The stoichiometry of life across the Precambrian-Cambrian boundary

Biogeochemical Cycles

IAP2-19-069

Overview

The Precambrian-Cambrian boundary witnessed some of the most profound transitions in Earth’s biosphere. These included a two-step rise of eukaryotic algae to ecological dominance (635Ma and 540Ma, Brocks et al. 2017), the emergence and extinction of the Ediacaran fauna (550Ma and 545Ma), and the Cambrian explosion of modern animals (~545 Ma) (Marshall 2006). These transitions have been linked to increasing levels of oxygen in the atmosphere and oceans (Och & Shields-Zhou 2012) followed by growing supplies of nutrients, in particular phosphorus and nitrogen (Reinhard et al. 2017, Wang et al. 2018). Fully oxygenated conditions paired with high nutrient levels may have enabled the development of complex body plans that had a high energy demand, culminating in the invention of predatory lifestyles.
However, complex organisms were likely also more sensitive to environmental perturbations than unicellular life forms. While some bacteria can adjust their cellular C:N:P ratios depending on nutrient supplies (Geider & La Roche, 2002, Planavsky, 2014) and others can transition from aerobic to anaerobic respiration pathways, eukaryotic life appears to be relatively inflexible in its stoichiometry and metabolic strategy. Therefore, if nutrient availability played a significant role in modulating the expansion and contraction of macrofauna over Earth’s history, then the evolutionary transitions that occurred in the Ediacaran and early Cambrian may be reflected in the stoichiometry (C:N:P ratios) of preserved biomass from that time. In other words, the transition to simple to complex life forms may be reflected in a change in the chemical composition of total biomass.
This project seeks to test that hypothesis with analyses of fresh drill cores from Namibia that span the Ediacaran-Cambrian boundary. These rocks are of very low metamorphic grade and represent a so far untouched archive of this time period. Sampling will focus on organic-rich black shales from onshore and offshore settings with the aim of (a) reconstructing local redox conditions and nutrient supplies across the basin during selected time slices, (b) evaluating relative abundances of eukaryotic and prokaryotic life during these intervals, and (c) deriving primary C:N ratios of biomass. Complementary (d) bulk, pyrolysis, molecular and kerogen microscopic analyses will characterise the richness, quality and maturity of the organic matter preserved in the sediments.

Methodology

The cores have been drilled within the framework of the GRIND programme and will be stored in Berlin for general access. Sampling will be guided by a detailed sedimentary log in order to identify coeval deep and shallow water settings during the Ediacaran and Cambrian. The sedimentological and paleontological characterisation of the rocks will be supervised by Prof Tony Prave and Dr Tim Raub.
Redox conditions and the availability of nitrogen during the time of deposition will be determined with sulphur and nitrogen isotope analyses at the University of St Andrews, led by Dr Catherine Rose and Dr Eva Stüeken. Particularly organic-rich horizons will be sub-sampled for bulk organic geochemistry incl extractions of biomarkers and kerogen microscopy, which will be carried out at Heriot-Watt University under the supervision of Prof Thomas Wagner. These data will reveal if ancient ecosystems were dominated by prokaryotic or eukaryotic life. Kerogen extractions from those horizons will be analysed for C:H and C:N ratios at St Andrews to assess the biochemical nitrogen demand of life during the time of deposition. Bulk elemental analyses of whole rocks will be used to monitor metasomatic alteration of the rocks.

Project Timeline

Year 1

Sedimentary logging and characterisation of facies, sampling of selected black shale horizons, preparation of rock powders and thin sections.

Year 2

Measurements of nitrogen, sulphur and carbon isotopes and bulk elemental and organic composition of rock powders; publication of results. Internship at with CASE partner at the end of Year 2.

Year 3

Organic biomarker analyses of selected samples; kerogen microscopy and analyses of kerogen isolates for elemental ratios. Publication of results.

Year 3.5

Compilation of the dissertation.

Training
& Skills

The student will gain training in geological techniques, including field sampling, sedimentary logging and petrography.

Furthermore, the project will provide extensive training in geochemical analyses in two different laboratories. A three-months internship with our CASE partner will provide additional opportunities to gain hands-on experience in cutting-edge analytical techniques.

Transferrable skills will include quantitative data analysis, time management, public presentations, scientific writing and teamwork.

References & further reading

Brocks, J.J., Jarrett, A.J., Sirantoine, E., Hallmann, C., Hoshino, Y. and Liyanage, T., 2017. The rise of algae in Cryogenian oceans and the emergence of animals. Nature, 548(7669), p.578.

Geider R, La Roche J (2002) Redfield revisited: variability of C:N:P in marine microalgae and its biochemical basis. European Journal of Phycology, 37, DOI: 10.1017/S0967026201003456.

Marshall, C.R., 2006. Explaining the Cambrian “explosionâ€ of animals. Annu. Rev. Earth Planet. Sci., 34, pp.355-384.

Och, L.M. and Shields-Zhou, G.A., 2012. The Neoproterozoic oxygenation event: environmental perturbations and biogeochemical cycling. Earth-Science Reviews, 110(1-4), pp.26-57.

Planavsky NJ (2014) The elements of marine life. Nature Geoscience, 7, 855-856.

Reinhard, C.T., Planavsky, N.J., Gill, B.C., Ozaki, K., Robbins, L.J., Lyons, T.W., Fischer, W.W., Wang, C., Cole, D.B. and Konhauser, K.O., 2017. Evolution of the global phosphorus cycle. Nature, 541(7637), p.386.

Wang, D., Ling, H.F., Struck, U., Zhu, X.K., Zhu, M., He, T., Yang, B., Gamper, A. and Shields, G.A., 2018. Coupling of ocean redox and animal evolution during the Ediacaran-Cambrian transition. Nature communications, 9(1), p.2575.

Further Information

For additional questions contact Dr Eva Stueeken (ees4@st-andrews.ac.uk).

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