Substrates and Substrate Modification

Basic Divisions of the Marine Realm

Benthic

Pelagic

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. Rock Outcrops2. Reef frameworks (built by organisms)3. Hardgrounds and Firmgrounds (Lithified Seafloor)4. Logs, Sunken Ships, etc.

Substrata in Marine Environments

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

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 Detrital (Siliciclastic) Sedimentary Rocks

Components of Marine Biochemical Sediments

Benthic Components:

1. Large calcium carbonate skeletons or skeletal elementsof 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 planktonicmicroorganisms (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 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 ofrivers, as observed in the formation of deltas.

Siliciclastic Sediment:

Sediment Contribution from Benthic Organisms

Reef framework organisms (corals, sponges, etc.)Preserved as in-situ reefs, butAlso contribute loose sediment(boulder- to silt-sized particles)

Non-framework organismsContribute loose particles(mostly pebble- to silt-sizedparticles)

Biochemical Sediment:

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 sedimentonce soft tissues of the plants have decayed

Sediment Contribution from Benthic Organisms

Calcareous ooze dominated by planktonic coccolithsscale bar: 10 microns 

Sediment Contribution from Pelagic Organisms:Carbonate skeletons accumulate as calcareous ooze in deep sea because siliciclastic input is extremely low (far from land)

Siliceous ooze dominated by planktonic diatoms

Siliceous ooze dominated by planktonic radiolaria

Sediment Contribution from Pelagic Organisms:Silica skeletons accumulate as siliceous ooze in deep seabecause siliciclastic input is extremely low (far from land)AND calcium carbonate is dissolved out in cold deep water

Composition of sediment in marine environments dependent on sediment supply from siliciclastic versus biogenic sources.

nearshore subtidal shelf or basin

Siliciclastic input (land-derived)

Reef buildups and algal sediment (tropics only)

Biochemical input from pelagic fauna (primarily microplankton)

Mostly siliciclastic but canbe biochemical-dominated intropics where biological activity is exceptionally high

Pure biogenic ooze

Biochemical input from benthic fauna

deep sea

siliciclastics withsome biogenic particles

General trends in sediment distribution on global scale

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

Substrate Modification by Marine Organisms:Soft Substrates

Examples of Substrate Modification by Marine Organisms:Stabilization of Soft Substrates

seagrass tube worms

Binding of grains by biofilms

diatom biofilm mat of archaea/bacteria consortium

cyanobacterialmat

Binding bylargerorganisms

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 explosionbrought about great modification of marine sediments

Vendian/Ediacaran:Little disturbance of sediment

By Middle Cambrian:Sediment below surface used by organisms to make homes and exploit for food

An interesting thought:How long has farmingbeen a way of life ?(possible that some of the first complex metazoans farmed sediment for bacteria)

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 brokendown 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 ofthese 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 sponge

Endolithic algae

Active Bioeroders: Erode hard substrata in the process of obtaining food

Sea urchins Parrotfish

Snails (e.g. limpets)

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 bioerodersmay be necessary for maintenance of diversity in reef systems.

How have hard substrate communities changed through time ?

END OF LECTURE