Gischler et al 2007

Giant Holocene Freshwater Microbialites, Laguna Bacalar,
Quintana Roo, Mexico
EBERHARD GISCHLER*, MICHAEL A. GIBSON and WOLFGANG OSCHMANN*
*Institut fu¨ r Geowissenschaften, J.W. Goethe-Universita¨t, 60438 Frankfurt am Main, Germany
(E-mail: gischler@em.uni-frankfurt.de)
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 14
C 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 14
C 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 life
on earth and known from deposits as old as Early
Archaean (Riding, 2000; Allwood et al., 2006).
The term microbialite is used here in the sense of
Burne & Moore (1987), describing an organosedi-
mentary deposit formed from the interaction
between benthic microbial communities and
detrital or chemical sediments. Stromatolites are
layered microbialites, whereas thrombolites have
rather 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
Ó 2008 The Authors. Journal compilation Ó 2008 International Association of Sedimentologists 1293

was interpreted as a consequence of an increased
grazing pressure from evolving metazoa during
Earth history (Garrett, 1970). Subsequently, this
view was modified due to the discovery of more
and more Precambrian and Phanerozoic micro-
bialite occurrences. These occurrences turned out
to be quite diverse regarding shape, texture and
organic content (Pratt, 1982; Riding, 2000). Not
only did microbes evolve and algae come into
play, environmental conditions such as the car-
bonate content of ocean water also changed
throughout the Phanerozoic. Carbonate saturation
is of great importance for microbialite formation
because non-enzymatic precipitation of calcium
carbonate in biofilms is only partially organically
controlled (Riding & Liang, 2005).
Modern microbialites, which can be used as
analogues for their fossil counterparts, occur in a
wide variety of environments. There are hyper-
saline examples, such as the classic Shark Bay
location in Western Australia (Reid et al., 2003),
sub-tidal stromatolites in the Bahamas, that are
forming in areas of extensive sediment redeposi-
tion (Dill et al., 1986; Reid et al., 2000), ‘kopara’ in
shallow Pacific atoll lagoons (De´farge et al., 1994),
microbialites in protected reef cavities (Reitner,
1993; Camoin et al., 1999), in brackish environ-
ments (Rasmussen et al., 1993) and in alkaline
lakes (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) and
Canada (Laval et al., 2000). In some of these
locations, metazoan grazers are rare; however,
there are also examples in which grazers are
present. Freshwater lagoon and lake waters
containing microbialites are usually characterized
by elevated carbonate content.
To understand the formation of fossil microbia-
lites, it is crucial to study modern examples.
However, not all fossil examples have modern
counterparts, and, likewise, not all modern occur-
rences have equivalents in the fossil record
(Golubic, 1991). Therefore, it is important to
increase the knowledge of modern and fossil
microbialite occurrences. In this paper, the newly
discovered 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 km
wide, NNE-trending freshwater lagoon located in
south-eastern Quintana Roo, Mexico (Figs 1 and
2); it is surrounded by flat-lying Cenozoic lime-
stone. The outline of Laguna Bacalar, adjacent
lagoons and former river beds suggests the influ-
ence of the tectonic grain in the area. The Rio
Hondo Fault Zone presumably acted as a pathway
for meteoric fluids during Pleistocene lowstands
of sea-level (Purdy et al., 2003). Laguna Bacalar
gets as deep as 15 m. Along large parts of the
lagoon, shallow, intermittently flooded areas with
plant growth separate Laguna Bacalar into a
western section and an eastern section. Four
cenotes (water-filled sinkholes) near the village
of Bacalar and the cenote near Xul-Ha have
depths of up to 90 m, according to local fisher-
men and boatmen. In the southern part of Laguna
Bacalar, there is a strong current to the north
originating at Xul-Ha cenote, through Canal de los
Piratas where it curves towards the south and into
the eastern part of the lagoon. The eastern lagoon
is connected to Chetumal Bay via the Rio Hondo
and the Chetumal aqueduct to the south. In the
north, the lagoon connects to Chetumal Bay
through a system of other lagoons including
Laguna Chile Verde and Laguna Guerrero. No
major currents were detected in these lagoon
parts. Water temperatures ranged from 25 to 28 °C
during field work in March 2005 and August
2006. Rainfall amounts to 1250 to
1500 mm year)1
(Purdy et al., 2003). There were
no fluctuations in the water level of the lagoon
observed during field work spanning two years,
nor does it vary according to local boatmen and
inhabitants of the village of Bacalar. The analyses
of water samples from the lagoon (Table 1, Fig. 3)
show that conditions for calcium carbonate pre-
cipitation are extraordinary. Ca2+
concentrations
are close to typical values of marine waters and
HCO3
)
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 freshwater
stromatolite 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 and
south-western part of Chetumal Bay near the Mexican–Belizean boundary. Freshwater stromatolites occur along the
lagoon shore for 10 km, from the village of Bacalar to the entrance of Xul-Ha cenote (crosses). Published brackish
stromatolite location in Chetumal Bay (Rasmussen et al., 1993) south of city of Chetumal near mouth of Rio Hondo is
also marked by a cross. Numbers 18, 21 and 22 are sample stations (see Fig. 2).
