CURRENT RIPPLES MICROBORING VERSUS RECRYSTALLIZATION: FURTHER INSIGHT INTO THE MICRITIZATION PROCESS R. PAMELA REID 1 AND IAN G. MACINTYRE 2 1 Rosenstiel School of Marine and Atmospheric Science, University of Miami, 4600 Rickenbacker Causeway, Miami, Florida 33149, U.S.A. e-mail: preid@rsmas.miami.edu 2 Department of Paleobiology, National Museum of Natural History, Smithsonian Institution, Washington, DC 20560, U.S.A. ABSTRACT: SEM observations of lightly etched thin sections of Bahamian sediments reveal an unusual process of micritization that involves carbonate precipitation in microborings concurrent with endolithic activity. A coccoid cyanobacterium, tentatively identified as Solentia sp., bores tunnels, which initially penetrate just beneath grain surfaces and eventually extend throughout the entire grain. These tunnels are filled by radial fibrous aragonite, which is precipitated as the microorganism advances. Extensive multicyclic repetitions of this process result in obliteration of original grain textures with almost complete preservation of grain margins and rare empty bore holes. The rapidly filled tunnels cannot be detected by resin cast embedding techniques that are commonly used to study microboring. This type of multicyclic boring and concurrent filling of bore holes forms micritized grains that can be difficult or impossible to distinguish from micritized grains formed by recrystallization. INTRODUCTION The term micritization was coined by Bathurst (1966) to refer to a process by which original fabrics of carbonate grains are altered to cryptocrystalline textures by repeated algal boring and filling of the bore holes with micritic precipitates. As envisioned by Bathurst (1966, 1975), micritization involves the following steps: (1) boring and colonization by an alga, (2) death of the alga and vacation of tube, and finally, (3) carbonate precipitation in the tube. In a classic study of micritization, Alexandersson (1972, p. 201) concluded that no organisms take a direct part in the precipitation of the carbonate (in borings) and the process is regarded as a form of marine carbonate cementation. Following Bathurst s initial report (Bathurst 1966), the term micritization was broadened to include other shallow marine diagenetic processes that result in obliteration of original carbonate microstructure by gradual alteration to cryptocrystalline textures (e.g., Alexandersson 1972; Sibley and Murray 1972; Land and Moore 1980; Reid et al. 1992; Reid and Macintyre 1998). On the basis of detailed studies using light microscopy, several authors concluded that cryptocrystalline textures in shallow marine sediments are commonly formed by recrystallization (e.g., Illing 1954; Purdy 1963, 1968; Pusey 1964; Winland 1969). Recrystallization is used here in a general sense for a reorganization in the size, shape, or composition of carbonate minerals, following Sorby (1882, in Folk 1965) and Purdy (1968). The importance of recrystallization as a micritization process has been a subject of controversy (see, for example, Purdy 1968; Bathurst 1975; Reid and Macintyre 1998). Recently, Reid and Macintyre (1998) provided SEM images supporting these earlier claims that recrystallization is a widespread process of micritization. Evidence for recrystallization in both SEM and light-microscope studies was based in part on an apparent lack of microborings in micritized areas. In this paper we document a previously unrecognized process of micritization that involves aragonite precipitation in microborings concurrent with endolithic activity. This precipitation, which appears to be biologically induced, results in the formation of cryptocrystalline grains with many of the same petrographic features as grains micritized by recrystallization. MATERIALS AND METHODS Ten surface samples of carbonate sand were collected in 2 10 m water depth in Abaco and Exuma Cays, Bahamas. Petrographic thin sections of these samples were examined and photographed with a light microscope. The thin sections were subsequently etched in 2% acetic acid for 2 seconds, lightly sputter coated ( 10 nm) with palladium, and examined with a Hitachi S-570 and a Philips XL-30 field emission environmental scanning electron microscope (SEM) equipped with an Oxford Link energy-dispersive spectrometer (EDS) for elemental analysis. In addition, three thin sections of individual foraminifera were similarly etched and examined with SEM. Observations of these etched foraminifera were compared with SEM images of unetched, fractured sections of the identical specimens, analyzed as part of our recent recrystallization study (Reid and Macintyre 1998). Previous X-ray diffraction analyses of these foraminifera indicate that they are composed of Mg-calcite and aragonite (Reid and Macintyre 1998); in the present study, detection of magnesium in EDS spectra was used to differentiate Mg calcite from aragonite. Descriptions of microboring types follow the terminology of Alexandersson (1972). RESULTS The sediments are composed predominantly of peloids, porcelaneous foraminifera, Halimeda, and molluscs; in some samples, grains have oolitic coatings. Micritized grains are common in all samples. SEM observations of lightly etched thin sections show a variety of microborings. Marginal pits and simple tunnels are common; most of these microborings are open, and the borings result in incomplete, irregular grain margins. In addition, and the focus of the present study, there is another type of boring, which is dominant in many grains, particularly foraminifera and coated grains. These borings begin as convolute marginal tunnels, which penetrate just beneath the