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Could Possible Changes in Global Groundwater Reservoir Cause Eustatic Sea-Level Fluctuations? 9 WILLIAM W. HAY and MARK A. LESLIE University of Colorado ABSTRACT The total pore space in sediments is about 116 x 106 km3; this volume constitutes a water reservoir significantly larger than the estimated volume of 24 x 106 km3 for present-day ice caps and glaciers. 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) changes in overall precipitation on the continents, (2) temporal changes in the volume of pore space in sediments on the continents, and (3) changes in the infiltration and discharge rates with time. Because significantly larger volumes of porous sediments may have been present on the continents at several intervals in the geologic past, there may have been a positive feedback between larger porous sediment volumes and greater infiltration rates to amplify the sea-level signal of climatic changes. Although this possible mechanism for sea-level change cannot be demonstrated to have happened, it implies episodic diagenetic changes and periodic high fluid flows, which would affect hydrocarbon migration; studies of these mechanisms might serve as independent tests of the hypothesis. INTRODUCTION The problem of explaining geologically rapid eustatic sea-level changes on an Earth without major ice caps is vexing because it has been difficult to understand where the water that left the ocean was stored. One possible storage reservoir that has not been previously investigated is the pore space in sediment on the continental blocks. Although there is general agreement that the volume of ]61 water in the world ocean is about 1370 x 106 km3 (=13,700 x 102g; Korzun, 1978, gives 1338 x 106 km3, which may be the most authoritative figure), estimates of the volumes of water in other reservoirs differ widely. Estimates of the volume of ice currently on the surface of the Earth vary from 30 x 106 km3 (Gavrilenko and Derpgol'ts, 1971) to 200 x 102g (GaITels and MacKenzie, 1971), with 24 x 106 km3 cited by L'vovich (1974) and Korzun (1978~. (For H2O, 1 km3 = 10'5 cm3 = 10~5 g; 106 km3 = 102~ g.) Esti

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162 mates of the amount of water in lakes and rivers are even more variable, from 1 0 x 106 km3 (Gavrilenko and Derpgol'ts, 1971), which is clearly too high by almost two orders of magnitude, to 0.03 x 106 km3 (Garrels and MacKenzie, 1971), with intermediate values being given by L'vovich (1974; 0.29 x 106 Kim. Estimates of volumes of water in the pore space of sediments are generally higher than the volume currently in ice, from 330 x 106 km3 (Garrels and MacKenzie, 1 97 1 ~ to 1 90 x 106 km3 (Gavrilenko and Dergopol'ts, 1971) to 64 x 106 km3 (L'vovich, 1974) to a minimum of 23.4 x 106 km3 esti- mated for groundwater by Korzun (1978~. From their calculations of sediment volumes and porosities, Southam and Hay (1981) indicate a total pore volume in sediments of 116 x 106 km3. Clearly, the pore space in sediments currently contains a large enough volume of water whereby any large-scale changes in the amount of pore space filled with water could affect sea level. The question is whether changes of the magnitude required are possible. In the equilibrium situation, generally assumed in hydro- logic studies, the amount of recharge to the groundwater reservoir is balanced by the discharge of water to the rivers and oceans so that the groundwater reserves remain constant, and there is no net change in the elevation of the groundwater table. However, if either inflow or outflow changes relative to the other, then the volume of water in the groundwater reservoir will change to compensate for the change in supply or release; an increase in groundwa- ter recharge relative to groundwater discharge will result in an increase in the volume of water in the reservoir and a rise of the groundwater table; and a decrease in recharge relative to discharge will result in a decrease of water in the reservoir and a fall of the groundwater table. Because the amount of water residing in the groundwater reservoir is related to the amount of water available to the oceans, large increases or decreases in the groundwater reservoir may raise or lower global sea level. By gradually filling up the reservoir with infiltrated meteoric water or by releas- ing groundwater to the oceans through runoff, the sedi- ments on the continents may play an important role in controlling the amount of water that is available to the world oceans. This study investigates the potential reser- voir 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 pre- cipitation back onto the ocean or onto land areas in the form of rain or snow. The