Reassessing evidence of life in 3,700-million-year-old rocks of Greenland
Abstract
Os cinturões supracrustais paleoarqueanos na Groenlândia contêm as rochas mais antigas da Terra e são um dos principais alvos na busca das primeiras evidências da vida na Terra. No entanto, o metamorfismo em grande parte obliterou texturas e composições originais da rocha, representando um desafio à preservação de assinaturas biológicas. Um estudo recente de rochas de 3.700 milhões de anos do cinturão supracrustal de Isua, na Groenlândia, descreveu uma zona rara em que baixa deformação e um sistema metamórfico fechado permitiam a preservação de características sedimentares primárias, incluindo supostos estromatólitos cônicos e domésticos1 (estruturas acrecionárias laminadas formadas por sedimentação mediada por micro-organismos).
The morphology, layering, mineralogy, chemistry and geological context of the structures were attributed to the formation of microbial mats in a shallow marine environment by 3,700 million years ago, at the start of Earth’s rock record.
Here we report new research that shows a non-biological, post-depositional origin for the structures. Three-dimensional analysis of the morphology and orientation of the structures within the context of host rock fabrics, combined with texture-specific analyses of major and trace element chemistry, show that the ‘stromatolites’ are more plausibly interpreted as part of an assemblage of deformation structures formed in carbonate-altered metasediments long after burial.
The investigation of the structures of the Isua supracrustal belt serves as a cautionary tale in the search for signs of past life on Mars, highlighting the importance of three-dimensional, integrated analysis of morphology, rock fabrics and geochemistry at appropriate scales.
The morphology, layering, mineralogy, chemistry and geological context of the structures were attributed to the formation of microbial mats in a shallow marine environment by 3,700 million years ago, at the start of Earth’s rock record.
Here we report new research that shows a non-biological, post-depositional origin for the structures. Three-dimensional analysis of the morphology and orientation of the structures within the context of host rock fabrics, combined with texture-specific analyses of major and trace element chemistry, show that the ‘stromatolites’ are more plausibly interpreted as part of an assemblage of deformation structures formed in carbonate-altered metasediments long after burial.
The investigation of the structures of the Isua supracrustal belt serves as a cautionary tale in the search for signs of past life on Mars, highlighting the importance of three-dimensional, integrated analysis of morphology, rock fabrics and geochemistry at appropriate scales.
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