external surfaces of the grains. These tunnels create extensive subsurface networks but leave the outermost grain margins perfectly intact (Fig. 1A, B). The tunnels are 5 10 m in diameter, FIG. 1. SEM photomicrographs of lightly etched thin sections showing convolute marginal tunnels filled with radiating bundles of fibrous aragonite; all photomicrographs except Part C are from a single specimen of the foraminifer Archaias (sample FOR95-C1). A) Low-magnification view showing infilled borings along the edges of septal walls; the small pits on the lateral wall at the upper edge of the photo are pores. B) Higher-magnification view of the boxed area in Part A; the edges of the upper tunnel are scalloped, a characteristic feature of many of these borings. Note perfect preservation of wall margins. C) An open pit at one end of a tunnel (left side of photo) is bordered to the right by bands of fibrous aragonite, which arch across the bore hole; this grain was microbored in a laboratory experiment (Macintyre et al., unpublished data; sample HC-C-20). D) Fibrous aragonite crystals forming a banded pattern similar to that shown in C are enveloped in mucous (arrow). E) High-magnification view showing infilling fibrous aragonite crystals composed of 0.05 m crystallites arranged in radial linear patterns. JOURNAL OF SEDIMENTARY RESEARCH, VOL. 70, NO. 1, JANUARY, 2000, P. 24 28 Copyright 2000, SEPM (Society for Sedimentary Geology) 1073-130X/00/070-24/$03.00
MICROBORING VERSUS RECRYSTALLIZATION 25
26 P. REID AND I.G. MACINTYRE and the edges of the tunnels are commonly scalloped (Fig. 1B). With the exception of open pits at one end of some tunnels (Fig. 1C), the tunnels are typically filled with fibrous aragonite. The aragonite crystals fan outward from the interior of the boring, commonly forming crescent-shaped banded patterns across the tunnels (Fig. 1C, D). These infilling crystals are in some cases enveloped in organic biofilms or mucous (Fig. 1D). The crystals show a highly irregular, composite growth form: they are composed of tiny crystallites about 0.05 m in diameter arranged in radial, linear patterns (Fig. 1E). Thin-section observations indicate that these aragonitic precipitates have a gray cryptocrystalline appearance in plane-polarized light. Advancing centripetally inward from grain margins, the rapidly filled tunnels eventually penetrate the entire grain. As endoliths rebore previously filled tunnels, it becomes impossible to distinguish individual borings (Fig. 2). SEM observations show that some grains consist almost entirely of bundles of radiating aragonite crystals, oriented in diverse directions (Fig. 2B, C). At this stage, it is difficult to differentiate microborings from possible recrystallization, as discussed in the following section. DISCUSSION AND CONCLUSIONS Our study has documented a microboring process in which precipitation in bore holes is concurrent with endolithic activity and appears to be biologically induced. Rapid precipitation during the life of the endolith is indicated by the complete filling of bore holes, except for empty pits at the ends of some of the tunnels presumably the space occupied or recently vacated by the boring microbe (Fig. 1C). The scalloped patterns of many tunnel margins (Fig. 1B) suggests a stepwise advancement of the microborer by formation of a series of pits. The banded nature of many fillings (Fig. 1C, D) is evidence of a series of precipitation events, which fill the vacated pits as the endolith advances. The close association of aragonite crystals with organic biofilms or mucous in some tunnels (Fig. 1D) argues that organic compounds may be involved in the precipitation process. Because microborings of equivalent size, but different boring types, in these same grains remain unfilled (Fig. 2A), inorganic precipitation based solely on water chemistry is unlikely. Concurrent microboring and filling of bore holes has also been observed in Bahamian stromatolites (Macintyre et al., unpublished data.). The coccoid cyanobacteria responsible for producing these filled microborings is tentatively identified as Solentia sp. Laboratory cultures of this endolith and observations of stromatolite accumulation rates indicate that entire grains are micritized in a matter of months (Macintyre et al., unpublished data). Although the process of precipitation in microborings concurrent with endolithic activity begins as a type of micrite envelope formation, it does not conform to the typical model of micritization in which precipitation takes place in vacated bore holes by processes of inorganic cementation (e.g., Alexandersson 1972). The microboring and filling patterns documented in this study are unusual in several respects (Fig. 3). In contrast to FIG. 2. SEM photomicrographs of a lightly etched thin section showing multicyclic microboring in a pervasively altered foraminiferal skeleton (sample FOR96- D7). A) Low-magnification view showing dark patches of original foraminiferal skeleton (F) surrounded by lighter areas that have been microbored and infilled; also note open microborings in the lateral wall at bottom of photo. B) Magnified view of the boxed area in Part A, showing isolated patches of high Mg-calcite (Mg), the mineralogy of the original foraminiferal skeleton, surrounded by radiating bundles of aragonite. C) An enlarged view of the boxed area in Part B, all aragonite. It is difficult to identify individual borings in Parts B and C or to differentiate between multicyclic boring and possible recrystallization. Fractured surfaces of the same septal wall illustrated in Parts B and C are shown in Reid and Macintyre (1998, figs. 4E and 4F).
