The ancient environment and modern analogues

By combining all the aspects of the Rhynie geology from the structural setting and sedimentology to the palaeontology and palaeobotany we can interpret the environment in the Rhynie area some 400 million years ago. As we have seen previously, the cherts and associated sediments were deposited in a narrow northeast-southwest trending basin or half-graben with a hinterland of Dalradian metamorphic and Ordovician basic igneous rocks (see Geology and Setting ). The basin sediments and rocks indicate a continental setting; generally an alluvial plain with an axial river system, floodplains and localised ponds and lakes. There appears to have been contemporaneous volcanic activity with the deposition of tuffs and the localised extrusion of andesitic lavas. We know, beyond a doubt, that the chert beds represent the surface expression of an active hydrothermal system which appears to have been fed by conduits along an active fault zone bounding the north western edge of the basin. From the suite of textures and the differing biota within individual chert beds it appears that many different sub-environments were present during the deposition of the chert, but how do we deduce these sub-environments?

We do not base our conclusions of the fossil environments on observations of the Rhynie chert alone. To help us better understand the palaeoenvironments represented by the cherts at Rhynie, modern analogues or comparative environments are particularly instructive.

Modern Analogues

We are fortunate in that there are a number of hydrothermally active areas in virtually all of the worlds volcanic regions that can be studied as possible analogues to the Rhynie chert, though only a few have the necessary requirements for geyser activity. The better known areas where geysers are seen include Rotorua in New Zealand and Yellowstone National Park in the USA. Although the structural and volcanic setting is rather different in both these areas compared with that of Rhynie, not to mention the differences in contemporary biotas, in terms of the actual surface processes and sub-environments Rotorua and particularly Yellowstone provide quite good modern analogues. By studying the sub-environments associated with these areas, the type of biota and the sediments and their textures, we can compare and contrast with similar features seen in the Rhynie chert and thus begin to interpret the ancient palaeoenvironments (e.g. Trewin et al. 2003 ).

For the purposes of this resource, we can look at these modern analogues in two main ways; in terms of 'large-scale features' such as the spatial arrangements of biota, their associations and style of preservation with respect to the hot springs and their deposits, and also in terms of 'micro-features' such as the textures formed in the modern sinters as observed under a microscope.

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.

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. 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 75^oC), probably colonised, if at all, by heterotrophic bacteria and Archaea. The narrow yellow to orange zone (a maximum distance of 55cm) ranges from 72^oC on the left to 46^oC on the right and is colonised primarily by photosynthetic cyanobacteria. Beyond this to the edge of the pool the temperature drops to approximately 30^oC in the 'milky white' water to 24^oC 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).

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

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. Photograph taken near the main overflow channel from Giantess geyser, Yellowstone National Park. Surface view of silicified plants in sinter. The 'rosette' shapes were formed from dead stems (probably of grasses) from earlier stages of growth, collapsed around an inner zone of growing shoots. Potts Basin, 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 (see 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.

Botryoidal clusters of silica (white) precipitated around the bases of stems and roots along a stream near White Dome 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).

Ponded run-off from Daisy geyser (middle distance) creating a localised wetland habitat on a degraded sinter surface, 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 insets below).

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.

Thick cyanobacterial mat growing across the surface of a hot spring pool in Yellowstone National Park. The mat is buoyed up by trapped gas bubbles and is cohesive enough to support the weight of small invertebrates such as the dragonfly in this image. The plant stems on the left of the image are held rigid within the microbial mat.

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.

Micro-features

Looking at thin sections of various types of modern sinters and comparing them with textures and preservation styles seen in thin sections of the Rhynie chert can also be instructive:

Note: The blue colour in the following images represents pore space in-filled by blue-dyed epoxy resin.

Straws of Aglaophyton and partially decayed plant stems held in an open framework of amorphous, 'clotted' chert. The two straws in this sample of Rhynie chert exhibit similar geopetal textures to that in the modern sinter shown in the image on the left (scale bar = 3mm).

Silicified, partially decayed plant stem missing much of the cortex, the resulting void is traversed by a mesh of silicified microbial filaments. Stream bed sinter near White Dome Geyser, Yellowstone National Park (scale bar = 250μm). Decayed stem of Aglaophyton with the xylem strand and remainder or the internal cells collapsed to the base of the straw. The resulting void is traversed by a mesh of filaments (probably fungal hyphae) (scale bar = 1mm).

A plant axis that has partially decayed and shrunk away from the cuticle before being silicified. Sinter from Elk Flats, Yellowstone National Park (scale bar = 500μm). Partially decayed axes of Rhynia gwynne-vaughanii that have shrunken away from the cuticle before being silicified (scale bar = 2mm).