Global System edit
Increased rates of sea-floor spreading (i.e. the expansion of the mid-ocean ridge) has caused global (eustatic) sea-level to rise over very long timescales (millions of years)[1].
The high sea levels that occurred during the Cretaceous period (144-65 Ma) can only be attributed to plate tectonics since thermal expansion and the absence of ice sheets by themselves cannot account for the fact that sea levels were 100-170 meters higher than today[2].
Increased sea-floor spreading means that the hot young crust at the mid-ocean ridge will form at a faster rate than it can be destroyed at subduction zones. The mid-ocean ridge will then expand and form a broader ridge, taking up more space in the ocean basin and causing sea levels to rise[2].
Sea-level change can be attributed to other factors (thermal expansion, ice melting). Over very long timescales, however, it is the result of changes in the volume of the ocean basins which are, in turn, affected by rates of sea-floor spreading along the mid-ocean ridges[3].
Seawater Chemistry edit
Mid-ocean ridges are global scale ion-exchange systems[4].
Rapid spreading rates will expand the mid-ocean ridge causing basalt reactions with seawater to happen faster. The Magnesium/Calcium ratio will be lower because more magnesium ions are being removed from seawater and consumed by the rock, and more calcium ions are being removed from the rock and released to seawater. A lower Mg/Ca ratio favors the precipitation of Low-Mg calcite polymorphs of calcium carbonate (calcite seas)[4].
Slow spreading at mid-ocean ridges has the opposite effect and will result in a higher Mg/Ca ratio favoring the precipitation of aragonite and High-Mg calcite polymorphs of calcium carbonate (aragonite seas)[4].
Experiments show that most modern High-Mg calcite organisms would have been Low-Mg calcite in past calcite seas[5], meaning that the Mg/Ca ratio in an organism's skeleton varies with the Mg/Ca ratio of the seawater in which it was grown.
The mineralogy of reef-building and sediment-producing organisms is thus regulated by chemical reactions occurring along the mid-ocean ridge, the rate of which is controlled by sea-floor spreading[5].
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- ^ Church, J.A.; Gregory, J.M. Sea Level Change. pp. 2599–2604. doi:10.1006/rwos.2001.0268.
- ^ a b Miller, Kenneth G. (2009). Encyclopedia of Paleoclimatology and Ancient Environments. Springer, Dordrecht. pp. 879–887. doi:10.1007/978-1-4020-4411-3_206.
- ^ Kominz, M.A. Sea Level Variations Over Geologic Time. pp. 2605–2613. doi:10.1006/rwos.2001.0255.
- ^ a b c Stanley, Steven; Hardie, Lawrence (February 1999). "Hypercalcification: Paleontology Links Plate Tectonics and Geochemistry to Sedimentology" (PDF). GSA TODAY. 9 (2): 1–7.
- ^ a b Ries, Justin B. (2004-11-01). "Effect of ambient Mg/Ca ratio on Mg fractionation in calcareous marine invertebrates: A record of the oceanic Mg/Ca ratio over the Phanerozoic". Geology. 32 (11). doi:10.1130/g20851.1. ISSN 0091-7613.