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DNA molecules are continuously released through decomposition of organic matter and are ubiquitous in most environments. Such DNA becomes fragmented and damaged (often

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 [...]

Recent high precision [sup.142]Nd isotope measurements showed that global silicate differentiation may have occurred as early as 30-75 Myr after the Solar System formation [Bennett V, et al. (2007) Science 318:1907-1910]. This time scale is almost contemporaneous with Earth's core formation at ~30 Myr [Yin Q, et al. (2002) Nature 418:949-952]. The [sup.182]Hf-[sup.182]W system provides a powerful complement to the [sup.142]Nd results for early silicate differentiation, because both core formation and silicate differentiation fractionate Hf from W. Here we show that eleven terrestrial samples from diverse tectonic settings, including five early Archean samples from Isua, Greenland, of which three have been previously shown with [sup.142]Nd anomalies, all have a homogeneous W isotopic composition, which is ~2[epsilon]-unit more radiogenic than the chondritic value. By using a 3-stage model calculation that describes the isotopic evolution in chondritic reservoir and core segregation, as well as silicate differentiation, we show that the W isotopic composition of terrestrial samples provides the most stringent time constraint for early core formation (27.5-38 Myr) followed by early terrestrial silicate differentiation (38-75 Myr) that is consistent with the terrestrial [sup.142]Nd anomalies. hafnium | tungsten | hidden reservoir | Hawaii | Iceland doi/ 10.1073/pnas.0913605107

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.