the Indo-Pacific region itself (central ocean vs. continental
margin sites), have been affected by the two processes
described above. In the early Holocene, fast rising sea level
in both regions provided accommodation space for reef
growth, but after the mid Holocene (c. 7–5 ka cal BP) their
broad histories diverge (see Chapter entries “Holocene
Reefs : Thickness and Characteristics”, “Sea Level
Changes and Effects on Reef Growth”, “Reef Classification, Response to Sea Level Rise,” and “Reef structure”).
Caribbean model
Fast rising RSL in the early Holocene allowed high
reef accretion rates, particularly in the Caribbean, where
the average reef growth rate was c. 6 m/kyr (Dullo, 2005).
Caribbean reef-building coral consists primarily of
Acropora cervicornis (which grows in 5–15 m water depth)
and Acropora palmata (which grows in <5 m water depth).
Therefore, most Caribbean reef growth occurred after RSL
rose to within 15 m of present (c. 9 ka cal BP) (Toscano and
Macintyre, 2003). The exact timing of reef initiation
depends on the depth of the antecedent surface, and can vary
by c. 5 ka in locations with different reef-base elevations
(Hopley et al., 2007). Caribbean reef-building corals were
able to keep pace with RSL rise during the early Holocene
(keep-up mode: Neumann and Macintyre, 1985), but as
RSL slowed in the mid Holocene, back reef zones developed
dominated by unconsolidated sand and rubble, representing
a late succession stage of Caribbean reef development
(Steneck et al., 1998; Gischler and Hudson, 2004). As
RSL continued to rise at a slower rate in the late Holocene,
Acropora palmata accretion continued to keep pace and
slowly developed into framework-coral-covered reef flats.
Indo-Pacific model
In the Indo-Pacific, RSL rose more rapidly in the early
Holocene than in the Caribbean, although initial inundation of the À15 m contour occurred at about the same time
(c. 9 ka cal BP: Pirazzoli, 1991). Unlike in the Caribbean,
reef-building coral (Acroporids and Poritids) was unable
to keep pace with RSL rise, entering instead a ‘catch-up’
mode where reef accretion lagged RSL rise (McLean
and Woodroffe, 1994). As RSL rose to present in the mid
Holocene, coral growth caught up with RSL, robust head
corals with a dense framework were lain down, and an
algal crust developed. The exact timing of reef attaining
sea level is dictated by the depth of the antecedent surface
and the amount of ‘catch-up’ it had to undergo, although
this age range is much less than in the Caribbean because
of the abrupt nature of RSL stabilization in the mid
Holocene.
Under stable RSL from the mid Holocene onwards, tectonically stable locations saw lateral reef development
as accommodation space for vertical reef growth disappeared. In many oceanic locations, this produced large
reef flats and detrital carbonate platforms (Hopley, 1985).
As RSL began to fall in the center of the Indo-Pacific ocean
basins in the mid-late Holocene, these platforms became
emerged at low tide causing any coral living on them to
die. There are many examples of coral microatolls from
mid to late Holocene time stranded above their growth position on exposed reef flats in the Indo-Pacific region (e.g.
Cocos Keeling Islands: Woodroffe, 2005).
These two idealized models of reef development under
RSL rise assume no factors other than eustatic sea level
affect reef growth. There are, however, some locations in
the Indo-Pacific (e.g. Tahiti) where RSL has continued
to rise through the Holocene and reef development has
followed more closely the Caribbean than Indo-Pacific
model. This is due to a variety of factors including
volcanic and tectonic subsidence, which have caused continual, gradual submergence of Tahiti as it migrates away
from a volcanic hotspot (see Chapter entry “Volcanic
loading and Isostasy”). Similarly, in the Caribbean there
are emerged mid Holocene corals on the island of Barbados, which are evidence of plate tectonic movement in this
seismically active zone (Schellmann and Radtke, 2004).
While sea level was an important influence on
the growth of coral reefs worldwide during the mid Holocene, other elements of the coral growth environment may
also have been different to today’s. The period is often
referred to as the Holocene Climatic Optimum and in
the tropics much of the climatic data comes from the
corals themselves (see Chapter entry “Palaeoclimate
from corals”).
For many areas such as the Great Barrier Reef (GBR),
a warmer climate of about 1.3
C, less variable wet seasons, fewer droughts, floods, and weaker ENSO events
produced higher growth rates of individual corals (Gagan
et al., 1998) and of coral reefs (see Hopley et al, 2007,
ch. 11) within the period 8–7 ka which saw the most rapid
reef growth on the GBR. Similar warmer, wetter conditions
(from stronger monsoons) have been recorded elsewhere in
the tropics in the mid Holocene (e.g. Gagan et al., 2000).
Case study: the Great Barrier Reef
The GBR provides an example of how different types
of reef morphology are dependent on different RSL conditions. The GBR shelf is wide (c. 230 km at its widest
point) and has undergone differential movement through
the Holocene. The offshore part of the shelf has subsided over time compared to the inner shelf due to hydroisostatic loading, resulting in continuous RSL rise at the
outer edge and a mid-late Holocene RSL fall on the inner
part of the shelf (Nakada and Lambeck, 1989; Woodroffe,
2009). The reefs at the outer edge of the shelf are primarily
made up of framework coral, most often found in keep-up
situations, like those of the Caribbean. Inner shelf reefs,
on the other hand, are described as ‘detrital piles with coral
caps’ (Hopley et al., 2007, p. 390), with unconsolidated sediment increasing significantly across the shelf. Inner shelf
reefs have also undergone emergence and erosion through
the mid-late Holocene under slowly falling RSL.
Another factor affecting reef growth and morphology
was the “mid Holocene High Energy Window” (see entry).
