The Coastal Oasis: Ice Age Springs on Emerged Continental Shelves

Global and Planetary Change 33 (2002) 47–56 www.elsevier.com/locate/gloplacha

The coastal oasis: ice age springs on emerged continental shelves

Hugues Faure a,*, Robert C. Walter b, Douglas R. Grant c

aINQUA Commission on Carbon, Universite´ de la Me´diterranee, Faculte des Science, P.B. 106, F-13406 Marseille Cedex 09, France bDepartment of Geosciences, Franklin and Marshall College, Lancaster, PA 17604-3003, USA cTerracon Geoscience International, 1703 Prospect Bay Road, Prospect, NS B3T 2B3, Canada

Received 6 February 2001; received in revised form 12 July 2001; accepted 13 September 2001

Abstract

As ice caps expanded during each of the last five glaciations, sea level fell at least 120 m below current levels, exposing continental shelves worldwide to create vast areas of new land. As a result of this exposure, the ecology, climate, pedology, and geology of global shorelines were dramatically transformed, which in turn altered the carbon cycle and biodynamics of this new landmass. In this paper, we focus on a little-known hydrogeological phenomenon that may have had profound influences on biodiversity, human evolution, and carbon storage during periods of severe climatic stress of the Pleistocene Ice Ages. We propose that freshwater springs appeared on emerged continental shelves because falling sea level not only drew down and steepened the coastal water table gradient, thus increasing the hydrostatic head on inland groundwater aquifers, but also removed up to 120 m of hydrostatic pressure on the shelf, further enhancing groundwater flow. We call this phenomenon the ‘‘coastal oasis’’, a model based on three well-established facts. (1) In all coastal areas of the world, continental aquifers discharge a continuous flow of fresh water to the oceans. (2) Many submarine sedimentary and morphological features, as well as seepages and flow of fresh water, are known on and below the shelves from petroleum explorations, deep-sea drilling programs, and mariners’ observations. (3) Hydraulic principles (Darcy’s law) predict increased groundwater flow at the coast when sea level drops because the piezometric head increases by the equivalent depth of sea-level lowering. Sea level is presently in a relatively high interglacial position. Direct observation and verification of our model is difficult and must rely on explorations of terrain that are now deeply submerged on continental shelves. For this reason, we draw parallels between our predicted model and simple, well-exposed terrestrial hydrological systems, such as present-day springs that appear on the exposed shores of lakes whose free- air water levels fell during periods of aridity. Such modern examples are seen in the Caspian Sea and Dead Sea, the Afar Depression, and the Sahara Desert. These modern analogues demonstrate the likelihood that underground water will be more abundant on emerged shelves during sea-level fall, causing springs, oases, and wetlands to appear. Our model creates an apparent paradox: in tropical and subtropical arid lands, such as most of Africa, sea-level fall during hyperarid glacial phases would produce abundant fresh water flow onto emerged continental shelves as the continental interior desiccated. Thus, emergent shoreline springs provided new habitats for terrestrial vegetation and animals displaced from the interior by increasingly arid conditions, shrinking ecosystems, and dwindling water supplies. Such a scenario would have had a profound influence on the vegetation that spreads naturally to colonize the emerged shelves during glacio-eustatic sea-level lowstands, as well as creating new habitats for terrestrial mammals, including early humans. D 2002 Elsevier Science B.V. All rights reserved.

Keywords: global climate change; glacial maxima; sea-level change; lake-level change; continental shelf; paleohydrogeology; coastal oases; coastal springs; Sahara; Quaternary; carbon storage; migrations; human evolution; Africa

* Corresponding author. E-mail address: [email protected] (H. Faure).

