View attachment 426823On last Sunday's work-party, John (Upstarts) asked me what causes the 'oily film' that occurs in small patches on the stagnant water around the edges of the flashes. These patches have exactly the same 'rainbow' appearance as when petroleum spirit is spilled on puddles of rain-water (
i.e. colourful interference patterns from two reflecting surfaces separated by small distances, of similar magnitude to the wavelength of the incident light). See image attached.
However, the cause in this case is
entirely natural and is directly related to the smelly 'marsh gas' that often arises when these areas are disturbed. Both are caused by anaerobic decomposition of accumulated vegetation (
i.e. plants decaying underwater in the absence of oxygen). Methanogenic bacteria digest this organic matter and convert it into methane, which escapes as gas bubbles into the atmosphere. Similar bacterial processes can also produce higher molecular-weight
liquid hydrocarbons, which float/collect on the surface of water. Chemically, these are indeed similar to the alkanes (paraffins) and alkenes (olefins) found in petrol.
Another (rarely seen) phenomenon is called
'ignis fatuus' , whereby similar anaerobic processes can form pyrophoric (spontaneously combusting)mixtures - thought to contain reduced phosphorus compounds (phosphine/diphosphane). These are said to have an eerie appearance and are thought to be responsible for the malevolent swamp sprites of folklore, known as 'Will o the Wisp' (and other superstitious phenomena associated with marshes/wetlands).
Personally, I'd love to see a 'Will o Wisp' - maybe on an 'all-night Baillons Crake' survey this spring/summer - eh, Des? :t: