Introduction
Tufas are freshwater carbonate deposits that form in karstic areas through a combination of abiotic (CO2 outgassing) and biotic (microorganisms’ metabolisms, trapping and binding) processes (Ford and Pedley, Reference Ford and Pedley1996; Arenas-Abad et al., Reference Arenas-Abad, Vázquez-Urbez, Pardo-Tirapu, Sancho-Marcén, Alonso-Zarza and Tanner2010; Gradziński, Reference Gradziński, Pedley and Rogerson2010), the latter playing a significant role in carbonate precipitation (Shiraishi et al., Reference Shiraishi, Bissett, de Beer, Reimer and Arp2008a, 2017; Dupraz et al., Reference Dupraz, Reid, Braissant, Decho, Norman and Visscher2009; Arp et al., Reference Arp, Bissett, Brinkmann, Cousin, De Beer, Friedl, Mohr, Pedley and Rogerson2010; Pedley, Reference Pedley2014). These deposits are characterized by the precipitation of CaCO3 from bicarbonate-rich fluids, often accompanied by CO2 outgassing, particularly during changes in substrate gradient or increased fluid flow velocity (Merz-Preiβ and Riding, Reference Merz-Preiβ and Riding1999; Andrews and Brasier, Reference Andrews and Brasier2005; Capezzuoli et al., Reference Capezzuoli, Gandin and Pedley2014).
Microbial activity has been widely documented as a component for the deposition of carbonate rocks, ranging from the early stages of Earth's history to modern water systems that exhibit carbonate deposition (Riding, Reference Riding2000). Microorganisms, particularly prokaryotic cells from the Bacteria domain and certain eukaryotes, are responsible for carbonate precipitation in various environments, including marine and lacustrine systems, fluvial tufa, travertines, and speleothems (Riding, Reference Riding and Riding1991, Reference Riding2000). Understanding the biotic processes involved in tufa formation has significant implications for biogeochemical cycles, paleoenvironmental records, and for possible microbial carbonate analogues (Andrews, Reference Andrews2006; Pedley et al., Reference Pedley, Rogerson and Middleton2009; Arenas et al., Reference Arenas, Vázquez-Urbez, Auqué, Sancho, Osácar and Pardo2014a; Shiraishi et al., Reference Shiraishi, Hanzawa, Okumura, Tomioka, Kodama, Suga, Takahashi and Kano2017).
Microorganisms play a significant role in tufa formation through various mechanisms. One such mechanism involves the secretion of extracellular polymeric substances (EPS) by microorganisms, which act as nucleation sites for mineral formation (Merz-Preiβ and Riding, Reference Merz-Preiβ and Riding1999; Riding, Reference Riding2000; Shiraishi et al., Reference Shiraishi, Bissett, de Beer, Reimer and Arp2008a; Dupraz et al., Reference Dupraz, Reid, Braissant, Decho, Norman and Visscher2009; Pedley et al., Reference Pedley, Rogerson and Middleton2009). Additionally, photosynthetic processes carried out by cyanobacteria and algae can induce carbonate disequilibrium by absorbing CO2 from the water, leading to an increase in pH and subsequent enhancement of CO3 saturation (Arp et al., Reference Arp, Reimer and Reitner2001; Shiraishi et al., Reference Shiraishi, Okumura, Takahashi and Kano2010; Shiraishi, Reference Shiraishi, Reitner, Queric and Arp2011).
The present study focuses on the Bonito region at Mato Grosso do Sul State, Brazil, which presents recent and inactive tufa deposits of Quaternary age. The tufas in this region are predominantly found along rivers, creeks, and abandoned meanders within the karstic system of the Corumbá Group. The Bonito region offers a unique opportunity to investigate these continental freshwater deposits. While recent works in the same region have provided valuable insights into tufa facies and their environmental forming conditions (Oste et al., Reference Oste, Rodríguez-Berriguete and Dal'Bó2021), as well as early diagenetic changes in these recent tufa deposits (Oste et al., Reference Oste, Rodríguez-Berriguete and Dal'Bó2023), the primary focus of the present study is to unravel the role of microorganisms in tufa formation. Specifically, we aim to investigate how the biogenic nature of tufa deposits influences the different facies and porosity arrangements.
Geological Setting
Located southwest of the Pantanal wetlands in the State of Mato Grosso do Sul (Brazil), the Serra da Bodoquena Hill (Fig. 1), up to 800 meters high, extends for 200 km in the north–south direction. The Serra da Bodoquena, within the Paraguai Fold Belt, is part of the Bodoquena Plateau and composed of a set of north–south trending mountains (Sallun Filho et al., Reference Sallun Filho, Karmann, Boggiani, Petri, Cristalli and Utida2009a). The Paraguai Fold Belt is a Brazilian–Panafrican tectonic unit, bordering the Amazon craton and the Rio Apa block (Boggiani and Alvarenga, Reference Boggiani, Alvarenga, Mantesso-Neto, Bartorelli, Carneiro and Brito-Neves2004; Campanha et al., Reference Campanha, Sallun Filho and Zuquim2011). Stratigraphically, this tectonic unit is subdivided into three other units: the lowermost part, formed by glaciogenic turbidites of the Cuiabá Group and Puga Formation; carbonates of the Araras and Corumbá groups; and siliciclastic rocks of the Alto Paraguai Group (Alvarenga and Trompette, Reference Alvarenga and Trompette1993; Riccomini et al., Reference Riccomini, Nogueira and Sial2007).
Figure 1. Geographic and geological settings of the study area. Taíka (21°00′24.1″S, 56°29′58.7″W) and Mimosa (21°00′00.1″S, 56°30′40″W and 20°59′58.9″S, 56°30′40.4″W) sites highlighted by red squares. Modified from Campanha et al. (Reference Campanha, Sallun Filho and Zuquim2011).
The Corumbá Group (Ediacaran Age) is subdivided into four units, including siliciclastic rocks at the bottom, followed by dolostones (Bocaina Formation), carbonate mudstones and limestones (Tamengo Formation), and lutites at the top (Almeida, Reference Almeida1965; Boggiani et al., Reference Boggiani, Fairchild and Coimbra1993; Romero et al., Reference Romero, Sanchez, Morais, Boggiani and Fairchild2016). Carbonate rocks from the Corumbá Group (Bocaina and Tamengo formations) are of great relevance because they are the source of CaCO3 for the modern tufa development (Sallun Filho, Reference Sallun Filho2005).
