Another good example of reef back-stepping in
a rapidly subsiding terrane has been reported from Hawaii
(Webster et al., 2004a, 2007). There, average long-term
subsidence associated with volcanic loading of the lithosphere has been measured at 2.7 mm/year over the last
$500 kyr (Sharp and Renne 2006). This submergence
coupled with glacio-eustatic sea-level change has produced 12 back-stepping linear ridges, of which 3 have
been confirmed as reefal in origin and have radiometric
ages (H7, 392 kyr; H2, 136 kyr; H1, 14.7 kyr; Webster
et al., 2009). The oldest of these ridges (H7) returned
U-series ages of 392–377 ka from corals at the base of
the sequence. But this sequence also showed evidence of
erosional breaks indicating the ridge may, in fact, be
a composite unit consisting of several superimposed episodes of reef development (Webster et al., 2009). This preliminary evidence of multicyclic ridge development is
also consistent with simulated reef development during
the last two glacio-eustatic sea-level cycles (Webster
et al., 2007).
The youngest of the Hawaiian ridges (H1) is a composite feature, consisting of three closely-spaced ridges
between 150 and 105 m water depth. The deepest ridge
has been investigated by ROV and samples from in-situ
corals collected from the reef crest, returned ages of
15.8–14.7 kyr indicating that it drowned following
Mwp-1a (Webster et al., 2004c). However, it is unknown
if these ridges consist of single reefs formed during
a single sea-level position or multicyclic sequences
formed during more than one sea-level position.
Cause of reef back-stepping
In both stable and subsiding terranes, the above evidence
indicates that the proximate cause of reef drowning and
back-stepping is rapid sea-level rise, particularly jumps
in sea level caused by pulses in meltwater and iceberg discharge during deglaciation (Blanchon and Shaw 1995;
Webster et al., 2004c). Direct measurement of the rise
rates during these jumps clearly falsifies early assumptions that reefs can easily outpace sea-level rise, and therefore largely negates the paradox of reef drowning
(Schlager 1981). Indeed, sea-level rise rates during these
jumps exceed the accretion potential of modern and late
Pleistocene reefs by as much as six times.
In addition to the rise rate, however, the magnitude of
the jump is also critical for the drowning and backstepping response of reefs. Evidence indicates that the
$15 m sea-level jump during Mwp-1a produced an ubiquitous back-stepping response from late Glacial reefs at all
sites investigated. Cored reef sequences from the oceanic
islands of Tahiti and Barbados both show that the position
of shallow-reef development switched to an upslope location in 500 years or less. On Barbados, A. palmata-dominated crests moved upslope between 11 and 16 m in
<450 years (although that duration could be significantly
reduced by dating the base of the reef-crest sequence in
RGF-12 in Figure 1). Whereas on Tahiti, new drilling on
the fore-reef slopes during IODP-310 has identified
drowned reefs related to the back-stepping following
Mwp-1a (Camoin et al., 2007), and indicates that reef relocation started at 14.6 and was complete by at least 14 ka
(although the precise timing and magnitude of this backstepping remains to be reported) (Figure 2). The isolated
oceanic nature of these two islands means that this backstepping event took place in healthy reef systems that were
largely unaffected by conditions that could suppress their
accretion potential and make them more susceptible to
drowning (such as rapid flooding of extensive continental
shelves). As a consequence, their back-stepping provides
clear support for the singular role of large-magnitude
jumps in sea level. It is likely that jumps of this magnitude
submerged reefs below an optimal accretion window and
re-established this window far enough upslope that the
original communities were unable to recover due to rapid
deterioration in light levels and/or sediment flux. This is
supported by core sequences from both islands, which
show an immediate switch to deeper-water communities
following Mwp-1a and indicate that only 10–15 m of subsequent deep-reef accretion took place. In the case of
Barbados, that accretion had ceased completely by
$12 ka when sea level had submerged the drowned reef
by $30 m (see RGF-15 in Figure 1, but note that accretion
in RGF-9 may have been supplemented by downslope
sedimentation from early reef growth at RGF-12).
The response of reefs to the subsequent smaller magnitude sea-level jumps, however, was not consistent
between oceanic reef provinces. In the Caribbean, for
example, Mwp-1b did trigger reef back-stepping. Barbados cores show that a reef-crest sequence at 50 m in
RGF-12 was abruptly replaced by a deeper A. cervicornis
unit, and shifted 5–10 m upslope in <314 years in RGF-8 -
(Figure 1). But at Tahiti, no evidence of back-stepping has
been found in cores through the modern reef crest, where
the sequence simply shows a switch from robustbranching to tabular-branching assemblages at $50 m. It
should be noted that this lack of evidence does not mean
that back-stepping did not occur, just that it is undocumented. It might be that the islands fringing reefs, which
have largely been ignored, initiated at this time.
