Glacial Eustasy

As has been indicated earlier on this site, the role that the varying position of relative sea level played in forming the sedimentary record has become well known. However geologists have always had difficulty in separating the influence that tectonism has played in controlling basin fill, versus the influence of eustasy.

Suess (1906) first coined the concept of "eustastic" or worldwide sea level change. He was influenced by work of Charpentier and Agassiz. Agassiz (1, 2, 3) (1840) inspired by the ideas of Charpentier and Venetz-Sitten had recognized that all of northern Europe and North America was covered several times by ice sheets and that simultaneously the sea level had dropped (Dott and Batton 1976). Agassiz became the main proponent of the study of the sedimentary response to the Quaternary Ice caps and this field of study accelerated in both Europe and North America. Glacially induced eustasy was widely recognized, and the isostatic uplift of landmasses once covered by ice was traced using the raised beaches of the Northern hemisphere.

Suess (1906) was one of the first geologists to recognize the importance of movements of eustasy and their effect on the deposition of marine sediments. He described eustatic movements as being changes in the level of the strand, which were of equal height and which occurred on a worldwide basis. He recognized the difficulty of separating "relative" sea level (Chambers, 1848) from eustatic changes, and wrote a review on the subject which begins by citing Dante speaking in 1320 on the problems of sea level with respect to the curvature of the earth and the elevation of the land.

Suess (1906) described three means of recognizing changes in sea level which include (1) the different positions of the old strandline sediments deposited by ancient seas on the continent, (2) the paleo-bathymetry of sedimentary formations, and (3) the elevation of old strand lines with respect to the present coast. His review focused on explanations of changes in eustasy and the problems of measuring them. He described the repeated synchronous transgressions and regressions found in the sedimentary record from all over the world and expressed the opinion that these were independent of secular oscillations in the continental crust. Significantly, he postulated that the unconformities used by William Smith to demark formations were related to eustatic sea-level falls.

In 1920 by Milankovitch proposed an origin of the four major glacial events and their associated high frequency perturbations in climate for the Quaternary (Milankovitch, 1941). His hypothesis formalized the ideas Croll (1864) had suggested, proposing that variations in solar radiation onto the earth were driven by variations in the cyclic patterns in the earth's orbit and so influenced climate change. These relationships were further modified by Broecker (1966) and Berger 1976, 1978). This hypothesis has since been extrapolated to, and correlated with, variation in the oxygen isotopes. Thus the periodicity of increase in the amount of the heavy oxygen isotope 18 have been tied to and used as a proxy for the glacial period sea level lows predicted by the Milankovitch cycles. Variations in 18O/16O ratios derived from planktonic microfossils from the deep sea can be dated and have been related to glacial eustatic events (Broeker and Van Donk, 1970:Shackleton and Opdyke, in 1973 Fillon and Williams, 1983, among others). Similarly Mesolella et al. (1969), Steinen et al. (1973) and Matthews (1984a) have shown that changes in eustasy are recorded in coral reef terraces around Barbados during its tectonic uplift in the Pleistocene. These terraces would dated using the U/Th method. Evidence for the same eustatic and events can be seen in reef terraces dated by the U/Th in New Guinea (Bloom et al., 197 Chappel, 1974) and Indonesia (Chappel and Veeh, 1978). Matthews (1984b) has come up with an elegant possible solution in which he suggests using the oxygen isotope record from the last 100 Ma to dimension the relative size of the events identified by Vail et al. (1977), Vail and Hardenbol (1979) and Vail et al. (1984). This presupposes (1) that there have been glacial events as far back as 100 Ma, and (2) that the oxygen isotopes of skeletal remains of benthic fauna solely represent these events. However Abreu et al (1988) have gone on to develop identify the variation of oxygen isotopes through to the Cretaceous, while Miller et al 1998, Miller et al 1996 and Kominz, et al 1998 have worked on the use of the variation in oxygen isotopes through the Mid and Late Tertiary. Hardie et al (1986, and 1987) and Goldhammer et al (1987, 1990 and 1994) explain the cyclic character of the Mesozoic sedimentary record in the Dolomites of Italy have been ascribed to Milankovitch driven cycles in sea level while the same effects seen in the Mesozoic sedimentary record of are ascribed to these same orbital perturbations D'Argenio and Raspini of Naples University are major propopents of this process and have published widely on it. Lately Matthews and Frohlich (2002) have explained the sequence stratigraphic record of the Cretaceous and Jurassic, particularly on the Arabian Plate, as a response to orbital forcing.

Elegant as it is to build models of the earth and as compelling as the glacial eustatic model is to explain much of the cyclycity of the earth's sedimentary secton, one needs to pause and recognise that an outlandish proportion of the geologic is lost. As we see patterns it is easy to ascribe them to our pre-concieved models but when we do we should always think about and make some statement about their limitations.