Downloaded from geology.gsapubs.org on March 3, 2010
The paradox of tropical karst morphology in the coral reefs of the
arid Middle East
S.J. Purkis1*, G.P. Rowlands1, B.M. Riegl1, and P.G. Renaud2
1
National Coral Reef Institute, Nova Southeastern University Oceanographic Center, Dania Beach, Florida 33004, USA
Khaled bin Sultan Living Oceans Foundation, 8181 Professional Place, Suite 215, Landover, Maryland 20785, USA
2
ABSTRACT
Despite 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 seafloors 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 fine-scale karstification is confined 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 climate 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.
A
B
100 km
Egypt
37°E
Ras
Qisbah
Red
Sea
Saudi
Arabia
Al
Wajh
52°E
100 km
Arabian Gulf
Qatar
Bu Tinah
25°N
24°N
U.A.E.
Extent of QuickBird satellite imagery
Figure 1. Locations of study sites. A: Red
Sea. B: Arabian Gulf. Ras Qisbah and Bu Tinah are detailed in Figure 2. U.A.E.—United
Arab Emirates.
*E-mail: [email protected].
Ras Qisbah (Red Sea)
Bu Tinah (Arabian Gulf)
Type-2
Linear
patch reefs
Type-2 depressions
Type-1 depressions
Sand
sheets
Sand
sheets
Type-1
0
(m)
1000
0
(m)
INTRODUCTION
The 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 archipelago (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, accretion is predominantly by coralline algae (Purkis and Riegl, 2005; Sheppard et al., 1992). In
both settings the patterned seafloor 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
250
2000
500
Figure 2. Top: Representative QuickBird (see
text) images of reticulated seabeds from Ras
Qisbah and Bu Tinah (locations in Fig. 1).
North is toward top. Bottom: Binary depictions of morphology. Positive relief (sills) are
black. Depressions (ponds) are white.
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 reticulated sills that surround polygonal sedimentfilled depressions (ponds) (Fig. 2).
We interpret the reticulated morphology created by the Type-2 depressions (Fig. 2) as due
to antecedent topography forming a template for
later reef growth. It is difficult to imagine such
a complex pattern developing from any reeflimiting 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 morphometry 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 generating 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 episodes when the surface is subaerially exposed.
SATELLITE REMOTE SENSING AND
GROUND SURVEY
We assembled 10,000 km2 of QuickBird satellite imagery for the eastern coast of the northern 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 necessary, 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 camera seafloor 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 imagery. 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 filmed using a remotely operated
vehicle (ROV), facilitating an appraisal of morphology up to (and beyond) the depth of the Last
Glacial Maximum (LGM) lowstand (~−130 m).
LANDFORM MORPHOMETRY
Areas within the QuickBird imagery identified as having Type-1 and/or Type-2 morphology 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 quantified using plots of exceedance
© 2010 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected].
GEOLOGY,
March
2010
Geology,
March
2010;
v. 38; no. 3; p. 227–230; doi: 10.1130/G30710.1; 5 figures; Data Repository item 2010062.
227
Downloaded from geology.gsapubs.org on March 3, 2010
10 0 10 1
10 2
10 3
Area m 2
10 4
B
10 5
Bu Tinah
Ras Qisbah
Al Wajh
10 -1
10 -2
EP =
e2.27(Area)-0.99
10 -3
Type-2
depressions
50
100
150
200
Kernel radius (m)
350
Number of moated ponds in kernel
Exceedance probability
A
Type-1 depressions
300
Bu Tinah (Arabian Gulf)
Ras Qisbah (Red Sea)
Al Wajh (Red Sea)
250
200
150
100
50
Figure 3. Morphometrics and depth of reticulated seafloors. A: Bi-logarithmic plots of exceedance probability (EP) versus area of ponds. Vertical broken lines delineate thresholds
between which the system is power-law distributed. B: Average number of ponds subtended
by 10 randomly seeded expanding circular kernels at sites detailed in Figure 2.
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 patterning 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, confirming a consistent
morphology.
probability (EP) (Fig. 3A). To test for common
patterning between sites, 10 points within each
binary representation were selected with a random 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 number 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 departure is observed for ponds with area exceeding
A
Southern Red Sea
Arabian
Gulf
Hyper-arid
PALEOCLIMATE AND SEA LEVEL
Reconstruction of late Pleistocene to Holocene sea level and climate reveals mechanisms
generating reticulated seafloors (Fig. 4A). The
inset in Figure 4A demonstrates that reef terraces <25 m below present sea level were
exposed from ca. 110 to 9 ka causing scars of
~100 k.y. of meteoric alteration and dissolution. 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
B
de Menocal et al. Lézine et al. (1998); McClure (1976) Davies (2006)
(2000) Neff et al. (2001); Parker et al. (2006) Gasse et al. (1990)
Arid
+10
2000
IOM incursion
4000
6000
0
8000
10000
Arabian Gulf
0
-5
SW Indian Ocean Composite
Camoin et al. (2004)
Depth (m)
-10
Red Sea, Siddall et al. (2003)
Sea level (m)
-20
SW Indian Ocean, Camoin et al. (2004)
-30
0
-40
5e pluvial
McKenzie
(1993)
Red Sea
-10
-15
Muscat (predicted)
Lambeck (1996)
-20
-25 m
-40
-25
-50
-80
-60
50000
100000 k.y. ago
mid-Holocene pluvial
period penetrates Arabia
McClure (1976); Parker et al. (2006)
0
Northern Red Sea
2000
4000
?
