Isua Greenstone Belt: Greenland's Ancient Rocks and Early Life

The Isua Greenstone Belt, also called the Isua Supracrustal Belt, is a strip of ancient rock in south-western Greenland that formed about 3.8 to 3.7 billion years ago. It contains some of the oldest known lavas and sediments laid down on Earth’s surface. Because of its age, it has become a focus of the scientific search for the earliest evidence of life.

Location and rocks

The belt lies about 150 kilometres north-east of Nuuk, close to the margin of the Greenland ice sheet. It forms an arc roughly 35 kilometres long and only a few kilometres wide, surrounded by younger granitic gneisses. Geologists call such rocks “supracrustal” because they formed on the surface, as lava flows and sediments, rather than deep in the crust.

The sequence includes basalts erupted under water, some still showing pillow shapes, along with chert, carbonate rocks and banded iron formation. Pillow lavas form only when lava erupts into water, so they show that oceans existed by this time. The rocks were later heated and deformed, which makes them hard to read but did not destroy everything.

Discovery and dating

Greenland’s geological survey mapped the area in the 1960s, partly because of a large iron-ore deposit at Isua. In the early 1970s the Oxford geologist Stephen Moorbath and colleagues used radiometric dating to show that rocks in the region were around 3.7 to 3.8 billion years old. At the time this made them the oldest rocks known on Earth.

Since then, older rocks have been found elsewhere, including the Acasta Gneiss in Canada, while tiny mineral grains such as the Jack Hills zircons of Australia are older still. Isua remains special because it preserves a varied surface environment rather than only deep-seated rock.

The search for early life

Carbon traces

In 1999 the Greenlandic geologist Minik Rosing reported tiny particles of graphite, a form of carbon, in Isua sediments at least 3.7 billion years old. The carbon was enriched in the lighter isotope carbon-12, a signature that living organisms tend to produce when they take up carbon. He argued that it came from plankton-like organisms living in the early ocean.

Such isotope evidence is suggestive but not conclusive. Non-biological processes, especially in hot, mineral-rich fluids, can also produce light carbon, and metamorphism can alter the original signal.

Disputed stromatolites

In 2016 a team led by Allen Nutman published a study in Nature describing small cone-shaped and domed structures, between about 1 and 4 centimetres high, in a newly exposed Isua outcrop. Melting snow had revealed the rock. They interpreted them as stromatolites, layered mounds built by microbial mats, which would make them the oldest fossils on Earth, about 220 million years older than the next accepted examples.

In 2018 a team led by Abigail Allwood, a NASA geologist, re-examined the outcrop and published a rebuttal, also in Nature. They concluded that the structures were produced by deformation of the rock and lacked the internal layering expected in stromatolites. The debate continues, and many scientists now treat the Isua stromatolites as unproven.

Why the evidence is so hard to read

Isua’s rocks have been buried, heated to several hundred degrees Celsius and folded more than once since they formed. This metamorphism recrystallised many minerals, moved carbon around in hot fluids and stretched original layers out of shape. Any fossil or chemical trace has to be shown to date from the time the sediment was deposited, not from a later event.

Scientists therefore look for several independent lines of evidence at once: the shape of a structure, its internal layering, its chemistry and its setting within the surrounding rock. At Isua, no single site has yet satisfied all of these tests to general agreement, which is why claims there attract such close scrutiny.

What Isua tells us about early Earth

Even setting the life debate aside, Isua reveals a great deal. Its pillow lavas, cherts and iron-rich sediments show that by 3.8 billion years ago Earth had a liquid ocean, active undersea volcanoes and chemical sediments forming on the sea floor. The banded iron formation indicates seawater rich in dissolved iron, which is only possible when the ocean contains very little oxygen.

Myths and facts

It is sometimes said that Isua “proves” life existed 3.7 billion years ago. In fact, both the carbon and the stromatolite evidence remain contested. What is widely accepted is that Isua preserves an ocean environment where life could have existed. The oldest generally accepted fossils are the Pilbara stromatolites of Western Australia, around 3.48 billion years old.

Isua also features in debates about plate tectonics. Some researchers have interpreted parts of the belt as fragments of ancient ocean crust pushed together by early subduction, while others argue the evidence is too altered to tell.

Timeline

• c. 3.8-3.7 billion years ago: Volcanic and sedimentary rocks of the Isua belt are laid down.
• 1960s: Geological mapping of the area, including the Isua iron-ore deposit.
• Early 1970s: Stephen Moorbath and colleagues date the region’s rocks at about 3.7-3.8 billion years.
• 1999: Minik Rosing reports isotopically light graphite in Isua sediments.
• 2016: Allen Nutman’s team reports possible 3.7-billion-year-old stromatolites.
• 2018: Abigail Allwood’s team argues the structures are not biological.

Why it matters

The Isua Greenstone Belt is one of the few windows onto Earth’s surface nearly 4 billion years ago. Arguments over its carbon and supposed fossils have sharpened the methods scientists use to recognise ancient life, methods that also guide the search for life on Mars.