Quaternary Research 51, 124 –132 (1999)
Article ID qres.1998.2029, available online at http://www.idealibrary.com on
Holocene Sea-Level Record on Funafuti and Potential Impact
of Global Warming on Central Pacific Atolls
William R. Dickinson
Department of Geosciences, Box 21077, University of Arizona, Tucson, Arizona 85721
Received April 8, 1998
Geomorphic features inherited from the mid-Holocene glaciohydro-isostatic sea-level highstand that affected the central Pacific
region influence the susceptibility of atoll islets to potentially
enhanced wave erosion associated with rise in sea level from global
warming. Shoreline morphology on multiple islets of Funafuti
atoll in central Tuvalu reflects a relative mid-Holocene sea-level
highstand 2.2–2.4 m above modern sea level. Typical islets are
composed of unconsolidated post-mid-Holocene sediment resting
disconformably on cemented coral rubble formed beneath nowemergent mid-Holocene reef flats. Exposed remnants of the lithified islet foundations serve as resistant buttresses protecting the
flanks of atoll islets from wave attack. Islets lacking cemented
mid-Holocene deposits as part of their internal structure are migratory sand cays with unstable shorelines. Any future sea-level
rise >0.75 m, bringing high tide above the elevation of midHolocene low tide, might trigger enhanced wave erosion of stable
atoll islets by overtopping the indurated mid-Holocene reef platforms. As analogous threshold relations are inferred for other
central Pacific atolls, the risk of future inundation of island nations cannot be evaluated solely in terms of expected sea-level rise
with respect to gross islet elevations. © 1999 University of Washington.
Key Words: atolls; Funafuti; hydro-isostasy; Pacific Ocean;
reefs; sea level; shoreline geomorphology; Tuvalu; wave erosion.
BACKGROUND
Low-lying atolls are inherently at special risk from coastal
damage caused by potential sea-level rise associated with projected global warming induced by anthropogenic emission of
greenhouse gases (Roy and Connell, 1989, 1991; Aalbersberg
and Hay, 1992; Yamada et al., 1995; Rabie et al., 1997). In
Tuvalu and neighboring island nations (Fig. 1), coastal geomorphic features partly inherited from a complex Holocene
history of changes in relative sea level within the Pacific Ocean
basin indicate that a regime of accelerated coastal erosion
could be fostered by a relatively modest rise in future sea level.
As similar Holocene sea-level fluctuations affected the whole
mid-Pacific region, appraisals of hazard to Pacific atolls from a
prospective sea-level rise based on the existing topographic
freeboard of atoll islets tend to understate the dangers that
could arise for atoll dwellers. Although projections of the
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Copyright © 1999 by the University of Washington.
All rights of reproduction in any form reserved.
magnitude of sea-level change over the coming century are
heavily model-dependent, with large uncertainties (Hoffman,
1984; Emery and Aubrey, 1991; Warrick, 1993), risk analysis
should take into account insights gained from understanding
the present islet morphology as the result of past fluctuations in
regional Holocene sea level.
INTRODUCTION
In September of 1997 an intensive study of Funafuti atoll
(Fig. 2) in central Tuvalu was undertaken to resolve disputed
issues of Holocene sea-level fluctuation in the central Pacific
region. Of the nearly 30 mapped islets of varying size, the
largest six, representing 70% of the cumulative length of
supratidal ground distributed along the annular atoll rim, were
examined in detail on foot, and another dozen islets (additional
15% cumulative islet length) were observed closely from offshore by boat. As is typical for Pacific atolls (Summerhayes,
1971; Trichet et al., 1984), the bulkiest and longest islets occur
on the windward side of Funafuti, with only isolated islets
separated by wide reef passages present to leeward (Fig. 2). All
the islets combined occupy ,10% of the area of the reef
platform surrounding the interior lagoon.
Funafuti has been a classic locale for atoll research since the
pioneering study of David and Sweet (1904), who were the first
to infer that key geomorphic features at Funafuti reflect an
intra-Holocene highstand in local relative sea level distinctly
higher than present sea level.
