MICROFACIES CHARACTERISTICS AND SINTER PROPERTIES

Microfacies 1 – Cup- to Ridge-Shaped Sinter

Microfacies 1 (Figure 3) sinter forms in slightly turbulent waters with high emissions of steam, and water temperatures ranging from 91°C at some vents to 64°C in the associated discharge channels (Figure 4.1). The water turbulence is caused by the ebullient discharge from nearby vents that produces pulses of thermal water (typically 85°C) washing onto the siliceous sinters. Neither splash nor spray was observed around these deposits. However, water level changes were recorded in this microfacies (Figure 4.1). Sinter forms subaerially on pumice clasts and pine cones that act as substrates above water level (Figure 5). Sinter accretion rates were fastest at this microfacies. An average of 0.24 g silica accumulated on glass slides over 67 days in this environment (Figure 4). It is unknown if Microfacies 1 sinter accreted at a uniform rate or not.

XRPD analyses of sinters from this and all other microfacies of this study site showed that they are composed almost entirely of opal-A, with minor traces of detrital quartz and/or feldspar (for full width at half maximum intensity (FWHM) values and density/porosity data, see Rodgers et al. 2004). DTA of sinters from this study also gave identical traces characteristic of opal-A, with no indications of clay content (cf. Herdianita et al. 2000a). TGA of these sinters indicated weight loss of approximately 5 to 6% resulting from opal-A dehydration (cf. Herdianita et al. 2000b; Handley et al. 2005).

Microfacies 1 sinter is vitreous, light grey in colour, and usually cup- or ridge-shaped with minute microspicules (0.5 cm high) visible on the uppermost sides of the rim or within cavities (Figure 5.2). The sinter surfaces have multiple, thin layers of silica with an irregular texture, and are composed of gnarled, broken, and isolated surface remnants (Figure 5.4). This irregular texture is gradational, occurring predominantly in the lowermost portions of the sinter, close to the air-water interface. Vertical sections through the sinters exhibit alternating series of light brown and light grey laminae (Figure 5.5). The laminae range from <2 to 50 µm in thickness and alternate between brown to dark brown and clear, translucent silica (Figure 5.6). No evidence for the presence of microorganisms or any mineral-microbe association was observed in thin-section. Laminae in the lower portions of the sinters are relatively flat, but become progressively more convex upwards. On and within these layers, conical microspicules may be present, but in a relatively lower frequency than in Microfacies 2. Under SEM, vertical sections appeared vitreous and massive, without any conspicuous laminae. Prokaryote-sized microorganisms were only rarely observed in these sections.

At the Parariki site, sinter morphology is affected by the subaerially exposed substrate dimensions. Substrates that are relatively wider and higher above water exhibit limited siliceous coating. This coating is confined to the outer fringes of the substrates, resulting in the formation of cup-shaped deposits (Figure 5.2). On substrates that are low-lying and less aerially extensive, silica deposited across the substrate and a ridge-shaped deposit arises (Figure 5.3).

Large areas of Microfacies 1 sinter surfaces are covered by irregularly lobed, coccoidal microorganisms (1-1.5 µm in diameter) (Figure 6.1). These microbes are linked to one another through a meshwork of mucosal to filamentous extracellular polymeric substances (EPS). Such microbe-EPS assemblages are known as biofilms (cf. Cady and Farmer 1996; Handley et al. 2005). Biofilms in the upper- and inner-most portions of the Parariki sinters typically assumed the shapes of isolated, conical cell clusters (Figure 6.2). Some of these clusters became progressively encrusted by nanospheres of silica (<250 nm in diameter) and eventually recolonised by a new succession of microbes (Figure 6.3-6.4). Note that individual nanospheres attached directly to the cells and the associated EPS, thereby starting the process of silicification. Cells that appeared free from these silica spheres may be tentatively regarded as unsilicified.

The continuous interplay between microcolony formation, silicification, and recolonisation appears to have formed vertically upright, pillar-like microspicules (Figure 7). The upper surfaces of these spicules displayed a knobby morphology (typically 1-2 µm in diameter) of similar dimensions to nearby coccoidal cells (Figure 7.3). Thus we infer that individual knobs formed by the silicification of these microbial cells. The knobby texture was only observed on spicules of Microfacies 1, where it coincided with the presence of coccoidal microbial cells. We did not observe this texture on spicules from other microfacies where bacilli (rod-shapes) predominate (see below).

