5 - Carbonates

2 types of biogenic sediments

Biogenic and Chemical

Biogenic carbonates

Metabolised by organisms

Skeletal material/ ‘Hard parts’→ carbonates, silicates and phosphates

‘Soft parts’ → organic matter

Chemical sediments

Evaporites precipitated directly from the atmosphere or water

What % of seawater is salt

3.5%

Where do sea salts come from

Metal species weathered from continental and mid ocean ridge processes

What makes up 86% of sea salts

Cl- and Na+ combining to form salt through supersaturation

Carbonate ion

CO3 2-

Aragonite

CaCo3

High pressure polymorph of calcium carbonate

Less heavier than calcite

Metabolised from organisms → more structurally sound shells (due to lack of cleavage)

Breaks down into calcite after death

Dolomite

CaMg(CO3)2

Most stable form of carbonate

Abiogenic equation for calcite production

Dependent on Ca2+ availability

Partial dissociation of CO2 in the water

Conditions to remove CO2 from seawater

Increase temperature

Decrease pressure

Agitating water

Autotrophs

Build organic molecules from inorganic molecules

Using chemicals or light

Base of food chain

Plants, algae, some bacteria

Heterotrophs

Consume organic molecules

Animals

Within the upper 1m of water

all red light is absorbed by green photoautotrophs

Factors affecting carbonate precipitation

Light levels

Temperature

Pressure

Best conditions for carbonate production

High temperatures

Shallow waters

Evaporation > freshwater input so salinity increases

Low rainfall poorly connected to the open ocean

Balancing salinity

Higher reduces CO2 solubility

Lower is better for majority of heterotrophs, and all but highly specialized autotrophs

Chlorozoan association

T>15°C

32-40% salinity

Tropical

Coral reef

Chloroalgal association

T>15°C

>40% salinity

Tropical Mediterranean

Green algae

Foramol association

T<15°C

Temperate

High diversity of heterotrophs

Molluscs, Formanifera, echinoderms, bryazoans, ostracods

Composition of carbonates

Grains

Matrix (micrite = carbonate mud matrix)

Cement

Pore space

Bioclasts

Grains/fossil fragments from living organisms

Encrusters

Usually reef builders

Adult phase anchored to sea floor

Buried in place after death

Encrusted by younger organisms

Frameworks form as organisms grow on top of each other

Non skeletal grains

Coated grains

Peloids

Clasts

Coated grains

Ooids

Oncoids

Ooids

<1mm

Micrite precipitates chemically in an ocilating flow with many layers of cement

Oncoids

Micrite precipitates by algal and microbial coatings on the outside grains → less regular structure

Peloids

Fish poo

100-500mm diameter

No spherical layers

Grain aggregates

Form by algal binding and cementation

Grapestones - spherical grains

Lumps - smooth outline and hollow interior

Botryoidal lumps - thin oolitic coating

Intraclasts

Reworked partially lithified material from locally within the basin

Extraclasts

Lithified material from outside the immediate depositional area