MID-HOLOCENE RELATIVE SEA LEVEL
124
The 1967 CARMARSEL Expedition (Curray et al., 1970) to
Micronesia, north and west of Tuvalu, concluded that relative
Holocene sea levels never stood significantly higher than at
present within the intra-Pacific island groups east of the tectonically active Izu–Bonin–Mariana arc–trench system. Atoll
exposures of cemented coral rubble within the intertidal zone,
and locally in supratidal position, were interpreted as stormwave deposits related to modern sea level (Newell and Bloom,
1970). In the Tuamotu Archipelago of French Polynesia, however, flat-topped terraces of analogous coral rubble flanking
HOLOCENE SEA LEVEL OF CENTRAL PACIFIC
125
FIG. 1. Map showing location of Funafuti atoll and island nations (denoted in all caps) composed mainly of atolls in central Pacific Ocean. Triangles are
atolls or isolated coral islands (solid triangles denote atolls discussed in text). Other islands are darkened (smaller ones are shown schematically as black dots).
atoll islets represent emergent reef flats formed at slightly
higher mid-Holocene sea levels (Montaggioni and Pirazzoli,
1984). Inferences of past Holocene sea levels derived in French
Polynesia from radiocarbon ages of emergent corals in growth
position and for coral-rubble deposits cemented into terraces
define the same mid-Holocene sea-level highstand extending
from ca. 4000 yr B.P. to some time during the interval 2000 –
1500 yr B.P. (Pirazzoli and Montaggioni, 1988, Fig. 11).
Radiocarbon ages (n 5 5; 2920 –2580 yr B.P.; Curray et al.,
1970, Table 2) reported by the CARMARSEL Expedition for
rubble-rampart terraces of atolls in the Marshall Islands, lying
northwest of Tuvalu (Fig. 1), suggest that the cemented coralrubble deposits of Micronesia also formed shortly after the
mid-Holocene hydro-isostatic sea-level highstand (peak ca.
4000 yr B.P.) that affected the entire equatorial Pacific region
(Mitrovica and Peltier, 1991). Indeed, a preliminary report
from the CARMARSEL Expedition noted that the midHolocene radiocarbon ages “may suggest a former slightly
higher sea level” (Shepard et al., 1967, p. 542). Widespread
preservation of Micronesian coral-rubble deposits of midHolocene age can most readily be understood as the reflection
of a subsequent decline in regional sea level (Scoffin, 1993),
and there is now widespread appreciation that a mid-Holocene
hydro-isostatic sea-level highstand affected both southern and
western Pacific arenas (Nunn, 1995).
The regional highstand stemmed from an effect, termed
equatorial ocean syphoning (Mitrovica and Peltier, 1991),
whereby water that was initially added to the equatorial Pacific
by the postglacial eustatic rise in global sea level was later
drawn away to fill the voids left by collapse of proglacial
forebulges that deformed seafloor within belts surrounding
regions glaciated during Pleistocene time. Although specific
estimates of the magnitude and timing of the mid-Holocene
highstand vary from place to place within the Pacific Ocean
basin depending upon different assumptions regarding mantle
rheology and the budget for deglaciation (Richmond, 1992, p.
83), the general tenor of the net effect is not in doubt (Nakiboglu et al., 1983; Hopley, 1987; Nakada and Lambeck, 1989;
Mitrovica and Peltier, 1991; McLean and Woodroffe, 1994).
At the time of the CARMARSEL Expedition, postglacial
hydro-isostatic effects on regional sea levels were not well
appreciated, and the prime goal of the expedition was to
resolve supposed discrepancies in the worldwide record of
Holocene eustasy. Expedition results were published several
126
WILLIAM R. DICKINSON
FIG. 2. Funafuti atoll of Tuvalu in central Pacific Ocean. Studied (named)
islets: all capitals 5 examined in detail on foot; lowercase 5 observed by boat
from close offshore; italics 5 sand cays built on modern reef flats (other islets
underpinned by emergent mid-Holocene reef platforms of indurated coral
rubble). Arrows denote islet sites where differential elevations of modern and
mid-Holocene shoreline features are well displayed (see text): (a) northern
Tengako (ocean shore), (b) Luamotu (lagoon shore), (c) southern Funafara
(ocean shore).
years before any general appreciation of the important influence of Holocene glacio-hydro-isostasy on global sea levels
(Walcott, 1972; Chappell, 1974). Allowance for hydroisostatic effects, as load was transferred from Pleistocene ice
sheets of restricted extent to ocean water distributed widely
over the globe, reconciles contrasting Holocene sea-level
records from different locales without the need to postulate
uniform behavior from place to place, as would be inferred
from eustasy alone (Bloom, 1967).
