Oceanography: Biogenic Sediments

Opal

Opal is the national gemstone of Australia which is the world's largest producer.

The carbonate system

-The addition of carbon dioxide to water creates carbonic acid! -Carbonic acid dissociates into hydrogen ion and bicarbonate ion -The free hydrogen ion "consumes" the carbonate ion and produces a bicarbonate ion -The less carbonate ion in the water makes calcium carbonate more likely to dissolve.

Key points

-The ocean is everywhere under-saturated with respect to silica -Only in places with very high diatom productivity are silica-rich sediments found -Deep ocean has high levels of dissolved carbon dioxide. -Deep ocean is under-saturated with respect to carbonate ion. Things are MORE LIKELY TO DISSOLVE in the deep ocean then. -Carbonate shells tend to dissolve in deep ocean, so carbonate-rich sediments are found on topographic highs (mid-ocean ridges).

Calcareous sediments

Shells of dead organisms rain through the water column. The highests CaCO3 sediments are NOT found in areas of high biological productivity. The distribution of carbonate sediments is controlled by preservation.

Calcium carbonate concentration

The concentration of calcium carbonate sediments is negatively correlated with seafloor depth. Calcium carbonate is low in sediments beneath cold waters in high latitudes.

Deep ocean carbon

The deep ocean has high levels of carbon dioxide because... 1. It's COLD 2. It accumulates carbon dioxide from respiration of organic matter. **As a results, carbonate ion content is low in deep water and calcium carbonate shells will tend to dissolve.

Carbonate dissolution

The deep ocean is under-saturated with respect to calcium carbonate. Calcium carbonate shells will tend to dissolve at depth.

Big point of that...

The less carbonate ion in the water, the MORE likely something is to dissolve.

Opal dissolution

The ocean is always under-saturated with respect to silica. Silica shells will always tend to dissolve so silica rich sediments will tend to form only in areas of high productivity.