The irregularly lobed cocci are regarded as microbial cells and not as abiogenic crystals, since they are highly irregular in shape, often associated with copious EPS (Figure 6) and are very similar in morphology under SEM to cultured archaeal thermoacidophiles (e.g., Chen et al. 2005, figure 5A). Indeed, 16S rRNA genes extracted from Microfacies 1 sinter is related to known thermoacidophiles that also have irregularly lobed sphere shapes (e.g., Thermoplasmatales; Schinteie 2005).

SEM examination revealed that silica deposited predominantly as spheres of opal-A that are primarily <10 nm to 50 nm in diameter. However, spheres can grow up to 500 nm in diameter through the aggregation of smaller spheres. Two types of sphere shapes were observed: (1) freshly deposited, round equidimensional spheres; and (2) more poorly defined spheres with interparticle "necks" (Figure 8.1-8.2) (cf. Iler 1979, figure 3.24). The poorly defined sphere shape-texture was pervasive on Microfacies 1 sinter. Associations between this texture and microorganisms were not observed under SEM. In addition to silica, well developed crystals of gypsum, barite, and sulphur were present. No clay minerals were observed anywhere upon or within sinter from this or any other microfacies at the Parariki site. The absence of clay minerals is in contrast to acid-derived sinters and residues reported elsewhere in New Zealand (Jones et al. 2000; Rodgers et al. 2002).

At the microscale, the surfaces of the lower portions of Microfacies 1 sinter are irregular and uneven. Anastomosing ridges, nodules, and associated cavities are common. Incipient ridges appear as surface irregularities that are bounded by small cavities (Figure 8.3-8.4). Within these cavities, nodules formed that eventually become mushroom-shaped, producing a neck-like structure (Figure 8.4-8.6). While incipient nodules are surrounded by small cavities, taller nodules are associated with deeper cavities and larger ridges.

The lower portions of Microfacies 1 sinters exhibit isolated patches of surficial silica sheets (Figure 9.1). These sheets appeared to have become progressively contracted and diminished, particularly around necks (Figure 9.2), so as to leave behind isolated remnants of a formerly continuous surface. The patches are bound by anastomosing ridges (Figure 9.3) that are larger towards the outer fringes of these isolated patches (Figure 9.4). Figure 9.1-9.4 show the surface of silica grown onto a glass slide for one month in a Microfacies 1 setting. Surfaces of natural Microfacies 1 sinter show even more extreme forms of isolated remnants (Figure 9.5-9.6), presumably because they have been in this environment for much longer than the slides. These remnants contain concentric laminae, mirroring the topography of the underlying layers, as well as ridges, which are larger on the fringes.

Microfacies 2 – Spiculose Sinter

Microfacies 2 (Figure 3) is characterised by quiescent thermal water discharge. Steam emission is also minor, while water temperatures range from ~85°C to ~30°C (Figure 4.1). As in Microfacies 1, neither splash nor spray was observed around these deposits. Microfacies 2 sinter forms subaerially on pumice clasts, wood, pine cones, and dead insects (Figure 10). Water level changes have also been observed in this microfacies (Figure 4.1). Steam condensate keeps the sinters moist. Silica accretion rates are slower than those in Microfacies 1; an average of 0.12 g silica accreted on the slides over 67 days (Figure 4).

The outer and lower portions of Microfacies 2 sinters are usually covered by a ubiquitous green microbial mat (Figure 10). Transmission electron microscopy (TEM) and rRNA gene analysis indicates that these mats are primarily composed of coccoidal algal cells related to the rhodophyte taxon Cyanidiophyceae (Cyanidium and Galdieria; Schinteie 2005). The mats occur where temperatures are between 52.5 and 30°C. On sinter, the mats are at their thickest (1 mm) at ~ 45°C. On the uppermost portions of the sinters, where temperatures are often slightly cooler and less exposed to thermal water, the mats become faint and disappear.

The sinters are white to vitreous in colour, and typified by significant spicular growth (Figure 10). The spicules are needle-like to conical in shape, 1 to 3 mm wide, and from <1 mm to ~1 cm long. Spicules are densely packed, accrete perpendicular to the substratum, and become progressively smaller outward towards the water. A rim forms where spicules progressively link laterally with each other through a continuous deposition of silica (Figure 10.2, 10.5; cf. Handley 2004). Vertical sections through these sinters revealed alternating light and dark laminae (Figure 10.6).