The tufas belonging to the Serra da Bodoquena system, are spread along rivers and creeks covering an erosive and angular unconformity with the underlying rocks of the Corumbá Group (Boggiani et al., Reference Boggiani, Coimbra, Gesicki, Sial, Ferreira, Ribeiro, Flexor, Schobbenhaus, Campos, Queiroz, Winge and Berbert-Born2002; Sallun Filho et al., Reference Sallun Filho, Karmann, Boggiani, Petri, Cristalli and Utida2009a, Reference Sallun Filho, Karmann, Sallun and Suguiob; Oliveira et al., Reference Oliveira, Rossetti and Utida2017). These tufa deposits have been informally included in the Pleistocene–Holocene Serra da Bodoquena Formation, which is subdivided into two members: the Rio Formoso Member and the Fazenda São Geraldo Member (Sallun Filho et al., Reference Sallun Filho, Karmann, Boggiani, Petri, Cristalli and Utida2009a; Oliveira et al., Reference Oliveira, Rossetti and Utida2017). Elements such as cascades and barrier tufas of the Rio Formoso Member, either active or not, occur exclusively in river channels (Sallun Filho et al., Reference Sallun Filho, Karmann, Boggiani, Petri, Cristalli and Utida2009a). Shells of gastropods and ostracodes are common, along with Characeae algae (green macro algae) or mosses, leaves. and branch fragments encrusted by carbonate (Boggiani et al., Reference Boggiani, Coimbra, Gesicki, Sial, Ferreira, Ribeiro, Flexor, Schobbenhaus, Campos, Queiroz, Winge and Berbert-Born2002). Unconsolidated micritic mud deposits from the Fazenda São Geraldo Member occur in ancient wetlands, lakes, and abandoned meanders (Boggiani et al., Reference Boggiani, Coimbra, Gesicki, Sial, Ferreira, Ribeiro, Flexor, Schobbenhaus, Campos, Queiroz, Winge and Berbert-Born2002; Sallun Filho et al., Reference Sallun Filho, Karmann, Boggiani, Petri, Cristalli and Utida2009a), with ostracodes and microgastropods (Utida et al., Reference Utida, Petri, Oliveira and Boggiani2012, Reference Utida, Oliveira, Tucker, Petri and Boggiani2017). In this study, we characterize the tufa deposits as discontinuous calcareous bodies and unconsolidated sediment, exhibiting limited distribution and no lateral continuity, despite the proposal of a stratigraphic unit encompassing them.
The climate of the study area is considered as humid tropical, with little difference in precipitation and temperature, which falls under the Aw category of the Köppen–Geiger classification. Mean air temperature is 23.8°C, with 1–3 dry months (June to August), also with the lowest temperatures. Annual precipitation (1684 mm) occurs mostly in December and January (https://pt.climate-data.org [accessed 14 October 2024]), which also corresponds to the months with highest temperatures.
Materials and Methods
Two sites, named Taika (21°00′24.1″S, 56°29′58.7″W) and Mimosa (21°00′00.1″S, 56°30′40″W and 20°59′58.9″S, 56°30′40.4″W), have been chosen for the present study. They are located along the Mimoso River near the city of Bonito, in Mato Grosso do Sul State, Brazil (Fig. 1). Oste et al. (Reference Oste, Rodríguez-Berriguete and Dal'Bó2021, Reference Oste, Rodríguez-Berriguete and Dal'Bó2023) previously investigated tufa deposits along the Mimoso River, however, our present work is focused on distinct fresh outcrops of waterfalls and pools–barrage–cascade subenvironments. Sampling was conducted during the dry season, at the end of autumn–winter period, in August and September of 2015 and 2016.
The depositional environment of active tufa precipitation was characterized during field campaigns, during which 25 samples were collected for laboratory analysis at the Laboratório de Análises de Minerais e Rochas (LAMIR), Universidade Federal do Paraná. Petrographic analysis included both mesoscopic and microscopic descriptions using handheld lenses and a petrographic microscope (Axio Zeiss Imager A.2) linked to a camera (AxioCam MRc) for image captures. Twelve thin sections were described. For lithological descriptions, the Dunham (Reference Dunham and Ham1962) classification, modified by Embry and Klovan (Reference Embry and Klovan1971), as well as studies by Arenas-Abad et al. (Reference Arenas-Abad, Vázquez-Urbez, Pardo-Tirapu, Sancho-Marcén, Alonso-Zarza and Tanner2010) and Gradziński et al. (Reference Gradziński, Hercman, Jaskiewicz and Szczurek2013), were followed. Additional petrographic studies and porosity analysis were conducted using a SkyScan X-ray 1172 microtomograph (90kV potential; 112 μA current, 12.8 μm/pixel resolution and 2000×1336 camera resolution) for 3D imaging of 10 sample cubes with a volume of 1 cm3. Porosity determination was optimized through binarization processes specific to each sample. The Choquette and Pray (Reference Choquette and Pray1970) classification was used for the porosity description. Additionally, six samples were analyzed using a scanning electron microscope (SEM) (JEOL JSM-6010LA model). The microscopic and microtomography analyses were conducted at LAMIR, Federal University of Paraná.
X-ray diffraction (XRD) analyses were performed on 25 powdered samples using a PANalytical diffractometer (EMPYREAN model) with a copper anode (Cu Kα1 = 1.5406 Å) at a Theta × 2Theta geometry, operating at 40 kV and 30 mA. Identification and percentages of the mineral phases (error of ± 5%) were estimated by using X'Pert High Score Plus software (PDF-2 database) and applying the RIR (reference intensity ratio) method for semi-quantitative analysis. A PANalytical X-ray fluorescence spectrometer (AXIOS MAX model, SuperQ 5.3 software) was used to analyze 10 major oxides and 4 trace elements, as well as for conducting LOI (lost on ignition) analysis on the same 25 powdered and dried samples. Isotopic bulk analyses of δ13C and δ18O were performed on 25 powdered samples, which were dissolved in 100% anhydrous phosphoric acid. The CO2 was extracted using a GasBench II system and IRMS Delta V Advantage mass spectrometer (Thermo Fisher Scientific). The isotopic values are expressed in δ notation in parts per thousand (‰) relative to the VPDB (Vienna Peedee Belemnite) standard. Analytical precision is 0.04‰ for δ13C and 0.08‰ for δ18O. Geochemical, mineralogical and isotopic analyses were conducted at LAMIR, at the Federal University of Paraná (UFPR).