CENTRAL PACIFIC HIGHSTAND
A mid-Holocene hydro-isostatic highstand for the central
Pacific region has now been confirmed by evidence for postmid-Holocene emergence of shorelines in the northwestern
Hawaiian islands by 1.4 –1.8 m (Jones, 1992, 1998; Calhoun
and Fletcher, 1996; Fletcher and Jones, 1996), in close accord
with the expectation of 1.3–1.9 m from global hydro-isostatic
theory (Mitrovica and Peltier, 1991, Fig. 8q). A recent analysis
of key paleoshoreline features on a small islet off Oahu indicates a mid-Holocene (ca. 3500 yr B.P.) hydro-isostatic highstand of 2.0 6 0.35 m (Grossman and Fletcher, 1998), close to
the maximum for Hawaii calculated from hydro-isostatic
theory.
Recent data from Kosrae (Fig. 1) demonstrate that the midHolocene highstand also affected sites westward across the
Pacific Ocean basin from Hawaii. Originally intertidal paleobeachrock, inclined gently (4°– 6°) seaward and rising to a
maximum elevation of 1.3–1.4 m above the modern high-tide
level at Sroanef Point on the northeast coast, as measured
personally, has yielded a radiocarbon age of 3280 6 70 yr B.P.
(Athens, 1995). The indicated post-mid-Holocene emergence
of 1.3–1.4 m is near the higher limit of the hydro-isostatic
estimate of 0.6 –1.5 m for Kosrae (Mitrovica and Peltier, 1991,
Fig. 8d). Independent analysis of emergent corals and paleobeachrock exposures at various other sites along the east
coast of Kosrae suggests a relative mid-Holocene sea level (ca.
3700 yr B.P.) approximately a meter above present mean sea
level (Kawana et al., 1995), at an elevation near the middle of
the range in hydro-isostatic estimates.
A decade after the CARMARSEL Expedition, Schofield
(1977a) argued that cemented coral-rubble terraces in Kiribati
and Tuvalu, southeast of Kosrae, represent emergent midHolocene reef flats stranded in the supratidal zone by postmid-Holocene sea-level drawdown; a radiocarbon age of
2760 6 70 yr B.P. was obtained for giant clam shells imbedded
within typical emergent reef-flat deposits exposed 2.25 m
above modern low-tide level on Tarawa in Kiribati (Fig. 1).
The indicated post-mid-Holocene emergence is near the middle
of the range (1.9 –2.7 m) in hydro-isostatic estimates for Kiribati (Mitrovica and Peltier, 1991, Fig. 8g). Coral boulders in
similar deposits elsewhere within Kiribati have yielded radiocarbon ages (n 5 6) of 2140 –3980 yr B.P. (Schofield, 1977a,
Table 1; Richmond, 1993, Table 2a) from sites at elevations of
1.5 to 2.4 m above modern low-tide level. Analogous deposits
farther north at Bikini and Enewetak atolls (Fig. 1) in the
Marshall Islands, yielding radiocarbon ages (n 5 10) of 3290 –
4360 yr B.P., have been interpreted to imply a mid-Holocene
sea-level highstand at least a meter above present sea level, but
the morphology of the deposits does not record the maximum
sea level attained during Holocene time (Tracey and Ladd,
1974; Buddemeier et al., 1975).
Others have regarded the evidence for a mid-Holocene highstand on central Pacific atolls as equivocal (Marshall and
Jacobson, 1985; Richmond, 1992, 1993), but there is general
agreement that present islets did not begin to form until 3000 –
4000 yr B.P. following widespread submergence of atoll reefs
by postglacial eustasy (Schofield, 1977b; Marshall and Jacobson, 1985; Dye, 1987; Roy and Connell, 1989, 1991; McLean
and Hosking, 1991; Connell and Maata, 1992; Richmond,
1992). Prior to the mid-Holocene inception of islet growth,
HOLOCENE SEA LEVEL OF CENTRAL PACIFIC
atoll reefs were still awash as a result of postglacial eustatic
rise in sea level.
