Gravitational tugs from Jupiter and Venus slightly elongate Earth's orbit every 405 000 years, an amazingly consistent pattern that has influenced our planet's climate for at least 215 million years. This pattern allows scientists to more precisely date geological events like the spread of dinosaurs, according to a Rutgers-led study.
"It's an astonishing result because this long cycle, which had been predicted from planetary motions through about 50 million years ago, has been confirmed through at least 215 million years ago," said lead author Dennis V. Kent, a Board of Governors professor in the Department of Earth and Planetary Sciences at Rutgers University-New Brunswick.
"Scientists can now link changes in the climate, environment, dinosaurs, mammals and fossils around the world to this 405 000-year cycle in a very precise way," Kent said.
The scientists linked reversals in the Earth's magnetic field - when compasses point south instead of north and vice versa - to sediments with and without zircons (minerals with uranium that allow radioactive dating) as well as to climate cycles.
"The climate cycles are directly related to how the Earth orbits the sun and slight variations in sunlight reaching Earth lead to climate and ecological changes," said Kent, who studies Earth's magnetic field.
"The Earth's orbit changes from close to perfectly circular to about 5% elongated especially every 405 000 years," he said.
The scientists studied the long-term record of reversals in the Earth's magnetic field in sediments in the Newark basin, a prehistoric lake that spanned most of New Jersey, and in sediments with volcanic detritus including zircons in the Chinle Formation in Petrified Forest National Park in Arizona. They collected a core of rock from the Triassic Period, some 202 million to 253 million years ago. The core is 63.5 mm (2.5 inches) in diameter and about 518 m (1 700 feet) long, Kent said.
The results showed that the 405 000-year cycle is the most regular astronomical pattern linked to the Earth's annual turn around the Sun.
Prior to this study, dates to accurately time when magnetic fields reversed were unavailable for 30 million years of the Late Triassic. That's when dinosaurs and mammals appeared and the Pangea supercontinent broke up. The break-up led to the Atlantic Ocean forming, with the sea-floor spreading as the continents drifted apart, and a mass extinction event that affected dinosaurs at the end of that period, Kent said.
"Developing a very precise time-scale allows us to say something new about the fossils, including their differences and similarities in wide-ranging areas," he said.
Rhythmic climate cycles of various assumed frequencies recorded in sedimentary archives are increasingly used to construct a continuous geologic timescale. However, the age range of valid theoretical orbital solutions is limited to only the past 50 million years. New U–Pb zircon dates from the Chinle Formation tied using magnetostratigraphy to the Newark–Hartford astrochronostratigraphic polarity timescale provide empirical confirmation that the unimodal 405-kiloyear orbital eccentricity cycle reliably paces Earth’s climate back to at least 215 million years ago, well back in the Late Triassic Period.
The Newark–Hartford astrochronostratigraphic polarity timescale (APTS) was developed using a theoretically constant 405-kiloyear eccentricity cycle linked to gravitational interactions with Jupiter–Venus as a tuning target and provides a major timing calibration for about 30 million years of Late Triassic and earliest Jurassic time. While the 405-ky cycle is both unimodal and the most metronomic of the major orbital cycles thought to pace Earth’s climate in numerical solutions, there has been little empirical confirmation of that behavior, especially back before the limits of orbital solutions at about 50 million years before present. Moreover, the APTS is anchored only at its younger end by U–Pb zircon dates at 201.6 million years before present and could even be missing a number of 405-ky cycles. To test the validity of the dangling APTS and orbital periodicities, we recovered a diagnostic magnetic polarity sequence in the volcaniclastic-bearing Chinle Formation in a scientific drill core from Petrified Forest National Park (Arizona) that provides an unambiguous correlation to the APTS. New high precision U–Pb detrital zircon dates from the core are indistinguishable from ages predicted by the APTS back to 215 million years before present. The agreement shows that the APTS is continuous and supports a stable 405-kiloyear cycle well beyond theoretical solutions. The validated Newark–Hartford APTS can be used as a robust framework to help differentiate provinciality from global temporal patterns in the ecological rise of early dinosaurs in the Late Triassic, amongst other problems.
Source: Rutgers University
Empirical evidence for stability of the 405-kiloyear Jupiter–Venus eccentricity cycle over hundreds of millions of years - Dennis V. Kent, Paul E. Olsen, Cornelia Rasmussen, Christopher Lepre, Roland Mundil, Randall B. Irmis, George E. Gehrels, Dominique Giesler, John W. Geissman, and William G. Parker - PNAS May 7, 2018. 201800891; published ahead of print May 7, 2018. https://doi.org/10.1073/pnas.1800891115
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