A similar lack of consistency also resulted from the
final sea-level jump, Mwp-1c, starting at 8 ka. In the
Caribbean, many areas including Barbados show
A. palmata reefs that established at the beginning Holocene had died off by $8 kyr, just as modern reef structures
were initiating (Figure 1; Blanchon et al., 2002). Initial
explanations of this early Holocene die-off were formulated before there were adequate data on the age of modern
reef initiation, and so the close timing between the two
generally went unappreciated. Subsequently, however,
the age of modern reef initiation has been reported from
many areas including, more recently, southeast Florida
and St. Croix where it occurred between 7.8 and 7.6 ka
at depths of 10–12 m (Figures 1 and 3). Drowned reefs
that died off during the 8 ka jump have also been reported
from several areas, most recently Grand Caymen and the
82
BACK-STEPPING
a rapidly subsiding terrane has been reported from Hawaii
(Webster et al., 2004a, 2007). There, average long-term
subsidence associated with volcanic loading of the lithosphere has been measured at 2.7 mm/year over the last
$500 kyr (Sharp and Renne 2006). This submergence
coupled with glacio-eustatic sea-level change has produced 12 back-stepping linear ridges, of which 3 have
been confirmed as reefal in origin and have radiometric
ages (H7, 392 kyr; H2, 136 kyr; H1, 14.7 kyr; Webster
et al., 2009). The oldest of these ridges (H7) returned
U-series ages of 392–377 ka from corals at the base of
the sequence. But this sequence also showed evidence of
erosional breaks indicating the ridge may, in fact, be
a composite unit consisting of several superimposed episodes of reef development (Webster et al., 2009). This preliminary evidence of multicyclic ridge development is
also consistent with simulated reef development during
the last two glacio-eustatic sea-level cycles (Webster
et al., 2007).
The youngest of the Hawaiian ridges (H1) is a composite feature, consisting of three closely-spaced ridges
between 150 and 105 m water depth. The deepest ridge
has been investigated by ROV and samples from in-situ
corals collected from the reef crest, returned ages of
15.8–14.7 kyr indicating that it drowned following
Mwp-1a (Webster et al., 2004c). However, it is unknown
if these ridges consist of single reefs formed during
a single sea-level position or multicyclic sequences
formed during more than one sea-level position.
Cause of reef back-stepping
In both stable and subsiding terranes, the above evidence
indicates that the proximate cause of reef drowning and
back-stepping is rapid sea-level rise, particularly jumps
in sea level caused by pulses in meltwater and iceberg discharge during deglaciation (Blanchon and Shaw 1995;
Webster et al., 2004c). Direct measurement of the rise
rates during these jumps clearly falsifies early assumptions that reefs can easily outpace sea-level rise, and therefore largely negates the paradox of reef drowning
(Schlager 1981). Indeed, sea-level rise rates during these
jumps exceed the accretion potential of modern and late
Pleistocene reefs by as much as six times.
In addition to the rise rate, however, the magnitude of
the jump is also critical for the drowning and backstepping response of reefs. Evidence indicates that the
$15 m sea-level jump during Mwp-1a produced an ubiquitous back-stepping response from late Glacial reefs at all
sites investigated. Cored reef sequences from the oceanic
islands of Tahiti and Barbados both show that the position
of shallow-reef development switched to an upslope location in 500 years or less. On Barbados, A. palmata-dominated crests moved upslope between 11 and 16 m in
<450 years (although that duration could be significantly
reduced by dating the base of the reef-crest sequence in
RGF-12 in Figure 1). Whereas on Tahiti, new drilling on
the fore-reef slopes during IODP-310 has identified
drowned reefs related to the back-stepping following
Mwp-1a (Camoin et al., 2007), and indicates that reef relocation started at 14.6 and was complete by at least 14 ka
(although the precise timing and magnitude of this backstepping remains to be reported) (Figure 2). The isolated
oceanic nature of these two islands means that this backstepping event took place in healthy reef systems that were
largely unaffected by conditions that could suppress their
accretion potential and make them more susceptible to
drowning (such as rapid flooding of extensive continental
shelves). As a consequence, their back-stepping provides
clear support for the singular role of large-magnitude
jumps in sea level. It is likely that jumps of this magnitude
submerged reefs below an optimal accretion window and
re-established this window far enough upslope that the
original communities were unable to recover due to rapid
deterioration in light levels and/or sediment flux. This is
supported by core sequences from both islands, which
show an immediate switch to deeper-water communities
following Mwp-1a and indicate that only 10–15 m of subsequent deep-reef accretion took place. In the case of
Barbados, that accretion had ceased completely by
$12 ka when sea level had submerged the drowned reef
by $30 m (see RGF-15 in Figure 1, but note that accretion
in RGF-9 may have been supplemented by downslope
sedimentation from early reef growth at RGF-12).
The response of reefs to the subsequent smaller magnitude sea-level jumps, however, was not consistent
between oceanic reef provinces. In the Caribbean, for
example, Mwp-1b did trigger reef back-stepping. Barbados cores show that a reef-crest sequence at 50 m in
RGF-12 was abruptly replaced by a deeper A. cervicornis
unit, and shifted 5–10 m upslope in <314 years in RGF-8 -
(Figure 1). But at Tahiti, no evidence of back-stepping has
been found in cores through the modern reef crest, where
the sequence simply shows a switch from robustbranching to tabular-branching assemblages at $50 m. It
should be noted that this lack of evidence does not mean
that back-stepping did not occur, just that it is undocumented. It might be that the islands fringing reefs, which
have largely been ignored, initiated at this time.
A similar lack of consistency also resulted from the
final sea-level jump, Mwp-1c, starting at 8 ka. In the
Caribbean, many areas including Barbados show
A. palmata reefs that established at the beginning Holocene had died off by $8 kyr, just as modern reef structures
were initiating (Figure 1; Blanchon et al., 2002). Initial
explanations of this early Holocene die-off were formulated before there were adequate data on the age of modern
reef initiation, and so the close timing between the two
generally went unappreciated. Subsequently, however,
the age of modern reef initiation has been reported from
many areas including, more recently, southeast Florida
and St. Croix where it occurred between 7.8 and 7.6 ka
at depths of 10–12 m (Figures 1 and 3). Drowned reefs
that died off during the 8 ka jump have also been reported
from several areas, most recently Grand Caymen and the
82
BACK-STEPPING