Του Γιώργου Λεκάκη
Δομές - απολιθώματα ενσωματωμένες
σε κρύσταλλο, που σχηματίσθηκαν γύρω από υδροθερμικά περιβάλλοντα, έως και 4.280.000.000
χρόνια πριν…
Στελέχη πλούσια σε ΣΙΔΗΡΟ, το
καθ΄ένα κλάσμα μεγέθους βλεφαρίδας (!), ίσως αποτελούν ένδειξη των πρώτων
μορφών ζωής, που κατοίκησαν τον (τότε) νεογέννητο πλανήτη Γη. Οι μικροσκοπικοί αυτοί
«σωλήνες αιματίτη» είναι εντυπωσιακά παρόμοιοι με τις δομές που παράγονται από
μικρόβια, τα οποία ζουν γύρω από τις υποθαλάσσια υδροθερμικά περιβάλλοντα.
ΔΙΑΒΑΣΤΕ: 3.950.000.000 χρόνων ευρήματα.
Η ομάδα συνέκρινε επίσης τα
μικρο-απολιθώματα με μικροβιακές δομές, ηλικίας 480.000.000 χρόνων, που
σχηματίσθηκαν στην νυν Νορβηγία, και δομές ηλικίας 186.000.000 χρόνων, που
βρέθηκαν στα μεσοδυτικά των ΗΠΑ και οργανισμούς που ζουν γύρω από υδροθερμικά περιβάλλοντα. Οι ομοιότητες ήταν εντυπωσιακές!
Τα αρχαιότερα γνωστά ίχνη της ζωής είναι κάποια μικροβιακά στρώματα, ηλικίας 3,7 δισεκατομμυρίων χρόνων, που βρέθηκαν στην Γροιλανδία.
ΠΗΓΕΣ: Matthew S. Dodd, Dominic Papineau, Tor Grenne, John F. Slack, Martin Rittner, Franco Pirajno, Jonathan O’Neil και Crispin T. S. Little «Evidencefor early life in Earth’s oldest hydrothermal vent precipitates», στο Nature, 543, sel. 60–64 (2017), 2.3.2017. Nat. Geographic, 1 ΜΑΡΤΙΟΥ 2017. ΑΡΧΕΙΟΝ ΠΟΛΙΤΙΣΜΟΥ, 2.3.2017.
Abstract
There are no confirmed microfossils older than 3,500 million years (Myr)
on Earth, probably because of the highly metamorphosed nature of the oldest
sedimentary rocks1. Therefore, studies have focused almost exclusively on
chemical traces and primarily on the isotopic composition of carbonaceous
material, which has led to con-troversies regarding the origin of isotopically
light reduced carbon2. Schists from the approximately 3,700-Myr-old Isua
supracrustal belt in southwest Greenland contain up to 8.8 wt% graphitic carbon
that is depleted in 13C, and this depletion has been attributed to biological
activity3,4. However, because non-biological decarbonation reactions and
Fischer-Tropsch-type synthesis5 can produce reduced carbon with similar
isotopic compositions, non-biological interpretations are possible2. Apatite with graphite coatings,
within iron formations from the approximately 3,830-Myr-old Akilia supracrustal
belt in southwest Greenland, has been interpreted as the metamorphosed product
of biogenic matter6, supported by the presence of biologi-cally important
heteroatoms within the graphite7. However, it has been suggested that some graphite in the Akilia
iron formations was deposited by
metamorphic fluids8,9. This latter interpretation is echoed in the Nuvvuagittuq
supracrustal belt (NSB) by the presence of poorly crystalline,
fluid-deposited graphite that coats apatite10, demonstrating that some
apatite–graphite occurrences are produced abiotically during fluid
remobilization and high-grade metamorphism.The NSB in northeastern Canada
represents a fragment of the Earth’s primitive mafic oceanic crust. The NSB is
composed predominantly of basaltic metavolcanic rocks (Extended Data Fig. 1),
which locally preserve pillow lava structures11 consistent with a submarine
setting, and of chemical sedimentary units including iron formations and minor
jasper (quartz-rich rocks with haematite and magnetite) and carbonate-bearing
iron formations. The iron formation units occur between sequences of tholeiitic
and calc-alkaline metabasalts, suggest-ing deposition at the same stage of
volcanic evolution and seafloor- hydrothermal activity. The minimum age for the
belt is constrained by cross-cutting trondhjemitic intrusive rocks that yield
U-Pb zircon ages of 3,774–3,751 Myr (refs. 12, 13). Furthermore, 146Sm-142Nd