FUNAFUTI GEOMORPHOLOGY
At Tarawa atoll in Kiribati, north of Tuvalu (Fig. 1), and also
still farther north in the Marshall Islands, Holocene reefs began
to grow on a substratum of degraded Pleistocene reef limestone
lying 12–15 m below present sea level shortly after 8000 yr
B.P. during the postglacial eustatic rise in global sea level
(Marshall and Jacobson, 1985; Szabo et al., 1985). McLean
and Hosking (1991) inferred a similar time frame for the
growth of the annular reef of Funafuti atoll, concluding that the
reef platform reached an elevation near modern sea level by
4000 –5000 yr B.P., but did not address possible fluctuations in
sea level over the past 4000 years. Initiation of Holocene reef
growth occurred only slightly earlier, at approximately 9000 yr
B.P., in the northern Cook Islands to the east of Tuvalu (Gray
et al., 1992).
The rapid postglacial rise in sea level occurred at a rate of
approximately 10 mm/yr prior to 7500 yr B.P. (Bard et al.,
1996, Fig. 2), slowing to a rate of approximately 5 mm/yr
during the subsequent two to three millennia when vigorous
Holocene reef growth was underway at Tarawa (Marshall and
Jacobson, 1985, Table 2), and by inference at Funafuti. Latest
Pleistocene and early Holocene rates of sea-level rise associated with postglacial eustasy far exceeded mean rates (,1
mm/yr) of post-mid-Holocene hydro-isostatic drawdown in
local relative sea level calculated for any island groups within
the Pacific region (Mitrovica and Peltier, 1991, Fig. 8). In
general terms, the hydro-isostatic fluctuation in regional sea
level, though important for islet morphology, was approximately an order of magnitude less than the preceding eustatic
change.
Funafuti islets atop the annular atoll rim include two basic
types (Fig. 2): (a) cays composed entirely of unconsolidated
coral rubble and associated calcareous sand, and (b) generally
larger islands where similar unconsolidated deposits are underlain by cemented coral-rubble reaching upward into the modern supratidal zone. The latter type, with consolidated underpinnings, can be termed motu (Stoddart and Steers, 1977;
Nunn, 1994, pp. 245–249), derived from the common Polynesian word for islet (McLean and Hosking, 1991), with no
distinction in spelling between singular and plural. A full
discussion of atoll islet morphology is beyond the scope of this
paper.
Unconsolidated materials on Funafuti islets probably represent amalgamated rubble sheets deposited by successive stormwave washover (McKee, 1959) of the islets. The transient
effects of tropical storms also include episodic construction of
subaerial rubble ridges, both along the flanks of selected islets
and atop the surfaces of fringing reefs offshore (Maragos et al.,
1973; Baines et al., 1974; Bayliss-Smith, 1988; Richmond,
1992; Scoffin, 1993). Offshore storm ridges of unconsolidated
127
debris are with time both degraded and shifted shoreward by
fairweather surf action that locally produces lateral additions to
islet coastlines as the storm ridges migrate to the shore under
wave attack (Baines and McLean, 1976, Fig. 1; Bayliss-Smith,
1988, Fig. 3). Along the oceanward flanks of some islets in
Funafuti, notably Fongafale (Fig. 2), accretionary storm-ridge
increments to exposed islets are consequently underlain only
by modern reef flats, standing at current low-tide level, with no
cemented supratidal coral-rubble present beneath the storm
deposits, which have been extensively quarried for aggregate
and fill. Incipient intertidal cementation of the basal parts of
storm deposits occurs within a few decades of their emplacement above modern reef flats (Richmond, 1992, p. 203), but the
supratidal portions of the storm deposits remain loose sediment
over comparable time intervals.
Beaches on the shifting cays atop modern reef flats expose
only loose sediment, except where a veneer of intertidal beachrock armor occurs locally (Fig. 3). In all cases, the sediment
cemented into beachrock has the same texture and composition
as loose sediment on the adjacent beach face (Richmond, 1992,
p. 159), and modern beachrock uniformly displays bedding
parallel to beach faces. The migratory behavior of cays through
time is documented by exposures on Fualifeke islet (Fig. 2) of
exhumed beachrock in crossbed sets dipping variously with
respect to the beach faces of present shorelines. Islets with
indurated foundations of cemented coral rubble are more stable
in morphology because the cemented materials anchor the
islets in place (McLean and Woodroffe, 1994) and form resistant buttresses protecting islet flanks from fairweather wave
attack (Cloud, 1952; Fosberg and Carroll, 1965; Richmond,
1992, p. 158).
