GEOLOGY, March 2010 227

INTRODUCTIONThe shallow waters of both the Arabian Gulf

and Red Sea display reticulated networks capped by cement-bound carbonate debris (Fig. 1). Such morphology has also been reported for Belize (Macintyre et al., 2000), Kiritimati (Woodroffe and McLean, 1998), Pearl and Hermes Atoll (Rooney et al., 2008), the Cocos (Keeling) Islands (Searle, 1994), the Maldives (Purdy and Bertram, 1993), the Tuamotu archi-pelago (Guilcher, 1988), and the Great Barrier Reef (Hopley et al., 2007), though unlike the hyperarid Red Sea and Arabian Gulf, these sites receive >1 m of rainfall per year. In the Red Sea the reticulated structures support a veneer of live coral, whereas in the Arabian Gulf, where modern coral growth is at best incipient, accre-tion is predominantly by coralline algae (Pur-kis and Riegl, 2005; Sheppard et al., 1992). In both settings the patterned seafl oor displays two characteristic formations (Fig. 2). Circular ponds of several hundred meters diameter that attain depths as great as 40 m are termed Type-1 depressions. These are rimmed on all sides or

coalesce to form networks of canals. Type-2 depressions have lesser relief, are smaller in aperture, and form a complex maze of reticu-lated sills that surround polygonal sediment-fi lled depressions (ponds) (Fig. 2).

We interpret the reticulated morphology cre-ated by the Type-2 depressions (Fig. 2) as due to antecedent topography forming a template for later reef growth. It is diffi cult to imagine such a complex pattern developing from any reef-limiting factor such as temperature, salinity, or sedimentation. These cannot be anticipated to vary in such a complicated or geometrically regular manner, suggesting substrate-controlled modern coral framework veneers over the sills (Purdy, 1974). Furthermore, the pattern mor-phometry is statistically consistent between Red

Sea and Arabian Gulf (Figs. 3A and 3B), despite different exposure, depth, salinity, temperature regimes, and dynamics and composition of reef builders (Purkis and Riegl, 2005; Sheppard et al., 1992). The morphology is, however, easily explained by karstic dissolution of carbonate rocks by mildly acidic rainwater, a process gen-erating a terrain pattern of enclosed depressions bounded by steep-walled sills (Fleurant et al., 2008). As would be the case during a sea-level lowstand, chemical erosion is restricted to epi-sodes when the surface is subaerially exposed.

SATELLITE REMOTE SENSING AND GROUND SURVEY

We assembled 10,000 km2 of QuickBird sat-ellite imagery for the eastern coast of the north-ern Red Sea at Ras Qisbah and Al Wajh, and 800 km2 for the Bu Tinah shoal in the Arabian Gulf (Fig. 1). These sites were a subset from a greater archive of >25,000 km2 of imagery covering an additional four sites split between the Red Sea and Arabian Gulf, all containing evidence for dissolution topography. The clear waters of the region allow morphology to be discerned to depths of up to 40 m. Where neces-sary, the attenuating effect of the water column in the satellite imagery was corrected. Field work was conducted on four occasions between 2006 and 2009. Remote sensing data were supplemented by 1200 tethered video cam-era seafl oor observations, which were used to verify the character of the seabed. A total track length of 250 km of 3 Hz single-beam acoustic bathymetry was acquired from a vessel, yielding >200,000 soundings against which bathymetry was spectrally derived from the satellite imag-ery. Reef terraces in the 2–30 m depth range were investigated for the presence of reticulated structure using Scuba. Those in the 30–150 m range were fi lmed using a remotely operated vehicle (ROV), facilitating an appraisal of mor-phology up to (and beyond) the depth of the Last Glacial Maximum (LGM) lowstand (~−130 m).

LANDFORM MORPHOMETRYAreas within the QuickBird imagery identi-

fi ed as having Type-1 and/or Type-2 morphol-ogy were processed to a binary representation of the seabed. Satellite pixels corresponding to sills were coded value 0 and ponds coded 1 (Fig. 2). The area-frequency distribution of ponds was quantifi ed using plots of exceedance

Geology, March 2010; v. 38; no. 3; p. 227–230; doi: 10.1130/G30710.1; 5 fi gures; Data Repository item 2010062.© 2010 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or editing@geosociety.org.

*E-mail: purkis@nova.edu.

