(Lagoons) may have few such patch reefs, but in other
atolls there can be an intricate mesh of reefs forming a
reticulate network of shallow reefs, such as Mataiva (see
also Reticulated Reefs). The narrow ridges of reef separate
deeper holes, termed “Blue Holes”. Lagoons vary considerably in depth; they can be shallow, or tens of meters deep
(Gischler, 2006). Reginald Daly believed that there was an
overall similarity in the depth of many atolls, which he
considered provided support for his theory of glacial control (described below, see also Daly). However, it is now
recognized that there is considerably more variation in
depth than envisaged by Daly. Atolls in the southern
Maldives, for example, contain lagoons that are more than
70 m deep, but other lagoons may partially dry at low tide
(the southern end of the lagoon of the Cocos (Keeling)
Islands, for example).
The lagoon is a prominent feature of most atolls.
Lagoons are gradually infilling both with sediment produced within the lagoon, but more especially with sediment derived from the more productive reef rim. Purdy
and Gischler (2005) propose an “empty bucket” model
of lagoon infill, capturing the stages of infill after the surrounding reefs have caught up with sea level (Neumann
and Macintyre, 1985). The pattern of infill is likely to
depend on the nature of the reef rim, and sediment production may be proportional to the atoll’s perimeter
(Tudhope, 1989).
The lagoon at Enewetak has been described in a
detailed study by Emery et al. (1954). It reaches a depth
of 55 m, and the benthic communities form a series of concentric zones (Colin, 1986), with foraminifera a conspicuous component of lagoon sediments, and a patch reef zone
(termed knolls or pinnacle reefs by Emery) (Wardlaw
et al., 1991). Similar concentric zones are known to occur
in the sediments of Rongelap and Bikini Atolls (Emery
et al., 1954) and also in the case of Kapingamarangi Atoll
(McKee et al., 1959). More recent studies reaffirm the
significance of foraminifera, particularly Calcarina and
Heterostegina in each of Kayangel in the Palau Islands,
and Enewetak and Majuro in the Marshall Islands
(Yamano et al., 2002). There is much variability in the
lagoon floor sediments of Tarawa Atoll, in Kiribati, but
foraminifera are a prominent component (Weber and
Woodhead, 1972; Lovell, 2000; Paulay, 2001). The calcifying alga Halimeda is one of the most prominent features
of the sediments of some of the deeper lagoons; it dominates much of the interior of the lagoon of Suwarrow
(Tudhope et al., 1985). In the case of Cocos, much of the
lagoon is dominated by sediments produced within the
lagoon (Smithers et al., 1992). Mud may accumulate in
localized embayments within the islands, termed teloks on
Cocos and barachois on Diego Garcia (Stoddart, 1971).
On highly enclosed atolls, such as Marakei in the Gilbert
chain of Kiribati, lagoon sediment is mud, with reef flat
foraminifera found only at the entrance to the narrow reef
passage on the eastern margin (Woodroffe, 2008). Mud is
typical of the lagoon floor within the reticulated reefs of
Mataiva Atoll in French Polynesia (Adjas et al., 1990).
Although active vertical reef growth implies that the
reef rim might be a constructional feature, it has been
apparent since the study of MacNeil (1954) that solution
of the interior of the lagoon occurs during subaerial exposure when the sea is lower than present during glaciations.
This view, examined further below in relation to the geological evolution of atolls, has been especially promoted
by Purdy and Winterer (2001).
The geomorphology and biogeography of atolls has been
mapped in detail where ground reconnaissance has been
undertaken, notably through the extensive studies of David
Stoddart (Stoddart, David Ross (1937–)), or in a few instances where aerial photography is available at suitable
scales (Woodroffe and McLean, 1994). Recently, highresolution satellite imagery has been used in conjunction
with state-of-the-art remote sensing algorithms to map reef
geomorphology and habitat distribution (Andréfouët et al.,
2001, 2003, Naseer and Hatcher, 2004; Yamano et al., 2006).
