1. Two approaches Sediment accumulation rates Two basic approaches:
a sediment component that changes at a known rate
after deposition (constant, or known, input)
Radioactive decay – U series, radiocarbon
a sediment component with a known, time-dependent
input function (constant, or known change,
after deposition)
excess 230Th , d18O chronostratigraphy
Two approaches
AA racemization;
Microtektites; ash layers; bomb isotopes

2. Radiometric dating Radiometric dating
Dating principles – covered in Isotope Geochemistry (Faure)
Two “simple” approaches:
Average slopes from age vs. depth plots
Absolute 14C dates for foraminiferal abundance maxima
Normalization to constant 230Th flux
Sediment focusing / winnowing
Point-by-point mass accumulation rates
Radiometric dating

3. Terminology Terminology
Radioactive parent  daughter + - (electron) or  (He nucleus)
(or + or  emission or e – capture or…)
Isotopes: Same number of protons, differing numbers of neutrons
chemically similar, different mass (kinetics), different radioactive
properties
Terminology

4. Radioactive decay rate proportional to number of atoms present 
N is the number of parent atoms in the sample
λ is the decay constant (units t-1)
λN gives the activity (disintegrations/time)
Decay rate

5. (Radioactive decay equation)
Half life
(Radioactive decay equation)

6. The time for 50% of atoms present to decay.
Half life
Half life
The time for 50% of atoms present to decay.
Rule of thumb = radioisotope activity can be measured for about 5 half-lives

7. l values – motivation for atom-counting methods vs decay-counting!
isotope
l (decay const.)
min -1
Half-life yr
238U
2.92 x
4.51 x 109
230Th
1.75 x
75,200
14C
2.30 x
5,730
210Pb
5.92 x 10 -8
22.3
234Th
2.00 x 10 -5
0.066 (24 dy)
l values – motivation for atom-counting methods vs decay-counting!

8. Radioactive parent decaying to stable daughter
ingrowth
decay

9. Cartoon 230Th profile (S = 2 cm/ky)
230TH cartoon
Regress ln(A) vs. depth (z)
ln(Az) = ln(Ao) – mz
Decay eqn
ln(Az) = ln(Ao) – lt
= ln(Ao) – l(z/S)
= ln(Ao) – (l/S)z

10. Assumptions for regressions of age vs. depth
Accumulation without mixing below the mixed layer
The isotope is immobile in the sediment
Constant input activity (reservoir age), or known as a function of time
  Recall activity at time = 0 in the decay equation:
How well do we know N(o) in the past?

11. Ku (in Broecker and Peng 1982)

12. Antarctic sediments – agreement between sed rates over two v
Antarctic sediments – agreement between sed rates over two v. different timescales.
DeMaster et al. 1991

13. Radiocarbon:
Produced where? How?
Natural variability in production?
Natural variability in atmospheric 14C content?
Human impacts on 14C budgets?

14. Produced in upper atmosphere,
modulated by solar wind, earth’s magnetic field
Faure

15. Natural variability in atmospheric 14C content? YES!
Production variations (solar, geomagnetic)
Carbon cycle (partitioning between atmosphere, biosphere, and ocean)
At steady state, global decay = global production
But global C cycle not necessarily at steady state,
and 14C offsets between C reservoirs not constant.

16. Human impacts on 14C budgets?
Seuss effect (fossil fuel dilution of 14C(atm))
Bomb radiocarbon inputs

17. 14C-free
14C produced in atmosphere, but most CO2 resides in (and decays in) the ocean

18. Radiocarbon dating of sediments.
Bulk CaCO3, or bulk organic C
standard AMS sample 25 mol C (2.5 mg CaCO3)
Specific phases of known provenance:
Planktic, benthic foraminifera
Specific (biomarker) compounds (5 mol C)
Dating known phases (e.g., foraminifera), at their abundance maxima,
improves the reliability of each date.
No admixture of fossil (14C-free) material.
Minimizes age errors caused by particle mixing
and faunal abundance variations.
But, reduces # of datable intervals.

19. Radiocarbon reporting conventions are convoluted!
14C data reported as Fraction modern, or age, or Δ14C
t 1/2 ~ 5730 y (half life)
λ= / yr (decay constant) (about 1% in 83 years)
Account for fractionation, normalize to 13C = -25
Activity relative to wood grown in pre-bomb atmosphere
Stable carbon isotopic composition

20. Peng et al., 1977 bulk carbonate 14C
Regress depth vs. age

21. 14C in the sediment “mixed layer”
Curvature of solid-phase profiles can in principle yield reaction rates (OM decomp, CaCO3 diss), if sediment accumulations rates are low and particle mixing rates are known.
But we’ll see that porewater profiles are often more sensitive indicators of the reactions taking place, and their rates.
14C in the sediment “mixed layer”

22. 14C of atmosphere, surface ocean, and deep ocean reservoirs in a model.
Ocean mixed layer reservoir age; lower 14C, damped high-frequency variations.
Stuiver et al., 1998

23. Modern mixed layer reservoir age corrections, R
Modern mixed layer reservoir age corrections, R. Reservoir age = 375 y +/- R. Large range; any reason R should stay constant?