1. Introduction Based on evidence from present-day environments below and above sea level, we demonstrate that Glacial/interglacial climatic cycles strongly modify terrestrial water fluxes are perturbed by sea-level fall terrestrial vegetation belts. The effect of these changes according to simple, well-understood hydraulic prin- on carbon storage on land, and thus, on the global ciples. Our model, if verified, would have consider- carbon cycle, is still vigorously debated. Carbon stor- able impact on tropical and subtropical arid belts of age on continents depends on changes in ecosystems the world, especially during hyperarid conditions of or biomes, and on the area occupied by these biomes. glacial periods, as their emerged shelves would con- As continental shelves emerge during glacial maxima, tain abundant, new sources of water (Fig. 1). Emerged roughly 15–20 million km2 of land is freshly exposed, continental shelves would also become privileged which must be considered when determining terrestrial ecosystems, a possible haven for terrestrial mammals carbon budgets at these times. and a route for their dispersal during periods of Little is known about ecological changes on newly hyperaridity inland. Emerged shelves during glacial emerged shelves. Assuming, however, a simple model maxima will have a higher biomass density than their that ascribes the ecosystem of the land adjacent to the contiguous inland deserts, depending on the number, shelf, roughly 200 Gton of carbon could have been extent, and longevity of these coastal oases. stored on emerged shelves during the Last Glacial Maximum (LGM) (Faure et al., 1996; Prentice and Fung, 1990). This is probably a conservative under- 2. Global and arid lands’ hydrogeological cycle estimate. Here, we demonstrate that a more complex phenomenon occurs when shelves emerge, because Nearly 97% of Earth’s liquid freshwater is groundwater availability is enhanced, so the potential for water (Church, 1996) occupying fissures and pores in carbon storage is increased by the creation of new soils, sediments, and rocks at various depths. This wetland habitats. This would have had a profound liquid mass is constantly renewed by infiltration from influence on vegetation distribution, and thus, on precipitation and running water. Except for closed biomass and carbon cycles, during periods of pro- (endorheic) basins, most groundwater moves toward nounced global climatic change. oceans. Rates of movement vary with depth, topog-

Fig. 1. Simplified model of springs associated with initial and final positions of water table during glacial sea-level fall. Solid triangles: springs (oases); wavy lines: paleosealevels (highstand: interglacial, e.g., 125 ka; lowstand: glacial, e.g., 20 ka); solid line: land surface and sea floor; broken line: groundwater table; small arrows: main groundwater flow; large arrows: groundwater flow to subaerial springs on emerged

