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Sedimentary Geology 1
Research paper
Silica-carbonate stromatolites related to coastal hydrothermal
venting in Bahıa Concepcion, Baja California Sur, Mexico
Carles Caneta,*, Rosa Marıa Prol-Ledesmaa, Ignacio Torres-Alvaradob, H. Albert Gilgc,
Ruth Esther Villanuevaa, Rufino Lozano-Santa Cruzd
aInstituto de Geofısica, Universidad Nacional Autonoma de Mexico, Cd. Universitaria, Coyoacan, 04510 Mexico, D.F., MexicobCentro de Investigacion en Energıa, Universidad Nacional Autonoma de Mexico, Temixco, Morelos, Mexico
cLehrstuhl fur Ingenieurgeologie, Technische Universitat Munchen, GermanydInstituto de Geologıa, Universidad Nacional Autonoma de Mexico, Cd. Universitaria, Coyoacan, 04510 Mexico, D.F., Mexico
Received 13 May 2004; received in revised form 25 November 2004; accepted 3 December 2004
Abstract
Submarine diffused seepage (from 5 to 15 m depth) and intertidal focused gasohydrothermal venting take place on the
West shore of the Bahıa Concepcion Bay, on Baja California, Mexico. The intertidal venting consists of a cluster of hot
springs that occur a few meters offshore, with vent temperatures up to 62 8C and a pH of 6.68. Two irregularly shaped
patches of silica-carbonate hot spring deposits occur around the main intertidal vent areas. In addition, a fossil bed of silica-
carbonate hot spring deposits of about 75 m long crops out along a cliff next to the active vent area. Both fossil and modern
silica-carbonate deposits are finely laminated, and form columnar, bulbous and smooth undulating microstromatolites up to
10 cm thick. Noncrystalline opal-A is the only silica phase present in the modern and fossil hot spring deposits and occurs as
microspheres up to 300 nm in diameter forming porous aggregates and irregular clusters, chains and spongy filament
networks. The silica supersaturation state of the thermal fluid necessary for opal precipitation is achieved by cooling when it
reaches the surface. The presence of preserved microbial remains (diatoms and possibly filamentous microbes) in both
modern and fossil deposits reflects the biological activity around the hot springs. The biological activity constrains the fabrics
and the textures of the deposit, and could mediate silica deposition. Calcite is the most abundant crystalline phase in the hot
spring deposits and forms discontinuous horizons of subhedral bladed crystals within the silica aggregates. Calcite crystals
are unusually enriched in 13C, with d13CV-PDB values between +3.0x and +9.3x. The large 13C enrichment is attributable to
a geothermal CO2 degassing process, which yields calcite supersaturation. The d18OV-PDB values in calcite, between �10.0xand �6.6x, indicate precipitation from a hot spring fluid that is a mixture of seawater and meteorically derived water. With
the methods applied in this study, no indication of biogenic influence on calcite precipitation has been found. Minor amounts
0037-0738/$ - s
doi:10.1016/j.se
* Correspon
E-mail addr
74 (2005) 97–113
ee front matter D 2004 Elsevier B.V. All rights reserved.
dgeo.2004.12.001
ding author. Tel.: +52 55 56 22 41 33; fax: +52 55 55 50 24 86.
ess: [email protected] (C. Canet).
C. Canet et al. / Sedimentary Geology 174 (2005) 97–11398
of barite occur in the fossil and modern hot spring deposits and precipitates when Ba2+-rich thermal water mixes with
seawater.
D 2004 Elsevier B.V. All rights reserved.
Keywords: Shallow hydrothermal vents; Microbialites; Sinter; Travertine; Opal-A; Carbon isotopes; Oxygen isotopes
1. Introduction
Silica and minor calcite finely laminated stroma-
tolitic deposits are a widespread surface feature of
many geothermal systems throughout the world.
Silica deposits (sinters) usually form in relation to
discharge of near-neutral alkali chloride waters, and
are composed mainly by opaline silica (e.g., Four-
nier and Rowe, 1966; Rodgers et al., 2004). For
example, silica stromatolitic sinters have been
described in the Taupo Volcanic Zone, New
Zealand (e.g., Renaut et al., 1996; Mountain et
al., 2003; Rodgers et al., 2004), in Iceland
(Konhauser et al., 2001), in the Yellowstone
National Park, Wyoming (e.g., Walter et al., 1972;
Walter, 1976; Guidry and Chafetz, 2003a), and in
the East African Rift (Renaut et al., 2002).
However, opaline stromatolitic sinters can also form
around acidic hot springs and, in this case, they
contain significant amounts of kaolinite, sulfur and
jarosite (Jones et al., 2000a).
Likewise, finely laminated calcium carbonate
deposits (tufa, travertine) are widely known in modern
subaerial and lacustrine thermal springs, for example,
in East African Rift basins (Renaut et al., 2002), in
Pyramid Lake, Nevada (Arp et al., 1999), and in the
Yellowstone National Park, Wyoming (Pentecost,
1990; Fouke et al., 2000). Additionally, several fossil
calcite travertine hot spring deposits have been
reported, for example, in Central Italy (Folk et al.,
1985) and in Mud Hills, California (Pedone and
Dickson, 2002). In hot spring environments, calcite
deposits form from neutral to alkali thermal waters
that are supersaturated with respect to calcite and
cannot precipitate in acidic conditions (Jones et al.,
2000b).
On the other hand, mixed silica-carbonate hot
spring deposits are much more rare (Campbell et al.,
2002). Among the best-studied silica-carbonate hot
spring deposits, those from Waikite (Jones and
Renaut, 1996; Jones et al., 2000b) and Ngatamariki
(Campbell et al., 2002), New Zealand, and a relict
deposit in Yellowstone, Wyoming (Guidry and Cha-
fetz, 2003b), can be mentioned.
Although many geyser and subaerial thermal
spring sinters and travertine deposits are well
documented, those formed in coastal and submarine
hydrothermal environments have received little
attention. Near Punta Mita, on the western coast
of Mexico, finely laminated calcite deposits occur in
10 m depth submarine gasohydrothermal vents
(Canet et al., 2003). Moreover, silica-rich precip-
itates have been reported in close association to
massive sulfides in a modern oceanic hydrothermal
vent system, in the Central Indian Ocean (Halbach
et al., 2002). Likewise, only few sublacustrine hot
spring deposits (sinter and travertine) have been
studied (e.g., Barrat et al., 2000; Renaut et al.,
2002).
