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Title: Experimental and computational studies on foraminiferal calcite
Author: Birse, S. E. A.
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 2005
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In this thesis, the effect of dissolution on the chemistry and structure of perforate planktonic foraminiferal tests was investigated. Atomistic simulations were undertaken to study the energetics of magnesium ions in calcite and their structural ordering in the {104} surface. Shell mass, δ18Oc and Mg/Ca measurements were obtained on core-top tests of foraminifera from depth transects in the Indian and Pacific oceans. The degree to which dissolution biased the temperature determined from δ18Oc and Mg/Ca measurements was found to be similar for G. sacculifer and G. ruber but greater for the Mg/Ca-temperature estimates for P. obliquiloculata and N. dutertrei. Corrections to the Mg/Ca ratios based on core-top depth, bottom water carbonate ion concentration, and shell mass were developed. For all species, bottom water carbonate ion concentration provided the best correction for the dissolution artifact. Crystallinity of the planktonic foraminiferal tests was investigated using x-ray powder diffraction from the {104} plane of calcite. Species offsets in crystallinity were attributed to differences in test structure, in particular the proportions of primary to secondary calcite composing the test wall. A correlation between tests which had undergone increased dissolution and a narrowing of the 104 Bragg peak was observed. This change was attributed to the preferential removal of smaller crystallites (the primary calcite). The crystallinity of undissolved tests correlated with growth rate. Faster test calcification could broaden the 104 Bragg peak because overall smaller crystallites were deposited or because more primary calcite was initially deposited. An empirical potential set was generated for calcium carbonate. Trends in metal cation defect energies between calcite and aragonite stemmed from the differences in bulk geometries. The {104} calcite surface, in vacuo and hydrated, was generated from the same potential set. Surface relaxation of the in vacuo surface was investigated by calculating within layer polarization and between layer strain.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available