by Douglas Page © 2000

In the late 1960s an airborne geophysical investigation of an area in west central Greenland conducted by a mining company detected a major magnetic anomaly. Subsequent investigation revealed a large iron ore deposit, which later proved to be part of the oldest supracrustal rocks (sediments and volcanics deposited at the surface) on Earth.

This rare outcrop of ancient rock, called the Isua Greenstone Belt (IGB), which somehow survived almost unscathed for as long as 3.8 billion years, offers a tantalizing view of conditions on Earth merely 750 million years after its formation.

There are older rocks elsewhere, but these older rocks were formed deep in the Earth's crust and cannot give any information as to the surface conditions of the early Earth. The Isua rocks, however, were formed at the surface of the earliest Earth and may thus provide a wealth of information on the surface of the Earth and the environment where life might have started.

In spite of its significance, in the 30 years or so since its discovery the 2 billion t, 30 km by 1 to 4 km Isua formation, located less than 150 km from Nuuk, the capital of Greenland, has attracted surprisingly little research attention.

"Except for a few important specialized studies on aspects of igneous petrogenesis, metamorphism, and stratigraphy published mostly in the 1980's, little modern work has been done on the detailed depositional environment of the IGB," says Peter W. U. Appel, of the Geological Survey of Denmark and Greenland (GEUS). "Sophisticated geochemical work of a type which has been so successful in many geologic terrains of all ages to delineate tectonic, petrogenetic, and sedimentary (marine and non-marine) environments is almost totally lacking."

Specifically, he says, an important recent review article, titled "Sedimentology of Archaean greenstone belts: signatures of tectonic evolution" (K. A. Eriksson et al, 1994, Earth Science Reviews, 37), doesn't even mention the IGB, "although it would have been highly relevant to the discussion."

The IGB was mapped in some detail by members of the GEUS during the 1970s and early 1980s, culminating in production of a geological map in 1986 by Allen Nutman (Bull. Grønlands geol. Unders. No. 154).

During these years, various small groups financed by GEUS and an assortment of foreign sources carried out research projects in the IGB.

"Of great relevance to all this was Vic McGregor's discovery in the early 1970s of locally abundant, small (meter-sized) enclaves of supracrustal lithologies in the Amîtsoq gneisses of the Nuuk region," Appel says. "He called these the 'Akilia suite', published a description in 1977, and suggested that they were broadly equivalent in age to the IGB some 150 km to the Northeast."

The significance of McGregor's discovery is 1) the finding suggested the IGB represents a fragment of a once much more extensive crustal block and 2) the discovery made it possible to constrain the youngest possible age for the IGB rocks because the gneisses cut them; it's easier to determine meaningful igneous and metamorphic ages from gneisses than from the supracrustal rock types.

Still, very little systematic field work has been done within the IGB to delineate localized regions of relatively low strain (i.e. deformation) and/or relatively low metamorphic temperatures in an attempt to look back in more detail at the original nature of the rocks, which is especially relevant in the search for biogenic (fossil) remnants, Appel says.

Appel is attempting to fill this research void. The veteran field geologist, who has clambered over the IGB for over 15 years, is currently leading the major Isua Multidisciplinary Research Project (IMRP), an international group of experts in structural geology, sedimentary processes and environments, igneous and metamorphic petrology, conventional geochemistry, isotope geochemistry, multi-method geochronology, and 'molecular' palaeontology. One of the project's objectives is the investigation of the environmental conditions at the Earth's surface during deposition of the Isua rocks, and to substantiate recent claims that life existed 3.8 billion years ago - as far back as these special rocks allow us to see.

The IMRP, which began in March, 1998, will end early in 2001. Each summer, between mid-June to mid-August, Appel and co-organizer Stephen Moorbath of Oxford University assemble their team from 21 research institutions at base camp near the gnarled nest of Archaean rocks, to be dispatched inland by helicopter and along the coast by boat to coax their clues from the ancient gneiss terranes.

"We know very little about the earliest surface of Earth, or the environments in which life may have arisen," says Christopher M. Fedo, assistant professor of geology at George Washington University, whose work focuses on Isua's sedimentary layers.

Some scientists believe the ingredients of life likely incubated in a superheated water environment, like those found around vent or geyser communities, Fedo says. Others favor a well-lit, shallow-water, tidal pool-type origin, where the chemicals necessary for the reproductive miracle didn't so much poach as mingle leisurely. Still others think life may have been delivered from somewhere in the cosmos, perhaps Mars, where there exists the possibility that life originated then was transported to Earth by a spray of meteoric debris sometime during the very early history of both planets.

