gischler et al 2007

Giant Holocene Freshwater Microbialites, Laguna Bacalar,Quintana Roo, Mexico

EBERHARD GISCHLER*, MICHAEL A. GIBSON� and WOLFGANG OSCHMANN**Institut fur Geowissenschaften, J.W. Goethe-Universitat, 60438 Frankfurt am Main, Germany(E-mail: [email protected])�Department of Geology, Geography & Physics, University of Tennessee, Martin, TN 38238, USA

Associate Editor: Christian Betzler

ABSTRACT

With more than 10 km of total length, Holocene microbialites in Laguna

Bacalar, Mexico, belong to the largest freshwater microbialite occurrences.

Microbialites include domes, ledges and oncolites. Domal forms can grow to

diameters and heights of 3 m. Microbialites are composed of low magnesium

calcite which is, to a large extent, precipitated due to the metabolic activity of

the cyanobacteria Homeothrix and Leptolyngbya, and associated diatoms.

Photosynthesis removes carbon dioxide and triggers carbonate precipitation.

Also, an elevated carbonate concentration in lagoon waters, derived from

dissolution of Cenozoic limestone in a karst system, supports carbonate

precipitation. Trapping and binding of detrital grains is also observed, but is

not as common as precipitation. Bacalar microbialites are largely thrombolitic,

however, stromatolitic sections occur as well. The bulk of Bacalar

microbialites probably formed in the Late Holocene (ca 1 kyr BP until

present). According to 14C dating, microbialites accreted 9 to 8 cal kyr BP;

however, these ages may be too old as a result of a strong hard water effect.

This effect is seen in 14C ages of living bivalve and gastropod mollusc shells

from Bacalar Lagoon, which are 8 to 7 cal kyr BP. The modern associated fauna

of microbialites is characterized by low diversity and high abundance of the

bivalve mollusc Dreissena sp. and the gastropod Pomacea sp. The abundant

grazing gastropods presumably hamper modern microbialite formation. A

comparison of Bacalar microbialites with other modern microbialite

occurrences worldwide shows only a few patterns: sizes, shapes, microbial

taxa, mineralogy, type of accretion and settings including water properties of

microbialite occurrences exhibit high variability. A trend may be seen in the

grazing metazoa, which are rare to absent in the marine and brackish examples,

but apparently present in all the freshwater occurrences of microbialites. Also,

freshwater examples are usually characterized by elevated concentrations of

carbonate and/or calcium ions in the surrounding waters.

Keywords Carbonate, Holocene, Mexico, microbialite.

INTRODUCTION

Microbialites are among the oldest traces of lifeon earth and known from deposits as old as EarlyArchaean (Riding, 2000; Allwood et al., 2006).The term microbialite is used here in the sense ofBurne & Moore (1987), describing an organosedi-

mentary deposit formed from the interactionbetween benthic microbial communities anddetrital or chemical sediments. Stromatolites arelayered microbialites, whereas thrombolites haverather clotted and unlayered textures. The rela-tively high abundance of microbialites in Pre-cambrian when compared with younger deposits

Sedimentology (2008) 55, 1293–1309 doi: 10.1111/j.1365-3091.2007.00946.x

� 2008 The Authors. Journal compilation � 2008 International Association of Sedimentologists 1293

was interpreted as a consequence of an increasedgrazing pressure from evolving metazoa duringEarth history (Garrett, 1970). Subsequently, thisview was modified due to the discovery of moreand more Precambrian and Phanerozoic micro-bialite occurrences. These occurrences turned outto be quite diverse regarding shape, texture andorganic content (Pratt, 1982; Riding, 2000). Notonly did microbes evolve and algae come intoplay, environmental conditions such as the car-bonate content of ocean water also changedthroughout the Phanerozoic. Carbonate saturationis of great importance for microbialite formationbecause non-enzymatic precipitation of calciumcarbonate in biofilms is only partially organicallycontrolled (Riding & Liang, 2005).

Modern microbialites, which can be used asanalogues for their fossil counterparts, occur in awide variety of environments. There are hyper-saline examples, such as the classic Shark Baylocation in Western Australia (Reid et al., 2003),sub-tidal stromatolites in the Bahamas, that areforming in areas of extensive sediment redeposi-tion (Dill et al., 1986; Reid et al., 2000), ‘kopara’ inshallow Pacific atoll lagoons (Defarge et al., 1994),microbialites in protected reef cavities (Reitner,1993; Camoin et al., 1999), in brackish environ-ments (Rasmussen et al., 1993) and in alkalinelakes (Kempe et al., 1991; Arp et al., 2003). Micro-bialites also occur in freshwater lakes and lagoons,for example, in Western Australia (Moore, 1987;Moore & Burne, 1994), western Mexico (Winsbor-ough et al., 1994; Garcia-Pichel et al., 2004) andCanada (Laval et al., 2000). In some of theselocations, metazoan grazers are rare; however,there are also examples in which grazers arepresent. Freshwater lagoon and lake waterscontaining microbialites are usually characterizedby elevated carbonate content.

To understand the formation of fossil microbia-lites, it is crucial to study modern examples.However, not all fossil examples have moderncounterparts, and, likewise, not all modern occur-rences have equivalents in the fossil record(Golubic, 1991). Therefore, it is important toincrease the knowledge of modern and fossilmicrobialite occurrences. In this paper, the newlydiscovered Bacalar location is described, which

represents one of the largest freshwater microbia-lite occurrences in the world.

