Peripatus Home Page  pix1Black.gif (807 bytes)  Paleontology Page >> Hadean Era Updated: 7 Sep 2005 

Hadean Era


Abstract

A brief description of the Hadean Era.

Keywords: Hadean, Archaean, Precambrian, earth history

Introduction

The Hadean Era was the first in Earth history, extending from the first formation of continental crust, which began some time around 4,200 Ma (Goodwin 1991, Condie 1997), becoming persistent soon after, to approximately 3,500 Ma.

The earliest terrestrial environments were harsh:

  • Levels of atmospheric oxygen around 1% were too low to sustain an ozone layer, without which there would have been little protection from solar radiation.
  • High levels of atmospheric carbon dioxide and methane would have created a strong greenhouse effect, with global temperatures estimated to have been between 30 and 50°C.

Oceans formed early, between 4,400 and 3,900, from condensation of atmospheric water vapour. Estimates suggest that the earliest oceans were hot (between 80 and 100°C) and acidic.

(After Willis & McElwain 2002, p. 40.)

 
 

Related Topics


Further Reading

  • xxx

Related Pages

Other Web Sites

  • xxx
 
 

Stratigraphy

The Hadean interval is widely recognised, in Europe and elsewhere, though not by the Precambrian Subcommission.

Type Section/Sections

...

Upper Boundary

In Europe, the Hadean is overlain by the Ishuan, the boundary being dated around 3,750 Ma. The Precambrian Subcommission, however, does not (at least not yet) recognise the Hadean; the oldest unit recognised by the Subcommission is the Archaean, with a lower boundary dated around 3,900 Ma. So defined, the Archaean is coeval with the Ishuan and also the uppermost part of the Hadean. Thus there is no simple geochronologic sequence which is recognised everywhere for rocks of this age.

Paleogeography

Major Tectonic Events

...

Land and Sea

...

Climate

...

Detrital uraninite and paleosol profiles indicate that the late Hadean ceiling for oxygen concentrations remained about 1-2% PAL.

 

SCIENCE, Volume 298, Issue 5602, December 20 2002

---------------------------------------------------------------------------
Reducing Schemes
---------------------------------------------------------------------------

Geological evidence suggests that Earth’s early atmosphere had little free oxygen, but direct evidence of oxygen content during Earth’s history has been unavailable. Two reports take advantage of the better record of sulfur isotope values for much of the Precambrian to infer the abundance of oxygen and the sulfur chemistry of the early Earth (see the Perspective by Wiechert). Isotopic fractionation is normally mass dependent, but exceptions are known, including the mass-independent photochemical fractionation of sulfur that occurs today in the upper atmosphere where ozone is absent. For sulfur, evidence for mass-independent effects that reflect a paucity of atmospheric oxygen has been found in Hadean crustal rocks. Farquhar et al. (p. 2369) now report evidence of mass-independent isotope effects in sulfide inclusions in ancient diamonds, which are derived from deep in Earth’s mantle. These data provide direct evidence that these diamonds were sampling material that reflected processes in Earth’s atmosphere and had been subducted into the mantle during the Hadean. Hadean sulfide rocks are much less depleted than are modern sulfides in 34S, an isotope that reflects bacterial processing. In culture experiments, Habicht et al. (p. 2372) show that the isotope data limit Hadean ocean sulfate concentrations to 200 micromolar or less, considerably lower than previously thought. These data also imply that the atmosphere had little free oxygen but an abundance of the greenhouse-gas methane produced through the action of more prevalent methanogens.

Paleontology

General Characteristics

The most ancient sedimentary rocks – those older than about 3,300 Ma – occur at only a few places on Earth: the Isua supracrustal belt in southwest Greenland, the Barberton area in eastern South Africa, and the Pilbara area of northwest Australia.

The greenstones of the Isua Supracrustal Group date from around 3,700 Ma and possibly more than 3,800 Ma. Unfortunately, although some sequences are thought to be of sedimentary origin, they are strongly metamorphosed (to amphibolite facies) and no fossils have been recovered from them.

