Transcript Substrates and Substrate Modification

Substrates and Substrate Modification
Basic Divisions of the Marine Realm
Pelagic
Substrata in Marine Environments
Loose Sediments (Generally Soft Substrata)
1. Detrital (siliciclastic): components derived from other rocks
(e.g. beach sand)
2. Biochemical: components produced by living things
(e.g. shells, teeth, bone, plant fragments)
3. Chemical: components precipitated from solution
(e.g. salt, gypsum, chert, banded iron formation)
Solid Substrata
1.
2.
3.
4.
Rock Outcrops
Reef frameworks (built by organisms)
Hardgrounds and Firmgrounds (Lithified Seafloor)
Logs, Sunken Ships, etc.
General Composition of Source Rocks (e.g. Granite)
Quartz
-effectively stable as-is
Feldspar
(e.g. Plagioclase, K-Feldspar)
-weathers to clays, silica in solution
Dark Ferromagnesian Silicates
(e.g. Olivine, Pyroxene, Amphibole, Biotite)
-weathers to clays, iron oxides, and silica in solution
decreasing
mineral
stability
Components of Detrital (Siliciclastic) Sedimentary Rocks
Most common components:
quartz (most common detrital mineral due to resistance
to physical and chemical breakdown)
clays (derived from weathering of feldspar and
ferromagnesian minerals)
Minor components:
rock fragments (fragments containing various minerals,
including ferromagnesian minerals, preserved in cases
where weathering and/or transport distance low)
feldspar (where weathering and/or transport distance is
low)
Components of Marine Biochemical Sediments
Benthic Components:
1. Large calcium carbonate skeletons or skeletal elements
of organisms (e.g. large shells, coral skeletons, algal plates)
2. Microscopic calcium carbonate skeletons or skeletal elements
(e.g. microelements of algae such as needles) and various benthic
microfossils.
3. Siliceous skeletal elements (e.g. sponges)
Pelagic Components
Plankton (drifters)
1. Calcium carbonate (calcite or aragonite) skeletons of planktonic
microorganisms (e.g. coccolithophores, foraminifera).
2. Silica skeletons of micro-organisms (e.g. radiolaria, diatoms)
Nekton (swimmers)
Bones and teeth of vertebrates (e.g. fishes), shells of cephalopods
Siliciclastic Sediment:
Siliciclastic sediment predominates in areas on and adjacent
to land masses. Weathering and erosion of rocks and soil
provide a constant source of siliciclastic detritus to the
seafloor. Much of this sediment is deposited at the mouths of
rivers, as observed in the formation of deltas.
Biochemical Sediment:
Sediment Contribution from Benthic Organisms
Reef framework organisms
(corals, sponges, etc.)
Preserved as in-situ reefs, but
Also contribute loose sediment
(boulder- to silt-sized particles)
Non-framework organisms
Contribute loose particles
(mostly pebble- to silt-sized
particles)
Sediment Contribution from Benthic Organisms
Carbonate mud produced in large volumes largely by calcareous green
algae in shallow-water environments:
-lots of lime mud deposited in sheltered areas such as lagoons
where algal productivity is high, and water is quiet.
Penicillus
(contains needles of aragonite)
Halimeda
(contain plates of aragonite)
Carbonate needles / plates are deposited as sediment
once soft tissues of the plants have decayed
Sediment Contribution from Pelagic Organisms:
Carbonate skeletons accumulate as calcareous ooze in deep sea
because siliciclastic input is extremely low (far from land)
Calcareous ooze dominated by planktonic coccoliths
scale bar: 10 microns
Sediment Contribution from Pelagic Organisms:
Silica skeletons accumulate as siliceous ooze in deep sea
because siliciclastic input is extremely low (far from land)
AND calcium carbonate is dissolved out in cold deep water
Siliceous ooze
dominated by planktonic diatoms
Siliceous ooze
dominated by planktonic radiolaria
General trends in sediment distribution on global scale
Composition of sediment in marine environments dependent on
sediment supply from siliciclastic versus biogenic sources.
nearshore
subtidal shelf or basin
deep sea
Siliciclastic input (land-derived)
Biochemical input from pelagic fauna (primarily microplankton)
Biochemical input from benthic fauna
Reef buildups and algal sediment (tropics only)
Mostly siliciclastic but can
be biochemical-dominated in
tropics where biological
activity is exceptionally high
siliciclastics with
some biogenic particles
Pure biogenic ooze
Substrate Modification by Marine Organisms:
Soft Substrates
Some organisms bind and stabilize soft sediment.
Other organisms are well-adapted to burrowing into soft
sediment
Common effect of burrowing: burrowers tend to increase the
water content of soft sediments, through their activities, often
rendering the sediment soupy and prone to remobilization by
the weakest disturbances
This soupy sediment can exclude organisms prone to
smothering
Examples of Substrate Modification by Marine Organisms:
Stabilization of Soft Substrates
Binding of
grains by
biofilms
diatom biofilm
mat of archaea/bacteria
consortium
cyanobacterial
mat
Binding by
larger
organisms
seagrass
tube worms
Examples of Substrate Modification by Marine Organisms:
Destabilization of Soft Substrates
burrowing fiddler crab
mudflat with faecal mounds
produced by lugworm
Ghost shrimp
(resin cast of burrow at bottom)
Biotic diversification resulting from the Cambrian explosion
brought about great modification of marine sediments
An interesting thought:
How long has farming
been a way of life ?
(possible that some of
the first complex
metazoans farmed
sediment for bacteria)
Vendian/Ediacaran:
Little disturbance of sediment
By Middle Cambrian:
Sediment below surface used by
organisms to make homes and
exploit for food
Another interesting trend: diversity of bivalves (clams) has generally
increased since the Paleozoic whereas that of brachiopods decreased
Possible answer: brachiopods never evolved past being stationary suspension
feeders, whereas some bivalves evolved strategies for sediment-removal
mechanisms. As burrowing (sediment disturbance) intensified through Phanerozoic,
brachs lost out.
Substrate Modification by Marine Organisms:
Hard Substrates
The usual doctrine taught to undergraduates: Earth materials are broken
down at the Earth’s surface by physical and chemical processes. It is
often assumed that all these processes are inorganic.
HOWEVER, biological activities are very influential in the breakdown of
these materials
EXAMPLE:
Many organisms have adapted to boring into hard substrates.
Boring accomplished by physical or chemical means
Result: substratum is structurally weakened and rendered more prone to
physical and chemical destruction
Some organisms that bore into hard substrata to make homes
Lithophaga: the rock-eating clam
“Christmas tree worms” (Polychaetes):
appendages extended (left) and retracted into boring made in coral (right)
Hard substrate dwellers, cont’d
Endolithic barnacle
Endolithic algae
Endolithic sponge
Active Bioeroders: Erode hard substrata in the
process of obtaining food
Sea urchins
Parrotfish
Snails (e.g. limpets)
How have hard substrate communities changed through time ?
It is possible that modern reefs are more diverse than
many ancient reefs as a result of higher rates of bioerosion
(i.e. prevention of hard substrate communities to reach
climax stage in ecologic succession)
Food for thought: “intermediate disturbance” by bioeroders
may be necessary for maintenance of diversity in reef
systems.
END OF LECTURE