Geological evolution of atolls
As the oceans were explored during the seventeenth and
eighteenth centuries, atolls became more broadly known.
We do not know when atolls first became a feature of
familiarity to European explorers, but they have certainly
been known since the Spaniard Mendana landed on an
atoll in Tuvalu. In the early nineteenth century, Charles
Lyell (Lyell, Charles (1797–1875)) promulgated the view
that atolls represented a coral veneer around the margin of
submerged volcanic craters, in his Principles of Geology
published in the 1830s, a book that was to have
a profound influence on Charles Darwin.
Charles Darwin (Darwin, Charles (1809–1882)) proposed an alternative theory, his theory of coral reef formation, after witnessing evidence of the vertical movement
of land during surveys by HMS Beagle on the coast of
South America. Darwin’s subsidence theory considered
that there “is but one alternative; namely the prolonged
subsidence of the foundations on which the atolls were
primarily based, together with the upward growth of the
reef-constructing corals. On this view every difficulty
vanishes; fringing reefs are thus converted into barrier
reefs; and barrier reefs, when encircling islands, are thus
converted into atolls, the instant the last pinnacle of land
sinks beneath the surface of the ocean” (Darwin, 1842,
p.109). This remarkable deduction that volcanic islands in
mid-ocean might undergo subsidence, and that reefs might
proceed through a sequence from fringing reef to barrier
reef to atoll, as a consequence of vertical reef growth to
sea level, had occurred to Darwin after observing the evidence of uplift in South America, before he ever saw
a reef. His hypothesis was reinforced when he viewed the
fringing reefs around Moorea from the slopes of Tahiti.
Although the Beagle passed atolls in the Pacific, it did not
stop at any. Darwin wrote the first draft of his theory of reef
development as the ship sailed to New Zealand (Stoddart,
1995). The Beagle had passed through the Tuamotu Archipelago in what is now French Polynesia, but the only atoll
ATOLLS
57
atolls there can be an intricate mesh of reefs forming a
reticulate network of shallow reefs, such as Mataiva (see
also Reticulated Reefs). The narrow ridges of reef separate
deeper holes, termed “Blue Holes”. Lagoons vary considerably in depth; they can be shallow, or tens of meters deep
(Gischler, 2006). Reginald Daly believed that there was an
overall similarity in the depth of many atolls, which he
considered provided support for his theory of glacial control (described below, see also Daly). However, it is now
recognized that there is considerably more variation in
depth than envisaged by Daly. Atolls in the southern
Maldives, for example, contain lagoons that are more than
70 m deep, but other lagoons may partially dry at low tide
(the southern end of the lagoon of the Cocos (Keeling)
Islands, for example).
The lagoon is a prominent feature of most atolls.
Lagoons are gradually infilling both with sediment produced within the lagoon, but more especially with sediment derived from the more productive reef rim. Purdy
and Gischler (2005) propose an “empty bucket” model
of lagoon infill, capturing the stages of infill after the surrounding reefs have caught up with sea level (Neumann
and Macintyre, 1985). The pattern of infill is likely to
depend on the nature of the reef rim, and sediment production may be proportional to the atoll’s perimeter
(Tudhope, 1989).