24. Substantial variation, slope not constant; non-unique 14C ages
Tree ring decadal 14C
Tree ring age
Stuiver et al., 1998

25. Production variations and carbon cycle changes through time
Atmospheric radiocarbon from tree rings, corals, and varves. Calendar ages from dendrochronology, coral dates, varve counting.
Stuiver et al., 1998

26. Bard et al. (’90; ’98) – U-Th on Barbados coral to calibrate 14C beyond the tree ring record. Systematic offset from calendar age.
Reservoir-corrected 14C ages
Calendar ages from dendrochronology and Barbados coral U-Th

27. The product of these radiocarbon approaches is an age-depth plot.
Regression gives a sedimentation rate;
linearity gives an estimate of sed rate variability.
Typically, sedimentation rates do vary.
How many line segments do you fit to your data?
How confident are you in each resulting rate estimate?
To estimate mass accumulation rates (MARs)
Calculate average sedimentation rates between dated intervals, and multiply by dry bulk density and concentration.
But:
Average sed rates can’t be multiplied by point-by-point
dry bulk density and concentration to yield time series.
The solution – 230Th-normalized accumulation rates

28. Two approaches Sediment accumulation rates Two basic approaches:
a sediment component that changes at a known rate
after deposition (constant, or known, input)
Radioactive decay – U series, radiocarbon
a sediment component with a known, time-dependent
input function (constant, or known change,
after deposition)
excess 230Th , d18O chronostratigraphy
Two approaches
AA racemization;
Microtektites; ash layers; bomb isotopes

29. Flux estimates using excess 230Th in sediments
(M. Bacon; R. Francois)
Assume:
230Th sinking flux
= production from 234U parent in the water column
= constant fn. of water depth
  (uranium is essentially conservative in seawater)
Correct sediment 230Th for detrital 230Th using measured 232Th
and detrital 232Th/238U.
Correct sediment 230Th for ingrowth
from authigenic U (need approximate age model).
Use an age model to correct the remaining,
“excess” 230Th for decay since the time of deposition.

30. Two applications:
Integrate the xs230Th between known time points (14C, 18O).
Deviations from the predicted (decay-corrected) xs230Th inventory
reflect sediment focusing or winnowing.
Sample by sample, normalize concentrations of sediment constituents
(CaCO3, organic C, etc.) to the xs230Th of that sample. Yields flux
estimates that are not influenced by dissolution, dilution.

31. Point by point normalization:
Activity(230) (dpm g-1) = Flux(230) (dpm m-2 y-1)
Bulk flux (g m-2 y-1)
So:
Bulk flux (g m-2 y-1) = Flux(230) (dpm m-2 y-1)
Activity(230) (dpm g-1)
= Prod(230) (dpm m-3 y-1) x (water depth)
And:
Component i flux (g m-2 y-1) = Bulk flux (g m-2 y-1) x (wt % i)

32. Simple examples (without focusing changes):
If % C org increases in a sample, but Activity(xs230Th) increases
by the same fraction, then no increase in C org burial –
just a decrease in some other sediment component.
If % C org stays constant relative to samples above and below, but
Activity(xs230Th) decreases, then the C org flux (and the bulk flux)
both increased in that sample (despite lack of a concentration signal).
But:
To assess changes in focusing, we’re stuck
integrating between (dated) time points.

33. Chronostratigraphy based on foraminiferal d18O values
Two premises (observed):
The d18O of seawater responds to changes in global ice volume
- high-latitude precipitation is strongly depleted in d18O
- more ice on continents => higher seawater 18O
Foraminiferal d18O reflects seawater d18O
- (but also temperature)
Foraminiferal d18O provides a global stratigraphy
Dating (radiometric, or orbital tuning) provides timescale

34. Eccentricity, Tilt, and Precession

35. Calculated orbital variations
Imbrie et al., 1984

36. Planktonic foraminiferal d18O (ice volume and water temperature)
Imbrie et al., 1984
Bruhnes-Matuyama geomagnetic reversal in some cores

37. d18O vs. time
Align control points (“wiggle matching”) to put cores on same time-scale.
Assumption – the d18O time series reflect global signal (ice volume, and SST)

38. Normalize, stack, smooth
Result:
A reference d18O stratigraphy
Timescale at base of stack set by radiometric dating (K/Ar on volcanic rock) of B/M reversal

39. K/Ar age of Bruhnes/Matuyama reversal ~730 ky
Shackleton and Opdyke 1973
K/Ar age of Bruhnes/Matuyama reversal ~730 ky
Age based on tuning to (assumed) orbital forcing is older.
Shackleton et al. (1990): “K/Ar-based timescale underestimates true age by 5 – 8 %”

40. Age estimates for magnetic reversals (Ma)
K/Ar (1)
Milankovich (2)
40Ar/39Ar (3,4)
Bruhnes /
0.73
0.78
> 0.746
Matuyama
0.783
< 0.78
Matuyama /
0.91
0.99
Jaramillo
(1) Berggren et al. (1985) K/Ar
(2) Shackleton et al (1990) orbital prediction
(3) Tauxe et al. (1992) Ar/Ar Kenya
(4) Baksi et al. (1992) Hawaii