Nickel and osmium isotope and trace element geochemistry of organic-rich sedimentary rocks: The first investigation of Ni isotope systematics in marine sediments

Abstract

Understanding the chemical composition of organic-rich marine sediments has the potential to: 1) allow evaluation of variations in ocean chemistry, enabling assessment of changes in global processes throughout geological time; and 2) provide an increased temporal and spatial understanding of petroleum systems. Herein two geologically distinct organic-rich sedimentary formations are explored utilising trace elements, and rhenium-osmium (Re-Os) and nickel (Ni) isotope systematics. Additionally, this thesis is the first study to investigate the behaviour of Ni isotope systematics in organic-rich marine sediments.
Osmium isotope profiling across the Sinemurian-Pliensbachian boundary GSSP indicates that there was a significant contribution of unradiogenic Os to the oceans at this time. Seawater 187Os/188Os(i) values range from ~0.20 – 0.48, becoming increasingly unradiogenic up-section. This progressive change in ocean chemistry is coincident with flooding of the Hispanic Corridor, formed during rifting of the Pangean supercontinent and creation of the Central Atlantic Ocean, evident from sudden levels of faunal exchange between the eastern Pacific and western Tethyan oceans. The Os isotope signal here reflects the onset of hydrothermal activity associated with formation of the Hispanic Corridor.
New Ni stable isotope data presented herein for the Sinemurian-Pliensbachian (S-P) GSSP and the Devonian-Mississippian Exshaw Formation, demonstrates that organic-rich marine sediments are characterised by δ60Ni values that are distinct to those of extraterrestrial and abiotic terrestrial samples. Further, the level of Ni isotope fractionation in organic-rich sediments (ranging from ~1.32 ‰ in the S-P sediments, and ~2.04 ‰ in the Exshaw Formation) is far greater than that seen in the other sample suites (ranges of ~0.17 – 0.37 ‰; Cameron et al., 2009). Although there are limited datasets available for comparison at present, the ranges of δ60Ni values for the S-P GSSP and Exshaw Formation are similar (0.28 ± 0.05 to 1.60 ±0.05 ‰ and 0.46 ± 0.04 to 2.50 ± 0.04 ‰, respectively), suggesting that such variation in Ni isotope fractionation may be characteristic of organic-rich sediments. This may be due to complexities that are ubiquitous to the sediment-seawater depositional environment. In addition, trace element ratios utilised to establish depositional paleoredox conditions demonstrate that redox did not exert control on the level of Ni isotope fractionation observed in these sediments. The study herein also demonstrates that thermal maturation of the Exshaw Formation has a negligible effect on Ni isotope systematics in mature source rocks, strongly suggesting that Ni isotopes may have the potential to be developed as an oil-source correlation tool.