Annual cycles of [formula omitted] in coral skeletons and [formula omitted] thermometry

Abstract

We have discovered annual variations in the 238U 40Ca ratio of Porites coral skeletons. Measurements were made using thermal ionization mass spectrometric techniques, yielding precisions of ±2ℵ. (2σ) for 238U 40Ca and ±l‰ (2σ) for 88Sr 40Ca . Coralline aragonite subsamples weighed ∼2 mg, enabling submillimeter sampling resolution corresponding to ∼monthly temporal resolution. The annual nature of the cycles was confirmed by comparison with annual banding observable in X-radiographs. For two modern and one fossil sample, the amplitude of the U Ca variation ranges from 6 to 23%, well outside of analytical error. As annual U Ca cycles appear to be a general feature of primary coralline aragonite, the preservation of such features will be important in identifying unaltered coral for U-series dating studies. U Ca variations mimic and are in phase with annual variations in 88 Sr 40 Ca . For a given fractional shift in Sr Ca , the fractional shift in U Ca is about 6 times larger. For the two modern corals, 238U 40Ca is strongly anticorrelated with measured temperature, suggesting that 238U 40Ca has potential as a paleothermometer. If temperature is the only significant control on coralline 238U 40Ca , we reach the following conclusions from analyses of fossil samples: a Vanuatu sample, which grew halfway through the last deglaciation, gives U Ca temperatures 4 to 5°C below modern values. U Ca thermometry applied to published data for Barbados corals indicates that (1) temperature generally correlates with sea level, (2) glacial temperatures (stages 2 and 4) were 4 to 6°C lower than interglacial temperatures (stages 7a, 5e, and 1), and (3) temperatures rose from glacial to interglacial values early in the last deglaciation. Thermometry applied to Papua New Guinea corals indicates that (1) temperatures were 5 to 6°C lower than interglacial temperatures from ∼13 to ∼10 Ky bp, then rose to present values ∼9 Ky bp, (2) the temperature depression between ∼13 and ∼10 Ky bp is consistent with low temperatures observed in Vanuatu during the same time interval, and (3) the Papua New Guinea deglacial temperature history differs from that of Barbados. The results generally support estimates of tropical temperatures obtained from Sr Ca thermometry and snow line elevation data, but disagree with those based on foram transfer functions. A thermodynamic model suggests that coralline 238U 40Ca may also be sensitive to marine carbonate ion concentration, raising the possibility that some of the observed glacial-interglacial 238U 40Ca variation may result from glacial-interglacial carbonate ion changes. However, the key experiments that might establish a coralline 238U 40Ca-carbonate ion relationship have yet to be performed.