(lowstand) shelf and to submarine reefs, knolls, mounds, and pockmarks; CH4,6H2O: possible gas hydrates whose dissociation can favor vertical expulsion of gas and water. Springs formed on the shelf as sea level fell and continued functioning until the piezometric level dropped below the ground surface. H. Faure et al. / Global and Planetary Change 33 (2002) 47–56 49 raphy, and physical properties (e.g., porosity and more temperate climates, bear testimony to episodes of permeability) of host materials. While groundwater higher rainfall in the past. During these temperate in karstic regions (Yuan, 1992) and some shallow interglaciations, increased rainfall recharged ground- aquifers exit rapidly and reach the ocean in days or water supplies through highly permeable dune fields or months, water from some deep aquifers and less other coarse sediments. porous material requires thousands or millions of years In central Sinai and southern Israel, ancient ground- to reach the ocean, as shown by isotopic dating of water of the Upper Nubian Sandstone aquifer may flow Saharan (Thorweihe, 1986) and Australian (Haber- as springs in the Dead Sea and Red Sea basins (Issar, mehl, 1980) desert aquifers. In such deep and wide 1990). On the coast of Oman, underground water a sedimentary basins as these, groundwater flow is only hundred meters deep is dated to 20 ka, having infil- 1–2 m/year. Models suggest that anomalies in the trated from mountains some 50 km inland (Weyhen- present water table surface of the vast Saharo–Sahe- meyer, 2000a,b). lian aquifer can be explained by Pleistocene/Holocene In other mountainous areas, such as the Hoggar climatic variations and by sea-level fall during the Last (Ahaggar, Central Sahara), groundwater occupies Glacial Maximum (LGM) (Dieng, 1987). This implies deep, fissured basement rocks. Its isotopic composi- that glacio-eustatic drawdown had a pronounced effect tion shows that it is mostly fossil water (Saighi on inland aquifers. and Mesbach, 1999). Only permeable sediments in dry riverbeds and weathered rocks contain relatively 2.1. Sahara—a reservoir of fossil water young water. In the Tibesti volcanic mountains of Chad, groundwater is also present at great depth. Here, Initial estimates of groundwater reservoirs in the the water table lies about 1000 m below the surface, as western Sahara were recently revised from 15,000 km3 demonstrated by rare seepage into the bottom of deep (Ambroggi, 1966) to nearly 100,000 km3 (Margat and calderas. Similarly, Teide and other volcanoes on the Saad Kamal, 1984). In comparison, the Nubian Sand- Canary Islands are saturated with freshwater at depths stone aquifer in eastern Sahara is nearly 4000 m thick, of 1000–2000 m, even on the arid side of the islands. covers an area of 2 million km2, and contains an Models, validated by thousands of kilometers of sub- estimated 150,000 km3 of groundwater. These massive horizontal galleries (water tunnels) dug in Tenerife, aquifers flow about 1 m/year and discharge 550 demonstrate that the continuous outflow of water to the million m3 of groundwater per year to the Mediterra- surrounding sea (220 million m3) is about equal to the nean Sea, to lowland depressions, and to the Nile annual recharge (240 million m3) of freshwater per drainage (Technical University of Berlin, 1998). The year (Custodio et al., 1988). Here, permeability is high enormous reservoir of groundwater in the Sahara and tall mountains descend rapidly to the sea, causing Desert is well established and contrasts vividly with the high rate of groundwater discharge of about the hyperaridity of its surface. If the modern flow rates 1000 m3 per linear meter of coastline per year. for groundwater in the Nubian Sandstone are correct, the residence time of groundwater in this aquifer is 2.2. Oases in the Sahara—a model for the coastal about 270 ka. oasis phenomenon In the Sahara, the great distance from mountainous recharge areas means that groundwater requires sev- A well-known feature of the present-day hyperarid eral hundred thousand years to reach the central part of Sahara Desert is the existence of oases where springs the aquifer, and thus, that most of the water in this bring fossil water to the surface. While there are many aquifer is fossil (Faure, 1967; Edmunds and Wright, types of springs related to local host-rock properties 1979; Issar et al., 1972; Fontes et al., 1991). It is and/or tectonic features, most oases occur in topo- inferred that Saharan aquifers were replenished several graphic depressions. Moreover, springs (or humid soils times during humid (interglacial) climatic episodes of or wells) in most oases are in sites formerly occupied the Quaternary (Faure, 1969; Petit-Maire, 1982). by lakes that existed during humid (interglacial) cli- Thousands of presently dry lakebeds in the Sahara matic phases, and their water exit points are situated and sub-Saharan Africa, containing fossil evidence of below the highest level of the fossil lake stand. Most 50 H. Faure et al. / Global and Planetary Change 33 (2002) 47–56

Saharan oases formed in direct response to lake regression: when aridity increased, the lakes evaporated rapidly, which released several meters of hydrostatic pressure on the surrounding water table depending on the former lake depth. Fossil lakes in the Sahara were probably not more than several tens of meters deep (Street-Perrot and Perrot, 1990), and yet, even this modest reduction in the free water column promotes flow of groundwater to the surface, creating springs and oases that persist to this day (Faure, 1967). Saharan lakes expanded several times between 12 and 6 ka ago (Gasse et al., 1990; Street-Perrot and Perrot, 1990). Arid surface conditions, however, have prevailed for the last 6 ka, indicating the longevity of these desert oases as a function of the large volume of groundwater available. Thus, Saharan oases provide an excellent model to consider similar conditions occurring along continental margins when sea levels fall an additional one hundred meters. By the same physical mechanism, groundwater, whether adjacent to lakes or oceans, flows toward the new topographic depression due to reduction of the water body. Additional examples of this can be seen today in places below sea level, such as the Caspian Sea, Dead Sea, and Danakil Depression or in places below fossil lake level: oases in the Sahara and in the East African rift valley (e.g., Lake Magadi; Fig. 2) (Casanova, 1985). Fig. 2. A calcium carbonate chimney was formed on the shore of a former lake that existed about 12–9 ka near present-day Lake Magadi in Kenya. Hydrostatic pressure from sublacustrine ground- 2.2.1. Springs and lakes in the Danakil Depression: water discharge was about 56 m higher than the present Lake examples of the phenomena Magadi, when the fossil chimney was active. Today, many active Continental areas that are presently below sea level springs surround the shallow to dry trona lakes, Magadi and Natron exhibit hydrogeological phenomena that illustrate (Casanova, 1985). This feature is close to a high lacustrine shoreline how marine shorelines behave during arid glacial and can illustrate the marine ‘‘coastal oases’’ hypothesis. periods. The Danakil Depression lies at the foot of the Red Sea escarpment of the Eritrean–Ethiopian plateau (C.N.R.–C.N.R.S. Afar Team, 1973). It is 2.2.1.1. Lake Assale. Lake Assale is a shallow, presently an arid desert with less than 100 mm of ephemeral sheet of brine covering the salt plain around mean annual rainfall and evaporation of about 5 m/yr the salt dome of Dallol, Ethiopia. Miocene salt depos- (200 in.) (Holwerda and Hutchinson, 1968). A rift its, several thousand meters thick, occur below the valley marking the northern end of the Afar triple- brine lake. Fresh water is present in alluvial fans at the junction, the Danakil is an elongated rift basin with foot of the escarpment to the west, and in aerial views, a central chain of active, axial volcanoes, and a series the groundwater table can be seen intersecting the of lakes at various depths below sea level: Lake surface west of the village of Dallol at various eleva- Badda (À50 m), Lake Assale (À118 m), Lake Bakili tions below sea level (Holwerda and Hutchinson, (À120 m), Lake Afdera (À111 m), and Lake Acori 1968). This fresh groundwater, heated at depth, dis- (À94 m). To the east, near the Gulf of Aden in Djibouti, solves potassium and magnesium salts from the thick Lake Assal (À155 m) is the lowest spot in Africa. Dallol evaporite sequence, and issues as brines from H. Faure et al. / Global and Planetary Change 33 (2002) 47–56 51 hot springs. One spring saturated with magnesium compared to its present, arid situation. It is likely that chloride has a temperature of 130 jC (Holwerda and the early Holocene wet phase increased the frequency Hutchinson, 1968). Potassium salt mines in the area of floods in the wadis descending along the escarp- frequently encounter saltwater intrusions. ment, which fed underground water reservoirs through thick alluvial fans. Input to underground water storage 2.2.1.2. Lake Afdera. Farther south, Lake Afdera was more frequent and water levels were higher (also called Giulietti or Afrera) is a well-defined lake during the early Holocene, as indicated by stromato- more than 85 m deep with a floor at about 200 m litic incrustations visible in many sectors of the Afar. below sea level. Numerous springs occur at the foot of A different water budget was established during the the Ethiopian escarpment, and around and within the early Holocene, with increased lake areas occurring lakes of the central depression. Many are saline and under different climatic parameters that maintained hot; for example, artesian springs around lake Afrera lake levels about 30 m above present. When lake are 40–53 jC (Martini, 1969). Springs on the west levels fell to present levels, roughly 7–5 ka ago, the side of Lake Afrera discharge about 0.25 million m3/ hydrostatic pressure over the underground water was day (Ku¨rsten et al., 1968), whereas, the total inflow to reduced by 3 atm (about 3 hPa). This explains the the lake, calculated from the evaporation on the lake, strong outflow of many springs around the coast of must be at least 0.6–1.2 million m3/day. Hence, each Lake Afrera during the present hyperarid climate. square meter of Lake Afdera should receive at least an The calculated annual sublacustrine groundwater additional 3 m3/year of underground water to com- flow into Lake Afrera of about 6600 m3/year per meter pensate for the 3 m of annual evaporation. The total of shoreline is comparable to the highest estimates for annual discharge of underground water is probably groundwater flow into the world’s oceans (see Section around 330 million m3. The lake is 20 km long, 3). The lifespan of these springs, however, depends on perpendicular to the supposed underground flow, the various parameters that control the piezometric and has a perimeter of about 50 km. Thus, the annual surface (level) of the underground reservoir. Only a flow under each linear meter of the lakeshore is about careful field survey and a complete water budget will 6600 m3, or about 0.21 l/s. This estimate may be high quantify these components. because of the possibility of direct flow of surface runoff from higher ground during brief periods of 2.2.2. Evidence from early human occupation in the exceptional rainfalls, but it compares well with the Danakil maximum estimates of present-day input of under- Modern lakes of the Danakil Depression, occur- ground water flow to the ocean (see Section 3). ring well below present sea level, serve as crude Fossil lake deposits occur to the west of Lake analogues for hydrological conditions that operated Afrera. These sediments are about 5 m thick, stand at along marine coasts during sea-level lowstands of 15–30 m above the present lake surface, and represent glacial maxima. Significantly, many Paleolithic tools deposition during a high-level stage between 10 and 7 of the Middle to Upper Pleistocene occur in or on ka ago. A white, powdery limestone contains 5% coastal marine strata, fluvial terraces, and alluvial diatoms that reveal a history of alkaline water that fans in the Danakil that presently lie well below sea later became eutrophic, then slightly salty (Gasse, level (Faure and Roubet, 1968; Walter et al., 2000), 1975). The period of relatively fresh water corresponds suggesting that these regions were habitable at var- to the early Holocene wet phase that was responsible ious times in the past as a result of freshwater for the maximum expansion of Lake Afrera and springs that flowed in this hyperarid climate. Coastal changing water budgets. Evaporation during the ini- oases may have been the key for the survival of early tiation of the dry Holocene climate phase briefly humans on emerged shelves during one or more of stabilized the level between 96 and 81 m below present the last five major glacial lowstands (e.g., oxygen sea level. It is now 111 m below sea level. isotope stages 2, 6, 8, 10, and 12), a period that During the early Holocene, a more important con- documents the most extreme global climatic changes tribution of rainwater and surface runoff accounts for since the onset of northern hemisphere glaciations in fresher conditions and higher levels of Lake Afrera the Late Pliocene. 52 H. Faure et al. / Global and Planetary Change 33 (2002) 47–56