Siliceous sinters are formed from hot spring
solutions below life’s upper temperature limit (e.g.,
Jones and Renaut, 1996; Konhauser et al., 2001),
and its close association with microbes has been
well established in many cases (Konhauser et al.,
2001 and references therein; Guidry and Chafetz,
2003c). For a long time, the role of microbes had
been considered passive in silica sinter precipita-
tion, assuming that it is an inorganic-physicochem-
ical process (Walter et al., 1972; Walter, 1976).
Nevertheless, recent research points out that micro-
organisms have a more active role in silica
precipitation, fixing silica and favoring its nuclea-
tion (e.g., Jones and Renaut, 1996; Konhauser et
al., 2001). In many cases, silica precipitation can be
attributed to a combination of both biotic and
abiotic mechanisms (Guidry and Chafetz, 2003a). In
the same way, calcite crystallization in thermal
spring deposits may be mediated by microbial
C. Canet et al. / Sedimentary Geology 174 (2005) 97–113 99
activity (e.g., Chafetz and Folk, 1984; Arp et al.,
1999; Chafetz and Guidry, 1999; Canet et al.,
2003). In any case, organo-mineralization products
contribute as a main component in many hot spring
deposits, and microorganisms usually are mineral-
ized and preserved as fossils (Walter and Des
Marais, 1993). Furthermore, there is no doubt that
the presence of microbes has a great influence on
the textures and development of hot spring deposits
(Mountain et al., 2003).
Thermophilic life forms, comparable to those
supported by the hot spring environments, have been
Fig. 1. Location of the studied silica-carbonate hot spring deposits of Bahıa
deposits.
proposed as the common ancestor of life on Earth
(e.g. Stetter et al., 1990). Therefore, the study of
organo-mineralization processes and their products in
hot spring deposits can yield information on the early
Earth lifestyles (Walter and Des Marais, 1993;
Konhauser et al., 2001, 2003; Guidry and Chafetz,
2003a).
In this paper, we provide morphological, minera-
logical, geochemical and carbon and oxygen stable
isotope data on unusual silica-carbonate stromatolitic
deposits related to coastal hydrothermal springs from
the Gulf side of the Baja California peninsula,
Concepcion. (a) Fossil hot spring deposits and (b) modern hot spring
C. Canet et al. / Sedimentary Geology 174 (2005) 97–113100
Mexico, and examine the role of microorganisms in
their precipitation.
2. Geological setting
The studied silica-carbonate stromatolitic deposits
and coastal hydrothermal springs are located at the
West shore of the Bahıa Concepcion Bay, on the
Gulf coast of the Baja California Peninsula (Fig. 1).
Bahıa Concepcion is one of the largest fault-bound
bays in Baja California, and its half-graben config-
uration developed during Late Miocene extension, in
relation with the Gulf of California opening
(Ledesma-Vazquez and Johnson, 2001). The evolu-
tion of Bahıa Concepcion during the Tertiary was
constrained by a subduction setting. Most of the
Bahıa Concepcion area is made up of the volcanic
series of the Comondu Group, which are related to
the Tertiary forearc magmatism (Ledesma-Vazquez
and Johnson, 2001). The Comondu volcanic rocks
unconformably cover Cretaceous granitoids. Accord-
ing to Umhoefer et al. (2001), the age of the
Comondu Group is restricted between upper Oligo-
cene and middle Miocene. Three different units form
the Comondu Group: (a) a lower unit containing
coarse detrital continental series with minor interlay-
ered volcanics, (b) a middle unit mainly composed
of andesite lava flows and (c) an upper unit
composed of andesite lava flows interlayered with
massive volcanic breccias (Umhoefer et al., 2001).
In the East shore of Bahıa Concepcion Bay, late
Miocene to early Pliocene marls of the Tirabuzon
Formation crop out (Moran, 1984). Fossiliferous
shallow water limestones, conglomerates and cherts
of the Infierno Formation, of late Pliocene age, crop
out in the southeastern corner of the bay (Ledesma-
Vazquez et al., 1997). The cherts of the Tirabuzon
Formation probably formed in a shallow environment,
due to the interaction between hydrothermal fluids
and mangrove carbonate muds (Ledesma-Vazquez et
al., 1997; Ledesma-Vazquez and Johnson, 2001).
Early Pleistocene basaltic flows and pyroclastic
deposits are the most recent magmatic events in the
area (Moran, 1984).
Plio–Quaternary tectonic activity caused uplift of
the bay area and generated extensive rocky shorelines
(Johnson and Ledesma-Vazquez, 2001).
3. Methodology
Samples of stromatolites were collected from
intertidal gasohydrothermal springs and in an adjacent
fossil deposit along a cliff. Thin polished sections
were prepared for examination with optical and
electronic microscope.
Unpolished samples were examined on a Jeol
JSM-35C Scanning Electron Microscope (SEM), on
secondary electron mode and operating at 20 kV, at
the Instituto de Geologıa of the Universidad Nacio-
nal Autonoma de Mexico (UNAM). These samples
received a thin gold coating before SEM examina-
tion. In addition, back-scattered electron images and
EDS qualitative analyses were obtained from thin
polished sections using a Jeol JXA-8900R electron
microprobe equipment at the Instituto de Geofısica
(UNAM).
XRD analyses were performed at the Instituto de
Geologıa of the UNAM, on a Philips PW130/96,
stepping 0.028 2h from 4.008 to 70.008, and using
copper X-radiation generated at 30 kV and 20 mA.
Whole-rock chemical analyses were performed in
selected stromatolite specimens. Major and minor
element analyses were obtained by X-ray fluorescence
at the Instituto de Geologıa (UNAM).
