Aivo Lepland

Reassessing the evidence for the earliest traces of life

The isotopic composition of graphite is commonly used as a biomarker in the oldest (>3.5 Gyr ago) highly metamorphosed terrestrial rocks. Earlier studies on isotopic characteristics of graphite occurring in rocks of the approximately 3.8-Gyr-old Isua supracrustal belt (ISB) in southern West Greenland have suggested the presence of a vast microbial ecosystem in the early Archean. This interpretation, however, has to be approached with extreme care. Here we show that graphite occurs abundantly in secondary carbonate veins in the ISB that are formed at depth in the crust by injection of hot fluids reacting with older crustal rocks (metasomatism). During these reactions, graphite forms from the disproportionation of Fe(II)-bearing carbonates at high temperature. These metasomatic rocks, which clearly lack biological relevance, were earlier thought to be of sedimentary origin and their graphite association provided the basis for inferences about early life. The new observations thus call for a reassessment of previously presented evidence for ancient traces of life in the highly metamorphosed Early Archaean rock record.

Isotopic Evidence for Massive Oxidation of Organic Matter Following the Great Oxidation Event

Analysis of two-billion-year-old rocks reveals an extreme carbon-cycle disruption after atmospheric oxygen increased. The stable isotope record of marine carbon indicates that the Proterozoic Eon began and ended with extreme fluctuations in the carbon cycle. In both the Paleoproterozoic [2500 to 1600 million years ago (Ma)] and Neoproterozoic (1000 to 542 Ma), extended intervals of anomalously high carbon isotope ratios (δ13C) indicate high rates of organic matter burial and release of oxygen to the atmosphere; in the Neoproterozoic, the high δ13C interval was punctuated by abrupt swings to low δ13C, indicating massive oxidation of organic matter. We report a Paleoproterozoic negative δ13C excursion that is similar in magnitude and apparent duration to the Neoproterozoic anomaly. This Shunga-Francevillian anomaly may reflect intense oxidative weathering of rocks as the result of the initial establishment of an oxygen-rich atmosphere.

Phosphate oxygen isotopic evidence for a temperate and biologically active Archaean ocean

Oxygen and silicon isotope compositions of cherts and studies of protein evolution have been interpreted to reflect ocean temperatures of 55–85 °C during the early Palaeoarchaean era (∼3.5 billion years ago). A recent study combining oxygen and hydrogen isotope compositions of cherts, however, makes a case for Archaean ocean temperatures being no greater than 40 °C (ref. 5). Ocean temperature can also be assessed using the oxygen isotope composition of phosphate. Recent studies show that 18O:16O ratios of dissolved inorganic phosphate (δ18OP) reflect ambient seawater temperature as well as biological processing that dominates marine phosphorus cycling at low temperature. All forms of life require and concentrate phosphorus, and as a result of biological processing, modern marine phosphates have δ18OP values typically between 19–26‰ (VSMOW), highly evolved from presumed source values of ∼6–8‰ that are characteristic of apatite in igneous rocks and meteorites. Here we report oxygen isotope compositions of phosphates in sediments from the 3.2–3.5-billion-year-old Barberton Greenstone Belt in South Africa. We find that δ18OP values range from 9.3‰ to 19.9‰ and include the highest values reported for Archaean rocks. The temperatures calculated from our highest δ18OP values and assuming equilibrium with sea water with δ18O = 0‰ (ref. 12) range from 26 °C to 35 °C. The higher δ18OP values are similar to those of modern marine phosphate and suggest a well-developed phosphorus cycle and evolved biologic activity on the Archaean Earth.