Results
The Northern part of Bonito city is characterized by the predominance of active and inactive tufas of the Rio Formoso Member, forming several-meter high waterfalls and dam–pool–cascade subenvironments, along rivers and creeks, in a stepped fluvial profile. For this study, we focused exclusively on outcrops along the Mimoso River, specifically at two sites: Taíka and Mimosa (Fig. 1). The Taíka site consists of small cascades and natural dams extending approximately 10–20 m, while the Mimosa site features waterfalls measuring ~3–5 m high, smaller cascades, and dams spanning ~10 m in length.
Tufa facies: stromatolitic boundstones
The stromatolitic boundstones occur as laminated tufa or conical structures with irregular laminae, both in small cascades and dams (Fig. 2A–D). The lamination is well defined, consisting of alternating dark- and light-colored layers that are millimeters thick. Internally, the light-beige laminae are thinner and primarily composed of micrite, while the darker layers are porous and formed by micritic fibro-radiated filaments.
Figure 2. Stromatolitic boundstone. (A) Stromatolitic tufa characterized by well-defined laminations. (B) A small cascade with slow-flowing water. (C) Small conical features are common at the surface of laminated tufa. (D) Sample of stromatolitic tufa with greenish conical structures at the top. (E) Microscope image; lamination of stromatolitic boundstone, with dark and thin micritic laminae, microspar to spar laminae, and thick lamina with clear spar shrub-like crystals; parallel polarizers. (F) Microscope image; lamination of stromatolitic boundstone with thick laminae with shrub-like spar crystals; crossed polarizers. (G) SEM image; semi-radial microstructures with oval shape. (H) SEM image; semi-radial calcified tubes.
Microscopically, the stromatolitic boundstones are characterized by planar to gently domed lamination. The laminae are irregular and marked by an intercalation of: (1) dark and thin micritic laminae with a thickness of 1–3 mm; (2) clear spar crystals; (3) peloidal and highly porous micrite lamina 2–4 mm thick, and (4) elongated calcified filaments of cyanobacteria and/or algae, formed by a micritic nucleus encrusted with spar crystals (Fig. 2E and F). There are three types of pores observed in the stromatolitic boundstones: fenestral porosity, characterized by elongated pores aligned parallel to the lamination; growth-framework porosity, which is related to pores between calcified filaments; and moldic porosity, related to empty tubes formed after the decay of algae cells.
The stromatolitic tufa displays alternating undulating laminations, resulting in semi-radial microstructures observed through SEM imaging (Fig. 2G). These structures exhibit an oval shape with a diameter of approximately 100 μm and form bush-like structures with radiated calcified tubes (Fig. 2H). These tubes, with a diameter of 10 μm, are composed of stacked trigonal calcitic subcrystals, likely related to Oocardium stratum cells. Extracellular polymeric filaments are commonly present between radii-fibers, although no bacterial cell remains have been detected. Occasionally, the EPS appears as honeycomb-like networks.
Stromatolitic boundstones interpretation
The stromatolitic boundstones occur in various conditions, including fast-flowing areas, such as cascades or low-slope zones, and calm fluvial areas (Arenas-Abad et al., Reference Arenas-Abad, Vázquez-Urbez, Pardo-Tirapu, Sancho-Marcén, Alonso-Zarza and Tanner2010; Gradziński et al., Reference Gradziński, Hercman, Jaskiewicz and Szczurek2013; Arenas et al., Reference Arenas, Vázquez-Urbez, Pardo and Sancho2014b; Berrendero et al., Reference Berrendero, Arenas, Mateo and Jones2016). Typically, stromatolitic boundstones are formed by the rapid precipitation of calcite caused by mechanical CO2 outgassing and microbial activity (Shiraishi et al., Reference Shiraishi, Reimer, Bissett, de Beer and Arp2008b, Gradziński, Reference Gradziński, Pedley and Rogerson2010; Arenas and Jones, Reference Arenas and Jones2017). The laminated structure of stromatolites (alternating porous and denser laminae) is often associated with seasonal changes in physicochemical conditions and/or microbial communities (Kano et al., Reference Kano, Matsuoka, Kojo and Fujii2003; Shiraishi et al., Reference Shiraishi, Reimer, Bissett, de Beer and Arp2008b; Arenas and Jones, Reference Arenas and Jones2017). In this study area, Oste et al. (Reference Oste, Rodríguez-Berriguete and Dal'Bó2021) concluded that these stromatolites are directly associated with the seasonality of rain in the region.
Tufa facies: phytoherm boundstones of algae filaments
The phytoherm facies is characterized by fibro-radiated structures associated with algal filaments (Fig. 3A). This facies develops as mats in pools and ponded areas. The calcified filaments are displayed parallel to each other, with a brushy aspect (Fig. 3B) and can reach up to 2 cm in length. Typically, this facies consists of stacked algal mats, forming irregular and non-planar layers. Microscopically, these structures mainly comprise micritic filaments, with 0.2 mm in diameter, calcified by microspar to clear spar crystals (Fig. 3C and D) in a micritic matrix. Growth framework porosity predominates between the calcified filaments, usually 1–2 cm, and moldic porosity related to empty tubes.
Figure 3. Phytoherm boundstone of algae. (A) Non-planar layers of fibro-radiated structures. (B) Sample of phytoherm boundstone of algae with calcified filaments. (C) Microscope image; micritic filaments with semi-parallel orientation and a micritic matrix. (D) Microscope image; filaments calcified with microspar to small clear spar crystals; circular shapes with micritic nuclei represent perpendicular cuts of filaments (yellow arrow). (E) SEM image; dendritic calcite forming triangular domains associated with EPS. (F) SEM image; rhomboidal and euhedral crystals formed by triangular domains of calcite.
SEM images revealed the presence of triangular domains of dendritic calcite as an early stage of crystal formation. The dendritic calcite grows in three regular directions and forms clusters of small microcrystal triads (Fig. 3E). These fiber microcrystals collectively create a triangular domain shape associated with EPS. The dendritic calcite forms a crust around the filaments, exhibiting partial dissolution at the center when viewed from the top, which results in circular moldic pores. This dissolution is likely associated with the consumption of organic content within filamentous cyanobacteria. Occasionally, the triangular domains of calcite develop into rhombs and euhedral crystals (Fig. 3F). The EPS displays filamentous structures related to the triangular domains of dendritic calcite, and no visible bacterial cells were preserved.