Indeed, the age of the Isua rocks seems to coincide generally with a cosmic volley of the inner solar system called the Late Heavy Bombardment. Such timing is critical for discussions dealing with the origins of life, no matter where life may have first formed.

"Recent age estimates for the termination of late major impact bombardment of the lunar surface - based on returned lunar sample age data - cluster around 3.80-3.83 Ga," says Appel. On the widely accepted assumption that the terrestrial surface shared this pelting, it appears that the deposition of the IGB post-dated the termination of the impact event by only some 30-60 Ma, although no unambiguous evidence for impact has been found in the IGB. Since the depositional age is not well known, there is even the possibility the Isua rocks overlapped the Late Heavy Bombardment.

Very large impactors would have the capacity to boil off the oceans and sterilize life, Fedo says, leading some scientists to believe that life may have had more than one origination, because even if life appeared much earlier, it may not have survived the assault.

Isotropic Clues

Significantly, the rocks at Isua and at the nearby Akilia Island contain isotopic evidence implying that life not only existed by the time the rocks were deposited, it had already evolved the capacity to photosynthesize.

Evidence for life at this time comes from carbon-isotope data taken from tiny grains of carbon found in minute graphite inclusions in a rock formation discovered on Akilia Island, It may be as old as 3.85 Ga, as reported in a Nature cover story (Mojzsis et al, Nature 384, 1996).

While it is unknown when life first appeared on Earth (which is estimated to be 4.5 billion years old), the previous earliest evidence for life was presented by UCLA paleobiologist J. William Schopf, who showed that on the basis of bacteria-like fossils, primitive life, in a form much like modern pond scum, existed on Earth 3.46 billion years ago.The Isua findings push the emergence of life on Earth back several hundred million years.

"Our evidence establishes beyond reasonable doubt that life emerged on Earth at least 3.85 billion years ago, and this is not the end of the story," said lead author of the Nature paper, Stephen J. Mojzsis, at the time a graduate geochemistry student at the Scripps Institution of Oceanography, now a member of the W.M. Keck Center for Isotope Geochemistry in the Department of Earth and Space Sciences at UCLA. "We may well find that life existed even earlier."

The carbon inclusions in the rock were analyzed at UCLA using a unique high-resolution ion microprobe, an instrument that enables scientists to discover the exact composition of samples. No other instrument is sensitive enough to reveal precisely the isotopic composition of these carbon inclusions. The microprobe shoots a beam of ions (charged atoms) at a specific area of the sample, releasing ions from the sample that are then analyzed by a mass spectrometer.

Surprisingly, a high ratio of one isotope of carbon to another was found, which, according to Mojzsis, provides the "signature of life".

The carbon aggregates in the rock have a ratio of about 100 to one of 12C (the most common isotope form of carbon, containing six protons and six neutrons) to 13C (a rarer isotopic form of carbon, containing six protons and seven neutrons). Since living organisms preferentially use the lighter carbon-12, rather than the heavier carbon-13, a lump of carbon that has been processed by a living organism has more carbon-12 atoms than one found elsewhere in nature.

"It's very difficult to find any other process that efficiently sorts these two isotopes of carbon that way," says Marcia Bjornerud, chair of the Lawrence University geology department. "People have done all kinds of geochemical modeling trying to figure out whether it would be possible simply through metamorphism, ground water flow, or other processes to end up with 12C-13C ratios that are as high as those seen in these graphite grains. So far, no one has been able to come up with anything other than the process of photosynthesis as the sorting mechanism for these carbon isotopes."

At Isua, there is nothing to be seen, no visible fossil. But the geochemical character - the isotopic ratio of carbon in the graphite - can't be explained other than by photosynthesis, Bjornerud says. "And, if that's true, photosynthesis is a pretty sophisticated operation, suggesting that life had been around for some time to have developed and evolved the capacity to do photosynthesis. That pushes the envelope back at least a few hundred million years, to say 4.0 Ga for life to appear on Earth."

The form of life discovered was probably a simple micro-organism, such as photosynthesizing bacteria, although its actual shape or nature cannot be ascertained because over time heat and pressure have annihilated any original physical structure of the organisms.

It had been thought that the Earth was uninhabitable for perhaps a billion years, but it now seems safe to assume that the Earth spawned life almost as soon as the Earth itself was born.

Much work in the form of biogenic remnant searches by field geologists and microscopists is needed to support these interpretations.