1294 E. Gischler et al.
Ó 2008 The Authors. Journal compilation Ó 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 sample
stations 18, 21 and 22 (see Fig. 1).

presumably a consequence of dissolution of
limestone bedrock and water circulation in a
connected karst system.
METHODS
A total of 13 push cores (aluminium core pipe of
7Æ5 cm diameter with core catcher), up to 1Æ8 m
long (total length 15 m), were taken in the
southern part of Laguna Bacalar in March 2005
and August 2006 (Fig. 2). In addition, surface
sediment and water samples were collected at 20
sample stations (Figs 1 and 2). Reconnaissance
trips were also made along the coast of Chetumal
Bay from the city of Chetumal to the opening of
Laguna Guerrero in the north. Two additional
water samples were taken in the bay. The
Chetumal Bay stromatolite occurrence south of
the mouth of Rio Hondo (Rasmussen et al., 1993)
was also visited and cored for comparison. In the
laboratory, cores were opened and lithologies
described. Samples were selected for thin sec-
tion preparation, scanning electron microscope
(SEM) studies and X-ray diffraction (XRD) anal-
yses to identify carbonate mineralogy. Surface
sediment samples from the lagoon ﬂoor were
washed, dried and investigated for composition
and mineralogy. Living microbial mats from the
surface of microbialites at location 2 were
collected and stored in 70% alcohol. These
samples were critically point dried and investi-
gated under SEM. A total of 28 bulk samples
were age dated using the standard 14
C radiocar-
bon method by Beta Analytic Inc., Miami, FL,
USA. Two mollusc shells were dated by the
accelerator mass spectrometry (AMS) method at
the same laboratory. Results are reported as
2-sigma calibrated ages (95% probability). Water
samples were analysed by atomic absorption
spectrometry (AAS) for cations Na+
, K+
, Mg2+
,
Ca2+
in the Laboratory of Geochemistry of the
Environment at J.W. Goethe-Universita¨t, 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)
SO4
2)
BAC 1 55Æ75 4Æ75 78Æ88 325Æ00 183Æ00 41Æ70 1072Æ00
BAC 2 56Æ25 4Æ25 72Æ13 322Æ50 165Æ00 44Æ50 1100Æ00
BAC 3 59Æ88 4Æ50 75Æ75 326Æ25 140Æ00 43Æ50 1060Æ00
BAC 4 55Æ13 4Æ25 72Æ88 322Æ50 177Æ00 70Æ40 1160Æ00
BAC 5 51Æ00 4Æ00 69Æ50 325Æ00 201Æ00 66Æ60 1137Æ00
BAC 6 49Æ13 3Æ63 71Æ75 322Æ50 183Æ00 65Æ80 1113Æ00
BAC 7 49Æ25 3Æ50 76Æ13 325Æ00 214Æ00 70Æ60 1139Æ00
BAC 8 48Æ63 3Æ75 72Æ38 313Æ75 207Æ00 48Æ40 1019Æ00
BAC 9 49Æ88 4Æ13 70Æ38 308Æ75 232Æ00 34Æ50 1031Æ00
BAC 10 48Æ75 3Æ50 73Æ75 313Æ75 238Æ00 50Æ90 1038Æ00
BAC 11 67Æ88 4Æ88 82Æ38 325Æ00 165Æ00 78Æ00 1211Æ00
BAC 12 72Æ00 5Æ00 81Æ50 320Æ00 146Æ00 79Æ00 1106Æ00
BAC 13 70Æ13 5Æ13 84Æ00 316Æ25 189Æ00 81Æ70 1147Æ00
BAC 14 75Æ63 5Æ88 84Æ88 313Æ75 116Æ00 102Æ00 1154Æ00
BAC 15 80Æ00 6Æ00 86Æ75 315Æ00 110Æ00 110Æ00 1194Æ00
BAC 16 77Æ50 5Æ50 85Æ38 312Æ50 104Æ00 102Æ00 1165Æ00
BAC 17 67Æ75 5Æ13 83Æ88 312Æ50 104Æ00 185Æ00 1154Æ00
