Geology Reference
In-Depth Information
Depth-related and species-related patterns of Holocene reef
accretion in the Caribbean and western Atlantic: a critical
assessment of existing models
DENNIS K. HUBBARD
Department of Geology, Oberlin College, Oberlin, OH 44074, USA (E-mail: dennis.hubbard@oberlin.edu)
ABSTRACT
Reef-accretion rate was measured in 151 core intervals from 12 Caribbean and
western Atlantic locations. Palaeowater depth for each core interval was determined
by comparing its position to the curve of Lighty
et al.
(1982), based on calendar years
before 1950. While the majority of the data points fell within the upper 15 m of the
water column, no clear depth-related pattern of reef accretion emerged. This is in sharp
contrast to the widely held assumption that reef accretion will decrease exponentially
with water depth at rates approximately an order of magnitude below the correspond-
ing rates of coral growth at the same depths. Similarly, reef-accretion rates from facies
dominated by branching
Acropora palmata
(3.83 m kyr
-1
) versus those associated with
massive corals (3.07 m kyr
-1
) were not signifi cantly different (
= 0.05), owing to high
variance across all water depths. Reef accretion showed a tendency to increase at
higher rates of sea-level rise, but that relationship was also non-signifi cant.
It is proposed that the known depth-related decrease in carbonate production may
be offset by a parallel drop in bioerosion. While available data generally support
this hypothesis, quantitative verifi cation must await careful measurements of both
biological degradation and transport along a depth gradient. Nevertheless, bioerosion
appears to not only play an important role in creating reef fabric, but to perhaps affect
patterns of reef accretion as much as initial calcifi cation. Regardless of the cause, the
patterns revealed by this study fl y in the face of the assumptions that underlie our most
widely accepted Holocene reef models. Clearly new ones are needed that emphasize
the varying contribution of biological material to what is largely a process of physical
aggradation - in short reef corals grow, coral reefs accrete.
Keywords
Reef accretion, Caribbean, Holocene, bioerosion, sea level,
Acropora.
INTRODUCTION
corallines and other organisms that secrete
calcium carbonate and 'build reefs'. Mention is
made of physical damage and bioerosion by graz-
ers and infauna (e.g. Ginsburg, 1958; Scoffi n, 1992)
but the role of construction was until recently
perceived as overwhelmingly dominant. As a
result, discussion of reefs throughout the later
twentieth century focused on reef 'framework' as
the architect of this rigidity (Lowenstam, 1950;
Newell
et al
., 1953) and the importance of 'large,
colonial or gregarious, intergrown skeletal organ-
isms in general growth position' in creating it
(Fagerstrom, 1987).
More recently, the interiors of some Caribbean
coral reefs have been likened to 'garbage piles'
that comprise as much sediment and toppled cor-
als as in-place or interlocking organisms (Hubbard
et al
., 1990). These ideas challenge the primacy
What emerges from the myriad defi nitions of
reefs is that they are resistant structures that
stand above their surroundings, thus exert-
ing some degree of infl uence over local circula-
tion. Reefs can be built by organisms as large as
modern corals or Cretaceous rudists, or as small
as Precambrian microbes. They might emerge as
fringing and barrier reefs or form submerged fea-
tures along shelf margins (Macintyre
et al
., 1981;
Hubbard
et al
., 1997, 2005), or even in very deep
water well below the photic zone (Fosså
et al
.,
2002; Reed
et al
., 2005). Disagreements focus on
how resistant these features need to be and what
is responsible for that rigidity. Throughout most
of the twentieth century, descriptions of modern
reefs emphasized the constructive role of corals,
