) −1 *1000   AON e ( Reporting of Radiocarbon Data 100 1.0 0 80 0.8 -200 0 40 0.6 0.4 -400 -600 2000 age (yr BP) 60 ∆14C (permil) 1000 Fraction Modern (Fm) • 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. Percent Modern Carbon (pMC) • ) 3000 4000 5000 14C • ) 10000 20 0.2 -800 0 0.0 -1000 15000 20000 30000 5 Radiocarbon age versus ∆14C ∆14C is useful for isotopic mass balance calculations Factors influencing radiocarbon abundances: 1. Atmospheric 14C variations • Variations in solar (cosmic ray flux) activity (long-term, ~ 103 yr variations in production rate). • Variations in Earth’s geomagnetic field strength (short-term, < 102 yr variation in production rate) • Climate induced variations - solubility of CO2 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 14C is assumed to be double that for 13C (reflecting mass difference relative to 12C). • Conventional radiocarbon ages are corrected to a single δ13C 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 14C) and deeper layers (lower 14C) 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. 6 14C variations and radiocarbon calibration 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 (102 yr) variations are due to geomagnetic field variations. Long-term (103 yr) variations are due to variations in cosmic ray flux. Recent variations of 14C in atmospheric CO2 1000 Fraction modern (fM) 2 1.8 800 The “Bomb Spike” 1.6 600 1.4 1.2 400 Maunder Minimum ∆ 14C (‰) 200 1 0 The “Suess Effect” 1600 1650 1700 1750 1800 1850 1900 1950 2000 Calender Year 7 Wiggle-matching, dendrochronologies and varve chronologies 14C variations vs varve age for Cariaco basin sediments, compared to those from tree rings Radio-carbon years BP Potential limitations in assigning Calendar ages from 14C data t+σ t-σ t+σ t+σ t-σ t-σ cal BC cal BC cal BC 8 Geochemical Applications of Radiocarbon • • • Development of sediment chronologies Tracer studies (e.g., bomb-spike) Isotopic mass balance The Global Organic Carbon Cycle (ca. 1950) ATMOSPHERIC CO2 (750) fossil fuel utilization SEDIMENTARY ROCKS (KEROGEN) (15,000,000) -20 to -35‰ t = infinite net terrestrial primary production (50) LAND PLANTS (570) p -25 to -30‰ (C3) we hysic ath al -14 to -20‰ (C4) riv (0. ering t = 0 yr er 2) tr or e humification (P ansp olian OC or (50) =0 t .15 ) SOIL HUMUS (1,600) uplift -25 to -30‰ (C3) -14 to -20‰ (C4) t =10-10,000 yr net marine primary production (50) OCEAN (BIOTA) (3) -20 to -30‰ (C3) t = 400 yr OC preservation (0.15) burial (units=1015gC) Modified after Hedges (1992) sediment redistribution RECENT SEDIMENTS (100) -20 to -25‰ t = ?? yr 9 Radiocarbon age of bulk OC in Long Island Sound sediments 14 0 1000 2000 C age (yr BP) 3000 4000 5000 6000 0 20 Depth (cm) 40 60 80 100 120 (Benoit et al., 1979) Variations in stable carbon isotopic and radiocarbon composition of organic matter as a function of particle size Fraction 1 "total sample <2000µm 20µm sieve 20µm sieve 0-20 µm 20-2000 µm 0-63 µm Fraction Fraction 2 3 63-2000 µm Fraction Fraction 4 5 Schematic representation of the sieving and settling procedure. 1. Total 70.9% -23.9 % Percent modern carbon δ13C 2. 0-20 µm 3. 20.2000 µm 51.7% 80.1 % -25.6 % -23.1% 4. 0-63 µm 0 20 40 74.2% -23.5 % 60 5. 