Geology Reference
In-Depth Information
Sites 1 and 2, as well as to the south and north of
the fringing reef system (Fig. 3). Given the active
hydrodynamics it is diffi cult to argue for differ-
ent nutrient concentrations (or seawater compo-
sition) over any sustained period. Undoubtedly,
on calm summer days signifi cant temperature
differences are measured in the backreef lagoon
versus reef edge but are rarely maintained for more
than a few days as tides and easterly winds mix
the waters. An ambient-water, time-series sam-
pling programme is currently underway to better
understand the temporal nutrient characteristics
along the fringing reef at Highborne Cay.
In summary, the physical environmental
conditions are currently ideal with respect to
prolifi c stromatolite formation in the back-reef
lagoon and dominant coralline algae develop-
ment on the outer reef crest. The limited growth
of small encrusting corals on the reef fl at imply
overall unfavourable conditions with respect to
scleractinian reef builders today. Although other
factors (i.e. nutrients, temperature, salinity) may
play a role, the observations presented here sug-
gest that sediment and sediment dynamics are the
key factors governing the distribution of microbial
versus metazoan reef builders in the Highborne
Cay and Stocking Island reef systems today.
of reef initiation and early reef development in
general (Macintyre, 1988; Gischler & Hudson,
2004). Although many factors such as tempera-
ture, salinity, nutrients and carbonate satura-
tion state may affect reef development (Camoin
et al.
, 1997; Montaggioni & Faure, 1997; Kleypas
et al.
, 1999), the discussion above indicates that
sedimentation is the dominant factor controlling
present distribution of reef biota at Highborne Cay
and Stocking Island. It is therefore of interest to
consider how sediment-reef interactions at these
sites evolved through time.
Initial fl ooding of the Pleistocene reef terraces
in the Bahamas during the late Holocene sea-level
transgression took place around 4600 yr
BP
, assum-
ing the base of the Pleistocene reef around 2 m
below the Holocene reef (Boardman
et al.
, 1989).
This time is also marked by an abrupt decrease in
the rate of sea-level change from ~15 cm 100 yr
1
before 3800 yr
BP
to ~2 cm 100 yr
1
thereafter
(Kindler, 1992). This rate of change seemed con-
ducive to overall reef initiation and development
in the Caribbean region (Neumann & Macintyre,
1985) and is supported by the basal core date in
the Stocking Island reef (3750
70 yr
BP
).
Sand and ooid shoals started forming in the
Bahamas soon after the late Holocene sea-level
transgression (Martin & Ginsburg, 1965; Harris,
1979). Signifi cant bioclast production and shore-
ward transport is proposed for the post-3800 yr
BP
sea-level rate slowdown (Kindler, 1992). On
Stocking Island, the Pleistocene terrace lies
roughly 2 m above the sea fl oor. Thus, mobile
sediments seaward of the fore-reef area would
have had to overcome this step in order to have
a signifi cant impact on initial reef development.
Presently, current ripples characterize the sand
in more than 2 m of water depth in front of the
outer reef edge, indicating that currents dominate
at these depths. Occasionally sand in the fore reef
accumulates into sand waves, and water depth
decreases to such an extent that suspended sedi-
ment has an impact on the reef. Based on the rate
of sea-level rise up until ~2000 yr
BP
(Boardman
et al.
, 1989; Kindler, 1992), there was ample
accommodation space to fi ll as reefs had not
caught up to sea level, and together with a lag
in sediment production, early reef development
was not much impacted by sediment. In this
context, Steneck
et al.
(1997) concluded that '. . .
the insular shelf off the Exuma Sound, at the time
of initial reef and ridge development (
c
. 3000 yr BP)
may have had considerably less sediment than it
does today'.
Late Holocene reef development, Highborne
Cay and Stocking Island
To what extent were the current controls of reef
growth important in the nucleation and initiation
of the reefs at Stocking Island and Highborne Cay?
Have physical factors been acting with the same
intensity and frequency as today? The observation
that the back-reef lagoon of the Highborne Cay
reef exhibits a basal coral surface, together with
the documented Holocene history of the Stocking
Island reef imply changes through time in the
relative dominance of environmental factors con-
trolling the distribution of reef biota. A major fac-
tor to consider when interpreting late Holocene
reef history is sea-level rise.
Sea level-sediment interactions
What was the effect of late Holocene sea-level rise
with respect to the amount of sediment production,
composition and accumulation? For any reef, sea
level marks the upper limit on growth, irrespec-
tive of the reef-building assemblage. Indeed, sea
level in combination with antecedent topography,
which provides substrate, are key to the timing
