dated indicating a history of sea-level rise during the past
8000 years (Ohde et al., 2002).
During the Quaternary, atolls have evolved in response
to a series of sea-level oscillations. It was Daly who gave
these Quaternary ice ages such prominence in geological
interpretations of oceanic islands. His glacial control theory involved the eradication of coral reefs from areas at
their poleward limits, which he called marginal seas during
successive glaciations (although this view is not supported
by study of reefs at their latitudinal limit). He also believed
that the reef rim had been planed off during glacial
lowstands, and by inference that the entire reef rim had
accreted during the postglacial. Daly’s views were further
promoted by Wiens (1959, 1962), but are no longer
supported (see Glacial Control Hypothesis).
Significant further studies on atolls occurred after World
War II. American research in the Pacific involved a focus on
the atolls of the Marshall Islands. Geological studies were
prominent and the stratigraphy of atolls became better
known because of selection of sites for atomic bomb testing,
as well as through scientific curiosity. Seismic studies in
1946 and 1950 provided the first hint that the carbonates
were underlain by volcanic rocks (Raitt, 1954), together
with the recovery of noncarbonate rocks dredged from
depths greater than 1,400 m on the flanks of these islands.
In 1951, drilling on Bikini Atoll encountered basalt at
depths of 1,287 and 1,411 m respectively in two boreholes.
Examination of the limestones indicated that they had been
deposited in shallow water, and the presence of solutional
unconformities supported the episodic exposure of these
during successive sea-level lowstands (Schlanger, 1963).
Subsequently drilling on Mururoa has revealed
400–500 m of carbonate over the volcanic basement that
underlies that atoll (q.v.), with a similar stratigraphy also
on neighboring Fangataufu (Lalou et al., 1966). A Quaternary history of the past 300,000 years has been derived
(Camion et al., 2001) implying that the atolls became
more atoll-like as a result of dissolution of the lagoon
and buildout of the periphery through reef growth. On
Midway Atoll in the Hawaiian Islands (Midway Atoll
(Hawaiian Archipelago)), volcanic basement has been
encountered at 55 m beneath Sand Island and 378 m
beneath reef to the north of the lagoon, further supporting
Darwin’s subsidence theory (Ladd et al., 1967, 1970).
A series of further studies were initiated by the Pacific
Science Board during the period 1946–1969. This
included fieldwork on Arno, Ifaluk and Kapingamarangi
Atolls in what are now the Federated States of Micronesia,
Onotoa in Kiribati, and Raroia in French Polynesia.
A compilation of this work led to the publication of the
book on atolls by Wiens in 1962. This was also a period
during which the Atoll Research Bulletin was initiated
(Fosberg and Sachet, 1953; Spencer et al., 2008).
Quaternary evolution of atolls
Shallower drilling on several atolls has encountered Pleistocene reef limestone, often dated to the Last Interglacial, at
depths of 10–20 m below the modern atoll rim. In the Cocos
(Keeling) Islands Pleistocene limestone, shown to be of
Last Interglacial age, occurs at depths of 8–13 m below
sea level beneath each of the major islands, and seismic
reflection profiling records a reflector that correlates with
this discontinuity beneath the lagoon (Searle, 1994;
Woodroffe et al., 1994). This karstified Pleistocene limestone underlies the rim composed of Holocene limestones.
Pleistocene limestone has been shown to underlie the rim
of Tarawa Atoll (Marshall and Jacobson, 1985), Funafuti
Atoll (Ohde et al., 2002), several atolls in the northern Cook
Islands (Gray et al., 1992), as well as atolls in the Maldives
and Chagos Archipelagoes (Woodroffe, 2005). In some that
have been drilled to deeper depths, such as Eniwetak and
Mururoa, it is apparent that the last interglacial limestone
is underlain by older reef limestones deposited during preceding highstands (Szabo et al., 1985; Camoin et al., 2001).
