Presentation on theme: "Radiocarbon9/18/12 Lecture outline: 1)radiocarbon dating principles 2)Atmospheric & ocean radiocarbon variability 3)The Calibration Curve 4)Radiocarbon."— Presentation transcript:
Radiocarbon9/18/12 Lecture outline: 1)radiocarbon dating principles 2)Atmospheric & ocean radiocarbon variability 3)The Calibration Curve 4)Radiocarbon as biogeochemical tracer The Shroud of Turin What makes 14 C the emperor of isotopes?
Radiocarbon ( 14 C) formation and decay -formed by interaction of cosmic ray spallation products with stable N gas -radiocarbon subsequently decays by β - decay back to 14 N with a half-life of 5730y Radiocarbon dating was first explored by W.R. Libby (1946), who later won the Nobel Prize. Most published dates still use the Libby half-life of 5568y to enable comparison of 14 C dates. The activity of radiocarbon in the atmosphere represents a balance of its production, its decay, and its uptake by the biosphere, weathering, etc. Which of these three things might change through time, and why?
Radiocarbon Dating where present-day, pre-bomb, 14 C activity = 13.56dpm/g C 1) As plants uptake C through photosynthesis, they take on the 14 C activity of the atmosphere. 2) Anything that derives from this C will also have atmospheric 14 C activity (including you and I). 3) If something stops actively exchanging C (it dies, is buried, etc), that 14 C begins to decay. So all you need to know to calculate an age is A 0, which to first order is 13.56dpm/g, BUT *small variations (several percent) in atmospheric 14 C in the past lead to dating errors of up to 20%! Sources of variability: 1)geomagnetic field strength 2)solar activity 3)carbon cycle changes
Radiocarbon Measurements and Reporting 2) Fact: Most living things do not uptake C in atmospheric ratios – i.e. they fractionate carbon, (lighter 12 C preferentially used), must correct for this fractionation because it affects the 14 C/ 12 C ratio 1) Radiocarbon dates are determined by measuring the ratio of 14 C to 12 C in a sample, relative to a standard, usually in an accelerator mass spectrometer. standard = oxalic acid that represents activity of 1890 wood 14 C ages are reported as 14 C years BP, where BP is ) The final step is to obtain a calibrated 14 C age using the atmospheric radiocarbon content when the sample grew. Researchers collect the 13 C/ 12 C ratio, use it to correct for missing 14 C So the less 13 C a sample has, the less 14 C it has, and so the uncorrected 14 C age will be _______ than the calendar age? Samples are normalized to a 13 C PDB value of -25
Convention: The atmospheric radiocarbon anomaly with respect to a standard is defined as 14 C Atmospheric radiocarbon variability through time -addition of isotopically light fossil fuel C to atmosphere -solar activity changes Note: the 14 C is 0 during 1890, b/c thats the activity of the oxalic acid standard time But how did somebody construct this curve?
Reconstructing atmospheric radiocarbon variability through time 1821A.D. by ring-counting tree cut in 1999A.D. radiocarbon sampling transect Most of the Holocene 14 C atmos variability derives from changes in the geomagnetic field What you need: absolute age & radiocarbon age What you get: history of 14 C atmos
Earths Carbon Reservoirs 14 C content (in Gigatons C): atmosphere748highest (site of production) terrestrial2,000high (some old organic matter) oceanic38,000high (surface) and low (deep) geologic4,000zero (isolated from atmos. for long time) Over longer timescales, whats controlling atmospheric Δ 14 C? Climate changes perturb the Es carbon cycle (and therefore 14 C atmos ) by: -changing the amount of C stored on land (ex ice sheets of LGM) -changing the ability of the deep ocean to sequester old carbon from atmosphere (ex increased stratification)
The ocean conveyor belt and radiocarbon Seawater radiocarbon measurements set a time-scale for whole-ocean mixing (~1500y), and identified the locations where deep-mixing occur The 14 C ages of waters along a N-S transect in the Atlantic. 14 C measurements from vertical profiles taken in worlds oceans the bomb spike the oldest waters are in the mid-depth pacifc (age~1500y) the youngest waters are at the surface and in the deep North Atlantic 4) If dating a marine sample, you will need to correct its calibrated 14 C age with a reservoir age, which ranges from 1000yrs in the mid-depth Pacific. note non-zero age at surface
Broecker et al., 2004 Adkins et al., 1998 Paleo-ventilation ages from 14 C in marine carbonates 1. compare 14 C ages of benthic and planktonic forams in same core horizon 2. compare U-series and 14 C ages for deep-sea corals 100y of growth (study modern corals) diff. in 14 C ages=670y! Pacific, LGM=large symbols GEOSECS=small dots Atlantic
