Substrates and Substrate Modification Basic Divisions of the

Substrates and Substrate Modification

Basic Divisions of the Marine Realm Pelagic nth Be ic

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.

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