MID-HOLOCENE FUNAFUTI HIGHSTAND
Geomorphic relations at Funafuti atoll provide strong support for a mid-Holocene hydro-isostatic highstand in local
relative sea level close on the heels of the postglacial eustatic
rise. Contacts between unconsolidated cover strata and underlying cemented coral-rubble breccia and conglomerate on Funafuti islets are sharp disconformities, with no gradation between overlying and underlying stratigraphic elements (Fig. 4).
The absence of gradational contacts argues against a submodern age for the indurated coral rubble, and wholesale supratidal
cementation of fragmental debris is unlikely in any case, although intertidal cementation is common (Newell and Bloom,
1970). Schofield (1977a) concluded, correctly in my view, that
the cemented coral-rubble deposits, which rise to a common
platform level where not degraded by modern wave erosion,
represent emergent mid-Holocene reef flats that were formed
below mid-Holocene low-tide level and persistently wavewashed before being exposed subaerially by subsequent drawdown in regional sea level. The modern submerged reef flats in
Funafuti are underlain by analogous rubble cemented into
subtidal breccia and conglomerate. Their surfaces, standing at
128
WILLIAM R. DICKINSON
shores of Tengako (Fig. 5) and Funafara islets (Fig. 1), where
both features are prominent. Given the local tidal range of
1.5–1.6 m (Schofield, 1977a; Rabie et al., 1997), the elevation
difference implies a post-mid-Holocene decline in local relative sea level of 2.2–2.4 m. Schofield (1977a, Fig. 5A) measured a similar elevation difference of 2.3 m between the
modern reef flat and an emergent mid-Holocene reef flat along
the lagoon shore of Luamotu (Fig. 2), where comparative
relations are well displayed. Observed emergence of Funafuti
by the indicated amount since mid-Holocene time closely
matches the mean of the best current hydro-isostatic estimates
(1.9 –2.7 m) for the mid-Holocene highstand in Tuvalu, as
interpolated from results for Kiribati and Fiji (Mitrovica and
Peltier, 1991, Figs. 8g and 8i), lying, respectively, to the north
and south of Tuvalu (Fig. 1).
FIG. 3. Intertidal beachrock on lagoon shore of Fualifeke islet, northern
Funafuti. Cemented beachrock laminae overlie unconsolidated beach sand and
dip parallel to beach face. Beachrock forms within the intertidal zone in the
tropics as alternate wetting and drying of beach faces allows daytime solar
warming of interstitial water within porous sand to promote saturation and
consequent intrastratal precipitation of calcium carbonate (Ginsburg, 1953).
modern low-tide level, are boulder-strewn and algal-encrusted
erosional or nondepositional pavements (Richmond, 1992),
which form both the base level for current intertidal erosion
and the ceiling for upward growth of modern coral (Newell and
Bloom, 1970).
The measured elevation difference between the upper surface of the cemented coral-rubble platform and modern shoreline notches incised into the indurated strata at modern hightide level is 0.7– 0.8 m at two key localities along the ocean
FIG. 4. View south toward northern end of Funafara islet, Funafuti atoll,
showing unconsolidated post-mid-Holocene reef detritus (light color) overlying dark cemented platform of coral-rubble forming emergent surface of
mid-Holocene reef flat. In left foreground, note erosional outlier of unconsolidated islet cover perched on dark emergent reef flat, exhumed and partially
eroded by wave washover of the atoll rim.
FUNAFUTI SHORELINE STABILITY
The relative stability of shorelines on inhabited islets of
Funafuti results from the protection afforded by indurated
mid-Holocene reef-flat deposits that extend through the intertidal and into the supratidal zone to form resistant buttresses on
islet flanks. By inference, the formation of stable islets along
the atoll rim was delayed until post-mid-Holocene hydroisostatic drawdown in regional sea level had carried high-tide
level below the upper surface of the cemented reef platform
developed at mid-Holocene low-tide level. Overlying unconsolidated deposits are effectively shielded, by emergent erosional remnants of cemented coral rubble, from wave attack by
surf that overtops the narrow fringing reef at high tide. Buildout of the edges of offshore fringing reefs may provide some
FIG. 5. Emergent mid-Holocene reef-flat composed of cemented coralrubble eroded into miniature tablelands along ocean shore of Tengako islet,
northeast Funafuti, with modern fringing reef offshore and undercut shoreline
notch incised into emergent limestone (dark color) by solution and bioerosion
at modern high-tide level; unconsolidated islet cover of uncemented coral
rubble and calcareous sand (beneath vegetation on left) disconformably overlies the partly exhumed mid-Holocene reef platform.