63-2000 µm 50.1% -25.5 % 80 2000 Particle size (µm) 10 Variations in stable carbon isotopic and radiocarbon composition of organic matter as a function of chemical compound class δ13C Percent Modem Carbon 14a 80 p MC 12δ-22.8 % CHCL3:MeOH H2O, 100 C E1 -21.1 R1 -23.4 -22.5 -22.8 70 81 07 77 Saponification 1 M H2SO4, 100 C E2 R2 -24.7 -20.0 -27.7 63 69 63 6 M HCL, 100 C R3 -25.9 E3 -21.9 49 87 CHCL3 extraction R4 -25.0 E4 -27.6 46 Extract 55 1 M KOH 100 C 1h R5 -24.3 Residue E5 -25.9 43 56 Scheme of the extraction procedure. R = residue; E = extract. See text for chemical identification Depth in sediment (cm) Height above bottom (m) Variations in stable carbon isotopic and radiocarbon composition of organic matter as a function of chemical compound class -4000 TOC THAA TCHO Lipid Acid-insol BC alkenone -3000 -2000 -1000 -10 plankton zooplankton sinking POM aggregates flocc 0 10 20 30 40 50 a 60 70 -26 -24 -22 -20 -18 -16 -14 13 δ C (‰) b -800 -600 -400 -200 0 14 ∆ C (‰) 11 POC Flux,mg/m2d Sediment Traps Plankton 0.01 1.0 100 0 Percent of Organic Carbon 20 40 60 80 100 Weight Percent 0 20 40 60 80 100 Plankton 105 m 105 m 1000 m 1000 m Uncharacterized Lipid > 3500 m > 3500 m Surface Sediment b a Amino acid Carbohydrate Lipid Amino acid Carbohydrate c Surface Sediment 12 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 14C and 13C allows for three OC source inputs (phytoplankton, vascular plant, relict organic matter) to be defined. Select compounds for 14C and 13C analysis using biochemical criteria, rather than characterizing OC pools based on operational definitions. Molecular 14C 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 14C in individual organic compounds in complex mixtures. Greater than 25 µg C required for reliable 14C 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 C26∆ 5,22 + C26∆ 22 Extract 37 µgC SiO2 chromatography 5 C27∆ 86 µgC Polarity fraction FID Intensity 5α-C27 54 µgC Derivatisation? 5,22 C28∆ 126 µgC C29∆ Target compound mixture PCGC 5 irm-GC-MS/GC-MS 113 µgC Trap 1 Trap 2 Trap 3 Trap 4 Trap 5 Trap 6 Trap 0 5α-C29 (sample lost) Total Combustion irm-GC-MS/GC-MS Carbon dioxide 30 35 40 Retention Time (min.) 45 50 Reduction Graphite AMS 13 Correction for derivative carbon Alkenone purification methodology Sediment Solvent extraction Total Lipid Extract O Saponification Neutral fraction SiO2 chromatography Ketone fraction Urea adduction Straight-chain ketones O Ag+ SiO2 chromatography Unsaturated ketones Al2O3 chromatography Purified alkenones Combustion Carbon dioxide irMS Reduction Graphite AMS 14 Bulk vs molecular 14C - isotopically homogeneous samples 400 ∆14C compound (% ) 200 0 -200 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)* -400 -600 -800 -1000 -1000 -800 -600 -400 -200 0 200 400 ∆14C bulk (% ) Pre-Anthropogenic 14C Distribution, Santa Monica Basin D 14C (%0 ) +200 +150 CO2 Mixed-layer DIC Riverine FOC Soil DOC +100 +50 +0 -50 -100 Photoautotrophs Heterotrophs DOC Chemoautotrophs Surface Sediment Deep Basin DIC -150 -200 -250 -300 -350 -400 -450 -500 -1000 Petroleum and Gas Seeps 15 Pre-Anthropogenic 14C Distribution, Santa Monica Basin D 14C (%0 ) +200 +150 CO2 Mixed-layer DIC Riverine FOC DOC Soil +100 +50 +0 -50 -100 Photoautotrophs Heterotrophs DOC Chemoautotrophs Deep Basin DIC -150 -200 -250 -300 -350 -400 -450 -500 -1000 Surface Sediment Petroleum and Gas Seeps 1000 Atmospheric CO2 Mixed-Layer DIC Surface Sediment TOC 800 ∆14C (% ) 600 400 200 0 -200 1950 1960 1970 1980 1990 2000 year 16 TOC and foram ∆14C in Santa Monica Basin sediments 14 ∆ C -250 -150 -50 50 150 0 1996 Post-Bomb interval DIC 1 1985 TOC 2 z (cm) Depth in sediment core 250 1961 3 Planktonic forams 1937 4 1913 5 Benthic forams 6 14C Pre-Bomb interval 1899 contents of algal sterols in CBB sediments 150 Post-bomb DIC 100 14 ∆ C 50 +59 ± 26 ‰ 0 -86 ± 19 ‰ -50 SMB, TOC -100 SMB 0-1 SBB 0-1 SMB 5-6 Pre-bomb DIC SMB, TOC -150 -200 -35 -30 -25 13 δ C -20 -15 17 14C contents of lipid biomarkers in CBB sediments 200 Land Plants, Fossil Carbon Surface DIC, 1996 100 0 -100 Surface DIC, Pre-1950 -200 Phytoplankton, Zooplankton, Bacteria -300 Archaea lk A n- en pr C4 0 iso an oi es ds s ol an yp H er ol s l 1, 0C3 St -d 15 -1 ne -o 15 0- C3 io l -o ol oh lc A n- Fa m es s -400 18