Reconstructing Nitrogen Sources to Earth’s Earliest Biosphere at 3.7 Ga

Earth’s sedimentary record has preserved evidence of life in rocks of low metamorphic grade back to about 3.2–3.5 billion years ago (Ga). These lines of evidence include information about specific biological metabolisms, permitting the reconstruction of global biogeochemical cycles in the early Archean. Prior to 3.5 Ga, the geological record is severely compromised by pervasive physical and chemical alteration, such as amphibolite-granulite facies metamorphic overprinting. Despite this alteration, evidence of biogenic organic matter is preserved in rare localities, including meta-turbidites from the 3.8 to 3.7 Ga Isua Supracrustal Belt, Western Greenland. But detailed insights into metabolic strategies and nutrient sources during the time of deposition of these Eoarchean meta-sedimentary rocks are lacking. Here we revisit the Isua meta-turbidites and provide new data for metal abundances as well as organic carbon and nitrogen isotope values. Our results reveal mixing between authigenic and detrital nitrogen phases with the authigenic phase likely fractionated by metamorphic degassing. Rayleigh fractionation models of these 3.7 Ga samples indicate pre-metamorphic δ15N values of between −1 and −10‰. The most plausible initial values are below −5‰, in agreement with a prior study. While the upper endmember of −1‰ could indicate biological N2 fixation at 3.7 Ga, the more plausible lighter values may point toward a distinct biogeochemical nitrogen cycle at that time, relative to the rest of Earth’s history. In light of recent experimental and phylogenetic data aligned with observations from the modern atmosphere, we tentatively conclude that lightning and/or high-energy photochemical reactions in the early atmosphere may have contributed isotopically light nitrogen to surface environment(s) preserved in the Isua turbidites. In this case, recycling of Eoarchean sediments may have led to the isotopically light composition of the Earth’s upper mantle dating back to at least 3.2 Ga.

The Isua (Greenland) relict stromatolites cannot be confidently interpreted as original sedimentary structures

<p>The biogenicity of proposed stromatolite structures from Eoarchean (ca. 3.71 Ga) rocks of the Isua Supracrustal Belt (ISB) in West Greenland is under debate. Our 2020 publication argues against biogenicity for the proposed stromatolites. The subsequent Comment to our work challenged some of our fundamental arguments for a tectonic origin to the structures. This Comment has been an opportunity for us to elaborate on these structures and further refine and solidify our initial conclusion that they represent the expected outcome of the tectonic deformation displayed in the ISB. This dialogue between groups is essential as the consequence of these structures being biogenic would move the date for complex microbial communities 200 million years closer to Earth's formation, to a time when Earth’s surface would have been even less habitable. Here we reexamine our four key observations that support our tectonic origin. First, we report detailed field characterization and structural analysis to show that the structures are linear inverted ridges aligned with azimuths of local and regional fold axes and parallel to linear structures; they were never primary linear, deformation-parallel stromatolites or deformed conical stromatolites. Second, our combined major element (e.g., Ca, Mg, Si) scanning μXRF maps fail to reveal internal laminations for the cores of these structures, but other authors argue layers are present. In the instance where layers appear to be preserved, we argue that an amorphous core is still present.  Also, layering on its own is inconclusive of a biogenic origin as relict internal laminations could be preserved. Third, the gross morphology of these structures being nearly identical in morphology and dimensions to clearly tectonic structures only tens of meters away is a more reliable indicator of a tectonic versus biogenic origin than internal laminations. Lastly, discontinuous field relationships and absence of primary sedimentary structures that could serve as way-up indicators preclude confident assignment of these outcrops as being structurally overturned, as originally argued. Collectively, our results reinforce that the Isua structures are the expected result of a tectonic fabric that preserves no fine-scale primary sedimentary structures and were probably never stromatolites.</p>

Structure and geochronology of Sargur schist belt, Western Dharwar Craton, southern India