In his years scrambling over the desolate shields of Earth's old bones, Appel and especially IMRP member John Myers, professor at Memorial University St Johns Newfoundland, have found the IGB comprises a large number of different rock types, mainly of volcanic, volcaniclastic, and chemical origin.

"The range of chemical compositions in both clastic (rocks formed of fragments of preexisting rocks) and chemical sediments is very wide," he says. "All rocks have been metamorphosed at temperatures in the range about 450-600°C. The degree of deformation is mostly intense, but there are areas that permit the preservation of original volcanic and sedimentary features, such as pillow lavas, graded bedding, and other depositional features."

McGregor and Moorbath visited the area together in 1971 and immediately realized from field evidence that the supracrustal belt was older than the surrounding granitoid gneisses, and that both units were cut by a major metadolerite dyke swarm, Appel says. They postulated that the gneisses might be equivalent in age to the so-called Amîtsoq gneisses near Nuuk 150 km to the southwest, which had already been mapped by McGregor in the late 1960's.

They also postulated that the metadolerite dyke swarm might be equivalent to the Ameralik dyke swarm in the Nuuk region. At that time (1971), preliminary age measurements (Moorbath et al. 1971: Earth Planet. Sci. Lett. 12, 245-249) had begun to show that the Amîtsoq gneisses in the Nuuk region were around 3600-3700 Ma, the oldest known rocks on Earth at that time. If the gneisses in the Nuuk and Isua regions were indeed equivalent in age, then the IGB would not only be older, but represent the oldest known supracrustal rocks on Earth.

This working hypothesis subsequently turned out to be correct, on the basis of both further field work and age measurements. The first direct age measurements on the IGB in the 1970's demonstrated that the rocks were between 3700 and 3800 Ma, while the lithology of the rocks showed beyond doubt that water already existed on the surface at this early stage of Earth history.

Fedo's work seems to confirm the presence of water.

Specifically, he has been working on a rock unit that has variously been interpreted as either a boulder-rich sedimentary rock or a rock made by deformational forces.

"I have concluded that the unit is in fact a sedimentary rock, in spite of its deformational history," he says. "The fact this turns out to be such a coarse-grained sedimentary rock has specific implications, such as exposure, erosion, and transport. These features demand that liquid water be present."

And water is generally believed to be life's womb.

Paradoxically, though, other Isua findings indicate that life had to survive an atmosphere devoid of oxygen, meaning that deadly ultraviolet light could not be screened and cells living in the upper layers of the seas would have perished. It took oxygen-producing (photosynthesizing) life, such as cyanobacteria, to gradually raise oxygen levels in the oceans and the atmosphere, increasingly screening out more UV.

Again, the clues are in the rocks.

While Isua includes mainly volcanic rocks, such as pillow basalts, and some sedimentary rocks, it also contains banded iron formations. As the name suggests, these are iron-rich rocks, consisting of magnetite and hematite.

"They no longer form on the Earth today," says Bjornerud. "They are believed to have precipitated out of seawater at a time when there was a lot of iron dissolved in the oceans."

Like today's limestone, these are chemical sedimentary rocks. However, in today's oceans there is virtually no iron dissolved in seawater because as soon as it combines with oxygen it becomes insoluble and precipitates out.

"The fact that in these ancient Isua rocks there are chemically precipitated iron sediments means that the atmosphere must have been basically reducing, so that iron could be in solution in the water," Bjornerud says. "That is one of the clues to the atmospheric story, that these rocks were deposited at a time when Earth's atmosphere had very low oxygen content."

SUGGESTED READING

Appel, P. W. U., Fedo, C. M., Moorbath, S. and Myers, J. S., "Recognizable primary volcanic and sedimentary features in a low-strain domain of the highly deformed, oldest known (~3.7-3.8 Gyr) Greenstone Belt, Isua, West Greenland", Terra Nova. 10, 57-62 (2000).

Mojzsis, S.J. and Harrison, T. M., "Vestiges of the beginning: Clues to the emergent biosphere recorded in the oldest known sedimentary rocks", GSA Today (newsletter of the Geological Society of America), 10, 1-6 (2000).

Rosing, M. T., et al., "Earliest part of Earth's stratigraphic record; a reappraisal of the >3.7 Ga Isua (Greenland) supracrustal sequence", Geology, 24, 43-46 (1996).

Whitehouse, M. J., Kamber, B. S. and Moorbath, S., "Age significance of U-Th-Pb zircon data from early Archean rocks of west Greenland: a reassessment based on combined ion-microprobe and imaging studies", Chem. Geol. 160, 201-24 (1999).

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