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Faunal, sedimentological and geochemical indicators of dysoxia in Cretaceous marine sediments

Faunal, sedimentological and geochemical indicators of dysoxia in Cretaceous marine sediments

Weavers, Ellen Louise (2010) Faunal, sedimentological and geochemical indicators of dysoxia in Cretaceous marine sediments. PhD thesis, University of Greenwich.

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Cretaceous marine sediments worldwide are characterised by the occurrence of 'Oceanic Anoxic Events' during which organic matter was extensively deposited in 'black shale' facies. Although diverse criteria for the recognition of low oxygen levels have been described previously they have not been fully calibrated. This project compares palaeooxygenation related features of lower Cretaceous rocks from the shallow sea muds of the Gault Clay (south east England) and Niveau Pacquier and Briestroffer sequences (south east France), and the Tarfaya Basin upwelling-zone Amma Fatma sediments (Morocco). The palaeo-oxygenation indicators investigated include:

- sedimentological proxies, including ichnology, sedimentology, taphonomy and pyrite framboid size distribution;

- faunal proxies including macrofaunal properties, and biofacies models;

- geochemical proxies, including trace-element abundances [Mo, V, Ni, Mn], element ratios and parameters [U/Th, authigenic U (U a ), V/Cr, Ni/Co, Ni/V, (Cu+Mo)/Zn, V/Sc and V/(V+Ni)] and Fe-S-C systematics [Total Organic Carbon (TOC), Total Sulphur/TOC ratio, Degree of Pyritisation and the Indicator of Anoxia].

The presence of lamination is found to be a key sedimentological indicator of restricted bottom water oxygenation conditions. New descriptive categories of pyrite morphology are proposed as the use of numerical categories of pyrite framboid diameters is found to have variable success for palaeo-oxygenation determination. Interpretation of faunal proxies can be limited by poor preservation, but taphonomy-related indicators are found to correlate well with proposed palaeo-oxygenation conditions. Molybdenum is found to be the most reliable of the trace-metal enrichment proxies studied and altered numerical boundaries are proposed for Ua , U/Th, V/Cr and Ni/Co. Of the Fe-S-C systematics indicators, Total Organic Carbon and the Indicator of Anoxia are found to provide the greatest definition between palaeo-oxygenation data sets. Combinations of trace metal and Fe-S-C indicators such as [Molybdenum Enrichment Factor x Indicator of Anoxia], and plots of Ua against TOC are proposed to provide the strongest palaeo-oxygenation proxies and distinguish between oxic/dysoxic, anoxic and euxinic conditions.

Item Type: Thesis (PhD)
Additional Information:
Uncontrolled Keywords: sedimentology, palaeo-oxygenation, geochemistry, water oxygenation,
Subjects: Q Science > QD Chemistry
Q Science > QE Geology
Faculty / School / Research Centre / Research Group: Faculty of Engineering & Science > School of Science (SCI)
Last Modified: 17 Oct 2016 09:11

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