Carbon and oxygen isotope analyses of calcite
were carried out at the isotope laboratory of Bayeri-
sche Staatssammlung fqr Pal7ontologie und Geologie,
Mqnchen, Germany. All analyses were performed
using an automated Thermo/Finnigan bGasbench IIQonline preparation device coupled to a Thermo/
Finnigan Deltaplus isotope ratio mass spectrometer
using a continuous flow mode. CO2 was produced by
reaction of CaCO3 with phosphoric acid at 72 8C. Allmeasurements are reported as d-values in per mil (x)
relative to V-PDB. The precision and accuracy of
isotope values is estimated at F0.2x.
Fluid mixing modeling and determination of the
saturation state of the hydrothermal solutions with
respect to mineral species were performed using the
program bThe Geochemist’s WorkbenchQ (Bethke,
1996). Activity coefficients were calculated using
the Debye-Hqckel model, considering that the ionic
strength of the thermal fluids range between 0.5 and
0.7 approximately. Saturation indexes were calculated
for modeled cooling reaction paths beginning at the
thermal fluid temperature and ending at 25 8C. No
C. Canet et al. / Sedimentary Geology 174 (2005) 97–113 101
mineral precipitation was allowed during these calcu-
lations (closed system).
4. Modern coastal hydrothermal activity at Bahıa
Concepcion
Submarine diffused seepage and intertidal focused
hydrothermal venting take place on the West shore of
the Bahıa Concepcion Bay, along a 500 stretch of a
rocky shoreline (Fig. 1). Locally, the shoreline cliffs
are exposed and controlled by the El Requeson fault
zone, which delineates the western margin of Bahıa
Concepcion, and acts as a conduit for hydrothermal
fluids (Forrest et al., 2003).
The submarine hydrothermally active area is easily
recognized by gas bubbling, and occurs at depths
between 5 and 15 m. It consists in diffuse fluid seepage
(gas and water) through the unconsolidated sediments
(sand and volcanic boulders and cobbles deposited
from the rocky cliffs) that cover the seafloor. The
Fig. 2. Photographs of the main vent sites. (A) Intertidal hot springs a
approximately 0.5 m. (B) Shallow submarine (about 0.5 m depth) silica-car
cm. (C) Detail of stromatolitic growths around intertidal hot spring ou
hydrothermal fluid seepage and gas bubbling at 5 m depth. Scale bar is a
temperature measurements of the submarine diffuse
vents yield values of 50 8C at the sea bottom, and up to
87 8C within the sediments, where pH attains 6.2. No
submarine mounds or chimneys are developed. How-
ever, in the areas with most intense submarine venting,
iron oxyhydroxides covers the volcanic boulders
(Canet et al., submitted for publication) and orange-
yellow biofilms veneer the sediments.
On the other hand, focused intertidal hot springs are
a shallower expression of the gasohydrothermal
activity in the bay. A cluster of hot springs occurs
about 500 m SE from the main submarine diffuse ven-
ting area (Fig. 1). These springs are located only a few
meters offshore. Two main vent areas are recognizable
and are separated about 20 m from each other. Both
areas are located along a fissure that can be followed for
a few tens of meters on the seafloor. Temperature
reaches 62 8C in the intertidal hot springs and pH is
6.68. A decimeter-thick chalcedony-calcite-barite
banded vein protrudes above sea level between the
two vent areas, roughly parallel to the fissure that hosts
nd associated silica-carbonate stromatolitic deposits. Scale bar is
bonate deposits covering the seafloor. Scale bar is approximately 15
tflow channels. Scale bar is approximately 5 cm. (D) Submarine
pproximately 25 cm.
C. Canet et al. / Sedimentary Geology 174 (2005) 97–113102
the hydrothermal vents. Two irregularly shaped patches
of hot spring deposits occur around the main intertidal
vent areas, each extending about 10 m2 (Fig. 2). These
deposits are partially subaerial during low tide.
4.1. Hydrothermal fluid chemistry
The hydrothermal activity in Bahıa Concepcion
consists of water and gas venting and seepage by
several intertidal hot springs and through the sedi-
ment-covered seafloor (Fig. 2). The composition of
the hydrothermal solution from submarine and inter-
tidal sites is shown in Table 1. The vent water is of
sodium-chloride type and has high concentrations of
Ca, Mn, Si, Ba, B, As, Hg, I, Fe, Li, HCO3 and Sr
(Prol-Ledesma1 et al., 2004). The liquid composition
of the vents and hot springs results of a mixing
between local seawater and a deep circulating
meteoric water (Prol-Ledesma1 et al., 2004). The
different degree of mixing with seawater causes the
compositional differences between submarine and
intertidal hot springs. Na/Li, Na–K–Ca and Si geo-
thermometers yield a reservoir temperature of approx-
imately 200 8C (Prol-Ledesma1 et al., 2004).
Mineral saturation state calculations allowed the
relation between the hydrothermal fluid composition
and the minerals that form the hot spring deposits of
Bahıa Concepcion to be evaluated. Fig. 3 presents the
results of these calculations for the hot spring water
and for a continuous mixing of hydrothermal and
marine water. The fluid from the intertidal hot springs
is supersaturated with respect to barite and silica
(quartz, chalcedony and amorphous silica) for the
complete temperature range (68 to 25 8C). Calcite
supersaturation takes place above about 53 8C (Fig.
Table 1
Water analyses (major ions) in Bahıa Concepcion
Sample Origin pH T (8C) Ca Mg K Na
BC1 Submarine venting 5.95 54–87 23.3 35.8 12.7 39
BC4 Submarine venting 6.02 54–87 19.4 41.9 12.5 41
BC6 Submarine venting 5.97 54–87 20.6 40.2 12.5 40
BC10 Intertidal hot spring 6.68 62 28.9 25.0 12.2 33
BC9 Seawater 7.75 ~25 9.8 58.3 12.48 48
Concentrations in mmolal. SI, calculated saturation index (log Q/K): Cal,
Calculated fugacity of CO2 ( fCO2) in atm (after Prol-Ledesma1 et al., 20
The detection limits (mmol/kg) are: Ca—0.02, Mg—0.002, K—0.01, Na—
3A). The mixture of marine and hydrothermal waters
is clearly subsaturated with respect to calcite and
supersaturated with respect to barite (Fig. 3B).
