In Lagoa Vermelha, Brazil, a lagoonal stromatolite and a saltpan microbial mat are investigated to understand the influence of environmental changes on the decomposition of microbial carbonates. The lagoonal stromatolite, composed mainly of magnesian calcite and aragonite, is developed on a dolomite-containing carbonate crust. While most stromatolites are eroded to the water surface level, some smaller, green stromatolites below the water surface retain a domal shape. The domal stromatolite surface is dominated by endolithic cyanobacteria with conspicuous microborings. In addition, microbial aerobic respiration causes carbonate dissolution in darkness, and metazoans grazing the inner surface of the stromatolite excrete fecal pellets. This suggests that the formational stage of lagoonal stromatolites has ceased and they are now decomposing, most likely because of environmental changes in recent years. The microbial mat, which is about 3 cm thick, developed in a saltpan pond precipitating carbonate and gypsum, and it contains quartz, magnesian calcite, aragonite, and gypsum. At the time of the investigation, the population of oxygenic phototrophs is low at the mat surface, and carbonate dissolution, rather than precipitation, is occurring by microbial metabolism deeper in the mat. This suggests that the formation of carbonate in the mat has ceased and is decomposing, probably due to the progressive salinity increase in the salt pan. This examination of two carbonate deposits in Lagoa Vermelha suggests that microbial metabolism is an important process for decomposing microbial carbonates in addition to grazing and microboring, and that environmental changes may alter microbial compositions from carbonate-constructive to carbonate-destructive communities.

Contributions of abiotic and biotic processes on travertine deposition are still not well‐understood due to technical difficulties, despite that the travertines draw attention as analogues for ancient microbial carbonates and oil reservoirs. To evaluate their contributions, this study examined eight hot springs in Japan. Water chemistry analyses showed common downstream trends: a decrease in CO 2 concentration and increases in CO 3 2− concentration and pH. Mineralogical analysis showed that the constituent minerals of travertines at six hot springs were both calcite and aragonite, while one was just calcite and another only aragonite. Microscopic observations of travertine surfaces indicated the dominance of cyanobacteria secreting extracellular polymeric substances without a detectable amount of carboxyl groups. Small particles were sometimes entangled/covered by these cyanobacteria. Microelectrode measurements showed the occurrence of abiotic CaCO 3 precipitation and photosynthetic induction/inhibition of CaCO 3 precipitation, the extent of which was different at each site. By integrating these results, the contributions of abiotic and biotic processes were evaluated. Cyanobacteria inhabiting travertine surfaces were generally not calcified regardless of an ambient high CaCO 3 saturation state; instead, they contributed to creating pore spaces and trap/bind suspended particles. Downstream CO 2 degassing increased the CaCO 3 saturation state by shifting carbonate chemical equilibrium and caused abiotic CaCO 3 precipitation. Suspended particles trapped by cyanobacteria increased the surface area for crystal growth to further accelerate precipitation. The contribution of photosynthesis‐induced CaCO 3 precipitation was low because of several factors, including variable cyanobacteria populations and photosynthetic inhibition of CaCO 3 precipitation. The average contributions of photosynthesis‐induced CaCO 3 precipitation, Ca 2+ adsorption and abiotic precipitation in the eight hot springs were 16%, 3% and 81%, respectively, indicating predominance of the abiotic process for travertine deposition. Mineralogical composition of travertines significantly correlated with concentrations of SO 4 2− and Mg 2+ , much more than with Mg/Ca ratio and water temperature, suggesting their importance for controlling CaCO 3 polymorphs in travertines.

Tufas are freshwater carbonate deposits that attracted attention as paleoclimatic archives and analogs of microbial carbonates. Recent geomicrobiological studies revealed that cyanobacterial photosynthesis significantly contributed to the precipitation of CaCO3 (photosynthesis-induced CaCO3 precipitation; PICP), and that the chemical properties of extracellular polymeric substances (EPS) secreted by cyanobacteria significantly influenced on the depositional fabrics. These depositional processes were revealed from tufas formed in cool and temperate climates; however, their applicability to tufa deposits in other climate conditions remains unclear. This study, therefore, investigated tufa deposits that formed in a tropical climate in Brazil and applied geomicrobiological techniques, including microelectrode measurements and lectin-binding analysis, to clarify their depositional processes. The surfaces of eight investigated tufa samples were dominated by cyanobacteria. Microelectrode measurements indicated the occurrence of PICP in all the samples. However, five samples additionally exhibited a Ca2+ concentration decrease at the tufa surface, even in dark conditions, which was difficult to explain by abiotic CaCO3 precipitation and light-independent microbial metabolisms. Based on the fluxes calculated from the results of microelectrode measurements, the contribution of PICP to the formation of CaCO3 precipitates was estimated as 100% for three samples, similar to tufa deposits in cool and temperate climates. For the remaining five deposits, the average contributions of PICP, Ca2+ adsorption to organic matter, and other processes were estimated as 36%, 3%, and 61%, respectively. Among the phototrophs living the tufa surface, Phormidium and Leptolyngbya secreted EPS abundantly containing carboxyl groups, and were heavily calcified. In contrast, Phormidesmis, Calothrix, Pleurocapsa, and diatoms secreted EPS without detectable amount of carboxyl groups, and they were largely not calcified and formed pore spaces. Such influence of microbial EPS on the depositional fabrics were common to cool and temperate tufas. Nonetheless, some features would be specific to tropical tufas, including the contribution of unidentified light-independent processes, conical and columnar surface morphologies, and unclear annual lamination.