BAC 18 108Æ38 7Æ63 88Æ88 310Æ00 104Æ00 43Æ60 1030Æ00
BAC 19 43Æ13 3Æ88 71Æ25 333Æ75 226Æ00 39Æ10 1023Æ00
BAC 20 43Æ88 3Æ88 72Æ50 328Æ75 220Æ00 46Æ50 1185Æ00
Mean 61Æ49 4Æ66 77Æ74 319Æ63 171Æ20 70Æ19 1111Æ90
CHE 21 4174Æ00 138Æ50 498Æ50 295Æ00 171Æ00 5730Æ00 1346Æ00
CHE 22 2535Æ00 85Æ50 344Æ50 351Æ25 153Æ00 3060Æ00 1544Æ00
Mean 3354Æ50 112Æ00 421Æ50 323Æ13 162Æ00 4395Æ00 1445Æ00
Ocean
water
10 760 385 1295 415 140 19 350 2700
Values for adjacent Chetumal Bay (CHE) and mean ocean water are given for
comparison. For locations of samples, see Fig. 2. Average ocean water values
are 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 error
ranges given in Table 1 are calculated based on
results from multiple analyses conducted by the
two 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 Laguna
Bacalar. These microbialites occur along a 10 km
stretch from the northern end of Xul-Ha cenote to
the southern end of the town of Bacalar, encom-
passing samples from all the stations from 1 in the
north to 9 in the south (Fig. 2). It is this lagoon
section that is characterized by strong northward
currents and elevated concentrations of calcium
and bicarbonate ions. Thin microbialite crusts are
also found in the south-eastern branch of Laguna
Bacalar around sample location 17 (Fig. 2). There
are no microbialite occurrences in the lagoon
north of Bacalar. Also, no microbialites were
found during reconnaissance trips to the western
coast 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 and
diameter (Fig. 4A). In some cases, microbialites
coalesce to form larger features. The largest and
most continuous occurrence is located in ‘The
Rapids’ (Fig. 4B), an area where the lagoon
narrows to ca 5 m wide and where currents
increase significantly compared with the rest of
the lagoon. Here, microbialites are up to 3 m in
diameter and height; they mostly coalesce to form
a continuous mass along the entire channel. Close
to the water surface, these microbialites also grow
into overhangs and ledges. At location 2, micro-
bialites exhibit circular layering on the exposed
flat upper surface (Fig. 4A); some look like micro-
atolls, in that growth is concentrated at the rim
enclosing a shallow pool. At the same location,
microbialites also appear to be in the process of
being recently exhumed from peat and soil (Figs 5
to 7).
Microbialites are usually better consolidated at
the surface when compared with sections ca
30 cm below the surface. When broken open,
microbialites are mostly unlayered with a throm-
bolitic texture. Layering (stromatolitic texture) is
seen in some cases and is caused by vertical
variation in the degree of cementation (Fig. 4C).
The occasional occurrence of layered sections
indicates that stromatolite mat growth may be a
recurrent phenomenon in the accretion of these
largely thrombolitic microbialites.
