Reports: ND251822-ND2: Ni Isotopes as Novel Methanogenesis Tracers in Organic-rich Sediments

Richard Pancost, University of Bristol

The stable isotopes of transition metals are promising geochemical tools for understanding the nature of organic-rich rocks, the environment of deposition and the biogeochemical processes that are central to their formation. Nickel is the most recently developed trace metal isotopic system [1]. The intimate association between nickel and organic matter is highlighted by, for example, its abundance in petroleum [2] and in the metabolisms of organisms that have a key role in the evolution of Earth’s biosphere and the carbon cycle [3]. The first nickel dataset published by Cameron et al. [1] demonstrated that methanogens isotopically fractionate nickel on uptake from the environment. In contrast, the abiotic terrestrial Earth represented by materials such as basalts, loess, marine sediments and meteorites show relatively insignificant isotopic variation.

Arising from those initial findings, we postulated that nickel stable isotopes could potentially serve as a sharp diagnostic tool, complementing lipid biomarker and sedimentary approaches, in studying the microbial processes associated with the formation of organic-rich rocks. As such, the goal of our ACS-PRF funded research was to apply this new geochemical tool to measure the nickel stable isotopic composition of a variety of organic-rich rocks, sediments and biological material and, to evaluate the biomarker potential of the system. We focused on two likely controls on the nickel isotopic composition of these systems: i) the imprint of elevated organic matter production and burial, i.e. signatures deriving from the biological pump and analogous to that expressed in carbon and nitrogen isotope systems; and ii) the effect of widespread ocean anoxia, particularly with respect to more widespread methanogenesis and methanotrophy. To this end:

1) We conducted a suite of (ongoing) experiments to measure the nickel isotopic compositions in terrestrial and marine non-methanogenic organisms, the first study of its kind. This is essential because Ni is not restricted to the enzymes of methanogens, and some of our ancient Ni isotope profiles (see below) could instead reflect other biotic changes, including changes in algal productivity of increased cyanobacterial production. The initial part of this work served as a project for a Masters student. The last segment is currently being carried out by the PDRA, V. Cameron. The outcome of this research is twofold. Firstly, the largest and most significant biological fractionation imparted on nickel isotopes thus far has been by methanogens. We have now shown that other organisms can also fractionate nickel isotopes. Secondly, a substantial element of this project involved the development of various biological methodologies, which in addition to the isotope work, provided a new and extremely valuable interdisciplinary learning platform for the Masters student (currently doing a PhD in geobiology).

2) We have determined the nickel isotopic compositions of seawater and rivers, which alongside the experimental work will be essential for interpreting the nickel isotope results from black shales and other organic-rich sediments.  This is a key study, representing the first attempt to establish the isotopic mass balance of Ni on the surface Earth. V. Cameron has recently submitted a manuscript on these data to GCA.

3) To inform our interpretation of Ni isotope signatures preserved during times of putative marine anoxia, we have produced the first nickel isotope dataset for sediments from the Black Sea and Cariaco Basin. Nickel concentrations are similar across oxic and euxinic domains at both sites. However, a pronounced shift in nickel isotopes across the chemocline is observed in the Cariaco Basin  - but intriguingly, not the Black Sea - suggesting that processes differ between the two environments.

(4) We have produced the first nickel isotopic dataset for Oceanic Anoxic Events (OAEs) sediments that span OAE1b (Aptian-Albian boundary) and OAE2 (the Bonarelli or Cenomanian-Turonian boundary). Isotope and element data was also generated for molybdenum and copper, which to the best of our knowledge, is the first isotopic dataset for copper in OAEs. This work was carried out by V. Cameron in collaboration with S. Westermann. In addition to samples provided by S. Westermann, material was also obtained from the IODP Bremen Core Repository. Results show positive nickel isotopic excursions for all events that are not entirely dissimilar to our new and previous published data for molybdenum. The molybdenum results were presented by S. Westermann at the Goldschmidt conference in Florence, Italy in 2013. In total, the outcome from this work (and those above) will generate several manuscripts, the first of which is currently being prepared.

This research represents the first systematic investigation of nickel isotopes on the Earth and is one of the very first applications to the geological record. In total, our papers in development lay the groundwork for the use of nickel isotopes in understanding both the modern biogeochemical cycling of trace metals, and particularly the use of nickel isotopes to understand the intricate relationships between life and the evolution of organic-rich rocks. This latter finding will be of great value to the petroleum industry as it represents a new organic-matter fingerprinting tool, as well as Earth scientists studying the evolution of life and oceanographic events or interpreting ancient depositional environments.

References Cited:

[1] Cameron et al. (2009) PNAS 106, 10944-10948.

[2] Filby, R. H. (1994) Geological Society, London, Special Publications 78, 203-219.

[3] Kasting, J.W. (2005) Precambrian Res. 137, 119-129.