Gustaf Arrhenius

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.

Fractionation and condensation in space

Abstract Observations in space suggest that the primordial condensation of solids in our solar system took place from a low density, partially excited gas, and that the gas temperatures were much higher than the temperatures of the solid grains growing from this medium. Laboratory simulation of such condensation processes has provided information on the characteristics of the ensuing solids. The materials in meteorites have properties which in some cases permit, in others strongly suggest or require the assumption that they are primary and largely unaltered solids grown in extreme thermal disequilibrium with the surrounding gas phase.

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.

Mineral induced formation of sugar phosphates

Glycolaldehyde phosphate, sorbed from highly dilute, weakly alkaline solution into the interlayer of common expanding sheet structure metal hydroxide minerals, condenses extensively to racemic aldotetrose-2,4-diphosphates and aldohexose-2,4,6-triphosphates. The reaction proceeds mainly through racemic erythrose-2,4-phosphate, and terminates with a large fraction of racemic altrose-2,4,6-phosphate. In the absence of an inductive mineral phase, no detectable homogeneous reaction takes place in the concentration- and pH range used. The reactant glycolaldehyde phosphate is practically completely sorbed within an hour from solutions with concentrations as low as 50 µm; the half-time for conversion to hexose phosphates is of the order of two days at room temperature and pH 9.5. Total production of sugar phosphates in the mineral interlayer is largely independent of the glycolaldehyde phosphate concentration in the external solution, but is determined by the total amount of GAP offered for sorption up to the capacity of the mineral. In the presence of equimolar amounts of rac-glyceraldehyde-2-phosphate, but under otherwise similar conditions, aldopentose-2,4,-diphosphates also form, but only as a small fraction of the hexose-2,4,6-phosphates.

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.

Neptunism and vulcanism in the ocean

Abstract The origin of authigenic minerals on the ocean floor has been extensively discussed in the past with emphasis on two major processes; precipitation from solutions originating from submarine eruptions, and slow precipitation from sea water of dissolved elements, originating from weathering of continental rocks. It is concluded that in several marine authigenic mineral systems these processes overlap. A diagnostic principle is suggested, permitting a qualitative or semiquantitative discrimination between marine authigenic minerals crystallized from dissolved species, which have spent a long time in solution on the one hand, and the same minerals generated from solutions, near their source on the other. Extensive data are available for the manganese and iron oxide minerals forming manganese nodules. It is indicated on the basis of their composition and structure that many of the nodules found in the vicinity of the continents are made up essentially of manganese derived from continental weathering. In contrast to this group, all of the nodules found in the Pacific area of submarine vulcanism display the criteria for rapid precipitation near the source of solution. The distribution of barium minerals over the deep ocean floor is discussed.The same diagnostic principle is suggested for application to these solids, in order to discriminate between baryte and harmotome crystallized near the source of barium- rich, acidic vulcanites, and the same minerals formed from continental solution with passage through the biosphere. In the case of the authigenic aluminosilicates it is found that many of the framework elements (Si and particularly Al) have low passage time through solution, and the major fraction of these elements is consequently removed from solution in the vicinity of the eruptive source materials. Extensive modification of the crystal structures, however, takes place over long periods of time, adding particularly cations from sea water, and probably to some extent silica from siliceous fossils, which on their decay on the ocean floor appear to contribute to the silicate framework of growing zeolites. The marked fractionation of the rare earth ions between coexisting phases is pointed out, with discussion of the potential use of this phenomenon to indicate the processes of formation. The use of the hafnium/zirconium ratio as a tracer for the igneous source type is suggested, and the application of ideally imperfect tracers to establish the varying relative importance of volcanic versus halmeic source of marine minerals is discussed in general.

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.

Rate of production, dissolution and accumulation of biogenic solids in the ocean

The equatorial current system, by its response to global circulation changes, provides a unique recording mechanism for long range climatic oscillations. A permanent record of the changes in rate of upwelling and organic production is generated in the equatorial deep sea sediments, particularly by such biogenic components which are unaffected by secondary dissolution. In order to determine the rates of accumulation of various sedimentary components, a reliable differential measurement of age of the strata must be obtained. Various approaches to this problem are reviewed, and sources of error discussed. Secondary dissolution of calcium carbonate introduces a substantial and variable difference between the dissolution-modified, and hence a priori unknown, rate of deposition on one hand and the rate of accumulation, derivable from the observed concentration, on the other. The cause and magnitude of these variations are of importance, particularly since some current dating schemes are based on assumed constancy in the rate of accumulation of this and, in some cases, also all other sedimentary components. The concepts used in rate evaluation are discussed with emphasis on the difference between the state of dissolution, an observable property of the sediment, and the rate of dissolution, a parameter that requires deduction of the carbonate fraction dissolved, and of the time differential. As a most likely cause of the enhanced state of dissolution of the interglacial carbonate sediments is proposed the lowered rates of biogenic production and deposition, which cause longer exposure of the carbonate microfossils to corrosion in the bioturbated surface layer of the sediment. Historical perspective is included in the discussion in view of the dedication of the Symposium to Hans Pettersson, the leader of the Swedish Deep Sea Expedition 1947-1948, an undertaking that opened a new era in deep sea research and planetary dynamics.

Origin of Oceanic Manganese Minerals.

A criterion is suggested for discrimination between ferromanganese oxide minerals, deposited after the introduction of manganese and associated elements in sea water solution at submarine vulcanism, and minerals which are slowly formed from dilute solution, largely of continental origin. The simlultaneous injection of thorium into the ocean by submarine vulcanism is indicated, and its differentiation from continental thorium introduced into the ocean by runoff is discussed.

β-FeOOH · Cln (akaganéite) and Fe1-xO (wüstite) in hot brine from the Atlantis II Deep (Red Sea) and the uptake of amino acids by synthetic β-FeOOH · Cln

Abstract A yellowish brown precipitate in samples of Red Sea hot brine has been identified as β-FeOOH · Cl n (akaganeite) by Guinier-Hagg X-ray diffraction techniques, transmission electron microscopy (TEM), electron diffraction and electron excited X-ray fluorescence. Microcrystals of Fe 1-x O (wustite) were also identified by electron diffraction. Synthetic β-FeOOH · Cl n characterized by the same techniques was identical with the brine precipitate. Bonding between β-FeOOH · Cl n and 14 C labeled amino acids is pH dependent: between pH 3 and pH 7.6 amino acids were bound to the extent of 40 to 60% or 0.6 to 0.9 mmol per kg of iron oxide hydroxide. Some selectivity in affinity for different amino acids was observed, however, the effect is insufficient to explain the unusual distribution in the Atlantis II Deep brine reported in the literature.