Vent gas is mainly composed of CO2 (44%) and N2
(54%), with minor amounts of CH4, Ar, He, H2 and
O2 (Forrest and Melwani, 2003). The mean d13C
value for CO2 is �6x (Forrest and Melwani, 2003).
5. Hot spring deposit occurrences and morphology
Both fossil and modern, finely laminated silica-
carbonate hot spring deposits of pale-yellow color are
present around the hydrothermally active areas on the
shoreline of Bahıa Concepcion (Fig. 1).
A 75 m long fossil bed of silica-carbonate
stromatolites crops out along a cliff next to the active
vent area. The morphology of the fossil hot spring
deposits is variable; they form small columnar,
bulbous and smooth undulating microstromatolites,
up to 10 cm thick (Fig. 4). The stromatolites occur
irregularly upon the volcanic bedrock and frequently
incorporate clastic fragments.
Silica and carbonate aggregates are presently
depositing in the surroundings of the intertidal hot
springs, about 150 m southeast from the above
mentioned fossil hot spring deposits. These modern
hot spring deposits are much less extensive than the
fossil beds that crop out along the cliffs and remain
submerged during the high tides. They usually form
crusts and coalescing rims over volcanic pebbles and
boulders (Fig. 2). Around the main hot spring outflow
conduits, the stromatolitic aggregates coat a structure-
less aggregate formed of allochthonous material
(detrital grains of plagioclase and volcanic rock
Cl SO4 HCO3 Si SI fCO2
Cal Brt Op
4.5 458.4 17.0 4.9 3.1 �0.61 0.43 �0.04 0.153
4.7 500.7 21.2 4.3 2.1 �0.76 0.52 �0.15 0.109
8.9 493.6 20.6 4.5 2.4 �0.76 0.53 �0.11 0.124
4.0 409.0 12.4 1.9 4.5 0.10 0.27 0.11 0.020
5.9 527.5 26.6 1.6 0.0 0.03 �0.42 �1.94 0.001
calcite; Brt, barite; Op, amorphous silica.
04).
0.04, Cl—0.03, SO4—0.03, HCO3—0.04 and Si—0.07.
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
0.5
25
2.0
1.5
1.0
0.5
0.0
-0.530 35 40 45 50 55 60 65 70
0.0
-0.5
-1.0
-1.5
-2.0
Barite
Calcite
62 58.3 54.6 50.9 47.2 43.5 39.8 36.1 32.4 28.7 25
Mixing fraction
Satu
ratio
n in
dex
(log
Q/K
)Sa
tura
tion
inde
x (l
og Q
/K)
Hydrothermal Seawater
B
A Intertidal hot springs
Quartz
Chalcedony
Barite
Amorphous silica
Calcite
Temperature (ºC)
Temperature (ºC)
Amorphous si ical
Fig. 3. Mineral saturation indexes (SI) for the fluids of the intertidal hot spring of Bahıa Concepcion. (A) Variation of SI with respect to
temperature for the hot spring thermal water. (B) Variation of SI for a continuous mixing of hydrothermal and marine waters. (Calculations were
performed with the program bThe Geochemist’s WorkbenchQ, Bethke, 1996).
C. Canet et al. / Sedimentary Geology 174 (2005) 97–113 103
fragments) cemented by silica with minor amounts of
calcite, barite and Mn oxides.
The modern stromatolitic aggregates attain 1 cm
thickness and show a fine internal lamination (Fig. 4).
They present bulbous to smooth undulating external
morphologies and columnar forms are absent.
Due to the low energy conditions prevalent in the
Bahıa Concepcion Bay, the presently forming stro-
matolites are not affected by high-energy waves.
6. Bulk mineralogy and chemical composition of
hot spring deposits
X-ray diffraction patterns of silica-carbonate stro-
matolites show a broad hump centered at about 3.9 2and extending from 168 to 328 2h, which corresponds
to noncrystalline opal-A (Herdianita et al., 2000;
Rodgers et al., 2004). Opal-A is the principal phase by
volume in both modern and fossil hot spring
Fig. 4. Hand samples of silica-carbonate stromatolites. (A) Columnar microstromatolites from fossil hot spring deposits. Scale bar=2.5 cm. (B)
Vertical section of columnar microstromatolites from fossil hot spring deposits showing fine laminations. (C) Bulbous microstromatolites from
moderns hot springs. Scale bar=2.5 cm. (D) Vertical section of bulbous microstromatolites from the modern hot springs. The silica-carbonate
microstromatolite is finely laminated. The base of the microstromatolite consists of volcanic pebble with sand-sized grains cemented by silica.
C. Canet et al. / Sedimentary Geology 174 (2005) 97–113104
stromatolites of Bahıa Concepcion. The width at half
maximum intensity of the ~4 2 diffraction line (D82h) is about 7.28 and 7.48 (0.125 and 0.131 nm) in
fossil and modern stromatolites, respectively. This
parameter indicates the degree of structural order/
disorder in opaline silica and, in both cases, it is in
Table 2
Chemical analyses of silica-carbonate microstromatolites collected in Bah
Sample Location SiO2 TiO2 Al2O3 Fe2O3
BCP-3 Fossil hot spring deposit 6.65 0.01 0.55 0.18
BCP-4 Fossil hot spring deposit 68.71 0.03 1.35 0.45
BCP-6 Fossil hot spring deposit 45.63 0.07 1.73 0.61
BCT-4 Modern hot spring deposit 75.83 0.02 0.47 1.12
(Analyses in wt.%)
Sample Rb Sr Ba Y Zr Nb
BCP-3 4 1481 91 b0.7 b0.7 b0.7
BCP-4 22 403 264 1 5 b0.7
BCP-6 17 917 166 b0.7 5 b0.7
BCT-4 10 335 921 b0.7 2 b0.7
(Analyses in ppm, b below limit of detection)
LOI, loss on ignition.
agreement with non-altered opal-A (Herdianita et al.,
2000).