2.2.3. Summary fresh water across a coastal perimeter of about 1100 The Holocene and recent history of Lake Afrera km. This is a flux of 500 m3 of fresh water per meter of (with its bottom at about 200 m below sea level) coastline per year, or a flow rate of about 0.016 l/s. illustrate the general hydrological mechanism by Assuming the porosity of the Nubian Sandstone is which Pleistocene coastal springs were likely initiated. 10%, this flow rate requires the aquifer to have a mean The main driving force of groundwater flow is the thickness of 2000 m and a mean transmissivity of 2.5 physical change of pressure, whether along ocean m/year. In sum, the sedimentary basins around the coasts or lakeshores, above or below sea level. Free- African coast contain abundant fossil groundwater air water levels can change rapidly, depending on the with a general piezometric (water table) gradient atmospheric parameters that control evaporation, rain- following topography and inclined towards the Atlan- fall, and runoff (or ice melt), but groundwater level— tic and Indian Oceans and the Mediterranean and Red especially of large aquifers—changes much more Seas. This general picture is valid despite local varia- slowly, depending on the physical properties of host tions in hydraulic parameters, discharge rates, and the rocks. It is this relative difference in elevation between slope of the water table relative to the topography. the free-air water surface and the groundwater table that produces the changes of pressure that create, 3.2. Effects of LGM sea-level fall control, and modify the flow of coastal springs. A glacio-eustatic sea-level fall of 120 m, the approximate level attained during the LGM 20 ka 3. Submarine groundwater discharge in Africa ago (Rohling et al., 1998), effectively steepened the hydraulic gradient, and thus, increased groundwater In general, groundwater flows to the sea. World- flow to the sea (Fig. 1). Based on long residence times wide, submarine groundwater discharge (SGWD), the of water in major inland aquifers, rates of glacio- direct flow of fresh water into the sea through porous eustatic sea-level fall are faster than drawdown rates rocks and sediments, has been detected on all con- of terrestrial aquifers. For example, between oxygen tinental shelves of the world (cf., Manheim, 1967; isotope stages 5a and 4, sea level fell at an average Buddemeier, 1996). high rate of 10 m/ka for nearly 6000 years (Edwards et Quantitative estimations of SGWD in most oceans al., 1999). During such a rapid sea-level fall, the water are difficult to measure and are still the subject of table behaves as an ‘‘infinite reservoir’’, appearing active research (Burnett et al., 2000). Global values always in a ‘‘high potential state’’ compared to its range from 400–4000 km3/year (Church, 1996). The falling base level. Therefore, although the water table highest value corresponds to 10% of the surface runoff surface also drops during arid periods, it remains or stream flow. Three different methods yield estimates significantly higher than sea level towards the coast. of approximately 2400 km3/year (Zekster and Dzha- Enormous volumes of groundwater can be dis- malov, 1981), which yields a mean SGWD across each charged into emerged coastal shelves due to sea-level meter of the world’s 400,000-km linear coast of 6000 fall. We calculate that during glaciations, every 50 km m3/year, or 0.19 l/s. A lower estimate of 1000 m3/year of coastal length receives an extra 40 km3 of water is calculated using the values of Kessler and Harvey annually. A significant part of this water will issue (2001). Thus, coastal aquifers supply at least 400– from springs and wetlands on the emerged shelf. 2400 km3 of fresh water per year to the world’s oceans. Significantly, a 120-m sea-level fall is equivalent to raising the continental water table 120 m above its 3.1. The Nubian Sandstone aquifer of eastern Sahara stable base level. In hydrologic terms, this is comparable to a climatic change to wet conditions along the Sahara discharges most of its water via submarine continental margins despite commensurate hyperarid groundwater flow northward into the Mediterranean conditions in the adjacent continental interiors. Sea. Only a fraction of this water enters the Nile River Discharge rates of coastal springs ultimately depend or is evaporated in depressions (Kattara). Thus, the on groundwater surface gradients (the distribution of Nubian aquifer annually moves 550 million m3 of potential heads) and on local lithology, tectonic struc- H. Faure et al. / Global and Planetary Change 33 (2002) 47–56 53 tures, and topography. Because of the large surface methane) feeds primary reef producers. It follows that areas of Saharan aquifers, discharge into the sea is a lowering of sea level, which effectively reduced the continually replenished from storage inland at higher hydrostatic pressure of the seawater column by as elevations. However, on very flat topography, such as much as 120 m, reactivated and increased the flow sedimentary basins in West Africa, the resulting of former submarine fluid circulation. depression of the water table extends to about 150 Confirmation of freshwater at depth under the sea km inland from the present coast, as shown by local comes mainly from offshore oilfield drilling. As a ge- anomalies in the present water table to depths of 40 m neral rule, oilfields include enormous volumes of (‘‘nappe en creux’’). Models confirm that such anoma- underground water. For example, in the offshore drilling lies are relic depressed water tables inherited from the of the Rospo Mare (Adriatic Sea, about 30 km off the LGM sea-level fall (Dieng, 1987). Moreover, exam- coast of Italy) at about 1300 m below sea-level, heavy ples of coastal oases (called ‘‘N’Niaye’’) are found on oil is trapped in Cretaceous karstic limestones. Most of the coast of Senegal, where pollen studies of early these karst cavities are filled with fresh or slightly Holocene peat and lacustrine beds, presently 10 m brackish water (Soudet et al., 1994; in Vernet, 2000). below sea level, reveal a refugial Guinean mesophilous forest (Lezine, 1987) localized in coastal oases sur- rounded by sand dunes covered by more arid vegeta- 5. A hydrogeological scenario for ice-age tion (Lezine, 1987). continental shelves

