The Ancient Environment and Modern Analogues:

Large-scale features

 

The spatial distribution of the biota with respect to the hot springs, apart from available soil moisture, nutrients and sunlight, is dependant primarily on two environmental factors: water temperature and pH. Both these factors are important, especially in hot spring areas, because different forms of life have specific tolerances to both, and thus temperature and pH tend to govern which plants and animals may flourish. These constraints would also have been applicable at Rhynie 400 million years ago.

The following table gives a general idea of the upper temperature limits of a number of animals, plants and micro-organisms (after Brock 1994):

Group Upper temperature limits (oC)
Animals
Fish 38 
Insects 45-50
Ostracods (crustaceans) 49-50
Plants
Vascular plants 45
Mosses 50
Eukaryotic micro-organisms
Protozoa 56
Algae 55-60
Fungi 60-62
Prokaryotes
Bacteria

Cyanobacteria (O2 producing photosynthetic bacteria)

70-73

Other photosynthetic bacteria (do not produce O2)

70-73

Heterotrophic bacteria (use organic nutrients)

90
Archaea

Methane-producing bacteria

110

Sulphur-dependant bacteria

115

Eukaryotic organisms are unable to adapt to high temperatures, the upper limit (for fungi) being 60-62oC and for plants and animals less than 50oC. Above 62oC only prokaryotes may live, of which the photosynthetic, thermophyllic, cyanobacteria can only tolerate temperatures up to 73oC. At the highest temperatures, over 100oC, where water is boiling, only the heat-adapted, hyperthermophyllic Archaea survive.

Not surprisingly, therefore, in these hydrothermal areas the thermal gradient can often be visually identified by the biota present. The hot pool in the inset right is a typical example, the clear blue water on the left represents the hotter parts of the pool (below boiling, at around 75oC), probably colonised, if at all, by heterotrophic bacteria and Archaea. The narrow yellow to orange zone (a maximum distance of 55cm) ranges from 72oC on the left to 46oC on the right and is colonised primarily by photosynthetic cyanobacteria. Beyond this to the edge of the pool the temperature drops to approximately 30oC in the 'milky white' water to 24oC at the waters edge where vascular plants, primarily sedges, abound. Notice the rapid change in temperature over less than one metre  to the pools edge and, therefore, how close plants and animals can actually live to these hydrothermal features. The pH also tends to become more alkaline, from a neutral pH 7 in the centre of the pool to pH 9 at the edge.

Similarly, changes in biota and the colour of cyanobacterial mats are also evident in overflow channels from geysers and hot springs, marking the temperature limits of the biota and hence the decrease in water temperature (see inset below).

 

Seismograph pool, Yellowstone National Park

Above: Seismograph pool at West Thumb Geyser Basin, Yellowstone National Park, showing the thermal gradient as the water temperature drops from the centre of the pool (left) to the waters edge. The yellow to orange band represents a sharp drop between 72 - 46oC, the colour caused by the growth of photosynthetic cyanobacteria.

Heart Spring

Above: Heart Spring, near the Lion Geyser Complex (centre right background), Yellowstone National Park. This image shows the changes in colours, from pale yellow, to orange to brownish green, created by cyanobacteria in the overflow channels, marking the progressive drop in water temperature from the spring. Notice how close plants are growing to the spring on the right of the photograph. Sinter is being precipitated around the edge of the spring and over the water surface (the bright white ledges) and also on the overflow apron in the centre left of the image. The foreground comprises degraded, desiccated and brecciated sinter.

Sinter colonised by Triglochin, Yellowstone National Park

In cooler areas of overflow aprons and around pools, plants can grow in variable numbers and in variable diversity depending upon the substrate, available nutrients, moisture and pH. For example, one of the earliest vascular plants to colonise sinters in many areas of Yellowstone National Park is Triglochin and typically occurs as patches of monotypic stands (see insets left and below left). Even as these plants are growing their roots and bases of stems may be coated in precipitated silica. Old exhumed sinters occasionally show these and similar plants preserved in silica (click on inset left for an example). Other mature sinter surfaces may become incorporated into organic and mineral-rich soils and are capable of supporting more diverse biotas.

 

Left: Sinter surface, with a thin veneer of standing water, colonised by a stand of Triglochin. The bases of the plant stems often show a thin powdery coating of precipitated silica (Click on the image for examples preserved in sinter). Photograph taken near the main overflow channel from Giantess geyser, Yellowstone National Park.

 

Often, during periods of heightened hydrothermal activity, together with changes in the subterranean 'plumbing system', water levels of springs may rise, and overflow courses may alter. During such events, areas colonised by plants, previously away from hot water discharges, may suddenly become inundated, killing the plants and any fauna that cannot escape. Rapid precipitation from cooling, flowing, silica-charged waters can, and do, preserve such 'death assemblages' in sinter.

Even in cool streams fed by overflow channels from springs and geysers 10's to 100's of metres up-steam, the evaporation of waters still super-saturated with silica will deposit silica crusts on stream beds and around the bases and roots of plants (Trewin et al. 2003) (see inset right).

 

 

Silicification at the bases of plants along the edge of a stream

Above: Botryoidal clusters of silica (white) precipitated around the bases of stems and roots along a stream near White Dome Geyser, Yellowstone National Park.

Ponded run-off from Daisy geyser, Yellowstone National Park

In some areas, the cooled overflow from geysers and hot springs may become ponded forming shallow, ephemeral bodies of water (see inset left), forming a variety of wetland habitats that, together with tolerant species of plants, may also be home to various aquatic invertebrates.

Hot springs and geyser vents may become dormant due to changes in the subterranean 'plumbing' of the hydrothermal system and may thus become 'cool pools' capable of supporting a variety of aquatic flora and fauna (see inset below).

 

Left: Ponded run-off from Daisy geyser (middle distance) creating a localised  wetland habitat on a degraded sinter surface, Yellowstone National Park.

 

Pool with surface microbial mat

Above: A hot spring with an outflow channel (bottom centre), Yellowstone National Park. When the photograph was taken, the centre of this pool had a maximum water temperature of 54oC, well within the temperature range for cyanobacteria, and also the upper temperature range for algae and protozoans. The surface of the pool is colonised by a thick, rubbery cyanobacterial mat, buoyed by trapped gas bubbles (click on image for a close up!). Plant stems are held rigid within this mat and it is cohesive enough to support the weight of small invertebrates. The white patches on the mat are comprised of very fine, precipitated silica.

In the few examples of habitats and subenvironments given here, given the right conditions and a continuous influx of silica-charged water (cooled or otherwise), the biota and deposits of all have the potential to become silicified and incorporated in sinter for inclusion in the rock record.

From the variety of textures and biota found within individual beds of the Rhynie chert, most, if not all of the subenvironments mentioned here, or their 400 million year old equivalents, are probably represented.