data from: corals (bright red) lake varves (green) marine varves (blue) speleothems (orange) tree rings (black) The Radiocarbon Calibration Curve (atmospheric 14 C history) Principle: compare radiocarbon dates with independent dates examples of independent dating: tree-ring counting, coral U-Th dates, varve counting, correlation of climate signals in varves with ice core Hughen et al., 2004 So was atmospheric 14 C larger or smaller at 20k (LGM) than today? equiline Observation: radiocarbon dates are consistently younger than calendar ages time
Beck et al., 2001 But what caused these large changes in atmospheric 14 C? red=observed 14 C black=modelled 14 C geomagnetic field from paleomag studies only geomagnetic field from paleomag + magnetic anomaly at 44k geomagnetic field + mag. anomaly + reduced sedimentation during glacial geomagnetic field + mag. anomaly + reduced sedimentation during glacial + change in overturning circulation Use a carbon cycle model that includes radiocarbon, play with different scenarios, check fit with reality. stop transferring radiocarbon into deep ocean
M. Stuiver, P.J. Reimer, and R. Reimer So what is the average geochemist to do? INTCAL09 – established one curve to use for 14C calibration: Reimer PJ, Baillie MGL, Bard E, Bayliss A, Beck JW, Blackwell PG, Bronk Ramsey C, Buck CE, Burr GS, Edwards RL, Friedrich M, Grootes PM, Guilderson TP, Hajdas I, Heaton TJ, Hogg AG, Hughen KA, Kaiser KF, Kromer B, McCormac FG, Manning SW, Reimer RW, Richards DA, Southon JR, Talamo S, Turney CSM, van der Plicht J, Weyhenmeyer CE IntCal09 and Marine09 radiocarbon age calibration curves, 0–50,000 years cal BP. Radiocarbon 51(4):1111–50. Trust the experts! Use their calibration program (current version = CALIB 6.0): Also, avoid contamination with post-bomb/tracer carbon at all costs! Ex: diagenesis may replace original C with post-bomb (modern) C or contamination with tracer (super-enriched) 14 C used by biologists (done in IBB)
The timing and structure of the bomb spike Bomb-produced radionuclides (in Bq (1Bq=1dps) * * * *
Trumbore, = 14 C doubles The radiocarbon bomb spike – atmosphere vs. other reservoirs Source of bomb 14 C: stratosphere, Northern Hemisphere Incorporation of bomb 14 C into various C reservoirs depends on the residence time of C in that reservoir Ex: short residence time = quick, high-amplitude response long residence time = delayed, low-amplitude response Why?
Seawater radiocarbon observations 1.GEOSECS – Geochemical Ocean Section Study transects through all major oceans 2.WOCE – World Ocean Circulation Experiment extensive coverage of the ocean
Rodgers et al., 1999 Ocean models of bomb 14 C Can ocean general circulation models accurately capture the spatial and temporal evolution of the bomb radiocarbon spike? *Our understanding of ocean mixing (esp. vertical mixing) is extremely limited, 14 C can help.
Randerson et al., Global Biogeochemical Cycles, 2002 Atmosphere-Ocean-Biosphere models of bomb 14 C evolution The contributions of ocean, terrestrial biosphere, fossil fuel combustion and bomb testing to atmospheric 14 C was investigated with an Atmospheric General Circulation Model (GCM) coupled to a carbon cycle model. Why does the terrestrial biosphere lag the atmosphere? Atmosphere Terrestrial biosphere w/ three different carbon residence times The detrended atmos 14 C, showing large seasonal cycle Why do Southern and Northern Hemisphere 14 C values differ in 1970?
Why does the Northern Hemisphere have lower Δ 14 C? Map of mean annual Δ 14 C relative to South Pole. Randerson et al., 2002
French Frigate Shoals Fiji Galapagos Nauru Model 14 C-coral 14 C comparisons Rodgers et al., 1999 Why are some records smoother than others (ie Galapagos vs. Fiji)?
Radiocarbon in the Biosphere Approach: Use bomb 14 C as a tracer of carbon cycling in complex environments. Or use natural level 14 C to date carbon in various pools. Gaudinski et al., 2000
Figure 1. Radiocarbon in different reservoirs of carbon at Station M in the North Eastern Pacific Ocean. Red circles represent values for HMW DOM (Aluwihare, 1999); blue bars at 0 m and 1500 m represent the range of values observed for individual monosaccharides isolated from HMW sugars (Aluwihare, 1999). DIC, total DOC, and POC data are from Druffel et al., 1996 and Bauer et al., Radiocarbon measurements of carbon cycling in the ocean dissolved organic carbon old – highly refractory dissolved inorganic carbon contains bomb 14 C at surface, not at deep sinking particulate organic matter contains bomb carbon