Biodata of R. Pamela Reid, A. Patricia Lopes Gaspar, Emily ... · Biodata of R. Pamela Reid, A....

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V.C. Tewari and J. Seckbach (eds.), STROMATOLITES: Interaction of Microbes with Sediments, Cellular Origin, Life in Extreme Habitats and Astrobiology 18, 407–425DOI 10.1007/978-94-007-0397-1_18, © Springer Science+Business Media B.V. 2011

Biodata of R. Pamela Reid, A. Patricia Lopes Gaspar, Emily M. Bowlin, Lilian Custals, and Dr. Miriam S. Andres, authors of “Microbialites and Sediments: A 2-Year Record of Burial and Exposure of Stromatolites and Thrombolites at Highborne Cay Bahamas”

Dr. R. Pamela Reid is Associate Professor in the Division of Marine Geology and Geophysics at the Rosenstiel School of Marine and Atmospheric Science, Uni-versity of Miami, USA. She obtained her Ph.D. from the University of Miami in 1985 and continued her research at the Smithsonian Institution. Her scien-tific interests are carbonate sedimentation and diagenesis, geomicrobiology and biomineralization, biogenesis of stromatolites, and shallow-water alteration of carbonate skeletons.

E-mail: [email protected]

A. Patricia Lopes Gaspar is in the maritime cartography division of CARIS, Eindhoven Area, Netherlands, where she works as a cartographic and GIS spe-cialist. She obtained her Masters degree in Marine Affairs from the University of Miami in 2007, with an internship involving sedimentation on a microbial reef.


R. Pamela Reid A. Patricia Lopes Gaspar

408 R. PAMELA REID ET AL.

Emily M. Bowlin is a graduate student in the final stages of her Ph.D. in the division of Marine Geology and Geophysics at the Rosenstiel School of Marine and Atmospheric Science, University of Miami, USA. She obtained her BS from the University of Miami in 2003. Her scientific interests are carbonate sedimentology, geobiology, and stromatolite morphogenesis.


Lillian Custals is currently in the Divisions of Marine and Atmospheric Chem-istry at the Rosenstiel School of Marine and Atmospheric Science, University of Miami, USA. She obtained her BS from the Universidad Nacional Pedro Hen-riquez Urena, Dominican Republic in 1981. Her scientific interests are transport, size distribution, and chemical composition of Aerosols and Mineral Dust in the atmosphere. She became involved in statistical analysis and data processing of the environmental data for this study.


Emily M. Bowlin Lillian Custals

409MICROBIALITES AND SEDIMENTS

Dr. Miriam S. Andres is currently a research scientist with the Carbonate Research and Development team of Chevron Energy Technology Company in San Ramon, California. She obtained her Ph.D. from the Swiss Federal Institute of Technology (ETH) – Zurich in 2002 and continued her research at the University of Miami. Her scientific interests include the evolution of microbialites through time and space. More specifically, the biogeochemistry of microbe–mineral interactions, isotopic fingerprinting of microbial pathways, as well as the role of biofilms in lithification processes.


411

MiCRoBiALitES AnD SEDiMEntS: A 2-YEAR RECoRD of BuRiAL AnD ExPoSuRE of StRoMAtoLitES AnD thRoMBoLitES At hiGhBoRnE CAY BAhAMAS

R. PAMELA REiD1, A. PAtRiCiA LoPES GASPAR2, EMiLY M. BowLin1, LiLLiAn CuStALS1, AnD MiRiAM S. AnDRES3 1University of Miami/RSMAS-MGG, 4600 Rickenbacker Causeway, Miami, FL 33149, USA2Division of Hydrography, Port of Lisbon Authority, Lisbon, Portugal3Chevron Energy Technology Company, San Ramon, CA, USA

Abstract An intensive 2-year field effort has characterized burial and exposure patterns for stromatolites and thrombolites currently forming in a fringing reef com-plex at Highborne Cay, Bahamas. Migrating sand waves in the nearshore environment bury and expose the microbialites for periods of days, weeks, or months. Intertidal, nearshore thrombolites and shallow subtidal stromatolites are buried for 30–70% of the year. Duration of exposure intervals is highly variable based on position along the reef tract. These distinctive burial and exposure patterns form a basis for understanding the distribution of microbial communities within the reef complex and the resultant microbialite accretion.