We propose a simple scenario to explain hydro- 4. Evidence of former groundwater flow within the geological conditions during the last glacial/intergla- continental shelf cial climatic cycle that should apply to any of the previous climate cycles of the last half million years or We are presently in a relatively high sea-level state more. About 125 ka ago, at the height of the last compared to the generally low glacio-eustatic sea interglaciation (oxygen isotope stage 5e), Africa was levels that prevailed during 80% of Pleistocene time in a wet phase and sea level was about 3–6 m higher (Shackleton, 1987). Today, evidence of paleohydro- than today (Shackleton, 1987). Groundwater tables geologic events on former emerged shelves can only were also higher, as suggested by elevated lake levels be examined under the sea. Present-day submarine in the Sahara at that time (Petit-Maire, 1982). From groundwater discharge is observed in many places 120 to 20 ka, as ice sheets grew at the expense of sea by mariners (Church, 1996) and is revealed by 226Ra level, the groundwater reservoir tended to adjust to enrichments (Moore, 1996). Evidence of fluid circu- descending sea level. First, brackish water (seawater lation in sediments at the shelf edge, mostly in the form mixed with freshwater during high sea stands on the of water and methane (occasionally from gas hydrates; littoral) was flushed out by the continuous flow of Laherrere, 2000) is well known around the world from inland freshwater into the emerged shelves (Fig. 1). Norway (Hovland, 1990) to Australia (Feary et al., Moreover, the reduction of pressure exerted by the 2000). Pockmarks (King and MacLean, 1970) and seawater column caused an increase in flow of existing cold-water carbonate mounds and knolls (Hovland et submarine springs (some emerged) and the inception al., 1987) and deep-sea Lophelia coral reefs several of new springs in favorable places on the shelf with tens of meters thick are found between 400 and 40 m favorable lithology, faults or local bathymetry, and below sea level. These reefs are abundant between topography. Oscillations in the rate of sea-level fall 200- and 350-m water depths. Radiocarbon dates would presumably cause variable groundwater flow, indicate that active mound growth occurred during with periods of accelerated discharge. sea-level lowstands. Carbonate mounds could be This phenomenon would only cease when the water linked to flow of saline fluids within the shelf sequence table equilibrated with sea level, depending on the induced by sea-level lowering (Feary et al., 2000) and drawdown rate of the water table. We calculate that supports the ‘‘hydraulic theory’’ of Hovland (1990); massive continental water tables such as the Nubian pore water seepage at these depths (e.g., groundwater, aquifer remain significantly higher than sea level even 54 H. Faure et al. / Global and Planetary Change 33 (2002) 47–56 when glacio-eustatic sea level is at its lowest and when eventually dispersing throughout the Old World fol- continental aridity is at its highest (e.g., about À 120 m lowing these littoral routes (Stringer, 2000; Walter et at the peak of the LGM 20,000 years ago). Afterwards, al., 2000). Our coastal oasis model can also be applied as glacial melting ensued, rapid sea-level rise caused a to emerged land surfaces around large East African global invasion of land by ocean water and a corre- lakes, which experienced major falls in water levels spondingly, fast rise of the coastal water table. The rate during glacial maxima (Gasse et al., 1989). Emerged of water table rise increased even further after 12,000 land surfaces on the fringe of these reduced lakes years ago as global climates returned to humid con- would have experienced the same hydrological effects ditions (Gasse et al., 1989). as marine shorelines, producing springs and wetlands Paradoxically, while freshwater tables fell on land as prime habitats for the survival of vegetation and in response to increasingly arid climates, water avail- terrestrial mammals during glacio-climatic arid phases. ability increased on emergent continental shelves. The coastal oasis model permits the following test- Ultimately, water availability depended on local con- able hypotheses: (1) antithetical to arid conditions on ditions on the emerged shelf, such as faults, porosity, land, emerged shelves may have been refugia for more and deep valley incisions. Generally, however, water humid, interglacial-like vegetation and, if so, may have was widely available during hyperarid glacial maxima had an effect on global biomass and carbon storage in favorable areas