Phytoherm boundstones of algae filaments interpretation
Ponded and dammed areas, with slow-flowing or stagnant waters, create favorable conditions for the preservation of algae and microbial mats. This facies is characterized by the presence of vertical or inclined bushes of algae that align parallel to the flow direction (Arenas-Abad et al., Reference Arenas-Abad, Vázquez-Urbez, Pardo-Tirapu, Sancho-Marcén, Alonso-Zarza and Tanner2010).
Tufa facies: phytoherm boundstones of bryophytes
The phytoherm facies is characterized by the presence of spongy calcified mosses, which have small leaves up to 1 cm in size. The bryophyte cushions occur near small cascades, in splash areas, or in dams with intermittent water flow (Fig. 4A and B). Microscopically, this facies exhibits a highly porous micritic matrix with moss leaves, whether preserved or not, coated by unclear microspar crystals (Fig. 4C). Large and empty moldic pores are commonly observed, occasionally containing remnants of organic matter.
Figure 4. Phytoherm boundstone of bryophyte. (A) Dams formed by bryophyte cushions (yellow arrow) in a splash-water area. (B) Bryophyte cushion. (C) Microscope image; leaves of moss covered by clear shrub-like spar crystals (yellow arrow) in a micritic matrix. (D) SEM image; moss leaf with with thin fungal filaments and some disperse micritic crystals. (E) SEM image; pennate diatoms (d) associated with micritic crystals. (F) SEM image; veneer of EPS covering living moss with micrite (mc).
SEM images show a cushion of living moss partially covered by filaments (Fig. 4D). Communities of pennate diatoms are attached to the moss leaves (Fig. 4E), interlocking with calcite crusts and encrusting diatoms. In the boundstones of bryophytes, the EPS may appear as a thin veneer (Fig. 4F) covering the moss leaves, incorporating diatoms frustules and calcite microcrystals. It is common to find fungal filaments inhabiting the bryophyte leaves.
Phytoherm boundstones of bryophytes interpretation:
Tufa environments, including the upper parts of barriers and dams with thin water laminae, offer favorable conditions for the growth of bryophytes, which thrive in well-lit areas. The calcitic crystals precipitated around the bryophyte leaves occur due to splashing and spraying of supersaturated water (Arenas et al., Reference Arenas, Osácar, Sancho, Vázquez-Urbez, Auqué, Pardo, Yoshida, Windley and Dasgupta2010; Arenas-Abad et al., Reference Arenas-Abad, Vázquez-Urbez, Pardo-Tirapu, Sancho-Marcén, Alonso-Zarza and Tanner2010; Gradziński et al., Reference Gradziński, Hercman, Jaskiewicz and Szczurek2013), or due to a temporary increase in water flow.
Tufa facies: phytoclastic rudstones
Phytoclastic rudstones occur at the tops of dams and cascades (Fig. 5A and B). They consist of plant fragments, such as leaves and small branches, covered by a thin calcitic crust and embedded in a micritic matrix (Fig. 5C). Gentle millimeter-thick laminations formed by oriented phytoclasts of leaves parallel to the flow can be observed.
Figure 5. Phytoclastic rudstone. (A) Dam formed by the accumulation of leaves and plant fragments (yellow arrow). (B) Phytoclasts deposited at the top of a dam. (C) Uncoated plant fragments in a greenish micritic matrix. (D) Microscope image; phytoclast with organic matter preserved (yellow arrow) calcified with clear spar crystals. (E) Phytoclast covered by micrite to microspar crystals; note the moldic porosity. (F) SEM image; plant stalk with a diameter of 25 μm, covered by calcite and associated with small calcified tubes.
Under microscopic examination, the phytoclasts are covered by a thin layer of dark micrite, occasionally grading to unclear microspar crystals or clear shrub-like spar crystals (Fig. 5D and E). The phytoclasts are often associated with calcified filaments of cyanobacteria or algae. The matrix primarily consists of dark, loose, and occasionally peloidal micrite. Moldic pores are related to phytoclasts, which typically retain a very thin remnant of organic matter (Fig. 5D).
In SEM images, phytoclastic rudstones reveal plant stalks with diameters of approximately 25 μm that are completely covered by calcite (Fig. 5F), with relatively low mucilage content (EPS). Phytoclasts (e.g. leaves and fragments of branches) appeared calcified by small crystals of micrite and/or microspar. Algal and/or cyanobacterial tubes are frequently present (Fig. 5F). Thin filaments (5–10 μm in diameter) connected to leaves and stalks are probably related to fungi.
Phytoclastic rudstones interpretation
Plant fragments can be transported from vegetated areas by flash floods, high velocity water flow (Gradziński et al., Reference Gradziński, Hercman, Jaskiewicz and Szczurek2013), and/or be carried by wind. While erosion might occur during high-energy episodes, deposition of this facies occurs on tops of barriers or in dammed conditions (Arenas-Abad et al., Reference Arenas-Abad, Vázquez-Urbez, Pardo-Tirapu, Sancho-Marcén, Alonso-Zarza and Tanner2010; Vázquez-Urbez et al., Reference Vásquez-Urbez, Arenas and Pardo2012) where micrite encrustation takes place in slow-flowing water.
Porosity analysis
Stromatolites, phytoherm boundstones, and phytoclastic rudstones generally have high values of total porosity in μ-CT analysis (Fig. 6A). The tomographic images for different sections within the tufa samples exhibit a porous morphology (Fig. 6B–E).
Figure 6. (A) Graph showing total porosity values; Note the high porosity values in the phytoherm boundstone of bryophytes. (B) Microtomographic images of the stromatolitic boundstone, which has fenestral pores aligned with the lamination; the arrow indicates a cut on the y-axis within the sample. (C) Microtomographic images of the phytoherm boundstone of algae, which has growth-framework porosity with pores between the filaments; the arrow indicates a cut on the z-axis within the sample. (D) Microtomographic images of the phytoclastic rudstone; the arrow indicates a cut on the y-axis within the sample. (E) Microtomographic images of the phytoherm boundstone of bryophyte, normally associated with moss leaves; the arrow indicates additional magnification of the sample.
Moldic, growth framework and fenestral pores are considered selective types of porosity (Choquette and Pray, Reference Choquette and Pray1970). Moldic porosity, the most common type in the tufas of Bonito, is formed by the oxidation of organic matter of phytoclasts (Fig. 6D), gastropod shells, moss leaves, and algal filaments. Growth framework pores are commonly observed between calcified filaments of cyanobacteria and/or algae in phytoherm boundstones of algae (Fig. 6C) and stromatolitic boundstones (Fig. 6B). These pores are vertical and can reach up to 1 cm in size, similar to the radii-fibers. Fenestral porosity, a primary type of pore, occurs parallel to the lamination in stromatolitic boundstones (Fig. 6B). This type of porosity is characterized by isolated and elongated pores, with dimensions of up to 0.1 cm in the vertical axis and 1 cm in the horizontal axis. Interparticle and intercrystalline porosities, also considered selective types (Choquette and Pray, Reference Choquette and Pray1970), occur in all described facies, although they are not exclusive to a particular tufa facies.