Keywords Stromatolites • Thrombolites • Sediments • Burial • Exposure • Microbial mats • Bahamas • Modern • Marine • Accretion • Bed load transport • Cyanobacteria • Photosynthesis • Schizothrix sp. • Dicothrix sp.

1. introduction

Despite the ubiquity of microbialites in the Precambrian world, modern analogs to these ancient structures are found in only a few remote locations (Grotzinger and Knoll, 1999). One such site is Highborne Cay, on the western margin of Exuma Sound, Bahamas. There, both stromatolitic (laminated) and thrombolitic (unlaminated, with clotted structures) microbialites are actively forming in a fringing reef in open marine conditions (Reid et al., 1999; Littler et al., 2005; Myshrall et al., 2010). The proximity of both microbialite types makes the Highborne reef an ideal site for studying contrasting styles of microbial deposition.


As the only known modern analog to Precambrian stromatolites that formed in open marine environments, Exuma stromatolites have been the subject of intense investigation for the past decade (e.g., Desnues et al., 2008; Eckman et al., 2008; Dupraz and Visscher, 2005; Reid et al., 2000; Visscher et al., 2000). Integrated studies relating stromatolite microstructure to microbial populations and processes have provided a model for lamination formation (Reid et al., 2000). The stromatolites are formed by lithifying microbial mats that consist of distinctly layered microbial populations, with each layer representing a community that was, at one time, at the stromatolite surface. Three “mat types” each charac-terized by a distinct surface community are recognized: a filamentous cyanobac-terial community, a biofilm community, and a coccoid cyanobacterial community. Each surface community is associated with the accretion of a distinct type of mineral deposit: trapped and bound sand grains, micritic laminae, and cemented layers of micritized grains. Subsurface lamination thus represents a chronology of former surface mats.

In 2003, the Research Initiative on Bahamian Stromatolites (RIBS) project initiated a comprehensive 5-year program to investigate intrinsic and extrinsic factors causing cycling of the surface microbial communities forming the stro-matolites at Highborne Cay, Bahamas. An important component of this study was an intensive field monitoring effort, which included “A Year in the Life of a Stromatolite.” The “year” evolved into 2 years of almost continuous monitoring of the field site from Jan 2005 to Dec 2006. During the field campaign, environ-mental and biological parameters influencing microbialite formation were closely monitored, and morphology and spatial distribution of microbialites were mapped. For a complete listing of papers published to date in the RIBS Contribution Series, see http://www.stromatolites.info.

Previous authors have speculated that growth of modern stromatolites in open ocean conditions requires periodic burial by sand to prevent overgrowth by epilithic algae and grazing/burrowing invertebrates (e.g., Dravis, 1983; Dill et al., 1986; Mac-intyre et al., 1996). Initial quantitative data on gross sedimentation in the Stocking Island reef complex, at the south end of the Exuma chain, showed that stromato-lites were buried to depths of 0.5 m for periods of up to 100 days, limiting recruit-ment and colonization by macroalgae and corals (Steneck et al., 1998).

A rigorous analysis of the wave and sediment dynamics along the High-borne Cay reef complex suggests that stromatolite growth may be restricted to a relatively narrow range of flow energy and sediment transport conditions (Eckman et al., 2008). Nearshore wave dynamics are tightly linked to local wind conditions, and stromatolites experience gross rates of deposition of suspended sediment of 7–10 g cm−2 day−1 (Eckman et al., 2008). Bedload transport, responsible for the migration of nearshore sand waves, was not described by Eckman et al. (2008) and is the focus of the study reported here.

In the present study, we document burial and exposure of the Highborne Cay reef complex by migrating sand waves. This report represents an initial phase in the compilation of a comprehensive data set pertaining to stromatolite

http://www.stromatolites.info


accretion. Forthcoming papers by Bowlin et al. (in preparation) will build on this sediment data to show how burial and exposure determine microbial mat composition, which in turn controls stromatolite accretion.

