The oceans are full of many varieties of microscopic organisms, but only several varieties are responsible for generating vast quantities of biogenous sediments.
Ooze is slimy mud sediment (soft and mushy) on the bottom of an ocean or lakebed formed from the accumulation of skeletal and organic remains of microscopic organisms (phytoplankton and zooplankton).
- Oozes can be dominantly calcareous or siliceous in composition.
- To be considered an "ooze" sediment must consist of >30% biogenous material (Figure 6.77).
- Oozes form slowly - accumulating at a rate of 1/2 to 2 1/2 inch per 1000 yrs.
- Oozes form in low energy environments and are very fine grained (clay sized particles).
Calcareous oozes are sediments dominantly composed dominantly of calcium carbonate (CaCO3). Two dominant groups of microorganisms that contribute carbonate remains: Coccolithopores (phytoplankton) and Foraminifera (zooplankton)
Coccolithopores are single-celled marine phytoplankton (microscopic plants) that live in large numbers throughout the upper layers of the ocean. Unlike any other plant in the ocean, coccolithopores secrete shells of microscopic plates made of calcite (CaCO3). These scales, known as coccoliths, are shaped like hubcaps and are only three one-thousandths of a millimeter in diameter (Figure 6.78). Coccolithopores are part of base of the food chain and contribute vast quantities of coccoliths as sediment to large regions of the ocean basins. Coccoliths are concentrated in calcareous ooze.
Coccoliths first appear in the fossil record in Triassic time. Because they are composed of low-magnesium calcite (the most stable form) they are easily fossilized and preserved in sedimentary rocks. What is a Coccolithopores? (NASA)
Foraminifera (or forams) are a large group of single-celled zooplankton, most species have calcareous shells (or tests). Their shells are commonly divided into chambers which are added during growth and form patterns including spirals, open tubes, or hollow spheres (Figure 6.79). Depending on the species, the shell may be made of crystalline calcite, organic compounds, or sand grains and other particles cemented together. They are usually less than 1 mm in size, but some species grow much larger, reaching up to 20 cm. The majority of foraminifera species are benthic (meaning they live on or within the seafloor sediment) while typically smaller varieties are floaters (planktonic) in the water column at various depths. Foraminifera are found in all depths of the ocean, although deep ocean varieties do not have calcareous tests. They contribute a significant volume of sediments to carbonate reefs and a major component of carbonate oozes throughout ocean basins.
Over 10,000 species are recognized, both living and fossil. They first appeared in the fossil record in Cambrian time.
|Figure 6.79. Examples of foraminifera tests.
|Figure 6.80. Pyramids of Giza, Egypt are constructed with foraminiferal limestone.