Geology Reference
In-Depth Information
of 15 cm 100
1
years (Boardman
et al.
, 1989;
Kindler, 1992), it would have taken an additional
660 years before fl ooding the Pleistocene reef ter-
race at Highborne Cay and subsequent reef initia-
tion. Incidentally, this coincides with the temporal
difference between the developmental stages of
Stocking Island versus Highborne Cay.
This study offers no formal explanation for
the existence of a coral surface below the
stromatolites at Highborne Cay, and apparent lack
thereof at Stocking Island. However, it could be
speculated that just as the basal coral surface, to
date, has only been observed at the southern end
of the Highborne Cay reef complex, the single core
transect across the Stocking Island reef may not
capture the full spatial subsurface variability.
Irrespective of temporal differences in reef
growth at Stocking Island and Highborne Cay,
both reefs exhibit a dominance in microbial
versus reef building organisms for the past 500-
1000 years. The extended range of the coral dates
(790-566 years) suggests that the demise of the
coral patch reef at Highborne Cay was not abrupt,
but rather a continuous change from metazoan to
microbial reef builders. This study proposes that,
in comparison to today, the relative dominance
of controls such as sediment stress, sediment
patterns, burial amplitude and frequency have
changed. The demise of the coral reef and rise
of the microbial reef corresponded to increasing
sediment input, and as sand fi lled the accommo-
dation space seaward of the fore reef, it spilled
over the reef crest into the back reef lagoon where
it buried reef builders for prolonged periods of
time. Although corals, coralline algae and ver-
metid gastropods might have coped with sedi-
ment input and abrasion, they could not cope
with month-long burial. Stromatolites, however,
depend on frequent sand burial, not only for ver-
tical growth, but also to limit eukaryotic surface
cover and bioerosion (Andres & Reid, 2006).
In summary, a simple model is proposed:
increased sediment stress and corresponding
increased frequency and duration of sediment
burial led to the demise of a coral reef and the
rise of a microbial reef at Highborne Cay and
extensive microbialite development in the back
reef at Stocking Island. Although proposing a
single dominant process might be considered a
reduced approach, it explains the late Holocene
to present development of the discussed reefs.
The lack of microbialite development on all
other windward margins of the Exuma Islands,
or for that matter in all sediment-dominated reefal
settings, indicates that other factors, such as
available substrate, act synergistically with
sedimentation in controlling reef initiation and
determining reef biota. The observed sediment
dynamics are necessary to limit or even exclude
metazoan reef builders yet possibly not suffi cient
to explain the presence of microbialites.
FUTURE REEF DEVELOPMENT
Whereas the Stocking Island reef has been degrad-
ing for the past ~500 years (Macintyre
et al.
, 1996),
the Highborne Cay reef appears to be at or slightly
beyond its apex. We predict continued micro-
bial growth in the Highborne Cay reef complex is
predicted in the future. With slow sea-level rise
or even stagnation and fi lling of accommodation
space, we expect lateral stromatolite expansion,
coalescing, and the formation of increasingly tab-
ular structures, such as those at Stocking Island is
expected. Alternatively, with predicted increased
rates of sea-level rise due to global change of
0.09-0.88 m by 2100 (Houghton
et al.
, 2001), there
might be more accommodation space, resulting in
predominant vertical rather than lateral growth.
Extensive bioerosion by sea urchins, clams,
bivalves and the roots of macroalgae is obvious
in both Highborne Cay and Stocking Island reef
systems and will continue to occur. The coral-
line algal ridge at Highborne Cay may degrade
to a submerged algal fl at as observed at Stocking
Island.
The patch reefs south of Site 1 at Highborne Cay
(Fig. 3a, g and h) present a good analogue with
respect to Highborne Cay roughly 800 years ago.
Here, NW to SE sand bars increasingly impinge
onto the patch reef resulting in white coral skele-
tons due to prolonged sand burial and subsequent
demise (Fig. 3g). This is in stark contrast to the
windward E-facing side, where an over 2 m high
relief hinders sediment impingement (Fig. 3h).
With continued sediment burial in the future the
patch reefs to the south might face the same fate
as their northern counterparts: coral demise and
microbialite rise.
Geological signifi cance
Modern settings offer the opportunity to charac-
terize the ambient physical environment of a reef
and, more importantly to characterize and identify
