The Building of A Reef

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The Building of A Reef

• Coral Growth - The Constructive Stage
• Bioerosion - The Destructive Stage
• Sediment Production and Redistribution

Wilson Ramirez
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Constructive Stage - Carbonate Production by
Corals and other Organisms
The most important processes in the marine
system can be described by the formula Ca
2HCO3- ltgt CaCO3 CO2 H2O The vigor with
which aragonite will form is thus related to the
abundance of free calcium (Ca) and HCO3-. The
addition of CO2 to water ultimately makes both of
these available through the following
process CO2 H2O ltgt H2CO3 ltgt H HCO3-
Ca Free H, left over from the calcification
process lowers the pH (makes the solution
acidic). Conversely, dissolution of carbonate
will increase pH. The ability of various
organisms to regulate pH within their tissues,
and drive the reaction toward the precipitation
of aragonite, may be an important factor in
biologically-mediated calcification.
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Bioerosion - Destructive Stage

• While corals and coralline algae are capable of
  producing massive structures over time, most
  other organisms living in and on the reef counter
  that process in their quest for food (grazers) or
  shelter (borers).
• This process was termed bioerosion by Neumann,
  and has subsequently been recognized as a major
  factor in both the biological and geological
  development of reefs.

Grazers and Predators
Borers
Rates of Bioerosion
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Bioerosion
After Scoffin (1972)
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Algae and Overgrowth

• All dead surfaces of the reef are rapidly
  overgrown by a thin film of filamentous green
  algae.
• These form broad algal turfs that are a favorite
  diet of many fishes and urchins. Some algae bore
  tiny but ubiquitous holes into the reef surface.

• These endolithic algae can weaken the substrate,
  making it more susceptible to damage by grazers.

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Grazers
While some grazers (i.e. damselfish) selectively
pluck turfs from the substrate, and actively
"farm" the turfs within their territories most
grazers are less selective.
The algae are digested, and the remainder is
passed through the gut, mostly as sand.
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Predators
Parrotfish bite off pieces of substrate and pass
them through a rasping structure, the pharyngeal
mill, which produces a mixture of algae and
sediment.

• Some predators feed on live coral.
• Along the Great Barrier Reef, the
  Crown-of-Thorns starfish (Acanthaster plancii)
  has been the focus of national concern each time
  its population reaches epidemic proportions and
  devastates large areas of live coral.
• In the Caribbean, coralliophyla (coral devoring
  snails) are becoming larger and more common and
  the number and size of fire worms is increasing.
  Both of these feed on coral.

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DANGEROUS PREDATORS ?
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Borers
Boring into reefal rock by Clinoid sponge
The upper core is of a relatively undisturbed
reef coral. The lower core shows extensive boring
by bivalves.
Lithophega can reach 30 cm in length and, in
isolated instances, over 50 individuals per cubic
meter can be found within a patch of reef.
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Rates of Bioerosion

• The best estimates of bioerosion come from
  controlled experiments in both the laboratory and
  the field.
• Based on these, grazers appear to be responsible
  for better than half of the bioerosion in
  Caribbean reefs.
• Ogden proposed a rate of 0.49 kg/m2-yr for a
  small reef system on the north side of St. Croix
  (Caribbean Sea).
• This was computed from the amount of sediment
  produced by an "average" fish (determined by
  divers collecting "samples" from numerous fishes)
  multiplied by the number of defecations per fish
  and the number of fish on the reef.

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Rates of Bioerosion

• At many locations, urchins produce larger amounts
  of sediment (up to 5 kg/m2-yr avg 2kg/m2-yr,
  equally split between sand and mud.
• The relative importance of sponge boring was
  determined for St. Croix by Moore and Shedd who
  measured rates averaging near 1.25 kg/m2-yr, with
  90 of this being mud.
• Rates exceeding 4 kg/m2-yr are certainly
  possible.

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Sediment Production Redistribution

• Sediment in the reef is derived from two primary
  sources.
• The most important is bioerosion.
• The other is the death and disintegration of
  skeletal remains of other organisms living on or
  around the reef.
• Primary among these are molluscs, foraminifera
  and upright, carbonate-producing algae

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Sediment Production Redistribution

• Character of the reef interior reflects a
  constant battle between coralgal (corals and
  algae) construction and subsequent degradation by
  bioeroders.
• As soon as a coral dies, it is aggressively
  attacked by bioeroders.
• When infauna die, their galleries are usually
  filled in by muddy sediment, which is in turn
• Bound together by chemically precipitated
  cements. This process may be repeated many times
  over even a decade, leaving a fabric that is very
  complex and in some instances retains little
  evidence of the original coral.

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Reef Accretion

• The nature of a reef and the rate at which it
  accretes is the result of this complex interplay
  of factors the term reef accretion is much more
  accurate that the more-commonly used reef
  "growth" in reflecting this constant battle
  between constructive and destructive processes.

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Carbonate Budget

• Recognizable coral comprises about 42 of the
  reef fabric the remainder was made up of
  sediment (41) and open void (17).
• Of the recognizable coral, only a small portion
  is usually in place.
• In most reefs, the proportion of recognizable
  coral is much smaller.
• As such, reefs are clearly not dominated by
  in-place and interlocking framework.

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Carbonate Budget

• Roughly half of the sediment produced by
  bioerosion is retained within the reef.
• The remainder has been deposited in the sand
  channels that cross the deeper sections of the
  reef.
• This sand is capable of accreting at a rate
  exceeding that of the intervening reef.
• Thus, periodic export is necessary.
• The volume moved from the channels under
  day-to-day conditions is insufficient to offset
  the inbalance predicted from the budget
  equation, and it has been proposed that storms
  are required to remove this excess.

