Our existence is governed by natural cycles, from the daily rhythms of sleeping and eating, to longer patterns such as the turn of the seasons and the quadrennial round of leap years.

After looking at seabed sediment stretching back 65 million years, we have found a previously undetected cycle to add to the list: an ebb and flow in deep sea currents, tied to a 2.4-million-year swell of global warming and cooling driven by a gravitational tug of war between Earth and Mars. Our research is published in Nature Communications.

Milankovitch cycles and ice ages

Most of the natural cycles we know are determined one way or another by Earth’s movement around the Sun.

As the German astronomer Johannes Kepler first realised four centuries ago, the orbits of Earth and the other planets are not quite circular, but rather slightly squashed ellipses. And over time, the gravitational jostling of the planets changes the shape of these orbits in a predictable pattern.

These alterations affect our long-term climate, influencing the coming and going of ice ages. In 1941, Serbian astrophysicist Milutin Milankovitch recognised that changes in the shape of Earth’s orbit, the tilt of its axis, and the wobbling of its poles all affect the amount of sunlight we receive.

Known as “Milankovitch cycles”, these patterns occur with periods of 405,000, 100,000, 41,000 and 23,000 years. Geologists have found traces of them throughout Earth’s deep past, even in 2.5-billion-year old rocks.

Earth and Mars

There are also slower rhythms, called astronomical “grand cycles”, which cause fluctuations over millions of years. One such cycle, related to the slow rotation of the orbits of Earth and Mars, recurs every 2.4 million years.

The cycle is predicted by astronomical models, but is rarely detected in geological records. The easiest way to find it would be in sediment samples that continuously cover a period of many millions of years, but these are rare.

Much like the shorter Milankovitch cycles, this grand cycle affects the amount of sunlight Earth receives and has an impact on climate.

Gaps in the record

When we went hunting for signs of these multimillion-year climate cycles in the rock record, we used a “big data” approach. Scientific ocean drilling data collected since the 1960s have generated a treasure trove of information on deep-sea sediments through time across the global ocean.

In our study, published in Nature Communications, we used sedimentary sequences from more than 200 drill sites to discover a previously unknown connection between the changing orbits of Earth and Mars, past global warming cycles, and the speeding up of deep-ocean currents.

Most studies focus on complete, high-resolution records to detect climate cycles. Instead, we concentrated on the parts of the sedimentary record that are missing — breaks in sedimentation called hiatuses.

A deep-sea hiatus indicates the action of vigorous bottom currents that eroded seafloor sediment. In contrast, continuous sediment accumulation indicates calmer conditions.

Analysing the timing of hiatus periods across the global ocean, we identified hiatus cycles over the past 65 million years. The results show that the vigour of deep-sea currents waxes and wanes in 2.4 million year cycles coinciding with changes in the shape of Earth’s orbit.

Astronomical models suggest the interaction of Earth and Mars drives a 2.4 million year cycle of more sunlight and warmer climate alternating with less sunlight and cooler climate. The warmer periods correlate with more deep-sea hiatuses, related to more vigorous deep-ocean currents.

Warming and deep currents

Our results fit with recent satellite data and ocean models mapping short-term ocean circulation changes. Some of these suggest that ocean mixing has become more intense over the last decades of global warming.

Deep-ocean eddies are predicted to intensify in a warming, more energetic climate system, particularly at high latitudes, as major storms become more frequent. This makes deep ocean mixing more vigorous.

Deep-ocean eddies are like giant wind-driven whirlpools and often reach the deep sea floor. They result in seafloor erosion and large sediment accumulations called contourite drifts, akin to snowdrifts.

Can Mars keep the oceans alive?

Our findings extend these insights over much longer timescales. Our deep-sea data spanning 65 million years suggest that warmer oceans have more vigorous eddy-driven circulation.

This process may play an important role in a warmer future. In a warming world the difference in temperature between the equator and poles diminishes. This leads to a weakening of the world’s ocean conveyor belt.

In such a scenario, oxygen-rich surface waters would no longer mix well with deeper waters, potentially resulting in a stagnant ocean. Our results and analyses of deep ocean mixing suggest that more intense deep-ocean eddies may counteract such ocean stagnation.

How the Earth-Mars astronomical influence will interact with shorter Milankovitch cycles and current human-driven global warming will largely depend on the future trajectory of our greenhouse gas emissions.