Microbialites also occur in the form of
centimetre-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 or
branches that fell into the water. The latter
occurrences suggest that microbialites in Laguna
Bacalar are in the process of formation or at least
that they formed in the recent past.
The accessory fauna of microbialites, including
oncolites, consists of mytiloid bivalves of the
genus Dreissena, and herbivorous gastropods of
the thin-shelled genus Pomacea, which occur
in very large numbers of individuals (Fig. 4E and
F).
Composition and microstructure of calcified
microbialites
Microbialites consist entirely of low magnesium
calcite. Calcite crystals include nano-sized grains,
short prismatic crystals (<10 lm length, 2 to 3 lm
diameter) and scalenohedral grains (around
10 lm diameter) and cement (Fig. 8A and B).
Detrital grains are mostly mollusc shell frag-
ments, based on the dense texture and style of
fragmentation of the grains. Throughout the core
material, calcified tubes of low magnesium calcite
are found (Fig. 8C to F); they are often upward
radiating in orientation and show some growth
rhythm seen in thin sections as a weakly
expressed lamination (Fig. 8C). Tubes are up to
several millimetres long and have diameters of up
to 50 lm. Tube walls are up to 10 lm thick and
are composed of scalenohedral crystals; they
represent the calcified moulds of Homeothrix
filaments. Organic remains of filaments can be
seen in thin sections as opaque dark lines
surrounded by calcareous deposits (Fig. 8D). In
several cores, scalenohedral cement is found to
have grown into millimetre-thick crusts (see
Fig. 4G). Cement crystals are up to 50 lm long
(Fig. 8F). Diatom tests are also very common
within microbialites (Fig. 8G). Lagoonal sediment
surrounding 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. is
lacking below the living biomat surface.

A B
C D
E F
G H

Structure of living microbial mat
At location 2, conspicuous living microbial mats
up to 1 cm thick are found on top of some
microbialite heads. These mats are tough, leath-
ery and almost cartilaginous. The living biomats
are layered and exhibit colour changes from
orange to green to reddish brown, from top to
bottom (see Fig. 4H). The mats are produced by a
fine filamentous cyanobacterium, which pro-
duces tough extracellular polysaccharide sheaths.
The organism is present mainly in the top layers
of the mat, whereas the interior of the mat is
comprised of interwoven sheaths. On the surface,
the filaments are prostrate (Fig. 9A) and in the
interior 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 Leptolyngbya
Anagnostidis & Komarek 1988; it presumably
belongs to a new species, which is being inves-
tigated in a separate study. These biomats are
actively growing as seen in a 10 · 10 cm2
piece
cut out with a knife in March 2005, which had
completely grown back by August 2006.
Nano-sized crystals of low magnesium calcite
are found on bacterial filaments (Fig. 9C). Within
the meshwork of bacterial filaments, short pris-
matic low magnesium calcite crystals of less than
10 lm diameter with hexagonal cross-section,
and detritus with grain-sizes of 10 to 20 lm are
found (Fig. 9D). Occasionally, platy crystals
occur with diameters of 10 to 20 lm and 1 to
2 lm thickness (Fig. 9E). Also, calcite crystals
with a spiked ball morphology that are only a few
microns in diameter are found attached to
filaments (Fig. 9F).
Radiometric ages
Radiometric dates from microbialites in the
western lagoon section range from 7Æ1 to 9Æ1 cal
kyr BP and average 8Æ5 cal kyr BP (Table 2, Figs 6
and 7). Holocene carbonate sediment ages in
between microbialites from the same lagoon
section are 8Æ1 to 9Æ2 cal kyr BP, only two samples
from core bases are Pleistocene in age (19Æ9 and
26Æ1 cal kyr BP). Peats and soils at location 2 in
the western lagoon were dated as ‘modern’. Two
shells from living molluscs collected at the same
location are 7Æ8 and 7Æ3 cal kyr BP respectively.
Microbialite and carbonate sediment samples
from the eastern lagoon section are younger than
their western lagoon counterparts, and ages range
from 6Æ3 to 7Æ0 cal kyr BP (Table 2, Fig. 7).