Calcite lines are also shown by bulk sample
diffraction analyses, being the most abundant crystal-
line phase in modern and fossil hot spring stromato-
lites. In addition, feldspar diffraction lines can be
ıa Concepcion hot springs
MnO MgO CaO Na2O K2O P2O5 LOI Total
0.02 0.48 50.84 0.35 0.08 0.11 40.16 99.44
0.24 0.81 10.08 1.63 0.48 0.09 15.27 99.15
0.03 0.47 25.92 0.83 0.39 0.03 24.18 99.88
0.23 0.42 8.97 0.80 0.22 0.01 11.53 99.64
V Cr Co Ni Cu Zn Th Pb
2 b2 2 b0.7 12 101 b3 b5
8 b2 5 b0.7 11 17 b3 5
18 b2 5 1 11 31 b3 b5
10 b2 3 b0.7 81 39 b3 22
C. Canet et al. / Sedimentary Geology 174 (2005) 97–113 105
identified, corresponding to detrital grains of plagio-
clase. Barite, although present in the silica-calcite
stromatolites, was not identified by X-ray diffraction.
Bulk rock chemical analyses of samples of fossil
and modern hot spring stromatolites are presented in
Table 2. All the samples show high concentration of
CaO (between 8.9 and 50.8 wt.%), accordingly with the
high content in calcite of these hot spring deposits.
Fig. 5. Thin section micrographs and scanning electron microscope images
plagioclase; Si, opaline silica. (A) Micrograph of fine-layered silica-carbon
light. Field width 2.6 mm. (B) Micrograph of fine laminated silica-carbonat
nichols. Field width 2.6 mm. (C) Porous, finely laminated silica and calcit
SEM-BSE image. (D) Thin calcite layers within opaline silica. Fossil hot
upon an aggregate of detrital grains cemented by opaline silica and barite.
crystals coated by opal-A. Fossil hot spring deposit. SEM-BSE image. (G
Modern hot spring deposit. SEM-BSE image. (H) Spore-like body (right ar
calcite. Modern hot spring deposit. SEM-BSE image.
These CaO contents in the hot spring deposits of Bahıa
Concepcion are higher than in the calcite bearing silica
sinters of New Zealand (Campbell et al., 2002).
Most of the analyzed samples show significant
enrichment in Ba (up to 921 ppm) and the highest Ba
contents correspond to the samples with greater SiO2
concentrations. These high values agree with the
occurrence of barite crystals within the stromatolites.
of Bahıa Concepcion hot spring deposits. Ba, barite; Cc, calcite; Pl,
ate stromatolites from intertidal modern hot springs. Plane polarized
e aggregates replaced by coarse calcite. Plane polarized light, crossed
e aggregate with barite and detrital grains. Fossil hot spring deposit.
spring deposit. SEM-BSE image. (E) Silica-carbonate stromatolite
Modern hot spring deposit. SEM-BSE image. (F) Subhedral calcite
) Opaline silica forming highly porous spongy filament networks.
row) and possible coccoid microbe remains (left arrow) embedded in
C. Canet et al. / Sedimentary Geology 174 (2005) 97–113106
Likewise, Mn is enriched (up to 0.23 wt.%) in the
samples with higher silica contents.
7. Microscopic description
The petrography and electron microscope obser-
vations of fossil and modern hot spring stromato-
Fig. 6. Scanning electron microscope images on secondary electron mode o
Calcite subhedral crystal aggregate. (B) Radial aggregates of calcite upon o
microspheres. (D) Pennate diatoms. (E) Opal-A microspheres coalesced i
microcrystalline aggregates probably of Fe oxyhydroxides.
lites from Bahıa Concepcion have not shown
significant differences on the minerals and textures
between them (Figs. 5 and 6). In both cases, the
stromatolites consist of stacked undulating fine
layers of opaline silica (between 5 and 10 Amthick), with high porosity and interlayered discon-
tinuous calcite horizons (Fig. 5). Barite is present in
minor amounts, and detrital bind grains and micro-
f the Bahıa Concepcion hot spring silica-carbonate stromatolites. (A)
pal-A microspheres. (C) Silica discrete crusts cemented by coalesced
n irregular clusters and chains. (F) Opal-A microspheres coated by
Table 3
Carbon and oxygen stable isotope compositions of calcite
Sample Site description d13C(V-PDB) d18O(V-PDB)
BCP-3a Fossil hot spring deposits 7.2 �3.7
BCP-3b 6.4 �2.9
BCP-7-a 4.6 �3.3
BCP-7-b 9.3 �2.6
BCT-4-a Modern hot spring deposits 5.1 �10.0
BCT-4-b 7.7 �5.1
BCT-4-c 5.3 �7.1
BCT-5-a 6.8 �7.6
BCT-5-c 3.0 �9.1
BCT-5-b 7.3 �6.4
BR-2 Aragonite marine cement 1.0 �4.2
C. Canet et al. / Sedimentary Geology 174 (2005) 97–113 107
fossils are common within the silica and calcite
laminae.
Opal-A silica occurs mainly as distinct micro-
spheres, up to 300 nm in diameter, which coalesce and
form porous aggregates of irregular clusters and
chains (Fig. 6B, C, E and F). In some cases, opal-A
microspheres are coated by microcrystalline aggre-
gates, probably of Fe oxyhydroxides (Fig. 6F).
Locally, the silica chains form highly porous spongy
filament networks (Fig. 5G). Silica also forms discrete
regular crusts, of about 500 nm in thickness, parallel
to the stromatolite lamination. These crusts are
cemented among them by coalesced microspheres
(Fig. 6C).
Calcite is the most common precipitate after
opal-A, and it is deposited in discontinuous layers
within opal laminae, mostly forming subhedral
bladed to equant crystals up to few tens of microns
in size (Figs. 5B, C and E and 6A). Calcite also
forms thin layers (of few microns in thickness)
within silica aggregates (Fig. 5D). Calcite crystals
are generally coated by silica aggregates (Fig. 5E).
Locally, calcite develops coarse granular sparitic
textures (grain size up to 500 Am), filling voids of
the stromatolitic aggregates as a late cementation
(Fig. 5B).
Minor amounts of barite occur in the fossil and
modern stromatolites and, in addition, in the struc-
tureless opal-cemented aggregates that underlies the
stromatolites around the modern hydrothermal dis-
charge channels (Fig. 5C and E). In the silica-
carbonate stromatolites, barite forms short tabular
aggregates (up to 500 Am in length) and scattered
radial and bow tie clusters of tabular euhedral
crystals.
