Changes in sea level and seawater chemistry

The sedimentary record in continental areas is characterized by marine transgressions and regressions due to changes in sea level throughout geologic time. One of the highest sea level stands occurred in late Cretaceous time when, for example, the very pure marine limestone, Chalk, was deposited throughout much of northwest Europe.

Major changes in sea level, of 100 m or more, are difficult to explain, except during ice ages, when large volumes of fresh water are locked up in land-bound ice sheets. However, for much of geologic time, there were no major glaciations, and yet there were major changes in sea level. The concepts of sea floor spreading, hot spots, and plumes provide plausible mechanisms to resolve this problem. The water depth above oceanic crust formed solely by sea floor spreading is related to the age of the crust (Section 6.4), younger crust occurring at shallower depths. Such crust has an essentially uniform thickness of 6-7 km (Section 2.4.4). However, if this crust is thickened, as a result of enhanced igneous activity above a hot spot or plume, the water depth will be shallower than that predicted by the age/depth relationship. Exceptionally, as in the case of Iceland and the Azores, the volcanic edifice rises above sea level. Thus, enhanced rates of sea floor spreading, hot spot or plume activity can produce elevated ocean floor that will displace the water upwards and cause a rise in sea level. During the Cretaceous period, for example, the high sea level stand might well be due to exceptionally high rates of sea floor spreading and plume activity, as discussed in Section 5.7.

Changes in the net rate of formation of oceanic crust, as a result of changes in spreading rates and/or the total length of actively spreading ridges, are a very effective way of changing the proportion of young, elevated ocean floor, and hence producing, in the long term, changes in sea level. Variations in net accretion rate also imply changes in the amount of igneous and hydrothermal activity at spreading centers that will have implications for the chemistry of seawater. Interactions between the circulating seawater and the hot basaltic rock at ridge crests are thought to remove magnesium and sodium from the seawater and to release calcium ions from the rock. It is also possible that the sulfate ion is removed from the water when it encounters the oxic conditions at or near the sea floor. These changes would predict that the Mg/Ca, SO4/Cl, and Na/K ratios in seawater decrease during periods of high rates of formation of oceanic crust and hydrothermal activity.

Stanley & Hardie (1999) suggest that such changes in seawater chemistry are reflected in the mineralogy of marine evaporites and carbonate sediments throughout the Phanerozoic. They assume that a first order sea level curve may be used as a proxy for the rate of production of oceanic crust, and hence the variation in hydrothermal brine flux, throughout the past 550 Ma (Fig. 13.1). From this the temporal variation in the

O

S

D

M

P

Pm

Tr

J

K

Pg

High

2 CT

550 500 450 400 350 300 250 200 150 100 50 0 Ma

High

2 CT

550 500 450 400 350 300 250 200 150 100 50 0 Ma

A

Calcite

Aragonite

Calcite

A

Mg

KCL evaporites

Mg SO 4 evap

KCL evap

Mg

Fig. 13.1 Variation in the Mg/Ca ratio in seawater, calculated by Hardie, 1996, from an assumed curve of long term changes in sea level, and (below) summaries of the mineralogy of nonskeletal carbonates, and marine evaporites, illustrating the correlation with the predicted changes in the Mg/Ca ratio in seawater during the past 550 Ma (based on figure 2 in Stanley & Hardie, 1999).

Mg/Ca ratio for seawater is calculated (Hardie, 1996). During the resulting periods of low Mg/Ca ratio, associated with high sea level stands, nonskeletal carbonates are composed of (low magnesium) calcite, and marine evaporites are characterized by late forming KCl (sylvite), and an absence of Mg salts. By contrast, the periods of high Mg/Ca ratio are characterized by non-skeletal carbonate deposits composed of high magnesium calcite and aragonite (a polymorph of calcite), and marine evaporites in which MgSO4 formed during the final stages of evaporation. The former periods have been termed periods of "calcite seas," and are thought to be associated with high pCO2 and high surface temperatures; i.e. a "Greenhouse Earth" such as that which probably characterized the Cretaceous. The periods of high Mg/Ca ratio have been designated as periods of "aragonite seas." These appear to correlate with times of low pCO2, and low surface temperatures, and include ice ages, i.e. an "Icehouse Earth".

Variations in pCO2 in the atmosphere in the geologic past are thought to have been largely due to the outgas-sing ofCO2 from volcanic activity. Thus eustatic changes in sea level, changes in seawater chemistry, and variations in the concentration of CO2 in the Earth's atmosphere in the past might all be related to variations in the rates of sea floor spreading and plume activity.

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