1.1. STUDY SITE

Highborne Cay (24°43¢N; 76°49¢W) at the north end of the Exuma chain of islands is approximately 4 km long and 2 km wide (Fig. 1). An unusual microbialite-algal ridge fringing reef complex stretches for 2.5 km along a sandy beach on the eastern shore of the island (Reid et al., 1999; Andres and Reid, 2006). The reef is best developed in the southern 1 km of the beach, where it was subdivided into ten research sites. Sites 1 and 2, in the south, where the beach is the most concave, are 20–50 m wide, with reef width decreasing to <10 m north of Site 2. North of Site 10, reef patches occur only as small isolated patches. Sediments in the swash zone and shallow subtidal portions of the reef at Sites 1–10 are oolitic sands with a median grain size of 175–275 mm; grain size coarsens significantly north of Site 10 (Eckman et al., 2008). Tides are semidiurnal with a maximum range of approximately 1 m.

Three distinct morphological zones are recognized within the reef (Fig. 2). From seaward to shoreward, these zones are described below.

Reef crest. This zone corresponds to the seaward edge of a coralline algal ridge formed by the branching coralline alga, Neogoniolithon strictum. Heads of the sediment tolerant N. strictum, up to 1 m in diameter, may be exposed at low tide. Coralline algal ridges were unknown in the Bahamas prior to reports of their discovery at Highborne Cay and Stocking Island by Steneck et al., 1997. All ten sites at the Highborne Cay reef, except Site 7, have a distinct reef crest.

Reef flat. This zone corresponds to the flat area of the algal ridge, shoreward of the reef crest. The reef flat is typically colonized by macroalgae, mainly Chondria littoralis, Laurencia obtuse, and Batophora occidentalis (Littler et al., 2005); microbial mats may also be present on the reef flat (Andres and Reid, 2006).

Back reef lagoon. A shallow lagoon extends from the reef flat to the beach. The dominant features in this back reef lagoon are stromatolites and thrombol-ites (Reid et al., 1999). The stromatolites form columnar heads and ridges in the subtidal areas of the back reef lagoon. The thrombolites form meter-sized plat-forms in the intertidal zone; thrombolites were further subdivided into nearshore thrombolites and seaward thrombolites, based on the difference in microbial mat communities of these structures (Myshrall et al., 2010).

To conduct an intensive field-monitoring program of the Highborne Cay reef, a small research station was constructed on the island in 2003. The initial plan was to monitor the reef 1 week per month. However, sediment movement along the beach proved far too dynamic for a 1-week per month sampling scheme to be effective; therefore the program evolved to almost continuous monitoring for a 2-year period from Jan 2005 to Dec 2006. Sporadic data sets are available from


figure 1. Map showing the location of the ten research sites established along the Highborne Cay stromatolite-algal ridge fringing reef complex. The study area extends from Site 1 in the south to Site 10 in the central area of the eastern shore of Highborne Cay.


June 2003 to Dec 2007, with almost continuous records for the period of 2005–2006. Methods and results pertaining to the investigation of burial and exposure patterns within the Highborne Cay reef are presented below.

2. MEthoDS

2.1. DATA COLLECTION

A total 84 markers (galvanized stakes and nails) were monitored within the ten research sites along the reef track. The stakes and nails were hammered into the top surfaces of thrombolites, stromatolites, reef flat, and reef crest. Visual obser-vations at daily to weekly intervals, describing the stakes and nails as “buried,” “exposed,” or “sand level”, were recorded on field sheets and entered into a database. “Buried” corresponds to greater than 2 cm of sediment covering the monitored surface, based on data showing that the mats are photosynthetically inactive at

figure 2. Photographs showing the distinct reef zones at Site 4. (a) A panoramic view of Site 4 with thrombolites in the foreground and the reef crest in the background. (b) Stromatolites in back reef lagoon. (c) Reef flat covered with the macroalgae Laurencia obtuse. (d) Reef crest with heads of the coralline alga, Neogoniolithon strictum at the seaward edge of the reef platform.


depths of 3 cm (Perkins et al., 2007). Actual depths of burial, as determined by exposed heights on the stakes or nails, were recorded in the database, but for the purposes of analysis in this study, these records are classified simply as buried. “Exposed” denotes a monitored surface free of sediment accumulation. “Sand level” denotes a surface at the level of the sand, or buried to a depth of less than or equal to 2 cm, and corresponds to a transition between burial and exposure events. Surfaces at sand level may be buried and unburied with shifting tides.

