Haq et al. eustatic sea level curve: implications for sequestered water volumes
Journal of Geology, v.100, p.703-715
Abstract
The Eustatic sea level curve of Haq and co-workers differentiates a long and a short term signal. The mechanism that controls the long term curve is primarily variation in the volume of the ocean basins, while the short term curve reflects changes in ocean water volume. Only glacial eustasy operates at the frequency and with sufficient magnitude to control the short term curve. At each increment of time the volume of water sequestered from the ocean basins to account for the difference between the long and short term curves is computed, incorporating both hypsometric and water loading factors. An amount equivalent to that of present day Antarctica was sequestered at an approximately 5 Ma frequency since 145 Ma, but direct geological evidence of Late Jurassic to Early Tertiary continental ice of this equivalent magnitude is lacking. Because the existing ice coverage of Antarctica inhibits direct observation, we use the known fractionation affects of sequestration of water on the oxygen isotopic composition of sea water as a direct proxy for evidence of removal of large volumes of water from the oceans. Using fractionation factors estimated for different modes of water extraction of between +0.4” and +1.6” for a drop in sea level of 120 to 130 m, teh changes in the isotopic composition of sea water oxygen are retrodicted using the volumes of sequestered water computed from the Haq et al. eustatic sea level curves. The retrodicted oxygen isotopic compositions for the interval between 60 and 30 Ma are compared with measured values of planktonic formanifera from several DSDP sites. The comparison reveals virtually no correspondence between the retrodicted and observed ¸18O records. This explicit test of the implicit requirements of the Haq et al. sea level curve indicates that the magnitudes and very existence of many of the short term eustatic oscillations are unsubstantiated. We then compute the coastal onlap history implicit in the long term curve, which shows many of the major shifts recorded by the coastal onlap curve of Haq et al. but lacks the highest frequency signals. We infer that the observed coastal onlap curve is dominated by the long term curve and that much more subtle changes in sedimentation and or subsistence are responsible for the sequences recorded by the coastal onlap curve. The short term may in part be an artifact of the assumed correlation of onlap with times of increased rate in sea level rise.
LEFT: The area of ice implied by the 3rd-order eustatic sea-level curve of Haq et al (1987). Left axis legend reads "Area (x 107 km2)"; right axis legend reads "Percentage of present ice area". Note that this implies that for many intervals in the Cretaceous and early Cenozoic, there were ice-sheets larger than those of the Recent. This is not borne out by the geological record.
