The Ancient Environment and Modern
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
||Upper temperature limits (oC)
Cyanobacteria (O2 producing photosynthetic bacteria)
Other photosynthetic bacteria (do not produce O2)
Heterotrophic bacteria (use organic nutrients)
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).
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
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.
||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).
Above: Botryoidal clusters of silica
(white) precipitated around the bases of stems and roots along a stream
near White Dome 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).
run-off from Daisy geyser (middle distance) creating a localised wetland
habitat on a degraded sinter surface, Yellowstone National Park.
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.