SETTING

Laguna Bacalar is a 40 km long and 1 to 2 kmwide, NNE-trending freshwater lagoon located insouth-eastern Quintana Roo, Mexico (Figs 1 and2); it is surrounded by flat-lying Cenozoic lime-stone. The outline of Laguna Bacalar, adjacentlagoons and former river beds suggests the influ-ence of the tectonic grain in the area. The RioHondo Fault Zone presumably acted as a pathwayfor meteoric fluids during Pleistocene lowstandsof sea-level (Purdy et al., 2003). Laguna Bacalargets as deep as 15 m. Along large parts of thelagoon, shallow, intermittently flooded areas withplant growth separate Laguna Bacalar into awestern section and an eastern section. Fourcenotes (water-filled sinkholes) near the villageof Bacalar and the cenote near Xul-Ha havedepths of up to 90 m, according to local fisher-men and boatmen. In the southern part of LagunaBacalar, there is a strong current to the northoriginating at Xul-Ha cenote, through Canal de losPiratas where it curves towards the south and intothe eastern part of the lagoon. The eastern lagoonis connected to Chetumal Bay via the Rio Hondoand the Chetumal aqueduct to the south. In thenorth, the lagoon connects to Chetumal Baythrough a system of other lagoons includingLaguna Chile Verde and Laguna Guerrero. Nomajor currents were detected in these lagoonparts. Water temperatures ranged from 25 to 28 �Cduring field work in March 2005 and August2006. Rainfall amounts to 1250 to1500 mm year)1 (Purdy et al., 2003). There wereno fluctuations in the water level of the lagoonobserved during field work spanning two years,nor does it vary according to local boatmen andinhabitants of the village of Bacalar. The analysesof water samples from the lagoon (Table 1, Fig. 3)show that conditions for calcium carbonate pre-cipitation are extraordinary. Ca2+ concentrationsare close to typical values of marine waters andHCO3

) values even exceed average marine con-centrations. These high ion concentrations are

Fig. 1. Location maps of the study area. (A) Location of Laguna Bacalar in Mexico. The other well-known freshwaterstromatolite location of Cuatro Cienegas is marked in the north-east of the country (Winsborough et al., 1994; Garcia-Pichel et al., 2004). B: Belize, G: Guatemala, H: Honduras, N: Nicaragua, S: San Salvador. (B) Laguna Bacalar andsouth-western part of Chetumal Bay near the Mexican–Belizean boundary. Freshwater stromatolites occur along thelagoon shore for 10 km, from the village of Bacalar to the entrance of Xul-Ha cenote (crosses). Published brackishstromatolite location in Chetumal Bay (Rasmussen et al., 1993) south of city of Chetumal near mouth of Rio Hondo isalso marked by a cross. Numbers 18, 21 and 22 are sample stations (see Fig. 2).

1294 E. Gischler et al.

� 2008 The Authors. Journal compilation � 2008 International Association of Sedimentologists, Sedimentology, 55, 1293–1309

A B

Freshwater microbialites, Bacalar, Mexico 1295


Fig. 2. Southern part of Laguna Bacalar showing sample stations between villages of Bacalar and Xul-Ha. For samplestations 18, 21 and 22 (see Fig. 1).



presumably a consequence of dissolution oflimestone bedrock and water circulation in aconnected karst system.

METHODS

A total of 13 push cores (aluminium core pipe of7Æ5 cm diameter with core catcher), up to 1Æ8 mlong (total length 15 m), were taken in thesouthern part of Laguna Bacalar in March 2005and August 2006 (Fig. 2). In addition, surfacesediment and water samples were collected at 20sample stations (Figs 1 and 2). Reconnaissancetrips were also made along the coast of ChetumalBay from the city of Chetumal to the opening ofLaguna Guerrero in the north. Two additionalwater samples were taken in the bay. TheChetumal Bay stromatolite occurrence south ofthe mouth of Rio Hondo (Rasmussen et al., 1993)was also visited and cored for comparison. In thelaboratory, cores were opened and lithologiesdescribed. Samples were selected for thin sec-

tion preparation, scanning electron microscope(SEM) studies and X-ray diffraction (XRD) anal-yses to identify carbonate mineralogy. Surfacesediment samples from the lagoon floor werewashed, dried and investigated for compositionand mineralogy. Living microbial mats from thesurface of microbialites at location 2 werecollected and stored in 70% alcohol. Thesesamples were critically point dried and investi-gated under SEM. A total of 28 bulk sampleswere age dated using the standard 14C radiocar-bon method by Beta Analytic Inc., Miami, FL,USA. Two mollusc shells were dated by theaccelerator mass spectrometry (AMS) method atthe same laboratory. Results are reported as2-sigma calibrated ages (95% probability). Watersamples were analysed by atomic absorptionspectrometry (AAS) for cations Na+, K+, Mg2+,Ca2+ in the Laboratory of Geochemistry of theEnvironment at J.W. Goethe-Universitat, Frank-furt, and by titration for anions Cl), SO4

2), HCO3)

in the Ala Analytisches Labor (Aachen,Germany). AAS analyses were conducted on a

Table 1. Results of water analyses.Sample Na2+ K+ Mg2+ Ca2+ HCO3

) Cl) SO42)

BAC 1 55Æ75 4Æ75 78Æ88 325Æ00 183Æ00 41Æ70 1072Æ00BAC 2 56Æ25 4Æ25 72Æ13 322Æ50 165Æ00 44Æ50 1100Æ00BAC 3 59Æ88 4Æ50 75Æ75 326Æ25 140Æ00 43Æ50 1060Æ00BAC 4 55Æ13 4Æ25 72Æ88 322Æ50 177Æ00 70Æ40 1160Æ00BAC 5 51Æ00 4Æ00 69Æ50 325Æ00 201Æ00 66Æ60 1137Æ00BAC 6 49Æ13 3Æ63 71Æ75 322Æ50 183Æ00 65Æ80 1113Æ00BAC 7 49Æ25 3Æ50 76Æ13 325Æ00 214Æ00 70Æ60 1139Æ00BAC 8 48Æ63 3Æ75 72Æ38 313Æ75 207Æ00 48Æ40 1019Æ00BAC 9 49Æ88 4Æ13 70Æ38 308Æ75 232Æ00 34Æ50 1031Æ00BAC 10 48Æ75 3Æ50 73Æ75 313Æ75 238Æ00 50Æ90 1038Æ00BAC 11 67Æ88 4Æ88 82Æ38 325Æ00 165Æ00 78Æ00 1211Æ00BAC 12 72Æ00 5Æ00 81Æ50 320Æ00 146Æ00 79Æ00 1106Æ00BAC 13 70Æ13 5Æ13 84Æ00 316Æ25 189Æ00 81Æ70 1147Æ00BAC 14 75Æ63 5Æ88 84Æ88 313Æ75 116Æ00 102Æ00 1154Æ00BAC 15 80Æ00 6Æ00 86Æ75 315Æ00 110Æ00 110Æ00 1194Æ00BAC 16 77Æ50 5Æ50 85Æ38 312Æ50 104Æ00 102Æ00 1165Æ00BAC 17 67Æ75 5Æ13 83Æ88 312Æ50 104Æ00 185Æ00 1154Æ00BAC 18 108Æ38 7Æ63 88Æ88 310Æ00 104Æ00 43Æ60 1030Æ00BAC 19 43Æ13 3Æ88 71Æ25 333Æ75 226Æ00 39Æ10 1023Æ00BAC 20 43Æ88 3Æ88 72Æ50 328Æ75 220Æ00 46Æ50 1185Æ00Mean 61Æ49 4Æ66 77Æ74 319Æ63 171Æ20 70Æ19 1111Æ90CHE 21 4174Æ00 138Æ50 498Æ50 295Æ00 171Æ00 5730Æ00 1346Æ00CHE 22 2535Æ00 85Æ50 344Æ50 351Æ25 153Æ00 3060Æ00 1544Æ00Mean 3354Æ50 112Æ00 421Æ50 323Æ13 162Æ00 4395Æ00 1445Æ00Oceanwater

