Principles of U-Th Dating of Corals

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1 5/10/10 Principles of U-Th Dating of Corals U isotopes: 238U, 235U & U 238U decays to U (via 2 short-lived intermediates) with a half-life of 4.5 Gyr U decays to 230 Th with 246 kyr half-life 230 Th decays to 226Ra with 75.4 kyr half-life 1

2 Principles of U-Th Dating of Corals UO 2 (CO 3 ) 3 4- Ca 2+ When CaCO 3 is produced by coral it incorporates a small amount of U from seawater as an impurity in the CaCO 3 crystal matrix, but almost no Th Ingrowth of 230 Th provides an age Must account for decay of 230 Th, initial [U], initial seawater U/ 238 U, initial [Th] (by measuring 232 Th) U 230 Th λ U = 246 kyr Sea Level from Submerged Coral: Principle Drill into reef and nearshore 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 Fairbanks (1989) Nature Vol. 342:

3 Sea Level from Submerged Coral: Application 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 Thomas et al. (2009) Science Vol. 324: Sea Level from Raised Reefs: Principle - 1 Tropical sites w/ rapid & stable tectonic uplift U-Th date reef terraces Correct for uplift Permits sea level determinations through full glacial cycle c.f., Chappell & Shackleton (1986) Nature, 324:

4 5/10/10 Sea Level from Raised Reefs: Principle - 2 Huon Peninsula (PNG) uplift rates: m/ky Estimated by assuming sea level 5 m above present at MIS 5e (120 ka) Corresponds to reef complex VIIb Esat et al. (1999) Science Vol. 283: Effect of Coral Diagenesis on U-Th Ages of Corals δu(t) = [(U/ 238U )-1]*eλ U 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 (U/238U) with increasing age of corals This implied either: 1. Seawater U/238U was changing (v. unlikely given long residence time of U in ocean) 2. Corals were accumulating U with high U/238U Henderson et al. (1993) Earth Planet. Sci. Lett. Vol. 115:

5 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. 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? Cutler (2003) Earth Planet. Sci. Lett. Vol. 206: Alpha Recoil Recoil ejection of Th from a spherical grain resulting from α-decay of 238 U, followed by β- decay of Th U Net effect is depletion in U (relative to 238 U) in surface layer of grain & enrichment of U (relative to 238 U) in water surrounding grain Similar recoil effects are associated with α-decay of U, 230 Th & 226 Ra One result of this effect is that surface & ground waters have U/ 238 U up to 20x higher than the secular equilibrium* ratio * U production rate = U decay rate DePaolo et al. (2006) Earth Planet. Sci. Lett. Vol. 248:

6 Screening Corals for Diagenesis: U/ 238 U Constraint When corals have U/ 238 U ratios substantially greater than the seawater ratio of 149 they are suspected of containing some diagenetic CaCO 3 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 U/ 238 U ratio (δ U) be within 4-10 of seawater ratio Stirling et al. (1998) Earth Planet. Sci. Lett. Vol. 160: More Sophisticated Screening Procedure for Diagenetic Alteration of U-series Ages Seek concordant ages from two U-Series isotopic systems [ 231 Pa/ 235 U]-[ 230 Th/ U] concordia diagram New Guinea (red, green) & Barbados (blue) corals Samples that plot on the curve (concordia) have identical 231 Pa & 230 Th ages, and are considered unaltered Cutler (2003) Earth Planet. Sci. Lett. Vol. 206:

7 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) 7

Geologic time and dating. Geologic time refers to the ages relevant to Earth s history

Geologic time and dating. Geologic time refers to the ages relevant to Earth s history Geologic time and dating Most figures and tables contained here are from course text: Understanding Earth Fourth Edition by Frank Press, Raymond Siever, John Grotzinger, and Thomas H. Jordan Geologic time