MID HOLOCENE
699
margin sites), have been affected by the two processes
described above. In the early Holocene, fast rising sea level
in both regions provided accommodation space for reef
growth, but after the mid Holocene (c. 7–5 ka cal BP) their
broad histories diverge (see Chapter entries “Holocene
Reefs : Thickness and Characteristics”, “Sea Level
Changes and Effects on Reef Growth”, “Reef Classification, Response to Sea Level Rise,” and “Reef structure”).
Caribbean model
Fast rising RSL in the early Holocene allowed high
reef accretion rates, particularly in the Caribbean, where
the average reef growth rate was c. 6 m/kyr (Dullo, 2005).
Caribbean reef-building coral consists primarily of
Acropora cervicornis (which grows in 5–15 m water depth)
and Acropora palmata (which grows in <5 m water depth).
Therefore, most Caribbean reef growth occurred after RSL
rose to within 15 m of present (c. 9 ka cal BP) (Toscano and
Macintyre, 2003). The exact timing of reef initiation
depends on the depth of the antecedent surface, and can vary
by c. 5 ka in locations with different reef-base elevations
(Hopley et al., 2007). Caribbean reef-building corals were
able to keep pace with RSL rise during the early Holocene
(keep-up mode: Neumann and Macintyre, 1985), but as
RSL slowed in the mid Holocene, back reef zones developed
dominated by unconsolidated sand and rubble, representing
a late succession stage of Caribbean reef development
(Steneck et al., 1998; Gischler and Hudson, 2004). As
RSL continued to rise at a slower rate in the late Holocene,
Acropora palmata accretion continued to keep pace and
slowly developed into framework-coral-covered reef flats.
Indo-Pacific model
In the Indo-Pacific, RSL rose more rapidly in the early
Holocene than in the Caribbean, although initial inundation of the À15 m contour occurred at about the same time
(c. 9 ka cal BP: Pirazzoli, 1991). Unlike in the Caribbean,
reef-building coral (Acroporids and Poritids) was unable
to keep pace with RSL rise, entering instead a ‘catch-up’
mode where reef accretion lagged RSL rise (McLean
and Woodroffe, 1994). As RSL rose to present in the mid
Holocene, coral growth caught up with RSL, robust head
corals with a dense framework were lain down, and an
algal crust developed. The exact timing of reef attaining
sea level is dictated by the depth of the antecedent surface
and the amount of ‘catch-up’ it had to undergo, although
this age range is much less than in the Caribbean because
of the abrupt nature of RSL stabilization in the mid
Holocene.
Under stable RSL from the mid Holocene onwards, tectonically stable locations saw lateral reef development
as accommodation space for vertical reef growth disappeared. In many oceanic locations, this produced large
reef flats and detrital carbonate platforms (Hopley, 1985).
As RSL began to fall in the center of the Indo-Pacific ocean
basins in the mid-late Holocene, these platforms became
emerged at low tide causing any coral living on them to
die. There are many examples of coral microatolls from
mid to late Holocene time stranded above their growth position on exposed reef flats in the Indo-Pacific region (e.g.
Cocos Keeling Islands: Woodroffe, 2005).
These two idealized models of reef development under
RSL rise assume no factors other than eustatic sea level
affect reef growth. There are, however, some locations in
the Indo-Pacific (e.g. Tahiti) where RSL has continued
to rise through the Holocene and reef development has
followed more closely the Caribbean than Indo-Pacific
model. This is due to a variety of factors including
volcanic and tectonic subsidence, which have caused continual, gradual submergence of Tahiti as it migrates away
from a volcanic hotspot (see Chapter entry “Volcanic
loading and Isostasy”). Similarly, in the Caribbean there
are emerged mid Holocene corals on the island of Barbados, which are evidence of plate tectonic movement in this
seismically active zone (Schellmann and Radtke, 2004).
While sea level was an important influence on
the growth of coral reefs worldwide during the mid Holocene, other elements of the coral growth environment may
also have been different to today’s. The period is often
referred to as the Holocene Climatic Optimum and in
the tropics much of the climatic data comes from the
corals themselves (see Chapter entry “Palaeoclimate
from corals”).
For many areas such as the Great Barrier Reef (GBR),
a warmer climate of about 1.3
C, less variable wet seasons, fewer droughts, floods, and weaker ENSO events
produced higher growth rates of individual corals (Gagan
et al., 1998) and of coral reefs (see Hopley et al, 2007,
ch. 11) within the period 8–7 ka which saw the most rapid
reef growth on the GBR. Similar warmer, wetter conditions
(from stronger monsoons) have been recorded elsewhere in
the tropics in the mid Holocene (e.g. Gagan et al., 2000).
Case study: the Great Barrier Reef
The GBR provides an example of how different types
of reef morphology are dependent on different RSL conditions. The GBR shelf is wide (c. 230 km at its widest
point) and has undergone differential movement through
the Holocene. The offshore part of the shelf has subsided over time compared to the inner shelf due to hydroisostatic loading, resulting in continuous RSL rise at the
outer edge and a mid-late Holocene RSL fall on the inner
part of the shelf (Nakada and Lambeck, 1989; Woodroffe,
2009). The reefs at the outer edge of the shelf are primarily
made up of framework coral, most often found in keep-up
situations, like those of the Caribbean. Inner shelf reefs,
on the other hand, are described as ‘detrital piles with coral
caps’ (Hopley et al., 2007, p. 390), with unconsolidated sediment increasing significantly across the shelf. Inner shelf
reefs have also undergone emergence and erosion through
the mid-late Holocene under slowly falling RSL.
Another factor affecting reef growth and morphology
was the “mid Holocene High Energy Window” (see entry).
MID HOLOCENE
699