MICROBORING VERSUS RECRYSTALLIZATION 27 FIG. 3. Diagram showing two distinctly different microboring patterns in carbonate grains. A) Conventional micritic rim formation, after Bathurst 1975. B) Concurrent filling, as documented in the present study. typical microbored grains, which have irregular, pitted outer surfaces, microboring processes such as those shown in Figure 1 leave grain margins almost completely intact (Fig. 1B). In addition, open bore holes, which are common in most microbored grains, are rare in these rapidly filled grains. Furthermore, instead of forming rim cements on bore-hole walls, crystals precipitated in the rapidly infilled tunnels are elongated parallel to tunnel margins, radiating from centers within the tunnel (Fig. 1C). Finally, interfaces between convolute marginal tunnels and the interior, unbored parts of the grains tend to be smooth, contrasting with the irregular interior edges of conventional micritic rims. To our knowledge, the process of filling of bore holes concurrently with endolithic activity has not previously been reported. One reason that it may have been overlooked in other studies is that these rapidly filled bore holes would not be recorded as resin casts in the embedding techniques commonly used for studying microboring patterns and identifying endoliths (e.g. Golubic et al. 1970; Radtke 1993; Perry 1998). As a consequence of rapid filling, lack of open tunnels, and intact grain margins, this type of microboring can be difficult to recognize. Because multicyclic repetitions of this process leads to development of cryptocrystalline textures with little evidence of boring, it can be mistaken for recrystallization. Indeed, we underestimated the extent of microboring in foraminiferal grains in our recent study of recrystallization (Reid and Macintyre 1998). Comparison of SEM images of etched thin sections and fractured sections of unetched specimens examined by Reid and Macintyre (1998) show that the precipitates in the filled borings (Fig. 1E) have a texture we previously termed pseudomicrite. This word was used to describe a mosaic of small crystallites, 0.02 0.05 m in size, which are typically aligned in blocky or radial patterns, forming domains 1 2 m in TABLE 1. Characteristic features associated with various micritization processes. Feature Traditional Micrite Envelopes (e.g. Bathurst 1966; Alexandersson 1972) Concurrent Infilling (This Paper) Recrystallization (e.g. Reid and Macintyre 1998; Purdy 1968) 1. open bore holes are common yes no no 2. crystals in bore holes line cavity walls yes no not applicable 3. grain margins are incomplete yes no no 4. micritization advances inward from yes yes yes grain margins 5. inward edge of micritized rim is irregular yes no no size and which are petrographically indistinguishable from micrite (Reid and Macintyre 1998, p. 931). We interpreted pseudomicrite as a recrystallization product, proposing that Mg-calcite in original foraminiferal skeletons recrystallized to aragonite. It is now apparent that at least some, if not all, pseudomicrite forms as a radial fibrous aragonite precipitate in bore holes. At this point, we remain uncommitted as to whether or not all pseudomicrite forms as the filling of microborings, or if this textural pattern extends beyond bore holes into the adjacent grain as a recrystallization front. Examine for example, Figures 2B and 2C (etched thin section), which show the septal wall identical to that illustrated in figures 4E and 4F (fractured section) of Reid and Macintyre (1998). Is all of the fibrous aragonite in this septal wall infilling bore holes, or could some of the radiating patterns be formed by recrystallization? Regardless of whether all textures previously described as pseudomicrite are primary precipitates or if this texture also forms as a recrystallization product, recrystallization remains an important process of micritization. Indeed, in later stages of micritization, the linear arrangements of tiny crystallites constituting the fibrous aragonite themselves recrystallize to form a blocky micrite (Reid and Macintyre 1998 and observations from the present study). In addition, pseudomicrite textures are not common in Halimeda, where micritized textures are formed by a recrystallization process involving a welding and growth of crystals to form blocky micrite (Reid and Macintyre 1998). In summary, and as predicted by Alexandersson (1972), the processes leading to a secondary cryptocrystalline fabric can involve several complex mechanisms. Three important processes of micritization are (1) traditional micrite envelope formation (e.g., Bathurst 1966; Alexandersson 1972), (2) precipitation in microborings concurrent with endolithic activity (this study) and (3) recrystallization (Reid and Macintyre 1998; Purdy 1968). Characteristic features of grains micritized by each of these three processes are summarized in Table 1. As shown in this table, grains micritized by concurrent filling of microborings have more features in common with recrystallized grains than with grains altered by traditional micrite envelopes. Consequently, it can be difficult or impossible to distinguish between concurrent filling of borings and recrystallization, particularly when using a petrographic microscope. ACKNOWLEDGMENTS Financial support for this study was provided by National Science Foundation grant OCE 9530215 to Reid. We are grateful to Dr. L. Prufert-Bebout for discussion of endolithic behavior throughout this study, and for identification of the microbor-
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