Competitive Exams: Ooid Formation
Ooids are spherical or ellipsoid concretions of calcium carbonate, usually less than 2mm in diameter (Donahue, 1969; Tucker and Wright, 1990). There have been examples in the Neoprotozoic of ooids that are 16mm in diameter (Sumner, 1993), but all modern ooids are 2mm or less.
The interior of an ooid is usually composed of a nucleus, which is surrounded by a cortex of calcite or aragonite crystals that are arranged radially, tangentially or randomly. These crystals are arranged in concentric lamina. The nucleus can be a shell fragment, quartz grain or any other small fragment (including an aragonite/calcite amalgamation).
The formation of these objects has been speculated from the early 19th Century and ideas for their origin range from crinoid eggs, insect eggs to the present day explanation of precipitated layers of CaCO3 (Simone, 1981).
Recent ooids are forming today in places such as the Bahamas (Tucker and Wright, 1990; Newell et al. 1966) and Shark Bay, Australia and are all composed of aragonite.
Ooids do not form continuously; instead they go through stages of growth and rest (Davis et al. 1978). Davies et al (1978). describe the typical life cycle of a Bahamian ooid:
- Suspension Growth Phase Nuclei introduced into a suitable location, with enough turbulence to keep them in suspension and water that is supersaturated in CaCO3, will induce a short lived inorganic precipitation of calcium carbonate on their surfaces. The precipitation is stopped by crystal poisoning, which is the addition of Mg2 + or H + onto the surface. If the proto-ooids remain in this environment the outer coating will be lost due to attrition. This means the suspension phase is short lived, but may be repeated several times.
- Temporary Resting Phase Coated nuclei resting in the marine environment will quickly equilibrate with the surrounding fluid. Removal of the ‘poisonous’ ions will reactivate the coated surface in such conditions. However, not all ‘poisonous’ ions are removed, so after several growth and temporary resting stages have been completed a third stage is required.
- Sleeping Stage A new surface is required in order to form a new coating. This membrane is probably organic in origin. Experiments show that this takes 1 − 3 weeks to form. The membrane forms a new, stable substratum for new CaCO3 precipitation.
The timing of these stages means that an ooid spends only 5% of its time actually growing; the rest is spent ‘sleeping’ (Davis et al. 1978; Bathurst, 1967).
As can be seen from the life cycle, the following factors will have an affect ooid growth: (Monoghan and Lytle, 1956; Newell, 1960; Bathurst, 1967; Davis et al. 1978; Deelman, 1978; Heller, 1980; Simone, 1981; Sumner and Grotzinger, 1993):
- Supersaturation: The supersaturation of the seawater is of vital importance (Monoghan and Lytle, 1956). Monoghan and Lytle (1956) investigated the effect of CO3 concentration on the formation of ooids. They found that the concentration needed to be above 0.002 moles/litre and below 0.0167 moles/litre for ooids to form successfully. Below 0.002 moles/litre only aragonite needles or poor ooids formed. Above 0.0167 moles/litre the ooids formed an amorphous mass. Other authors have stressed the importance of supersaturation, but they give no quantitative information (Bathurst, 1967; Davies et al. 1978; Simone, 1981).
- Nuclei: The type of nuclei affects the rate of growth and the size of each lamination (Davies et al. 1978). Organic coating on the nuclei give faster and longer precipitation, while using oxidised quartz show much slower and shorter precipitation. Davies et al (1978). show their results as a change of pH (a negative pH change is assumed to indicate precipitation), rather than growth or precipitation rates.
- Agitation: The agitation an ooid undergoes must be enough to keep it in suspension for the growing phase followed by removal to a non-supersaturated fluid (the rest phase) (Newell, 1960; Davies et al. 1978; Heller, 1980). Davies et al (1978). conducted a study using two different speeds of water current to test this: 5cm/s and 10 cm/s. The ooids were kept in suspension by this water flow, and in other experiments involving horizontal shaking and tumbling motion formed, the ooids were non-existent or more like those formed in non-agitated water in the presence of organic compounds. In all cases of different nuclei the larger water current increase precipitation rates, but the time that precipitation changed depending on the nuclei type. Agitation may also control ooid size (Sumner and Grotzinger, 1993). As the ooid grows the mass lost per impact with another object increases as the cube of the radius. The mass gained from growth is proportional to the square of the radius. Eventually, the mass loss will equal or exceed the mass gained, limiting the size of the ooid. Sumner and Grotzinger (1993) performed numerical modelling on ooid formation. Their model gave a higher ooid radius in higher velocity flows, with a decrease that looks like an exponential or a power law with decreasing velocity (Sumner and Grotzinger, 1993, their fig 6). They did not include the impact of ooids to limit size. The agitation can come from waves or tidal movements. Storms provide that mixing of ooids in the rest stage and those that can no longer precipitate. There is some change in crystal orientation with the amount of agitation (Donahue, 1969). Ooids can form in quiet waters, but organic CaCO3 precipitation is needed for them to form (Suess and Fatterer, 1972). These ooids will show radial crystals. Ooids formed in agitated waters have crystals arranged tangentially. The change between suspension to bedload transport may also initiate this change (Deelman, 1978).
- Location: The location off ooid formation is important. They must be kept in the same area throughout the formation, in order that their life cycle can be completed (Simone, 1981).
- Water Depth: Most ooids form in water less than 2m deep (Simone, 1981), but this may have more to do with wave agitation and tidal movements than water depth itself. Newell et al (1960). surveyed sediment at various depths and calculated the % fraction of ooids in the sediment. All the sediments that are near 100% ooids are formed with 8m of the surface.