Abundant detrital grains occur within the silica and
carbonate layers. These trapped grains are mainly
sand-sized, angular to sub-angular, plagioclase and
volcanic rock fragments. In addition, some chalced-
ony fragments occur within the fossil silica-carbonate
stromatolites.
The hot spring stromatolites of Bahıa Concep-
cion show preserved and fossilized microorganisms
and microbial features. Pennate diatoms, up to 30
Am long and 8 Am wide, are scattered throughout
the hot spring stromatolites, and are especially
abundant in porous silica aggregates (Fig. 6D). In
addition, some spore-like bodies with possible
internal septum have been found within the stroma-
tolites, and small ellipsoidal structures embedded in
calcite and silica, which can be attributed to coccoid
microbes (Fig. 5H).
8. C and O stable isotopes
Eleven carbonate-rich bulk samples were selected
for stable carbon and oxygen isotopic analyses. Four
samples correspond to the fossil hot spring stroma-
tolites and six samples to the modern hot spring
stromatolites. In addition, one sample of marine
aragonite cement was analyzed as a reference. This
sample corresponds to a heterometric, polimictic
breccia cemented by aragonite, which was collected
in the intertidal zone next to the submarine diffuse
vent area.
Stable carbon and oxygen isotopic analyses of
calcite are summarized in Table 3. All the analyzed
samples are enriched in 13C with respect to normal
marine carbonates, with d13C values up to +9.3x.
The d 8O values range between �2.6x and
�10.0x. The samples more depleted in 18O
correspond to modern stromatolites, whereas samples
of fossil hot spring stromatolites show a narrow
range of d18O variation, with values between �2.6xand �3.7x.
In a d18O vs. d13C plot, fossil and modern
stromatolite analyses define two different linear trends
(Fig. 7). Fossil stromatolite samples fall along a near-
horizontal regression trendline. In contrast, modern
stromatolite analyses define a trendline with higher
35
30
25
20
15
10
5
0
-5-7 -5 -3 -1 1 3 5 7 9 11
Seawater
Hot spring water
Aragonite marine cement
Modern hot spring
Fossil hot spring
∆ at
62º
C
CO
2
∆ at
62º
C
∆ at
25º
C
∆ at
25º
C
5
0
-5
-10
-15
-20
-25
-30
δ13C(PDB)
δ18O
(SM
OW
)
δ18O
(PD
B)
Fig. 7. Plot of carbon and oxygen stable isotope composition of calcite of the hot spring deposits. D represents calcite-water equilibrium
fractionations according to O’Neil et al. (1969).
C. Canet et al. / Sedimentary Geology 174 (2005) 97–113108
slope, because of the positive correlation between
d18O and d13C.
9. Discussion
Atypical silica-carbonate deposits are presently
forming in a coastal environment, around the intertidal
hot springs of Bahıa Concepcion. However, these
deposits do not form in the submarine diffuse
gasohydrothermal vents (at 5–15 m depth) of the same
area. In addition, a fossil equivalent of these deposits is
found onshore, close to the hydrothermally active area.
The subaerial exposure of that fossil deposit can be
attributed to the Plio–Quaternary tectonic uplift of the
bay area (Johnson and Ledesma-Vazquez, 2001).
Both modern and fossil silica-carbonate deposits
can be considered intertidal stromatolitic sinters. The
term stromatolite is applied here according to the
definition of Riding (1991) bmicroscopically lami-
nated deposits that can be built by a variety of
microbesQ, in the same way that has been applied
before for some hot spring sinter deposits by several
authors (e.g., Walter and Des Marais, 1993; Jones et
al., 2000a; Campbell et al., 2002).
9.1. Mineralogy and processes of formation of the hot
spring deposits
Although silica (sinters) and calcite (tufa, traver-
tine) deposits usually form in geothermal systems
elsewhere, silica-carbonate deposits are uncommon
(Jones and Renaut, 1996; Jones et al., 2000b; Camp-
bell et al., 2002). In hot spring deposits, the
mineralogy is mainly controlled by thermal water
chemistry (e.g., Jones et al., 2000a; Campbell et al.,
2002). Thus, silica is deposited from near-neutral and
alkaline pH chloride-rich and seldom from acid sulfate
thermal waters (e.g., Jones et al., 2000a; Rodgers et
al., 2004). According to Fournier and Rowe (1966),
deep reservoir temperatures above 175 8C are
necessary to form extensive silica deposits in the hot
spring environments.
In the hot springs of Bahıa Concepcion, the
thermal fluid (gas and water) chemistry allows silica,
calcite and barite precipitation. The deep reservoir
C. Canet et al. / Sedimentary Geology 174 (2005) 97–113 109
temperatures calculated by Prol-Ledesma1 et al.
(2004) are sufficiently high to produce hot spring
silica deposits. The saturation state calculations show
that the intertidal hot spring water is supersaturated
with respect to barite and silica (quartz, chalcedony
and amorphous silica) for a range of temperatures
between 68 and 25 8C (Fig. 3A). Both temperature
and fluid composition are responsible for mineral
saturation. Silica solubility increases with rising
temperature and pH; thus, silica supersaturation in
rising hydrothermal solutions is easily achieved by
cooling before mixing with seawater (Renaut et al.,
2002). Likewise, Guidry and Chafetz (2002) proposed
that cooling is the predominant process in silica
precipitation in the hot spring sinters of Yellowstone
(Wyoming). Theoretical calculations for a continuous
mixing of hydrothermal and marine water show that
amorphous silica saturation index decreases with
increasing seawater dilution (Fig. 3B). This fact
agrees with the absence of silica deposits in the
subtidal vents, where extensive mixing with seawater
prevents its precipitation.
