A study, published in Geochemical Perspectives Letters, reveals the existence of minimum oxygen zones in ancient oceans. These zones, also known as dead zones, are of great scientific interest today because they are expanding due to global warming.

The study also offers insight into one of the great ‘chicken and egg’ mysteries of the Earth’s development: did the evolution of animal life cause the oceans to oxygenate, or did the oxygenation of oceans enable animals to evolve?

In modern oceans the minimum oxygen zones occur in the sloping, shallower parts of the oceans, near the edge of continents. They form distinct layers at mid depth, with oxygenated waters above and below them. This is exactly the structure the scientists discovered in the ancient oceans in the early Cambrian period 540 million years ago. What’s more, these oceans were already dominated by animal life.

Lead author of the study, Dr Romain Guilbaud from Lancaster Environment Centre (LEC), said: “Up until now the results we have on the oxygenation of the oceans in the early Cambrian period were contradictory, some suggesting the oceans were fully oxygenated and others that they were fully anoxic (without oxygen).”

These contradictions occur, Romain believes, because there are very few places in the world where a complete profile of the sedimentary rock laid down in the oceans during the early Cambrian period survives. So scientists are often dealing with an incomplete picture.

For this research, the scientists studied the very well preserved Baltic Basin, obtaining 100-150 metre long core samples from Russia, Sweden and Estonia. Using three different techniques they analysed chemical markers and fossil remains to identify the presence of both oxygen and animal life at different depths.

“It’s interesting that where we have the rise of animals, we also have first establishment of something that looks like the modern ocean,” said Romain.

“In modern oceans the oxygen minimum zones are sustained by animal activity - animals carry organic matter which consumes oxygen. Because we now know the zones existed in the early Cambrian period, and because this is when animals start to dominate the ocean ecosystem, we suggest that the animals are shaping their environment, not the other way around,” said Romain.

“Oxygen minimum zones are highly topical in today’s science literature. They are expanding as a direct effect of global warming, because oxygen is less soluble when we have warmer temperatures.

“So although we are looking at something in the very deep past it is a very topical subject. Animals now dominate the ecosystems so understanding the relationships between animals and oxygen structure in the ocean is extremely important. If we understand this relationship through time we have keys to make models of how they will interact in the future.”

The paper’s co-authors are: Dr Ben Slater from the University of Cambridge and Uppsala University; Professor Simon Poulton from the University of Leeds; Dr Thomas Harvey from the University of Leicester; Dr Jochen Brocks and Dr Benjamin Nettersheim from the Australian National University; and Professor Nicholas Butterfield from the University of Cambridge.