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OCR for page 161
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 102°g; 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 102°g (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
OCR for page 162
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.
OCR for page 163
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~.
OCR for page 164
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
OCR for page 165
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
OCR for page 166
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
OCR for page 167
167
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OCR for page 168
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
OCR for page 169
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
OCR for page 170
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.
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Representative terms from entire chapter:
pore space
