This article is concerned with colours due to cyanobacteria and algae at the entrance zones of caves in carbonate rocks such as:
- Limestone
- Aeolian calcarenite
- Marble
The rocks forming a typical limestone or marble landscape appear to be shades of grey, exept where there is a cave entrance. The colours of rocks and calcite deposits seen around cave entrances are usually shades of orange, brown, green and grey.
If you examine the surface rocks, you may notice the grey is due to a surface coating of small plants: algae, lichens and bryophites. Cyanobacteria may also be noticed.
If you stand at the cave entrance, you may notice different colours corresponding to where rainwater falls on the rock and where it stays dry most of the time. There are other colours, too, correspoding to the amount of light that falls on the rock. As you go further into the cave, away from the influence of the weather, there is a tendency for the bedrock to appear first grey (same as outside) then orange, then white (in the dark zone). Even speleothems take on these colours. Exceptions to this are where something has rubbed the surface coating away (eg streamway, animal movements) or where it has been coated with mud.
See also "The cave with the pink ceiling and the white dots" (note on colours in the Abercrombie Arch entrance).
Let's examine some of the colours in more detail (hopefully your browser makes sense of it! I am using X colour names):
| Group | Colour | Moisture Levels | Light levels | Closeness to Entrance | Surfaces | Community | Discussion |
|---|---|---|---|---|---|---|---|
| A | Grey (gray69) | Only wet during rain; follows drip lines. | Often in direct sunlight | Right at the entrance, where rainwater runs from the surface rocks. | It usually occurs on both vertical and horizontal surfaces where there is no soil. | Most likely a community of lichens, algae and cyanobacteria. | It needs both light and rainwater to maintain the coloration. This is the same as the limestone surface rock coloration. Under the microscope one sees light calcite grains and dark plant colonies. |
| B | Black | Usually damp; follows drip lines. | Often in direct sunlight | Right at the entrance, where rainwater runs from the surface rocks. | It usually occurs on vertical surfaces where there is no soil. | Most likely algae | It needs both light and high moisture levels to maintain the coloration. Ferns sometimes grow on it. |
| C | Dark green (ForestGreen) (varies) | Usually wet and dripping | Either direct or indirect sunlight where the light level is enough to see by without artificial light. | Close to the entrance | It usually occurs on both vertical and horizontal surfaces where there is no soil. | Most likely algae and bryophytes (mosses and liverworts). | It needs light and wet conditions to maintain the coloration. If moisture level is too low, community B tends to form. If the coating is thick enough, bryophyte communities predominate, followed by ferns. |
| D | Light Green (PaleGreen) | Usually dry | Either direct or indirect sunlight where the light level is enough to see by without artificial light. | Close to the entrance | It occurs on both vertical and horizontal surfaces | Most likely bryophytes. | It needs light to maintain the coloration. If moisture level is too high, community C tends to form. |
| E | Light blue-green (LightSeaGreen) | Periodically wet from drips; usually dry | Indirect sunlight where the light level is enough to see by without artificial light. Also direct sunlight at times of the day or year. | Anywhere in the entrance zone | Leading (light side) edges of craybacks and other speleothems. | Cyanobacteria. | Promotes calcite deposition due to organism's absorption of CO2 during day. Presumably there is not enough light over a year to promote the growth of more complex organisms, alternatively calcite deposition may be too rapid to allow other organisms such as bryophytes to establish. |
| F | Darker blue-green (CadetBlue) | Periodically wet from drips; usually dry | Indirect light where the light level is enough to see by without artificial light. | Anywhere in the entrance zone | Shaded parts of craybacks and other speleothems. | Cyanobacteria. | Calcite deposition rate is less than that of E due to lower light levels and lower photosynthesis. |
| G | Pink Brown (RosyBrown) | Usually dry | In dimly lit areas. | Anywhere in the entrance zone. | Mostly on ceilings and upper walls. | This may be another species of cyanobacteria or red algae better adapted to darker, dry conditions. | This is the pink roof in Abercrombie Arch (NSW). Usually associated with groups E and F, depending on light levels. Apparently red chlorophylls work at lower light levels compared with green chlorophylls, but are easily out-competed by the more efficient green chlorophylls at higher light levels. |
| H | Brown-orange (DarkSalmon) | Damp | In darker areas. | Anywhere in the entrance zone, also in dark zone chambers close to the surface. | Mostly on speleothems. | Most likely humic acid colouring; possibly with some lublinite-forming soil bacteria. | Occurs with I and J |
| I | Light Brown-orange (peachpuff) | Usually dry | Can be direct sunlight right through to dark zone. | Anywhere in the entrance zone. | It usually occurs on both vertical and horizontal surfaces where there is no soil, typically on bedrock. | Seems to be a red-brown organism e.g. red algae with fine powdery calcite. | Occurs with H and J. Under the microscope, appears to be colonies of reddish organisms and an off white deposit. My tests with HCl infer it is mostly CaCO3 with a small residue of red-brown material. |
| J | Off white (Ivory) | Ranges from dry to wet and dripping | Can be direct sunlight right through to dark zone. | Often where there is a breeze. | Can occur on any surface, but more likely where there is seepage (porous substrate). | Deposits in direct sunlight may contain some algae. Cave deposits may contain lublinite-forming soil bacteria. | Travertines may occur anywhere in light areas; moonmilk tends to occur in darker areas. Both depend on a source of water containing high carbonate ion levels. Outgasing of CO2 results in calcite deposition as "moonmilk"; often as needle form calcite (lublinite). |
Cosmetic update, January 2006. Content updated 17th February 2006.