10 760 385 1295 415 140 19 350 2700

Values for adjacent Chetumal Bay (CHE) and mean ocean water are given forcomparison. For locations of samples, see Fig. 2. Average ocean water valuesare from Milliman (1974); his table 3). Values are in mg l)1. Analysis stan-dard deviations: Na 2Æ5%, K 2Æ5%, Mg 3Æ5%, Ca 4Æ0%, HCO3 3Æ0%, Cl 2Æ5%,SO4 4Æ5%.



Fig. 3. Ca2+ and HCO3) concentrations at sample stations in Laguna Bacalar. Note that the carbonate concentration in

the south-west of the lagoon near the cenotes is much higher when compared with the rest of the lagoon.



Perkin-Elmer AAnalyst 300 spectrometer (Per-kin-Elmer, Waltham, MA, USA). Analytical errorranges given in Table 1 are calculated based onresults from multiple analyses conducted by thetwo laboratories during the past several years.

RESULTS

Occurrence and appearance of microbialites

Field mapping shows that microbialites are con-centrated in the south-western branch of LagunaBacalar. These microbialites occur along a 10 kmstretch from the northern end of Xul-Ha cenote tothe southern end of the town of Bacalar, encom-passing samples from all the stations from 1 in thenorth to 9 in the south (Fig. 2). It is this lagoonsection that is characterized by strong northwardcurrents and elevated concentrations of calciumand bicarbonate ions. Thin microbialite crusts arealso found in the south-eastern branch of LagunaBacalar around sample location 17 (Fig. 2). Thereare no microbialite occurrences in the lagoonnorth of Bacalar. Also, no microbialites werefound during reconnaissance trips to the westerncoast of Chetumal Bay between the city of Chetu-mal and the entrance to Laguna Guerrero (Fig. 1).

The microbialites are usually domal or pillow-formed structures of up to 2 m height anddiameter (Fig. 4A). In some cases, microbialitescoalesce to form larger features. The largest andmost continuous occurrence is located in ‘TheRapids’ (Fig. 4B), an area where the lagoonnarrows to ca 5 m wide and where currentsincrease significantly compared with the rest ofthe lagoon. Here, microbialites are up to 3 m indiameter and height; they mostly coalesce to forma continuous mass along the entire channel. Closeto the water surface, these microbialites also growinto overhangs and ledges. At location 2, micro-bialites exhibit circular layering on the exposedflat upper surface (Fig. 4A); some look like micro-atolls, in that growth is concentrated at the rimenclosing a shallow pool. At the same location,microbialites also appear to be in the process ofbeing recently exhumed from peat and soil (Figs 5to 7).

Microbialites are usually better consolidated atthe surface when compared with sections ca30 cm below the surface. When broken open,microbialites are mostly unlayered with a throm-bolitic texture. Layering (stromatolitic texture) isseen in some cases and is caused by verticalvariation in the degree of cementation (Fig. 4C).

The occasional occurrence of layered sectionsindicates that stromatolite mat growth may be arecurrent phenomenon in the accretion of theselargely thrombolitic microbialites.

Microbialites also occur in the form ofcentimetre-sized to decimetre-sized oncolites.Oncolites are found in a larger area at location 2(Fig. 4D) and on the channel floor of ‘The Rapids’.Other forms of microbial activity include encrus-tations around mangrove roots and tree trunks orbranches that fell into the water. The latteroccurrences suggest that microbialites in LagunaBacalar are in the process of formation or at leastthat they formed in the recent past.

The accessory fauna of microbialites, includingoncolites, consists of mytiloid bivalves of thegenus Dreissena, and herbivorous gastropods ofthe thin-shelled genus Pomacea, which occurin very large numbers of individuals (Fig. 4E andF).

Composition and microstructure of calcifiedmicrobialites

Microbialites consist entirely of low magnesiumcalcite. Calcite crystals include nano-sized grains,short prismatic crystals (<10 lm length, 2 to 3 lmdiameter) and scalenohedral grains (around10 lm diameter) and cement (Fig. 8A and B).Detrital grains are mostly mollusc shell frag-ments, based on the dense texture and style offragmentation of the grains. Throughout the corematerial, calcified tubes of low magnesium calciteare found (Fig. 8C to F); they are often upwardradiating in orientation and show some growthrhythm seen in thin sections as a weaklyexpressed lamination (Fig. 8C). Tubes are up toseveral millimetres long and have diameters of upto 50 lm. Tube walls are up to 10 lm thick andare composed of scalenohedral crystals; theyrepresent the calcified moulds of Homeothrixfilaments. Organic remains of filaments can beseen in thin sections as opaque dark linessurrounded by calcareous deposits (Fig. 8D). Inseveral cores, scalenohedral cement is found tohave grown into millimetre-thick crusts (seeFig. 4G). Cement crystals are up to 50 lm long(Fig. 8F). Diatom tests are also very commonwithin microbialites (Fig. 8G). Lagoonal sedimentsurrounding microbialites is very similar to sed-iment within microbialites (Fig. 8H). In addition,the bivalve mollusc Dreissena sp. is found occa-sionally cemented into the microbialite frame-work, whereas the gastropod Pomacea sp. islacking below the living biomat surface.