DISCUSSION
Formation of Bacalar microbialites
Mechanisms of formation of microbialites in
general 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 living
mat (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 sample
is 15 cm. (D) Underwater photograph of oncolites at location 2. Diameter of oncolites is up to 20 cm. (E) Surface of
microbialite 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) Thick
crust 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 microbialite
heads and oncolites. (B) Close-up of microbialite area
with core locations.

Fig. 6. Schematic cross-section at location 2 including 14
C-dated cores. For locations of cores, see Fig. 5.
Fig. 7. Core logs from other core locations including 14
C data. For locations, see Fig. 2.

detritus by biofilms and precipitation of calcium
carbonate, either directly or indirectly controlled
by microbes (Burne & Moore, 1987; Reid et al.,
2000). The associated biofilms usually consist of
cyanobacteria, micro-algae, heterotrophic bacteria
and chemoautotrophic bacteria. Extrapolymeric
substances (EPS) produced by biofilms are crucial
for carbonate accumulation (Krumbein et al.,
2003).
A major process forming microbialites in
Laguna Bacalar was, and remains, the precipita-
tion of calcium carbonate at cyanobacterial fila-
ments of Homeothrix and Leptolyngbya.
Withdrawal of CO2 during photosynthesis of
these oxygenic phototrophs and pH elevation
presumably trigger carbonate precipitation. Evi-
dence of carbonate precipitation is found under
SEM of living microbial mats and calcified
microbialites. Similarly, photosynthesis of dia-
toms has presumably contributed to calcium
carbonate precipitation. Also, some precipitates
may have originated inside microbialites in the
process of degradation of organic matter (Sprach-
ta et al., 2001). A crucial factor of carbonate
precipitation in Laguna Bacalar clearly is, and
probably always has been, the high carbonate
content in south-western lagoon waters which, in
turn, is a consequence of karst aquifer circulation
through cenotes. Away from cenotes, the carbon-
ate content in lagoonal waters is significantly
lower and microbialites are absent. Water agita-
tion and flushing is of importance as seen in the
dense 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 and
in calcified microbialites as observed under SEM.
The great abundance of herbivorous Pomacea
gastropods in Laguna Bacalar and around micro-
bialite occurrences supports the contention that
grazing does occur and currently is an impor-
tant factor for microbialite bioerosion. This
factor is clearly outweighed by microbialite
accretion and cementation, however, in the
carbonate-rich waters of the lagoon. It is not
entirely clear whether or not the existence
versus absence of grazers has always been of
great importance for microbialite formation in
Laguna Bacalar. The high abundance of grazing
gastropods of the genus Pomacea in the modern
lagoon and the rare occurrences of gastropods in
the core material suggest that grazing has only
recently become important among Bacalar
microbialites. The paucity of fossil gastropods
could also be interpreted as a consequence of
the high solubility of aragonitic gastropod
shells. However, no gastropod moulds were
found in the core. Also, two thin gastropod
layers in core 17 (Fig. 7) contain millimetre-
sized unidentified gastropods with the aragonite
shells preserved.
The seemingly large time gap between living
microbial mats at the surface and underlying
microbialites presents an interpretation problem.