6000
Age ( k.y. ago)
8000
0
0
-70
228
from the penultimate glacial to the last interglacial 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 present 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 following the LGM, both tracking the rise of the
Indian Ocean. To chart the Holocene inundation of Arabia, we consider the transgression
from the perspective of two sea-level curves, an
earliest possible flooding (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,
seafloors 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 Arabia (Davies, 2006; deMenocal et al., 2000; Fleitmann et al., 2003; Neff et al., 2001; Parker et al.,
2006). The IOM shift may not have been sufficient to induce monsoonal rains in the northern
10000
25
50
75
5
10
15
20
25
Area of depositional system
characterized by Type-2
morphology ( km2)
100
30
125
Figure 4. A: Sea level and
climate reconstructions
for the Holocene transgression in Arabia. Inset
graphs sea level for the
past 125 k.y. in the Red
Sea (Siddall et al., 2003).
B: Bars illustrate distribution by depth of Type-2
reticulates.
12000
Arz et al. (2003)
GEOLOGY, March 2010
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GEOLOGY, March 2010
tens of meters observed in the field. These structures 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 reticulates must have formed in the last pluvial period,
because if they were older, any exposure exceeding 8 k.y. with even moderate rainfall would
force a shift to Type-1 morphology. Type-2
reticulated karst is therefore a transient condition, 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 realworld seabeds [i.e., EP = e–2.27(area)–0.99; Fig. 3A].
This relationship is mirrored after 2 k.y. of simulation (gray circles, Fig. 5D), proving the model
capable of emulating reticulated network formation with the same structure as quantified from
QuickBird. As the plotted simulations demonstrate, differential solution promotes topography
having plan-view patchiness that is power-law
distributed, supporting the premise that predictable 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 submergence by the transgression, the vertical relief
of the Type-2 karst would have been 1 m or
less. Upon flooding, this low-relief patterning
A
Ponds
Ras Qisbah
Sills
50
0m
500
-15 m
-20 m
Simulation
8000 yr
Type-2
fully developed
1m
5m
-10 m
B
Type-1
absent
Simulation
2000 yr
Type-2
absent
50
m
C
Type-1
fully developed
0m
m
500
100
Exceedance probability
LANDFORM MODELING
To numerically simulate the effects of subaerial exposure on a limestone terrace, we employ
the CHILD (channel-hillslope integrated landscape development) landform model (v.8.12;
Kaufmann and Braun, 2001), which is capable
of simulating karst formation in soluble landscapes (Tucker et al., 2001) and can be modified
to apply well-known calcite dissolution kinetics
to calculate mass loss as water flow 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 rainfall 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 concentrated 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 current northern limit of the IOM, and is in agreement 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 geomorphic evolution are unchanged (see the GSA
Data Repository1).
Simulations demonstrate that 2 k.y. of exposure is sufficient 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 morphology 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 demonstrates that Type-1 sinkholes require considerably more time to develop. By 8 k.y. of simulation (Fig. 5C), Type-1 formations are present,
but their relief is limited to <5 m as compared to
Depth (m)
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 proceeded slowly during the 100 k.y. of aridity that
separated the last and present interglacial. Dissolution 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 midHolocene 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 patterning 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 confidence 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 Holocene monsoon climate.
10-1
10-2
100
D
Simulations
8000 yr
6000 yr
4000 yr
2000 yr
EP =
e2.27(area)-0.99
Type-2 depressions
Red Sea and/or
Arabian Gulf
101
102
103
Area (m2)
104
105
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 simulations. Broken line is the EP versus pond-area relationship harvested from Figure 3A.
1
GSA Data Repository item 2010062, validation of the CHILD landform model, is available online at
www.geosociety.org/pubs/ft2010.htm, or on request from [email protected] or Documents Secretary,
GSA, P.O. Box 9140, Boulder, CO 80301, USA.
229
Downloaded from geology.gsapubs.org on March 3, 2010
survived the erosive processes of submergence
by serving as a template for reef initiation.
Coral growth preferentially focused on topographic highs (i.e., sills) while being inhibited
in the lows (ponds) by an abundance of unconsolidated sediment. Hence, reef growth accentuates 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 field, 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 accretion that averages 3–6 m/k.y. in the Indo-Pacific,
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
confirm that the area was subjected to a Holocene humid interval, despite likely being beyond
the reach of the IOM. The presence of patterning reaffirms a Mediterranean pluvial influence
on the northernmost Red Sea (Arz et al., 2003).
CONCLUSIONS
The complex maze of reticulated sills surrounding polygonal sediment-filled 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 evidence 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.
ACKNOWLEDGMENTS
We 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. Financial 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 2009
Revised manuscript received 30 September 2009
Manuscript accepted 2 October 2009
Printed in USA
GEOLOGY, March 2010
Ras Qisbah (Red Sea)
Bu Tinah (Arabian Gulf)
Type-2
Linear
patch reefs
Type-2 depressions
Type-1 depressions
(m)
0
1000
(m)
0
250
2000
500
Figure 2
Sand
sheets
Sand
sheets
Type-1