The lagoon at Enewetak has been described in a
detailed study by Emery et al. (1954). It reaches a depth
of 55 m, and the benthic communities form a series of concentric zones (Colin, 1986), with foraminifera a conspicuous component of lagoon sediments, and a patch reef zone
(termed knolls or pinnacle reefs by Emery) (Wardlaw
et al., 1991). Similar concentric zones are known to occur
in the sediments of Rongelap and Bikini Atolls (Emery
et al., 1954) and also in the case of Kapingamarangi Atoll
(McKee et al., 1959). More recent studies reaffirm the
significance of foraminifera, particularly Calcarina and
Heterostegina in each of Kayangel in the Palau Islands,
and Enewetak and Majuro in the Marshall Islands
(Yamano et al., 2002). There is much variability in the
lagoon floor sediments of Tarawa Atoll, in Kiribati, but
foraminifera are a prominent component (Weber and
Woodhead, 1972; Lovell, 2000; Paulay, 2001). The calcifying alga Halimeda is one of the most prominent features
of the sediments of some of the deeper lagoons; it dominates much of the interior of the lagoon of Suwarrow
(Tudhope et al., 1985). In the case of Cocos, much of the
lagoon is dominated by sediments produced within the
lagoon (Smithers et al., 1992). Mud may accumulate in
localized embayments within the islands, termed teloks on
Cocos and barachois on Diego Garcia (Stoddart, 1971).
On highly enclosed atolls, such as Marakei in the Gilbert
chain of Kiribati, lagoon sediment is mud, with reef flat
foraminifera found only at the entrance to the narrow reef
passage on the eastern margin (Woodroffe, 2008). Mud is
typical of the lagoon floor within the reticulated reefs of
Mataiva Atoll in French Polynesia (Adjas et al., 1990).
Although active vertical reef growth implies that the
reef rim might be a constructional feature, it has been
apparent since the study of MacNeil (1954) that solution
of the interior of the lagoon occurs during subaerial exposure when the sea is lower than present during glaciations.
This view, examined further below in relation to the geological evolution of atolls, has been especially promoted
by Purdy and Winterer (2001).
The geomorphology and biogeography of atolls has been
mapped in detail where ground reconnaissance has been
undertaken, notably through the extensive studies of David
Stoddart (Stoddart, David Ross (1937–)), or in a few instances where aerial photography is available at suitable
scales (Woodroffe and McLean, 1994). Recently, highresolution satellite imagery has been used in conjunction
with state-of-the-art remote sensing algorithms to map reef
geomorphology and habitat distribution (Andréfouët et al.,
2001, 2003, Naseer and Hatcher, 2004; Yamano et al., 2006).
Geological evolution of atolls
As the oceans were explored during the seventeenth and
eighteenth centuries, atolls became more broadly known.
We do not know when atolls first became a feature of
familiarity to European explorers, but they have certainly
been known since the Spaniard Mendana landed on an
atoll in Tuvalu. In the early nineteenth century, Charles
Lyell (Lyell, Charles (1797–1875)) promulgated the view
that atolls represented a coral veneer around the margin of
submerged volcanic craters, in his Principles of Geology
published in the 1830s, a book that was to have
a profound influence on Charles Darwin.
Charles Darwin (Darwin, Charles (1809–1882)) proposed an alternative theory, his theory of coral reef formation, after witnessing evidence of the vertical movement
of land during surveys by HMS Beagle on the coast of
South America. Darwin’s subsidence theory considered
that there “is but one alternative; namely the prolonged
subsidence of the foundations on which the atolls were
primarily based, together with the upward growth of the
reef-constructing corals. On this view every difficulty
vanishes; fringing reefs are thus converted into barrier
reefs; and barrier reefs, when encircling islands, are thus
converted into atolls, the instant the last pinnacle of land
sinks beneath the surface of the ocean” (Darwin, 1842,
p.109). This remarkable deduction that volcanic islands in
mid-ocean might undergo subsidence, and that reefs might
proceed through a sequence from fringing reef to barrier
reef to atoll, as a consequence of vertical reef growth to
sea level, had occurred to Darwin after observing the evidence of uplift in South America, before he ever saw
a reef. His hypothesis was reinforced when he viewed the
fringing reefs around Moorea from the slopes of Tahiti.
Although the Beagle passed atolls in the Pacific, it did not
stop at any. Darwin wrote the first draft of his theory of reef
development as the ship sailed to New Zealand (Stoddart,
1995). The Beagle had passed through the Tuamotu Archipelago in what is now French Polynesia, but the only atoll
ATOLLS
57