How Does Mantle Convection Affect Eustatic Sea Level: Ocean Ridge Volume is Only Part of the Answer
Abstract
Consider "water world"...that is consider an Earth with a volume of water sufficient to completely cover all land masses and ocean basins. A period of rapid convection in the mantle will not increase eustatic sea-level...being measured as the radius of the surface of the ocean. Rapid convection leads to an increase in ridge volume and decrease in the volume of the "ocean basins." However, the volume of material moving into the "ocean basins" must come from elsewhere within the mantle as the volume of the mantle is, to first order, a constant. Therefore, the ocean ridge volume increase is equaled by a volume decrease elsewhere in the solid Earth and the overlying sea-level is unchanged. We do not have a "water world"...the continents rise above the level of the oceans. Part of the heated mantle material comes from underneath the continents and moves to the ocean ridges. Consequently, during ocean ridge growth there must be a decrease of the average elevation of the continents. That we have been finding strong worldwide synchronicity in rises/falls of relative sea-level as recorded by local continental stratigraphic records is no surprise as the increase in ridge volume is synchronized with lowering of continental masses. That we are finding that the amplitudes of the relative increases/decreases are not constant for different localities is, at least in part, due to the fact that the convection responsible for increasing an ocean ridge volume does not draw equally from underneath of all continents or all parts of a given continent. I say "to first order" above because I am considering convection to be a nearly constant volume process. In steady state/rate convection the volume of the convecting mantle must be very nearly constant, varying only by exchange of material with the lithosphere, oceans and atmosphere. Over the time scale of changes of rates of convection, there is a nearly constant rate of heat transfer at the core/mantle boundary and nearly constant rate of heat generation within the mantle. Therefore, the Earth must expand during slow convection as the heat builds up and mantle materials expand. The Earth must then contract during rapid convection as the heat/volume is relatively rapidly lost at the surface. Eustatic sea-level of "water world" would record these increases/decreases in the volume of the underlying solid Earth. In fact, the eustatic sea-level would increase during slow convection, as the Earth expanded, and decrease during rapid convection, as the Earth contracted...regardless of the ocean ridge volume and continental elevation change. Even in the "non-water world" case, the present day case, the average radius of the Earth must react as in the paragraph above to periods of slow and rapid convection. Thus expansion/contraction alone would result in eustatic sea-level falls/rises during slow/rapid convection as the expansion/contraction increased/decreased the volume of the ocean basins. Note that the eustatic sea-level falls/rises of this effect are in phase with those due to the changes in continental elevation and ocean ridge volume. Convection driven eustatic sea-level changes are not solely caused by ocean ridge volume changes.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2009
- Bibcode:
- 2009AGUFM.T43D2153K
- Keywords:
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- 8011 STRUCTURAL GEOLOGY / Kinematics of crustal and mantle deformation;
- 8120 TECTONOPHYSICS / Dynamics of lithosphere and mantle: general;
- 8199 TECTONOPHYSICS / General or miscellaneous