Other
Scientific paper
May 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995gecoa..59.2025m&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 59, Issue 10, pp.2025-2042
Other
29
Scientific paper
We have discovered annual variations in the 238 U/ 40 Ca ratio of Porites coral skeletons. Measurements were made using thermal ionization mass spectrometric techniques, yielding precisions of ±2 . (2 ) for 238 U/ 40 Ca and ±l (2 ) for 88 Sr/ 40 Ca. 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, 238 U/ 40 Ca is strongly anticorrelated with measured temperature, suggesting that 238 U/ 40 Ca has potential as a paleothermometer. If temperature is the only significant control on coralline 238 U/ 40 Ca, 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 , then rose to present values ~9 Ky , (2) the temperature depression between ~13 and ~10 Ky 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 238 U/ 40 Ca may also be sensitive to marine carbonate ion concentration, raising the possibility that some of the observed glacial-interglacial 238 U/ 40 Ca variation may result from glacial-interglacial carbonate ion changes. However, the key experiments that might establish a coralline 238 U/ 40 Ca-carbonate ion relationship have yet to be performed.
Beck Warren J.
Edwards Lawrence R.
Gallup Christina D.
Min Rong G.
Recy Jacques
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