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Author(s): Abigail C. Allwood [sup.1] , Minik T. Rosing [sup.2] , David T. Flannery [sup.1] , Joel A. Hurowitz [sup.3] , Christopher M. Heirwegh [sup.1] Author Affiliations:(1) Jet Propulsion Laboratory, [...], The Palaeoarchean supracrustal belts in Greenland contain Earth's oldest rocks and are a prime target in the search for the earliest evidence of life on Earth. However, metamorphism has largely obliterated original rock textures and compositions, posing a challenge to the preservation of biological signatures. A recent study of 3,700-million-year-old rocks of the Isua supracrustal belt in Greenland described a rare zone in which low deformation and a closed metamorphic system allowed preservation of primary sedimentary features, including putative conical and domical stromatolites.sup.1 (laminated accretionary structures formed by microbially mediated sedimentation). 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.In contrast to a previous study of 3,700-million-year-old rocks of the Isua supracrustal belt in Greenland, which presented fossil evidence of stromatolites (macroscopic remains of layered microbial communities), this study shows that these 'stromatolites' are features of deformation unconnected to the processes of organic life.

The first indisputable evidence for very early differentiation of the silicate Earth came from the extinct [sup.146Sm]]-[sup.142]Nd chronometer. I42Nd excesses measured in 3.7-billion-year (Gyr)-old rocks from Isua (1, 2) (southwest [...]

The Earth's surface environment over the approximately 4 billion years (Gyr) recorded in geologic formations appears to have been maintained within a relatively narrow range in which liquid water was [...], Environmental niches in which life first emerged and later evolved on the Earth have undergone dramatic changes in response to evolving tectonic/geochemical cycles and to biologic interventions (1-3), as well as increases in the Sun's luminosity of about 25 to 30 per cent over the Earth's history (4). It has been inferred that the greenhouse effect of atmospheric C[O.sub.2] and/or C[H.sub.4] compensated for the lower solar luminosity and dictated an Archaean climate in which liquid water was stable in the hydrosphere (5-8). Here we demonstrate, however, that the mineralogy of Archaean sediments, particularly the ubiquitous presence of mixed-valence Fe(II-III)oxides (magnetite) in banded iron formations (9) is inconsistent with such high concentrations of greenhouse gases and the metabolic constraints of extant methanogens. Prompted by this, and the absence of geologic evidence for very high greenhouse-gas concentrations (10-13), we hypothesize that a lower albedo on the Earth, owing to considerably less continental area and to the lack of biologically induced cloud condensation nuclei (14), made an important contribution to moderating surface temperature in the Archaean eon. Our model calculations suggest that the lower albedo of the early Earth provided environmental conditions above the freezing point of water, thus alleviating the need for extreme greenhouse-gas concentrations to satisfy the faint early Sun paradox.

The 94 naturally occurring chemical elements can combine into an endless number of compounds. In nature, however, such compounds come as a few thousand mineral species (1), each with its [...]

Author(s): Abigail C. Allwood [sup.1] , Minik T. Rosing [sup.2] , David T. Flannery [sup.1] , Joel A. Hurowitz [sup.3] , Christopher M. Heirwegh [sup.1] Author Affiliations:(1) Jet Propulsion Laboratory, [...], In Extended Data Fig. 1 of this Letter, the map showed the field-work location incorrectly; this figure has been corrected online.

The first indisputable evidence for very early differentiation of the silicate Earth came from the extinct 146Sm-142Nd chronometer. 142Nd excesses measured in 3.7-billion-year (Gyr)-old rocks from Isua (southwest Greenland) relative to modern terrestrial samples imply their derivation from a depleted mantle formed in the Hadean eon (about 4,570-4,000?Gyr ago). As dictated by mass balance, the differentiation event responsible for the formation of the Isua early-depleted reservoir must also have formed a complementary enriched component. However, considerable efforts to find early-enriched mantle components in Isua have so far been unsuccessful. Here we show that the signature of the Hadean enriched reservoir, complementary to the depleted reservoir in Isua, is recorded in 3.4-Gyr-old mafic dykes intruding into the Early Archaean rocks. Five out of seven dykes carry 142Nd deficits compared to the terrestrial Nd standard, with three samples yielding resolvable deficits down to ?10.6 parts per million. The enriched component that we report here could have been a mantle reservoir that differentiated owing to the crystallization of a magma ocean, or could represent a mafic proto-crust that separated from the mantle more than 4.47?Gyr ago. Our results testify to the existence of an enriched component in the Hadean, and may suggest that the southwest Greenland mantle preserved early-formed heterogeneities until at least 3.4?Gyr ago. [ABSTRACT FROM AUTHOR]