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5/10/10
1
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)