Columnar travertines: bio-influenced genesis, Porcelana Geysers, Northern Patagonia, Chile

Abstract

Porcelana Geysers are located on the slopes of Barranco Colorado volcano, southern Chile, and is characterized by having a lateral hydrothermal fluid transport and an important CO2 content, having high gas exsolution rates on the surface at temperatures above 80°C. But it does not seem to be enough to explain the genesis of columnar travertines more than 2 meters high, considering that precipitation rates counteracts erosive rates due to rainy climate of the zone and the high slope gradient (>10 mm/h). The presence of thermophilic microorganisms revealed by EPS (exopolysaccharide) and microscopic textures together with the redox conditions of the hot springs waters, could explain the genesis of the exceptional travertine morphology found in Porcelana by a biological contribution.

Full text

V51C-0371: Columnar travertines: bio-influenced gen-
esis, Porcelana Geysers, Northern Patagonia, Chile
Bárbara Ruiz-Velásquez [1]; Diego Morata [1]; Linda Daniele [1]; Beatriz Díez [2]
[1] Department of Geology and Andean Geothermal Center of Excellence (CEGA), Universidad de Chile
[2] Department of Microbiology Pontificia Universidad Católica de Chile
Porcelana Geysers are located on the slopes of Barranco Colorado volcano, southern Chile, and is characterized by having a lateral hydrothermal fluid transport and an im-
portant CO2 content, having high gas exsolution rates on the surface at temperatures above 80°C. But it does not seem to be enough to explain the genesis of columnar
travertines more than 2 meters high, considering that precipitation rates counteracts erosive rates due to rainy climate of the zone and the high slope gradient (>10
mm/h). The presence of thermophilic microorganisms revealed by EPS (exopolysaccharide) and microscopic textures together with the redox conditions of the hot springs
waters, could explain the genesis of the exceptional travertine morphology found in Porcelana by a biological contribution.
Abstract
Introduction
Travertine deposits are very common not only in marine environments but also they
accumulate in continental depositional settings (Pentecost, 2005). Some morpholo-
gies are usual to see them forming terraces, mounds, cascades and dams, and it is
possible to find them associated to springs along fault systems. Southern Chile is a
high favorability zone to find geothermal systems. However, travertine deposits in
Los Lagos district are rare except for Porcelana Geysers, which present anomalous
travertine pinnacles up to 2.5 m height (Fig. 1). Many factors could be contributing
to the growth of these singular structures, physicochemical settings that control CO2
exsolution (high temperature groundwaters and bubbles formation) (Asta et al.,
2017; Jones, 2017; Ladd and Ryan, 2016; Pentecost, 2005) and biological settings
as microbial activity (Capezzuoli et al., 2014; Fouke, 2011; Fouke et al., 2003; Gao
et al., 2013; Okumura et al., 2011; Okumura et al., 2013). In consequence, this
study is relevant to consider not only physicochemical factors but also biological
conditions to understand the different morphologies of travertine deposits and the
fragile ecological balance existing in this zone.
FONDAP
Fondo de Financiamiento de Centros de
Investigación en Áreas Prioritarias
A1
A0
d18O ‰ (PDB)
d13C ‰ (PDB)
0
-5
-25 -20
-4
-3
-2
-1
-15 -5-10
1
2
3
4
5
6
7
8
9
10
11
Hydrothermal
travertine
Fluvial
tufa
Geological settings
Porcelana Geysers are located in southern Chile, Los Lagos dis-
trict. This area belongs to the Southern Volcanic Zone (Stern, 2004)
and both volcanoes and springs are distributed along two mean
fault systems: N-S trend Liquiñe-Ofqui Fault System (LOFS) and
sinistral NW-SE trend Arc oblique Long-lived basement Fault
System (ALFS).
North Patagonian Batholith (NPB) and Bahía Mansa Metamorphic
Complex (BMMC) are the mean lithological units (Duhart et al.,
2001).
Porcelana Geysers are located over Barranco Colorado Volcano
slope, in the Peninsula of Huequi, over ALFS and LOFS structures
therefore anomalous permeability is a possibility for this area.
Travertines
In the geysers area dthere are different geometries of travertines:
