Geology Reference
In-Depth Information
Site 1 (Fig. 4), yet the other dates do not suggest
progressive oceanward coral demise. Combined
ages from the three platy brain corals (H-0410-
bsc- 1.1, -1.3, and -1.4a) indicate no pattern,
instead random coral death within a patch reef.
Noteworthy is the spread of coral ages of roughly
300 years with respect to the lack of vertical growth.
The youngest coral sample, H-0410-bsc -1.2 is a
small encrusting
Siderastraea radians
, growing at
the basal part of a stromatolite column (Fig. 6d).
Ages from the vertically cut stromatolite are
seemingly in reverse stratigraphic order: samples
from 10 and 20 cm depth date at 1570 years (on
average), whereas the underlying initial lamina at
30 cm depth is ~884 year old, or 690 years younger
than the overlying material at 10 and 20 cm depth.
This can be explained by considering the source
of the dated material: the material dated at 30 cm
was derived from micritic laminae consisting of
authigenically precipitated carbonate; in contrast
the older samples from 10 and 20 cm depth were
mainly trapped and bound surrounding sediment.
Indeed, the 1500-year-old dates for these ooids and
skeletal grains agree with previously published
ages for oolitic sands elsewhere in the Bahamas
(Martin & Ginsburg, 1965) and demonstrate that
the trapped and bound sediment in stromatolites
represent an old, and well-mixed population.
not, however, supported by the other dates. On
the other hand, the fi rst occurrence of stroma-
tolitic micritic laminae over coarse grainstone,
dated at 884 yr
BP
, indicates the presence of
coarse-grained sediment and implies a process
promoting the stabilization of grains. Coral death
around 970 yr
BP
and initial stromatolitic laminae
could be explained by decreasing hydrodynamic
energy due to the emerging algal ridge as pro-
posed for Stocking Island (Macintyre
et al.
, 1996)
resulting in more sediment, and/or the presence
of a stabilizing agent such as microbial mats.
Whatever cause or combination thereof, stromato-
lite laminae overlying this grainstone package are
evidence that conditions were right for the growth
of microbialites such as stromatolites. Coral dates
younger than 884 yr
BP
(samples H-0410-bsc-1.1,
-1.2, and -1.3), in particular the presence of small
encrusting corals at the base of some of the stro-
matolite columns imply the co-habitation of coral
and microbial build-ups during this phase of reef
growth. All the coral species found in the top of
the basal surface (
Siderastraea
and
Diploria
) are
tolerant with respect to sediment, wave energy and
shallow water; noteworthy that this is an adequate
description of the current environment. Today,
however, corals such as
Siderastraea
and
Diploria
are predominantly found on the reef fl at, reef
crest and on the seaward edge of the reef platform
as even tolerant coral species cannot cope with
months-long burial below 30 cm of sand, which
is a common scenario in the back-reef lagoon.
Furthermore, the present-day corals on the reef
fl at are small, only 5-10 cm across, very similar to
the 566-year-old
Siderastraea
sampled at the base
of the stromatolite (Fig. 6d). We conclude that
compared with today, environmental conditions
deteriorated with respect to corals but improved
with respect to microbialite development.
In summary, up until 1000 years ago, a shallow-
water coral community dominated the Highborne
Cay reef. The time from 1000 to 500 years ago was
characterized by the co-habitation of stromatolites
and corals, but with increasingly deteriorating
conditions for corals. For the past 500 years,
the back-reef lagoon at Highborne Cay has been
dominated by microbial build-ups.
Demise of a coral reef, rise of a microbial reef
Finding and dating an exposed basal surface
in the Highborne Cay reef complex is central to
answering one of the fundamental outstanding
questions pertaining to the growth history: what
is the age of the stromatolites? Previously, it was
unclear if the modern stromatolites were growing
on an antecedent Holocene or Pleistocene surface.
Combining outcrop stratigraphy with the new age
dates of the corals below the stromatolites allows
this question to be addressed and provides a basis
for discussion of Highborne Cay reef development
within a temporal domain. Identifying and under-
standing the factors currently controlling reef
development offers an opportunity to explore and
speculate on the controls that have been acting in
the past and future.
Based on the age of the oldest coral (~970 yr
BP
),
the back-reef lagoon in the southern Highborne
Cay reef was dominated by a coral patch reef
1000 years ago. The demise of the most shore-
ward coral around 970 yr
BP
raises the possibility
of increased sediment input due to a prograding
beach. Progressive seaward demise of corals is
CONTROLS ON MICROBIAL VERSUS
METAZOAN REEF BUILDERS
Integrating and comparing data and observations
from the mixed microbial-coralline algae-coral