(Authors:Adriana Dutkiewicz, ARC Future Fellow, University of Sydney; Dietmar Müller, Professor of Geophysics, University of Sydney, and Slah Boulila, Associate lecturer, Sorbonne Université)

(Disclosure Statement:Adriana Dutkiewicz receives funding from the Australian Research Council. Dietmar Muller and Slah Boulila do not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and have disclosed no relevant affiliations beyond their academic appointment)

This article is republished from The Conversation under a Creative Commons license. Read the original article.
 

(Except for the headline, this story has not been edited by NDTV staff and is published from a syndicated feed.)

While science knows what kind of rocks were being formed on different parts of the planet 3.5 billion years ago, they are still understanding which geological processes drove these formations. Image for Representation.
| Photo Credit: Getty Images

Our Earth is around 4.5 billion years old. Way back in its earliest years, vast oceans dominated. There were frequent volcanic eruptions and, because there was no free oxygen in the atmosphere, there was no ozone layer. It was a dynamic and evolving planet.

Scientists know all of this – but, of course, there are still gaps in our knowledge. For instance, while we know what kind of rocks were being formed on different parts of the planet 3.5 billion years ago, we are still understanding which geological processes drove these formations.

Luckily the answers to such questions are available. Evidence is preserved in ancient volcanic and sedimentary rocks dating back to the Archaean age, between 4 billion and 2.5 billion years ago.

These rocks are found in the oldest parts of what are today the continents, called cratons. Cratons are pieces of ancient continents that formed billions of years ago. Studying them offers a window into how processes within and on the surface of Earth operated in the past. They host a variety of different groups of rocks, including greenstones and granites.

One example is the Singhbhum Craton, in the Daitari Greenstone Belt in the state of Odisha in eastern India. This ancient part of the Earth’s crust has been found in previous research to date back to 3.5 billion years ago. The craton’s oldest rock assemblages are largely volcanic and sedimentary rocks also known as greenstone successions. Greenstones are rock assemblages made up mostly of sub-marine volcanic rocks with minor sedimentary rocks.

My research team and I recently published a study in which we compared the Singhbhum Craton to cratons in South Africa and Australia. We chose these sites because they preserve the same kinds of rocks, in the same condition (not intensely deformed or metamorphosed), from the same time period – about 3.5 billion years ago. They are the best archives to study early Earth surface processes.

Our key findings were that explosive-style volcanic eruptions were common in what are today India, South Africa and Australia around 3.5 billion years ago. These eruptions mostly occurred under oceans, though sometimes above them.

Understanding these early Earth processes is vital for piecing together the planet’s evolutionary history and the conditions that may have sustained life during different geological epochs. This kind of research is also a reminder of the ancient geological wonders that surround us – and that there is much more to discover to understand the story of our planet.

The research

We sampled some rocks from the Singhbhum Craton so we could study them in our laboratory. Existing data from the same site, as well as sites in South Africa and India, were used for comparison purposes.

Our detailed field-based studies were complemented by uranium-lead (U-Pb) radiometric-age dating. This common and well-established method provides information as to when a magma crystallised; in other words, it tells us when a rock formed. In this way we were able to establish key geological timelines to illustrate what processes were underway and when.

We also found that the geology of this area shares stark similarities with the greenstone belts documented in South Africa’s Barberton and Nondweni areas and the Pilbara Craton in western Australia.

Most particularly, all these areas experienced widespread submarine mafic – meaning high in magnesium oxide – volcanic eruptions between 3.5 and 3.3 billion years ago, preserved as pillowed lava and komatiites.

This differs from silicic (elevated concentration of silicon dioxide) volcanism, which research has shown was prevalent around 3.5 billion years ago.

These findings enrich our understanding of ancient volcanic and sedimentary processes and their significance in the broader context of Earth’s geological as well as biological evolution.

Our planet’s formative years

Our discoveries are pivotal for several reasons. First, they offer a clearer picture of Earth’s early tectonic activities during the Archaean times, contributing to our understanding of the planet’s formative years.

Second, the Singhbhum Craton’s unique geological features, including its greenstone belts, provide invaluable information about Earth’s surface and atmospheric processes. This is crucial for hypothesising early habitable conditions and the emergence of life on Earth.

Additionally, comparing the Singhbhum Craton with similar cratons in South Africa and Australia allows us to construct a more comprehensive model related to geological processes that operated during the Archaean. This can help to shed light on ancient geodynamic processes that were prevalent across different parts of the young Earth.

This research emphasises the need for further exploration into the geological history of ancient cratons worldwide. Understanding these early Earth processes is vital for piecing together the planet’s evolutionary history and the conditions that may have sustained life.

