One of the four main Milankovitch cycles is a 405,000 year fluctuation in the Earth’s orbital eccentricity. This means that the Earth’s orbit gets more and less circular over that period, affecting the extremes of climate. The variation is due to the effects of Jupiter and Venus which are the two planets with the greatest tidal effect.
“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.
The 27-My Newark–Hartford astrochronostratigraphic polarity timescale (APTS) (1) is one of the longest continuous APTS segments presently available and calibrates much of the Late Triassic and earliest Jurassic geologic timescale. It relies on the geomagnetic polarity and cycle stratigraphies of over 6,900 m of coring in the Newark basin (2⇓–4) and an overlapping 2,500-m-thick outcrop section in the nearby Hartford basin (5), both in eastern North America. The composite continental record is paced by 66 McLaughlin lithologic cycles spanning 27 My of the Norian and Rhaetian of the Late Triassic and Hettangian and Sinemurian of the Early Jurassic, reflecting the response of climate to the long astronomical eccentricity variation with a presumed 405-ky period. The 405-ky period cycle is related to the gravitational interaction of Jupiter and Venus (g2–g5 cycle) and is the prominent and most stable term in the approximation of eccentricity of Earth’s orbital variations on geologic timescales despite chaotic behavior of the Solar System (6). The record also encompasses 51 Poisson-distributed geomagnetic polarity intervals (with an additional 15 polarity intervals in the fluvial noncyclic sediments toward the base of the section, which by extrapolation of sediment accumulation rates extend the record an additional ∼6 My into the Carnian), providing a template for global correlation. However, the astronomically paced polarity sequence is anchored at essentially only one level. High-precision U–Pb zircon dates in lavas and intrusions of the Central Atlantic Magmatic Province (CAMP), which are clustered in close spatiotemporal proximity to the Triassic–Jurassic boundary (7), were collapsed to a calibration age of 201.6 Ma for the onset of Chron E23r close to the base of a McLaughlin cycle (Ecc405: k = 498.25, where k is the inferred number of 405-ky eccentricity cycles projected back from the most recent maximum at 0.216 million years ago as k = 1), which immediately underlies the oldest CAMP basalts. In the absence of other directly dated horizons, the APTS relies on the assumption of a continuous section with no substantial hiatus(es) in deposition of these continental sediments, which would potentially result in one or more unrecorded 405-ky cycles. The specific timing of the APTS is also dependent on the untested reliability at this distant age range of the 405-ky cycle.
The developing APTS has been successfully used for global correlations in marine and nonmarine facies (e.g., refs. 8 and 9; see ref. 1). Nevertheless, there have been persistent suggestions made largely on the basis of nonmarine biostratigraphic correlations that several million years of Rhaetian (latest Triassic) time are missing in a cryptic unconformity that supposedly occurs just above Chron E23r in the Newark Supergroup basins (e.g., refs. 10 and 11). If true, this would have consequences of comparable magnitude in the timing of events based on anchoring the astrochronology below the alleged cryptic unconformity to the U–Pb-dated CAMP lavas above it. Therefore, a test is needed of the conjoined assumptions of stratigraphic continuity and the 405-ky periodicity for the long eccentricity cycle that are implicit in the construction of the Newark–Hartford APTS.
The Late Triassic-age Chinle Formation (Fm.) of the American Southwest (Fig. 1A) consists of fluvial and minor lacustrine facies interfingered with paleosols and, importantly for the task at hand, has numerous sandstone horizons with volcaniclastic detritus containing detrital zircons amenable for U–Pb dating (12). The Black Forest Bed (BFB) within the Petrified Forest Member (Mb.) of the Chinle Fm. at Chinde Point in the northern sector of Petrified Forest National Park (PFNP) was the first unit to be successfully U–Pb zircon dated in the PFNP section (13). Recent high-precision U–Pb zircon dating of the BFB (12) made it an attractive target for calibration of the APTS. Sampling of outcrop sections demonstrated the feasibility of obtaining a magnetostratigraphy (14) even though revised long-distance lithostratigraphic correlations of the Sonsela Mb. of the Chinle Fm. (15) indicate that there may be a large gap in the composite magnetostratigraphic section. A main scientific objective of the inaugural drilling effort of the Colorado Plateau Coring Project (16) at PFNP (core CPCP-PFNP-13-1A; henceforth PFNP-1A) (Fig. 1B) was to obtain a magnetostratigraphic sequence directly supported by high-precision U–Pb detrital zircon dates for the Chinle Fm. with unequivocal superposition and a diagnostic polarity signature for correlation to the APTS. Here, we report on paleomagnetic and geochronologic results from the upper ∼280 m of the 519-m-long core recovering a section of the Chinle Fm. down from the lower Owl Rock Mb. to the Mesa Redondo Mb., the Moenkopi Fm. and the uppermost Coconino Sandstone”