Noncrystalline opal-A, deposited mainly as
microspheres, is the only silica phase encountered
in the modern and fossil hot spring deposits of
Bahıa Concepcion. According to Fournier (1985),
the silica phase that is deposited depends on the
degree of supersaturation of the solutions. Silica
microspheres grow from highly supersaturated sol-
utions, in which homogeneous nucleation and
colloidal growth takes place (Fournier, 1985; Renaut
et al., 2002). So, the precipitation of opal-A micro-
spheres agrees with the calculation of saturation state
for amorphous silica. Noncrystalline opal-A is the
initial silica product that forms in most hot springs
when the hydrothermal solutions become super-
saturated (Rodgers et al., 2004). After opal-A
deposition, several maturation stepwise transforma-
tions take place: opal-A changes to opal-CTFopal-C
and finally yields microcrystalline quartz (chalcedo-
ny)Fmoganite (Herdianita et al., 2000; Rodgers et
al., 2004). The alteration of opal-A to opal-C and its
subsequent transformation to chalcedony take thou-
sands of years and at least 50,000 years, respectively
(Herdianita et al., 2000; Rodgers et al., 2004).
Therefore, considering that opal-A is the only
identified silica phase of the studied fossil hot
spring deposit, it must be very recent, in spite of
the existence of hydrothermally derived cherts
(Ledesma-Vazquez et al., 1997) in the vicinity of
the vents that suggest that the present coastal
hydrothermal activity has been long-lived.
On the other hand, calcium carbonate deposits on
hot springs form from neutral alkali thermal waters
with high Ca2+ and CO32� activities (Jones et al.,
2000b) and, in most cases, carbonate supersaturation
is attained by CO2 degassing (e.g., Friedman, 1970;
Julia, 1983; Arp et al., 1999) or by heating of ambient
seawater (Barrat et al., 2000). The slightly low pH
values measured in vent fluids (between 5.95 and
6.68) are near neutral at vent temperatures (Kw at 60
8C is 13.1076, Harned and Owen, 1958). Only for
temperatures above 53 8C, the thermal water of Bahıa
Concepcion is slightly supersaturated in calcite and
even the saturation indexes are too low to explain
calcite precipitation (Fig. 3A). Furthermore, the
mixture of marine and hydrothermal water is clearly
subsaturated with respect to calcite (Fig. 3B). Thus,
calcite precipitation should be mainly triggered by
CO2 degassing. In many travertine systems, the loss of
CO2 by degassing is the main cause of calcite
supersaturation (Chafetz et al., 1991). CO2 degassing
is an ongoing process as the collected gas samples
from the active vents have a high CO2 concentration
(Forrest and Melwani, 2003). It produces a drop of
CO2 fugacity and increases the saturation index of the
waters with respect to calcite in the intertidal hot
springs (Table 1).
The Bahıa Concepcion hot springs yield a rare
example of authigenic precipitation of barite on
stromatolitic deposits. Barite precipitates when Ba2+-
rich hydrothermal water mixes with cold SO42�-rich
seawater. Therefore, as evidenced by theoretical
calculations, mixing of hydrothermal and marine
waters increases de saturation index of water with
respect to barite, causing its precipitation (Fig. 3B).
Microbial remains and microfossils identified in
the studied silica-carbonate stromatolitic deposits
evidence the biological activity around the hot
springs, and comprise diatoms and possibly, coccoids
and spores. Furthermore, the chains and filaments that
built up the porous spongy silica networks could be
attributed to filamentous microorganisms (cyanobac-
teria or fungi) that have been silicified. Diatoms are
common in hot spring waters elsewhere (e.g., Jones et
al., 2000a; Renaut et al., 2002; Rodgers et al., 2004;
C. Canet et al. / Sedimentary Geology 174 (2005) 97–113110
Guidry and Chafetz, 2003b); however, thermal envi-
ronment biota is dominated by fungi at acidic pH, and
by bacteria at near-neutral pH (e.g., Jones et al.,
2000a). Microbial mats can mediate silica precipita-
tion serving as templates for fixing and nucleation of
silica. This way of microbial mediation in silica
precipitation has been suggested in several hot spring
deposits with filamentous opaline textures (e.g., Jones
et al., 2000a; Konhauser et al., 2001). In contrast, the
lack of microbial fabrics on calcite, as calcitic shrubs
and clumps (Chafetz and Folk, 1984; Chafetz and
Guidry, 1999), fibrous crystals (Julia, 1983) and
porous aggregates (Riding, 1991), suggests that
CaCO3 precipitation in Bahıa Concepcion hot springs
is an inorganic-physicochemical process. In the same
way, for the Ngatamariki silica-carbonate hot spring
deposits, New Zealand, Campbell et al. (2002)
suggest a microbially mediated deposition process
for silica and an abiogenic process for calcite.
In the vicinity of the main hot spring outflow
conduits, a structureless aggregate of silica, calcite
and barite, which cements detrital fragments, under-
lies the stromatolites (Fig. 5E). Similarly, Renaut et al.
(2002) reported laminated silica crusts overlying
massive silica cemented detrital agglomerates in the
sublacustrine hot springs of Lake Baringo, Kenya. In
Bahıa Concepcion, the abiogenic deposition of silica,
calcite and barite forming structureless agglomerates
around the hot springs indicate that those minerals can
precipitate without microbial mediation, as predicted
by equilibrium thermodynamic calculations. This
implies that, where microorganisms are present,
although they do not actively induce mineral precip-
itation they are responsible of the finely laminated
fabrics of the deposit.
9.2. Carbon and oxygen isotopes
All the analyzed carbonates from the silica-
carbonate hot spring deposits are unusually enriched
in 13C, with d13C values up to +9.3x V-PDB. These
values indicate high 13C contents in the dissolved
inorganic carbon, considering the small fractionation
(about 1x at low temperatures) between HCO3� and
calcite (Romanek et al., 1992).
High d13C carbonates have been reported mainly
in lakes with high evaporation rates (Stiller et al.,
1985) and in hot spring-related carbonates (Valero-
Garces et al., 1999; Fouke et al., 2000; Melezhik and
Fallick, 2001). The precipitation of hot spring
carbonates is usually triggered by degassing of CO2
(e.g., Friedman, 1970; Arp et al., 1999; Chafetz et al.,
1991). Through CO2 degassing, a kinetic fractionation
process yield a strong 13C enrichment of dissolved
inorganic carbon and, therefore, of carbonate precip-
itates (Michaelis et al., 1985; Chafetz et al., 1991).