water that falls on land is later WILLIAM W. HAY AND MARK A. LESLIE TABLE 9.1 Annual Global Water Balance (modified after data from Budyko, 1974; L'vovich, 1974) Element of Water Balance Volume (km3) Percent of Total Precipitation, land Precipitation, ocean Evaporation, land Evaporation, ocean Runoff, surface Runoff, subsurface 111 x 103 410 x 103 67 x 103 454 x 103 30 x 103 14x 103 21.3 78.7 12.9 87.1 68.2 31.8 Note: Total precipitation equals total evaporation. Runoff coefficient (proportion of precipitation that becomes runoff) = 39 percent. Subsurface runoff coefficient (proportion of precipita- tion that becomes subsurface runoff) = 12 percent. returned to the oceans through reevaporation, which may be direct or through transpiration, precipitation, or runoff (see Table 9.1~. Over 111 x 103 km3 of water is estimated to fall annually on land in the form of precipitation. About 67 x 103 km3, or 60 percent of the total precipitation on land, is evaporated, and 44 x 103 km3 is left on the land surface in the form of potential runoff. Of this amount, 30 x 103 km3, or 69 percent, of potential runoff is stored temporarily as snow and ice or is shed directly by the land in the form of rivers or glaciers, and is commonly referred to as surface runoff. The other 14 x 103 km3, or 31 percent, of the potential runoff, is absorbed into the pore spaces and fissures of the sediments and rocks of the continent and moves downward to the ground- water system. Here, it becomes part of the groundwater reservoir, eventually to be discharged and reappear in marshes, streams, and rivers; i.e., about two-thirds of the flow of rivers measured at the points where they enter the sea is surface runoff and about one-third of the flow is groundwater discharge. The global average residence time for shallow groundwater that is discharged into rivers is not at all well constrained, but has been estimated by L'vovich (1974) to be about 330 yr. Although there are some notable exceptions, such as the Snake River basalts, we consider the groundwater system to be limited essentially to the pore space in the major sedimentary bodies on the continental blocks. The water-bearing sediments may act both as conduits for the transmission of water through the Earth's crust and as long term storage reservoirs for the groundwater (Todd, 198G). Consequently, the potential variations in the amounts of water that can be stored in, transported through, or re- leased from the groundwater reservoir depends on the physical characteristics of these sedimentary bodies.

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COULD POSSIBLE CHANGES IN Gf OBOE GROUNDWATER RESERVOIR CAUSE EUSTATIC SEA-LEVEL FLUCTUATIONS? ~ 63 SEDIMENTARY RESERVOIRS OF THE CONTINENTAL BLOCKS The sites of residence of sediments on the Earth's continental blocks can be grouped into three major catego- nes: cratonic, geosynclinal, and the coastal plains and continental shelves (Southam and Hay, 1981~. The sedi- mentary bodies at these sites cover approximately 80 per- cent of the land surface (Ronov, 1982) and have a total volume of over 790 x 106 km3 (Southern and Hay, 1981; Ronov and Yaroshevsky, 1977~. Their dimensions and characteristics are summarized in Table 9.2. Cratonic Sediments Cratonic sediments fringe the stable shield areas of the continental interior and the adjacent continental platforms. Because the central shield areas contain little or no sedi- ment cover, the majority of the cratonic sediments reside on the peripheral continental platforms (Southam and Hay, 19811. Using data compiled by Gilluly et al. (1970) and Ronov and Yaroshevsky (1977), Southam and Hay (1981) calculated the combined total area of the world's cratonic shields and cratonic platforms to be 96.3 x 106 km2, the cratonic shields occupying an area of 29.4 x 106 km2 and the platforms having an extent of 66.9 x 106 km2. How- ever, because most of the sedimentary cover resides on the peripheral areas of the platforms, they estimated that only 55 x 106 km2 of the total cratonic area contains appreciable sediment cover. They assumed the average global thick ness of the cratonic sediments to be 3 km so that the total volume is 165 x 106 km3. This accounts for almost 21 percent of the total sedimentary volume on the continental blocks (see Table 9.2~. Ronov (1982) calculated the percentages of the major rock types occurring on the continental blocks based on volume estimates of the different rock types of North America, Europe, and the Soviet Union (see Table 9.2~. He estimated that almost half of the cratonic platform sediments are clays and shales with carbonates and sand- stones almost 25 percent each. 