In addition to the visual surveys, photographs were taken at weekly to monthly intervals to document changes in surface mat communities at each stake and to contribute to the overall understanding of sediment movement along the reef complex; an example showing burial and exposure of a stromatolite nail at Site 1 is shown in Fig. 3. A total of 13,000 photographs for 2005 and 2006 are compiled in an image database.

figure 3. A photographic time series showing different states of burial and exposure at nail N1.2, Site 1 (nail height is 11 cm). The nail was hammered into the top of a columnar stromatolite. (a) June 19, 2005, only the head of the nail is exposed, sand load is almost 11 cm. (b) October 10, 2005, stromatolite column is totally exposed. (c) November 11, 2005, the top surface of the stromatolite column is at sand level. (d) March 13, 2006, the stromatolite is buried, nail is partially buried; the stromatolite surface is buried by approximately 3 cm of sand.


2.2. DATA ANALYSIS

2.2.1. Burial and Exposure2.2.1.1. Sedimentation Patterns for Individual StakesEach entry recorded in the sedimentation database was categorized as exposed (0 cm), sand level (0–2 cm), or buried (>2 cm). The time series of sedimentation data for each marker was then compiled into a bar graph that spanned the 2-year monitoring period, with each category represented by a distinctive color block. Missing data intervals were left blank, but subsequently some of the missing inter-vals were interpolated, with a “best guess” as to burial or exposure state, based on our accumulated knowledge of the dynamics of each site after 5 years of field observations.

The burial and exposure data for all stakes and nails within a single reef zone at each site were used to define average sedimentation patterns of different reef zones at each site, and average duration of exposure and burial events expe-rienced by the microbialites, as presented below.

2.2.1.2. Sedimentation Patterns by Reef ZoneThe exposure/burial visual survey data for the 84 markers were grouped by site and classified by reef zone (nearshore thrombolites, seaward thrombolites, stro-matolites, reef flat, and reef crest) within each site. Each day for 2005–2006 was classified as either buried or exposed; sand level was grouped with exposed. Days lacking recorded data were handled as follows: if the marker showed the same burial or exposure state on both ends of the data gap, the days within the gap were assigned this same value; if burial state changed during the missing time interval, burial state was changed at the mid-point of the gap. The number of days each marker was exposed or buried in 2005 and 2006 was then calculated as a percent of the 2-year time span. The percentages for all markers at a given site within a specific reef zone were then averaged. The averages of percent of time buried vs. percent of time exposed for each site within each reef zone were graphed as histograms.

2.2.1.3. Duration of Microbialite Exposure and Burial EventsThe average burial and exposure data for thrombolites and stromatolites at each site, as calculated in Sect. 2.2.1.3, were classified into event types according to the duration of the burial and exposure periods. Burial or exposure episodes lasting less than 7 days were classified as daily events; burial or exposure episodes of 7–30 days were classified as week-long events, and burial or exposure episodes of >30 days were classified as month-long events. The frequency of each event type was then calculated as a percent of total time exposed or buried, and averaged for all markers from a single site within each zone. These averages were graphed as histograms showing percent of each event type (i.e., month-long, week-long, or daily burial as a percent of total time buried; and month-long, week-long, or daily exposure as a percent of total time exposed) for each site within each of the three zones (nearshore thrombolites, seaward thrombolites, and stromatolites).


3. Results

3.1. SEDIMENTATION PATTERNS AT INDIVIDUAL STAKES

Burial and exposure data for each of the 84 individual stakes and nails for the 2005 and 2006 monitoring period were plotted as “bar” graphs; examples for stro-matolite nails N1.3 at Site 1 and N8.3 at Site 8 are shown in Fig. 4. Blue on these figures represents periods of exposure, black represents buried, and red represents burial depths of less than 2 cm, termed “sand level,” where the surface of the microbialite is typically buried and unburied with each tidal cycle. Interpolated (i.e., “best guess”) intervals of burial and exposure are indicated by horizontal white bars overlying the blue and black; intervals of missing data that we are not comfortable interpolating were left white.