This non-equilibrium process can explain the 13C
enrichment in calcite of both modern and fossil silica-
carbonate hot spring deposits of Bahıa Concepcion. In
the same way, this process has been proposed by
Valero-Garces et al. (1999) to explain extreme 13C
enrichments in authigenic carbonates from Andean
lakes with geothermal activity.
The low d18O values of calcite from the modern
hot springs are due to the presence of a meteorically
derived hydrothermal fluid that is involved in calcite
precipitation. The range of d18O variation indicates
that calcite precipitates from a mixture of seawater
(cold, with d18O close to 0x V-SMOW) and hydro-
thermal fluid (62 8C, �3.11x V-SMOW, Prol-
Ledesma1 et al., 2004) in variable proportions. The
lowest d18O values (�10.0x V-PDB) can be
achieved by precipitation in isotopic equilibrium from
the hydrothermal fluid at temperatures near 60 8C,according to the fractionations established by O’Neil
et al. (1969) (Fig. 7). Otherwise, the highest d18O
values imply lower temperatures and a major seawater
component on calcite deposition. d18O and d13C
roughly define a positive correlation, so that the
analyses less depleted in 18O are the most enriched in13C (Fig. 7). This fact suggests an inverse temperature
dependence of the process of 13C/12C fractionation by
CO2 degassing. Kim and O’Neil (1997) observed that
the 18O/16O fractionation between calcite and water is
higher at high HCO3� concentration. In the modern
intertidal hot springs of Bahıa Concepcion, where the
hydrothermal fluid is enriched in HCO3� with respect
to seawater, this phenomenon could explain the poor
correlation between d18O and d13C.
In contrast, d18O values of calcite from the fossil
hot spring deposits show a very limited variation
(�2.6x to �3.7x V-PDB). The d18O values are
slightly higher than the values from calcite crystals of
the modern hot springs and are not correlated with
d13C variations. Taking into account that d18O on
calcite depends on the temperature and the isotope
C. Canet et al. / Sedimentary Geology 174 (2005) 97–113 111
composition of the water, the oxygen isotope compo-
sition of fossil hot spring calcite requires an 18O-
enriched water component and/or lower temperatures
of crystallization. The narrow range of variation of
d18O values suggests a limited process of mixing of
fluids during calcite precipitation. However, the
thermal fluid itself is a mixture of seawater and deep
circulating groundwater (Prol-Ledesma1 et al., 2004).
Thus, the high oxygen isotope values of the fluid can
be produced by a greater seawater component. More-
over, the seawater composition could have changed
throughout time and is subjected to local variations, so
that high evaporation rates and restricted water
exchange can produce heavier d18O in bay waters.
Dettman et al. (2004) measured d18O values up to
7.6x V-SMOW in tide pools in the northern Gulf of
California.
Other possible explanation of the O isotopic
composition observed in the fossil hot spring deposits
is to consider that the original d18O was not preserved
due to exchange reactions with seawater after cessa-
tion of the hot spring activity. However, at low
temperatures, rates of volume diffusion are very small;
therefore, simple isotopic exchange with fluids is
sluggish (O’Neil, 1987). Moreover, there are no
evidences of polymorphic changes (e.g., aragonite-
calcite) that could explain an isotopic exchange by
recrystallization.
10. Conclusions
The intertidal hot springs of Bahıa Concepcion
provide a rare example of silica-carbonate stromato-
litic sinter. In addition, a silica-carbonate deposit found
onshore can be considered the fossil equivalent of the
modern hot spring deposits. Moreover, a structureless
abiotic aggregate of silica, calcite and barite underlies
the stromatolites around the main hot springs.
Microbial remains and microfossils are abundant in
both modern and fossil silica-carbonate stromatolitic
deposits of Bahıa Concepcion, and provide evidence
that biological activity takes place around the hot
springs. The biological activity has an important role
in the development of the laminated fabrics and the
textures of the deposit.
Noncrystalline opal-A is the only phase of silica
present in the modern and fossil hot spring deposits
of Bahıa Concepcion. Opal-A is mainly deposited as
microspheres, suggesting that it precipitates from a
highly supersaturated solution. The silica supersatu-
ration state of the thermal fluid must be achieved by
cooling. Opal-A is also the only silica phase in the
fossil hot spring deposit, so that it must be very
recent (as maximum few thousands of years), as it
has not yet recrystallized to more stable silica
polymorphs.
In spite that the physicochemical conditions of the
hot springs allow a pure inorganic precipitation of
silica, microbes could mediate silica deposition serv-
ing as templates for fixing and nucleation of silica.
Calcite is the most abundant crystalline phase in
the hot spring deposits. The calculation of saturation
state shows that the mixed hydrothermal and marine
water is subsaturated in calcite, thus, calcite precip-
itation would be related to a loss of CO2 by degassing.
Calcite crystals are unusually enriched in 13C, with
d13C values up to +9.3x PDB. The 13C enrichment is
attributable to a CO2 degassing process, which yields
calcite supersaturation and triggers a kinetic fractio-
nation between CO2 and dissolved inorganic carbon.
The range of d18O values of calcite is in agreement
with precipitation from a mixture of seawater and
meteorically derived hydrothermal fluid. Unlike opal,
the lack of biological fabrics on calcite suggests that
its precipitation is an inorganic-physicochemical
process.
Acknowledgements
Funding was provided by the CONACyT projects
I39170-T and 32510-T, and PAPIIT IN-107003. The
SEM-EDS analyses were done in the Instituto de
Geologıa and in the Instituto de Geofısica of the
Universidad Nacional Autonoma de Mexico. We
thank M. Reyes Salas for her assistance during the
SEM analyses. X-ray diffraction and fluorescence
analyses were carried out in the Instituto de Geologıa
(UNAM). We acknowledge the help of U. Struck with
isotope analyses. We thank A. Camprubı for the
photographs. We thank D.I. Norman and J. Tritlla for
their explanations that improved the manuscript. The
authors thank M.A. Torres Vera, M.J. Forrest and J.
Ledesma Vazquez for their help during the collection
of the samples.
C. Canet et al. / Sedimentary Geology 174 (2005) 97–113112
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