• Lucy

PalAss Research: Testing the Biogenicity of Ancient Stromatolites Using Magnetic Susceptibility

During the summer I have been researching and data collecting for an undergraduate research project, with the support of my supervisors; Dr Sarah Greene, Dr Marco Maffione and Dr Carl Stevenson. I was successful in securing a bursary for this work from the Palaeontological Association. The research project is titled ‘Testing the Biogenicity of Ancient Stromatolites using Magnetic Susceptibility’. In short, the purpose of this research is to determine whether a stromatolite has been formed by abiotic (non-biological interactions) or biotic (biological interactions) influences.

What Are Stromatolites?

Cotham Marble Stromatolite. Sourced by Dr Sarah Greene.

Stromatolites are layered, domed, sedimentary structures that typically form in shallow marine environments. They are common throughout the geological record, dating back to the Archean eon (~3.5 billion years ago). The morphology of stromatolites throughout geological time are assumed to have formed under biological influences known as microbial mats which are complex layers of microbes, similar to the ones found today in Shark Bay, Australia and Exuma Sound, Bahamas. Therefore, stromatolites are amongst the earliest evidence for life on Earth [1]. However, it has been discovered that stromatolite-like morphologies can form from abiotic processes such as chemical precipitation, sedimentation and random noise[2].

Ancient stromatolites lack preserved microbial fossils. This leaves the biogenicity open for debate.

Svalbard Stromatolite. Sourced by Dr Tom Dunkley Jones.

What does Magnetic Susceptibility tell us about the Biogenicity of a Stromatolite?

Magnetic Susceptibility is a dimensionless measure of magnetisation of a material in response to a magnetic field (it essentially is a way to determine the concentration of magnetic particles in a particular specimen). Biogenicity refers to the biological influence on the morphology of a rock particularly layered morphologies. So how does this translate?

Fine detrital grains, including magnetic particles are present in shallow marine environments and will accumulate throughout the water column and settle upon growing stromatolites. If the loose sediment falls on a steep-sided (>~45°) stromatolite with no microbial mat present the sediment will slide off, resulting in magnetic particles building up only on the top and bottom of a stromatolite lamination; this is known as the angle of slide relationship. However, if a microbial mat is present, the sticky biofilm will trap and bind detrital grains in excess of the angle of repose (>45°)[3]. Filamentous microbial communities are known to trap sediment up to a 180° angle. Hence, the variation in the concentration of magnetic particles throughout a stromatolite lamination can be used as an indicator to distinguish between abiotic and biotic formation processes.

Illustration of an abiotic vs. biotic formation. Diagram adapted from Petryshyn et al., 2016 Testing the biogenicity of stromatolites using magnetic susceptibility has been trialled in the lab by Petryshyn, et al. with successful results. However, this test is yet to be widely applied to the fossil record. Therefore, in my research I will be focusing on two stromatolites from different geological periods; the Cotham Marble stromatolite dating back to the end-Triassic mass extinction from Bristol, England and a stromatolite from the Precambrian period found in Svalbard, Norway.

[1]Reid et al., 2000. The role of microbes in accretion, lamination and early lithification of modern marine stromatolites. Nature, 406, p. 989-992.

[2]Grotzinger and Rothman, 1996. An abiotic model for stromatolite morphogenesis. Nature, 383, p. 423-425. [3]Petryshyn et al., 2016. Magnetic susceptibility as a biosignature in stromatolites. Earth and Planetary Science Letters 437, p. 66-75.

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