Chizuru Takashima

Cold water coral mounds revealed

The discovery of mounds and reefs hosting cold-water coral ecosystems along the northeastern Atlantic continental margins has propelled a vigorous effort over the past decade to study the distribution of the mounds, surface sediments, the ecosystems they host, and their environments [Hovland et al., 1994; Freiwald and Roberts, 2005].This effort has involved swath bathymetry, remotely operated vehicle deployments, shallow coring, and seismic surveys. Global coverage is difficult to gauge, but studies indicate that cold-water corals may cover as large an area as the better known warm-water corals that form shallow reefs (284,300 square kilometers) [Freiwald et al., 2005]. Cold-water corals occur in a variety of forms and settings, from small isolated colonies or patch reefs to giant mound structures such as those found west of Ireland.

Microbial Processes Forming Daily Lamination in an Aragonite Travertine, Nagano-yu Hot Spring, Southwest Japan

An aragonite travertine at Nagano-yu hot spring, SW Japan, exhibits clear sub-millimeter-order lamination that resembles ancient ministromatolites. Thirty-three hours of continuous observation showed that the lamination is formed daily with no changes in physicochemical properties except light intensity. Phylotype analysis and fluorescence in situ hybridization indicate that Hydrogenophaga sp. is dominant and concentrated in diurnal layers containing abundant extracellular polymeric substances. Growth of Hydrogenophaga sp. is activated in the daytime, likely due to extracellular polymeric substance production by cyanobacterial photosynthesis. Daytime development of Hydrogenophaga-dominant biofilms, and the concurrent inhibiting effect on aragonite precipitation, explains the daily lamination observed.

Laminated Iron Texture by Iron-Oxidizing Bacteria in a Calcite Travertine

An iron-rich travertine at Shionoha hot-spring in western Japan, displays a sub-millimetre order lamination consisting of microbially-incduced ferrihydrite and calcite-matrix. The ferrihydrite occurs as 5- to 10-μ m thick filaments that extend and apparently branch upwards. A sheathed morphotype having a meshwork of a rod-like organic substance and phylogenetically identified species of genus Siderooxidans were responsible for precipitation of the ferrihydrite. The iron oxidizers are microaerophilic and thrive on Fe(II) and a redox gradient, that are available at the study site near the vent. Bacterial activity enhanced ferrihydrite deposition at rate of ∼ 10 μ m/day, and formed the laminated texture. The bacteria increased their density upward in each lamina and suddenly decreased the density at the top of the lamina. This change may have resulted from a deficiency of metabolic substances at the sediment–water interface when the iron-oxidizers became very dense or when other chemoautotrophs, such as methanotrophs, consumed oxygen on the surface. The metabolism of the microaerophilic iron-oxidizing bacteria growing in neutral pH environments contribute significantly to the precipitation of iron mineral deposits. Because the laminated textures observed in this study have a great preservation potential, they help to identify the contributions of iron-oxidizers to ancient BIFs and provide an idea for pO2 and pH of the ancient ocean.

Processes Forming Daily Lamination in a Microbe-Rich Travertine Under Low Flow Condition at the Nagano-yu Hot Spring, Southwestern Japan

A daily rhythm of microbial processes, in terms of sub-mm order lamination, was identified for a microbe-rich aragonite travertine formed at a low-flow site of the Nagano-yu Hot Spring in Southwestern Japan. Continuous observation and sampling clearly showed that the lamination consisted of diurnal microbe-rich layers (M-layers) and nocturnal crystalline layers (C-layers). The M-layers originated from biofilm formed by growth and upward migration of filamentous cyanobacteria related to Microcoleus sp., which can rapidly glide and secrete extracellular polymeric substances (EPS). During the daytime, cyanobacterial biofilm development inhibited aragonite precipitation on the travertine surface due to the calcium-binding ability of EPS. After sunset, aragonite precipitation started on the surface where aerobic heterotrophic bacteria decomposed EPS, which induced precipitation of micritic crystals. This early stage of C-layer formation was followed by abiotic precipitation of fan-shaped aragonite aggregates. Despite their major role in lamina formation, the cyanobacteria were readily degraded within 6–10 days after embedding, and the remaining open spaces in the M-layers were sparsely filled with crystal clots. These lamina-forming processes were different from those observed in a high-flow site where the travertine has a dense texture of aragonite crystals. The microbial travertine at Nagano-yu is similar to some Precambrian stromatolites in terms of in situ mineral precipitation, regular sub-mm order lamination, and arrangement of filamentous microbes; therefore, the lamination of these stromatolites possibly occur with a daily rhythm. The microbial processes demonstrated in this study may revise the interpretation of ancient stromatolite formation.