smooth slopes, small cascades and waterfall in millimeter-scale
and numerous pinnacles which vary from few centimeters to 2.5 m
height. Also, there is abundant exopolysaccharide (EPS) on the
travertine surface (cyanobacteria) (Mackenzie et al., 2013).
The average rate of travertine precipitation is 0.5-0.8 cm/year, and
in the spouters it is even higher (>1.5 cm/year).
An inactive pinnacle (21 cm long and 10 cm diameter) was ob-
tained. 4 layers was recognized according to their color (Fig. 2). Fi-
brous aragonite (Fig. 3) and silica are the most present minerals
in all of them except for the layer A1, where calcite and the silica
(as tridymite) were found. An unidentified filamentous thermophilic
organism was observed in layer A1 (Fig. 4).
Negative δ13C and δ18O values were measured (Fig. 5).
Figure 5. Isotopic ratios compared
to hydrothermal travertine and tufa
ratios (Della Porta, 2015). A0 corre-
spond to EPS layer, and A1 to the
outermost travertine layer
Figure 2. Travertine pinnacle
sampled.
Figure 1. Travertine pinnacles form Porcelana Geysers.
Hydrogeochemistry
The hot waters are clasified as chloride waters with volcanic
fluids input, which are mixing with groundwaters enriching boron
and bromine (Ruiz and Morata, 2016) and fjord influence.
The higher temperatures were recorded in the emerging waters
of the pinnacles zone, measuring between 85.1 and 81.4°C,
while the proximal and distal hot springs recorded 65,7°C and
53.4°C on average, respectively. All the waters present reducing
conditions (from -284 to -71.1 mV, decreasing in the time).
The values of partial pressure of CO2 in all hot water samples
are near 0.0 bar (0,00371 – 0,06165 bar), all of them lower than
pCO2 of CO2-enriched springs waters which use to be higher
than 1 bar (Minissale et al. 2004). Saturation indices of arago-
nite and calcite were calculated using LLNL databse (Appelo
and Postma, 2005), which are very low in all waters, however
only in the geysers they are positive (0.86 and 1.00 on average,
respectively.
Discussion and conclusion
After carbonate precipitation three elements are generated: travertine deposit, depleted
water and steam, thereby, joining their chemical compositions is possible to have an idea
about the deep fluid that is generating these travertine pinnacles. Thus, is important to
consider that the analyzed water correspond to the remaining fluid after first carbonate
precipitation stage, which let us to suppose that the mean cause of high precipitation rate
is a very high CO2 exsolution during the precipitation of travertine pinnacles.
According to water chemistry, a volcanic origin of CO2 could be proposed. However, by
observing the δ13C values these deposits cannot be classified as hydrothermal travertine,
which use to have positive δ13C values (Della Porta, 2015). This means that CO2 ex-
change from the hydrosphere to the atmosphere is happening possibly caused by high
rainfall rates of the zone. Respect to microbial presence, all indicates that the microbial
cells are present during carbonate precipitation, particularly in the pinnacles vents, be-
cause the carbonate textures show dark microcrystalline bands and needles among radi-
ating acicular aragonite crystals formed by rapid gas exsolution (Fig. 3), thus CO2 exsolu-
tion occurs contemporaneous to the presence of microbial filaments (i.e. cyanobac-
teria), which are being replaced by CaCO3 (Fouke, 2011). This microbial activity increas-
es the carbonate precipitation rates (Fouke, 2011; Kandianis et al., 2008), contributing to
high pinnacles formation. The unidentified filamentous thermophilic organism that was
found in the outermost layer could represent a current activity of cyanobacteria, which
could be dissolving and reprecipitating calcium carbonate.
Figure 4. An unidentified filamen-
tous thermophilic organism was
observed in layer A1 by SEM
image, possibly a filamentous het-
erocystous or non-heterocystous
cyanobacteria (Mackenzie et al.,
2013).
Figure 3. Polarized-light photomicrographs of
fibrous texture aragonite. The radiating arago-
nite needles nucleate from dark irregular mi-
cron-scale bands and present dark inter-
spersed needles.
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Contact: bruiz@ug.uchile.cl