Algally induced alteration of carbonate

As with other Holocene shallow water carbonate settings, some of the sediments of the Khor al Bazam show a distinctive type of alteration that affects all carbonate grains, irrespective of their origin (Kendall et al., 1966a and b). The texture of the grains is altered to a homogeneous microcrystalline fabric of aragonite. In the final stage of development, the grains are virtually indistinguishable from each other. In many instances the external shape may be the only indication of the grains' origin. This process has been observed in the Bahamas (llling, 1954; Purdy, 1963a and b), Portuguese Timor (Wolf, 1965b), the Arabian Gulf off Qatar (Houbolt, 1957) and in British Honduras (Purdy, 1965).
Illing (1954) attributed the alteration to bacteria or algae. In contrast, Newell et al. (1960) ascribed altered zones in ooids to the effects of decaying colonies of boring algae. Later, Purdy (1963a and b) concluded that organic matter trapped within the grains promotes the process. Bathurst (1966) ascribes this alteration to algae.
In Holocene sediments of the Khor al Bazam, foraminifera and red calcareous algae seem particularly susceptible to alteration. In contrast, thick walled shells and shells with a more coarsely crystalline fabric are less susceptible to alteration. Alteration in these shells generally is confined to the surface, but a few lobes of fine grained carbonate extend deep into the shell.
Alteration also occurs in aggregates of the microcrystalline carbonate that forms fecal pellets and grapestones. These aggregates are initially poorly bound, open textured grains that are rapidly hardened and cemented as they alter. oolites are similarly affected.
This process of alteration can be seen in the peneroplids of Quala Bay. Peneroplid foraminifera are particularly susceptible to alteration (Murray, 1966). Detailed study of specimens collected from the Khor al Bazam showed all stages of alteration. The tests show a series of surface changes, the ranging from translucent and porcelaneous on fresh tests to opaque and saccharoidal on altered specimens. These changes are accompanied by rounding and pitting of tests. Many of the pits are seen to contain blue green algae. Alteration may be so pronounced that tests are almost indistinguishable from aragonite ovoids of fecal and accretional origin. Both Illing (1954) and Purdy (1963a and b) noted similar changes in the foraminifera of the Bahamas.
Thin sections of fresh peneroplid tests under plane polarized light show a brown body color. They contain no crystal shapes apart from a faint structure parallel with the walls and a very faint granulation under high power. In reflected light they are milky white. Through crossed nicols the tests are seen to be composed of parallel sheaths of crystals which show low polarization colors and lie parallel with the curvature of the walls. The sheaths are embedded in a fine, granular matrix of crystals which show pin point polarization in grays and yellows of the first order.
This process may be accompanied by the infilling of the chambers by microcrystalline aragonite. The altered areas are characteristically microgranular and lack the brown body color shown by fresh tests. Under crossed nicols the altered areas show high polarization colors, but do not extinguish.
Hand picked peneroplids from the medium sand fraction of one of the samples were separated into five groups on the basis of the extent of external alteration. X ray analyses showed that the five groups progressively changed from high magnesian calcite to aragonite.
The algae growing on some of the peneroplids also were found by John Twyman (personal commun., 1964) on the oolites from Abu Dhabi. They were identified by Dr. Stewart of Westfield College, who found them to be Entophysalis deusta (Menegh), Drouet and Daily. Newell et al. (1960) found this genus common in the Bahamas.
Algae collected in Abu Dhabi survived 3 years in storage jars. Upon exposure to sunlight they started growing vigorously.
The algal cells and filaments were found to lie just beneath the surface of the altered tests, as shown by (1) progressively dissolving the carbonate of the tests in 5 percent acetic acid or 0.2 percent HCL, and (2) rendering the surface transparent with 40 percent hydrofluoric acid. Algae could be made even more apparent by staining with malachite green, which stains algal cells and the mucilage with different intensities.
If the unaltered tests are completely decalcified, they leave behind a thin, diaphanous, soft elastic membrane that retains the original form of the foraminifera. This is the "tectin" of Hyman (1940) and is believed to be the original organic material of the test which in life, occurs in conjunction with the calcite.
Decalcified altered foraminifera leave behind a much more translucent material, which also molds the test and fills its chambers, but is markedly different in appearance from the "tectin." In all ways it resembles the mucilage that is present in association with the blue green algae. It contains algal cells and small quantities of minute mineral grains which probably adhered to the mucilage as the test rolled on the sea floor.
