ABSTRACT

Many carbonate particles from the tropical Brazilian continental shelf contain intragranular magnesian calcite cement, with 13 mole percent MgCO₃ (d (104) lattice spacing of 2.99 A) in solid solution in the calcite. In aragonitic Halimeda plates, precipitation occurs within the utricles; upon complete infilling, as much as 50 percent of the plate is composed of secondary magnesian calcite. Filling of these Halimeda plates by magnesian calcite can play an important role in the preservation of the plates: mechanical removal of the primary aragonitic skeleton exposes the cemented utricles, resulting in thinner knobby plates with greater resistance to subsequent abrasion. The wide distribution of intragranular cementation on the Brazilian shelf suggests that cemen ation by magnesian calcite is potentially a significant pathway for the flux of carbonate from seawater into sediments.

GENERAL ASPECTS OF INTRAGRANULAR CEMENTATION

Present-day shelf sediments are thought to contain little, if any, chemically precipitated calcium carbonate. Only in hot and/or highly saline waters, such as the shallow Persian Gulf, do substantial amounts of non-skeletal carbonate form in seafloor sediments. Even the commonly supersaturated levels of CaCO₃ in shallow shelf waters do not support a flux of carbonate into sediments.

The numerous reports of cementation of reefs and algal ridges in shallow seawater do not really alter this view. In such instances, cementation is restricted to the space available within the various biogenic structures, whereas sediments on the adjoining seafloor appear uncemented (Alexandersson, 1969, 1974; Ginsburg et al., 1971; Schroeder, 1972; Macintyre, 1977; James and Ginsburg, 1979). It seems clear that the special microenvironment inside platform-margin reefs and algal ridges is especially conducive for carbonate precipitation.

More common are secondary aragonite and magnesian calcite fillings that form within carbonate particles on the shelf. We term these precipitates intragranular cements, realizing that while they do not hold particles together, they are morphologically (and possibly genetically) similar to intergranular cements. Most warm-sea shelf sediments usually contain at least some evidence of such cementation, either as cement in hardened fecal pellets or as fillings in the hollows of skeletal grains. Suitable hollows include vacated algal and fungal microborings, and constructional hollows in foram tests, algal skeletons, and small mollusk shells (Glover and Pray, 1971; Alexandersson, 1972; Boyer, 1972). A number of generalizations have been made concerning these intragranular ceme ts: 1) Cementation is most pronounced in shallow turbulent environments where grains are repeatedly in contact with supersaturated seawater (for example, Bathurst, 1971; Milliman, 1974); 2) crystal growth can occur in clean empty voids but locally may involve catalysis through microbial decay of organic substances (Bathurst, 1966; Milliman and Muller, 1977); 3) the mineralogy

FOOTNOTE 2. Contribution Number 4771 from Woods Hole Oceanographic Institution.

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of the cement may be controlled by the mineralogy of the host (for example Glover and Pray, 1971) or it may be independent of the host.

INTRAGRANULAR CEMENT IN HALIMEDA GRAINS FROM BRAZIL

The Brazilian continental shelf represents one of the longest continuous carbonate-dominated shallow-water areas in the world. Sediments from the outer Amazon shelf to the Uruguay border (more than 4000 km distant) are characterized by a wide range of carbonate sediments. Throughout most of the area (north of Cabo Frio; 23°S) the constituents are biogenic and tropical in nature: coralline algae, bryozoans, the green codiacean alga Halimeda, mollusks and benthic foraminifera (compare Vicalvi and Milliman, 1975). Petrographic inspection of more than 500 sediment samples from the Brazilian shelf (for example, Milliman and Summerhayes, 1975) shows that most samples have some degree of intragranular cementation.

In this paper we report on the study of six representative samples (Fig. 1) from the much larger sample suite, the six samples coming from a variety of locations, water depths, and carbonate contents and representing a variety of ages (Table 1). In nearly all instances the cement is magnesian calcite. Sample 3778 (water depth of 22 m) is so dominated by this cement that more than 90 percent of the total carbonate is magnesian calcite. Since the sample contains numerous aragonitic mollusks, bryozoans, and Halimeda plates, this high content of magnesian calcite can only be explained by the abundance of internal cements, accounting for perhaps 20 to 25 percent of the total carbonate in this sample.

Closer inspection of the Halimeda plates reveals that these occur in a continuum of increasing cementation: from absolutely fresh plates with all internal voids empty, to thoroughly cemented grains where mechanical erosion has exposed the fillings at the grain surface (Fig. 2). As a working classification, we found it convenient to divide the material into four stages of alteration which are easily recognized under the petrographic microscope: 1) fresh plates, purely aragonitic and with utricles

FIG. 1. Positions of sampling stations on the Brazilian shelf. The dotted line indicates the shelf edge; the arrow in upper center points to the area with Mg-calcite ooids described by Milliman and Barretto (1975).

TABLE 1. Sampling stations and sedimentary characteristics of deposits having internal Mg-calcite cement

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and other hollows empty (Fig. 2A); 2) slightly cemented plates, with some fillings but still with numerous voids; 3) strongly cemented plates, where virtually all original internal hollows are filled with cement (Fig. 2B); and 4) utricle plates, strongly cemented plates where mechanical abrasion has removed much of the surrounding aragonite, leaving a knobby relief of cemented utricles exposed at the grain surfaces (Fig. 2C, D). Under incident light these latter plates are reddish brown in color, as compared to the whitish color of fresh Halimeda plates.