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 conti- nental margins subsequently exposed by uplift and erosion as a result of plate tectonic processes. Geosynclines have been estimated by Ronov and Yaroshevsky (1977) to cover an area of 59 x 106 km2 and contain sediments to an average depth of 9 km. They represent over 67 percent of the sediment found on the continental block. Ronov (1982) compiled percentage estimates of the main rock types found in the Geosynclines as shown in Table 9.2. The relative proportions of 40.9 percent clays and shales, 19.2 percent carbonates, and 19.2 percent sands TABLE 9.2 Average Dimensions, Porosity, Pore Volumes, and Composition of the Major Continental Sedimentary Reservoirs (modified after data from Ronov and Yaroshevsky, 1977; Southam and Hay, 1981; Ronov, 1982) Sedimentary Reservoir Sediment Estimated Pore Volume and Percentage of Major Rock Types Reservoir Area Thickness Volumea Volumeb Porosity Volume Shale Sandstone Carbonate Volcanic Other Cratonic 55 3 165 157 20% 31.6 76.4 36.8 407.26 4.4 platforms 46.3% 22.3% 24.3%4.4% 2.7% Geosynclines 59 9 531 422 13% 54.9 217 102 102108.8 1.06 40.9% 19.2% 19.2%20.5%C 0.2% Passive margin, 31 3 95 91 shelves, and coastal plains Total 146 5.4 791 670 15.6% 20% 18.2 Note: Thickness is in km, areas are in 106 km2, and volumes are in 106 km3. 44 21 46.3% 22.3% 23 24.3% 4.2 2.5 4.4% 2.7% aIncludes volcanic rocks. bMinus volcanic fraction; Ronov (1982) assumed the passive margin shelves and coastal plains to have a composition equivalent to the cratonic sediment. CA compromise between 19.4% (Ronov, 1982) and 21.9% (Ronov and Yaroshevsky, 1977~.

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164 and sandstones are very similar to those of cratonic sedi- ments, but together they account for only 79 percent of the sedimentary volume of the geosynclines; the remainder is almost entirely volcanics. Coastal Plain and Continental Shelf Sediments Most of the world's coastal plains and continental shelves form the periphery of the continental blocks along the passive trailing margins of the continents formed by the breakup of Pangea. They are underlain by sediment that has been eroded from the continental interior. Southam and Hay (1981) calculated the total area of the coastal plains and continental shelves to be 31.8 x 106 km2. The sediments form a wedge that is thin inshore and thickens offshore. Emery and Uchupi's (1972) map of sediment thickness shows an average maximum of 6 km for the shelves. From this measure, Southam and Hay (1981) estimated the average thickness to be 3 km and hence the sediment volume to be 95.4 x 106 km3, representing ap- proximately 12 percent of the total sediment on the conti- nental blocks. Because the coastal plain and continental shelf sedi- ments 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. FIGURE 9.1 Porosity versus burial depth for sandstone, limestone, and shale. (Af- ter Baldwin and Butler, 1985~. WILLIAM W. HAY AND MARK A. LESLIE SEDIMENTS AS HYDROLOGIC RESERVOIRS Contained within these bodies of sedimentary material is a substantial volume of pore space (Table 9.21. The pore space is not equally distributed between the different rock types but is mostly in the aquifers, i.e., rock bodies that are sufficiently permeable and porous such that they are able to yield large quantities of groundwater (Todd, 19803. According to Muskat (1937), Manger (1963), Morris and Johnson (1967), and others, the most common aqui- fers are unconsolidated gravels and sands, sandstones, and limestones. Rocks of lesser permeability and porosity may act as barriers to groundwater flow; they are termed "aquifuges" if they neither store nor transmit water, "aquicludes" if they store but do not transmit water, or "aquitards" if they store water but transmit only small amounts of water over long periods of time (Davis and DeWiest, 19661. In this chapter, we consider all clays and shales and all volcanic rocks to be aquifuges and assume that they do not participate in the transmission or storage of underground water. In the potential aquifers, the sands and carbonates, the permeability and porosity of the rocks decrease steadily with depth of burial and age (Chilingar, 1964; Maxwell, 1964; Choquette and Pray, 19701. This is due chiefly to the increases in lithostatic pressure and temperature with depth, both of which act to reduce pore space and close off the interconnecting pore throats. A number of other fac POROSITY (percent) o 2 - A - I ~ 4 cr m 6 8 1 30 80 60 < p 20 20 0 A//// Maxwell, 1964 (sandstone envelope) Pryor, 1973 (sand) Sclater-Christie, 1980 (ss) Schmoker-Halley, 1982 (Is) Baldwin-Butler, 1985 (shale envelope) .~ I I I I l 0 20 40 60 SOLIDITY (percent) 80 100 o 2 I 4 6 8

