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|Title:||Microplankton changes through the early Silurian Ireviken extinction event on Gotland, Sweden|
|Authors:||Gelsthorpe, David Neil.|
|Presented at:||University of Leicester|
|Abstract:||This thesis documents and analyses the extinction and origination patterns of acritarchs and prasinophyte algae at the Llandovery/Wenlock boundary in the Lower Silurian on the island of Gotland, Sweden. Closely spaced samples were collected from two sections: Lusklint 1 and Lickershamn 2, spanning the upper part of the Lower Visby Beds and almost all of the Upper Visby Beds. Errors associated with the palynological processing technique have been assessed and a new photographic technique has been developed. Five new species are described.;At least eight extinctions affecting the conodont record have been reported at this level (named the Ireviken Extinction Event (Aldridge et al. 1993, Jeppsson 1997)). The Ireviken Event has been interpreted as an example of the change from a P to an S climate state (Jeppsson 1993).;The data show a significant turnover in the phytoplankton at this time, with most of the extinctions at the end of the event (86.3% in the top four metres of the Lusklint 1 section), after many of the conodont extinctions had already taken place. The originations are more numerous than the extinctions (54 species originate at Lusklint 1 as opposed to 44 that became extinct) and they are distributed through the whole of the Ireviken event. There is an uneven distribution across the event with more originations in the Lower Visby Beds forming a convex pattern.;The two section analysed were compared using graphic correlation, but palynomorph range end data show considerable scatter. Peaks in the number of palynomorphs per gram of sediment suggest that the two sections completely overlap. Deposition of the thickest bentonite produced a marked drop in the number of palynomorphs per gram of sediment, but a marked rise in numbers in the following few centimetres, probably caused by a fertilization effect. d13C values at Lusklint 1 remain stable in the Lower Visby Beds, but show a marked gradual rise in the Upper Visby Beds. The d18O values for the same interval vary little.;The P and S model does not successfully explain all the changes recorded, but is the most comprehensive model available at this time. Additions to the model, incorporating planktonic dimenthyl sulphide production and iron fertilization are presented.|
|Rights:||Copyright © the author. All rights reserved.|
|Appears in Collections:||Theses, Dept. of Geology|
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