405 kyr Jupiter-Venus driven Earth eccentricity cycle linked to paleoclimatic variation

A recent review article on PNAS titled ‘Astronomical metronome of geological consequence’ by Linda Hinnov makes interesting reading for talkshoppers.

A Brief Retrospective
In geology, a reliable “metronome” in the geologic record with a sufficiently short repeat time would greatly enhance the resolving power of the geologic timescale. Astronomers recognized the potential importance of a dominant 405-ky cycle in Earth’s orbital eccentricity variation for supplying such a metronome (2, 3), leading geologists to turn to the stratigraphic record of astronomically forced paleoclimate change to search for this cycle. In fact, one of the first geological studies to describe 405-ky scale stratigraphic cycling was on the Triassic–Jurassic Newark Basin lacustrine strata (4, 5) recovered in the National Science Foundation-funded Newark Basin Coring Project, in which each of the prominent 60-m-thick McLaughlin cycles in the cored sequence was assigned a 412.885- ky periodicity based on a now-legacy analytical astronomical solution, BRE74/BER78 (6, 7). Since the 1990s, there have been dozens of reports for strong 405-ky scale cycles in stratigraphic sequences from around the world that appear to bear out this astronomical calculation (8).

Kent et al. (1) demonstrate that assigning a 405-ky periodicity to the McLaughlin cycles is verified by high-precision geochronology. Radioisotopic dates of 209.93 and 210.08 Ma in the Chinle Formation are precisely positioned within the Newark Basin by correlating the reverse polarity magnetozone (PF2r) in which the dates occur to the reverse polarity Chron E16r in the Newark Basin. E16r is dated as 209.95– 210.25 Ma by counting the 405-ky McLaughlin cycles from the chron boundaries to the zircon-dated 201.6-Ma Orange Mountain Basalt (OMB) in the upper part of the core series. Thus, PF2r and E16r have indistinguishable ages and affirm the long-suspected 405-ky periodicity of the McLaughlin cycles. Other radioisotope dates in the Chinle Formation at lower (older, 215 Ma) stratigraphic levels provide additional support. Kent et al. (1) also demonstrate that the McLaughlin cycles are phase-locked with the 405-ky orbital eccentricity cycle of the LA2004 astronomical solution for its nominal [+5, −15 My] integration (table 6 in ref. 3) projected back to 215 Ma.

Revisiting Solar System Chaos
Tuning with the 405-ky metronome should resolve other, higher order cycles related to the quasi–100-ky orbital eccentricity terms. Modulating quasi–100-ky scale cycles are pervasive and prominent in the Newark Basin (5), but they likely do not match any of the theoretical astronomical solutions. This is because the contributing quasi–100-ky terms are thought to be susceptible to disturbance by (unknown) chaotic behavior of the planetary orbits. Assuming that the Chinle dates approximate stratigraphic age, the results presented by Kent et al. disentangle multiple issues concerning Triassic chronostratigraphy, the geologic timescale, paleoclimate, evolution, and Solar System behavior, and open new lines of inquiry. However, chaotic behavior in Earth and Mars orbits predicted by LA2004 from 82 to 90 Ma has recently been observed in the quasi–100-ky and obliquity modulations (g4 −g3 and s4 −s3, respectively) of cyclic stratigraphy from the Western Interior Seaway, United States (25). In that study, all of the chaotic behavior of Earth and Mars occurred as modulation disturbances in quasi–100-ky terms related to g4 −g5, g4 −g2, g3 −g5, and g4 −g2 (table 6 of ref. 3), and detected as a g4 −g3 modulation rapidly transitioning from a 2,400-ky periodicity to a 1,200-ky periodicity, and back (while s4 −s3 maintained a 1,200-ky periodicity). This harkens to Olsen and Kent (26) who described a persistent 1,750-ky periodicity along the entire Newark Basin core sequence that was interpreted as evidence for a g4 −g3 term (i.e., 2,400 ky shortened to 1,750 ky) of the orbital eccentricity, and it is worth further probing Solar System dynamics to explain this behavior. The absence of an obliquity cycle in the Newark Basin strata (to measure s4 − s3) necessitates seeking another coeval astronomical record, with one excellent candidate section emerging in northwest China (27).

Confidence in Astrochronology
The independent confirmation of the 405-ky orbital eccentricity cycle in these ancient sedimentary deposits has been long-sought and will be enthusiastically welcomed by geologists. It represents an important “benchmark” in the rapidly expanding annals of cyclostratigraphy, the branch of stratigraphy dedicated to the study of astronomically forced climate change and the development of an astrochronology to support and refine the geologic timescale.