A B

C D

E F

G H



Structure of living microbial mat

At location 2, conspicuous living microbial matsup to 1 cm thick are found on top of somemicrobialite heads. These mats are tough, leath-ery and almost cartilaginous. The living biomatsare layered and exhibit colour changes fromorange to green to reddish brown, from top tobottom (see Fig. 4H). The mats are produced by afine filamentous cyanobacterium, which pro-duces tough extracellular polysaccharide sheaths.The organism is present mainly in the top layersof the mat, whereas the interior of the mat iscomprised of interwoven sheaths. On the surface,the filaments are prostrate (Fig. 9A) and in theinterior they alternate in orientation from pre-dominantly vertical to horizontal and are perpen-dicular to each other layer by layer (Fig. 9B). The

filamentous cyanobacterium is currently classi-fied under the polyphyletic genus LeptolyngbyaAnagnostidis & Komarek 1988; it presumablybelongs to a new species, which is being inves-tigated in a separate study. These biomats areactively growing as seen in a 10 · 10 cm2 piececut out with a knife in March 2005, which hadcompletely grown back by August 2006.Nano-sized crystals of low magnesium calciteare found on bacterial filaments (Fig. 9C). Withinthe meshwork of bacterial filaments, short pris-matic low magnesium calcite crystals of less than10 lm diameter with hexagonal cross-section,and detritus with grain-sizes of 10 to 20 lm arefound (Fig. 9D). Occasionally, platy crystalsoccur with diameters of 10 to 20 lm and 1 to2 lm thickness (Fig. 9E). Also, calcite crystalswith a spiked ball morphology that are only a fewmicrons in diameter are found attached tofilaments (Fig. 9F).

Radiometric ages

Radiometric dates from microbialites in thewestern lagoon section range from 7Æ1 to 9Æ1 calkyr BP and average 8Æ5 cal kyr BP (Table 2, Figs 6and 7). Holocene carbonate sediment ages inbetween microbialites from the same lagoonsection are 8Æ1 to 9Æ2 cal kyr BP, only two samplesfrom core bases are Pleistocene in age (19Æ9 and26Æ1 cal kyr BP). Peats and soils at location 2 inthe western lagoon were dated as ‘modern’. Twoshells from living molluscs collected at the samelocation are 7Æ8 and 7Æ3 cal kyr BP respectively.Microbialite and carbonate sediment samplesfrom the eastern lagoon section are younger thantheir western lagoon counterparts, and ages rangefrom 6Æ3 to 7Æ0 cal kyr BP (Table 2, Fig. 7).

DISCUSSION

Formation of Bacalar microbialites

Mechanisms of formation of microbialites ingeneral include baffling, trapping and binding of

Fig. 4. Outcrop photographs from the southern part of Laguna Bacalar. (A) Location 2 with coalescing microbialites.Diameter of heads is 2 to 3 m. Note micro-atoll-type appearance of some heads. Some heads are covered with livingmat (orange). (B) ‘The Rapids’ with giant coalescing microbialites. Diameter of creek is 5 m. (C) Microbialite top from‘The Rapids’ cut open. Note the laminated texture at the base and thrombolitic texture at the top. Diameter of sampleis 15 cm. (D) Underwater photograph of oncolites at location 2. Diameter of oncolites is up to 20 cm. (E) Surface ofmicrobialite from location 2 with abundant Dreissena sp. epigrowth. Note millimetre scale at top of picture. (F)Pomacea sp. shells on top of microbialite head at location 2. Gastropods are 3 cm in diameter on average. (G) Thickcrust of calcite cement in core at location 9. Diameter of picture is 3 cm. (H) Living microbial mat broken open;location 2. Note orange, green and red layers (from top to bottom). Hand for scale.

A

B

Fig. 5. (A) Map of location 2 including microbialiteheads and oncolites. (B) Close-up of microbialite areawith core locations.



Fig. 6. Schematic cross-section at location 2 including 14C-dated cores. For locations of cores, see Fig. 5.

Fig. 7. Core logs from other core locations including 14C data. For locations, see Fig. 2.



detritus by biofilms and precipitation of calciumcarbonate, either directly or indirectly controlledby microbes (Burne & Moore, 1987; Reid et al.,2000). The associated biofilms usually consist ofcyanobacteria, micro-algae, heterotrophic bacteriaand chemoautotrophic bacteria. Extrapolymericsubstances (EPS) produced by biofilms are crucialfor carbonate accumulation (Krumbein et al.,2003).

A major process forming microbialites inLaguna Bacalar was, and remains, the precipita-tion of calcium carbonate at cyanobacterial fila-ments of Homeothrix and Leptolyngbya.Withdrawal of CO2 during photosynthesis ofthese oxygenic phototrophs and pH elevationpresumably trigger carbonate precipitation. Evi-dence of carbonate precipitation is found underSEM of living microbial mats and calcifiedmicrobialites. Similarly, photosynthesis of dia-toms has presumably contributed to calciumcarbonate precipitation. Also, some precipitatesmay have originated inside microbialites in theprocess of degradation of organic matter (Sprach-ta et al., 2001). A crucial factor of carbonateprecipitation in Laguna Bacalar clearly is, andprobably always has been, the high carbonatecontent in south-western lagoon waters which, inturn, is a consequence of karst aquifer circulationthrough cenotes. Away from cenotes, the carbon-ate content in lagoonal waters is significantlylower and microbialites are absent. Water agita-tion and flushing is of importance as seen in thedense microbialite formation in ‘The Rapids’where high water currents are observed. In addi-tion to precipitation, there is evidence of sedi-ment trapping in both living microbial mats andin calcified microbialites as observed under SEM.

The great abundance of herbivorous Pomaceagastropods in Laguna Bacalar and around micro-bialite occurrences supports the contention thatgrazing does occur and currently is an impor-tant factor for microbialite bioerosion. Thisfactor is clearly outweighed by microbialiteaccretion and cementation, however, in thecarbonate-rich waters of the lagoon. It is notentirely clear whether or not the existenceversus absence of grazers has always been ofgreat importance for microbialite formation inLaguna Bacalar. The high abundance of grazinggastropods of the genus Pomacea in the modernlagoon and the rare occurrences of gastropods inthe core material suggest that grazing has onlyrecently become important among Bacalarmicrobialites. The paucity of fossil gastropodscould also be interpreted as a consequence of

the high solubility of aragonitic gastropodshells. However, no gastropod moulds werefound in the core. Also, two thin gastropodlayers in core 17 (Fig. 7) contain millimetre-sized unidentified gastropods with the aragoniteshells preserved.