At location 2, microbialites indeed appear to be in
the 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 growth
comparable with forms in Lake Clifton microbia-
lites (Burne & Moore, 1993). Exhumation of
microbialites is not seen at any of the other
microbialite locations in the lagoon. The most
meaningful explanation would be to consider the
bulk of Bacalar microbialites as fairly recent or
late Holocene structures. The fact that shells of
living molluscs in the lagoon have radiometric
ages of 7Æ5 cal kyr BP on average in combination
with the narrow range of 14
C ages of microbialites
between 9 and 8 cal kyr BP supports the conten-
tion that there is a strong hard water effect in
Laguna Bacalar. The hard water or old carbon
effect is a common problem in radiometric dating
of lake sediments, especially in regions of calcar-
eous and coal-bearing bedrock or spring-fed lakes
associated with large aquifers (MacDonald et al.,
1991). In Laguna Bacalar, old or 14
C-deficient
carbon presumably originates from the dissolu-
tion of Neogene–Pleistocene limestone, which
forms the bedrock of the karst aquifer feeding the
lagoon. This assumption is corroborated by the
observation of younger radiometric ages in the
eastern lagoon, where the water has lower car-
bonate concentrations when compared with the
western lagoon section. Elevated sodium and
chlorine concentrations suggest some marine
water influence in this part of the lagoon. There
is no general solution for detection and correction
of erroneous dates caused by the hard water effect
(MacDonald et al., 1991). Also, organic material,
e.g. pieces of wood, for dating is not present
within the cores. However, based on the age
difference between average microbialite and mod-
ern mollusc shells, the currently exposed micro-
bialites in Laguna Bacalar are estimated to have
formed during the past ca 1000 years. Core
penetration in microbialites does not exceed
1Æ5 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 deposits
were reached in two cores in between microbia-
lites at location 2 at only 1Æ5 and 2Æ0 m below the
present water level. This observation indicates
that microbialite thickness does not exceed ca 2
to 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 radial
orientation 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 is
1 mm. (E) Detail showing tubular nature of filaments. (F) Close-up of tube structure with short prismatic crystals of
low 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 are
arranged perpendicular to each other forming a three-dimensional network. (C) Close-up of microbial filaments
shows precipitated nanocrystals of calcite. (D) Peloidal detrital grains and platy low magnesium calcite crystals in
microbial 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 model
in which microbialites are fossil structures,
which formed under special conditions during
the Early Holocene. Special conditions could
include even higher carbonate content of lagoonal
waters, possibly due to a relatively dry Early
Holocene climate with high evaporation levels,
which became more humid during the Middle
Holocene, according to the studies of lake sedi-
ments of the region (Hodell et al., 1991; Curtis
et al., 1998), thereby hampering carbonate forma-
tion. Drier conditions since the Late Holocene
would again allow microbial activity at the
surface of the fossil microbialites causing the
observed 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 the
present level (Gischler, 2006), i.e. well below the
investigated microbialites. A lagoonal water level
several metres above sea-level can be largely
excluded in a coastal limestone karst terrain.
The earlier Holocene development of Laguna
Bacalar is largely unknown at this point. Given
the up to 15 m deep parts of Laguna Bacalar, the
lagoonal basin presumably already existed during
the Early Holocene. Likewise, a number of other
lakes in Peten and the Yucatan peninsula came
into existence no later than 9 cal kyr BP due to the
rise in groundwater levels in response to sea-level
rise (Whitmore et al., 1996; Curtis et al., 1998).
Some lagoons in the area, such as Laguna de
Cocos, located close to the Rio Hondo some
40 km SW of Laguna Bacalar (Bradbury et al.,
1990), even show evidence of an Early Holocene
marine inﬂuence. Further studies, including the
deeper parts of Laguna Bacalar, would be neces-
sary to reconstruct the entire Holocene history of
the lagoon.
Comparison with other freshwater
occurrences
A comparison with other occurrences (Table 3)
offers the possibility to draw more far-reaching
conclusions regarding microbialite formation.
Laguna Bacalar stromatolites are quite similar to
the structures from the brackish Chetumal Bay
located ca 15 km to the south-east (Rasmussen
et al., 1993), in that morphologies and sizes are
the same. However, the internal textures and
microbial consortia are different. Also, the
Chetumal Bay structures are probably older and
formed when the rising Holocene sea inundated
the area some 2Æ3 cal kyr BP; their existence was
to a large part explained by the exclusion of
abundant grazers in brackish conditions
(Rasmussen et al., 1993). Bacalar microbialites
are also comparable with the lacustrine
stromatolites of Cuatro Cienegas (Fig. 1) in
north-eastern Mexico (Winsborough et al., 1994;
Garcia-Pichel et al., 2004). These occurrences
exhibit similar morphologies, elevated calcium
and carbonate contents of the lake waters are
observed and abundant grazers are present. On
the other hand, their microbial consortia include
in part different cyanobacterial taxa. Strikingly
similar microbialite occurrences are also found in
Lake 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 core
samples of Laguna Bacalar.
