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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

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

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

Decay rate

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)

Half life

(Radioactive decay equation)

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

l values – motivation for atom-counting methods vs decay-counting!

230TH cartoon

Cartoon 230Th profile (S = 2 cm/ky)

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

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?

Antarctic sediments – agreement between sed rates over two v. different timescales.

DeMaster et al. 1991

Produced where? How?

Natural variability in production?

Natural variability in atmospheric 14C content?

Human impacts on 14C budgets?

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.

Seuss effect (fossil fuel dilution of 14C(atm))

Bomb radiocarbon inputs

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

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.

Radiocarbon reporting conventions are convoluted!

14C data reported as Fraction modern, or age, or Δ14C

t 1/2 ~ 5730 y (half life)

λ= 0.00012097 / 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

Peng et al., 1977 bulk carbonate 14C

Regress depth vs. age

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

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

Substantial variation, slope not constant; non-unique 14C ages

Tree ring decadal 14C

Tree ring age

Stuiver et al., 1998

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

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

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

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

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.

- 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.

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)

Activity(230) (dpm g-1)

And:

Component i flux (g m-2 y-1) = Bulk flux (g m-2 y-1) x (wt % i)

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.

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

Calculated orbital variations

Imbrie et al., 1984

Planktonic foraminiferal d18O (ice volume and water temperature)

Imbrie et al., 1984

Bruhnes-Matuyama geomagnetic reversal in some cores

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)

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

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 %”

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