arXiv subjects

Richard E. Zeebe

Publications and source records attributed to Richard E. Zeebe.

12 recordsLinked to original sources

arXiv2025

No influence of passing stars on paleoclimate reconstructions over the past 56 million years

Passing stars (also called stellar flybys) have notable effects on the solar system's long-term dynamical evolution, injection of Oort cloud comets into the solar system, properties of trans-Neptunian objects, and more. Based on a simplified solar system model, omitting the Moon and the Sun's quadrupole moment J2, it has recently been suggested that passing stars are also an important driver of paleoclimate before ~50 Myr ago, including a climate event called the Paleocene-Eocene Thermal Maximum (~56 Myr ago). In contrast, using a state-of-the-art solar system model, including a lunar contribution and J2, and random stellar parameters (>400 simulations), we find no influence of passing stars on paleoclimate reconstructions over the past 56 Myr. Even in an extreme flyby scenario in which the Sun-like star HD 7977 (m = 1.07 M_Sun) would have passed within ~3,900 au about 2.8 Myr ago (with 5% likelihood), we detect no discernible change in Earth's orbital evolution over the past 70 Myr, compared to our standard model. Our results indicate that a complete physics model is essential to accurately study the effects of stellar flybys on Earth's orbital evolution.

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arXiv2025

Reduced solar quadrupole moment compensates for lack of asteroids in long-term solar system integrations

State-of-the-art long-term solar system integrations include several second order effects such as the Sun's quadrupole moment J2 and a contribution from asteroids (plus the Moon and general relativity). We recently showed that including 10 asteroids and a reduced J2 in our astronomical solutions provides the best match with geologic data to -58 Myr. However, the rationale for the reduced J2 remained ambiguous and may suggest that parameters for long-term integrations compatible with geologic observations are not fully compatible with our knowledge of the current solar system (specifically J2). Here we show that a reduced J2 compensates for a diminished asteroid population in long-term solar system integrations, which may appear surprising. We present an analysis and offer a mechanism for the long-term compensating effects of J2 and asteroid mass in the solar system (not planetary systems in general). Our analysis suggests that "differential effects" on specific secular frequencies involved in resonant terms (i.e., (g4-g3) and (s4-s3)), are critical in the long term, rather than short-term effects on the orbital elements of individual planetary orbits across the board. Also, our results indicate that if long-term intergrations including the full asteroid population were computationally feasible, a J2 value (within errors) compatible with our current knowledge of the solar system could be used. Attempts to improve the long-term accuracy of astronomical solutions by, e.g., tinkering with initial conditions using current/future astronomical observations are futile unless asteroid deficiencies in the solar system model are addressed.

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arXiv2024

Applying Astronomical Solutions and Milankovi{ć} Forcing in the Earth Sciences

Astronomical solutions provide calculated orbital and rotational parameters of solar system bodies based on the dynamics and physics of the solar system. Application of astronomical solutions in the Earth sciences has revolutionized our understanding in at least two areas of active research. (i) The Astronomical (or Milankovic) forcing of climate on time scales > ~10 kyr and (ii) the dating of geologic archives. The latter has permitted the development of the astronomical time scale, widely used today to reconstruct highly accurate geological dates and chronologies. The tasks of computing vs. applying astronomical solutions are usually performed by investigators from different backgrounds, which has led to confusion and recent inaccurate results on the side of the applications. Here we review astronomical solutions and Milankovic forcing in the Earth sciences, primarily aiming at clarifying the astronomical basis, applicability, and limitations of the solutions. We provide a summary of current up-to-date and outdated astronomical solutions and their valid time span. We discuss the fundamental limits imposed by dynamical solar system chaos on astronomical calculations and geological/astrochronological applications. We illustrate basic features of chaotic behavior using a simple mechanical system, i.e., the driven pendulum. Regarding so-called astronomical "metronomes", we point out that the current evidence does not support the notion of generally stable and prominent metronomes for universal use in astrochronology and cyclostratigraphy. We also describe amplitude and frequency modulation of astronomical forcing signals and the relation to their expression in cyclostratigraphic sequences. Furthermore, the various quantities and terminology associated with Earth's axial precession are discussed in detail. Finally, we provide some suggestions regarding practical considerations.