However, carbon isotope signatures recovered from these rocks provide indirect evidence that life may have existed in Isua times. "This isotopic evidence stems from the fact that the carbon atom has two stable isotopes, carbon-12 and carbon-13. The 12C/13C ratio in abiotic mineral compounds is 89. In biological syntheses, the processing of carbon [in] CO2 and carbonates gives a preference to the lighter carbon isotope and raises the ratio to about 92. Consequently, the carbon residues of previously living matter may be identified by this enrichment in 12C. A compilation has been made of the carbon isotopic composition of over 1,600 samples of fossil kerogen (a complex organic macromolecule produced from the debris of biological matter) and compared with that from carbonates in the same sedimentary rocks. This showed that biosynthesis by photosynthetic organisms was involved in all the sediments studied. In fact, this enrichment is now taken to be one of the most powerful indications that life on Earth was active nearly 3.9 billion years ago because the sample suite encompasses specimens right across the geological time scale" (Brack 2002).

Mojzsis et al. 1996 (p. 55) claims to have identified biological carbon isotope signatures from >3,800 Ma aged, chemically precipitated sediments, including banded iron formations (BIFs) and chert, on Akilia Island, southwestern Greenland. However, this interpretation has been challenged by Fedo & Whitehouse (2002) who regard the contested unit as a younger hydrothermal vein. The original claim has been vigorously defended and the final conclusion is as yet unresolved.

More certain is the report from Rosing (1999) of a biological carbon isotope signature from ~3,780 Ma (3,779 ± 81 Sm-Nd date) greywackes and slates with well-preserved sedimentary structures from the Garbenschiefer Formation in the Isua belt.

Some molecular clock analyses suggest an even earlier origin: Hedges 2002 estimates the divergence of Bacteria and Archaea at >4 Ga, noting however, that "the fidelity of genetic replication and repair systems in the early history of life is unknown, and the different environment of early Earth might have affected rates of molecular change. It is for these reasons that we have less confidence in the time estimates for the earliest splitting events" (p. 842). A phylogenetic tree constructed from highly conserved portions of the iron/manganese superoxide dismutase enzyme sequence (Kirschvink et al. 2000, p. 1404) suggests an age for this divergence of 3 to 4 Ga.

Lagerstätten

Lagerstätten (sing. lagerstätte) are fossil localities which are highly remarkable for for either their diversity or quality of preservation; sometimes both.

Both these criteria are relative and can only be appreciated in some sort of context. In fact, any form of fossil preservation is remarkable in rocks as ancient as these. Thus, although the term is hardly ever applied, there is a case to be made for calling the following Hadean fossil beds ‘lagerstätten.’

 

supplement from Cradle

Apex Chert (Pilbara Supergroup): Eleven species of filamentous fossil microbes comprising the oldest diverse microbial assemblage now known in the geological record were discovered in cherts from the Pilbara greenstone belt, northwest Australia. This prokaryotic assemblage establishes that cyanobacterium-like microorganisms were extant and both morphologically and taxonomically diverse at least as early as ~3.465 billion years ago.

Barberton: (= Fig Tree?) Bacteria microfossils dating back 3.3 to 3.4 billion years have also been discovered in rocks from the Barberton greenstone belt, South Africa.

Strelley Group: Long, fine filaments probably representing thermophilic microorganisms living in the vicinity of a hydrothermal vent have been found in a massive sulfide deposit from the Early Hadean Strelley Group (about 3.235 billion years old) of the Pilbara greenstone belt, northwest Australia. Although the temperature of the hydrothermal fluids was about 300°C, the microorganisms more likely developed at temperatures below 110°C and at water depths of about 1000 m. Under such environmental conditions, the microorganisms would have been anaerobic chemotrophs metabolizing in a reducing environment and obtaining their energy and nutrients from the hydrothermal fluids. This deep environment would have provided the microbiota with protection from the harmful UV radiation prevalent at the surface of the Earth during the Hadean, when there was no protective ozone layer.