X-ray microtomography revealed the highest porosities in phytoherm boundstones of bryophytes (Fig. 6E) and phytoclastic facies (Fig. 6D), with values of 66.32% and 43.80%, respectively (Table 1, Fig. 6A). The percentage of isolated pores was used as a parameter to indirectly analyze connectivity, with higher values indicating lower connectivity. The phytoclastic facies also exhibited the lowest values for isolated pores. In contrast, stromatolitic tufas and phytoherm boundstones of algae had the lowest total porosity rates, averaging 14%, due to their denser fabric.
Table 1. Summary of total porosity, isolated pores, and open porosity for tufa samples of each facies
It was possible to distinguish the internal part of moldic pores within phytoclasts from the radiated features on phytoherm boundstones of algae. The stromatolite facies exhibited some lamination and had pore sizes larger than the detection capacity of the equipment. The phytoclastic framework was typically chaotic, with no preferential direction of pores, most of which were not connected.
Porosity interpretation
The characterization of porosity using micro-CT predominantly revealed primary and selective types of pores, indicating that the arrangement of components is primarily influenced by depositional events. It is important to note that the remarkably high values of total porosity observed in all tufa facies are due to limited compaction and minimal burial influence (Oste et al., Reference Oste, Rodríguez-Berriguete and Dal'Bó2023). Diagenetic features that typically reduce porosity, such as cementation and aggrading neomorphism, are minimal in active tufa facies (Oste et al., Reference Oste, Rodríguez-Berriguete and Dal'Bó2023) and have negligible effects on the overall pore volume and connectivity.
Mineralogical, chemical, and isotopic results
The 25 samples were analyzed using XRD (X-ray diffraction) (Fig. 7A) and XRF (X-ray fluorescence) (Fig. 7B) techniques to obtain geochemical results. The samples were classified into four facies, and the mean values of semi-quantitative mineralogical results, major oxides, and volatiles were calculated for each facies group. Minor oxides, such as Na2O, K2O, TiO2, MnO, and P2O5, comprised less than 0.5% of each sample and are not considered in the graph (Fig. 7).
Figure 7. (A) XRD results by facies. (B) XRF results by facies. Minor oxides (< 0.5%) are not shown in the graphs.
The results of the chemical analysis indicated similar and frequent content of CaO and volatiles, including water, CO2, and organic matter (volatiles obtained through lost on ignition [LOI]) across all tufa facies (Fig. 7B). Some samples exhibited an increase in silica concentration, contrasting with the depletion of CaO. The presence of aluminum silicates, likely clay minerals such as illite, was suggested by the correlation between increased Al2O3 and silica values. Chemical results obtained through XRF were confirmed by XRD analysis, which revealed that all samples were composed primarily of calcite (mean 96–98%) with occasional detrital quartz (mean 2–4%) (Fig. 7A).
Isotopic analysis was conducted using 25 selected samples, which exhibited variations in δ18O values ranging from −6.35 to −8.65‰ VPDB (average of −7.52‰ VPDB), and δ13C values ranging from −6.16 to −9.52‰ VPDB (average of −8‰ VPDB). A positive correlation (R 2 = 0.41) was observed between the two isotopes across all samples (Fig. 8A). The δ18O and δ13C plot by facies (Fig. 8B) revealed that phytoherm boundstone of algae exhibited the lowest δ13C values, while certain phytoclastic rudstones samples had the highest δ13C values. Stromatolitic boundstones were scattered through the δ13C range, and phytoherm boundstones of bryophytes displayed the most consistent δ18O values.
Figure 8. Cross plot of stable carbon and oxygen data for 25 selected samples. The δ18O values range from −6.35 to −8.65‰ VPDB (average of −7.52‰ VPDB). The δ13C values range from −6.16 to −9.52‰ VPDB (average of −8‰ VPDB). A positive correlation (R 2 = 0.41) is observed between the two isotopes. The dotted highlights indicate the phytoclastic boundstone of algae with the lowest δ13C values (bottom), some samples of phytoclastic rudstone with the highest δ13C values (top), and phytoherm boundstone of bryophyte with the most consistent δ18O values (center).
Mineralogical, chemical, and isotopic interpretation
There are only two major chemical compounds in tufa samples: calcium, which forms calcite, and silica. Silica appears in freshwater tufa system as: (1) detrital quartz (Ribeiro et al., Reference Ribeiro, Sawakuchi, Wang, Sallun Filho and Nogueira2015), deposited into pools or dam environments, and is likely related to flooding events or windblown activity; (2) remains of diatom frustules; (3) vadose silt or clay infiltration in inactive tufa, related to pedogenetic processes; and (4) clay minerals, such as illite (aluminosilicate).
The highest values of SiO2, normally associated with the presence of quartz, are observed in phytoclastic rudstones, indicating that detrital grains are transported with phytoclasts and deposited at the bottoms of pools. In contrast, the lowest SiO2 values correspond to boundstones (stromatolites and phytoherm facies), which indicates an autochtonous process of tufa formation, without the presence of detrital grains.
The δ18O value of meteoric water in Campo Grande City (near Bonito) is reported to be −7.36‰ VSMOW (Vienna Standard Mean Ocean Water), based on data from GNIP (Global Network for Isotopes in Precipitation) stations (Paula, Reference Paula2012). A negative trend in δ18O represents meteoric fluid with rainout effect (Andrews, Reference Andrews2006), which is consistent with the isotopic values of meteoric water in the region. The δ18O values obtained in this study are similar to those reported by Boggiani et al. (Reference Boggiani, Coimbra, Gesicki, Sial, Ferreira, Ribeiro, Flexor, Schobbenhaus, Campos, Queiroz, Winge and Berbert-Born2002).
