Radiocarbon 9/30/10

Lecture outline: 1) radiocarbon dating principles 2) 3) Atmospheric & ocean radiocarbon variability

What makes 14 C the “emperor” of isotopes?

The Calibration Curve 4) Radiocarbon as biogeochemical tracer

The Shroud of Turin

Radiocarbon (

14

C) formation and decay

-formed by interaction of cosmic ray spallation products with stable N gas 1 0 n  14

N

7  14

C

6  1 1

H

-radiocarbon subsequently decays by β decay back to 14 N with a half-life of 5730y 14

C

6  14

N

7 

Q

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

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.

A



A e

0  

t

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

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 1950 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 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 ‰  13

C

    13

C

/ 12

C

  

spl

 13

C

/ 13

C

12

C



std

/ 12

C



std A corr



A meas

  1  2(25   13

C PDB

1000 )     3) The final step is to obtain a “calibrated 14 C age” using the atmospheric radiocarbon content when the sample grew.

Atmospheric radiocarbon variability through time

Convention

: The atmospheric radiocarbon anomaly with respect to a standard is defined as D 14 C D 14

C

      14

C

14

C

/ /

12

C

12

C

 

spl std

  -

solar activity changes

Note: the D 14 C is

0

during 1890, b/c that’s the activity of the oxalic acid standard -

addition of isotopically light fossil fuel C to atmosphere

time

But how did somebody construct this curve?

Reconstructing atmospheric radiocarbon variability through time

What you need: absolute age & radiocarbon age

A



A e

0  

t

What you get: history of 14 C atmos

tree cut in 1999A.D.

1821A.D. by ring-counting Most of the Holocene 14 C atmos variability derives from changes in the geomagnetic field

Over longer timescales, what’s controlling atmospheric

 14

C?

Earth’s Carbon Reservoirs (in Gigatons C):

atmosphere 748 terrestrial oceanic geologic 2,000 38,000 4,000

14 C content

highest (site of production) high (some old organic matter) high (surface) and low (deep) zero (isolated from atmos. for long time) Climate changes perturb the E’s 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.

note non-zero age at surface the youngest waters are at the surface and in the deep North Atlantic the “bomb” spike the oldest waters are in the mid-depth pacifc (age~1500y)

14 C measurements from vertical profiles taken in world’s oceans

4) If dating a marine sample, you will need to correct its calibrated 14 C age with a

reservoir age

, which ranges from <100yrs in the stagnant subtropical gyres to >1000yrs in the mid-depth Pacific.

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 Atlantic Pacific, LGM=large symbols GEOSECS=small dots

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

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 data from: corals (bright red) lake varves (green) marine varves (blue) speleothems (orange) tree rings (black) Observation: radiocarbon dates are consistently younger than calendar ages

time

Hughen et al., 2004

So was atmospheric 14 C larger or smaller at 20k (LGM) than today?

But what caused these large changes in atmospheric

14

C?

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

geomagnetic field from paleomag studies only red=observed 14 C black=modelled 14 C geomagnetic field from paleomag + magnetic anomaly at 44k geomagnetic field + mag. anomaly + reduced sedimentation during glacial

stop transferring radiocarbon into deep ocean

geomagnetic field + mag. anomaly + reduced sedimentation during glacial + change in overturning circulation

Beck et al., 2001

So what is the average geochemist to do?

Trust the experts!

INTCAL98 – established one curve to use for 14C calibration: Stuiver, M., Reimer, P.J., Bard, E., Beck, J.W., Burr, G.S., Hughen, K.A., Kromer, B., McCormac, G., van der Plicht, J., & Spurk, M. 1998. INTCAL98 Radiocarbon Age Calibration, 24,000-0 cal BP. Radiocarbon 40(3):1041-1084. Use their calibration program (current version = CALIB 6.0):

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

M. Stuiver, P.J. Reimer, and R. Reimer 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 (next lecture)

The timing and structure of the “bomb” spike

Bomb-produced radionuclides (in 10 18 Bq (1Bq=1dps)

* * * *

The radiocarbon bomb spike – atmosphere vs. other reservoirs

+1000 ‰ = 14 C doubles

Trumbore, 2000

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?

1.

Seawater radiocarbon observations

GEOSECS –

Section Study Geochemical Ocean 1972-1978 transects through all major oceans

2.

WOCE –

World Ocean Circulation Experiment 1990-2002 extensive coverage of the ocean

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.

Rodgers et al., 1999

Model

D 14

C-coral

D 14

C comparisons

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

Galapagos

Rodgers et al., 1999

Nauru 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

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 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.

Atmosphere-Ocean-Biosphere models of bomb

14

C evolution

Atmosphere Terrestrial biosphere w/ three different carbon residence times The contributions of ocean, terrestrial biosphere, fossil fuel combustion and bomb testing to atmospheric D 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?

The detrended atmos D 14 C, showing large seasonal cycle

Randerson et al., Global Biogeochemical Cycles, 2002

Why do Southern and Northern Hemisphere

D

14 C values differ in 1970?