11 - Chemical & Biogenic Sediments

Why are carbonates typically of warm, shallow marine settings?

Because most of the carbonates are formed by organisms. Which usually reside in warm tropical climates.

What are the 4 basic ways that biogenic carbonates are made?

Biogenic: Direct extraction of CaCO4 from water to form skeleton, Photosynthesis by marine plants/bacteria because CO2 is removed from water, thus promoting CaCO3 precipitation, Bacterial mediation - calcification takes place on the outer cell walls, Fecal Pellets - "re-packing" of sediment + organics

What properties control the degree of carbonate precipitation at groundwater springs?

Fluctuations in temperature, water chemistry, or flow rates can cause precipitation

What are the compositions of Gypsum and Anhydrite? How/when does conversion occur? What is Selenite?

Gypsum (CaSO4+H2O): Precipitated in restricted marine bodies or saline lakes Anhydrite (CaSO4): Gypsum converts to Anhydrite by dehydration, occurs at high temperatures (burial in depth or hot, arid evaporative environments) CaSO4 • 2H2O ↔ CaSO4 Can go back to gypsum in presence of water Selenite: Giant Selenite crystals, bladed variety of Gypsum

What are examples of inorganic and biogenic carbonate precipitation?

Inorganic: lakes & restricted marine bodies, Ooids, "Whitings", Archean - much greater oversaturation of CaCO3

What are the 2 most common Bitter Salts?

KCl and MgCl are the most common halides formed with extreme evaporation of sea water.

What controls the level of CO2 in sea water?

Loss of CO2 is caused by increased temperature and decrease in water pressure.

What is the influence of siliciclastic input to the ocean on carbonate precipitation?

Minimal clastic input = great reefs (Great Barrier Reef, NE Australia) Abundant clastic input = no reefs (Indian cost)

What is the composition of Halite? Crystal form?

NaCl, Halite "hopper" - mold of halite in crystal sediment

Where and how do geodes form?

Often form slowly in phreatic (saturated) zone. Mineral replacement is also very common.

What is karst?

Vadose zone limestone dissolution

What are speleothems?

Features of precipitation (stalactite or stalagmite)

What is the correlation between ocean temperature, latitude, and calcium carbonate saturation?

+ and - 30 degrees latitude. Warm near the equator which is where they form, and carbonate saturation is highest there because warm (cold?) waters cant hold as much carbonate.

How much dissolved salts are present in average seawater? What are the most abundant elements/compounds present? Why does seawater have so much dissolved salt? Why is this amount largely constant over time?

3.5% dissolved salts in seawater. 85 elements have a measurable value in sea water. 99.9% of the salts consist of 9 elements/compounds:- Sodium (Na+) - Magnesium (Mg+) - Calcium (Ca2+) - Potassium (K+) - Chloride (Cl-) - Sulfate (SO4-) - Bicarbonate (HCO3-) - Bromide (Br-) - Borate (BO33-) Because it's the depository for all the sediments coming off the continents. Salinity is relatively constant because there's a constant rate of erosion and deposition through time

What is chert?

Amorphous Silica - SiO2 Precipitated abiotically or as biotic hard parts Marine and freshwater occurances

What does "clastic" mean?

Any grains that are transported, so that chemical/biogenic sediments are commonly clastic

How much seawater has to be evaporated to get calcium carbonate, gypsum and halite to precipitate?

Calcium Carbonate 19% Gypsum 9.5% Halite 4%

What is the correlation between ocean temperature, latitude, and carbonate precipitation?

Decreases in lower salinities: more precipitation at lower salinity because "foreign" ions (Mg2+) interfere with precipitation. Decreases with increasing temperature: more precipitation with increased temp

What is travertine?

Dissolution & precipitation of carbonates. Often in thin lamina with detrital mud, etc.

What is the difference between terrigenous, chemical and biogenic sediment?

Terrigenous- form from weathering/ erosion of pre-existing rocks (usually transported into the basin of deposition) Chemical - direct chemical precipitation of minerals in the depositional basin Ex: halite, gypsum, carbonate Biogenic- precipitate/ form directly in the depositional basin through a biogenic agent, including bacteria.

How does the pH of water control carbonate precipitation/dissolution? Why does adding CO2 to water decrease the pH? How should carbonate precipitation/dissolution respond to increased/decreased CO2 in water?

increase in CO2 lowers pH and causes CaCO3 dissolution Decrease in CO2 raises pH and causes CaCO3 perception The same way in water

What is the phreatic zone?

under the water table

What is the vadose zone?

where the water enters