Isotopic values of δ13C are consistent with values from recent tufas around Bonito (Boggiani et al., Reference Boggiani, Coimbra, Gesicki, Sial, Ferreira, Ribeiro, Flexor, Schobbenhaus, Campos, Queiroz, Winge and Berbert-Born2002). Those values indicate influence of C3 plants and cyanobacteria (Schidlowski, Reference Schidlowski, Riding and Awramik2000), along with possible contribution of streams and waterfalls coming from wooded areas where local groundwater contains isotopically light carbon. Some highly negative δ13C values are also associated with groundwater enriched with light soil carbon (Andrews et al., Reference Andrews, Riding and Dennis1997). The isotopic values of δ13C correspond to forested lowland streams (Deocampo, Reference Deocampo, Alonso-Zarza and Tanner2010). Measurements of Bonito samples confirm the typical isotopic values for tufa systems (δ13C: −2 to −12‰ VPDB) (Pentecost, Reference Pentecost1995; Özkul et al., Reference Özkul, Kele, Gökgöz, Shen, Jones, Baykara, Fórizs, Németh, Chang and Alçiçek2013; Capezzuoli et al., Reference Capezzuoli, Gandin and Pedley2014).
Discussion
Relation between microorganisms and CaCO3 precipitation in different tufa facies
Recent freshwater tufa deposits are formed as a result of the interaction between physicochemical and biological processes (Pedley et al., Reference Pedley, Andrews, Ordonez, Garcia del Cura, Martin and Taylor1996; Kano et al., Reference Kano, Matsuoka, Kojo and Fujii2003; Shiraishi et al., Reference Shiraishi, Bissett, de Beer, Reimer and Arp2008a, 2010; Gradziński, Reference Gradziński, Pedley and Rogerson2010; Arenas et al., Reference Arenas, Vázquez-Urbez, Auqué, Sancho, Osácar and Pardo2014a, Reference Arenas, Piñuela and García-Ramos2015; Arenas and Jones, Reference Arenas and Jones2017). The primary cause of calcite supersaturation is the loss of CO2, which occurs through two pathways: (1) a physicochemical process, where mechanical CO2 outgassing, induced by water turbulence, leads to calcite precipitation; or (2) a biological process, involving photosynthesis. In fast-flowing waters, CO2 outgassing typically occurs through the mechanical pathway due to water turbulence (Arenas et al., Reference Arenas, Vázquez-Urbez, Auqué, Sancho, Osácar and Pardo2014a), whereas the photosynthetic process is more significant in slow-flowing waters or stagnant areas.
Generally, CaCO3 precipitation in tufa deposits has been attributed to abiotic factors, primarily intense CO2 degassing in fast-flowing waters. Biotic precipitation has been limited to areas with slow-flowing water, with extracellular polymeric substances (EPS) considered as nucleation sites for mineralization (Freytet and Plet, Reference Freytet and Plet1996; Merz-Preiß and Riding, Reference Merz-Preiβ and Riding1999; Turner and Jones, Reference Turner and Jones2005). However, recent studies have revealed the significance of microorganisms, particularly filamentous cyanobacteria, in tufa formation (Shiraishi et al., Reference Shiraishi, Bissett, de Beer, Reimer and Arp2008a, Reference Shiraishi, Reimer, Bissett, de Beer and Arpb, Reference Shiraishi, Okumura, Takahashi and Kano2010, Reference Shiraishi, Hanzawa, Okumura, Tomioka, Kodama, Suga, Takahashi and Kano2017; Arp et al., Reference Arp, Bissett, Brinkmann, Cousin, De Beer, Friedl, Mohr, Pedley and Rogerson2010; Pedley, Reference Pedley2014), even in areas with rapid and turbulent flow. The surfaces of stromatolitic boundstones are densely colonized by cyanobacteria, which stimulates CO2 uptake through photosynthesis, leading to an increase in the saturation state within the diffusive boundary layer and subsequent CaCO3 precipitation. Studies have shown that during dark conditions, despite high fluid supersaturation, CaCO3 precipitation ceases, indicating biologically induced mineralization driven by photosynthesis (Shiraishi et al., Reference Shiraishi, Bissett, de Beer, Reimer and Arp2008a, Reference Shiraishi, Okumura, Takahashi and Kano2010, Reference Shiraishi, Hanzawa, Okumura, Tomioka, Kodama, Suga, Takahashi and Kano2017; Arp et al., Reference Arp, Bissett, Brinkmann, Cousin, De Beer, Friedl, Mohr, Pedley and Rogerson2010).
The Taika and Mimosa sites, both located in the Mimoso River, display a stepped longitudinal profile (Arenas-Abad et al., Reference Arenas-Abad, Vázquez-Urbez, Pardo-Tirapu, Sancho-Marcén, Alonso-Zarza and Tanner2010) characterized by small waterfalls, cascades, and dam–pool–cascade environments (Oste et al., Reference Oste, Rodríguez-Berriguete and Dal'Bó2021). Tufa boundstones and rudstones were collected from these depositional systems, where sedimentation occurs under high CaCO3 precipitation rates and mechanical CO2 outgassing (Arenas et al., Reference Arenas, Vázquez-Urbez, Auqué, Sancho, Osácar and Pardo2014a). Considering that the Mimoso River has intense mechanical CO2 outgassing due to its stepped riverbed (Oste et al., Reference Oste, Rodríguez-Berriguete and Dal'Bó2021), the question arises regarding the actual relevance of biological processes in tufa formation and their effect on different facies (Fig. 9).
Figure 9. Schematic diagram with the pool–barrage–cascade depositional subenvironment, the distribution of tufa facies, and the main process related to the formation of each facies.
Previous research in the study area allowed recognition of a diverse community of microorganisms formed by filamentous algae (Oocardium stratum and Vaucheria geminata), cyanobacteria (Phormidium incrustatum), filamentous fungi, and pennate diatoms (Oste et al., Reference Oste, Arai, França, Cury and Bahniuk2018, Reference Oste, Rodríguez-Berriguete and Dal'Bó2021). Among the phototrophs observed in the stromatolites of Bonito, those containing abundant carboxyl groups within EPS, such as Phormidium and Leptolyngbya, were considered controllers of CaCO3 nucleation (Shiraishi et al., Reference Shiraishi, Hanzawa, Asada, Cury and Bahniuk2022). According to Shiraishi et al. (Reference Shiraishi, Hanzawa, Asada, Cury and Bahniuk2022), within well-developed biofilms of stromatolitic and laminated tufa, the CaCO3 precipitation is induced by photosynthesis; nevertheless, abiotic precipitation remains the major process for tufa formation outside the biofilm (Shiraishi et al., Reference Shiraishi, Hanzawa, Asada, Cury and Bahniuk2022).