When sea level was high during the last interglacial
(and presumably former interglacials) an atoll rim similar
to the modern existed, although no evidence remains as to
whether it contained islands (Perrin, 1990). During glaciation the reef limestones were exposed by the lower sea
level, and the emergent limestone underwent solution
(karstification). Atolls appear to be undergoing gradual
subsidence associated with plate migration (Scott and
Rotondo, 1983, see Subsidence Hypothesis of Reef Development), so when sea level rose again during postglacial
times it flooded the platform around 8000 years ago. There
are several atolls on which last interglacial limestone is
exposed at the surface (e.g., Aldabra in the western Indian
Ocean, Braithwaite et al., 1973; Anaa in French Polynesia,
Pirazzoli et al., 1988; and Christmas Island in eastern
Kiribati, Woodroffe and McLean, 1998). The extent to
which lowering and reshaping of the surface results from
subsidence or from solution remains an issue of debate
(Purdy and Winterer, 2001, 2006); erosion appears to have
accentuated lagoon morphology on many atolls.
Accretion of the reef rim
Holocene reef growth has been constrained by the pattern
of sea-level change; there appears to have been a lag
before corals reestablished over the Pleistocene substrate
around 8000 years ago. The reef grew in an effort to
catch-up with sea level, as revealed in the case of Cocos
in the Indian Ocean (Woodroffe et al., 1994) and Tarawa
in the Pacific Ocean (Marshall and Jacobson, 1985). After
reefs caught up with sea level, lateral progradation of the
reef seems to have occurred, particularly in those situations such as Suwarrow and Nukutipipi where there are
fossil algal rims abandoned behind the modern reef crest.
The majority of polar ice melt appears to have been
completed by 6000 years ago, and the volume of water
in the ocean at that time is likely to have been similar
to that of today. However, hydro-isostatic adjustments
mean that the details of relative sea-level history vary geographically (Lambeck, 2002, see Mid Holocene). In particular, a fall of sea level occurred relative to far-field
ATOLLS
59
8000 years (Ohde et al., 2002).
During the Quaternary, atolls have evolved in response
to a series of sea-level oscillations. It was Daly who gave
these Quaternary ice ages such prominence in geological
interpretations of oceanic islands. His glacial control theory involved the eradication of coral reefs from areas at
their poleward limits, which he called marginal seas during
successive glaciations (although this view is not supported
by study of reefs at their latitudinal limit). He also believed
that the reef rim had been planed off during glacial
lowstands, and by inference that the entire reef rim had
accreted during the postglacial. Daly’s views were further
promoted by Wiens (1959, 1962), but are no longer
supported (see Glacial Control Hypothesis).
Significant further studies on atolls occurred after World
War II. American research in the Pacific involved a focus on
the atolls of the Marshall Islands. Geological studies were
prominent and the stratigraphy of atolls became better
known because of selection of sites for atomic bomb testing,
as well as through scientific curiosity. Seismic studies in
1946 and 1950 provided the first hint that the carbonates
were underlain by volcanic rocks (Raitt, 1954), together
with the recovery of noncarbonate rocks dredged from
depths greater than 1,400 m on the flanks of these islands.
In 1951, drilling on Bikini Atoll encountered basalt at
depths of 1,287 and 1,411 m respectively in two boreholes.
Examination of the limestones indicated that they had been
deposited in shallow water, and the presence of solutional
unconformities supported the episodic exposure of these
during successive sea-level lowstands (Schlanger, 1963).
Subsequently drilling on Mururoa has revealed
400–500 m of carbonate over the volcanic basement that
underlies that atoll (q.v.), with a similar stratigraphy also
on neighboring Fangataufu (Lalou et al., 1966). A Quaternary history of the past 300,000 years has been derived
(Camion et al., 2001) implying that the atolls became
more atoll-like as a result of dissolution of the lagoon
and buildout of the periphery through reef growth. On
Midway Atoll in the Hawaiian Islands (Midway Atoll
(Hawaiian Archipelago)), volcanic basement has been
encountered at 55 m beneath Sand Island and 378 m
beneath reef to the north of the lagoon, further supporting
Darwin’s subsidence theory (Ladd et al., 1967, 1970).
