Oceanographic Setting
The deposition of carbonate material depends on the oceanographic setting of the area; this includes the water chemistry, water depth, and the hydrography of the area. The carbonate slope is a very dynamic region, each of which experiences different amounts of light, nutrients, temperature, pressure, chemistry, and biota.
Ocean Chemistry:
Lysocline: the depth at which a rapid increase in dissolution occurs; it separates the well preserved (above) from the poorly preserved (below) calcareous microfossil assemblages . The typical depth of the lysocline is between 3700-4500m and varies are a result of the carbonate ion concentration in the deep and intermediate water masses. It is shallowest when the CO2 concentration of the water is highest because those waters are the most corrosive to the calcareous microfossils.
Compensation Depth (of aragonite or calcite): the depth at which the amount of calcium carbonate delivered to the seafloor is equal to the amount removed by dissolution. The average depth of the calcite compensation depth (CCD) is 4500 m in the Pacific and 5500 m in the Atlantic and shallows when there is a greater supply of carbonate material to the seafloor. Only above the CCD can carbonate materials be deposited (below the CCD they dissolve and do not reach the sea floor). The deep depth of the CCD results only in dissolution on the lower parts of the slope.
Factors that affect the depth of the lysocline and the compensation depth include:
- Water temperature
- Depth
- CO2 concentration
- pH (high pH values aid in carbonate preservation)
- Amount of carbonate sediment supply
- Amount of terrigenous sediment supply
Calcium carbonate solubility increases with increasing carbon dioxide content, lower temperatures, and increasing pressure. Other factors that can influence the dissolution of calcium carbonate: organic coatings on the grains, size of the grain (smaller grains dissolve faster).
The depth of the lysocline (dotted line) and the CCD (solid line) in the Equatorial Pacific, along with the amount of water column saturation by carbonate (red line).
Different water masses that can affect the carbonate slope:
- Most commonly: Deep water travels between 1500 to 4000 meters (not in contact with the sea floor).
- Sometimes: Intermediate water is that water below the surface mixed layer to 1500 meters depth.
- Bottom water is that water below 4000 meters.
References and Sources:
Coniglio, M., Dix, G.R. carbonate Slopes in Facies Models: Response to Sea Level Change. Ed. R.G. Walker and N. P. James. Geological Association of Canada, St.Johns: 1992. pp 349-373.
Wilson, J.L. carbonate Facies in Geologic History. Springer-Verlag, New York: 1975.