The laminae alternate among laterally continuous green, brown, and translucent silica (Figure 11.1). As in Microfacies 1, laminae in the lower sinter portions are relatively flat but become progressively convex upwards (Figure 11.2), culminating in spicules composed of parabolic laminae. The fate of incipient spicules varied over time; some became reinforced by the deposition of successive laminae, whereas others were smothered or dampened by the lamination process (Figure 11.3-11.4). Overall, the relief of the sinters increased over time, with surfaces characterised by abundant, erect, and free-standing spicular structures with parabolic laminae. Numerous spicules also exhibit branching and small (~0.5 mm) projections with internal convex laminae.

The green laminae observed in thin section (Figure 11.5) are composed of spheres that are similar in size (2-10 µm) and appearance to the coccoidal cells present in the green biomats (Figure 11.6). These cells are restricted to the lower and middle portions of Microfacies 2 sinter, corresponding to the distribution of the living green mats. Diatom tests occur throughout the sinter.

As in Microfacies 1, sinter morphology also is controlled by substrate shape and dimensions. Subaerial substrate portions that are relatively wider and higher above water exhibit less siliceous encrustation relative to substrate size than deposits that are low-lying and have smaller widths. While wider substrates display sinter with cup-shaped morphologies (Figure 10.2), smaller substrates are covered in spicules that coat much of the subaerial portions (Figure 10.3). The extent of siliceous covering in Microfacies 2 is less than that of comparable substrate sizes and shapes in Microfacies 1, where relatively higher energy conditions occur.

Bacilli (i.e., rod-shaped microorganisms) (1-2.3 µm long) dominate the sinter biota of this microfacies (Figure 12.1). The microbes are usually associated with a meshwork of EPS that includes both fibrous and mucosal textures. These biofilms became progressively silicified by coalesced to partially coalesced opal-A nanospheres (100 nm) and microspheres (250 nm) (Figure 12.2). Eventually, the films were completely obliterated by the deposition of overlying spheres.

The bacilli also tended to form clumps of microcolonies which grew perpendicular to the silica surface and were associated with a network of EPS (Figure 12.3). In places, diatom tests provided areas of positive relief onto which new clusters of bacilli aggregated. As in Microfacies 1, such microcolonies became silicified, forming areas of positive relief (Figure 12.4). This consistent interplay between bacterial colonisation and silicification also resulted in the formation of microspicules. The possible outlines of bacilli inside spicules are preserved (Figure 12.6). The occurrence of vertically upright microcolonies in Microfacies 2 is more widespread than in Microfacies 1.

Pennate diatoms, predominantly Pinnularia acoricola Hustedt and P. champmaniana Foged, constitute a major component of the sinter biota (Schinteie 2005; cf. Foged 1979; Cassie 1989; Cassie and Cooper 1989). These diatoms preferentially occupied areas of low microrelief, such as crevices and cracks, as well as overlying areas of positive relief (Figure 13.1-13.3). The mode of attachment for these benthic diatoms is adnate, or closely appressed to the substratum, with the entire valve attached to a substrate by a coating of EPS (Figure 13.4). In open areas of the deposits that do not offer sheltering by surrounding sinter, diatom tests were often found fractured and amassed into clumps (Figure 13.5). Diatom assemblages may eventually become part of the sinter deposit, starting with the precipitation of silica spheres onto the tests, which eventually result in their complete cementation (Figure 13.6) (cf. Jones et al. 2000; Campbell et al. 2004). In this and in other microfacies of the Parariki site, early silicification occurs preferentially on the edges of diatom tests, EPS sheets, and fibres.

The green algal-dominated mats present on the lower surfaces and in thin section of Microfacies 2 sinters are largely composed of colonies of spherical cells (2-10 µm in diameter) that are covered in membranous sheets (Figure 14.1). The cell surfaces of these mats can become encrusted and incorporated into lower portions of the sinter (Figure 14.2-14.4; cf. Figure 11.5-11.6). Vertical sections through these portions exhibit numerous horizontal laminae (~100-180 µm thick) of dense, vitreous sinter, alternating with layers (~80-120 µm thick) dominated by silicified cells that are equal in size to cells of the living green mats (Figure 14.5). Several silicified cells also exhibited endospores (i.e., internal division of parental algal cells). In some instances, cellular impressions were preserved (Figure 14.6). The boundary between the predominantly abiotic and biotic layers is sharp and planar, showing that cells of the green mats initially colonised a flat surface before becoming silicified (Figure 14.4).

Unlike sinter surfaces of Microfacies 1, those from Microfacies 2-4 are generally not irregular in appearance. Ridges, nodules, isolated remnants, or associated cavities were rarely observed. Furthermore, their opal-A spheres (also <10 nm to 50 nm in diameter) tended to be more spherical in shape.