If foraminifera are treated with 40 percent hydrofluoric acid for more than 10 minutes, all the calcium carbonate is replaced, molecule by molecule, by transparent calcium fluoride (Grayson, 1956). The mucilage envelope and algae resist solution (Fischer, 1897) and consequently stand out as translucent and green in altered areas. Treatment with 40 percent hydrofluoric acid for 5 minutes makes the surfaces of the foraminifera transparent and reveals the network of radiating filaments and globular algae. Fresh foraminifera become completely transparent. The best optical results with the calcium fluoride specimens are obtained by viewing them under water.
Partially altered foraminifera examined in thin section show the algal filaments penetrating both the unaltered and altered areas. The filament "bores" generally are filled with microcrystalline aragonite, which preserves the filament shapes. Thus the unaltered test may be penetrated by tubes of altered material. The boundaries separating the unaffected high magnesian calcite areas of the test from the aragonitic altered areas commonly resemble the boring of algal filaments (Bathurst, 1966). Under plane polarized light, parts of the test adjacent to the bore shapes may show faint granulation, which is believed to be the first sign of alteration.
Staining thin sections of the altered material with aqueous malachite green results in a patchy effect. Deeper colors are present at the outside edges and in the chambers devoid of carbonate. Etching thin sections with weak acid solutions of malachite green dissolves the calcium carbonate, leaving the stained insoluble algae and mucilage. The deeper stain is usually at the edge of the former test and in some chambers, showing algal cells and filaments both at the surface and penetrating the interior. Generally, the parts of the chambers not filled with algal cells are stained less intensely, an indication that they are filled with mucilage. Dissolution and staining of the altered walls of the foraminifera leave only a trace of mucilage that stains lightly. This most likely results from the displacement of mucilage by the formation of microcrystalline aragonite crystals. The presence of this finely dispersed mucilage is confirmed by gently dissolving the whole tests. If the dissolution is too vigorous, the mucilage is ruptured and removed.
The intimate relation of blue green algae with alteration can be demonstrated. All plants photosynthesize and respire. Dalrymple (1965) observed the ability of blue green algae to precipitate calcium carbonate. Parks and Curl (1965) showed in the laboratory that a culture of blue green algae in sea water produces measurable change in the conductivity of the water between day and night. They inferred that, in light, photosynthesis transforms bicarbonate ions to carbonate, and in the dark, respiration causes the opposite effect.
In the peneroplids of the Arabian Gulf, the mucilaginous envelope generally creates a microenvironment within each test. Carbon dioxide given off during respiration would promote solution of calcium carbonate in this restricted environment. Conversely, the carbon dioxide utilized during photosynthesis would cause precipitation. Although high magnesium calcite is dissolved, it is aragonite that is precipitated because it is apparently the stable form of calcium carbonate under such conditions (Kinsman, 1964b). In this way, the high magnesium calcite of the foraminifera test is progressively dissolved and replaced by aragonite on a piecemeal basis. Once a foraminifer is altered, dissolution and reprecipitation of aragonite continue, ultimately destroying all evidence of its origin.
This process alone could account for alteration. Bathurst (1964) suggested that alteration is accomplished by algae boring and reboring the calcite test and that solution of the test occurs only where it is in direct contact with the algal filaments. However, the fact that the algae are restricted to the area near the surface of the test and do not necessarily extend into all the affected parts, argues against alteration as the result of boring alone. Also, the mucilaginous envelope secreted by the algae confines the carbon dioxide. This carbon dioxide can react at any point within the envelope.
The alteration of carbonate grains could be controlled by several factors: the length and frequency of exposure of the grains to direct sunlight, the dimensions of the grains and the size of their component crystals. For example, if the grain is buried quickly, it may not have time to alter or will only alter if it has a thin, delicate shell and small component crystals.
The shoal areas of the Khor al Bazam are rippled where the altered foraminifera were collected. If the ripples migrate slowly, the foraminifera being altered will be exposed only occasionally to direct sunlight. Consequently, the alteration process may be slow. Alteration is most effective on the weedcovered shoals of the coastal terrace and parts of the offshore bank. skeletal grains from those areas have abundaformformformformformnt algal filled pits. The prolific plant growth must upset the carbon dioxide balance of the sea water and alteration therefore may be more rapid.
Wolf and Conolly (1965, p. 108) pointed out that algae are generally good indicators of shallow water because photosynthesis is depth controlled. Thus, alteration would not be expected to take place below the photic zone. Off Qatar, bioclastic material is rounded and the surface character obliterated in water less than 20 m deep (Houbolt, 1957).
In rocks from similar ancient carbonate environments, the microcrystalline aragonite contained within the altered material generally is replaced by a mosaic of microcrystalline calcite in which algal alteration is still recognizable (Shearman and Skipwith, 1965). The altered material commonly forms Bathurst's (1966) micritic envelope.