of newly emerged coastal plains. during glacial times, depending on the extent and We call this phenomenon ‘‘the coastal oasis’’, keeping longevity of the emerged freshwater wetlands; (2) in mind that emerged coastal plains can be 100 km fauna, particularly large mammals, could have found wide and freshwater seeps may occur at any point on refuge on the shelf, and thus, survived in these coastal the newly exposed shelf according to local hydro- oases; (3) hominids, following these fauna, may have geological conditions. migrated onto the emerged shelves where they sur- We propose that the best place to find freshwater vived hyperarid climatic phases, perhaps adapting to a during hyperarid glacial cycles is on the emerged mix of terrestrial and marine food resources (Walter continental shelf. During interglacial high stands, such et al., 2000). A key to understanding early human as today, these springs are submerged. Modern exam- evolution may be locked up in these coastal deposits, ples of underwater seeps are found in the Mediterra- many now well below sea level (or lake level), or in nean and Red Seas and the Persian Gulf where fortuitously uplifted coastal regions. (4) Biomass esti- confined artesian aquifers host submarine rivers with mates, carbon storage modeling, geochemical and upwelling plumes offshore, which ancient mariners isotopic systematics should be reevaluated in light of exploited to restocked fresh water supplies (Church, the likelihood of enhanced freshwater flow on 1996). During glacial lowstands, many of these seeps emerged shelves during glacial maxima; and (5) a would be exposed on the emerged shelf, with their flow new multidisciplinary approach is necessary to solve rates enhanced. This new environment, with wetlands the many questions arising from our model of shelf from underground seeps, created a refugia for terres- paleohydrogeology and paleoenvironmental change trial vegetation (Lezine, 1987), and would have had a during glacial lowstands, and its consequences for significantly higher biomass and carbon density than biological, geochemical, and human diversity. would be expected from a dry and barren desert shelf, dramatically enhancing the capacity for carbon storage on emerged shelves. Acknowledgements Our model predicts that Africa’s coasts became havens for terrestrial plants and animals while inland H.F. thanks BRGM for providing many years of habitats shrank. Emergent coastal springs, resulting field research in the Sahara, as well as the CNRS and from glacio-eustatic sea-level fall, provided abundant ORSTOM (now IRD), Universite´ de la Mediterranee freshwater resources necessary for humans and other for facilities. R.C.W. thanks his colleagues on the animals to survive periods of extreme aridity over the Eritrean Research Project, especially Seife Berhe, past 500 ka. Once adapted to shoreline habitats, by ca. Yoseph Libsekal, Dick Buffler, Henrich Bruggemann, 125 ka, humans spread rapidly along Africa’s coast, and Mireille Guillaume for discussions, the Depart- H. Faure et al. / Global and Planetary Change 33 (2002) 47–56 55 ment of Geology at the University of Asmara, the Time Scale of Decades to Millennia. University of Hawaii, Department of Mines of the Eritrean Ministry of Honolulu, HI, USA, pp. 77. Faure, H., 1967. Le proble´me de l’origine et de l’age de l’eau des Energy and Mines. R.C.W. acknowledges the La Jolla oasis sahariennes du Niger. Memoire Reunion de l’Association Initiative for Explaining the Origins of Humans for Internationale d’Hydrologie 7, 277–278. discussions and comments on the first presentation of Faure, H., 1969. Lacs Quaternaires du Sahara. Mitteilungen-Inter- the Coastal Oasis model in August 2000. We thank nationale Vereinigung fuer Theoretische und Angewandte Lim- James Aronson and William Calvin who provided nologie 17, 131–146. Faure, H., Roubet, C., 1968. Dećouverte d’un biface Acheuleén valuable comments on the manuscript, as well as dans les calcaires marins du golfe Pleístoce`ne de l’Afar (Mer Paolo Pirazzoli, Liliane Faure-Denard and Constanze Rouge, E´ thiopie). Comptes Rendus Hebdomadaires des Seances Weyhenmeyer on an early version of the manu- de l’Academie des Sciences Paris 267, 18–21. script. We also thank two anonymous reviewers for Faure, H., Adams, J.M., Debenay, J.P., Faure-Denard, L., Grant, useful improvements. This is a joint contribution to D.R., Pirazzoli, P.A., Thomassin, B., Velichko, A.A., Zazo, C., 1996. Carbon storage and continental land surface change ‘‘CHANGES’’ (IGCP-404/459, 396/464, 413, 423, during the Last Glacial Maximum. 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