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COULD POSSIBLE CHANGES IN GLOBAL GRouNDwaTER RESERVOIR CAUSE EUSTATIC SEA-LEVEL FLUCTUATIONS? ~ 65 tors also act to reduce porosity and permeability with depth, such as the increase in quartz solubility and clay mineral authigenesis; however, these are all related to and dependent upon increased temperatures and pressures (Blatt et al., 19721. Using porosity versus depth data from a number of studies (Dickinson, 1953; Maxwell, 1964; Baldwin, 1971; Pryor, 1973; Sclater and Christie, 1980; Schmoker and Halley, 1982), Baldwin and Butler (1985) constructed general sediment compaction curves for the three major rock types (Figure 9.1~. Pryor (1973) reported that the porosities of Holocene river point bar and beach and dune sands range from 41 percent to 49 percent, respectively. Sclater and Christie (1980) studied the subsurface sand- stones of the North Sea, giving porosity-depth values up to a depth of 10 km. Maxwell (1964) studied the porosity characteristics of Paleozoic and Cenozoic quartzose sand- stones; his data cover a wide range of values and vary up to 25 percent at any given depth, producing the "sandstone envelope" values of Baldwin and Butler (1985~. The sand- stone curve of Sclater and Christie (1980) is almost the midline of the Maxwell sandstone envelope values of Baldwin and Butler (1985~. Hence, we consider the Sclater and Christie curve to approximate the average porosity of sandstones for any given depth up to 5 km (see Figure 9.11. The projection of the Sclater-Christie curve to the surface intersects Pryor's range of values for surficial sands at the lower end of the porosity range, at approximately 45 percent. Sands retain most of their original porosity down to a depth of 1 km. Porosities of approximately 48 percent at the surface show little change for the initial 100 m of burial, and then begin to decrease slightly with depth: to 45 percent at 300 m and 37 percent at 1 km. Porosities decrease rapidly at burial depths greater than 1 km; poros- ity values are 5 percent or less at depths approaching 10 km. Schmoker and Halley (1982) studied the porosity of limestones in south Florida and plotted porosity-depth values for depths to 5.5 km. They indicated that lime- stones have high initial porosities (over 40 percent) that decrease in much the same way as do sandstones, but limestones exhibit 5 to 10 percent lower average porosities than sandstones at any given depth. POTENTIAL WATER-BEARING CAPACITY OF THE MAJOR SEDIMENT RESERVOIRS Using the values for porosity cited above, we have calculated the possible ranges of pore-space volume ver- sus depth for the sandstone and carbonate (aquifer) frac- tion of the major sedimentary bodies to estimate their water-storage capacity, i.e., the amount of water that might be withheld from the oceans as a result of temporary storage. The available pore volume can be divided by the area of the ocean basins (325 x 106 km2) to give the hypothetical sea-level change that would occur if all of the pore space were empty and then filled with water, or vice versa. If the sandstone and carbonate fractions of the global cratonic sediments have an average porosity of 20 percent, the available pore space is 15.4 x 106 km3 and the potential sea-level change is +47 m. Considering only the aquifer fraction of the geosynclines (the sandstones and carbon- ates) the total pore volume is 26.5 x 106 km3, equivalent to a potential sea-level change of +81.5 m. The sandstone and carbonate fractions of the shelf and coastal plain res- ervoir have a volume of 44.5 x 106 km3 and, if assumed to have an average porosity of 20 percent (Atwater and Miller, 1965), have a combined pore space volume of 8.9 x 106 km3, for a potential sea-level change of +27.4 m. WATER-BEARING CAPACITY AND CONTINENTAL ELEVATION The total water-bearing capacity of the aquifers on the continental blocks is thus 50.8 x 106 km3, enough to change sea level by +156 m. However, this situation is hypotheti- cal, and in it, all the sandstones and limestones are ideal aquifers, possessing equal porosities, permeabilities, and other hydrological properties and forming a single ho- mogenous reservoir, capable of responding to the com- plete filling-up or emptying of their pore spaces with water, thereby raising or lowering global sea level. This scenario might be possible if all of the sediment bodies of the continents were to reside above sea level so their pore spaces were not permanently filled with water. This is obviously not the case; the continental shelf sediments are at present submerged and saturated with water while the adjacent coastal plain sediments are slightly above sea level and have a groundwater content reflecting the local climate. The submerged continental shelf sediments can- not store more water than they do at present, but they could release water if sea level were to fall. Coastal plain sediments could store more water if sea level were to rise and could release water if sea level were to fall. Clearly, sediments in this geologic setting can play a modulating role in sea-level changes by releasing water as sea level falls and filling with water as sea level rises, but the maximum possible effect is equivalent to less than 30 m of sea-level change. 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 continen- tal blocks lies below sea level, except possibly in some