The paradox of tropical karst morphology in the coral reefs of the arid Middle EastS.J. Purkis1*, G.P. Rowlands1, B.M. Riegl1, and P.G. Renaud2

1National Coral Reef Institute, Nova Southeastern University Oceanographic Center, Dania Beach, Florida 33004, USA2Khaled bin Sultan Living Oceans Foundation, 8181 Professional Place, Suite 215, Landover, Maryland 20785, USA

ABSTRACTDespite differences in reef growth between the Arabian Gulf and the Red Sea, a common

distinctive pattern of polygonal sills surrounding ponded depressions consistently occurs in shallow water. Viewed from a satellite, these seafl oors are reticulated and maze like. Despite little current rainfall, this patterning is best explained by karst dissolution of limestone during periods of lower sea level. This is a paradox since such fi ne-scale karstifi cation is confi ned to areas with considerably more precipitation than currently observed in Arabia. We resolve this apparent contradiction by developing a Pleistocene–Holocene chronology of sea level and cli-mate for the Red Sea and Arabian Gulf, and through the use of pattern analysis and computer simulation, reveal the mechanism of formation for these structures. We demonstrate that this patterning can be taken as a Quaternary signature of paleohumidity in the now hyperarid Red Sea and Arabian Gulf.

Figure 1. Locations of study sites. A: Red Sea. B: Arabian Gulf. Ras Qisbah and Bu Ti-nah are detailed in Figure 2. U.A.E.—United Arab Emirates.

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228 GEOLOGY, March 2010

probability (EP) (Fig. 3A). To test for common patterning between sites, 10 points within each binary representation were selected with a ran-dom number generator. Each served as a seed atop which a circular kernel was centered and expanded from an initial radius of 25 m, to a maximum of 250 m, with 20 m increments. The number of ponds subtended by the kernel was counted at each iteration. This metric examines local patterns of topographic relief and their variation with measurement scale. If landforms between sites are similarly patterned, the num-ber of ponds per sampling area will increase in concert (i.e., Fig. 3B).

Figure 3A represents the probability (y axis) that a given pond will be of an area greater than or equal to a given area (x axis). As previously observed for the topography of karst (Purkis and Kohler, 2008), this plot of EP versus area of depressions follows a power law. A clear depar-ture is observed for ponds with area exceeding

1000 m2, marking the transition from Type-2 to Type-1 depressions. This behavior is consistent for areas of 10 m2 to 10,000 m2, and Type-2 pat-terning of all sites is inseparable on the basis of its area-frequency relations. As evidenced by Figure 3B, density of ponds across scale is also similar between sites, confi rming a consistent morphology.

PALEOCLIMATE AND SEA LEVELReconstruction of late Pleistocene to Holo-

cene sea level and climate reveals mechanisms generating reticulated seafl oors (Fig. 4A). The inset in Figure 4A demonstrates that reef ter-races <25 m below present sea level were exposed from ca. 110 to 9 ka causing scars of ~100 k.y. of meteoric alteration and dissolu-tion. Reconstructions of Saharan climate for this period (Fig. 4A; colored bar above inset) reveals a brief (~8 k.y.) wet phase for both the Red Sea and Arabian Gulf during the transition

from the penultimate glacial to the last intergla-cial period, followed by the onset of 100 k.y. of extreme aridity (McKenzie, 1993; Preusser et al., 2002), before a return to wet conditions of ~5 k.y. duration in the early Holocene (Parker et al., 2006). As documented by Figure 4A, at the same time that sea level approximated its pres-ent position 3–6 k.y. ago, the climate of the Red Sea and Arabian Gulf shifted toward extremely hot and dry (Arz et al., 2003). These hyperarid conditions persist today with annual average rainfall <10 cm in the Arabian Gulf and half that in the Red Sea (Sheppard et al., 1992).

It is reasonable to assume a similar sea-level history of the Red Sea and Arabian Gulf fol-lowing the LGM, both tracking the rise of the Indian Ocean. To chart the Holocene inunda-tion of Arabia, we consider the transgression from the perspective of two sea-level curves, an earliest possible fl ooding (Camoin et al., 2004) and a latest (Lambeck, 1996) (Fig. 4A). At the onset of the Holocene pluvial period in Arabia, the most recent abrupt switch to a cooler and wetter climate in the arid Middle East ca. 10 ka (Parker et al., 2006), sea level was between −35 m (Lambeck, 1996) or even −45 m below present (Camoin et al., 2004). Irrespective of the sea level used to reconstruct the transgression, seafl oors displaying Type-2 morphology were exposed 10 ka. At that time, the climate entered a pluvial period, peaking ca. 9 ka and persisting until at least 6 ka (Lézine et al., 1998; McClure, 1976; Neff et al., 2001; Parker et al., 2006). This was caused by the migration of the Indian Ocean Monsoon (IOM) (Davies, 2006; Gasse et al., 1990; Lézine et al., 1998), extending the limit of the monsoon rainfall belt far north of its modern location, the southern shoreline of Ara-bia (Davies, 2006; deMenocal et al., 2000; Fleit-mann et al., 2003; Neff et al., 2001; Parker et al., 2006). The IOM shift may not have been suffi -cient to induce monsoonal rains in the northern