Graphite and carbonates in the 3.8 Ga old Isua Supracrustal Belt, southern West Greenland

We present a systematic study of abundance, isotopic composition and petrographic associations of graphite in rocks from the ca. 3.8 Ga Isua Supracrustal Belt (ISB) in southern West Greenland. Most of the graphite in the ISB occurs in carbonate-rich metasomatic rocks (metacarbonates) while sedimentary units, including banded iron formations (BIFs) and metacherts, have exceedingly low graphite concentrations. Regardless of isotopic composition of graphite in metacarbonate rocks, their secondary origin disqualifies them from providing evidence for traces of life stemming from 3.8 Ga. Recognition of the secondary origin of Isua metacarbonates thus calls for reevaluation of earlier interpretations that suggested the occurrence of 3.8 Ga biogenic graphite in these rocks. Thermal decomposition of siderite; 6FeCO3 = 2Fe3O4 + 5CO2 + C, is the process seemingly responsible for the graphite formation. The cation composition (Fe, Mg, Mn, and Ca) of the carbonate minerals, carbon isotope ratios of carbonates and associated graphite and petrographic assemblages of a suite of metacarbonates support the conclusion that multiple pulses of metasomatism affected the ISB, causing the deposition of Fe-bearing carbonates and subsequent partial disproportionation to graphite and magnetite. Equilibrium isotope fractionation between carbonate and graphite in the rocks indicates peak metamorphic temperatures between 500 and 600 ◦ C, in agreement with other estimates of metamorphic temperature for the ISB.

Questioning the evidence for Earth's earliest life—Akilia revisited

It has been argued that apatite crystals containing inclusions of isotopically light graphite in a quartz-pyroxene rock from the island of Akilia, southwest Greenland, represent the earliest (older than 3.85 Ga) traces of life on Earth. Although the age and protolith of this rock have been subjects of vigorous discussions, the occurrence of isotopically light graphite inclusions in Akilia apatite has so far not been debated in the literature. We present here the results of petrographic analysis of 17 different Akilia samples, including the actual sample (G91-26) used in the original study. Our finding that none of the apatite crystals in these samples contain graphite inclusions indicates that the Akilia apatite has no bearing on claims pertaining to a past record of life on Earth.

Apatite in early Archean Isua supracrustal rocks, southern West Greenland: its origin, association with graphite and potential as a biomarker

Abstract Rare earth element (REE) abundances in individual apatite crystals in banded iron formations (BIFs), metacherts, metacarbonates and mafic dykes in the Isua supracrustal belt (ISB) have been determined by laser ablation inductively coupled plasma mass spectrometry. The results together with petrographic observations on the distribution of graphite have been used to track the origin of the different compositional types of apatite and to evaluate the potential, proposed in earlier studies, for use of the apatite-graphite association as a biomarker. The chondrite-normalized distribution patterns of apatite in metasedimentary BIFs and metacherts fall into three groups. Relatively flat profiles with distinct positive Eu anomaly are interpreted as characterizing sedimentary (diagenetic) apatite that carry the REE signature of the Archean ocean. Secondary apatite in Isua metasdiments with either middle REE enriched profiles or with light REE depleted profiles is interpreted to have crystallized from percolating carbonate-rich metasomatic fluids or from fluids derived from cross-cutting mafic dykes, respectively. The occurrence together of these different genetic types of apatite with distinct REE signatures within cm-scale samples shows the immobility of REE in preexisting apatite during metamorphic episodes. Apatite crystals in Isua rocks of uncontested chemical sedimentary origin (BIF and metachert samples) do not have graphite inclusions or coatings. Graphite inclusions and coatings on the other hand characterize apatite in secondary metacarbonate rocks. In these rocks graphite is produced by thermal-metamorphic reduction of carbonate ion, derived from dissociation of the metasomatic ferrous carbonate where iron serves as electron donor, oxidizing to form magnetite. In view of the non-sedimentary, metasomatic origin of Isua metacarbonates and the abiogenic source of graphite, the apatite–graphite assemblage can not be considered as a biomarker and does not provide information on early Archean life in the ISB.

Fluid-deposited graphite and its geobiological implications in early Archean gneiss from Akilia, Greenland.