<p>The Sargur schist belt (SSB) - one of the oldest supracrustal belt (>3.4 Ga) - occurs as discontinuous band along the south-eastern part of Western Dharwar Craton of Indian peninsula. It is a 320 km long belt present in form of lenses, sheets, enclaves, pockets, patches and disrupted layers within the peninsular gneisses, tectonically interleaved, deformed and metamorphosed together with the associated supracrustal rocks (Janardhan et al., 1978; Srikantappa et al., 1984, 1985; Bidyananda and Mitra, 2005; Jayananda et al., 2008). The SSB shows a wide variation in lithology ranging from metapelites, metamafites, metaultramafites, quartzites, calc-silicates etc. with a varying metamorphic grade from greenschist to granulite facies. The major rock types in the study area include garnet-biotite±muscovite±staurolite schist, talc-tremolite-chlorite schist, banded magnetite quartzite, micaceous quartzite, hornblende-biotite±garnet gneiss, amphibolite schist, pyroxene granulites, foliated/deformed granite etc. The fabric in schistose rocks is mainly defined by the shape preferred aggregates of biotite-muscovite (in metapelites) and tremolite-talc-chlorite/amphibole (in metamafites/ultramafites). Whereas the gneissic fabric is defined by the quartzo-feldspathic rich leucocratic layers and biotite-garnet-amphibole-pyroxene rich melanocratic layers.</p><p>In the northern part, the SSB trends roughly N-S but towards the southern part the fabric orientation changes to E-W, whereas the dip is nearly vertical through-out the belt. The belt has undergone at least three phases of deformations. In the northern part the most penetrative fabric is a crenulation cleavage S<sub>1</sub>. The S<sub>1</sub> fabric describes open asymmetric folds having sub-vertical N-S and NNE-SSW axial plane (S<sub>2</sub>). The F<sub>2</sub> fold plunges gentle to moderately towards NNE to SSW. A set of E-W trending shears (S<sub>3</sub>) truncating the S<sub>2</sub> axial zones are zonally developed. In the southern part, as the E-W trending Moyar shear zone approaches, the early fabrics are obliterated or brought into parallelism with the E-W trending penetrative S<sub>3</sub> fabric. U-Th-total Pb dating of texturally controlled metamorphic monazites have yielded mainly two different age peaks at 2.2-2.3Ga and 2.4-2.5Ga with few older ages of ~2.7Ga ages along the northern part while the sample from the southern part (near to the E-W trending Moyar shear zone) gave younger ages ranging from 700-850 Ma and 500-600 Ma.</p><p>From the integration of structural and chronological data the D<sub>2</sub> deformation corresponds to the E-W shortening during the East and West Dharwar Craton accretion is syn- to post-tectonic with respect to the 2.4-2.6 Ga monazite growth. The 700-850 Ma and 500-600 Ma monazite growths post-tectonic with respect to the D<sub>3</sub> deformation indicates that the Neoproterozoic accretionary events affected the whole Southern Granulite Terrain and recrystallize the monazites present in the Moyar shear zone.</p>

Iron isotopes trace primordial magma ocean cumulates melting in Earth’s upper mantle

The differentiation of Earth ~4.5 billion years (Ga) ago is believed to have culminated in magma ocean crystallization, crystal-liquid separation, and the formation of mineralogically distinct mantle reservoirs. However, the magma ocean model remains difficult to validate because of the scarcity of geochemical tracers of lower mantle mineralogy. The Fe isotope compositions (δ57Fe) of ancient mafic rocks can be used to reconstruct the mineralogy of their mantle source regions. We present Fe isotope data for 3.7-Ga metabasalts from the Isua Supracrustal Belt (Greenland). The δ57Fe signatures of these samples extend to values elevated relative to modern equivalents and define strong correlations with fluid-immobile trace elements and tungsten isotope anomalies (μ182W). Phase equilibria models demonstrate that these features can be explained by melting of a magma ocean cumulate component in the upper mantle. Similar processes may operate today, as evidenced by the δ57Fe and μ182W heterogeneity of modern oceanic basalts.