Page 423 - Global Tectonics

IMPLICATIONS OF PLATE TECTONICS 405

13.1 ENVIRONMENTAL between the circulating seawater and the hot basaltic
rock at ridge crests are thought to remove magnesium
CHANGE 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

13.1.1 Changes in sea level and predict that the Mg/Ca, SO 4 /Cl, and Na/K ratios in
seawater decrease during periods of high rates of for-
seawater chemistry mation of oceanic crust and hydrothermal activity.
Stanley & Hardie (1999) suggest that such changes

The sedimentary record in continental areas is charac- in seawater chemistry are reflected in the mineralogy
terized by marine transgressions and regressions due to of marine evaporites and carbonate sediments through-
changes in sea level throughout geologic time. One of out the Phanerozoic. They assume that a first order sea

the highest sea level stands occurred in late Cretaceous level curve may be used as a proxy for the rate of pro-
time when, for example, the very pure marine lime- duction of oceanic crust, and hence the variation in

stone, Chalk, was deposited throughout much of north- hydrothermal brine flux, throughout the past 550 Ma
west Europe. (Fig. 13.1). From this the temporal variation in the
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 C O S D M P Pm Tr J K Pg Ng

spots, and plumes provide plausible mechanisms to
High
resolve this problem. The water depth above oceanic
Sea level
crust formed solely by sea floor spreading is related to

the age of the crust (Section 6.4), younger crust occur-
ring at shallower depths. Such crust has an essentially Low
uniform thickness of 6–7 km (Section 2.4.4). However,
6
if this crust is thickened, as a result of enhanced igneous
activity above a hot spot or plume, the water depth will Aragonite + high Mg calcite 5
be shallower than that predicted by the age/depth rela- Mg / Ca 4
tionship. Exceptionally, as in the case of Iceland and the 3 Mg / Ca mole ratio

Azores, the volcanic edifice rises above sea level. Thus,
2
enhanced rates of sea floor spreading, hot spot or plume

activity can produce elevated ocean floor that will dis- Calcite 1
place the water upwards and cause a rise in sea level. 0
During the Cretaceous period, for example, the high sea 550 500 450 400 350 300 250 200 150 100 50 0 Ma
level stand might well be due to exceptionally high rates

of sea floor spreading and plume activity, as discussed A Calcite Aragonite Calcite A Seas
in Section 5.7.
Changes in the net rate of formation of oceanic Mg KCL evaporites Mg SO evap KCL evap Mg Evap
crust, as a result of changes in spreading rates and/or 4
the total length of actively spreading ridges, are a very Fig. 13.1 Variation in the Mg/Ca ratio in seawater,
effective way of changing the proportion of young, calculated by Hardie, 1996, from an assumed curve of
elevated ocean fl oor, and hence producing, in the long long term changes in sea level, and (below) summaries
term, changes in sea level. Variations in net accretion of the mineralogy of nonskeletal carbonates, and marine
rate also imply changes in the amount of igneous and evaporites, illustrating the correlation with the predicted
hydrothermal activity at spreading centers that will have changes in the Mg/Ca ratio in seawater during the past
implications for the chemistry of seawater. Interactions 550 Ma (based on figure 2 in Stanley & Hardie, 1999).