Microfacies 3 – Parallel Laminated Sinter

Microfacies 3 sinter (Figure 3) is confined to the southern portion of the study site, which slopes at ~10° E from the horizontal. Although these deposits are usually submerged under flowing thermal water (Figure 15), the water level in this area is lower (~6-10 mm) than that flowing on flatter areas (>10 mm) elsewhere at the site (Figure 4.1). Therefore, this microfacies is affected by changing water levels, completely exposing the sinters to the air in times of lower water levels. Water temperatures range from ~60-54°C (Figure 4.1).

Sinter from this environment is flat and parallel-laminated (Figure 15.1). Surfaces are irregular and even rippled in places, which appears to be due to the patchy nature of ongoing silica deposition (Figure 15.2). During lower water levels, small, isolated puddles of thermal water occur in the irregular surface crevices of the sinters. Continuous patchy silica deposition on these sinters results in silica rims (Figure 15.3) that eventually grow into cup-like deposits like those of Microfacies 2 (Figure 15.4). Indeed, pumice clasts that rest partially submerged in these waters and on top of the planar sinter deposits act as substrates for Microfacies 2 sinters (Figure 15.1). Due to the thinness of Microfacies 3 and 4 sinter (~<3 mm and 2 mm, respectively), no thin sections were made of these two deposit types. Because of difficulties in permanently placing glass slides horizontally, silica accretion rate measurements were also not conducted for this microfacies. Any silica that deposits on vertically oriented slides would have represented Microfacies 2 conditions.

SEM revealed that the upper surfaces of Microfacies 3 sinters are colonised by bacilli (1-8 µm long) (Figure 16.1). Silicification of these microbes was patchy, with nanospheres (<100 nm) of silica precipitating onto the uppermost portions of cells, whereas those beneath were unsilicified (Figure 16.2). Vertical sections of the sinters reveal silica with a predominantly massive, vitreous texture (Figure 16.3). Cavities, cemented diatoms, and spherical cells (2-10 µm in diameter), resembling those from the green living mats, are the only discernable features in these sections (Figure 16.3-16.4).

On vertical portions of the sinters that have subaerial rims (Figure 15.3-15.4), diatoms clustered together in large groups with the tips of their tests aligned perpendicular to the approximate air-water interface (Figure 16.5). These clusters also became progressively silicified and incorporated into the deposit (Figure 16.6).

Microfacies 4 – Thin Siliceous Rim Sinter

Sinters from Microfacies 4 (Figure 3) are characterised by thin (~2 mm thick), orange, cup-like rims formed on small (<2 cm in diameter) pumice clasts that rest upon moist sandy substrates well above the outflow channels (Figure 17.1-17.3). The deposits can reach heights of 1-6 mm, with rare or absent spicular textures. Vertical sections through the rims reveal thin internal laminae (~0.5 mm thick) that are convex rather than horizontal (Figure 17.4). The outer sides of these sinters are typically covered with green algal mats. Rarely, anastomosing ridges occur on the inner sides of the sinter rims (Figure 17.4).

Thermal water was not observed to wash, splash, or spray onto these deposits. Small holes dug into the sandy substrates revealed warm thermal pore water seeping upward through the sands (Figure 17.3). This water may derive from surrounding thermal discharges nearby, or from small unidentified vents underneath the sandy alluvium. The temperature of the seeping water was recorded to range from 45-67°C (Figure 4.1). Silica accretion rates in this microfacies averaged only 9.5 x 10^-3 g over 67 days (Figure 4).

Sinter surfaces are covered by biofilms of diatoms (Figure 18.1-18.2), spherical microorganisms (2-6 µm in diameter) (Figure 18.3-18.4), and bacilli (1-2.3 µm long) (Figure 18.5). It is likely that the spherical cells are also algae, belonging to the green mats that cover the sinter rims on the margins of the deposits. Microbial silicification is likewise patchy, with some microbes completely covered in silica spheres, while others nearby are unsilicified. The bacilli also had the tendency to form vertically upright microcolonies, but were rarely observed to be silicified (Figure 18.5).

SEM showed that vertical sections of these sinters comprise a massive, vitreous texture with no visible laminae. Silicified spherical cells (2-6 µm in diameter), often at the endospore stage, were commonly incorporated into the sinter (Figure 18.6). However, these organisms do not form layers as in Microfacies 2, but are scattered across the sinter.