Stromatolites at Sites 1 and 8 experienced very different sedimentation pat-terns, as shown on Fig. 4. Site 1 stromatolites were typically exposed (blue) for long periods at a time, often several months in duration, with occasional sand level inter-vals (red). In contrast, Site 8 stromatolites experienced relatively short periods of exposure, typically lasting for days to weeks. Burial episodes at Site 1 are typically week-to-month long events, with burial at Site 8 being day, week, or month long in duration.

3.2. SEDIMENTATION PATTERNS BY REEF ZONE

Histograms in Fig. 5 show sedimentation patterns for each reef zone, by site, for the combined 2005 and 2006 monitoring period. Color codes of Fig. 5 indicate percent of total time buried (black) vs. exposed (blue). Data are averages of stakes

figure 4. Bar graphs showing burial and exposure data for stromatolite nails N1.3 at Site 1 and N8.3 at Site 8. Blue represents periods of exposure, black represents buried, and red represents burial depths of less than 2 cm, termed “sand level,” where the surface of the microbialite is often buried and unbur-ied with each tidal cycle. Interpolated intervals are indicated by white horizontal bars; areas of missing data that cannot be interpolated are left white. Note that Site 1 stromatolites are typically buried and exposed for periods of 1 month or longer, whereas Site 8 stromatolites are typically exposed for periods of weeks to days.


from each zone at each site. Results show that the nearshore thrombolites and stromatolites in the back reef were much more buried than the reef flat and reef crest. Nearshore thrombolites (Fig. 5a) and stromatolites (Fig. 5c) were typically buried for 25–70% of the total observation period, contrasting with 5–25% burial of the reef flat (Fig. 5d) and almost continuous exposure of the reef crest zone (Fig. 5e). At sites with both nearshore and seaward thrombolites (Sites 4, 5, and 8), the seaward thrombolites (Fig. 5b) were much less buried (5–15%) than the near-shore equivalents (30–70% buried; Fig. 5a).

Sedimentation within the back reef also shows distinct patterns along the length of the reef. Stromatolites at the south and north ends of the study area,

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figure 5. Histograms showing percentage of time the various reef zones at each site were exposed (blue) or buried (black) for the combined 2005 and 2006 observation period. Nearshore thrombolites (a) and stromatolites (b and c) in the back reef were exposed for much less than the reef flat (d) and reef crest (e).


Sites 1–4 and Site 10, were more exposed (65–75% of the year) than stromatolites in the reef’s middle section at Sites 5–9 (30–50% exposed; Fig. 5c).

Results for 2005 and 2006 plotted as separate years show the same overall trends as the summaries for the combined data sets, although some individual zones at specific sites exhibit distinct yearly variability. For example, stromatolites at Site 8, which average 50:50 exposure vs. burial for the combined 2-year record (Fig. 5c), showed 60:40 exposure vs. burial in 2005, and 40:60 exposure vs. burial in 2006. Similarly stromatolites at Site 1, which averaged about 60:40 exposure vs. burial for the 2-year record, showed 50:50 exposure vs. burial in 2005 and 70:30 exposure vs. burial in 2006.

3.3. DURATION OF MICROBIALITE ExPOSURE AND BURIAL EVENTS

Figure 6 shows the duration of exposure and burial events for thrombolites and stromatolites in the back reef zone, defined as month-long (>30 days), week-long (7–30 days), and daily (<7 days) events. Since the reef flat and reef crest were almost continuously exposed, exposure duration is close to 100% month-long events. The following results emerged from analysis of the more complex burial and exposure patterns of the thrombolites and stromatolites.

Nearshore thrombolites were generally exposed for about 30–40% of the year, except for Site 5, which is exposed about 70% of the time (Fig. 5a). Exposure episodes were mainly 7–30 day (week-long) events, except for Site 5, where >30 day (month-long) long exposure was common (Fig. 6). Burial of nearshore thrombolites (60–70% of the year, 30% at Site 5; Fig. 5a) was dominated by month-long events (55–75%; Fig. 6d).

