Sea-Level Change (1990) / Chapter Skim
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9 Could Possible Changes in Global Groundwater Reservoir Cause Eustatic Sea-Level Fluctuations?
9 Could Possible Changes in Global Groundwater Reservoir Cause Eustatic Sea-Level Fluctuations?
Pages 161-170
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From page 161... ...
Discounting sediments that currently reside 2000 m below sea level, and including only sands, sandstones, and carbonates as part of the aquifer system, there is about 25 x 106 km3 of pore space in the groundwater system, which might respond to changing inputs and outputs; this corresponds to a change in sea level of over 76 m without or 50 m with isostatic adjustment. The time scale for effecting major changes in the volume of the groundwater reservoir is probably 104 to 1os yr; possible mechanisms include (1)
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From page 162... ...
This study investigates the potential reservoir capacities of the major sedimentary bodies of the continents now and in the past, and the role they may have played in affecting global sea level. THE PRESENT DAY HYDROLOGIC CYCLE AND RUNOFF The hydrologic cycle is the process of global water circulation by which evaporation of water results in precipitation back onto the ocean or onto land areas in the form of rain or snow.
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From page 163... ...
These three rock types make up nearly 93 percent of the total volume of cratonic platform sediments, the remainder being mostly volcanics and evaporites. Geosynclinal Sediments Geosynclinal sediments, occupying elongate regions of the Earth's continental crust, usually represent sites of thick sediment accumulation on passive or active continental margins subsequently exposed by uplift and erosion as a result of plate tectonic processes.
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From page 164... ...
Because the coastal plain and continental shelf sediments were the least well known in terms of composition, Ronov (1982) assumed that the distribution of rock types would closely approximate the general composition of the continental cratons and platforms and hence used identical rock type percentages as shown on Table 9.2.
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From page 165... ...
The average elevation of the continents is less than 1 km above sea level, but the average thickness of the major sedimentary bodies that reside on the continents is about 3 km (see Table 9.24. Since most sediment on the continental blocks lies below sea level, except possibly in some
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From page 166... ...
If this pore space were initially empty but then filled by infiltration of precipitation, a further global drop in sea level of over 76 m would result, but would be reduced to 50 m as isostatic adjustment occurred. In reality, it is impossible for the groundwater table to be reduced to sea level even if the hydrologic cycle were to cease; capillary forces alone would cause the water table to be some finite distance above sea level.
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From page 168... ...
Porosity Range 10% 20% 30% 40% 5.578 11.167 16.755 413 1 827 1241 4.962 9.945 14.923 367 736 1105 4.35 8.72 13.085 322 646 969 3.736 7.489 11.237 277 555 832 3.12 6.253 9.383 231 463 695 2.501 5.012 7.521 185 371 557 1.879 3.767 5.651 139 279 418 1.255 2.516 3.775 93 186 279 0.629 1.26 ~1.891 47 93 140 22.349 1655 19.906 1 474 17.453 1293 14.989 1110 12.516 927 10.033 743 7.537 558 5.035 373 2.522 187 cat Community Climate Global Atmospheric Circulation Model at the National Center for Atmospheric Research for several different idealized paleogeographies have suggested that the global hydrologic cycle may vary considerably depending on the distribution of land, sea, and mountain ranges. In these experiments, runoff varied by an order of magnitude, with the present-day situation being close to the maximum.
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From page 169... ...
Furthermore, the late Cenozoic fluctuations in sea level in response to glaciation and deglaciation have resulted in modification of the continental shelves and coastal plains, offloading sediment from those regions into the continental slopes and rises and the abyssal plains. The potential groundwater reservoir that exists today may well represent the minimum that has existed for much of geologic history.
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From page 170... ...
The surficial sediments at these times were mostly young and highly porous, and may have had infiltration rates significantly greater than those of today. Clearly, changes in the global volume of groundwater with time are a possible mechanism for the changes in sea level on the order of one or a few million years as postulated by seismic stratigraphers.
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