Geology Reference
In-Depth Information
Coral cover is rarely 100% on any reef. It has
been long understood that bioerosion and phys-
io-chemical damage occur on reefs (Grant, 1826),
and can easily reduce 70% of the carbonate pro-
duced in a reef to sediment (Stearn & Scoffi n,
1977; Hubbard
et al
., 1990; Conand
et al
., 1997).
Much of this is exported from the reef, especially
by storms (Woodley
et al
., 1981; Hubbard, 1992),
leaving only a portion of the original carbon-
ate fi xed by coral, corallines and other calcifi ers
within the reef edifi ce. What is surprising is the
apparent disconnection between the depth-related
patterns of coral growth and reef accretion that
has emerged from these analyses. The presump-
tion that the dominance of light-mediated calcifi -
cation will remain as the fundamental underlying
control of reef accretion has been widely held.
The results of this synthesis indicate otherwise.
Shallow < 5 m
10
Max = 16.66 m kyr
−
1
0
Shallow-reef accretion (
d
< 5 m)
Mid-depth = 5-10 m
10
Max = 13.47 m kyr
−
1
0
Deep >10 m
10
Max = 9.73 m kyr
−
1
0
Deep-reef accretion (
d
= 10-20 m)
0
5
10
15
20
Accretion rate (m kyr
−
1
)
Possible sources of error
Fig. 9.
Depth-related patterns of reef-accretion rates
from this study (blue curves), compared with rates suggested
by Schlager (1981; brown bars) for reefs in water depths
less than 5 m and greater than 10 m (
d
As in any budgeting exercise, an unexpected
result begs for consideration of two possibilities:
(1) the measurements are incorrect or somehow
inappropriate, or (2) some factor may have been
omitted from consideration. There are numer-
ous sources of inaccuracy in the data used for the
calculations. Precisely determining the position
of a sample within a core can be diffi cult, and
surely some of these reported values are less than
totally accurate. However, these are undoubt-
edly evenly distributed around the true values
and would logically cancel out one another. Also,
palaeowater depth is based on a sea-level curve
that is by necessity generalized, and represents a
regional average. As discussed above, honest dis-
agreements persist over the details of the Caribbean
sea-level curve over the past 12,000 years. Some
of this stems from different interpretations of the
same data. Toscano & Macintyre (2003) chose
to place their curve above the majority of the
A.
palmata
and below the mangrove peats from
Florida, Jamaica and Belize. Gischler (2006)
argued that the curve should be placed over both.
One might reasonably argue that there is some
validity in both arguments and that the truth
lies somewhere in between. If this is the case,
then the maximum variation from the curve used
here would be in the order of 1.5 m. The curve
used in this paper (from Hubbard
et al
., 2005)
is based on both mangrove peat (Florida and
Bermuda) and
A. palmata
. In that case, the verti-
cal variation of the peat, which was short-rooted,
was small and agreed with the line skirted over
depth). Means and
medians for each distribution are also shown. The range of
Holocene sea level proposed by Schlager (up to 7 m kyr
1
)
is shown by the diagonal pattern. Maximum accretion
rate decreased with depth (16.66 m kyr
1
shallower than
5 m; 9.73 m kyr
1
deeper than 10 m). However, the primary
mode of reef accretion remained below 4 m kyr
1
(dashed
black line) for all depths. Mean (large red dots) and median
accretion rates (yellow lines) for the three depth ranges are
nearly identical for all three depth ranges.
=
for this study fell below 4 m kyr
1
, regardless of
palaeowater depth. Moreover, mean accretion
rates for the three depth intervals were virtually
identical, as were the medians and the positions
of the primary modes.
If these patterns are representative of what is
actually happening on a larger scale, then we are
faced with a number of questions. What is respon-
sible for this surprising and seemingly counter-
intuitive fi nding? How do we explain a system
in which branching corals grow faster than their
massive counterparts, but where a depth-related
decrease in accretion is clearly absent? Can reefs
in water depths greater than 10 m actually keep
pace with those in shallow water where light
intensity and coral growth are at least an order of
magnitude higher? And, if so, how? Finally, what
are the implications for reefs keeping up or being
left behind by the sea-level rise in the distant past,
or the near future?
It comes as no surprise that reef-accretion rates
are signifi cantly below the rate of coral growth.
