LAB 10
Reefs and Corals
Why study reefs
What a reef is
Reef terms
Reef structure
Reef zonation
Reefs through time
Corals
Other framework-builders
Why reefs?
There are three scientific justifications for studying reefs
1. Site of very high diversity of organisms. Study of reefs may suggest what causes and/or maintains high biodiversity.
2.. Organism-environment interaction. Illustrate the manner in which organisms affect their environment as well as being affected by their environment.
3. Economically important. High porosity and proximity to preserved organic matter make them important petroleum reservoirs.
(4). They are wonderful places to go to study
What is a reef?
A biological/geological definition:
An organically constructed, wave resistant structure that stands above the surrounding sea floor.
Three basic ingredients to this definition:
1. Rigid framework of organic skeletons
2. Has relief above the surrounding sea floor - from 1 to 10s of meters
3. Wave resistant (affecting surrounding environment – causes zonation of depth and water energy: fore reef/back reef lagoon)
Further observations about modern corals and reefs:
--shallow water mostly. Reef-building corals today have symbiotic algae within their soft tissues. The algae use the CO2 given off by the corals. Experiments have shown that corals calcify faster with the algae than without. Reef-building corals today are therefore, light-limited.
--subtropical and tropic latitudes. Found within 30 degrees of equator. Related to need for light and to temperature. (easier to secrete calcium carbonate in warmer water)
--deep-water and high latitude corals are not reef-builders.
-- in shipping, the term "reef" refers to any shallow water obstacle to shipping; not just to coral reefs.
Figure 10.1 A sketch
depicting the environmental window in which modern coral-algal reefs grow
best. Numbers for temperature and
salinity define the growth limits of corals, boxes enclose optimum values (from
Walker and James, 1992).
Reef structure
Most reefs can be thought of as being formed by three categories of organisms. The actual organisms may differ from reef to reef, and from geologic period to geologic period, but this may be a useful way of thinking about reef construction.
1. Framework builders - the makers of the rigid framework that allows the reef to stand above the bottom.
Typical framework builders: corals, sponges;
Less typical: oysters, rudist bivalves, algae, archaeocyathids
2. Sediment producers - organisms that attack the framework or contribute their own hard parts to the loose sediment of the reef.
sponges, coral, forams, calcareous algae, molluscs.
3. Cementers - organisms that bind the loose sediment together - the glue of the reef. Sponges, algae, bryozoa, coral.
Figure 10.2 (A) Schematic
drawing of the role of various calcareous organisms in reef construction. Subordinate taxa in these three roles are
listed in parentheses (after R.N. Ginsburg and H.A. Lowenstam, 1958). (B)
Ancient reefs often had different kinds of organisms but they performed much
the same structural function. As a reef
develops, local patterns of sedimentation are altered; major subenvironments
within a reef complex are the reef barrier and the environments in front of
(more turbulent) and behind (less turbulent) the barrier (from Newton and
Laporte, 1989).
Reef zonation
Reefs affect their surrounding habitat. As a consequence, they cause an environmental zonation in their vicinity controlled by wave energy and light availability.
A simplified zonation scheme
· fore-reef - from about 70 m depth on; little live coral, but lots of debris from higher in the reef. Often lots of sand.
· reef front - zone of most vigorous growth and highest diversity of coral. Colony shape shifts from encrusting to massive/domal, to branching and platy as wave energy and light decrease
· reef flats - high energy , rubble zone (storm debris, encrusting organisms)
· back reef/lagoon - quiet, sheltered water, sparse corals
Figure 10.3 Cross section through hypothetical, zoned
marginal coral (mainly Cenozoic). Rock
types and growth forms are illustrated (from from Newton and Laporte, 1989).
Reefs through time
Different framework builders at different times in geologic history
In general:
Reef framework builders thru time: See illustration.
Cenozoic - scleractinian corals
Mesozoic - scleractinian corals and rudist bivalves
Paleozoic - sponges, tabulate corals, rugose corals
Proterozoic – stromatolites
Figure 10.4 An idealized
stratigraphic column representing geological time and illustrating periods when
there were only mounds and those times when there were both mounds and
reefs. Numbers indicate different
associations of reef- and mound-building biota ((from from Newton and Laporte,
1989).
Corals
Although corals are not the only framework-builders in reefs, now or in the past, they are certainly the single most important group of framework builders.
Phylum
Cnidaria
·
The ultimate
objective here is to consider corals, but corals are only one part of the
Phylum Cnidaria, so some basics are in order.
· True multicellular organisms, in which the cells are well-differentiated into tissues that have specialized functions
·
Basic morphology:
Often
two phases and forms
Polyp Medusa
polyp is benthic, medusa is planktic (or nektic)
tentacles - extensions bearing nematocysts
(stinging cells)
mouth
digestive cavity
(no anus)
Hard part morphology:
corallite - one individual polyp
corallite wall or theca
tabula - horizontal partitions added as corallite
grows
septum - a radially arranged, vertical sheet
dissepiment - curved, or domed plates along inner
corallite wall.
calice - upper most cup being the soft tissues
Corals may be either solitary or colonial
(consisting of many individuals and their adjacent skeletons.)
