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Principles of U-Th Dating of Corals

•  U isotopes: 238U, 235U & 234U

  238U decays to 234U (via 2 short-lived intermediates) with a half-life of 4.5 Gyr

  234U decays to 230 Th with 246 kyr half-life

  230 Th decays to 226Ra with 75.4 kyr half-life

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Principles of U-Th Dating of Corals

234U 230 Th λ234U = 246 kyr

•  When CaCO3 is produced by coral it incorporates a small amount of U from seawater as an impurity in the CaCO3 crystal matrix, but almost no Th

•  Ingrowth of 230 Th provides an age

•  Must account for decay of 230 Th, initial [U], initial seawater 234U/238U, initial [Th] (by measuring 232Th)

Ca 2+

UO2(C

O 3) 34

-

Sea Level from Submerged Coral:

Principle

Fairbanks (1989) Nature Vol. 342:637-642

•  Drill into reef and near-shore sediments to recover continuous vertical sequence of coral deposited during last deglaciation •  Radiometrically date fossil coral strata •  Correct for any tectonic uplift or subsidence •  Paleo-sea level inferred from depth of sample

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Sea Level from Submerged Coral: Application

Thomas et al. (2009) Science Vol. 324: 1186-1189

•  New Tahiti submerged coral from penultimate deglaciation •  Large distance from ice sheets makes sea-level at Tahiti primarily a function of changing ocean volume (meltwater) rather than ice loading •  Constant subsidence of 0.25 m/kyr results from load of island on oceanic plate

Age (ka) • Conclusions: 1.  Sea level reached 85 mbsl

by 127 ka 2.  Preceded increase in 65°N

insolation

Sea Level from Raised Reefs: Principle - 1

c.f., Chappell & Shackleton (1986) Nature, 324: 137-140

•  Tropical sites w/ rapid & stable tectonic uplift

•  U-Th date reef terraces •  Correct for uplift

•  Permits sea level determinations through full

glacial cycle

http://www.ncdc.noaa.gov/paleo/pubs/tudhope2001/pic1.jpg

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Sea Level from Raised Reefs: Principle - 2

Esat et al. (1999) Science Vol. 283: 197-201

•  Huon Peninsula (PNG) uplift rates: 1.6-1.9 m/ky •  Estimated by assuming sea level 5 m above present at MIS 5e (120 ka)

•  Corresponds to reef complex VIIb

Effect of Coral Diagenesis on U-Th Ages of Corals

•  By early ‘90s it became clear that corals were not closed systems w.r.t. U & Th •  One of the 1st indications of a problem was an observed increase of initial (234U/238U) with increasing age of corals •  This implied either: 1.  Seawater 234U/238U was

changing (v. unlikely given long residence time of U in ocean)

2.  Corals were accumulating U with high 234U/238U

Henderson et al. (1993) Earth Planet. Sci. Lett. Vol. 115: 65-73

δ234U(T) = [(234U/ 238U )-1]*eλ234U

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Diagenetic Alteration of U-Series Ages

‘Alterations of uranium isotopic composition in raised reefs are likely associated with a complex set of processes involving adsorption of nuclides from

groundwater, dissolution/precipitation reactions with groundwater, & leaching into groundwater of nuclides

from recoil-damaged sites.’

Cutler (2003) Earth Planet. Sci. Lett. Vol. 206: 253-271

I.e., exposed coral is not a closed system. Gain or loss of U or its daughters (e.g., Th) can & does occur.

So how does groundwater acquire a uranium isotopic composition so vastly different than seawater?

Alpha Recoil •  Recoil ejection of 234Th from a spherical grain resulting from α-decay of 238U, followed by β-decay of 234Th 234U •  Net effect is depletion in 234U (relative to 238U) in surface layer of grain & enrichment of 234U (relative to 238U) in water surrounding grain •  Similar recoil effects are associated with α-decay of 234U, 230Th & 226Ra

DePaolo et al. (2006) Earth Planet. Sci. Lett. Vol. 248: 394-410

•  One result of this effect is that surface & ground waters have 234U/238U up to 20x higher than the secular equilibrium* ratio

* 234U production rate = 234U decay rate

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Screening Corals for Diagenesis: 234U/238U

Constraint

•  When corals have 234U/ 238U ratios substantially greater than the seawater ratio of 149‰ they are suspected of containing some diagenetic CaCO3 that formed in the presence of surface or ground water •  A common constraint on whether a coral specimen is suitable for U/Th dating is therefore that its 234U/ 238U ratio (δ234U) be within 4-10‰ of seawater ratio

Stirling et al. (1998) Earth Planet. Sci. Lett. Vol. 160: 745-762

More Sophisticated Screening Procedure for Diagenetic Alteration of U-series Ages

•  [231Pa/235U]-[230Th/234U] concordia diagram

•  New Guinea (red, green) & Barbados (blue) corals

•  Samples that plot on the curve (concordia) have identical 231Pa & 230 Th ages, and are considered unaltered

Cutler (2003) Earth Planet. Sci. Lett. Vol. 206: 253-271

•  Seek concordant ages from two U-Series isotopic systems

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Advantages & Disadvantages to Coral-Based Sea Level Reconstructions

•  Reef-building coral live at or near sea surface

•  Reefs tend to be far from locations of substantial isostatic rebound

•  U-Th isotope analyses by Thermal Ionization Mass Spectrometry (TIMS) is extremely sensitive, accurate & precise

•  Diagenesis can alter U-series ages

•  Coral from lower sea level stands difficult to obtain

•  Raised reefs can be covered by corals that grew subsequently, during times of higher sea level

•  Continuous sea level histories not possible

•  Require knowledge of tectonic history

•  U-series measurements difficult (TIMS)