High-precision uranium-series analyses of fossil corals and Nicaragua rise sediments: The timing of high sea levels and the marine delta U-234 value during the past 200,000 years.

摘要

I present high-precision thermal-ionization mass spectrometric (TIMS) {dollar}sp{lcub}230{rcub}{dollar}Th ages and uranium-isotopic measurements of fossil corals from Barbados that record periods of high sea level during the last 200 ka (thousand years) and have sufficient resolution to test the Milankovitch ice age theory and to constrain the past marine {dollar}deltasp{lcub}234{rcub}{dollar}U value. I also present uranium-isotopic measurements of carbonate-rich sediments from the Nicaragua Rise, Caribbean Sea, that constrain the last-interglacial marine {dollar}deltasp{lcub}234{rcub}{dollar}U value.; I establish a criterion for Barbados coral {dollar}sp{lcub}230{rcub}{dollar}Th ages that are accurate to {dollar}pm{dollar}2 ka based on the coral initial {dollar}deltasp{lcub}234{rcub}{dollar}U value and an empirical model for the behavior of uranium and thorium isotopes during diagenesis. I apply this criterion to my own and previously published TIMS-{dollar}sp{lcub}230{rcub}{dollar}Th data and find that the Barbados coral terraces record high sea levels at 83.3 {dollar}pm{dollar} 0.3 ka, 98.4 {dollar}pm{dollar} 1.2 to 100.5 {dollar}pm{dollar} 1.1 ka, 120.2 {dollar}pm{dollar} 1.4 to 129.1 {dollar}pm{dollar} 0.8 ka, and 193.8 {dollar}pm{dollar} 2.8 to 200.0 {dollar}pm{dollar} 1.5 ka. Thus, for the last three interglacial and the two intervening interstadial periods, sea level peaked at or after peaks in summer insolation in the high latitudes of the Northern Hemisphere, providing support for the Milankovitch theory. When combined with published Bahamian speleothem data, the Barbados coral data indicate that sea-level fall at the end of the penultimate interglacial, the last interglacial, and a subsequent interstadial period lagged behind the decrease in insolation by 5 to 10 ka.; The initial {dollar}deltasp{lcub}234{rcub}{dollar}U values of several Barbados corals are within error of the modern marine {dollar}deltasp{lcub}234{rcub}{dollar}U value; these data suggest that the marine {dollar}deltasp{lcub}234{rcub}{dollar}U value at 83 and 200 ka was within 2 per mil of the modern value. New methods for chemical seperation of primary uranium from carbonate-rich bulk sediments from the Caribbean Sea constrain the last-interglacial marine {dollar}deltasp{lcub}234{rcub}{dollar}U value to within 6 per mil of the modern value. Together, the coral and sediment data show no evidence that the marine {dollar}deltasp{lcub}234{rcub}{dollar}U value has differed from its modern value for the last 200 ka.