The seemingly large time gap between livingmicrobial mats at the surface and underlyingmicrobialites presents an interpretation problem.At location 2, microbialites indeed appear to be inthe process of exhumation; however, this repre-sents a very recent process of shore erosion.Likewise, the micro-atoll shape of some micro-bialites is not interpreted as an erosional feature,but as a depositional form of lateral growthcomparable with forms in Lake Clifton microbia-lites (Burne & Moore, 1993). Exhumation ofmicrobialites is not seen at any of the othermicrobialite locations in the lagoon. The mostmeaningful explanation would be to consider thebulk of Bacalar microbialites as fairly recent orlate Holocene structures. The fact that shells ofliving molluscs in the lagoon have radiometricages of 7Æ5 cal kyr BP on average in combinationwith the narrow range of 14C ages of microbialitesbetween 9 and 8 cal kyr BP supports the conten-tion that there is a strong hard water effect inLaguna Bacalar. The hard water or old carboneffect is a common problem in radiometric datingof lake sediments, especially in regions of calcar-eous and coal-bearing bedrock or spring-fed lakesassociated with large aquifers (MacDonald et al.,1991). In Laguna Bacalar, old or 14C-deficientcarbon presumably originates from the dissolu-tion of Neogene–Pleistocene limestone, whichforms the bedrock of the karst aquifer feeding thelagoon. This assumption is corroborated by theobservation of younger radiometric ages in theeastern lagoon, where the water has lower car-bonate concentrations when compared with thewestern lagoon section. Elevated sodium andchlorine concentrations suggest some marinewater influence in this part of the lagoon. Thereis no general solution for detection and correctionof erroneous dates caused by the hard water effect(MacDonald et al., 1991). Also, organic material,e.g. pieces of wood, for dating is not presentwithin the cores. However, based on the agedifference between average microbialite and mod-ern mollusc shells, the currently exposed micro-bialites in Laguna Bacalar are estimated to haveformed during the past ca 1000 years. Corepenetration in microbialites does not exceed1Æ5 m below the present lagoonal water level,and Pleistocene deposits were not reached below



A B

C D

E F

G H



the microbialites. Even so, Pleistocene depositswere reached in two cores in between microbia-lites at location 2 at only 1Æ5 and 2Æ0 m below the

present water level. This observation indicatesthat microbialite thickness does not exceed ca 2to 3 m.

Fig. 8. Thin section and SEM photographs from microbialite core. (A) Scalenohedral cement in microbialite core. (B)Calcified tube with prismatic crystals and nanograins. (C) Upper part of microbialite in thin section. Note the radialorientation of filaments showing stromatolitic texture with weak lamination. Diameter of sample is 10 mm. (D) Close-up of the same sample in thin section. Note filamentous structures surrounded by carbonate. Diameter of sample is1 mm. (E) Detail showing tubular nature of filaments. (F) Close-up of tube structure with short prismatic crystals oflow magnesium calcite and detrital grains. (G) Lagoonal sediment with diatoms. (H) Lagoonal sediment with pris-matic crystals and some detrital grains. Note similarity of microbialite sediment and lagoon floor sediment.

A B

C D

E F

Fig. 9. Scanning electron microscope photographs of microbialite mats. All samples were critically point dried. (A)Microbial Leptolyngbya mat, a view from above. (B) Microbial mat in cross-section. Note that filament layers arearranged perpendicular to each other forming a three-dimensional network. (C) Close-up of microbial filamentsshows precipitated nanocrystals of calcite. (D) Peloidal detrital grains and platy low magnesium calcite crystals inmicrobial mat. (E) Platy calcite crystal, which precipitated around microbial filament. (F) Spiked ball calcite crystals,precipitated at microbial filament.



Neglecting a hard water effect and using radio-metric ages as measured would support a modelin which microbialites are fossil structures,which formed under special conditions duringthe Early Holocene. Special conditions couldinclude even higher carbonate content of lagoonalwaters, possibly due to a relatively dry EarlyHolocene climate with high evaporation levels,which became more humid during the MiddleHolocene, according to the studies of lake sedi-ments of the region (Hodell et al., 1991; Curtiset al., 1998), thereby hampering carbonate forma-tion. Drier conditions since the Late Holocenewould again allow microbial activity at thesurface of the fossil microbialites causing theobserved hiatus. A major problem with this

model would be the fact that 9 to 8 cal kyr BP,sea-level in the area was 13 to 7 m below thepresent level (Gischler, 2006), i.e. well below theinvestigated microbialites. A lagoonal water levelseveral metres above sea-level can be largelyexcluded in a coastal limestone karst terrain.

The earlier Holocene development of LagunaBacalar is largely unknown at this point. Giventhe up to 15 m deep parts of Laguna Bacalar, thelagoonal basin presumably already existed duringthe Early Holocene. Likewise, a number of otherlakes in Peten and the Yucatan peninsula cameinto existence no later than 9 cal kyr BP due to therise in groundwater levels in response to sea-levelrise (Whitmore et al., 1996; Curtis et al., 1998).Some lagoons in the area, such as Laguna deCocos, located close to the Rio Hondo some40 km SW of Laguna Bacalar (Bradbury et al.,1990), even show evidence of an Early Holocenemarine influence. Further studies, including thedeeper parts of Laguna Bacalar, would be neces-sary to reconstruct the entire Holocene history ofthe lagoon.

Comparison with other freshwateroccurrences

A comparison with other occurrences (Table 3)offers the possibility to draw more far-reachingconclusions regarding microbialite formation.Laguna Bacalar stromatolites are quite similar tothe structures from the brackish Chetumal Baylocated ca 15 km to the south-east (Rasmussenet al., 1993), in that morphologies and sizes arethe same. However, the internal textures andmicrobial consortia are different. Also, theChetumal Bay structures are probably older andformed when the rising Holocene sea inundatedthe area some 2Æ3 cal kyr BP; their existence wasto a large part explained by the exclusion ofabundant grazers in brackish conditions(Rasmussen et al., 1993). Bacalar microbialitesare also comparable with the lacustrinestromatolites of Cuatro Cienegas (Fig. 1) innorth-eastern Mexico (Winsborough et al., 1994;Garcia-Pichel et al., 2004). These occurrencesexhibit similar morphologies, elevated calciumand carbonate contents of the lake waters areobserved and abundant grazers are present. Onthe other hand, their microbial consortia includein part different cyanobacterial taxa. Strikinglysimilar microbialite occurrences are also found inLake Clifton, Western Australia (Moore, 1987;Burne & Moore, 1993; Moore & Burne, 1994).There, marine-derived lake water has elevated

Table 2. Radiometric ages from sediment and coresamples of Laguna Bacalar.