A series of further studies were initiated by the Pacific
Science Board during the period 1946–1969. This
included fieldwork on Arno, Ifaluk and Kapingamarangi
Atolls in what are now the Federated States of Micronesia,
Onotoa in Kiribati, and Raroia in French Polynesia.
A compilation of this work led to the publication of the
book on atolls by Wiens in 1962. This was also a period
during which the Atoll Research Bulletin was initiated
(Fosberg and Sachet, 1953; Spencer et al., 2008).
Quaternary evolution of atolls
Shallower drilling on several atolls has encountered Pleistocene reef limestone, often dated to the Last Interglacial, at
depths of 10–20 m below the modern atoll rim. In the Cocos
(Keeling) Islands Pleistocene limestone, shown to be of
Last Interglacial age, occurs at depths of 8–13 m below
sea level beneath each of the major islands, and seismic
reflection profiling records a reflector that correlates with
this discontinuity beneath the lagoon (Searle, 1994;
Woodroffe et al., 1994). This karstified Pleistocene limestone underlies the rim composed of Holocene limestones.
Pleistocene limestone has been shown to underlie the rim
of Tarawa Atoll (Marshall and Jacobson, 1985), Funafuti
Atoll (Ohde et al., 2002), several atolls in the northern Cook
Islands (Gray et al., 1992), as well as atolls in the Maldives
and Chagos Archipelagoes (Woodroffe, 2005). In some that
have been drilled to deeper depths, such as Eniwetak and
Mururoa, it is apparent that the last interglacial limestone
is underlain by older reef limestones deposited during preceding highstands (Szabo et al., 1985; Camoin et al., 2001).
When sea level was high during the last interglacial
(and presumably former interglacials) an atoll rim similar
to the modern existed, although no evidence remains as to
whether it contained islands (Perrin, 1990). During glaciation the reef limestones were exposed by the lower sea
level, and the emergent limestone underwent solution
(karstification). Atolls appear to be undergoing gradual
subsidence associated with plate migration (Scott and
Rotondo, 1983, see Subsidence Hypothesis of Reef Development), so when sea level rose again during postglacial
times it flooded the platform around 8000 years ago. There
are several atolls on which last interglacial limestone is
exposed at the surface (e.g., Aldabra in the western Indian
Ocean, Braithwaite et al., 1973; Anaa in French Polynesia,
Pirazzoli et al., 1988; and Christmas Island in eastern
Kiribati, Woodroffe and McLean, 1998). The extent to
which lowering and reshaping of the surface results from
subsidence or from solution remains an issue of debate
(Purdy and Winterer, 2001, 2006); erosion appears to have
accentuated lagoon morphology on many atolls.
Accretion of the reef rim
Holocene reef growth has been constrained by the pattern
of sea-level change; there appears to have been a lag
before corals reestablished over the Pleistocene substrate
around 8000 years ago. The reef grew in an effort to
catch-up with sea level, as revealed in the case of Cocos
in the Indian Ocean (Woodroffe et al., 1994) and Tarawa
in the Pacific Ocean (Marshall and Jacobson, 1985). After
reefs caught up with sea level, lateral progradation of the
reef seems to have occurred, particularly in those situations such as Suwarrow and Nukutipipi where there are
fossil algal rims abandoned behind the modern reef crest.
The majority of polar ice melt appears to have been
completed by 6000 years ago, and the volume of water
in the ocean at that time is likely to have been similar
to that of today. However, hydro-isostatic adjustments
mean that the details of relative sea-level history vary geographically (Lambeck, 2002, see Mid Holocene). In particular, a fall of sea level occurred relative to far-field
ATOLLS
59