| mhorn | Дата: Thursday, 12.03.2015, 08:54 | Сообщение # 1 |
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| Jordan, N., Allison, P.A., Hill, J., Sutton, M.D. (2015) Not all aragonitic molluscs are missing: taphonomy and significance of a unique shelly lagerst€atte from the Jurassic ofSW Britain. Lethaia, DOI: 10.1111/let.12126.
http://onlinelibrary.wiley.com/doi/10.1111/let.12126/epdf
The Blue Lias Formation at Lyme Regis (Dorset, UK) includes an exceptional pave-ment of abundant large ammonites that accumulated during a period of profoundsedimentary condensation. Ammonites were originally composed of aragonite, anunstable polymorph of calcium carbonate, and such fossils are typically prone to dis-solution; the occurrence of a rich association of aragonitic shells in a condensed bed ishighly unusual. Aragonite dissolution occurs when pore-water pH is reduced by theoxidization of hydrogen sulphide close to the sediment-water interface. Evidence sug-gests that, in this case, the oxygen concentrations in the overlying water column werelow during deposition. This inhibited the oxidation of sulphides and the associatedlowering of pH, allowing aragonite to survive long enough for the shell to be neomor-phosed to calcite. The loss of aragonite impacts upon estimates of past biodiversityand carbonate accumulation rates. The preservational model presented here impliesthat diagenetic loss of aragonite will be greatest in those areas where dysoxic-anoxicsediment lies beneath an oxic waterbody but least where the sediment and overlyingwater are oxygen depleted. Unfortunately, this implies that preservational bias througharagonite loss will be greatest in those biotopes which are typically most diverse andleast where biodiversity is lowest due to oxygen restriction
Middle Jurassic - Lower Cretaceous ammonites & aptychi
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