Sample,
location Material
Calibrated age,
cal yr BP
Western lagoon
Core 2-1 base Microbialite 8875 ± 115
Core 2-1 top Microbialite 8245 ± 115
Core 2-1 top Soil ‘Modern’
Core 2-2 base Carbonate sediment 9265 ± 165
Core 2-2 top Carbonate sediment 8770 ± 240
Core 2-2 top Peat ‘Modern’
Core 2-3 base Carbonate sediment 26 190 ± 260
Core 2-3/50 Carbonate sediment 9095 ± 130
Core 2-4 base Carbonate sediment 19 950 ± 590
Core 2-5/1.5 Carbonate sediment 8800 ± 200
Core 2-5 top Peat ‘Modern’
Core 2-5 top Soil ‘Modern’
Core 2-6 base Microbialite 9135 ± 190
Core 2-6 top Microbialite 9190 ± 115
Core 6 base Microbialite 8350 ± 150
Core 6/75-78 Microbialite 8775 ± 225
Core 20 base Carbonate sediment 8155 ± 125
Location 2 Living Dreissena,
shell
7845 ± 85*
Location 2 Living Pomacea,
shell
7320 ± 80*
Eastern lagoon
*AMS date.
Date outside of calibration range.

Table3.Characteristicsofmodernmicrobialiteoccurrencesmentionedinthetext.
Occurrence
Ind.
size
Water
depth
Microbialite
shapes;textureOrganisms
Mineralogy,
cements
Typeof
accretion
Grazers
presentWaterchemistry
Freshwater
LagunaBacalar
(sub-tropical)
)3m)3mHeads,ledges,
oncolites;
thrombolitic,
(laminated)
Homeothrix,
Leptolyngbya,
diatoms,
molluscs
CalcitePrecipitates,
bafﬂing
YesHighcarbonate
andcalcium
concentration
Cuatro
Cienegas(arid)
)0Æ2mNearshore,
shallow
Heads,mats,
oncolites;
Laminated
Homoeothrix,
Schizothrix,
diatoms
CalcitePrecipitatesYesVeryhigh
alkalinityand
calcium
concentration
LakeClifton
(arid)
)1Æ5mNearshore,
shallow
Heads,mats;
thrombolitic,
(stromatolitic)
Scytonema,
diatoms
AragoniteMainly
precipitates
YesRichin
bicarbonateand
calcium
PavilionLake
(temperate)
)3m10to
>30m
Digitateheads;
dentritic
Synechococcus,
Fischerella,
Oscillatoria
CalciteTrappingYesSlightlyalkaline
LakeVan
(temperate)
)40m10to
>100m
Towers;irregular
layering
Pleurocapsa,
CFBgroup,
diatoms
Aragonite,
calcite
PrecipitationYesHighlyalkaline
(SodaLake)
SatondaCrater
(tropical)
)1m)23mCementedmatsPleurocapsa
group,redalgae
greenalgae,
serpulids
Aragonite,
Mgcalcite
Biol.-induced
calciﬁcation
YesHighlyalkaline
Brackish
ChetumalBay
(sub-tropical)
)1Æ5m)1Æ5mHeads,oncolites;
laminar,
columnar
Scytonema,
Phormidium,
bivalves,diatoms
CalciteMainly
precipitates
NoSupersaturated
withregardto
calcite
Marine
SharkBay
(arid)
)0Æ5mIntertidal
toshallow
sub-tidal,
)4m
Heads;laminatedSchizothrix,
Entophysalis,
Solentia
AragoniteTrappingand
binding,
precipitation
NoSupersaturated
withregardto
CaCO3,
hypersaline
Exumas,
Bahamas
(sub-tropical)
>2mSub-tidal
7to8m
Heads;laminatedSchizothrix,
Solentia,diatoms
Aragonite,
Mgcalcite
Trapping,
precipitation
RareNormalmarine,
strongcurrents,
sediment
movement
Kopara
(tropical,
sub-tropical)
10s
ofcm
<1mCrusts;laminatedPhormidiumMgcalcite,
(aragonite)
PrecipitatesRareVariable
mixturesoffresh
andseawater
ReefCavities
(tropical,
sub-tropical)
Several
cm
Sub-tidalCrusts;laminated,
clotted
Bacteria(?)MgcalciteOrg.-induced
precipitates
RareNormalmarine
Ind.¼individual;Biol.¼biologically;Org.¼organically

contents of calcium and carbonate, metazoan
grazers are common and morphologies include
micro-atoll-type forms like in Laguna Bacalar.
