Geological alteration of organic macromolecules by irradiation: Implication for organic matter occurrence on Mars
Shengyu Yang1,2, Hans-Martin Schulz2, Brian Horsfield3, Niels H. Schovsbo4, Kliti Grice5 and Jinchuan Zhang6 1School of Geosciences, China University of Petroleum (East China), 266580 Qingdao, China 2Department of Geochemistry, Section 3.2 Organic Geochemistry, Helmholtz Centre Potsdam GFZ German Research Centre for Geosciences, Telegrafenberg, D-14473 Potsdam, Germany 3GEOS4 GmbH, Peter-Huchel-Chaussee 88, 14552 Michendorf, Germany 4Department of Reservoir Geology, Geological Survey of Denmark and Greenland, Øster Voldgade 10, DK-1350 Copenhagen K, Denmark 5School of Earth and Planetary Sciences, Curtin University, Perth, WA 6485, Australia 6School of Energy Resources, China University of Geosciences, 100083 Beijing, China
ABSTRACT The harsh radiation environment on Mars is widely believed to destroy organic matter, but elucidating the systematic degradation pathway and its controlling factors has proved elusive. Here we show the alteration of macromolecular organic matter in the ∼500-m.y.-old uranium-rich Alum Shale Formation (northwestern Europe), which is suggested as a geological analogue on Earth of the Martian surface samples, in response to α-particle irradiation. The composition and quantity of pyrolysates as well as the extractability of immature Alum Shale are governed by uranium content, and this has been confirmed independently using uranium-rich shales of younger geological age. During the course of irradiation, macromolecules undergo cross-linking, demethylation, and aromatization processes, which are active at different stages in the alteration of the organic matter. A comparison of the Martian surface samples with Kolm (carbonaceous nodules with very high uranium content intercalated in the Alum Shale) samples in terms of irradiation dosage and organic geochemical characteristics reveals that they have received similar levels of irradiation. A model of the radiation-induced alteration of organic matter has been developed, which opens a window for understanding the possible pristine nature and composition of the irradiated organic matter on Mars and offers arguments to qualitatively predict potential organic matter in the Martian paleo-lacustrine subsurface.
CITATION: Yang, S., et al., 2020, Geological alteration of organic macromolecules by irradiation: Implication for organic matter occurrence on Mars: Geology, v. 48, https://doi.org/10.1130/G47171.1
