WEven though we are in the space age, our ideas about the sky are still mixed with ancient Greek thoughts. Similar to the classical Greek thinkers, we tend to imagine the sky as a place of order and harmony, with planets and moons in graceful, unchanging orbits.
As Johannes Kepler and Isaac Newton later proved, this is approximately true. However, in detail, the movements of the planets are messy and unpredictable. Like the arguing gods the Greeks once imagined them to be, the planets play and pull at each other, and these gravitational actions cause them to tilt, wobble, and nod as they orbit the sun. Although science has rejected the Greek belief in astrology—the idea that celestial bodies control human destinies—the Earth as a whole truly does experience the pull of other planets. In fact, the sky may be responsible for some of Earth’s more disorderly behaviors and even what we, following the Greeks, call “disasters”—literally, bad stars.
Examination of ice cores has revealed that gravitational impacts on the Earth from the sun and nearby planets lead to cyclical changes in our planet’s orientation and movement over thousands of years, influencing long-term climate outcomes. Referred to as Milankovitch cycles, they are named after the Serbian geophysicist and astronomer Milutin Milankovitch who calculated their mathematical complexities in the 1920s. They appear to have regulated, for instance, the glacial-interglacial fluctuations during the Pleistocene, or the Ice Age, which lasted from about 2.5 million to 12,000 years ago.
It’s the geologic equivalent of a Rube Goldberg machine.
The Milankovitch cycles involve changes in how elongated Earth’s orbit around the sun is (referred to as its eccentricity); the tilt of the Earth’s rotation axis (its obliquity); and which hemisphere inclines toward the sun at different points in its orbit (precession). Each of these, over periods ranging from 19,000 to 400,000 years, impacts the way that sunlight reaches the Earth, which in turn supervises processes in our atmosphere, oceans, and ecosystems. These orbital cycles are similar to “super-seasons” lasting not months but tens of thousands of years.
Recently, French and Australian geoscientists discovered evidence that longer-term orbital changes have also influenced Earth in the more ancient geologic past. Known as “astronomical grand cycles,” these changes have durations of 1 million years or more. This makes them too lengthy to detect even in the oldest ice in Antarctica, which dates to about 800,000 years ago. The new research, published in Nature Communications, instead uses data from a different natural archive: deep-sea sediments, which accumulate slowly and provide a high-quality record of climate and ocean conditions over geologic timescales.
The group, headed by Adriana Dutkiewicz at the University of Sydney, collected data from nearly 300 deep-sea cores from around the world that hold records of Earth’s history spanning the past 70 million years. Although previous researchers have sought signs of Milankovitch rhythms in sediments and sedimentary rocks (an approach known as cyclostratigraphy), the new study is among the first to look for evidence of the longer-term astronomical grand cycles in sediments.
In the past, cyclostratigraphy research viewed gaps in sedimentary layers as mistakes, but Dutkiewicz and coauthors realized these gaps could be important signals of powerful deep-sea currents eroding sediments on the seafloor.
The team discovered a previously unrecognized cyclical pattern in the behavior of the world’s oceans by studying the frequency of periods of “silence” in the sediment record. They found evidence of recurring changes in global ocean currents over millions of years, driven indirectly by the planet Mars.
At the core of the scientists’ work is a statistical method called Fourier analysis, which separates complex time-series data into different frequencies. Fourier analysis showed that the gaps in the sedimentary record have a strong 2.4-million-year cycle, indicating that every 2.4 million years, ocean circulation becomes more vigorous and erases the sedimentary record of the preceding interval.
Dutkiewicz and colleagues propose a connection between tiny changes in the gravitational pull of Mars on Earth and the formation of hiatuses in the sedimentary record. When Earth and Mars are at critical points in the 2.4 million year cycle, global temperatures tend to be around 1.75 degrees Celsius higher than average, triggering changes in ocean currents and leading to the formation of hiatuses.
There was no ice at the poles and sea level was more than 325 feet higher than today.
Geoscientists view the conclusion of the new paper as controversial, suggesting that the “Paleocene-Eocene Thermal Maximum” was triggered by a disruption in the 2.4 million-year Earth-Mars cycle. The researchers note that the grand cycles are periodically interrupted by times of “chaos” before settling into longer-term rhythms. They also suggest that the 2.4 million-year signal in their hiatus data breaks down at the time of the PETM, indicating a link between the climate catastrophe and an interval of what they call “Solar System chaos.”
The PETM period is notable and scary because carbon dioxide in the air suddenly increased, the oceans became very acidic, and both marine and land-based ecosystems were destabilized. An analysis of carbon-bearing rocks like limestones from that time shows that the sudden increase of carbon dioxide came from plants or phytoplankton and later changed to oil, gas, peat, or coal (known to humans as fossil fuels). It's possible that volcanic activity in the North Atlantic region caused this by igniting coal beds in many places. This led to a rise in global temperatures by almost 10 degrees Celsius and stayed high for about 170,000 years.
Many geologists see the PETM as a worrying look into what the next few thousand years might be like on Earth. During the PETM, there was no ice at the poles, sea level was more than 325 feet higher than today, and there were palm trees in Wyoming.
The new study from Dutkiewicz and her team lacks a detailed explanation for how an extreme event like the PETM might have been caused by the disturbance of weak gravitational interactions between Earth and Mars. They recognize that the narrowing of a submarine corridor in the Norwegian-Greenland sea, due to plate tectonic movements around 56 million years ago, likely had a bigger impact on ocean circulation at the time. This constriction would have limited the volume of ocean water entering and leaving the Arctic ocean basin.
If there is any connection to the PETM from space, it can only be because a small orbital "nudge" was greatly amplified in some complex way by Earth itself through more localized knock-on effects involving interactions between the ocean, atmosphere, rocks, and lifeforms. However, the study doesn't explain how a tiny tug from Mars could have released the huge amounts of carbon dioxide that turned Earth into a Hades-like hothouse for over 100,000 years.
It's definitely attractive to find a clear astronomical signal in the murky record of deep-sea sediments. However, it's also risky to see Earth as a helpless puppet moving in space at the gravitational influence of other objects. If we believe that Earth’s climate is mainly controlled by astronomical forces, we may be inclined to think we don't need to worry about the large-scale effects of our own actions. The climate system is extremely complex, and thinking that its variability can be linked to a single cause is arrogance, the fatal flaw of many heroes of Greek myth.
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