However, cyanobacteria of the genus Scytonema
are predominant. Another major difference to
Bacalar is the aragonite mineralogy of microbia-
lites. The freshwater microbialites of the slightly
alkaline Pavilion Lake, British Columbia, Canada
(Laval et al., 2000) occur in deeper waters, have
digitate morphologies and again different micro-
bial taxa, when compared with shallow water
Bacalar occurrences. Likewise, occurrences in
Lake Van, Turkey (Kempe et al., 1991), differ in
bacterial consortia and general size and morphol-
ogy, which include mostly huge branched or
digitate towers.
Among the marine examples, the Shark Bay
(Reid et al., 2003) and Bahamas (Dill et al., 1986)
structures are quite similar regarding sizes and
shapes; however, internal textures, microbial taxa
and mineralogy differ significantly from the
Bacalar examples. Microbialites in reef cavities
(Camoin et al., 1999) and ponds (De´farge et al.,
1994) exhibit even more differences when com-
pared with the Bacalar structures. Furthermore,
most of the marine locations mentioned are
characterized 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 of
modern microbialite occurrences are highly var-
iable. A general pattern appears to be the fact that
metazoan grazers are rare or absent in most of the
marine examples when compared with the fresh-
water microbialite locations, which usually have
abundant grazing organisms. Likewise, fresh-
water lakes and lagoons are characterized by
extraordinarily high concentrations of carbonate
and/or calcium ions, which apparently help
microbialites to outcompensate the grazing effects
by strong carbonate accretion.
CONCLUSIONS
• One of the largest modern/Holocene fresh-
water occurrences of microbialites occurs in
the south-western part of Laguna Bacalar,
Mexico, with a total length of >10 km.
• Microbialites are largely thrombolitic and
consist of low magnesium calcite. Carbonate
accretion is largely induced by CO2 with-
drawal by the cyanobacteria Homeothrix
and Leptolyngbya, and associated diatoms.
Elevated carbonate content in lagoon waters
is a precondition of carbonate precipitation
and results from dissolution of Cenozoic
limestone in a karst aquifer. Trapping and
binding of carbonate sediment in the pro-
cess of microbialite formation is observed to
be less important than precipitation.
• The bulk of microbialites presumably formed
in the Late Holocene, and not in the early
Holocene as indicated by 14
C ages. Old
radiometric ages are probably a result of the
strong hard water effect in Laguna Bacalar.
• Like in other modern freshwater microbialite
occurrences, grazers are abundant in Laguna
Bacalar. Even so, elevated carbonate concen-
trations in the lagoon waters are sufficient for
abundant microbialite formation. There ap-
pears to be a difference in the abundance of
grazers between freshwater and marine/
brackish microbialite occurrences in general,
as seen in a comparison of several prominent
locations worldwide.
ACKNOWLEDGEMENTS
The authors thank the Deutsche Forschungsgeme-
inschaft (project Gi 222/15) and the Faculty
Research and Development grant programmes at
the University of Tennessee at Martin for their
financial support. The authors are indebted to
Franscisco Vega (Ciudad Universitaria, Mexico)
for bringing these microbialites to the attention of
Michael Gibson and Wintfred Smith. Beth
Rhenberg (Martin, TN) assisted during the first
sample collection. Jose Leal (Sanibel Island, FL)
kindly identified gastropods. Rainer Petschick ran
the XRD analyses and Doris Bergmann-Do¨rr
(Frankfurt) conducted the AAS analyses. Gabriela
Meyer (Frankfurt) assisted during field work in
2006. The authors thank S. Golubic (Boston) and
an anonymous reviewer for their constructive
comments, which improved the paper. S. Golubic
also helped by identifying the cyanobacterium in
the living biomats.
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19 November 2007

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