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166 interior basins enclosed by high basement, it is perma- nently saturated with water. Only those sediments that lie above sea level can be potentially filled or emptied of groundwater, and only the aquifers are able to absorb, store, and transmit water through their pore spaces and thus participate in the process. However, the sediments above sea level are younger than average, hence less compacted and more porous. Specifically, the porosity and depth curves for sandstones of Sclater and Christie (1980) and curves for limestone of Schmoker and Halley (1982) suggest that average porosities of 30 to 40 percent are reasonable for such sediments buried to depths of 1 km or less. A HYPOTHETICAL MODEL OF CONTINENTAL ELEVATION AND SEA-LEVEL CHANGE The potential water-bearing capacity of sediments on the continental blocks and the possible effect on world- wide sea level can be evaluated as a model taking into account continental elevation and sea level. Because the present-day average elevation of the continents excluding ice-covered Antarctica is approximately 75Q m (Southam and Hay, 1981), a lowering of sea level to the current global shelf break would add another 200 m to the conti- nental elevation, so that the average continental elevation would be almost 1000 m above sea level. Assuming the sediments to be randomly distributed, and assuming a higher than average porosity of 40 percent for the near-surface sediments, this average elevation would indicate that over 24.7 x 106 km3 of pore space would reside above sea level in the major sedimentary aquifers at a sea-level stand 200 m below that of today. 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. Because the water in the upper part of the groundwater reservoir, i.e., that participating most actively in aquifer flow, is fresher than sea water, it is lighter, and therefore its sur- face must be above sea level. A lens of freshwater, being lighter, would of course depress the freshwater-saltwater interface in the groundwater system, and cause discharge of saltwater, which must eventually return to the ocean. Although this complication is important for the freshwa- ter-saltwater mass balance, it is immaterial in the discus- sion of sea level and will not be discussed further here. Figure 9.2 shows the effects of varying the elevation of the groundwater table to simulate different degrees of satura WILLIAM W. HAY AND MARK A. LESLIE lion and thereby to estimate the different volumes of un- saturated sediment that might respond to the introduction of infiltrated groundwater. For example, assuming an average continental elevation of 1000 m, and an average groundwater table of only 200 m elevation above sea level, there is an average of 800 m of unsaturated sediment above the groundwater table. A 40 percent porosity in these sediments would yield a total aquifer pore volume of 19.9 x 106 km3 that, if filled with water, could lower sea level initially by more than 61 m, or 40 m after isostatic adjustment. RESIDENCE TIMES We can estimate a residence time for water in the aqui- fer system by assuming that there is no subsurface dis- charge to the rivers and oceans, only surface infiltration into the empty sedimentary reservoirs until they are filled. As shown in Table 9.1, the present annual volume of water Precipitated on land is 111 x 106 km3/yr, but only 12 percent of this, or 13.5 x 106 km3/yr, infiltrates into the subsurface reservoir. Figure 9.3 shows residence times in years obtained by dividing the volume of pore space con- tained within the aquifers by the annual infiltrate volume; the filling times vary from about a 100 to a 1000 yr. This situation is not realistic because there is always subsurface discharge out of the groundwater reservoir to the rivers and oceans to compensate for the incoming infiltrate re- charge, and the rate of discharge must increase with in- creasing hydrostatic head. From this we can guess that the length of time required to fill or empty a groundwater reservoir after a step function change in the global hydro- logic cycle would probably be on