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arXiv2024

Milanković Forcing in Deep Time

Astronomical (or Milanković) forcing of the Earth system is key to understanding rhythmic climate change on time scales >~ 10 kyr. Paleoceanographic and paleoclimatological applications concerned with past astronomical forcing rely on astronomical calculations (solutions), which represent the backbone of cyclostratigraphy and astrochronology. Here we present state-of-the-art astronomical solutions over the past 3.5 Gyr. Our goal is to provide tuning targets and templates for interpreting deep-time cyclostratigraphic records and designing external forcing functions in climate models. Our approach yields internally consistent orbital and precession-tilt solutions, including fundamental solar system frequencies, orbital eccentricity and inclination, lunar distance, luni-solar precession rate, Earth's obliquity, and climatic precession. Contrary to expectations, we find that the long eccentricity cycle (previously assumed stable and labeled ''metronome'', recent period ~405 kyr), can become unstable on long time scales. Our results reveal episodes during which the long eccentricity cycle is very weak or absent and Earth's orbital eccentricity and climate-forcing spectrum are unrecognizable compared to the recent past. For the ratio of eccentricity-to-inclination amplitude modulation (frequently observable in paleorecords) we find a wide distribution around the recent 2:1 ratio, i.e., the system is not restricted to a 2:1 or 1:1 resonance state. Our computations show that Earth's obliquity was lower and its amplitude (variation around the mean) significantly reduced in the past. We therefore predict weaker climate forcing at obliquity frequencies in deep time and a trend toward reduced obliquity power with age in stratigraphic records. For deep-time stratigraphic and modeling applications, the orbital parameters of our 3.5-Gyr integrations are made available at 400-year resolution.

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arXiv2024

A secular solar system resonance that disrupts the dominant cycle in Earth's orbital eccentricity (g2-g5): Implications for astrochronology

The planets' gravitational interaction causes rhythmic changes in Earth's orbital parameters (also called Milanković cycles), which have powerful applications in geology and astrochronology. For instance, the primary astronomical eccentricity cycle due to the secular frequency term (g2-g5) (~405 kyr in the recent past) utilized in deep-time analyses is dominated by Venus' and Jupiter's orbits, aka long eccentricity cycle. The widely accepted and long-held view is that (g2-g5) was practically stable in the past and may hence be used as a "metronome" to reconstruct accurate ages and chronologies. However, using state-of-the-art integrations of the solar system, we show here that (g2-g5) can become unstable over long time scales, without major changes in, or destabilization of, planetary orbits. The (g2-g5) disruption is due to the secular resonance $σ_{12}$ = (g1 - g2) + (s1 - s2), a major contributor to solar system chaos. We demonstrate that entering/exiting the $σ_{12}$ resonance is a common phenomenon on long time scales, occurring in ~40% of our solutions. During $σ_{12}$-resonance episodes, (g2-g5) is very weak or absent and Earth's orbital eccentricity and climate-forcing spectrum are unrecognizable compared to the recent past. Our results have fundamental implications for geology and astrochronology, as well as climate forcing because the paradigm that the longest Milanković cycle dominates Earth's astronomical forcing, is stable, and has a period of ~405 kyr requires revision.

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arXiv2023

orbitN: A symplectic integrator for planetary systems dominated by a central mass -- Insight into long-term solar system chaos

Reliable studies of the long-term dynamics of planetary systems require numerical integrators that are accurate and fast. The challenge is often formidable because the chaotic nature of many systems requires relative numerical error bounds at or close to machine precision (~1e-16, double-precision arithmetic), otherwise numerical chaos may dominate over physical chaos. Currently, the speed/accuracy demands are usually only met by symplectic integrators. For example, the most up-to-date long-term astronomical solutions for the solar system in the past (widely used in, e.g., astrochronology and high-precision geological dating) have been obtained using symplectic integrators. Yet, the source codes of these integrators are unavailable. Here I present the symplectic integrator orbitN (lean version 1.0) with the primary goal of generating accurate and reproducible long-term orbital solutions for near-Keplerian planetary systems (here the solar system) with a dominant mass M0. Among other features, orbitN-1.0 includes M0's quadrupole moment, a lunar contribution, and post-Newtonian corrections (1PN) due to M0 (fast symplectic implementation). To reduce numerical roundoff errors, Kahan compensated summation was implemented. I use orbitN to provide insight into the effect of various processes on the long-term chaos in the solar system. Notably, 1PN corrections have the opposite effect on chaoticity/stability on 100-Myr vs. Gyr-time scale. For the current application, orbitN is about as fast or faster (factor 1.15-2.6) than comparable integrators, depending on hardware. The orbitN source code (C) is available at github.com/rezeebe/orbitN.

