Carbonates

Question 1

why are carbonates so important

Answer 1

• major carbon sink
• organisms remove CO2 from atmosphere by photosynthesis, shell secretion
• 99% of CO2 emitted by volcanoes removed by organisms
• 3.2 gigatons rest in calcite and aragonite
• more than 50% of oil and gas fields in carbonate rocks
• major economic resource (cement and building stone)
• host for ore minerals
• some freshwater (lakes and flowstones)
• 10-15% of sedimentary rocks
• many in shallow water (carbonate factory)
• some deeper marine e.g. oozes

Question 2

Carbonate Production

Answer 2

• shallow sea water is generally saturated in calcium carbonate
• high temp, low pressure and breaking waves favour carbonate production
• dissolved ions precipitated as sea water warms, loses CO2 & evaporates
• Organism generate shells & Skeletons from dissolved irons

Question 3

Carbonate Compensation Depth

Answer 3

• deep ocean waters under-saturated with calcium carbonate
• biogenic particles dissolve in water column and on sea floor
• calcareous oozes absent below CCD depth
• CCD varies from ocean to ocean (4000m in atlantic, 500-1500m in pacific

Question 4

carbonate minerals

Answer 4

• calcite (high magnesium calcite
• aragonite (less stable in geological record)
• Dolomite

Question 5

Origins of carbonate sediments: biological

Answer 5

• lime mud from faecal material or shell crystallites
• remains of shells and skeletons
• frame building organisms (e.g. coral)

Question 6

Origins of carbonate sediments: chemical

Answer 6

• precipitation from sea water

- modern seawater almost saturated in CaCO3

Question 7

Frame building organisms

Answer 7

• corals
• stromatoporoids
• stromatolites

Question 8

Carbonate Grains (organisms)

Answer 8

• Brachiopods
• Molluscs (sea shells, snails)
• Arthropods
• Sponges and algae
• Echinoderms (sea urchins)

Question 9

Carbonate grains

Answer 9

• pellets (many are faecal material)
• precipitated grains (ooids, grapestones)
• intraclasts (ripped up bits of sediment

Question 10

sources of carbonate lumps

Answer 10

1) accumulation of loose carbonate sand
2) Precipitation of marine carbonate cement while at rest
3) break up of layer by storm into lumps

Question 11

what holds grains together?

Answer 11

• lime mud - micrite

- cement - calcite (spar)

Question 12

interparticle porosity

Answer 12

each grain is separated, giving a similar pore space arrangement as sandstone

Question 13

Intergranular porosity

Answer 13

pore space is created inside te individual grains which are interconnected

Question 14

intercrystalline porosity

Answer 14

produced by spaces between carbonate crystals

Question 15

Mouldic porosity

Answer 15

pores created by the dissolution fo shells etc.