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The Boring Billion

Sedimentary rocks such as these hold the key to understanding variations in ocean trace elements and atmospheric oxygen.

Sedimentary rocks such as these hold the key to understanding variations in ocean trace elements and atmospheric oxygen.

By Ross Large

Trace element levels in the ocean over the past 3.5 billion years explain important evolutionary events such as the Cambrian explosion of life and a “boring” billion years when evolution stood still.

Four years ago I was sitting at a research seminar listening to one of my colleagues talk about the chemistry of the ancient oceans. He was discussing how a group of scientists in the USA had used computer models to predict the trace element chemistry of oceans 1–2 billion years old.

The idea flashed into my mind that we have the technology in our research laboratory to actually measure trace elements in the ancient oceans using the rock record. It was one of those rare light bulb moments in science that are few and far between.

At that time I was heading a team of scientists studying the chemistry of pyrite in gold ore deposits. We were mapping the trace element variations in pyrite in micron detail in order to understand the chemical history of the fluids that formed the gold ore deposit.

My eureka moment was realising that if we could track the history of trace elements in ancient ore fluids then we should also be able to track the chemistry of the ancient oceans by analysing sedimentary pyrites that formed on the ancient sea floor. The only problem was that we would need to analyse thousands of sedimentary pyrites from all around the world in rocks ranging back to 3.5 billion years ago.

So we formulated the Trace Elements in the Oceans (TEO) project. Our aim was to track the changes in 25 trace elements in the oceans from 3.5 billion years ago to the present day. This had never been done before, but if we could do this it would have major implications, not only for understanding the formation of ore deposits on the ancient ocean floors but also for understanding the evolution of life in the oceans.

I immediately contacted several international geologists to join the project and help with the collection of samples. Professor Leonid Danyushevsky, who leads our technology program, also joined me to devise new ways to analyse sedimentary pyrite and Dr Jacqueline Halpin joined us to be in charge of dating the pyrite samples so that the chemical changes could be tracked through time.

Trace Elements and Evolution

Scientist have known for some time that evolution of life in the oceans was strongly influenced by trace metal concentrations, as metals like copper, zinc, chromium, vanadium, cobalt , molybdenum and selenium are taken up by marine species and are critical for life and evolutionary change. In fact, up to 20 separate elements are required to maintain life, and this is likely to have been the case throughout evolutionary history since the origin of life.

In the 2013 book Evolution’s Destiny, Oxford professors Robert Williams and Ros Rickarby contend that evolutionary change, defined by Darwin as “random selection”, was far from random because chemical changes in the oceans controlled evolution. These chemical changes are now predicable.

This is why the research results from our TEO team were so important. We could now test the theory that trace elements in the ocean were a driver of evolutionary change. Since we started the project we have analysed more than 3000 pyrites, and although we need to analyse about 10,000 to get the full picture, the early results have already generated some major surprises.

Arsenic in the Oldest Oceans

The first surprise is that the oldest oceans, from 3.5 to 2.5 billion years ago, were much more enriched in certain trace elements than originally expected. For example, these early oceans contained 10–100 times more nickel, cobalt, arsenic, mercury and gold than in the present oceans. The arsenic and mercury levels alone would have made them toxic to marine life compared with current levels.

However, bursts or pulses of nutrient trace elements such as molybdenum and selenium were detected as far back as 3 billion years ago. These elements are both oxygen proxies and important for building selenoproteins and other organic molecules necessary for life.

This supports other scientific studies that have shown evidence of single-celled life and atmosphere oxygen pulses some three billion years ago.

The Great Oxidation Events

The second surprise was that the abundance of many of the trace elements in the ocean correlates with increases in atmospheric oxygen. We have known for some time that there was virtually no oxygen in the atmosphere in the Archean period (2.5–4.5 billion years ago), and that oxygen increased in two steps know as the Great Oxidation Events (GOE). GOE1 was around 2.3–2.5 billion years ago, and our work showed that selenium, molybdenum, thallium, uranium and manganese increased whereas nickel, cobalt, arsenic and mercury decreased. This suggests that the oceans became less toxic and thus more accommodating for life as we know it after GOE1.

The second jump in oxygen, GOE2, occurred around 700–500 million years ago. This was also when a large number of trace elements increased in concentration, including those necessary for life – molybdenum, selenium, zinc, cadmium, copper, manganese, copper and nickel. This correlation allowed us to identify certain trace elements as proxies for changes in atmospheric oxygen. This time period includes that vital time on Earth when complex metazoan life first diversified in size and variety, beginning in the Ediacaran and exploding with diversity in the Cambrian period.

The Boring Billion Years

Another surprise from our research relates to the middle years of the Earth’s history 1.8–0.8 billion years ago. Due to the “boring” nature of the geology and the stability of the tectonic environment at the time, this period was termed the “boring billion” by Dick Holland, a prominent economic geology professor from Harvard University.

Previous scientific data had suggested that the amount of oxygen in the atmosphere during this period was about 5% of what it is today, and it remained fairly static. However, our new research indicates that both oxygen levels in the atmosphere and trace element levels in the oceans gradually declined through the boring billion years, reaching a minimum about 800 million years ago. Selenium, an element critical for life, declined to about one-tenth of its value at the start of the period.

Other trace elements show a significant decline, including molybdenum, chromium, silver and vanadium. These decreases in nutrient elements and oxygen in the ocean would have placed considerable stresses on the evolutionary pathways of the single-celled species throughout this period, causing evolution to slow and in some periods stop entirely.

The trace element minimum in the oceans around 800 million years ago, called the Cryogenian Trough, marked the end of the boring billion. This period may be considered the equivalent of an extended period of mass extinction when evolution might have stood still – although at the time there was little life left in the oceans to become extinct.

Recovery from the Boring Billion and the Cambrian Explosion

How did life on Earth survive this devastatingly boring period of stasis? We know that in the Cambrian, starting about 540 million years ago, life in the oceans was rampant. This period is thus known as the Cambrian Explosion.

So what happened to change things between 800 million years ago, when life was virtually extinct, and 500 million years ago when it was rampant? We believe trace elements in the ocean and oxygen in the atmosphere were the key again.

The results from our ocean pyrite tracking research show a steady increase in nutrient trace elements in the oceans from about 700 to 520 million years ago. The cause of this increase is not fully understood, but it likely relates to increased continental erosion where the breakdown of rocks to their mineral components caused the release of trace elements into the river systems and eventually the oceans. The increased erosion may have been triggered by continental amalgamation and mountain-building as the supercontinent Gondwana formed around this time from the collision of several smaller continental plates.

Whatever caused the increased nutrient element supply to the oceans set in place a chain of events that, once started, was difficult to stop. We speculate that the increased trace elements stimulated evolution back into the game, leading to a rapid increase in bacteria and plankton in the oceans. This, in turn, released oxygen to the atmosphere and sped up oxidative erosion of the continents.

The increased ocean productivity led to increased burial of carbon on the seafloor, which drew down trace elements and sulfur into the seafloor muds within sedimentary pyrite (the key to the ocean pyrite tracking technology). The carbon and sulfur burial increased the atmospheric oxygen even more. A runaway oxygen scenario along with abundant nutrient trace elements was maintained to further stimulate evolution, eventually resulting in the Cambrian explosion of life.

These exciting results are only the start for our TEO team. We still need to collect more samples from around the globe, and analyse another 7000 more pyrites to generate a robust scientific picture of ocean chemistry and its effect on evolution and ore deposit formation.

Ross Large is Distinguished Professor at the ARC Centre of Excellence in Ore Deposits at The University of Tasmania.