Radiocarbon 9/18/12

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 14C the “emperor” of isotopes?

Radiocarbon (14C) formation and decay

1 14 14 10 7 6 1n N C H

-formed by interaction of cosmic ray spallation products with stable N gas

14 146 7C N Q

-radiocarbon subsequently decays by β- decay back to 14N 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 14C 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 threethings might change

through time,and why?

Radiocarbon Dating

0tA A e

where present-day, pre-bomb,14C activity = 13.56dpm/g C

1) As plants uptake C through photosynthesis, they take on the 14C activity of the atmosphere.2) Anything that derives from this C will also have atmospheric 14C activity (including you and I).3) If something stops actively exchanging C (it dies, is buried, etc), that 14C begins to decay.

So all you need to know to calculatean age is A0, which to first order is 13.56dpm/g, BUT

*small variations (several percent) in atmospheric 14C in the past lead to dating errors of up to 20%!

Sources of variability:1) geomagnetic field strength2) solar activity3) carbon cycle changes

Radiocarbon Measurements and Reporting

13 12 13 12

13

13 12

/ /1 *1000

/spl std

std

C C C CC

C C

2) Fact: Most living things do not uptake C in atmospheric ratios – i.e. they fractionate carbon, (lighter 12C preferentially used), must correct for this fractionation because it affects the 14C/12C ratio

1) Radiocarbon dates are determined by measuring the ratio of 14C to 12C in a sample, relative to a standard, usually in an accelerator mass spectrometer.

standard = oxalic acid that represents activity of 1890 wood

14C ages are reported as “14C years BP”, where BP is 1950

3) The final step is to obtain a “calibrated 14C age” using the atmospheric radiocarbon content when the sample grew.

Researchers collect the 13C/12C ratio, use it to correct for “missing” 14C

So the less 13C a sample has, the less 14C it has,and so the uncorrected 14C age will be _______than the calendar age?

Samples are “normalized” to a 13CPDB value of -25‰

132(25 )1 /

1000PDB

corr meas

CA A dpm g

Convention:The atmosphericradiocarbonanomaly with respect to a standardis defined as 14C

14 12

14

14 12

/1 *1000

/spl

std

C CC

C C

Atmospheric radiocarbon variability through time

-addition of isotopically lightfossil fuel C to atmosphere

-solar activity changes

Note:the 14C is 0during 1890,b/c that’sthe activity of theoxalic acid standard

time

But how did somebodyconstruct this curve?

Reconstructing atmospheric radiocarbon variability through time

1821A.D. by ring-counting

tree cut in 1999A.D.radiocarbonsamplingtransect

Most of the Holocene 14Catmos

variability derives from changesin the geomagnetic field

What you need:absolute age & radiocarbon age

What you get:history of 14Catmos

0tA A e

Earth’s Carbon Reservoirs 14C content (in Gigatons C):atmosphere 748 highest (site of production)terrestrial 2,000 high (some old organic matter)oceanic 38,000 high (surface) and low (deep)geologic 4,000 zero (isolated from atmos. for long time)

Over longer timescales, what’s controlling atmospheric 14C?

Climate changes perturb the E’s carboncycle (and therefore 14Catmos) by:

-changing the amount of C stored on land(ex ice sheets of LGM)

-changing the ability of the deepocean to sequester old carbon fromatmosphere (ex increased stratification)

The ocean “conveyor belt” and radiocarbon

Seawater radiocarbon measurementsset a time-scale for whole-oceanmixing (~1500y), and identifiedthe locations where deep-mixing occur

The 14C ages of waters along a N-Stransect in the Atlantic.