Genetic and diagenetic processes of travertine were examined at Futamata hot spring, where both active and inactive (subfossil and fossil) travertines are present. Geochemical and geomicrobiological analyses of active trayertines reveal that abiotic process of CaCO3 precipitation is predominant. Photosynthetic inhibition of CaCO3 precipitation occurs at the upstream, while the cyanobacterial population is low in the downstream, resulting in photosynthesis-induced CaCO3 precipitation only in the midstream. Nonetheless, microorganisms have an effect upon depositional fabrics, and filamentous cyanobacteria possessing non-acidic sheaths contribute to the particle trapping/binding and the generation of pore space. Both active and inactive travertines are mainly composed of calcite; however, active travertine in the up-/midstream also contains some aragonite due to high rates of CO3 degassing. Active and subfossil travertines are characterized by domal topography, and relatively high flow rates are assumed due to the dominance of slope facies. On the other hand, fossil travertines are characterized by fissure-ridge topography and relatively low flow rates are assumed due to the dominance of crest and marsh fades. The elemental composition of travertines primarily reflects their constituent minerals, and the active travertines consisting of both aragonite and calcite show higher Sr. Ba. Na, and K contents, whereas travertines consisting solely of calcite show higher Mg and Mn contents. Due to elevated Mn content, bright cathode luminescence is exhibited by the primary calcite of active travertines. Most fossil travertines have experienced prominent diagenetic alteration in the vadose zone and are commonly cemented and recrystallized. In addition to meteoric water, percolating hot spring water was also involved in the diagenetic process. Combined with the results of U-Th dating, our results suggest that the formation of fissure-ridge travertines at Futamata hot spring began approximately 20 thousand years ago (ka) via fault activity, which terminated at around 7 ka and the domal travertines were formed by hot spring water discharging from several vents. (C) 2020 Elsevier B.V. All rights reserved.

The relative influences of biotic and abiotic processes on travertine fabrics are still not well understood, despite increasing interest in the last decade to better understand the record of ancient microbial life and sedimentary fabrics in microbial hydrocarbon reservoirs. This study examines travertines at Satono‐yu hot spring in Japan (the temperature of water flowing over the travertine was ca 35°C), to better understand the interaction between depositional, hydrochemical and microbial parameters at different flow settings. Characteristics of the bulk hydrochemistry, mineralogy (exclusively aragonite) and the driving force for precipitation (primarily abiotic CO2 degassing with some photosynthetic microbial contribution) were similar among all of the flow settings. Conversely, the increase in flow velocity suppressed the influence of photosynthesis and enhanced the abiotic precipitation due to the thinner diffusive boundary layer at the travertine surface–water interface. Additionally, the increase in flow velocity changed the microbial composition and decreased the bacterial diversity by reflecting their adhesion efficiency on the travertine substrate. The acidity of the cyanobacterial sheaths controls the aragonite nucleation rate and the resulting calcification, even at significantly high equilibrium CO2 partial pressure (ca 22 to 28 matm), high dissolved inorganic carbon concentration (ca 35 to 38 mmol l−1), and elevated aragonite saturation state (ca 20‐fold to 34‐fold). Therefore, the increase in flow velocity suppresses the microbial influence with respect to the increase in the saturation state, the nucleation site supply and pore space generation. Overall, this results in the predominance of abiotic precipitation under high flow velocities. Consequently, a sparse‐micritic fabric with abundant interlamina porosity forms under lower flow velocity where the microbial influence is effective, while a dense‐sparitic fabric with little inter‐crystalline porosity forms under higher flow velocity where abiotic precipitation prevails. These findings provide an essential base for assessing the formation processes of ancient travertines and comparable deposits from petrological fabrics.