Basic Knowledge of Geochemical Processes

Travertines (or thermogene travertines in Pentecost 2005) are formed from hydrothermal water with an initial high concentration of Ca2+ and CO2 partial pressure (Ford and Pedley 1996; Gandin and Capezzuoli 2008, 2014; Capezzuoli et al. 2014). In this type of water, the active CO2 degassing immediately after discharging on the ground increases pH and saturation state with respect to CaCO3 of the water. Precipitation (and dissolution) of CaCO3, which is associated with CO2 degassing (and uptake), is often simply represented in the following reaction 2.1:

Sedimentology of Travertine

Sediment bodies of travertine exhibit unique geomorphology that results from its rapid sedimentation rate. As described in Chap. 2 and will be discussed in Chap. 6, the rapid sedimentation rate is closely associated with rapid CO2 degassing from water, which elevates the level of supersaturation with respect for CaCO3. Intensity of the CO2 degassing is generally related with hydrological conditions: more CO2 degassing in turbulent conditions. Therefore, rapidly flowing water is a site of active deposition in travertine settings. This is a distinct difference from an ordinary fluvial sedimentary system, in which rapidly flowing water generally erodes sediment. In travertine systems, erosion is an unusual process unless flow rate is excessively developed by some reasons like flooding. In our study in Pancuran Pitu in Indonesia, a flow rate of 2 m/s is not enough to erode travertine (Okumura et al. 2012). Initially depressed watercourse is filled up, and the watercourse shifts to flow along a newly developed depressed route (Fig. 3.1). Sedimentation of travertine usually occurs at the interface of water and sediment substrate. The substrate is normally preexisted travertine but can be sedimentary grains. Mode of sedimentation is therefore accretion of newly precipitated crystals of calcite or aragonite and somehow similar with coral reef crest where a sediment body grows forward by mineral secretion of reef corals.

Geomicrobiological Properties and Processes of Travertine: With a Focus on Japanese Sites

Introduction -- Basic knowledge of geochemical processes -- Sedimentology of travertine -- Methods -- Geomicrobiologial processes for laminated textures -- Geochemical model for rapid carbonate precipitation of travertines -- Travertines in Japan -- Concluding remarks.

Travertines in Japan

In this chapter, we describe several representative travertine sites in the Japanese islands. According to our extensive search, there are at least 30 hot springs that developed calcareous deposits (Fig. 7.1, see also Figs. 7.13 and 7.15). Many of them are simple localities having only a single onsen facility, and an example is Furofushi hot spring on northern coast of the Honshu Island (Fig. 7.1), where you can see the beautiful sunset on the Japan Sea. Hirokawara hot spring in Yamagata Prefecture is a relatively new geyser, and you have to drive 20 km on dirt road to reach there. Among the onsens listed on Fig. 7.1, only four locations, Masutomi, Shirahone, Arima, and Nagayu, have a large capacity and a wide facility for accepting many visitors. These onsens have also attracted many scientists for a study subject.

Geochemical Model for Rapid Carbonate Precipitation of Travertines

In the last chapter, we demonstrate the daily processes of the sub-mm-scale lamination. When favorable geochemical and hydrological conditions sustain, travertine can grow at a rate of tens of centimeters per year and tens of meters per a thousand years. Our founding strongly supports the previous statement that one of the most notable features of travertine is its rapid growth (or carbonate precipitation) rate (Kitano 1963; Folk et al. 1985; Pentecost 2005). In addition, recognition of the daily lamination enables to determine the growth rate of travertine that is enormous in comparison with the growth rate for tufa in non-hydrothermal karst settings (Ford and Pedley 1996). Andrews (2006) suggested that the tufa growth rate seldom exceeds 10 mm/year, and comprehensive studies in southwestern Japan revealed that the rate ranges from 3 to 8 mm/year (Kano et al. 2007; Kawai et al. 2009). Growth rate of a typical travertine is two orders of magnitude higher than that of a typical tufa.

Geomicrobiological Processes for Laminated Textures

A laminated deposit is a record of cyclic changes of physical, geochemical, and microbiological conditions. The laminated pattern is often quite regular. In lacustrine verves, regular seasonal changes in weather conditions accumulate a characteristic alternation of siliciclastic muddy particles and authigenic minerals. Based on patient search of the lamina counting and 14C dating, time series of geochemical and paleontological proxies in the lacustrine verves have decoded an excellent record of ancient climates (e.g., Nakagawa et al. 2003). Some stalagmites are also laminated annually, associated with seasonal change in physical and geochemical properties of dripwater. Distinct patterns in the drip rate, the saturation states, and fulvic acid content between summer and winter or between wet and dry seasons leave annual change in carbonate crystal fabrics and fluorescence (Shopov et al. 1994; Baker et al. 1999). Because U-Th isotopes support accurate dating, stalagmites have been used for paleoclimatic studies (e.g., Fairchild and Baker 2012). Another example of annual lamination is observed in tufas, non-hydrothermal carbonate precipitates developed in streams and rivers in limestone area. Origin of the lamination in tufas is somehow similar with that of the stalagmite lamination. However, tufas have one- or two-order wider lamination (or growth rate) than stalagmite. Because of the thicker lamination, tufas are available for higher-resolution analysis of the past climates (e.g., Kano et al. 2004).