The Coccoliths of Portsdown Hill
Most of the Chalk of Portsdown Hill is made up of coccoliths, the microscopic 'armour' of a sea-dwelling organism.
The following discussion covers:
- Introduction to coccoliths
- Coccolithophore structure
- Coccolithophore Life (Style)
- Coccolithophore Death (and Chalk Formation)
- The Geological significance of coccolithophores
- Coccolithophores Today
Introduction to coccoliths
A coccolith is the tiny calcite plate, that (with others), forms the "armour-plating" of a single-celled photosynthesising organism known as a coccolithophore. It is a biomineral; i.e. it is a mineral that, whilst inorganic itself, was produced as an integral [part] of a biological structure.
It is so small that an electron microscope is needed to see it.
Today's Emiliania huxleyi produces coccoliths that are approximately 2.5 x 10-6 metres in diameter. (For comparison, the wavelength of visible light is in the range 0.4-0.7 x 10-6 metres, and a pinhead is about 2000 x 10-6 metres in diameter.)
Despite this microscopic size at an individual level, coccoliths constitute the bulk of chalk, and the whole of the Cretaceous period, a span of Earth history covering 78 million years, is named for that chalk (Cretaceous = Latin for 'chalk').
The coccolith itself is named for the shape of the whole organism (which is commonly spherical); "kokko" being the Greek for "seed", and "lithos" the Greek for "stone".
It not only [forms] a significant rock that has shaped the landscape of Southern Britain and elsewhere; but can also affect global climate; so has been much studied.
Various species of the organism have been abundant since the Jurassic period (208 -144Ma ago), and they thrive today.
However; they were most [prevalent] in the Cretaceous, and also, at that time, other factors [encouraged|promoted|allowed] the formation of chalk - which is not an inevitable product, even where coccolithophores abound.
Coccolithophore structure
The coccolithophores that live in today's oceans take many different forms, with their calcite exteriors forming into spines or a variety of shapes (such as baskets, trumpets, and arrow-heads).
Similarly, those of previous time showed huge variation in their design.
The external calcite plates overlap and interlock, and although this interlocking is random, it forms a robust and relatively strong structure around the living cell inside.
There may be a single layer of coccoliths around the cell, or many; so each organism may have just one, or hundreds, of calcite plates around it.
The cells extrude the calcium that forms the plates, and whilst different varieties may have different forms, the most common Cretaceous coccolithophores appear to have been roughly spherical in shape, with a single layer of these plates around it. This complete spherical plating is thus known as a coccolithosphere
These single-celled organisms are tiny - the individual plates being of the order of 10µm (microns) in diameter. This means that if laid edge to edge in a single row, a centimetre-long string would contain about 1000 individual coccoliths (100 per millimetre), which is about 1/250th the size of a grain of sand.
A single plate is flat and disk-like, and a lump of chalk the size of a sugar lump contains around 80 billion individual coccoliths.
Coccolithophore Life (Style)
Coccolithophores live in the photic zone (the surface waters, where sunlight reaches) and are photosynthesising; so are at the bottom of the marine food chain.
Some have flagella (whip-like appendages) so unlike plants, are capable of movement; furthermore, they don't simply float around, but can swim.
Despite their [lowly status | position in ???? ], and although they are photosynthesising, some have been known to eat bacteria.
Because they have characteristics of both plants and animals, a new Kingdom was defined for them, the Monera Kingdom. However, they are sometimes referred to as 'algae', which is acceptable for 'common usage'.
The coccolith plates were probably shed during life as well as on death; so many individual plates formed and detached during life, adding to the rain of calcite to the sea floor that included the whole organism on death.
Being tiny, their life-span was short - 1 day to a few weeks.
Coccolithophore Death (and Chalk formation)
On death, the calcite plates sank to the sea floor. In cold waters, they would dissolve; and near continental landmasses, where terrigenous (land-derived) sediments also accumulated, they would become swamped by the clay and silts; so could not accumulate in sufficient concentrations and purity to form chalk.
However; in the conditions of the Cretaceous - warm, with vast expanses of shallow ocean and little exposed land, coccoliths accumulated. As this water-laden ooze was covered with the weight of more microscopic debris, the water was squeezed out and increasing pressure led to the lithification of the calcareous ooze in Chalk - some beds of greatly-compressed plates being 100's meters thick, that we see today.
The Geological significance of coccolithophores
Apart from forming the chalk of our landscape, coccoliths are particularly useful in [marking] geological time.
Unlike plants and land-based fossils, they are not constrained by area, and evolve worldwide at the same time. This allows long-distance correlation and the determination of geological ages worldwide.
As they reproduced in such huge numbers, any evolutionary mutation would be spread rapidly, carried by ocean currents to every marine area on Earth.
Despite their tiny size, coccolithophores are one the largest producers of calcium carbonate on Earth today, and probably have been since the Late Jurassic.
Their was the Cretaceous, when conditions combined to [allow] Chalk formation - very high sea levels and temperatures meant that there were huge areas of warm, shallow continental shelf ocean well away from exposed land.
The vast number of carbonate plate constituting the Chalk (from the calcite plates) is a form of calcium carbonate - and all that organic carbonate was produced by the photosynthesis of carbon dioxide, dissolved into the sea from the atmosphere. The level of CO2 in the atmosphere was then much higher than today (although we are trying!)
This high CO2 level led to higher temperatures, and was partially responsible for the very high sea levels (as there were no ice caps, and thermal expansion increased the volume of the water). (For more about this, please refer to The Cretaceous ... ).. This high sea-level mean that much low-lying land was submerged, so there were few and isolated areas of land from which terrigenous sediments were dumped into the sea. Thus, the formation of pure white chalk as we see today at Portsdown Hill was [facilitated|allowed]
Coccolithophores today
Coccolithophores are abundant throughout the oceans today, and can be found from tropical to sub-arctic waters, even where the water temperature drops below 0°C.
However; even with time, conditions in today's planet are not ideal for the formation of chalk, which would only form in certain restricted areas, mainly around places of shallow rift systems, away from areas accumulating terrigenous sediment, and fairly warm - such as [ where?! ] These areas can be seen on sea-sediment maps, where it shows as a nanofossil (or globigerina) ooze! [find one and give a link - s'ton uni?] ]
"Blooms" of coccoliths today can be seen from satellites, as huge white patches in the sea, across the world's oceans, and can be found in all but polar waters.
For any comments, suggestions or contributions, please e-mail me at: portsdown@bbm.me.uk