the order of tens of thousands to hundreds of thousands of years. MECHANISMS FOR CHANGING THE VOLUME OF GROUNDWATER It is obvious that groundwater levels depend on cli- matic conditions because climate determines the amount of precipitation that will fall onto the continents. High rates of precipitation onto the continents will increase the volume of water that infiltrates the groundwater reservoir and subsequently will cause the groundwater table to rise. An increase in the volume of water held by the continents means that there is less water available to the oceans and the sea level drops. Similarly, decreased precipitation onto the continents means decreased volumes of water available to the groundwater reservoir, resulting in lower: ing of the groundwater table; hence, less water is retained by the continents, more water becomes available to the oceans, and the sea level rises. Consequently, changes in the climate regimes of the Earth over time can have an

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168 900 800 700 600 500 FIGURE 9.3 Pore 400 volume (lO6 km3) of sediments residing above groundwater table and time (years) required to 300 fill at an infiltration rate of 13.5 x 103 km3/yr (assuming no discharge). Pore 200 volumes are in up per left-hand portion of each box, and . . times are In lower 100 r~ght-hand portion of each box. effect on sea level by storing water in the subsurface continental reservoir or releasing water to the oceanic reservoir through subsurface runoff. Presumably, any long-term change in the amount or temporal distribution of precipitation on land will induce some change in the size of the groundwater reservoir. The major question is whether global or extensive changes in the hydrologic cycle do occur in such a way as to signifi- cantly alter the amount of precipitation on land or the way in which it is temporally distributed. Experiments by W. W. Hay, E. J. Barron, and S. Thompson using the numeri WILLIAM W. HAY AND MARK A. LESLIE Thickness of Variable Sandstone and Carbonate Fraction Groundwater Lens (meter) 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 sug- gested that the global hydrologic cycle may vary consid- erably 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. Clearly, if conditions did change so that runoff became an order of magnitude less than at present and the climate remained stable for many thou- sands of years, the groundwater reservoir would become

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COULD POSSIBLE CHANGES IN GLOBAL GROUNDWATER RESERVOIR depleted and a net transfer of water to the sea would occur. It should be noted that these model experiments assumed an atmosphere with present-day composition (i.e., rela- tively low CO2 concentrations). Other compositions might change the hydrologic cycle significantly; specifically, with higher CO2 content, the amount of precipitation on land might increase significantly. Evaporation rates would also be higher, but it is not at all clear what would happen to infiltration rates under such conditions. If rainfall on land were concentrated into relatively short periods rather than being more evenly distributed throughout the year, more instantaneous runoff would occur and infiltration of the groundwater reservoir would be- come less effective. However, it is not clear whether this could produce a change in the volume of the groundwater reservoir large enough to affect sea level. A second possibility for changing the volume of ground- water lies in the temporal variations in the pore space contained in aquifers above sea level. Ronov's (1982) data have been analyzed by Hay and Wold (1986) who noted significant variations in the abundances of both shallow-water carbonates and nonmarine rocks with time. Such sediments formed in abundance in the mid-Paleo- zoic, late Paleozoic-early Mesozoic, and mid-Cretaceous. At these times their abundance was more than twice that of such sediments at the present time. The present is a peculiar moment in the history of the Earth because the relatively young, porous sediments that covered much of North America and Asia in the earlier Cenozoic have been eroded and stripped off because of uplift of broad areas of these continents during the late Cenozoic. 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 groundwa- ter reservoir that exists today may well represent the minimum that has existed for much of geologic history. In the times of abundant nonmarine and shallow carbonate deposition, the potential fluctuating global groundwater storage space could have been double the 50-m (after isostatic adjustment) estimate presented above. A third possibility for changing the volume of ground- water lies in the possibility that the infiltration rate may change with time. It seems likely that the infiltration rate would be higher if larger areas of land were covered by relatively porous unconsolidated sediment. Significantly larger than average land areas were covered by unconsoli- dated nonmarine sediments in the mid-Paleozoic, late Paleozoic-early Mesozoic, and mid-Cretaceous as noted above. The potential for increased infiltration rate as well as the larger potential pore volume available for fluctuat- ing groundwater supplies may well have constituted a CAUSE EUSTATIC SEA-LEVEL FLUCTUATIONS? ~ 69 feedback system that would amplify changes in the hydro- logic cycle. Finally, there is a possibility that discharge rates from the groundwater reservoir could change with time. This is perhaps most easily envisioned as a result of canyon- cutting with concomitant exposure of aquifers as a re- sponse to either uplift or lowering of sea level. The effect would be to supply more water to the ocean, and hence raise sea level. This is a possible negative feedback that could act to damp falling sea levels. However, it seems evident that because this mechanism must of necessity affect smaller areas, it is likely to be much less effective in changing groundwater levels than any of the other three possibilities discussed above. SIGNIFICANCE OF CHANGES IN THE GROUNDWATER TABLE FOR ACCUMULATION OF HYDROCARBONS AND FOR MINERALIZATION Periodic filling and emptying of the pore space in aqui- fers implies times of relatively rapid and slow ground- water migration, which in turn should have significant implications for hydrocarbon migration, diagenesis, and mineralization. These effects could be locally very impor- tant even if the global effects were small. Assuming that when the Earth is ice free, sea-level changes are caused by filling and draining of the groundwater reservoir, a sea- level drop would imply filling of the pore space; conse- quently, generally higher hydrostatic heads would mean higher rates of fluid flow, and flushing of the system followed by lower concentrations of solutes. These would be times when hydrocarbons would be more likely to migrate and when secondary porosity might be produced. Sea-level rises would correspond to times of draining of the groundwater reservoirs, lowered hydrostatic head, lower rates of fluid flow, and higher concentrations of solutes. Compaction, diagenesis, and mineralization might be expected to occur during these times, with concomitant irreversible reductions in porosity. It is interesting to specu- late that if such changes do occur, there is a complex feedback between the groundwater system and the ocean, which might be an important factor in concentrating natu- ral resources. The changes in freshwater-saltwater mass balance, alluded to above, would become an important consideration, but that discussion is beyond the scope of this chapter. SUMMARY AND CONCLUSIONS The pore space in aquifers within the upper 1 km of average elevation of the continents is about 25 x 106 km3, or equivalent to the volume of ice in glaciers on land

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170 today. If this pore space could be alternately filled with and emptied of water instantaneously, it would change sea level by +76 m or +50 m after isostatic adjustment. It seems likely that some fraction of this pore volume is subject to filling and draining as a result of climatic changes, which vary the amount of precipitation on land. The response times for the changes in the reservoir would be on the order of tens to thousands of years after a step function change in climate. Furthermore, there have been times in the geologic past (mid-Paleozoic, late Paleozoic- early Mesozoic, and mid-Cretaceous) when the pore vol- ume of sediments residing above sea level may have been as much as twice its size today. 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. In order to estimate the likely fluctuations in the ground- water reservoir more accurately, it will be necessary to determine the volumes of aquifer sediment more accu- rately, using more specific data for area-elevation-sedi- ment type than are currently available. One can expect that within 5 to 10 yr, enough of the required information will become available so that it will be possible to criti- cally assess the roles of fluctuating volume of the ground- water reservoir in effecting sea-level change. ACKNOWLEDGMENTS This work was supported by NSF Grant NSF OCE- 8409369. REFERENCES Atwater, G. I., and E. E. Miller (1965J. The effect of decrease in porosity with depth on the future development of oil and gas reserves in south Louisiana, Am. Assoc. Petrol. Geol. Bull. 49, 334. 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