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Figure 4. A: Sea level and climate reconstructions for the Holocene trans-gression in Arabia. Inset graphs sea level for the past 125 k.y. in the Red Sea (Siddall et al., 2003). B: Bars illustrate distri-bution by depth of Type-2 reticulates.

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GEOLOGY, March 2010 229

reaches of the Red Sea (Ras Qisbah; Fig. 2), where evidence for equally wet conditions exists, but the onset of westerly winter rainfall originating in the Mediterranean is implicated (Arz et al., 2003). Chemical erosion of exposed limestone terraces can be expected to have pro-ceeded slowly during the 100 k.y. of aridity that separated the last and present interglacial. Dis-solution would have initiated quickly after the onset of the Holocene wet phase (Fig. 4A), and been well under way by its peak 9 ka.

Evidence of Type-2 depressions having mid-Holocene age derives from the −25 m maximum depth below present sea level at which they are observed. In the Red Sea and Arabian Gulf, a pronounced increase in the prevalence of Type-2 morphology shallower than 10 m is observed, peaking at −5 m (Fig. 4B). If Type-2 pattern-ing were to owe its origin to meteoric alteration that occurred prior to the mid-Holocene (i.e., during the 100 k.y. interglacial before the most recent transgression), it would also be expected at water depths >25 m. This is not the case, as the −25 m depth limit can be constrained with high confi dence from the visual analysis of 25,000 km2 of QuickBird imagery, coupled with exhaustive ground-truthing. Terraces situated 5 m below present sea level (that display the highest prevalence of Type-2 patterning) would have been subjected to meteoric erosion until at least 7 ka and perhaps to 5 ka. This would allow between 3 and 5 k.y. of exposure to the Holo-cene monsoon climate.

LANDFORM MODELINGTo numerically simulate the effects of subaer-

ial exposure on a limestone terrace, we employ the CHILD (channel-hillslope integrated land-scape development) landform model (v.8.12; Kaufmann and Braun, 2001), which is capable of simulating karst formation in soluble land-scapes (Tucker et al., 2001) and can be modifi ed to apply well-known calcite dissolution kinetics to calculate mass loss as water fl ow across and/or under a terrain surface (Fleurant et al., 2008).

For the simulation, a landscape consisting of 10,000 nodes was subjected to 1 m/yr of rain-fall for 10 k.y. The initial model surface was roughened with ±0.5 m random topographic variation, deemed realistic heterogeneity for an Arabian reef terrace. Precipitation was concen-trated in storms of 5 h duration, occurring every 30 days. Present-day rainfall in the Arabian Gulf averages 10 cm/yr and even less in the Red Sea (Sheppard et al., 1992). The model parameters were, however, set to mimic the conditions at the northern limit of the IOM today. This honors the premise that during the mid-Holocence the IOM extended northward over Arabia. Rainfall was therefore set one order of magnitude greater than present. This value equals the current yearly average for the Arabian margin of the

Indian Ocean (Fleitmann et al., 2003), the cur-rent northern limit of the IOM, and is in agree-ment with the predicted rate of precipitation for the region during the Holocene wet phase (Lézine et al., 1998). The proportion of moisture lost to evaporation was neglected and the rate of tectonic uplift assumed zero. We developed a validation of the model, showing that with precipitation rates as low as 0.7 m/yr and with realistic tectonic shifts, our conclusions on geo-morphic evolution are unchanged (see the GSA Data Repository1).

