Radiometric dating and sediment accumulation rates

1. Radiometric dating and sediment accumulation rates Dating principles – covered in Isotope Geochemistry (Faure) Two radiocarbon approaches: Average slopes from age vs. depth plots Absolute dates for foraminiferal abundance maxima Normalization to constant 230Th flux Sediment focusing / winnowing Mass accumulation rates

2. Terminology Radioactive parent  daughter + - (electron) or  (He nucleus) (or + or  emission or e – capture) Isotopes: Same number of protons, differing numbers of neutrons chemically similar, different mass (kinetics), different radioactive properties 12C (stable), 13C (stable), 14C (t1/2~ 5730 yr) 230Th (t1/2~ 75,000 yr), 232Th (t1/2~ 1.4 x 1010 yr), 234Th (t1/2~ 24 dy)

3. Radioactive decay (from Faure, Principles of Isotope Geology) N is the number of parent atoms in the sample λ is the decay constant ( units t-1) mean life is (1/)

4. To solve for parent remaining as a function of time, rearrange and integrate: if N = N0 at t = 0, C = - ln N0 (need to know initial activity, N0 , for absolute age)

5. Activity at time = t Half life

6. Radioactive parent decaying to stable daughter ingrowth decay

7. 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

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

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

10. Natural variability in atmospheric 14C content? Production variations (solar, geomagnetic) Carbon cycle (partitioning between atmosphere, biosphere, and ocean) At steady state, global decay = global production Human impacts on 14C budgets? Seuss effect (fossil fuel dilution of 14C(atm)) Bomb radiocarbon inputs

11. 14C-free 14C produced in atmosphere, but most CO2 resides in (and decays in) the ocean

12. 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.

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

14. 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) (14C atm) in the past?

15. 14C of atmosphere, surface ocean, and deep ocean reservoirs in a model. Mixed layer reservoir age; lower 14C, damped high-frequency variations. Stuiver et al., 1998

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

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

18. 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

19. 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

20. 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

21. 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. Use an age model to correct the remaining, “excess” 230Th for decay since the time of deposition.

22. 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.

23. 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) Activity(230) (dpm g-1) And: Component i flux (g m-2 y-1) = Bulk flux (g m-2 y-1) x (wt % i)

24. 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.

32. 3280 m 4675 m 14C in the “mixed layer”