Highlight-5

Highlight 5: Milanković forcing through deep time, 0 to 3.5 billion years ago

From Zeebe and Lantink (2024a): Variations in the Earth’s orbital parameters arising from Solar System planetary motions, axial tilt (T) (also called obliquity) and climatic precession (P) contribute to the Milanković forcing of Earth’s paleoclimate system. State-of-the-art modeling back in time, 0 to -3.5 Gyr, yields internally consistent orbital and precession-tilt (PT) solutions for fundamental Solar System frequencies gi and si, Earth’s orbital eccentricity (E), orbital inclination, Earth-Moon distance, Earth rotation rate, Earth precession rate (Ψ), and T and P periodicities. The results reveal the following complexities:

(1) From deep time forward, the T and P periodicities are lengthened by tidal dissipation acting on Earth’s axial precession; E periodicities maintain values mostly close to their recent (0 Gyr) values, but episodically deviate (Fig. 1). Importantly, P, T, and E do not retain their 0 Gyr period ratio in deep time (Table 1).

(2) At -3 Gyr, mean T is ~20º with ±0.1º variations; forward in time, mean T progressively increases to present-day ~24º with ±1.0º variations (Fig. 2). Importantly, this evolution indicates that deep time T has a weak Milanković forcing potential, leaving P (modulated by E) as the principal Milanković forcing mechanism. Only after -1 Gyr does T take on a discernible role, i.e., once it attains ~50% power of 0-Gyr T.

(3) The 405-kyr (g2-g5) orbital eccentricity cycle is weak or absent for extended deep time intervals due to s12 secular resonance interference (Highlight 6). This results in Milanković forcing that is unrecognizable compared to that at 0 Gyr. The episodic loss of this cycle, which is commonly assumed to be stable and used as a tuning “metronome” in astrochronology, presents a previously unrecognized challenge for cyclostratigraphy.

(4) Orbital inclination s4-s3 and orbital eccentricity g4-g3 periodicities have a ratio that is not confined to s43 secular resonance states with 2:1 or 1:1 ratios, but a wide distribution of ratios around 2:1 that are not related to resonance state transitions (Highlight 7).

Table 1: Ratio of Milanković cycle periods in deep time. P=precession, T=tilt (obliquity); E1=100-kyr orbital eccentricity; E2=405-kyr orbital eccentricity. The “period ratio” method is commonly used in cyclostratigraphy to test for the presence of Milanković cycles; the ratio must account for geologic age.

Fig. 1: Evolution of Earth’s ETP periodicities, 0-3.5 Gyr. Orbital eccentricity E (orange and red) is distributed into E1 (~100 kyr) and E2 (405 kyr) periods, comprised of multiple gi combinations. T (green) and P (blue) periods show gi and si of all 64 orbital solution ensemble members; Ψ is from PT solution ensemble member R06 (and is nearly identical for all ensemble members). Tidal dissipation acts through geologic time to decrease Ψ from 176“/y at -3.5 Gyr to 50.38”/y at 0 Gyr; the long-term curvature in T and P is from fitting to cyclostratigraphic estimates of Ψ at -2.5 Gyr (Brockman Iron Formation, Australia), -1.4 Gyr (Xiamaling Formation, China), -0.5 Gyr (Alum Shale, Scandinavia), and -0.055 Gyr (ODP Site 1262, Walvis Ridge). (Figure 8 of Zeebe and Lantink, 2024a).

Fig. 2: Earth’s E, T, and P time series and ETP spectra from solution R02. Orange lines designate common time intervals. A: E over 2-Myr intervals centered from left to right on -3, -2, and -1 Gyr, and from 0 to -0.4 Myr. B: T over 400-kyr intervals. C: P over the same 400-kyr intervals. D: FFT spectra of ETP time series of the intervals shown in A-C; downward-pointing black arrows indicate T power increasing from -3 Gyr to 0 Gyr. Rotated numbers are periods in kyr of major spectral peaks for E, T and P. (Modified from Figures 6 and 7 of Zeebe and Lantink, 2024a.)

References:

Zeebe, R.E., Lantink, M. (2024a), Milanković Forcing in Deep Time, Paleoceanography and Paleoclimatology, 39(5), e2024PA004861, https://doi.org/10.1029/2024PA004861

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