Simulations demonstrate that 2 k.y. of expo-sure is suffi cient to form reticulated pond and sill patterning, even with the moderately low rainfall of 1 m/yr. Unlike the initial topography used in the model, this patterning is not random, but has evolved to display the ordered morphol-ogy observed in the QuickBird data (Fig. 5). Based on this correspondence, we conclude that rainfall in the region during the Holocene wet phase likely did not exceed 1 m/yr. Furthermore, the simulated 0.06 m/k.y. rate of denudation is in concert with comparable literature studies of karst in reefal limestones (Marshall and Davies, 1984; Spencer, 1985). The model also demon-strates that Type-1 sinkholes require consider-ably more time to develop. By 8 k.y. of simu-lation (Fig. 5C), Type-1 formations are present, but their relief is limited to <5 m as compared to

tens of meters observed in the fi eld. These struc-tures therefore must have developed over much longer periods of subaerial denudation, likely initiating during the penultimate pluvial period 120 ka (see the Data Repository). Type-2 reticu-lates must have formed in the last pluvial period, because if they were older, any exposure exceed-ing 8 k.y. with even moderate rainfall would force a shift to Type-1 morphology. Type-2 reticulated karst is therefore a transient condi-tion, persisting only for a few thousand years, prior to the development of Type-1 morphology.

The broken line in Figure 5D depicts the slope of the EP versus area relationship for the real-world seabeds [i.e., EP = e–2.27(area)–0.99; Fig. 3A]. This relationship is mirrored after 2 k.y. of simu-lation (gray circles, Fig. 5D), proving the model capable of emulating reticulated network forma-tion with the same structure as quantifi ed from QuickBird. As the plotted simulations demon-strate, differential solution promotes topography having plan-view patchiness that is power-law distributed, supporting the premise that predict-able scaling in coral reefs can at least in part be attributed to karstic processes (Purkis and Kohler, 2008; Purkis et al., 2007).

The model indicates that at the point of sub-mergence by the transgression, the vertical relief of the Type-2 karst would have been 1 m or less. Upon fl ooding, this low-relief patterning

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1GSA Data Repository item 2010062, validation of the CHILD landform model, is available online at www.geosociety.org/pubs/ft2010.htm, or on request from editing@geosociety.org or Documents Secretary, GSA, P.O. Box 9140, Boulder, CO 80301, USA.

Figure 5. Actual and modeled topography following karstic erosion. A: Three-dimensional representation of reticulated seabed from Ras Qisbah derived from QuickBird (see text). B, C: CHILD (see text) simulations. D: Exceedance probability (EP) for ponds arising from simu-lations. Broken line is the EP versus pond-area relationship harvested from Figure 3A.

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230 GEOLOGY, March 2010

survived the erosive processes of submergence by serving as a template for reef initiation. Coral growth preferentially focused on topo-graphic highs (i.e., sills) while being inhibited in the lows (ponds) by an abundance of uncon-solidated sediment. Hence, reef growth accen-tuates underlying karst topography (Macintyre et al., 2000; Purdy et al., 2003; Searle, 1994). To reconcile the differences for the simulated karst with the several meters of vertical relief observed in the fi eld, a rate of accretion atop the sills of ~1.5 m/k.y. is required. This is in broad agreement with the pace of Holocene reef accre-tion that averages 3–6 m/k.y. in the Indo-Pacifi c, depending on water depth and rate of sea-level rise (Montaggioni, 2005). The comparatively slow rate of 1.5 m/k.y. is explained both by the low accretion potential of the foliaceous and encrusting coral communities typical to the area, and the inevitable decline in reef vigor ca. 5 ka imposed by a reduction in accommodation space through the stabilization of sea level at that time. Since reticulated karst is evident in the northern limits of the Red Sea (Ras Qisbah; Fig. 2), we confi rm that the area was subjected to a Holo-cene humid interval, despite likely being beyond the reach of the IOM. The presence of pattern-ing reaffi rms a Mediterranean pluvial infl uence on the northernmost Red Sea (Arz et al., 2003).

CONCLUSIONSThe complex maze of reticulated sills sur-

rounding polygonal sediment-fi lled ponds on the shallow seabed of the Arabian Gulf and Red Sea is indicative of a brief period of subaerial chemical erosion followed by submergence and initiation of reef growth. There is strong evi-dence that the timing of this short episode of karst weathering occurred during the Holocene pluvial period in Arabia. We demonstrate that aspects of the reef morphology in the region are controlled by antecedent topography formed as recently as the mid-Holocene.

ACKNOWLEDGMENTSWe thank K. Kohler, S. Dunn, and A. Dempsey for

their help in assimilating the data and A. Wright and K. Verweer for helpful discussions. We are grateful for comments by three anonymous referees. Finan-cial support was provided by the National Coral Reef Institute (NCRI) and the Living Oceans Foundation. This is NCRI contribution 113.

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Manuscript received 21 September 2009Revised manuscript received 30 September 2009Manuscript accepted 2 October 2009

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