CLASSIFICATION
Phylum Cnidaria
Class Hydrozoa; dominantly polyp stage, short medusa;
hydroids; fresh and marine.
Limited
record; Camb-Recent
Class Scyphozoa: dominantly medusa stage, short
polyp; few calcified; the jellyfish,
rare
fossils;
Vendian - Recent
Class
Anthozoa: polyp stage only. soft and
"stony" or hard corals; Cambrian-Recent
Subclass Octocorallia - the soft corals- sea
fans, sea whips and the like.
Camb-(Vendian?)
-
Recent
Subclass Zoantharia - the hard corals. Camb-Rec.
Three orders of corals you should know:
Order Scleractinia: Triassic-Recent. Solitary and colonial. Aragonitic hard
parts. No or indistinct
walls
in colonial forms.
--Geometry
and order of septal insertion differ from the rugose corals.
Order Tabulata: Cambrian thru Permian. Strictly colonial, prominent tabulae and
poorly developed
septae;
calcitic hard parts
Order Rugosa: Cambrian thru Permian. Solitary and colonial; the "horn
corals" when solitary.
Calcitic
hard parts. Distinct walls between
corallites in colonial forms
GEOLOGICAL SIGNIFICANCE
Reefs are important paleoclimatic and
paleogeographic indicators. Tropical
latitudes, warm water.
When building reefs, corals exert a strong effect
on the surrounding environment and sediments.
Reefs are important as reservoirs for oil and
natural gas (accumulation sites) and are often “targets” for exploration.
Corals and reefs
1.
Fossil
specimens. The trays contain examples
of the three major groups of corals.
a.
Sketch one of each
Tabulate coral:
Rugose coral
Scleractinian coral
b.
Name two features
that differ among the three groups of coral.
In other words, how could you tell if you were looking at a rugose,
tabulate or scleractinian coral?
2.
Reef-building
scleractinian corals have symbiotic algae living in their soft parts. They help the coral grow quickly. How could you tell if a fossil coral,
especially a tabulate or a rugose coral, had symbiotic algae in its soft parts?
3.
Coral calendars
Many living and fossil corals display, either on
the outer surface or internally, what appears to be growth-banding, as
if there were some daily cycle, of rapid growth followed by slow or no
growth. In addition, some the band
widths vary systematically; a bunch of thick ones followed by a bunch of thin
ones. This may represent rapid growth
during the warmer times of the year and slower growth during the cooler parts
of the year.
One of the specimens of the solitary rugose coral
displays growth lines rather well..
Take a look.
Many years ago now, a Cornell paleontologist,
John Wells, looked into the phenomenon of growth banding in corals. Wells found that a Recent scleractinian Manicina areolata laid down about 360
bands in one year. Hmmmm......
He then looked at some fossil corals and came up
with the following estimates of the number of fine-scale increments (=days?)
between major growth slowdowns (=winters?):
Recent 360
65 Ma
Cretaceous 371
135 Ma Cretaceous 377
230 Ma Triassic 385
300 Ma Pennsylvanian 390
400 Ma Devonian
400
500 Ma Ordovician 412
Using the graph paper provided, plot the number
of days per year vs. the geologic age.
a.
Assuming that the speed and size of earth’s orbit around the sun has
remained the same during the past 600 Ma (which is what astronomers tell us and
we should probably believe them, how can you account for there being more days
per year in the geologic past? Would
you really have had more time to get things done in the Devonian?
b.
Geophysicists, on theoretical grounds, have suggested that the Earth's
rate of rotation has been slowing (because of tidal friction) at a rate of
0.0016 seconds per century. Is this the
rate of decrease shown in the coral
growth-banding data?
4.
Estimating abundances.
While in the field this coming weekend, you will
be estimating the relative abundances of the major types of corals, rudist
bivalves, and matrix (mud, stromatolitic mud, sand, rubble). To make this a little easier, you will be
provided with some diagrams that illustrate accurate abundance estimates (i.e.
helps you visualize what 5% abundance looks like as compared to 15% adundance).
[Note: please make sure you bring these diagrams with you this weekend.].
Ok, let’s practice:
Using the schematic meter-square “quadrat” (fancy
term for a square sampling area) that your artistically gifted T.A. has
prepared, what is the percent area covered by (remember, your total should be
100%):
Platy coral
Massive coral
Branching coral
Caprinid rudists
Monopleurid rudists
Stromatolitic mud
Mud
Sand
5. Now,
you’re on your own.
a.
Sketch a quadrat
(on this paper; it need not be one meter on a side) that shows 60% platy coral,
20% monopleurid rudists, 10% sand, and 10% caprinid rudists.
b. Sketch
a quadrat that shows 40% branching coral, 20 % platy coral, 20 % stromatolitic
mud and 20% caprinid rudists.