Sample,location Material

Calibrated age,cal yr BP

Western lagoonCore 2-1 base Microbialite 8875 ± 115Core 2-1 top Microbialite 8245 ± 115Core 2-1 top Soil ‘Modern’Core 2-2 base Carbonate sediment 9265 ± 165Core 2-2 top Carbonate sediment 8770 ± 240Core 2-2 top Peat ‘Modern’Core 2-3 base Carbonate sediment 26 190 ± 260�Core 2-3/50 Carbonate sediment 9095 ± 130Core 2-3 top Carbonate sediment 8810 ± 200Core 2-4 base Carbonate sediment 19 950 ± 590Core 2-4 top Carbonate sediment 8840 ± 190Core 2-5/1.5 Carbonate sediment 8800 ± 200Core 2-5 top Peat ‘Modern’Core 2-5 top Soil ‘Modern’Core 2-6 base Microbialite 9135 ± 190Core 2-6 top Microbialite 9190 ± 115Core 6 base Microbialite 8350 ± 150Core 6/75-78 Microbialite 8775 ± 225Core 6/10-13 Microbialite 8050 ± 110Core 10 base Microbialite 8365 ± 95Core 10/10-13 Microbialite 8180 ± 170Core 19 base Microbialite 8785 ± 195Core 19/53-56 Microbialite 8880 ± 250Core 19/21-24 Microbialite 7115 ± 115Core 20 base Carbonate sediment 8155 ± 125

Location 2 Living Dreissena,shell

7845 ± 85*

Location 2 Living Pomacea,shell

7320 ± 80*

Eastern lagoonCore 16 base Carbonate sediment 7005 ± 145Core 17 base Carbonate sediment 6345 ± 85Core 17/15-16 Microbialite 6790 ± 150

*AMS date.�Date outside of calibration range.



Table

3.

Ch

ara

cte

rist

ics

of

mod

ern

mic

robia

lite

occu

rren

ces

men

tion

ed

inth

ete

xt.

Occu

rren

ce

Ind

.si

ze

Wate

rd

ep

thM

icro

bia

lite

shap

es;

textu

reO

rgan

ism

sM

inera

logy,

cem

en

tsT

yp

eof

accre

tion

Gra

zers

pre

sen

tW

ate

rch

em

istr

y

Fre

shw

ate

rL

agu

na

Bacala

r(s

ub-t

rop

ical)

)3

m)

3m

Head

s,le

dges,

on

coli

tes;

thro

mboli

tic,

(lam

inate

d)

Hom

eoth

rix,

Lep

toly

ngbya,

dia

tom

s,m

oll

usc

s

Calc

ite

Pre

cip

itate

s,baffl

ing

Yes

Hig

hcarb

on

ate

an

dcalc

ium

con

cen

trati

on

Cu

atr

oC

ien

egas

(ari

d)

)0Æ2

mN

ears

hore

,sh

all

ow

Head

s,m

ats

,on

coli

tes;

Lam

inate

d

Hom

oeoth

rix,

Sch

izoth

rix,

dia

tom

s

Calc

ite

Pre

cip

itate

sY

es

Very

hig

halk

ali

nit

yan

dcalc

ium

con

cen

trati

on

Lake

Cli

fton

(ari

d)

)1Æ5

mN

ears

hore

,sh

all

ow

Head

s,m

ats

;th

rom

boli

tic,

(str

om

ato

liti

c)

Scyto

nem

a,

dia

tom

sA

ragon

ite

Main

lyp

recip

itate

sY

es

Ric

hin

bic

arb

on

ate

an

dcalc

ium

Pavil

ion

Lake

(tem

pera

te)

)3

m10

to>

30

mD

igit

ate

head

s;d

en

trit

icS

yn

ech

ococcu

s,F

isch

ere

lla,

Osc

illa

tori

a

Calc

ite

Tra

pp

ing

Yes

Sli

gh

tly

alk

ali

ne

Lake

Van

(tem

pera

te)

)40

m10

to>

100

mT

ow

ers

;ir

regu

lar

layeri

ng

Ple

uro

cap

sa,

CF

Bgro

up

,d

iato

ms

Ara

gon

ite,

calc

ite

Pre

cip

itati

on

Yes

Hig

hly

alk

ali

ne

(Sod

aL

ake)

Sato

nd

aC

rate

r(t

rop

ical)

)1

m)

23

mC

em

en

ted

mats

Ple

uro

cap

sagro

up

,re

dalg

ae

gre

en

alg

ae,

serp

uli

ds

Ara

gon

ite,

Mg

calc

ite

Bio

l.-i

nd

uced

calc

ificati

on

Yes

Hig

hly

alk

ali

ne

Bra

ckis

hC

hetu

mal

Bay

(su

b-t

rop

ical)

)1Æ5

m)

1Æ5

mH

ead

s,on

coli

tes;

lam

inar,

colu

mn

ar

Scyto

nem

a,

Ph

orm

idiu

m,

biv

alv

es,

dia

tom

s

Calc

ite

Main

lyp

recip

itate

sN

oS

up

ers

atu

rate

dw

ith

regard

tocalc

ite

Mari

ne

Sh

ark

Bay

(ari

d)

)0Æ5

mIn

tert

idal

tosh

all

ow

sub-t

idal,

)4

m

Head

s;la

min

ate

dS

ch

izoth

rix,

En

top

hysa

lis,

Sole

nti

a

Ara

gon

ite

Tra

pp

ing

an

dbin

din

g,

pre

cip

itati

on

No

Su

pers

atu

rate

dw

ith

regard

toC

aC

O3,

hyp

ers

ali

ne

Exu

mas,

Bah

am

as

(su

b-t

rop

ical)

>2

mS

ub-t

idal

7to

8m

Head

s;la

min

ate

dS

ch

izoth

rix,

Sole

nti

a,

dia

tom

sA

ragon

ite,

Mg

calc

ite

Tra

pp

ing,

pre

cip

itati

on

Rare

Norm

al

mari

ne,

stro

ng

cu

rren

ts,

sed

imen

tm

ovem

en

tK

op

ara

(tro

pic

al,

sub-t

rop

ical)

10s

of

cm

<1

mC

rust

s;la

min

ate

dP

horm

idiu

mM

gcalc

ite,

(ara

gon

ite)

Pre

cip

itate

sR

are

Vari

able

mix

ture

sof

fresh

an

dse

aw

ate

rR

eef

Cavit

ies

(tro

pic

al,

sub-t

rop

ical)

Severa

lcm

Su

b-t

idal

Cru

sts;

lam

inate

d,

clo

tted

Bacte

ria

(?)

