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Title: "Vital effects" in the 13C-18O clumped isotopic composition of marine macro-invertebrates : exploring biologically mediated disequilibrium effects in echinoids and cephalopods
Author: Davies, Amelia Jane
ISNI:       0000 0004 7969 8738
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2019
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The stable oxygen isotopic composition of marine carbonate has historically been widely used for paleoclimate reconstruction (Emiliani 1955; Zachos 2001). This technique however applies an a priori assumption of δ18O of the seawater. Furthermore, while the stable oxygen isotopic composition of some taxa faithfully reflects both the temperature and the δ18Oseawater other taxa show specific discrepancies known as "vital effects" (Epstein et al. 1951). The carbonate clumped isotope paleothermometer examines thermodynamically dependent "clumping" of 13C and 18O within the carbonate crystal lattice. This allows calculation of carbonate formation temperature independent of the δ18O of seawater (Ghosh et al. 2006; Eiler 2007). Initially inorganic and biogenic calibrations of the clumped isotope thermometer appeared reasonably congruent (Eiler 2011, Ghosh et al. 2006; Tripati et al. 2010a). Further studies however revealed vital effects in the clumped isotopic composition of corals (Thiagarajan et al. 2011; Saenger et al. 2012) with potential vital effects indicated in molluscs (Henkes et al. 2013). In this study we assess how robust the clumped isotope thermometer is to biologically mediated non-equilibrium precipitation (vital effects) by (1) measuring the clumped isotope composition of the previously uninvestigated echinoderm group and (2) verifying and further quantifying offsets in the clumped isotope composition of modern cephalopod molluscs. This aim necessitated further refinements CO2 purification for clumped isotope analysis, also presented within this thesis. The results of this thesis indicate that non-equilibrium signatures in the clumped isotope composition of biogenic materials are more common than initially assumed (Eiler, 2011). Furthermore the manifestation of vital effects does not to occur by a single mechanism, but varies dependent on biomineralization pathway. This has significant implications for the way in which the clumped isotope composition of biogenic carbonate is used in paleoclimate reconstruction. We can no longer unreservedly assume equilibrium precipitation in the 13C-18O composition of biogenic carbonate necessitating the creation of group specific calibrations using modern genera prior to the use of previously unmeasured groups in paloeclimatic reconstruction. Small non-equilibirum offsets in the 13C-18O compositions in echinoids may be corrected. Nautili however display inter-individual variation in 13C-18O such that correction for non-equilbirum effects may be not be possible, precluding the use of the clumped isotopic composition of groups such as the ammonites in paleoclimatic reconstruction.
Supervisor: John, Cedric Sponsor: Engineering and Physical Sciences Research Council
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral