About Early Earth and Biosignatures

Early plate tectonics, geodynamics and sub-seafloor processes: our understanding of the nature and onset of plate tectonics in the early Archaean is incomplete and fragmentary. The generation of new crust, subsequent cooling and fluid-rock interactions represent the “bedrock” for life. This controls the distribution of hydrothermal environments, energy and nutrient gradients in the seafloor and geochemistry of the overlying oceans. The recycling of oceanic crust in subduction zones is also important for global biogeochemical cycles, yet the extent and architecture of early subduction zones remain poorly characterized. Our future research plan includes the following objectives:

• Investigate the geological evolution of the Barberton Greenstone Belt. Understand early continental crustal growth and formation of Archean oceanic lithosphere.
• Field mapping and systematic sampling of subseafloor profiles and major tectonic breaks in the stratigraphy. Combine with metamorphic studies to characterize pressure, temperature and fluid gradients in the early crust.
• Investigate the protolith geochemistry of mafic-ultramafic rocks.
• Obtain new geochronological constraints upon Archaean geodynamic processes by radiometric dating of for example, zircon and titanite by LA-ICP-MS and SIMS. Compare and contrast modern and Archean hydrothermal systems, their physiochemical regimes and the habitats they provide.

KEY PUBLICATIONS:

Grosch, E.G., Vidal, O., Abu-Alam, T., McLoughlin, N., (2012). PT-Constraints on the metamorphic evolution of the Paleoarchean Kromberg type-section, Barberton Greenstone Belt, South Africa. Journal of Petrology, 53(3), 513-545.

Grosch, E.G., Kosler, J., McLoughlin, N., Drost, K., Slama, J., Pedersen, R.B. (2011). Paleoarchean detrital zircon ages from the earliest tectonic basin in the Barberton Greenstone Belt, Kaapvaal craton, South Africa. Precambrian Research 191, 85-99.

Alteration of Archaean volcanic glass and a hypothesized subseafloor biosphere: further test the biogenicity of candidate bioalteration textures in 3+ billion year old pillow lavas from the BGB by integrating textural studies with in-situ geochemical and petrological investigations. These results will be compared to younger geological analogues from non-metamorphosed volcanic sequences where we can map key controls on subseafloor alteration. Our research plan is built around the following objectives:

• Quantify the 3D distribution and abundance of alteration textures in the Archaean subseafloor as preserved in the Barberton Scientific Drilling Programme (BSDP) drill cores and investigate the physio-chemical controls on their distribution.
• Reconstruct subseafloor environments in the Archaean through a continuous section obtained in the BSDP drill core combining geochemistry with igneous and metamorphic petrology.
• Develop in-situ nanoSIMS and synchrotron based techniques to investigate isotopic signatures associated with the alteration of Archean volcanic glass.
• Investigate candidate bioalteration textures from the Troodos ophiolite of Cyprus and potential controls on their distribution, for comparison with the Archaean (PhD student).

KEY PUBLICATIONS:

Grosch, E.G., Mcloughlin, N. (2014). Reassessing the biogenicity of Earth's oldest trace fossil with implications for biosignatures in the search for early life. Proceedings of the National Academy of Sciences 111, 8380-8385.

McLoughlin, N., Grosch, E.G., Kilburn, M.R. and Wacey, D. (2012). Sulfur isotope evidence for a Paleoarchean subseafloor biosphere, Barberton, South Africa. Geology 40, 1031-1034.

Carbonaceous remains in Archaean cherts: this will focus on the Footbridge Chert of the BGB and additional cherts from South Africa and West Australia. We aim to identify robust microfossils and to constrain their metabolisms and habitats. This will be combined with fabric and geochemical studies of Archaean cherts to better understand their deposition environment, and to distinguish biologically derived carbonaceous compounds from abiotically derived carbon. More broadly, we will also develop in-situ geochemical techniques to investigate putative metabolisms at the cellular scale. The new research plan contains the following objectives:

• Integrate fabric mapping, bulk elemental and isotopic analysis on drill core and field samples to constrain seafloor environments recorded by the cherts (temperature, redox, energy levels) the source and timing of silicification.
• Develop raman spectroscopy, Focused Ion Beam – TEM, (nano)SIMS and synchrotron based X-ray technique to distinguish biogenic carbon from abiotic and highly-altered carbonaceous matter. Use a range of well-preserved Precambrian microfossil localities (e.g. the Gunflint Chert) to develop these techniques before applying them to the Archean (postdoc project).
• Measure cellular-scale C and S isotopic fractions in-situ by (nano)SIMS to infer the microbial metabolisms that were employed by early oceanic microorganisms.
• Design controlled laboratory silicification experiments to investigate the preservation of microbial matter in ancient cherts using living microbes collected from modern geomicrobiological laboratories established by CGB (PhD student).

KEY PUBLICATIONS:

McLoughlin, N. and Grosch, E.G. (2014). Carbonaceous matter and putative microfossils of the mid-Archean Kromberg type-section re-visited, Barberton Greenstone Belt, South Africa. European Geophysical Union abstract, Vienna May.

Wacey, D., McLoughlin, N., Kilburn, M.R., Saunders, M., Cliff, J.B., Kong, C., Barley, M.E., and Brasier, M. (2013). Nanoscale analysis of pyritized microfossils reveals differential heterotrophic consumption in the c.1.9 Ga Gunflint chert. Proceedings of the National Academy of Sciences 110, 8020-8024.