The oldest fossils known to date derive from the Apex Chert, a formation of the Pilbara Supergroup occuring in northwestern Western Australia, and dated at 3,465 Ma (± 5 Ma, see Schopf 1999, pp. 88-89). However, the fossils occur in fragments of rock within the chert; thus they are even older, though by how much, is unknown. The organisms themselves are filamentous, composed of distinct, organic walled cells occurring as a uniserial string, and are interpreted as cyanobacteria.

Questioning the evidence for Earth’s oldest fossils
MARTIN D. BRASIER*, OWEN R. GREEN*, ANDREW P. JEPHCOAT*, ANNETTE K. KLEPPE*, MARTIN J. VAN KRANENDONK?, JOHN F. LINDSAY?, ANDREW STEELE & NATHALIE V. GRASSINEAU

Nature 416, 76 - 81 (2002)

Correspondence and requests for materials should be addressed to M.D.B. (e-mail: martinb@earth.ox.ac.uk).

Structures resembling remarkably preserved bacterial and cyanobacterial microfossils from 3,465-million-year-old Apex cherts of the Warrawoona Group in Western Australia currently provide the oldest morphological evidence for life on Earth and have been taken to support an early beginning for oxygen-producing photosynthesis. Eleven species of filamentous prokaryote, distinguished by shape and geometry, have been put forward as meeting the criteria required of authentic Hadean microfossils, and contrast with other microfossils dismissed as either unreliable or unreproducible. These structures are nearly a billion years older than putative cyanobacterial biomarkers, genomic arguments for cyanobacteria, an oxygenic atmosphere and any comparably diverse suite of microfossils. Here we report new research on the type and re-collected material, involving mapping, optical and electron microscopy, digital image analysis, micro-Raman spectroscopy and other geochemical techniques. We reinterpret the purported microfossil-like structure as secondary artefacts formed from amorphous graphite within multiple generations of metalliferous hydrothermal vein chert and volcanic glass. Although there is no support for primary biological morphology, a Fischer-Tropsch-type synthesis of carbon compounds and carbon isotopic fractionation is inferred for one of the oldest known hydrothermal systems on Earth.

EARLY Hadean (3500-3000 Ma)
Lithostratigraphic Unit Age (Ma) Location References
Warrawoona Group 3500-3400 Australia Schopf & Packer (1987); Schopf (1992, 1993)
Onverwacht & Fig Tree Group 3500-3400 South Africa Knoll & Barghoorn (1977); Walsh & Lowe (1985); Walsh (1992)

 

LATE Hadean (3000-2500 Ma)

Lithostratigraphic Unit Age (Ma) Location References
Fortescue Group 2800 Australia Schopf & Walter (1983)
Transvaal Supergroup 2500 South Africa Lanier (1986); Klein et al. (1987)

Major Evolutionary Events

"There are only three places on Earth with sedimentary rocks older than 3300 million years: the greenstone belts of Isua in southwest Greenland, the Barberton area east of South Africa, and the Pilbara area of northwest Australia. The oldest sediments on Earth have been found in Greenland. The isotopic signatures of the organic carbon in these sediments provide indirect evidence that life may be 3.85 billion years old" (Brack 2002).

...

Major Taxa

"The early Hadean fossil record at best contains a handful of morphotypes, none of which is taxonomically diagnostic or physiologically informative. Indeed the low apparent diversity of early Hadean fossils cannot itself be taken at face value. Studies of Proterozoic formations show that with increasing diagenetic/metamorphic alteration, the apparent diversity of microfossil assemblages decreases (Knoll et al.1988). Thus, when subjected to lower greenschist facies metamorphism, an assemblage with an original diversity comparable to, say, the Gunflint Formation might well yield a morphological record much like that actually seen in early Hadean cherts."

