Inner-shelf reefs
As the inner shelves continued to flood and sea level slowed
further, the locus of reef development shifted to progressively shallower sites. As was the case for the deep reefs
off Barbados, this “back-stepping” often began before the
shelf-edge reefs were abandoned. On Lang Bank, reef
accretion along the margin continued until ca. 6,500–
6,000 cal BP, over a thousand years after now-emergent
reefs to the west had already started up closer to the shore
(Adey, 1975; Burke et al., 1989; Hubbard et al., 2005;
Macintyre et al., 2008). Similarly, when the shelf-edge reefs
off southwestern Puerto Rico were abandoned ($6,800 cal
BP), the mid-shelf reefs had already been active for nearly
a millennium (Hubbard et al., 1997). In many instances,
these shallower reefs sat atop antecedent highs, but Adey
(1988) showed that some early reefs stabilized mobile
bioclastic sand shoals – perhaps equivalent to the detrital
intervals described above for many shelf-edge reefs. It is
probably not coincidence that presently emergent reefs
started to build to sea level at the same time when barrier
islands were forming all around the world. While the
sources of material were different, slowing of sea level facilitated the filling of accommodation space in both instances.
What have we learned?
While this section focuses on the southeastern Caribbean,
lessons have been learned that are applicable to the
broader Caribbean. Cores off Barbados (Fairbanks,
1989) have verified earlier suggestions that reefs below
the edges of many platforms are probably relict Holocene
features left behind by rapidly rising sea level (Vaughan,
1916; Macintyre, 1972). The potential importance of sudden and rapid upward shifts in sea level has also been
clearly demonstrated.
The role of “inimical bank waters”
The earlier models of Adey (1978) and Macintyre
(1988) introduced the idea that turbid water associated
with platform flooding retarded reef building for up to
4,000 years along Lang Bank until water quality
improved. While poor water quality undoubtedly played
an important role in early shelf-edge reef accretion on St.
Croix, its effects were more complex and perhaps
shorter-lived that has been assumed. Throughout the
region, active A. palmata reefs built uninterrupted across
much of the interval for which reef-building was previously characterized as minimal. Many of these reefs continued to flourish until 7,000 cal BP and beyond.
Bioclastic debris near the base of many cores may
reflect an additional mechanism that is more indirectly
related to flooding – intense bioerosion driven by elevated
nutrient levels (Schlager, 1981; Hallock and Schlager,
1986). Adey and Burke (1976) described significant detrital intervals beneath the shallow “bank-barrier” reefs on
northeastern St. Croix and concluded that this represented
stabilization by an early coral community followed by
active coral-dominated accretion. Similar patterns have
been documented along the shelf margins described above
and may represent an early standoff between carbonate
production and bioerosion. An important lesson here is
that the oldest preserved corals need not reflect the first
appearance of coral communities with reef-building
potential. Thus, water quality may have been adequate to
support widespread coral growth but not reef accretion.
St. Croix as a “Caribbean model”
Because many shelf and platform margins throughout the
Caribbean sit at water depths similar to Lang Bank, the
scenario from St. Croix was extended to western Atlantic
platforms under the influence of medium to high energy:
“Where the shelf edge is between 10 and 30 m, early Holocene reefs were killed off owing to turbidity, and continuous Holocene reef building did not occur adjacent to deep
oceanic waters” (Adey, 1988). “By the time water conditions returned to normal, the advancing seas were too deep
to allow shallow-water communities of A. palmata to
reestablish. The rapid reef growth of these communities
was thus transferred to shallow-water substrates adjacent
to coastlines” (Macintyre, 1988). More recent studies have
shown that local tectonic and varying oceanographic processes have resulted in significant variability in antecedent
topography, both regionally (see Fig. 3 in Adey, 1978) and
locally (Burke et al., 1989; Macintyre et al., 2008; Hubbard et al., 2005, 2008; Bard et al., 2010). It, therefore,
seems more realistic to expect nonsynchronous patterns
of shelf flooding, wide variations in the importance of
“inimical bank waters,” and varying accretionary histories
controlled by the details of antecedent topography and
local oceanographic conditions (Hubbard et al., 2008;
Macintyre et al., 2008).
The role of sea level
Figure 8 summarizes our present understanding of Caribbean sea-level rise (a) and reef development (b). The lower
sea-level curve is based on core data from Fairbanks
(1989) while the upper curve is modified from Lighty
et al. (1982). While some studies have challenged the
details of this curve (e.g., Gischler and Hudson, 2004),
all agree that sea level rose rapidly (avg. $5–7 m/ky,
with shorter-lived episodes of faster rise) until ca.
8,000–6,000 years ago, after which it decreased to rates
below 2–3 m/ky.
