Oxygen Isotope Record of Ice Volume History: 100 M.Y of Glacio-Eustatic Sea Level Fluctuation: ABSTRACT

The reef-crest coral Acropora palmata from late Pleistocene reefs on Barbados has recorded the same global variations in oxygen isotopes as planktonic and benthonic foraminifera. Although the record of oxygen isotopes in Acropora palmata is discontinuous, it offers several advantages over the isotope records from deep-sea sediments: (1) the coral grows at water depths of less than 5 m; (2) the samples are unmixed; (3) specimens may be sampled from various elevations of paleo-sea level; and (4) aragonitic corals are suitable for 230Th/234U and He/U dating techniques. The latter advantage means that direct dating of the marine oxygen isotope record is possible. Oxygen isotope stage 5e corresponds to Barbados III, dated at 125,000 ± 6000 yr BP. Petrographic and geochemical evidence from five boreholes drilled into the south coast of Barbados indicates a major eustatic lowering (greater than 100 m below present sea level) occurred between 180,000 and 125,000 yr BP. The age and isotopic data suggest correlation of this change in sea level to Emiliani's oxygen isotope stage 6. Acropora palmata deposited at various elevations of sea level during oxygen isotope stage 6 vary by 0.11 ‰ δ18O for each 10 m of change in sea level. We further hypothesize a minimum drop of 2°C in the average temperature occurred during the regressive phase of oxygen isotope stage 6. These data indicate that temperature lowering of surface water near Barbados lagged behind a major glacial buildup during this time period. Using the δ18O vs sea level calibration herein derived, we estimate the relative height of sea stands responsible for Barbados coral reef terraces in the time range 80,000 to 220,000 yr BP.

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Oxygen-Isotope Analyses and Pleistocene Ice Volumes

Quaternary Research ◽

10.1016/0033-5894(84)90085-1 ◽

1984 ◽

Vol 21 (1) ◽

pp. 1-20 ◽

Cited By ~ 123

Author(s):

Alan C. Mix ◽

William F. Ruddiman

Keyword(s):

Isotopic Composition ◽

Oxygen Isotope ◽

Volume Change ◽

Past History ◽

Model Parameters ◽

Ice Volume ◽

Oxygen Isotope Record ◽

Isotope Record ◽

History Of ◽

Selective Preservation

The oxygen-isotope record from fossil foraminifera in deep-sea sediments is commonly used as a proxy for global ice volume. The linkage between δ18O and ice volume, however, is probably nonlinear. We have developed a simple numerical model of the isotopic response of the oceans to ice-volume change. The major features it simulates are (1) the changing mean isotopic composition of snow as a function of ice volume (colder snow temperatures forced by climate change and higher-elevation accumulation areas imply more negative mean δ18O); (2) the nonequilibrium isotopic composition of ice sheets (the past history of an ice sheet is integrated into its mean isotopic composition, which introduces a lag of isotopic “ice volume,” i.e., the measured δ18O record, scaled to ice-volume units, behind true ice volume); (3) selective preservation of isotopically more negative (colder, higher-latitude) ice (this geographic effect can selectively amplify or dampen the isotopic response to the ice-volume signal). We illustrate the response of our model to simple hypothetical ice-volume transitions of ice growth and ice decay. Sensitivity tests are illustrated for all model parameters. The results suggest that oxygen-isotope records reproduce the general patterns of ice-volume change fairly accurately. The foraminiferal isotope record, however, may misrepresent the true amplitude of the ice-volume signal and lag true ice volume by 1000 to 3000 yr.

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