Stromatolitic boundstones are formed by an intercalation of micritic laminae and fibro-radiated laminae, which are influenced by seasonal changes in physicochemical conditions and/or microbial communities (Kano et al., Reference Kano, Matsuoka, Kojo and Fujii2003; Arenas et al., Reference Arenas, Osácar, Sancho, Vázquez-Urbez, Auqué, Pardo, Yoshida, Windley and Dasgupta2010, Reference Arenas, Vázquez-Urbez, Auqué, Sancho, Osácar and Pardo2014a; Arenas and Jones, Reference Arenas and Jones2017). In the studied area, stromatolitic lamination is directly correlated with rainfall seasonality and, consequently, water discharge and microbial community preservation (Oste et al., Reference Oste, Rodríguez-Berriguete and Dal'Bó2021). Under SEM, microscopic facies of radiated tubes corresponding to Oocardium stratum (Chlorophyceae) (Golubić et al., Reference Golubić, Violante, Plenkovic-Moraj and Grgasovic2008; Gradziński, Reference Gradziński, Pedley and Rogerson2010; Rott et al., Reference Rott, Holzinger, Gesierich, Kofler and Sanders2010, Reference Rott, Hotzy, Cantonati and Sanders2012) are encrusted with trigonal-shaped calcitic mesocrystals (Oste et al., Reference Oste, Rodríguez-Berriguete and Dal'Bó2021). These tubes are covered by thick sheets of EPS, confirming biologically induced mineralization (Dupraz et al., Reference Dupraz, Reid, Braissant, Decho, Norman and Visscher2009).
In laminated tufas in fast-flowing waters, there is an intrinsic relation among micrite, spar crystals, and EPS (Pedley, Reference Pedley2014). Within the EPS, or intra-EPS, precipitation of calcite is controlled by the external calcium ion supply and the biofilm, which produces small calcite crystals. In contrast, outside the EPS, the high ion supply favors the crystallization of well-organized calcite crystals (Pedley, Reference Pedley2014). According to Oste et al. (Reference Oste, Rodríguez-Berriguete and Dal'Bó2021), a crystallization sequence occurs around tubes of Oocardium stratum: small crystals form around the filament within the EPS, while well-developed crystals such as rhombs (Fig. 3F) form outside the influence of EPS (extra-EPS). Although Oocardium stratum does not control the mineralization around the tubes, it accelerates the calcite precipitation (Tran et al., Reference Tran, Rott and Sanders2019).
Stromatolitic tufa develops in areas with both fast-flowing and slow-flowing water (Arenas-Abad et al., Reference Arenas-Abad, Vázquez-Urbez, Pardo-Tirapu, Sancho-Marcén, Alonso-Zarza and Tanner2010; Gradziński et al., Reference Gradziński, Hercman, Jaskiewicz and Szczurek2013), with intense calcite precipitation in the former due to rapid mechanical CO2 outgassing and enhanced microbial activity, such as photosynthesis, in the slow-flowing water areas. Thus, the formation of stromatolitic boundstones results from the combined influences of inorganic and microbial factors on mineralization.
Phytohermal boundstones of bryophytes are formed in splash-water zones, with thin water laminae (Arenas-Abad et al., Reference Arenas-Abad, Vázquez-Urbez, Pardo-Tirapu, Sancho-Marcén, Alonso-Zarza and Tanner2010; Gradziński et al., Reference Gradziński, Hercman, Jaskiewicz and Szczurek2013). Calcite precipitation around mosses primarily occurs through a mechanical process, driven by the high supersaturation of the splashed water. These boundstones with bryophytes exhibit calcite precipitation around the moss leaves, indicating nucleation within a biofilm and subsequent progressive calcite encrustation via an inorganic pathway. SEM images confirm the precipitation of micrite around bryophytes, with limited or no detectable EPS remnants. These findings, which are supported by Shiraishi et al. (Reference Shiraishi, Reimer, Bissett, de Beer and Arp2008b), revealed that the contributions of other phototrophs, such as diatoms and mosses, are much less significant on biotic precipitation compared to filamentous cyanobacteria.
Phytohermal boundstones of algae are the main facies that reflects the intense biological process of precipitation. This facies forms in stagnant waters, such as pools and small ponds (Arenas-Abad et al., Reference Arenas-Abad, Vázquez-Urbez, Pardo-Tirapu, Sancho-Marcén, Alonso-Zarza and Tanner2010), where mechanical CO2 outgassing is minimal. Preserved algal mats exhibit Oocardium stratum filaments encrusted with calcitic mesocrystals characterized by trigonal shapes, which are typically associated with microorganisms (Turner and Jones, Reference Turner and Jones2005; Spadafora et al., Reference Spadafora, Perri, Mckenzie and Vasconcelos2010; Manzo et al., Reference Manzo, Perri and Tucker2012; Jones, Reference Jones2017). Consequently, biotic activity is considered the principal process driving precipitation in these facies.
Phytoclastic rudstones, consisting of plant fragments, are typically deposited in calm areas, often at the tops of barriers, cascades, or in dammed conditions (Arenas-Abad et al., Reference Arenas-Abad, Vázquez-Urbez, Pardo-Tirapu, Sancho-Marcén, Alonso-Zarza and Tanner2010; Vázquez-Urbez et al., Reference Vásquez-Urbez, Arenas and Pardo2012). Slow-flowing waters facilitate the preservation of algal and/or cyanobacterial filaments, contributing to tufa formation. The micrite encrustation of phytoclasts primarily occurs as an inorganic process, although cyanobacterial filaments, which can induce biotic precipitation, are common in the phytoclastic rudstones. Therefore, phytoclastic rudstones result from an interaction between mechanical and biological processes of mineralization.
The micro-CT characterization of porosity predominantly revealed primary and selective pore types, providing insights into the arrangement of components primarily influenced by depositional events. Boundstones of algae and stromatolites have the lowest rates of total porosity, due to a close fabric. Growth-framework porosities, which are formed between microbial filaments, occur extensively in both facies, indicating a biological influence on porosity development. In contrast, bryophytes and phytoclastic have the highest porosities, which are formed mainly by rapid calcite encrustation of molds of leaves and plant fragments. The micro-CT results contribute to a comprehensive understanding of porosity types, revealing the interplay between mechanical processes and biological influence in shaping the porosity characteristics within the studied tufa facies.