HOLOCENE SEA LEVEL OF CENTRAL PACIFIC
additional protection for islet shorelines by pushing the surf
zone progressively offshore, but rapid progradation of steep
atoll reef fronts is unlikely.
Any future rise in sea level of $0.75 m would carry high
tide back above the level of the cemented mid-Holocene reef
platform and significantly alter conditions for wave activity on
Funafuti shorelines. Current estimates of the expected rise in
sea level over the next century, with allowance for global
greenhouse warming, project sea level 0.3–1.1 m higher than
today (Warrick and Oerlemans, 1990). Such a rise in sea level
would not overtop the larger Funafuti islets, although expected
wave runup would surely increase the incidence of storm
inundation (Rabie et al., 1997). The upper reaches of the
predicted range would be sufficient, however, to impact all
Funafuti shorelines adversely by allowing fair weather surf to
impinge directly on the unconsolidated cover capping islets
now underpinned by emergent mid-Holocene reef platforms of
indurated material. The buttressing effect of erosional remnants of emergent reef platforms along islet flanks would no
longer prevail.
COMPARATIVE REGIONAL RELATIONS
Although broad regional warping of the Pacific plate of
lithosphere occurred during Holocene time from hydroisostatic flowage of the mantle in response to shifting global
loads of ice and water, there is no reason to suspect differential
Holocene vertical movements, from local tectonism, among
central Pacific atolls of the Marshall Islands, Kiribati, Tuvalu,
Tokelau, and the northern Cook Islands (Fig. 1). In the absence
of hydro-isostatic effects, slow thermotectonic subsidence of
Pacific lithosphere would be expected to raise relative sea
levels on all the atolls through time as they subside with the
adjacent seafloor in which the volcanic underpinnings of the
atolls are imbedded (Nunn, 1994, pp. 128 –129). The rate of
thermotectonic subsidence is expected to be 51 exp (2t/62.8)
3 10 23 mm/yr, as derived from the expression [6400 –3200
exp (2t/62.8) m] for the depth of the seafloor adjusted isostatically to compensate for sediment load (Parsons and Sclater,
1977), where t is the age of the seafloor in millions of years.
The age of underlying Pacific seafloor in the region of the atoll
nations lies in the approximate range of 100 –150 myr yielding
an inferred rate of thermotectonic subsidence of no more than
0.01 mm/yr, which would cause the atolls to sink, relative to a
constant sea level, by ,0.05 m since mid-Holocene time. The
range of anticipated post-mid-Holocene subsidence lies within
the inherent limits of uncertainty in the elevations of paleoshoreline indicators, as observed in the field, and shows that
basinal tectonism, apart from hydro-isostatic effects, should
not introduce measurable scatter into paleoshoreline data from
different atolls or atoll groups.
As the magnitude of the mid-Holocene hydro-isostatic highstand was generally comparable throughout the region
(Mitrovica and Peltier, 1991, Fig. 8), shoreline relations anal-
129
ogous to those observed on Funafuti are inferred to be widespread. The morphology of atoll islets, apart from shifting
sand– gravel cays of ephemeral character, may be uniformly
metastable and dependent for persistence on the protection
from wave erosion afforded by emergent cemented reef flats
inherited from mid-Holocene time. If future high-tide levels
exceed mid-Holocene low-tide levels, bringing the sea above
flanking buttresses of cemented coral-rubble, an inherent
threshold for enhanced islet erosion might alter conditions for
islet preservation on almost all Pacific atolls.