14C measurements from verticalprofiles taken in world’s oceans

the “bomb” spike

the oldestwaters are

in the mid-depthpacifc (age~1500y)

the youngestwaters are

at the surfaceand in the deepNorth Atlantic

4) If dating a marine sample, you will need to correct its calibrated 14C age with a reservoir age, which ranges from <100yrs in the stagnant subtropical gyres to >1000yrs in the mid-depth Pacific.

note non-zero age at surface

Broecker et al., 2004Adkins et al., 1998

Paleo-ventilation ages from 14C in marine carbonates

1. compare 14C ages of benthic and planktonicforams in same core horizon

2. compare U-series and 14C agesfor deep-sea corals

100

y of

gro

wth

(st

udy

mod

ern

co

rals

)

dif

f. i

n 14

C a

ge

s=

67

0y

!Pacific,LGM=large symbolsGEOSECS=small dots

Atlantic

data from:corals (bright red)lake varves (green)marine varves (blue)speleothems (orange)tree rings (black)

The Radiocarbon Calibration Curve (atmospheric 14C 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 atmospheric14C larger or

smaller at 20k (LGM)than today?

equiline

Observation:radiocarbon datesare consistently younger than calendar ages

time

Beck et al., 2001

But what caused these large changes in atmospheric 14C?

red=observed 14Cblack=modelled 14Cgeomagnetic

field frompaleomagstudiesonly

geomagneticfield frompaleomag+ magneticanomalyat 44k

geomagneticfield +mag. anomaly+ reducedsedimentationduring glacial

geomagneticfield +mag. anomaly+ reducedsedimentationduring glacial+ change in overturningcirculation

Use a carbon cycle model that includes radiocarbon, play with different scenarios, check fit with reality.

stop transferringradiocarboninto deep ocean

http://radiocarbon.pa.qub.ac.uk/calib

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. 2009. 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) 14C used by biologists (done in IBB)

The timing and structure of the “bomb” spike

Bomb-produced radionuclides (in 1018 Bq (1Bq=1dps)

**

*

*

Trumbore, 2000

+1000‰ = 14C doubles

The radiocarbon bomb spike – atmosphere vs. other reservoirs

Source of bomb 14C: stratosphere, Northern Hemisphere

Incorporation of bomb 14C into various C reservoirs depends on the residence timeof C in that reservoirEx: short residence time = quick, high-amplitude response long residence time = delayed, low-amplitude response Why?

Seawater radiocarbon observations

1. GEOSECS – Geochemical Ocean Section Study

1972-1978transects through all major oceans

2. WOCE – World Ocean Circulation Experiment

1990-2002extensive coverage of the ocean

Rodgers et al., 1999

Ocean models of bomb 14C

Can ocean general circulation modelsaccurately capture the spatial and temporalevolution of the bomb radiocarbon spike?

*Our understanding of ocean mixing (esp. vertical mixing) is extremely limited, 14C can help.

Randerson et al., Global Biogeochemical Cycles, 2002

Atmosphere-Ocean-Biosphere models of bomb 14C evolution

The contributions of ocean, terrestrial biosphere, fossil fuel combustion and bomb testing to atmospheric 14C was investigated with an Atmospheric General Circulation Model (GCM) coupled to a carbon cycle model.

Why does the terrestrial biospherelag the atmosphere?

Atmosphere

Terrestrial biospherew/ three differentcarbon residence times

The detrended atmos 14C,showing large seasonal cycle

Why do Southern and NorthernHemisphere 14C values differ in 1970?

Why does the Northern Hemisphere have lower Δ14C?

Map of mean annual Δ14C relative to South Pole. Randerson et al., 2002

French FrigateShoals

Fiji

Galapagos

Nauru

Model 14C-coral 14C comparisons

Rodgers et al., 1999

Why are some records “smoother”than others (ie Galapagos vs. Fiji)?

Radiocarbon in the Biosphere

Approach:Use bomb 14C as a tracer of carbon cycling in complexenvironments.Or use natural level 14C todate 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., 1998.

Radiocarbon measurements of carbon cycling in the ocean

dissolvedorganic carbonold – highlyrefractory

dissolvedinorganic carboncontains bomb14C at surface,not at deep

sinking particulateorganic mattercontains bombcarbon