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Carbonate budget
The general model for Jamaican reefs proposed by
Land represents a milestone in reef-budgeting
studies and has served as the basis for most
subsequent attempts Pg - Pn Sedp - Seds,
where Pg gross carbonate production Pn
net carbonate production (including
reincorporated sediment) Sedp sediment produced
within the reef Seds sediment stored within the
reef channels (sediment not reincorporated)
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Gross production
Gross production (Pg) is the total amount of
carbonate produced on a reef over some period of
time. In a sense, it represents the "potential
accretion rate" of a reef before bioerosion and
sediment export are considered. Estimates of
gross production generally range between 0.8 and
1.4 kg/m2-yr for whole reefs and from 2.1 to 8.9
kg/m2-yr for specific reef zones. At any one
time, the reef surface is occupied by some
percentage of live coral, dead and algal-covered
surface and loose sediment. Carbonate production
by live coral will depend on total cover by each
species and its depth-dependent growth rate
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Surveying Coral ReefsBasic photographic survey
The technique we use employs use of a Nikonos
underwater camera with a 28mm lens for
photography of the bottom from a vertical
distance of 1.2m (giving a photo area of 70x100
cm). The size of the quadrat used is 70x100 cm
for an area of 0.7m2. This is smaller than the
standard meter square quadrat but it reflects the
dimensions of the photo print and a transect of
10 quadrats can be taken in the field in the time
of recording a single meter quadrat by
inspection. Color negative film (Kodak 200ASA)
is used it is developed and printed as 4x6"
prints.
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How Fast?
Coral Growth Rate 1 - 30 cm/ year (10 - 300
m/1000 yrs) Reef Accretion 1 - 10 m/1000 yrs
(1-3 is average)
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Problems with Ancient Reefs

• 1. Evolution

22
http//www.ucmp.berkeley.edu/porifera/poriferafr.
html/
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Evolution

• Reef builders werent always corals

How do we compare a modern coral reef to an
ancient stromatoporoid reef?
Corals
Molluscs
Sponges
Stromatoporoids
Algae/bacteria
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Our Best Guess

• FUNCTIONAL MORPHOLOGY

Reinhold Leinfelder
Stromatoporoid
Devonian Seas
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The guild concept

• Fagerstrom (1991) identified five basic guilds
  to place reef organisms
• Constructors The constructors provide the
  building blocks of the reef, whatever their
  ultimate fate
• Binders constructors can be overgrown and bound
  together by algae, forams and other members of
  the binder guild
• Bafflers are those organisms that affect
  accretion by interrupting the flow of water,
  thereby encouraging sedimentation
• Destroyers include grazers and borers that
  break down the primary framework in various ways
• Dwellers passive inhabitants that contribute to
  the ecologic diversity of the reef but often have
  little to do with the actual accretionary
  process, except to help fill in the spaces
  within the reef interior

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Some organisms are extinct
How do we understand organisms that no longer
exist
Halucenogenia
Conway Morris (1977)
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Problems with Ancient Reefs
2. Taphonomy
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What is taphonomy?
Rapid Sedimentation
Burial
Karla Parsons-Hubbard
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Taphonomy is.

• Everything between death and fossilization
• Decay
• Disarticulation
• Dissolution
• Abrasion
• Overgrowth
• Ubiquitous removal

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It is affectd by

• Organisms involved
• Platy vs massive vs branching
• Organism mix
• Resistance to loss
• Processes that occur
• Biological
• Physical
• Chemical
• Rate
• Time

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Taphonomic Effects
Start with 4 of each
The Taphonomic Filter
3 mollusc species 2 coral species 16 species -gt
5 species
4 jellyfish 4 grasses 4 corals 4 molluscs -gt
16 species
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Which morph will disappear faster? How will
organism loss be different?
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The End Result

• Evolution - different organisms
• Soft bodies gone
• Skeletal organisms filtered
• Reduced diversity
• Harder to discuss interactions
• More apparent (?) stability
• Modern - ancient comparisons?

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So, what does a fossil reef tell us?

• What is left
• When those fauna were first deposited (?)
• How true is the history left behind?

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Reef Framework ?
Hubbard et al., 1997 and 2000
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Location Percent Recovery   Shallow-water
reefs Buck Island (north reefs) 29 Buck Island
(south reefs) 18 Buck Island Bar 20 Tague Bay
(St. Croix) 4 Salt River Canyon (St. Croix)
6   Mid-depth reefs Puerto Rico (Island
reefs) 16 Puerto Rico (mud mounds) 6 Puerto
Rico (mid-shelf) 24   Deeper-water reefs Salt
River (St. Croix) 32 Cane Bay (St.
Croix) 32 Lang Bank (St. Croix -
north) 30 Lang Bank (St. Croix -
south) 20 Puerto Rico (shelf edge) 14   Average
Shallow-water reefs 14 Mid-depth and
Deeper-water reefs 25 Grand Average 19
Recovery of coral in cores
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Internal structure of a reef
Seismic section across a shelf edge reef at La
Parguera, Puerto Rico and the results of a core
hole drilled on the inner of two ridges A
Holocene reef about 15 m thick accumulated over
the older Pleistocene reef (age 30,780 yBP) The
present surface of the reef is covered with a
thin layer of massive coral living at a water
depth of 20 m
Average Accretion rate of the reef 0.27 cm/yr
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Basic concepts

• True reefs need not be comprised of in-place
  and interlocking framework
• Process and not product are the common
  denominator that we seek in linking modern
  carbonate buildups to examples in the rock record