I like these subjects and I can contribute a little peace: in fact are the White cliffs of Dover and on the French side Cap Blanc Nez part of the Cuesta landscape of the Basin of Paris: this Cuesta Landscape covered almost the whole northern part of France until Luxemburg but also influenced England as far as the Cotwolds.
I'd think diatoms would be as readily one source of silica as radiolarians. A quick check around did not reveal any definitive studies to say exactly what makes up the flint, microscopically, but there probably has been such work done. I think sponge spicules are fairly well established as an important contributor of silica to the flint, but it would be a little surprising if silica-forming microorganisms did not also contribute. One sees references to generic "siliceous plankton" which would include diatoms. There's also a moderate chance that some silica entered the system as very fine clastic sediment, rarely but episodically; that's one way of addressing the layered nature of a lot of the flint - either as relatively thin beds, or as strings of nodules. Alternatively, if the silica source is just organisms, the layering might be related to episodic temperature/depth/chemistry changes in the system that restricted the coccolithophore blooms and/or enhanced the possibility of silica precipitation. I've seen that a slight excess of iron in such systems sometimes gives preference to diatoms over coccolithophores. All that is speculation (reasonable I think) but I get the feeling that a consensus explanation for the silica layers has not been reached.
I have a flint specimen from these deposits that has a cavity lined with quartz crystals! very uncommon!
Nice!
I like these subjects and I can contribute a little peace: in fact are the White cliffs of Dover and on the French side Cap Blanc Nez part of the Cuesta landscape of the Basin of Paris: this Cuesta Landscape covered almost the whole northern part of France until Luxemburg but also influenced England as far as the Cotwolds.
Excellent, thanks for sharing this.
No surviving chalk here in Scotland. In S England I've not walked enough chalk to find any urchins, but have come across ammonite casts in Dorset.
Very interesting. It would be fun to get some of this chalk to look at with a microscope. Could diatoms have been one of the sources of silica?
I'd think diatoms would be as readily one source of silica as radiolarians. A quick check around did not reveal any definitive studies to say exactly what makes up the flint, microscopically, but there probably has been such work done. I think sponge spicules are fairly well established as an important contributor of silica to the flint, but it would be a little surprising if silica-forming microorganisms did not also contribute. One sees references to generic "siliceous plankton" which would include diatoms. There's also a moderate chance that some silica entered the system as very fine clastic sediment, rarely but episodically; that's one way of addressing the layered nature of a lot of the flint - either as relatively thin beds, or as strings of nodules. Alternatively, if the silica source is just organisms, the layering might be related to episodic temperature/depth/chemistry changes in the system that restricted the coccolithophore blooms and/or enhanced the possibility of silica precipitation. I've seen that a slight excess of iron in such systems sometimes gives preference to diatoms over coccolithophores. All that is speculation (reasonable I think) but I get the feeling that a consensus explanation for the silica layers has not been reached.
Thank you for the additional information. I think that diatoms also have blooms which could also be consistent with layering.
Fun and highly informative!
Good overview.