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arXiv2022

Reduced variations in Earth's and Mars' orbital inclination and Earth's obliquity from 58 to 48 Myr ago due to solar system chaos

The dynamical evolution of the solar system is chaotic with a Lyapunov time of only $\sim$5 Myr for the inner planets. Due to the chaos it is fundamentally impossible to accurately predict the solar system's orbital evolution beyond $\sim$50 Myr based on present astronomical observations. We have recently developed a method to overcome the problem by using the geologic record to constrain astronomical solutions in the past. Our resulting optimal astronomical solution (called ZB18a) shows exceptional agreement with the geologic record to $\sim$58 Ma (Myr ago) and a characteristic resonance transition around 50 Ma. Here we show that ZB18a and integration of Earth's and Mars' spin vector based on ZB18a yield reduced variations in Earth's and Mars' orbital inclination and Earth's obliquity (axial tilt) from $\sim$58 to $\sim$48 Ma -- the latter being consistent with paleoclimate records. The changes in the obliquities have important implications for the climate histories of Earth and Mars. We provide a detailed analysis of solar system frequencies ($g$- and $s$-modes) and show that the shifts in the variation in Earth's and Mars' orbital inclination and obliquity around 48 Ma are associated with the resonance transition and caused by changes in the contributions to the superposition of $s$-modes, plus $g$-$s$-mode interactions in the inner solar system. The $g$-$s$-mode interactions and the resonance transition (consistent with geologic data) are unequivocal manifestations of chaos. Dynamical chaos in the solar system hence not only affects its orbital properties, but also the long-term evolution of planetary climate through eccentricity and the link between inclination and axial tilt.

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arXiv2021

Stepsize errors in the $N$-body problem: discerning Mercury's true possible long-term orbits

Numerical integrations of the Solar System have been carried out for decades. Their results have been used, for example, to determine whether the Solar System is chaotic, whether Mercury's orbit is stable, or to help discern Earth's climate history. We argue that all of the past studies we consider in this work are affected by numerical chaos to different degrees, affecting the possible orbits and instability probability of Mercury, sometimes significantly. We show how to eliminate the effects of numerical chaos by resolving Mercury's pericentre passage. We also show that several higher order symplectic maps do not exhibit significant differences in resolving pericentre passage of Mercury (at fixed time step), making their advantages suspect for calculating long-term orbits. Resolving pericentre passage affects a wide array of orbital numerical studies, like exoplanet studies, studies of the galactic centre, and other $N$-body problems.

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arXiv2019

Solar system chaos and the Paleocene-Eocene boundary age constrained by geology and astronomy

Astronomical calculations reveal the solar system's dynamical evolution, including its chaoticity, and represent the backbone of cyclostratigraphy and astrochronology. An absolute, fully calibrated astronomical time scale has hitherto been hampered beyond $\sim$50 Ma, because orbital calculations disagree before that age. Here we present geologic data and a new astronomical solution (ZB18a), showing exceptional agreement from $\sim$58 to 53 Ma. We provide a new absolute astrochronology up to 58 Ma and a new Paleocene-Eocene boundary age (56.01 $\pm$ 0.05 Ma). We show that the Paleocene-Eocene Thermal Maximum (PETM) onset occurred near a 405-kyr eccentricity maximum, suggesting an orbital trigger. We also provide an independent PETM duration (170 $\pm$ 30 kyr) from onset to recovery inflection. Our astronomical solution requires a chaotic resonance transition at $\sim$50 Ma in the solar system's fundamental frequencies.

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arXiv2017

Numerical Solutions for the orbital motion of the Solar System over the Past 100 Myr: Limits and new results

I report results from accurate numerical integrations of Solar System orbits over the past 100Myr with the integrator package HNBody. The simulations used different integrator algorithms, step sizes, initial conditions, and included effects from general relativity, different models of the Moon, the Sun's quadrupole moment, and up to sixteen asteroids. I also probed the potential effect of a hypothetical Planet 9, using one set of possible orbital elements. The most expensive integration (Bulirsch-Stoer) required 4~months wall-clock time with a maximum relative energy error <~3e{-13}. The difference in Earth's eccentricity (DeE) was used to track the difference between two solutions, considered to diverge at time tau when max|DeE| irreversibly crossed ~10\% of mean eE (~0.028x0.1). The results indicate that finding a unique orbital solution is limited by initial conditions from current ephemerides and asteroid perturbations to ~54Myr. Bizarrely, the 4-month Bulirsch-Stoer integration and a symplectic integration that required only 5~hours wall-clock time (12-day time step, Moon as a simple quadrupole perturbation), agree to ~63Myr. Internally, such symplectic integrations are remarkably consistent even for large time steps, suggesting that the relationship between time step and tau is not a robust indicator for the absolute accuracy of symplectic integrations. The effect of a hypothetical Planet~9 on DeE becomes discernible at ~65Myr. Using tau as a criterion, the current state-of-the-art solutions all differ from previously published results beyond ~50Myr. I also conducted an eigenmode analysis, which provides some insight into the chaotic nature of the inner Solar System. The current study provides new orbital solutions for applications in geological studies.

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arXiv2015

Highly stable evolution of Earth's future orbit despite chaotic behavior of the Solar System

Due to the chaotic nature of the Solar System, the question of its dynamic long-term stability can only be answered in a statistical sense, e.g. based on numerical ensemble integrations of nearby orbits. Destabilization, including catastrophic encounters and/or collisions involving the Earth, has been suggested to be initiated through a large increase in Mercury's eccentricity (eM), with an estimated probability of ~1%. However, it has recently been shown that the statistics of numerical Solar System integrations are sensitive to the accuracy and type of numerical algorithm. Here I report results from computationally demanding ensemble integrations (N=1,600 with slightly different initial conditions) at unprecedented accuracy based on the full equations of motion of the eight planets and Pluto over 5Gyr, including contributions from general relativity. The standard symplectic algorithm produced spurious results for highly eccentric orbits and during close encounters, which were hence integrated with a suitable Bulirsch-Stoer algorithm, specifically designed for these situations. The present study yields odds for a large increase in Mercury's eccentricity that are less than previous estimates. Strikingly, in two solutions Mercury continued on highly eccentric orbits (after reaching eM values >0.93) for 80-100Myr before colliding with Venus or the Sun. Most importantly, none of the 1,600 solutions led to a close encounter involving the Earth or a destabilization of Earth's orbit in the future. I conclude that Earth's orbit is dynamically highly stable for billions of years, despite the chaotic behavior of the Solar System.

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arXiv2015

Dynamic stability of the Solar System: Statistically inconclusive results from ensemble integrations

Due to the chaotic nature of the Solar System, the question of its long-term stability can only be answered in a statistical sense, for instance, based on numerical ensemble integrations of nearby orbits. Destabilization of the inner planets, leading to close encounters and/or collisions can be initiated through a large increase in Mercury's eccentricity, with a currently assumed likelihood of ~1%. However, little is known at present about the robustness of this number. Here I report ensemble integrations of the full equations of motion of the eight planets and Pluto over 5 Gyr, including contributions from general relativity. The results show that different numerical algorithms lead to statistically different results for the evolution of Mercury's eccentricity (eM). For instance, starting at present initial conditions (eM ~= 0.21), Mercury's maximum eccentricity achieved over 5 Gyr is on average significantly higher in symplectic ensemble integrations using heliocentricthan Jacobi coordinates and stricter error control. In contrast, starting at a possible future configuration (eM ~= 0.53), Mercury's maximum eccentricity achieved over the subsequent 500 Myr is on average significantly lower using heliocentric than Jacobi coordinates. For example, the probability for eM to increase beyond 0.53 over 500 Myr is >90% (Jacobi) vs. only 40-55% (heliocentric). This poses a dilemma as the physical evolution of the real system - and its probabilistic behavior - cannot depend on the coordinate system or numerical algorithm chosen to describe it. Some tests of the numerical algorithms suggest that symplectic integrators using heliocentric coordinates underestimate the odds for destabilization of Mercury's orbit at high initial eM.

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