U-rich Archaean sea-£oor sediments from Greenland ^ indications of s 3700 Ma oxygenic photosynthesis Minik T. Rosing a;b; Ã , Robert Frei b;c a Geologisk Museum, Òster Voldgade 5^7, 1350 Copenhagen K, Denmark b Danish Lithosphere Center, Òster Voldgade 10, 1350 Copenhagen K, Denmark c Geologisk Institut, Òster Voldgade 10, 1350 Copenhagen K, Denmark Received 19 March 2003; received in revised form 18 September 2003; accepted 21 October 2003 Abstract s 3700 Ma metamorphosed pelagic shale from West Greenland contains up to 0.4 wt% reduced carbon with N 13 C values down to 325.6x [PDB, PeeDee Belemnite]. The isotopic signature and mode of occurrence suggest that the carbonderivedfromplanktonicorganisms.ThePbisotopiccompositionshowsthattheshalehadhighprimaryU/Th. This indicates that organic debris produced a local reducing environment which precipitated U transported to the site of sedimentation by oxidized ocean water. The existence of highly productive plankton that fractionated C isotopes strongly and set up oxidation contrast in the environment suggests that oxygenic photosynthesis evolved before 3700 Ma. ß 2003 Elsevier B.V. All rights reserved. Keywords: Isua; oxygenic photosynthesis; carbon; Archaean; Pb isotopes; U; Th 1. Introduction The most important evolutionary step for the proliferation of life on Earth was the development of oxygenic photosynthesis. The advent of this metabolic strategy marked the beginning of global atmospheric management by life, and the determi- nant in£uence of life on Earth climate. This node on the tree of life is thus one of the most impor- tant geological events to date. Living organisms generally display decreasing levels of structural complexity the deeper they are rooted on the phylogenetic tree. Life forms that existed during deposition of the oldest sedi- ments on Earth left no morphological fossils that could sustain the strong metamorphic recrystalli- zation and deformation that have a¡ected all rocks older than 3600 Ma. Early life must thus be identi¢ed and characterized by its metabolic interaction with Earth’s chemical environments. The interaction of life with its environment causes characteristic geochemical fractionations, which can be identi¢ed in geologic deposits. Metabolic processes can also cause isotope fractionations that are diagnostic of speci¢c enzymatic catalysts. Characterization of pre-3600 Ma life must thus be based on the identi¢cation and interpretation of 0012-821X/03/$ ^ see front matter ß 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0012-821X(03)00609-5 * Corresponding author. Tel.: +45-3532-2345; Fax: +45-3532-2325. E-mail address: minik@savik.geomus.ku.dk (M.T. Rosing). Earth and Planetary Science Letters 217 (2004) 237^244 R Available online at www.sciencedirect.com www.elsevier.com/locate/epsl