Seaward thrombolites were exposed for 65–95% of the 2-year monitoring record (Fig. 5b). Exposure events are mainly >30 days (month-long; Fig. 6b). The few burial events (5–35% of the year) were mainly 7–30 days (week-long) in dura-tion (Fig. 6e).

As in overall patterns of burial vs. exposure, stromatolites show distinct differences in duration of exposure and burial events along the reef. Exposure ofa stromatolites at Sites 1–4 and Site 10 (65–75% of the year; Fig. 5c) was dominated by month-long episodes, whereas exposure episodes at sites 5–9 (35–50% of the year; Fig. 6b) were week- and month-long, with significant daily exposure events at Sites 5, 7, 8, and 9 (Fig. 6c). In addition, burial events at Sites 1 through 8 were dominantly month-long, whereas burial at Sites 9–10 was mainly week-long (Fig. 6f).

It should be noted that in all cases daily burial and exposure events might be underestimated because the typical time between observation periods was several days to a week. In addition, as in total burial vs. exposure, individual records of duration events in years 2005 and 2006 show overall trends that are


similar to those of the 2-year record, but specific patterns for any individual sites may vary from year to year. For example, for Site 8 stromatolites, approximately 50% of exposure in 2005 plus 2006 consisted of month-long events (Fig. 6c), but month-long events comprise 60% of the 2005 record, and only 30% of the 2006 record. Similarly, month-long burial of Site 1 stromatolites averaged 60% of total burial for the 2-year record (Fig. 6f), but was 85% of total burial in 2005 and 35% in 2006.

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figure 6. Histograms showing duration of exposure (a, b, c) and burial events (d, e, f) for thrombolites (a, b, d, e) and stromatolites (c, f) by site. Events are classified as month-long (>30 days), week-long (7–30 days), and daily (<7 days).


4. Discussion

The burial and exposure patterns described earlier reflect migrating sand waves in the nearshore region of the Highborne Cay beach. These sand waves rarely encroach on the reef crest, the highest portion of the reef; sand waves sometimes blanket the reef flat, and they regularly bury and unbury microbialites in the back reef lagoon. Like suspended sediment in Highborne reef (Eckman et al., 2008), bedload transport is linked to local wind conditions, but to date, we have been unable to predict burial and exposure at individual sites, based on the records of wind strength, direction, and duration. Sediment movement along the reef will also be affected by the complicated and variable morphologies of each individual site. Further analysis involving consideration of the physics of bedload transport in areas with high rugosity may help to predict burial and exposure patterns, but is beyond the scope of the present study, which is aimed at understanding affects of burial and exposure on microbialite accretion.

Alternating burial and exposure of the stromatolites and thrombolites in back reef lagoon of the Highborne Cay reef profoundly affects the communities that inhabit this reef zone. Of particular importance for stromatolite biogenesis is the ability of cyanobacteria to survive burial (Kromkamp et al., 2007; Perkins et al., 2007). Studies of cyanobacterial photophysiology using variable fluores-cence showed that artificially buried samples of stromatolite cyanobacterial communities inactivated their photosynthetic electron transport when oxygen concentrations in the darkened sample decreased to less than 30% saturation. Post burial, the cyanobacterial community reactivated its photosynthetic activity within 1–2 h, but reactivation required both light and oxygen (Kromkamp et al., 2007). Based on these observations, Kromkamp and coauthors speculated that the redox state of the plastoquinone pool determines inactivation/reactivation process.

Results of photosynthesis in artificially buried stromatolite mats (i.e., Kromkamp et al., 2007) are supported by studies of natural burial processes (Perkins et al., 2007). Prokaroyotic cyanobacterial mats reactivated their quan-tum efficiency after 7–16 days of burial, although reactivation was slower with longer burial. In contrast, mats with surface communities dominated by diatoms did not have high tolerance for burial, with two out of three samples showing no recovery, and the third sample only low recovery after 7 days of burial (Perkins et al., 2007). Thus burial periods of longer than 1 week are likely to kill diatom communities on stromatolite surfaces, but cyanobacterial communities will be largely unaffected by burial.