systematics suggest an age of 4,280 Myr for the metabasaltic unit of the
NSB14,15. The exact age of the NSB is debated (see Supplementary Information);
nonetheless, the NSB includes one of the oldest—if not the oldest—iron
formations known on Earth. The iron forma-tion has seawater-like chemical
signatures16,17 and heavy Fe isotopic
compositions17,18, consistent with formation by the precipitation of
iron derived from hydrothermal fluids associated with volcanism19. Bulk-rock
rare earth element data for the NSB jasper and carbonate iron
formations—especially the presence of positive Eu anomalies—suggest a link to
seafloor-hydrothermal activity (Extended Data Fig. 2). The presence of
well-preserved, 20–3,000-µ m chalcopyrite crystals within the NSB jasper and
carbonate iron formations (Extended Data Fig. 3a) demonstrate the lack of
post-depositional oxidation.Most NSB rocks were subjected to upper
amphibolite-facies met-amorphism around 2,700 Myr ago14,20. Here we describe
parts of the NSB that were less affected by deformation (Supplementary Table 4)
and focus on sites where the metamorphic grade appears not to have exceeded
lower amphibolite facies17. This setting is evidenced by local outcrops in the
southwestern margins of the belt that preserve primary chert, diagenetic
calcite rhombohedra with poikilitic textures, and min-erals of low metamorphic
grade such as euhedral stilpnomelane and minnesotaite in chert that lack
pseudomorphic retrograde textures.Haematite tubes and filamentsModern
hydrothermal Si-Fe vent deposits host communities of micro-organisms, some of
which are Fe-oxidizing bacteria that form distinctive tubes and filaments21–26.
Epifluorescence imaging of modern vent sam-ples has shown that cylindrical
casts composed of iron oxyhydroxide are
formed by bacterial cells and are undeniably biogenic25. Hence, morphologically
similar tubes and filaments in ancient jaspers may be taken as
biosignatures25,27–30 that can survive elevated temperatures Although it is not
known when or where life on Earth began, some of the earliest habitable
environments may have been submarine-hydrothermal vents. Here we describe putative
fossilized microorganisms that are at least 3,770 million and possibly 4,280
million years old in ferruginous sedimentary rocks, interpreted as
seafloor-hydrothermal vent-related precipitates, from the Nuvvuagittuq belt in
Quebec, Canada. These structures occur as micrometre-scale haematite tubes and
filaments with morphologies and mineral assemblages similar to those of
filamentous microorganisms from modern hydrothermal vent precipitates and
analogous microfossils in younger rocks. The Nuvvuagittuq rocks contain
isotopically light carbon in carbonate and carbonaceous material, which occurs
as graphitic inclusions in diagenetic carbonate rosettes, apatite blades
intergrown among carbonate rosettes and magnetite–haematite granules, and is
associated with carbonate in direct contact with the putative microfossils.
Collectively, these observations are consistent with an oxidized biomass and
provide evidence for biological activity in submarine-hydrothermal environments
more than 3,770 million years ago.
ΛΕΞΕΙΣ: 4.280.000.000 χρόνων, δισεκατομμυρια χρονια πριν, αρχαιοτερο σημαδι ζωης στη Γη, απολιθωματα, κρυσταλλος, υδροθερμικο περιβάλλον, 4.280.000.000 χρονια πριν, ΣΙΔΗΡΟΣ, πρωτη μορφη ζωης, πλανητης Γη, αιματιτης, μικροβια, υποθαλασσιο περιβαλλον, βραχος, Κεμπεκ Καναδα, απολιθωμα, μικροβιο, 480.000.000 χρονια πριν, Νορβηγια, 186.000.000 χρονια πριν, ΗΠΑ, οργανισμος, 3.700.000.000 χρονια πριν, Γροιλανδια
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