Mg

calc

ite

Org

.-in

du

ced

pre

cip

itate

sR

are

Norm

al

mari

ne

Ind

.¼

ind

ivid

ual;

Bio

l.¼

bio

logic

all

y;

Org

.¼

org

an

icall

y



contents of calcium and carbonate, metazoangrazers are common and morphologies includemicro-atoll-type forms like in Laguna Bacalar.However, cyanobacteria of the genus Scytonemaare predominant. Another major difference toBacalar is the aragonite mineralogy of microbia-lites. The freshwater microbialites of the slightlyalkaline Pavilion Lake, British Columbia, Canada(Laval et al., 2000) occur in deeper waters, havedigitate morphologies and again different micro-bial taxa, when compared with shallow waterBacalar occurrences. Likewise, occurrences inLake Van, Turkey (Kempe et al., 1991), differ inbacterial consortia and general size and morphol-ogy, which include mostly huge branched ordigitate towers.

Among the marine examples, the Shark Bay(Reid et al., 2003) and Bahamas (Dill et al., 1986)structures are quite similar regarding sizes andshapes; however, internal textures, microbial taxaand mineralogy differ significantly from theBacalar examples. Microbialites in reef cavities(Camoin et al., 1999) and ponds (Defarge et al.,1994) exhibit even more differences when com-pared with the Bacalar structures. Furthermore,most of the marine locations mentioned arecharacterized by the absence or paucity of meta-zoan grazers.

The comparisons show that size, shape, inter-nal texture, taxonomy, mineralogy, type of accre-tion, abundance of grazers and general setting ofmodern microbialite occurrences are highly var-iable. A general pattern appears to be the fact thatmetazoan grazers are rare or absent in most of themarine examples when compared with the fresh-water microbialite locations, which usually haveabundant grazing organisms. Likewise, fresh-water lakes and lagoons are characterized byextraordinarily high concentrations of carbonateand/or calcium ions, which apparently helpmicrobialites to outcompensate the grazing effectsby strong carbonate accretion.

CONCLUSIONS

• One of the largest modern/Holocene fresh-water occurrences of microbialites occurs inthe south-western part of Laguna Bacalar,Mexico, with a total length of >10 km.

• Microbialites are largely thrombolitic andconsist of low magnesium calcite. Carbonateaccretion is largely induced by CO2 with-drawal by the cyanobacteria Homeothrixand Leptolyngbya, and associated diatoms.

Elevated carbonate content in lagoon watersis a precondition of carbonate precipitationand results from dissolution of Cenozoiclimestone in a karst aquifer. Trapping andbinding of carbonate sediment in the pro-cess of microbialite formation is observed tobe less important than precipitation.

• The bulk of microbialites presumably formedin the Late Holocene, and not in the earlyHolocene as indicated by 14C ages. Oldradiometric ages are probably a result of thestrong hard water effect in Laguna Bacalar.

• Like in other modern freshwater microbialiteoccurrences, grazers are abundant in LagunaBacalar. Even so, elevated carbonate concen-trations in the lagoon waters are sufficient forabundant microbialite formation. There ap-pears to be a difference in the abundance ofgrazers between freshwater and marine/brackish microbialite occurrences in general,as seen in a comparison of several prominentlocations worldwide.

ACKNOWLEDGEMENTS

The authors thank the Deutsche Forschungsgeme-inschaft (project Gi 222/15) and the FacultyResearch and Development grant programmes atthe University of Tennessee at Martin for theirfinancial support. The authors are indebted toFranscisco Vega (Ciudad Universitaria, Mexico)for bringing these microbialites to the attention ofMichael Gibson and Wintfred Smith. BethRhenberg (Martin, TN) assisted during the firstsample collection. Jose Leal (Sanibel Island, FL)kindly identified gastropods. Rainer Petschick ranthe XRD analyses and Doris Bergmann-Dorr(Frankfurt) conducted the AAS analyses. GabrielaMeyer (Frankfurt) assisted during field work in2006. The authors thank S. Golubic (Boston) andan anonymous reviewer for their constructivecomments, which improved the paper. S. Golubicalso helped by identifying the cyanobacterium inthe living biomats.

REFERENCES

Allwood, A.C., Walter, M.R., Kamber, B.S., Marshall, C.P. and

Burch, I.W. (2006) Stromatolite reef from the Early Ar-

chaean era of Australia. Nature, 441, 714–718.

Anagnostidis, K. and Komarek, J. (1988) Modern approach to

the classification system of cyanophytes 5. Oscillatoriales.

Arch. Hydrobiol. Algol. Stud., 50–53 (Suppl. 80), 327–472.



Arp, G., Reimer, A. and Reitner, J. (2003) Microbialite for-

mation in seawater of increased alkalinity, Satonda Crater

Lake, Indonesia. J. Sed. Res., 73, 105–127.

Bradbury, J.P., Forester, R.M., Bryant, A.W. and Covich, A.P.(1990) Paleolimnology of Laguna de Cocos, Albion Island,

Rio Hondo, Belize. In: Ancient Maya Wetland Agriculture

(Ed. M.D. Pohl), pp. 119–154. Westview Press, Boulder, CO.

Burne, R.V. and Moore, L.S. (1987) Microbialites: organose-

dimentary deposits of benthic microbial communities. Pal-

aios, 2, 241–254.

Burne, R.B. and Moore, L.S. (1993) Microatoll microbialites of

Lake Clifton, Western Australia: morphological analogues of

Cryptozoon proliferum HALL, the first formally-named

stromatolite. Facies, 29, 149–168.

Camoin, G.F., Gautret, P., Montaggioni, L.F. and Cabioch, G.(1999) Nature and environmental significance of microbia-

lites in Quaternary reefs: the Tahiti paradox. Sed. Geol., 126,271–304.