In contrast to burial, which kills diatoms, diatom communities flourish on stromatolite surfaces during exposure events of several weeks to months, with different diatoms dominating seasonally (unpublished observations). Thus diatoms can become well established on the surfaces of Site 1 stromatolites, which experi-ence characteristic exposure events with durations of >30 days. In contrast, diatoms are insignificant colonizers of stromatolites at Site 8, where exposure is typically limited to day- or week-long events. The balance of burial and exposure


events is thus likely to be an important determinant in the net accretion of modern marine stromatolites.

Similarly, differences in burial patterns of nearshore and seaward thrombo-lites may account for differences in the microbial communities on the surfaces of these structures. Seaward thrombolites, which experience little to no burial, are colonized by “button mats,” dominated by Cyanobacteria (mainly filamentous Dicthrix sp.; Planavsky et al., in press) and Alphaproteobacteria (Myshrall et al., 2010). In contrast, nearshore thrombolites, which are typically buried for up to 60–70% of the year, have surfaces colonized by “pink mat,” so named because of an abundance of pigmented spore-forming cells that dominate the upper mm of the mat. The pink thrombolitc mats demonstrate a significantly higher level of diversity than the button mats, with the dominant phyla being Alphaproteobac-teria and Deltaproteobacteria (Myshrall et al., 2010). As in the stromatolites, these differences in microbial surface mats are likely to influence accretion and subsurface microstructure of the microbialite.

5. Summary and Conclusions

Analysis of burial and exposure data for the Highborne Cay reef complex has identified characteristic sedimentation patterns for the various sites and reef zones of the microbialite-algal ridge reef system. Our findings are as follows:

• Stromatolites and thrombolites in back reef lagoon are frequently buried for periods ranging from days to months. The reef flat is occasionally blanketed by sediment, and the reef crest is almost never buried.

• At sites with both nearshore and seaward thrombolites (Sites 4, 5, 8), nearshore thrombolites were buried more (30–70% burial) than the seaward equivalents (5–15% burial).

• Nearshore thrombolites experienced frequent burial events longer than 30 days (month-long). Exposure of the nearshore thrombolites was mainly for 7–30 days (week-long), except for Site 5, where exposure events were commonly month-long.

• Seaward thrombolites were exposed mainly for periods greater than 30 days (month-long). The few burial events were mainly 7–30 days (week-long) in dura-tion. At Site 10 burial events lasting 7–30 days (week-long) and greater than 30 days (month-long) were equally common.

• Stromatolites at the south and north ends of the reef (Sites 1–4 and Site 10) were more exposed than stromatolites in the reef’s middle section (Sites 5–9).

• Exposure of stromatolites at Sites 1–4 and Site 10 (65–75% exposure) was dominated by month-long episodes, whereas exposure at Sites 5–9 (35–50% exposure) was characterized by week- and month-long episodes.

• Stromatolite at Sites 5, 7, 8, and 9 experienced significant daily exposure events.


These burial and exposure patterns form a basis for understanding the distribution of microbial communities within the reef complex, resulting in microbialite accretion. As shown by previous studies, cyanobacterial communities can survive periods of prolonged burial, whereas diatom communities cannot. Diatoms become established during prolonged exposure events. On the basis of the burial and exposure patterns described in this study, it is likely that diatoms play a role in stromatolite accretion at Site 1, but not at Site 8; these predictions will be verified by studies of microbial mats and accretion in forthcoming papers by Bowlin et al. (in preparation). Similarly, increased burial of nearshore throm-bolites compared with seaward thrombolites may account for the differences in composition of the surface microbial mats on these structures, with seaward thrombolites dominated by Cyanobacteria and Alphaproteobacteria, and near-shore thrombolites dominated by Alphaproteobacteria and Deltaproteobacteria (Myshrall et al., 2010). In summary, bedload sediment transport in the form of migrating sand waves results in distinct patterns of burial and exposure for the microbialites, which are likely to be important determinant of microbial mat composition, and, in turn microbialite accretion.

6. Acknowledgments

We thank members of the RIBS team, who provided lively discussion and support for all aspects of the monitoring program. The crew of the RV Walton Smith and the management of Highborne Cay Plantations are also gratefully acknowledged for logistical support. Research was supported by National Science Foundation grant EAR-0221796. This is RIBS Contribution Number 56.

7. References

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