Isotopic signature due to microbial influence
The isotopic composition of freshwater tufa can be related to the environment and CO2 degassing. Mechanical CO2 degassing is more pronounced in areas with turbulent flow, such as waterfalls and cascades, leading to rapid calcite precipitation. This is reflected in higher δ13C values compared to slow-flowing water areas (Arenas et al., Reference Arenas, Cabrera and Ramos2007). However, there are several factors that can affect this correlation, including the influence of underground and surface water, variations in flow conditions, water velocity, changes in soil CO2 contributions, biological activity, photosynthesis, degree of CO2 degassing, and isotopic composition of the parental rock (Andrews, Reference Andrews2006; Arenas-Abad et al., Reference Arenas-Abad, Vázquez-Urbez, Pardo-Tirapu, Sancho-Marcén, Alonso-Zarza and Tanner2010).
In our study, we did not observe a clear trend between the carbon isotope ratios of tufa facies and water velocity, suggesting that other factors are influencing the isotopic composition. These factors may include rainfall patterns, the contribution of C3 and C4 plants, and the uptake of 12CO2 by cyanobacteria in stromatolites (Schidlowski, Reference Schidlowski, Riding and Awramik2000; Arenas-Abad et al., Reference Arenas-Abad, Vázquez-Urbez, Pardo-Tirapu, Sancho-Marcén, Alonso-Zarza and Tanner2010).
Photosynthesis is associated with the preferential uptake of 12CO2, leading to an enrichment of 13C in dissolved inorganic carbon (DIC) and, subsequently, in the precipitated carbonate. However, the magnitude of this enrichment does not significantly affect the overall carbon distribution, which can vary depending on the environment and the types of plants present (Andrews et al., Reference Andrews, Pedley and Dennis2000). The δ13C enrichment due to photosynthesis is particularly more significant in microenvironments dominated by algae and cyanobacteria (Arp et al., Reference Arp, Reimer and Reitner2001), which is evident in the most-negative δ13C values that were observed in boundstones of algae in our study. Negative δ13C values in these boundstones may imply that the uptake of light 12C from mosses and algae combines with the signal originating from soil sources.
Isotopic δ13C values do not appear to distinguish between induced or influenced microbial precipitation. This lack of differentiation is attributed to the absence of enzymatic fractionation of carbon isotopes associated with microbial respiration or photosynthesis in these processes (Della Porta, Reference Della Porta, Bosence, Gibbons, Le Heron, Morgan, Pritchard and Vining2015). Although the δ13C signature in tufa does not accurately indicate a direct correlation between microbial precipitation and the presence of biological activity, it also does not necessarily imply that the precipitation is exclusively inorganic (Shiraishi et al., Reference Shiraishi, Reimer, Bissett, de Beer and Arp2008b).
More-negative isotopic δ18O values primarily indicate a meteoric fluid with rainout effect (Andrews, Reference Andrews2006). Laminated tufa is commonly used in several paleoclimatic studies (Andrews, Reference Andrews2006; Arenas et al., Reference Arenas, Osácar, Sancho, Vázquez-Urbez, Auqué, Pardo, Yoshida, Windley and Dasgupta2010; Arenas and Jones, Reference Arenas and Jones2017), where there is typically a good correlation between δ18O values and seasonal changes in water temperature. In our work, we were unable to identify any significant climatic change, only confirming the meteoric origin of the fluid and minimal influence from intense evaporation, owing to the typical climate in the region with high annual precipitation.
The observed negative trend in δ13C and δ18O within tufas from the Bonito region aligns with the typical range found in calcareous tufa deposits globally (Pentecost, Reference Pentecost2005). The effect of diagenesis on recent tufa deposits from Bonito appears to be minimal (Oste et al., Reference Oste, Rodríguez-Berriguete and Dal'Bó2023), suggesting that, besides the tufa formation being regarded as an open system, the isotopic signal is closely related to the original precipitation conditions. It is noteworthy that the δ13C isotopic composition incorporates contributions of various local-scale fractionation processes (Arenas et al., Reference Arenas, Osácar, Sancho, Vázquez-Urbez, Auqué, Pardo, Yoshida, Windley and Dasgupta2010). The primary factors contributing to the δ13C and δ18O signatures include mechanical outgassing of CO2, the presence of light CO2 derived from soil, biological activity within facies rich in moss and algae, and input from meteoric sources.
Conclusions
Six conclusions emerge based on the facies and geochemical analysis conducted in this study. (1) The formation of stromatolitic boundstones is influenced by both biotic and abiotic processes. The presence of Oocardium stratum and EPS filaments indicates the involvement of microbial activity, while rapid CO2 degassing plays a significant role in mineralization in fast-flowing waters. (2) Calcite precipitation around mosses in phytohermal boundstones of bryophytes is primarily driven by a mechanical process due to the high supersaturation of splashed water. In boundstones of algae, biotic activity is identified as the principal precipitation-driving process. The formation of phytoclastic rudstones results mainly from a mechanical process, although the presence of microorganisms is common and may influence the precipitation. (3) Porosity varies among different tufa facies. Stromatolites and phytoherm boundstones of algae exhibit lower porosity due to their compact fabric, whereas phytoherm boundstones of bryophytes and phytoclastic rudstones display higher porosity resulting from intense and rapid encrustation of phytoclasts and mosses. (4) The presence of calcified tubes of Oocardium stratum and abundant EPS associated with trigonal calcitic crystals strongly supports the biogenic-influenced origin of tufas, even in turbulent environments. (5) The isotopic composition of freshwater tufa in the Bonito region is influenced by CO2 outgassing and environmental factors, such as water sources and flow conditions. The negative trend of δ13C is likely attributed to photosynthetic processes and the contribution of light carbon-enriched groundwater. (6) Photosynthesis contributes to isotopic enrichment, particularly in microenvironments dominated by algae and cyanobacteria, as evinced by the lighter isotopic values of phytoherm boundstones of algae. However, it is important to note that the isotopic composition does not always directly correlate with photosynthesis. These findings enhance our understanding of the influence of microorganisms on tufa formation and highlight the complex interplay between biotic and abiotic processes in the development of different tufa facies.
Acknowledgments
We wish to thank INPEX Corporation, especially Dr. Jiro Asada, for the fomentation of this work, including financial support for field trips and laboratory analyses. We also thank all the colleagues and professionals of LAMIR at Universidade Federal do Paraná. We are pleased to acknowledge Dr. Fumito Shiraishi and his students at Hiroshima University for the support and all the constructive comments. This paper is based on a master's study conducted at Universidade Federal do Paraná that was supported by the Geobiocal Project/LAMIR and University of Hiroshima.