Whether reefs remain submerged or are close to sea
level today was determined primarily by the rate of sealevel rise at the time when they started to build. Hubbard
(2009a) calculated an average accretion rate for Caribbean
reefs between 3 and 4 m/ky using data from all published
Caribbean core logs. The mean accretion rate for reefs
dominated by A. palmata versus massive species varied
by less than 0.5 m/ky at any depth (see their Fig. 6). Similarly, the accretion rate for all paleo-depths between À2.5
and À22.5 m was $3.5 m/ky, and the differences were statistically insignificant (R
2 = 0.003). Short-term accretion
rates (i.e., between successive dated samples) were
344
EASTERN CARIBBEAN CORAL REEFS
As the inner shelves continued to flood and sea level slowed
further, the locus of reef development shifted to progressively shallower sites. As was the case for the deep reefs
off Barbados, this “back-stepping” often began before the
shelf-edge reefs were abandoned. On Lang Bank, reef
accretion along the margin continued until ca. 6,500–
6,000 cal BP, over a thousand years after now-emergent
reefs to the west had already started up closer to the shore
(Adey, 1975; Burke et al., 1989; Hubbard et al., 2005;
Macintyre et al., 2008). Similarly, when the shelf-edge reefs
off southwestern Puerto Rico were abandoned ($6,800 cal
BP), the mid-shelf reefs had already been active for nearly
a millennium (Hubbard et al., 1997). In many instances,
these shallower reefs sat atop antecedent highs, but Adey
(1988) showed that some early reefs stabilized mobile
bioclastic sand shoals – perhaps equivalent to the detrital
intervals described above for many shelf-edge reefs. It is
probably not coincidence that presently emergent reefs
started to build to sea level at the same time when barrier
islands were forming all around the world. While the
sources of material were different, slowing of sea level facilitated the filling of accommodation space in both instances.
What have we learned?
While this section focuses on the southeastern Caribbean,
lessons have been learned that are applicable to the
broader Caribbean. Cores off Barbados (Fairbanks,
1989) have verified earlier suggestions that reefs below
the edges of many platforms are probably relict Holocene
features left behind by rapidly rising sea level (Vaughan,
1916; Macintyre, 1972). The potential importance of sudden and rapid upward shifts in sea level has also been
clearly demonstrated.
The role of “inimical bank waters”
The earlier models of Adey (1978) and Macintyre
(1988) introduced the idea that turbid water associated
with platform flooding retarded reef building for up to
4,000 years along Lang Bank until water quality
improved. While poor water quality undoubtedly played
an important role in early shelf-edge reef accretion on St.
Croix, its effects were more complex and perhaps
shorter-lived that has been assumed. Throughout the
region, active A. palmata reefs built uninterrupted across
much of the interval for which reef-building was previously characterized as minimal. Many of these reefs continued to flourish until 7,000 cal BP and beyond.
Bioclastic debris near the base of many cores may
reflect an additional mechanism that is more indirectly
related to flooding – intense bioerosion driven by elevated
nutrient levels (Schlager, 1981; Hallock and Schlager,
1986). Adey and Burke (1976) described significant detrital intervals beneath the shallow “bank-barrier” reefs on
northeastern St. Croix and concluded that this represented
stabilization by an early coral community followed by
active coral-dominated accretion. Similar patterns have
been documented along the shelf margins described above
and may represent an early standoff between carbonate
production and bioerosion. An important lesson here is
that the oldest preserved corals need not reflect the first
appearance of coral communities with reef-building
potential. Thus, water quality may have been adequate to
support widespread coral growth but not reef accretion.
St. Croix as a “Caribbean model”
Because many shelf and platform margins throughout the
Caribbean sit at water depths similar to Lang Bank, the
scenario from St. Croix was extended to western Atlantic
platforms under the influence of medium to high energy:
“Where the shelf edge is between 10 and 30 m, early Holocene reefs were killed off owing to turbidity, and continuous Holocene reef building did not occur adjacent to deep
oceanic waters” (Adey, 1988). “By the time water conditions returned to normal, the advancing seas were too deep
to allow shallow-water communities of A. palmata to
reestablish. The rapid reef growth of these communities
was thus transferred to shallow-water substrates adjacent
to coastlines” (Macintyre, 1988). More recent studies have
shown that local tectonic and varying oceanographic processes have resulted in significant variability in antecedent
topography, both regionally (see Fig. 3 in Adey, 1978) and
locally (Burke et al., 1989; Macintyre et al., 2008; Hubbard et al., 2005, 2008; Bard et al., 2010). It, therefore,
seems more realistic to expect nonsynchronous patterns
of shelf flooding, wide variations in the importance of
“inimical bank waters,” and varying accretionary histories
controlled by the details of antecedent topography and
local oceanographic conditions (Hubbard et al., 2008;
Macintyre et al., 2008).
The role of sea level
Figure 8 summarizes our present understanding of Caribbean sea-level rise (a) and reef development (b). The lower
sea-level curve is based on core data from Fairbanks
(1989) while the upper curve is modified from Lighty
et al. (1982). While some studies have challenged the
details of this curve (e.g., Gischler and Hudson, 2004),
all agree that sea level rose rapidly (avg. $5–7 m/ky,
with shorter-lived episodes of faster rise) until ca.
8,000–6,000 years ago, after which it decreased to rates
below 2–3 m/ky.
Whether reefs remain submerged or are close to sea
level today was determined primarily by the rate of sealevel rise at the time when they started to build. Hubbard
(2009a) calculated an average accretion rate for Caribbean
reefs between 3 and 4 m/ky using data from all published
Caribbean core logs. The mean accretion rate for reefs
dominated by A. palmata versus massive species varied
by less than 0.5 m/ky at any depth (see their Fig. 6). Similarly, the accretion rate for all paleo-depths between À2.5
and À22.5 m was $3.5 m/ky, and the differences were statistically insignificant (R
2 = 0.003). Short-term accretion
rates (i.e., between successive dated samples) were
344
EASTERN CARIBBEAN CORAL REEFS