by Viviane Callier Friday, May 11, 2018
Earth’s rock record preserves evidence of numerous natural processes, from evolution and extinction to catastrophes and climate change, and sometimes even planetary configurations.
In a new study, a well-preserved sequence of Triassic lake sediments bearing evidence of cyclical patterns of climate change in the Newark Basin confirms the existence of a Milankovitch cycle — a periodic change in the shape of Earth’s orbit caused by, in this case, Earth’s gravitational interactions with Jupiter and Venus. The finding can be used to precisely date other events in the geological record, and inform climate and astronomical models.
Geologists previously suggested the patterns found in the Newark Basin, an ancient rift basin covering northern New Jersey, southeastern Pennsylvania and southern New York, could reflect the climatic effects of a predicted 405,000-year Milankovitch cycle, but the Newark Basin sediments were unable to be dated precisely enough to confirm the link.
Other Milankovitch cycles — a 23,000-year cycle related to the wobble of Earth’s axis, a 41,000-year cycle related to the tilt of the axis, and a 100,000-year cycle related to orbital eccentricity — are relatively well established based on glaciological and sedimentary records. Such astronomical cycles influence how much solar energy the planet receives and thus alters climate, for example by producing wet and dry periods, which leave their mark in the rock record.
In the study, published in Proceedings of the National Academy of Sciences, geologist Dennis Kent of Columbia University’s Lamont-Doherty Earth Observatory and Rutgers University and colleagues assigned dates to the Newark Basin rock record by correlating it with rocks cored from the Upper Triassic Chinle Formation in Petrified Forest National Park in Arizona. Uranium-lead dating of zircons found in volcanic ash layers produced precise ages for the Arizona rocks, which the team then correlated to the Newark Basin rocks by matching the pattern of Earth’s magnetic field reversals recorded in both places. The key is that “the Newark core has the [climatic] cycles. The Arizona core has the dates,” Kent says.
The researchers found that the dates of the climate cycles recorded in the Newark Basin rock record match the 405,000-year cycle predicted by astronomical models, providing the first empirical evidence for the cycle’s existence and stability over the last 215 million years.
“This is the first time we can actually confirm the theoretical description of the 405,000-year cycle,” says geologist Linda Hinnov of George Mason University, who was not involved in the study. “The astronomers have given us this model but we’ve never been able to establish that it’s actually accurate until now.”
However, a concern raised by Spencer Lucas, curator of paleontology at the New Mexico Museum of Natural History and Science, is that while the Newark record was deposited in an ancient lake, the Arizona sequence was deposited by a river, which makes it more likely to have gaps in the magnetic field history it records. “If you want a complete record of magnetic polarity, you have to have a complete pile of sediments. If you don’t have that, then you’re not going to get a complete record,” Lucas says.
Such gaps are to be expected and they are small compared to the overall length of time recorded in the section, which suggests that the overall pattern of magnetic field reversals is still likely to have been recorded accurately, says co-author Paul Olsen, a paleontologist at Columbia University’s Lamont-Doherty Earth Observatory. Also, the gaps do not affect the precision of the zircon dating, he adds.
Confirming the existence of the steady, regular astronomical cycle that has been operating in the same way for more than 200 million years provides astronomers reconstructing the history of the solar system with a predictable marker, much like the beat of a metronome, to calibrate their models.
Current astronomical models apply the laws of Newtonian dynamics and general relativity to reconstruct the solar system’s past configurations, but model predictions fail beyond about 50 million years. That’s because it is difficult to predict the motion of more than two moving bodies in space over very long periods of time. Astronomers are limited to computing the positions of the planets incrementally, and at each increment, errors accumulate.
“In addition, the physics of the solar system is chaotic,” Olsen says, meaning that modern astronomical models are highly sensitive to initial conditions, and therefore, the picture of planetary positions hundreds of millions of years ago can vary widely depending on the initial assumptions and conditions input to a model.
However, because astronomers know the configuration of Earth, Venus and Jupiter when the 405,000-year cycle is at its minimum and maximum, and because those events are evident in the rock record and have now been well-dated, astronomers have more solid data on which to base estimates of planetary configurations going back more than 200 million years.
The discovery represents a linchpin in a hypothesis that Olsen and his colleagues have been developing for decades called the “Geological Orrery,” after the 18th-century mechanical models of the solar system. It holds that climatic change recorded in the geologic record could be used to infer the former positions and motion of planets in the solar system going back hundreds of millions of years.
Olsen suggests that careful examination of the geological record could help refine and point to gaps in astronomical models. The geological record “is a new world of empirical data that allows for tests of large-scale solar system theory,” he says.
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