The internal cement in our Halimeda material was first identified by means of powder X-ray diffraction analyses of single grains and later confirmed by semi-quantitative analysis using EDAX-SEM. Based on X-ray diffraction data the lattice spacing of the calcite is uniform and well defined; the 2 value of the main calcite peak is 29.85 ± 0.02° (a lattice spacing of 2.99 A), corresponding to about 13 mole percent MgCO₃ using the idealized conversion curve of Goldsmith and Graf (1958) (see Milliman et al., 1971, for a review of problems in converting lattice spacings to MgCO₃ values).

SEM analyses of fractured plates show that the algal skeletal fabric of micrometer-sized aragonite needles remains unaffected during cementation (Fig. 3). The cement forming in utricles and other hollows (such as, occasional microborings) has the typical characteristics of marine intragranular Mg-calcite cement (as described, for example, by Alexandersson, 1972). The crystals become better defined with increased cementation, with maximum crystal size averaging 5 to 10 µm (Fig. 3F).

DISCUSSION

The presence of intragranular magnesian calcitic cement changes the fossilization potential of the Halimeda plates in two fundamental ways. First, it is known that diagenesis of Halimeda and other codiacean algae is dichotomous,

FIG. 2. [Grey Scale] Petrographic photographs of Halimeda grains in various stages of alteration. The width of the scale bar in C is 100 µm for A, B and C, and 25 µm for D. A) Fresh Halimeda with utricles empty of cement. B) Filled Halimeda plate although the parent is still recognizable, corresponding to stage 3 described in the text. C) Completely filled and altered utricle plate, all semblence with the original Halimeda having been lost. D) High magnification of utricle plate, showing the surface exposure of cemented utricles.

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with one portion of the skeletons disintegrating into free needles and another portion surviving as sand-sized grains which become internally cemented (for example, Neumann and Land, 1975). The cemented utricle plates in the Brazilian sediment represent a further stage in the diagenesis of the sand-sized portion, showing that when cemented

FIG. 3. [Grey Scale] SEM micrographs of Halimeda grains and intragranular Mg-calcite cement. Artificial fractures. The width of field in the three pictures in the left row is 2 mm; in B and D the width is 500 µm; in F the scalebars indicate 10 µm. A, B) Fresh Halimeda plate with utircles empty of cement. C, D) Slightly cemented plate with considerable internal porosity. D) Strongly cemented plate, very dense and with conchoidal fracture. F) Boundary between filling of blocky Mg-calcite (upper left) and skeletal fabric of aragonite needles (lower right). We have found no evidence of recrystallization of the original algal aragonite.

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grains are exposed to mechanical erosion, the internal cement may outlast most of the outer host structure. Second, the resultant particles probably are more likely to retain their primary structure. Non-marine diagenesis of carbonate particles can involve the loss of magnesium from the calcite lattice without loss of structural detail (Land, 1967; Towe and Hemleben, 1976), whereas conversion of aragonite to calcite involves solution and reprecipitation. Thus the cemented magnesian calcite utricle plates have a better chance of preservation than do aragonitic Halimeda plates.

An important problem concerns the mode of intragranular cementation and the extent to which such cements may occur within aragonitic codiacean algae. It is well known that utricles within individual Halimeda plates can fill rapidly with secondary cryptocrystalline carbonate, often before the entire plant has died (for example, Bathurst, 1971; Milliman, 1974), but all published observations indicate that this secondary carbonate is aragonite. Although the mechanisms controlling the mineral form of carbonate precipitates in seawater are not well understood, a number of factors are considered influential: water temperature, Mg/Ca ratio in seawater, spectrum of organic substances in solution, mineralogy of seed particles, and nature of organic matrices in the host grains (see discu sions, for example, in Glover and Pray, 1971; Alexandersson, 1972). The first three factors reflect environmental control, the last two relate to "host control." The fact that intragranular Mg-calcite cements dominate Brazilian shelf carbonates, no matter what the host mineralogy, suggests an environmental control; host control on mineralogy appears to be absent.

Related to this problem is the problem of seawater/sediment interactions. As mentioned at the beginning of this paper, the supposed lack of carbonate precipitation in carbonate-supersaturated seawaters into marine sediments is enigmatic; local fluxes occur only in particularly favorable environments (compare Milliman, 1974; Alexandersson, 1978). From a thermodynamic point of view, it would be comforting to find that carbonate supersaturation in shallow seawater indeed can lead to precipitation on a wider scale. Our Brazilian data, in fact, suggest that such precipitation does occur: in one of our samples (3778), the amount of intragranular cement approaches 20 to 25 percent of the total carbonate, thus representing a considerable diagenetic flux. Apparently this flux is concentrated i shallow depths and may not necessarily be related to time of exposure: the most heavily internally cemented sample (3778) comes from a water depth of 22 m and has an age of 2770 years, whereas sample 3281, which is 5 times older and comes from a depth of 102 m, exhibits a lesser degree of cementation. More global considerations of magnesian calcite flux into shelf sediments is a subject we hope to return to in a following article.

NOTE IN PROOF

Since writing this article, we have found intragranular Mg-calcite cement in Halimeda plates from Discovery Bay, Jamaica, and Miami Beach, Florida. Moreover, H. tuna plates from the North African shelf contain up to 40 percent Mg-calcite. Clearly the growth of Mg-calcite fillings in aragonitic Halimeda grains is a more universal process than previously documented.