Around 2,500 years ago, at the western end of the island of Kadavu in the southern part of Fiji, the ground shook, the ocean became agitated, and clouds of billowing smoke and ash poured into the sky. When the clouds cleared, the people saw a new mountain had formed, its shape resembling a mound of earth in which yams are grown. This gave the mountain its name – Nabukelevu, the giant yam mound. Image for Representation.
| Photo Credit: The Hindu

Can you imagine a scientist who could neither read nor write, who spoke their wisdom in riddles, in tales of fantastic beings flying through the sky, fighting each another furiously and noisily, drinking the ocean dry, and throwing giant spears with force enough to leave massive holes in rocky headlands?

Our newly published research in the journal Oral Tradition shows memories of a volcanic eruption in Fiji some 2,500 years ago were encoded in oral traditions in precisely these ways.

They were never intended as fanciful stories, but rather as the pragmatic foundations of a system of local risk management.

Life-changing events

Around 2,500 years ago, at the western end of the island of Kadavu in the southern part of Fiji, the ground shook, the ocean became agitated, and clouds of billowing smoke and ash poured into the sky.

When the clouds cleared, the people saw a new mountain had formed, its shape resembling a mound of earth in which yams are grown. This gave the mountain its name – Nabukelevu, the giant yam mound. (It was renamed Mount Washington during Fiji’s colonial history.)

So dramatic, so life-changing were the events associated with this eruption, the people who witnessed it told stories about it. These stories have endured more than two millennia, faithfully passed on across roughly 100 generations to reach us today.

Also Read | Kilauea, one of the world’s most active volcanoes, begins erupting after three-month pause

Scientists used to dismiss such stories as fictions, devalue them with labels like “myth” or “legend”. But the situation is changing.

Today, we are starting to recognise that many such “stories” are authentic memories of human pasts, encoded in oral traditions in ways that represent the worldviews of people from long ago.

In other words, these stories served the same purpose as scientific accounts, and the people who told them were trying to understand the natural world, much like scientists do today.

Battle of the vu

The most common story about the 2,500-year-old eruption of Nabukelevu is one involving a “god” (vu in Fijian) named Tanovo from the island of Ono, about 56km from the volcano.

Tanovo’s view of the sunset became blocked one day by this huge mountain. Our research identifies this as a volcanic dome that was created during the eruption, raising the height of the mountain several hundred feet.

Enraged, Tanovo flew to Nabukelevu and started to tear down the mountain, a process described by local residents as driva qele (stealing earth). This explains why even today the summit of Nabukelevu has a crater.

Also Read | One year after volcanic blast, many of Tonga’s reefs lay silent

But Tanovo was interrupted by the “god” of Nabukelevu, named Tautaumolau. The pair started fighting. A chase ensued through the sky and, as the two twisted and turned, the earth being carried by Tanovo started falling to the ground, where it is said to have “created” islands.

We conclude that the sequence in which these islands are said to have been created is likely to represent the movement of the ash plume from the eruption, as shown on the map below.

‘Myths’ based in fact

Geologists would today find it exceedingly difficult to deduce such details of an ancient eruption. But here, in the oral traditions of Kadavu people, this information is readily available.

Another detail we would never know if we did not have the oral traditions is about the tsunami the eruption caused.

In some versions of the story, one of the “gods” is so frightened, he hides beneath the sea. But his rival comes along and drinks up all the water at that place, a detail our research interprets as a memory of the ocean withdrawing prior to tsunami impact.

Other details in the oral traditions recall how one god threw a massive spear at his rival but missed, leaving behind a huge hole in a rock. This is a good example of how landforms likely predating the eruption can be retrofitted to a narrative.

Our study adds to the growing body of scientific research into “myths” and “legends”, showing that many have a basis in fact, and the details they contain add depth and breadth to our understanding of human pasts.

The Kadavu volcano stories discussed here also show ancient societies were no less risk aware and risk averse than ours are today. The imperative was to survive, greatly aided by keeping alive memories of all the hazards that existed in a particular place.

Australian First Peoples’ cultures are replete with similar stories.

Literate people, those who read and write, tend to be impressed by the extraordinary time depth of oral traditions, like those about the 2,500-year old eruption of Nabukelevu. But not everyone is.

In early 2019, I was sitting and chatting to Ratu Petero Uluinaceva in Waisomo Village, after he had finished relating the Ono people’s story of the eruption. I told him this particular story recalled events which occurred more than two millennia ago – and thought he might be impressed. But he wasn’t.

“We know our stories are that old, that they recall our ancient history,” he told me with a grin. “But we are glad you have now learned this too!”