Curtis, J.H., Brenner, M., Hodell, D.A., Balser, R.A., Islebe,G.A. and Hooghiemstra, H. (1998) A multi-proxy study of

Holocene environmental change in the Maya lowlands of

Peten, Guatemala. J. Paleolimnol., 19, 139–159.

Defarge, C., Trichet, J., Maurin, A. and Hucher, M. (1994)

Kopara in Polynesian atolls: early stages of formation of

calcareous stromatolites. Sed. Geol., 89, 9–23.

Dill, R.F., Shinn, E.A., Jones, A.T., Kelly, K. and Steinen, R.(1986) Giant subtidal stromatolites forming in normal mar-

ine seawaters. Nature, 324, 55–58.

Garcia-Pichel, F., Al-Horani, F.A., Farmer, J.D., Ludwig, R.and Wade, B.D. (2004) Balance between microbial calcifi-

cation and metazoan bioerosion in modern stromatolitic

oncolites. Geobiology, 2, 49–57.

Garrett, P. (1970) Phanerozoic stromatolites: noncompetitive

ecologic restriction by grazing and burrowing animals. Sci-

ence, 169, 171–173.

Gischler, E. (2006) Comment on ‘‘Corrected western Atlantic

sea-level curve for the last 11,000 years based on calibrated14C dates from Acropora palmata framework and intertidal

mangrove peat’’ by Toscano and Macintyre. Coral Reefs 22:

257–270 (2003), and their response in Coral Reefs

24:187–190 (2005). Coral Reefs, 25, 273–279.

Golubic, S. (1991) Modern stromatolites: a review. In: Cal-

careous Algae and Stromatolites (Ed. R. Riding), pp.

541–561. Springer, Berlin.

Hodell, D.A., Curtis, J.H., Jones, G.A., Higuera-Gundy, A.,Brenner, M., Binford, M.W. and Dorsey, K.T. (1991)

Reconstruction of Caribbean climate change over the past

10,500 years. Nature, 352, 790–793.

Kempe, S., Kazmierczak, J., Landmann, G., Konuk, T.,Reimer, A. and Lipp, A. (1991) Largest known microbia-

lites discovered in Lake Van, Turkey. Nature, 349,605–608.

Krumbein, W.E., Paterson, D.M. and Zavarzin, G.A. (2003)

Fossil and Recent Biofilms, a Natural History of Life on

Earth. Kluwer, Boston, MA, 482 pp.

Laval, B., Cady, S.L., Pollack, J.C., McKay, C., Bird, J.S.,Grotzinger, J.P., Ford, D.C. and Bohm, H.R. (2000) Modern

freshwater microbialite analogues for ancient dendritic reef

structures. Nature, 407, 626–629.

MacDonald, G.M., Beukens, R.P. and Kieser, W.E. (1991)

Radiocarbon dating of limnic sediments: a comparative

analysis and discussion. Ecology, 72, 1150–1157.

Milliman, J.R. (1974) Marine Carbonates. Springer, New York,

375 pp.

Moore, L. (1987) Water chemistry of the coastal saline lakes of

the Clifton-Preston lakeland system, south-western Austra-

lia, and its influence on stromatolite formation. Aust.J. Freshw. Res., 38, 647–660.

Moore, L.S. and Burne, R.V. (1994) The modern thrombolites

of Lake Clifton, Western Australia. In: Phanerozoic Stro-matolites II (Eds J. Bertrand-Sarfati and C. Monty), pp. 3–29.

Kluwer, Dordrecht.

Pratt, B.R. (1982) Stromatolite decline – a reconsideration.

Geology, 10, 512–515.

Purdy, E.G., Gischler, E. and Lomando, A.J. (2003) The Belize

margin revisited. 2. Origin of Holocene antecedent topog-

raphy. Int. J. Earth Sci., 92, 552–572.

Rasmussen, K.A., Macintyre, I.G. and Prufert, L. (1993)

Modern stromatolite reefs fringing a brackish coastline,

Chetumal Bay, Belize. Geology, 21, 199–202.

Reid, R.P., Visscher, P.T., Decho, A.W., Stolz, J.F., Bebout,B.M., Dupraz, C., Macintyre, I.G., Paerl, H.W., Pinckey,J.L., Prufert-Bebout, L., Steppe, T.F. and DesMarais, D.J.(2000) The role of microbes in accretion, lamination and

early lithification of modern marine stromatolites. Nature,

406, 989–992.

Reid, R.P., James, N.P., Macintyre, I.G., Dupraz, C.P. and

Burne, R.V. (2003) Shark Bay stromatolites: microfabrics

and reinterpretation of origins. Facies, 49, 299–324.

Reitner, J. (1993) Modern cryptic microbialite/metazoan facies

from Lizard Island (Great Barrier Reef, Australia): formation

and concepts. Facies, 29, 2–40.

Riding, R. (2000) Microbial carbonates: the geological record

of calcified bacterial–algal mats and biofilms. Sedimentol-

ogy, 47 (Suppl. 1), 179–214.

Riding, R. and Liang, R. (2005) Geobiology of microbial car-

bonates: metazoan and seawater saturation state influences

on secular trends during the Phanerozoic. Palaeogeogr.

Palaeoclimatol. Palaeoecol., 219, 101–115.

Sprachta, S., Camoin, G.F., Golubic, S. and Le Campion, T.(2001) Microbialites in a modern lagoonal environment:

nature and distribution, Tikehau Atoll (French Polynesia).

Palaeogeogr. Palaeoclimatol. Palaeoecol., 175, 103–124.

Whitmore, T.J., Brenner, M., Curtis, J.H., Dahlin, B.H. and

Leyden, B.W. (1996) Holocene climatic and human influ-

ences on lakes of the Yucatan peninsula, Mexico: an inter-

disciplinary, palaeolimnological approach. The Holocene,

6, 273–287.

Winsborough, B.M., Seeler, J.S., Golubic, S., Folk, R.L. and

Maguire, B. Jr. (1994) Recent fresh-water lacustrine stro-

matolites, stromatolitic mats and oncoids from northeastern

Mexico. In: Phanerozoic Stromatolites II (Eds. J. Bertrand-

Sarfati and C. Monty), pp. 71–100. Kluwer, Dordrecht.

Manuscript received 12 March 2007; revision accepted19 November 2007



gischler et al 2007

Technology