Imagine a world of nothing but water from pole to pole. If you were around three billion years ago, no imagination would be needed. All land on the earth was fully submerged beneath one planet-covering ocean. This at least is the claim made by a new study, just published in the journal Nature Geoscience. So how do the scientists know that and how did they even start studying it?

The answer is to be found in a remote part of Western Australia called Panorama. Here the land is composed of an ancient ocean crust created around 3.24 billion years ago which tectonic action has since raised above sea level.

Benjamin Johnson, Assistant Professor of Geological and Atmospheric Sciences at Iowa State University is the lead author of this latest study. He collected samples from the ancient ocean crust at Panorama and studied the ratio of two stable isotopes of oxygen - oxygen-16 and oxygen-18 in these samples.

"You can really see the whole profile of the ocean crust from this time exposed at this place," said Johnson of Panorama's, well, panorama.

Naturally occurring oxygen is composed of three stable isotopes, ¹⁶O, ¹⁷O, and ¹⁸O, with oxygen-16 being the most abundant followed by the heavy oxygen-18.

Johnson and his colleague Boswell Wing (an Associate Professor of Geological Sciences at Colorado U.), analysed samples of the rocks collected from Panorama by quantifying the amount of ¹⁶O and ¹⁸O that these rocks contained. Their results showed that the ancient ocean crust had much more of the heavy oxygen-18 isotope than oceans do today. Extrapolating from this, it appears that ancient seawater was much richer in ¹⁸O relative to modern oceans. In fact the ancient seawater was enriched with about four parts per thousand more of the heavy isotope of oxygen (oxygen-18) than is an ice-free ocean of today. So heavy oxygen (¹⁸O) was more predominant in ancient seawater - but why?

The most attractive and simplest explanation for these findings is that early Earth was covered with water, and there was no dry land to filter heavy oxygen from the oceans. It was only with the emergence of continents in the late Archaean era (between 3 and 2.5 billion years ago), that the heavy clays and soils of the continental crust developed. Clay and heavy soil acts as a filtering mechanism which removes a proportion of heavy oxygen particles as water passes through in the form of rain or snow. Heavy clays and soils absorb more oxygen-18 than oxygen-16, so that the water that returns to a modern ocean does so depleted in the heavier oxygen isotope.

"If there are no continents above sea level, no weathering, no soils, no clays, all that heavy oxygen-18 that would otherwise be in the continents stays in the ocean. And that's exactly what we see at Panorama," Johnson explained.

Journal Reference:

1. Benjamin W. Johnson & Boswell A. Wing. Limited Archaean continental emergence reflected in an early Archaean ¹⁸O-enriched ocean. Nature Geoscience (2020), 13, pp. 243–248 https://www.nature.com/articles/s41561-020-0538-9