A brief reconnaissance, in January 1998, of Majuro atoll in
the Marshall Islands (Fig. 1) revealed widespread shoreline
relations comparable to those observed on Funafuti. Both
ocean and lagoon shores of the elongate islets along the southern flank of the atoll, where the atoll rim is readily accessible
by road for 45 km, are commonly armored by resistant buttresses of cemented coral rubble best interpreted as eroded
remnants of emergent mid-Holocene reef flats. Near the airport, somewhat degraded but flat surfaces capping the cemented breccia and conglomerate stand 0.75– 0.85 m above
modern high-tide level, which is marked by solutional shoreline notches or shoreline angles at the inner edges of modern
wavecut platforms on the lagoon shore. Given the local tidal
range of 1.5–1.6 m (Curray et al., 1970; Tracey and Ladd,
1974), shoreline geomorphology thus implies emergence of
Majuro by 2.3–2.4 m since mid-Holocene time, within the
range (1.6 –2.5 m) estimated from hydro-isostatic theory
(Mitrovica and Peltier, 1991, Fig. 8f) and essentially identical
to the 2.2–2.4 m of post-mid-Holocene emergence inferred for
Funafuti. From analogous observations farther north in the
Marshall Islands, others have previously inferred a comparable
post-mid-Holocene drawdown in local relative sea level (Fosberg and Carroll, 1965).
On Majuro, some supratidal segments of shoreline are additionally armored by cuestas of emergent paleobeachrock
distinctly different from the algae-covered modern beachrock
restricted to the intertidal zone. The mid-Holocene paleobeachrock is darkened in color and eroded by intricate microkarst,
with shoreline angles incised sharply into the flanks of the
emergent cuestas by modern wave erosion (Fig. 6). At one
locality on the lagoon shore, paleobeachrock laminae dip
oceanward, beneath the islet rim of the atoll, showing clearly
that the paleobeachrock exposure is not related to the present
shoreline. It was evidently formed instead on a mid-Holocene
ocean shore and has been exhumed by removal of unconsolidated islet cover during migration of the islet under wave
erosion of the lagoon shore, which is impacted by surf induced
by waves driven across the broad lagoon by the prevailing
strong tradewinds.
At Suwarrow atoll in the northern Cook Islands (Fig. 1),
significant emergence since mid-Holocene time has also been
documented (Scoffin et al., 1985; Woodroffe et al., 1990).
Dated emergent features include corals in growth position with
radiocarbon ages of 4650 –3420 yr B.P. (n 5 5) and boulders
130
WILLIAM R. DICKINSON
independently, from rheological models and local geologic
indicators, for individual Pacific atolls or atoll groups (Roy and
Connell, 1991). Relations at Funafuti and Majuro can be regarded provisionally, however, as a bellwether of general behavior.
ACKNOWLEDGMENTS
J. J. Dickinson assisted all field work, and citizens of Tuvalu were unfailingly courteous during our extended stay on Funafuti. P. D. Nunn encouraged
my analysis and provided selected references difficult to acquire in Arizona.
Jim Abbott of SciGraphics (Tucson) prepared the figures.
REFERENCES
FIG. 6. Dark cuesta of emergent mid-Holocene paleobeachrock, now
exposed at an anomalous supratidal elevation up to a meter above modern
high-tide level, on lagoon shore of Majuro atoll in Marshall Islands (figure for
scale). Note degraded surface of paleobeachrock exposure and modern shoreline angle incised at high-tide level into downdip (left) flank of cuesta by
currently active wave erosion associated with surf from tradewind chop within
the lagoon.
in cemented coral-rubble deposits with comparable radiocarbon ages of 4460 –3560 yr B.P. (n 5 3). Somewhat younger
radiocarbon ages of 2420 –2000 yr B.P. (n 5 4) were also
obtained for other emergent corals and boulders. The dated
emergent reef features on Suwarrow suggest post-midHolocene emergence of perhaps a meter (Woodroffe et al.,
1990), reflecting a magnitude and timing for the mid-Holocene
highstand comparable to that observed in the Tuamotu atolls
farther east (see above).
CONCLUSION
The metastable state of atoll islets places a special premium
on accurate prediction of the effects that global greenhouse
warming may have on future global sea level. Existing appraisals of the risk of atoll flooding that highlight gross islet elevations (Holthus et al., 1992; Rabie et al., 1997), without taking
into account geomorphic features inherited from mid-Holocene
conditions, may underestimate the overall hazard to island
habitability. A modest (,1 m) rise in sea level could threaten
the most populous current islets by triggering a regime of
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