Fossil & Subfossil diatoms in Ireland
There seems to be a lack of accord as to when a diatom is defined as subfossil or fossil so the following may work here:-
Fossil - pre Holocene - earlier than 8000bc
Subfossil - Holocene - 8000bc to anthropocene (18th century onwards)
Recent - Anthropocene - (late 18th cent. to present)
The above presupposes that we accept the term Anthropocene which describes the period in the late 18th century when anthropogenic activity first began to have a significant effect on Earth's climate and ecosystems. This date coincides with James Watt's invention of the steam engine in 1784 and the Industrial Revolution.
David Richman forwarded the following definition which is tighter:-
'Sub-fossil usually implies early to mid Holocene (including Pleistocene), but basically means that the material has not become mineralised and is old, but not altered in chemical composition or hardened into rock (in this case diatomite). I think there is a bit of a fuzzy area in the Pliocene-Pleistocene and thus (because mineralisation is a slow process) there may be some argument over this. Diatom remains are "mineralised" from the start- being made essentially of opal (hydrated silicon dioxide), but, as I understand it, you can gauge this by the lack of diatomite rock (usually relatively fine-grained ) formation, although the diatoms may be layered.'
The industrial archaeology of diatomite working along the Lower Bann river is notable. For over a century diatomite was extracted, made into bricks locally or exported for other uses such as making insulation bricks, abrasives (car polish, toothpaste, etc.) and filters (cider, beer production). The first diatomite factories were built at Newferry in 1906 and Portglenone in 1912. During the second World War large quantities of diatomite were exported to English munitions factories to act as an absorbent for explosives (nitro glycerine). Commercial extraction virtually ceased in the 1960s and finally ended in the mid 1990s.