Graphite, interpreted as altered bioorganic matter in an early Archean, ca. 3.83-Ga-old quartz-amphibole-pyroxene gneiss on Akilia Island, Greenland, has previously been claimed to be the earliest trace of life on Earth. Our petrographic and Raman spectroscopy data from this gneiss reveal the occurrence of graphitic material with the structure of nano-crystalline to crystalline graphite in trails and clusters of CO₂, CH₄ and H₂O bearing fluid inclusions. Irregular particles of graphitic material without a fluid phase, representing decrepitated fluid inclusions are common in such trails too, but occur also as dispersed individual or clustered particles. The occurrence of graphitic material associated with carbonic fluid inclusions is consistent with an abiologic, fluid deposited origin during a poly-metamorphic history. The evidence for fluid-deposited graphitic material greatly complicates any claim about remnants of early life in the Akilia rock.

Methanotrophy in a Paleoproterozoic oil field ecosystem, Zaonega Formation, Karelia, Russia.

Organic carbon rich rocks in the c. 2.0 Ga Zaonega Formation (ZF), Karelia, Russia, preserve isotopic characteristics of a Paleoproterozoic ecosystem and record some of the oldest known oil generation and migration. Isotopic data derived from drill core material from the ZF show a shift in δ(13) C(org) from c. -25‰ in the lower part of the succession to c. -40‰ in the upper part. This stratigraphic shift is a primary feature and cannot be explained by oil migration, maturation effects, or metamorphic overprints. The shift toward (13) C-depleted organic matter (δ(13) C(org) < -25‰) broadly coincides with lithological evidence for the generation of oil and gas in the underlying sediments and seepage onto the sea floor. We propose that the availability of thermogenic CH(4) triggered the activity of methanotrophic organisms, resulting in the production of anomalously (13) C-depleted biomass. The stratigraphic shift in δ(13) C(org) records the change from CO(2) -fixing autotrophic biomass to biomass containing a significant contribution from methanotrophy. It has been suggested recently that this shift in δ(13) C(org) reflects global forcing and progressive oxidation of the Earth. However, the lithologic indication for local thermogenic CH(4) , sourced within the oil field, is consistent with basinal methanotrophy. This indicates that regional/basinal processes can also explain the δ(13) C(org) negative isotopic shift observed in the ZF.

Two-billion-year-old evaporites capture Earth’s great oxidation

A strongly oxidizing Paleoproterozoic era Two billion years ago, marine sulfate concentrations were around one-third as high as modern ones, constituting an oxidizing capacity equivalent to more than 20% of that of the modern ocean-atmosphere system. Blättler et al. found this by analyzing a remarkable evaporite succession more than 1 billion years older than the oldest comparable deposit discovered to date. These quantitative results, for a time when only more qualitative information was previously available, provide a constraint on the magnitude and timing of early Earths response to the Great Oxidation Event 2.3 billion years ago. Science, this issue p. 320 The oxidizing capacity of the ocean was one-fifth of modern values in the Paleoproterozoic era. Major changes in atmospheric and ocean chemistry occurred in the Paleoproterozoic era (2.5 to 1.6 billion years ago). Increasing oxidation dramatically changed Earth’s surface, but few quantitative constraints exist on this important transition. This study describes the sedimentology, mineralogy, and geochemistry of a 2-billion-year-old, ~800-meter-thick evaporite succession from the Onega Basin in Russian Karelia. The deposit consists of a basal unit dominated by halite (~100 meters) followed by units dominated by anhydrite-magnesite (~500 meters) and dolomite-magnesite (~200 meters). The evaporite minerals robustly constrain marine sulfate concentrations to at least 10 millimoles per kilogram of water, representing an oxidant reservoir equivalent to more than 20% of the modern ocean-atmosphere oxidizing capacity. These results show that substantial amounts of surface oxidant accumulated during this critical transition in Earth’s oxygenation.

6.2 The Pechenga Greenstone Belt

Geology and stratigraphy of the Pechenga Greenstone Belt is described in detail in Chap. 4.2. The brief geological outline presented here provides a scientific context and background information for the FAR-DEEP implemented in this area.