"Stromatolites shed some light on the nature of early Hadean life, but again there are uncertainties and differing interpretations of reported structures."

"[T]he microbial mat origin of the stratiform mats remains well supported. Complex communities of microorganisms including phototactic mat builders, certainly colonized early Hadean coastal environments. It is reasonable to suggest that these communities included photoautotrophs, but this is not beyond question (Walter 1983)."

"The issue of cyanobacterial antiquity is directly related to the question of Earth’s atmospheric history. Most students agree that prior to the evolution of cyanobacterial photosynthesis there could have been only trace amounts of oxygen in the atmosphere, perhaps 10-10 atm 02, (Holland 1984). Higher oxygen concentrations must have been generated photosynthetically. The presence of hematite iron formations has been used as prima facie evidence for cyanobacterial oxygen production, but it is possible that these sedimentary deposits were generated by the photooxidation of ferrous iron dissolved in anoxic early Hadean oceans."

"… Despite low global pO2, relatively high concentrations of oxygen could have accumulated locally in association with high cyanobacterial productivity; however, oxygen oases would have been transient in time and space. Therefore, obligately aerobic organisms could not have evolved until pO2 reached stable and global levels of 1-2% PAL [present atmospheric level] (Chapman & Schopf 1983)."

The late Hadean record (3000-2500 Ma)

"Schopf & Walther (1983) reported rare trichomes from the ca. 2800 Ma Fortescue Group, Western Australia. The fossils resemble oscillatorian cyanobacteria, but they are not taxonomically diagnostic; similar morphologies occur among both sulfur-oxidizing and sulfate-reducing bacteria. More diverse microfossils have been reported from the ca 2500 Ma Transvaal Supergroup, South Africa. Silicified microstromatolites and associated intraclasts from platform environments contain 1-5 m m diameter coccoids and thin filamentous sheaths interpreted as primary producers as well as tiny rods interpreted as heterotrophic bacteria (Lanier 1986). Chert nodules in deeper basinal limestones contain carbonate-lined filamentous sheaths up to 27 m m in cross-sectional diameter (Klein et al. 1987)."

"Stromatolites become increasingly abundant in younger Hadean successions, a pattern as likely to reflect craton growth as evolutionary change. By the end of the eon, extensive carbonate platforms supported widespread mat-building communities that almost certainly included cyanobacteria."

(After Knoll 1996.)

Extinctions

...

 

Conclusion

"The early Hadean record tells us that life was present at least 3500 Ma ago. Microbial ecosystems were driven by autotrophy, most likely photoautotrophy, and oxygenic cyanobacteria may already have appeared. Heterotrophs included prokaryotes and, possibly, primitive amitochondrial eukaryotes capable of feeding by phagocytosis. Depending on the amount of 02 available, the biota could also have included aerobic prokaryotes and mitochondrion bearing eukaryotic heterotrophs (but perhaps not eukaryotic algae; see below, and Knoll & Holland 1995). Although impossible to test empirically, the possibility that early communities included organisms unlike anything represented in the modern biota cannot be excluded. Clearly, early Hadean ecosystems remain poorly understood" (Knoll 1996, p. 55).

References

Brack, André 2002 (in press): Origin of Life. In Encyclopedia of Life Sciences. Nature Publishing Group, Macmillan.

Kazmierczak, Józef; Altermann, Wladyslaw 2002: NeoHadean Biomineralization by Benthic Cyanobacteria. Science 298: 2351.

Knoll, A.H. 1996: Chapter 4. Hadean and Proterozoic Paleontology. In Jansonius, J.; McGregor, D.C. (eds.) 1996: Paleontology: Principles and Applications. American Association of Stratigraphic Palynologists Foundation, v. 1, pp. 51-80.

Willis, K.J.; McElwain, J.C. 2002: The evolution of plants. Oxford University Press. 378 pp.


 Peripatus Home Page  pix1Black.gif (807 bytes)  Paleontology Page >> Hadean Era Contact me.