Exploring the depths of Solar System evolution

Fortunately, there is a rich source of empirical data that records the evolution of Solar System dynamics in ways that are sensitive to the spin rate of the Earth and therefore the Earth–Moon distance. It is an indirect astronomical observatory that is locked up in the depths of ancient strata, preserved as astronomically influenced rhythms in the properties of sediments. A study by Lantink et al. (2) in PNAS highlights how 2.46-billion-y-old rocks—known as banded iron formations—have preserved key information about the evolution of the Earth–Moon system and may yet yield deeper insight into astronomical interactions between Earth and its planetary neighbors.

Banded iron formations, or BIFs, are silica- and iron-rich rocks that formed in ancient oceans, typically displaying a distinct lamination and regularly repeated thin banding. They are relatively abundant in sedimentary strata older than about 2 billion y, and early work attributed them to low oxygen levels, prior to its buildup in the ocean via oxygenic photosynthesis and organic carbon burial (3). This was an ancient dys- or anaerobic world very different from today, and the mechanisms responsible for the deposition of BIFs are diverse and a function of their age, with submarine hydrothermal Fe input and the evolution of microbially mediated pathways for Fe(II) oxidation playing key roles (4). A number of studies have proposed that the distinctly banded and bedded nature of some BIFs is a signature of paleoenvironmental changes that are forced by Earth’s orbit and rotation, ranging from daily cycles to longer-period perturbations (5–7). The latter, also known as “Milankovitch cycles,” are astronomical variations that influence the distribution of sunlight hitting the planet’s surface, which in turn impacts the climate system. Such astronomical cycles have paced the ice ages of the Quaternary, and traces of their influence appear to be a pervasive feature of the sedimentary rock record (8). Nevertheless, documenting Milankovitch cycles in the deepest stretches of Earth history has been challenging. Among the key issues is large uncertainty in the theoretical astronomical models—due to Solar System chaos, and uncertainty in the tidal-dissipation history of the Earth–Moon system (8–10)—which raises the question: What exactly should we be looking for? Add to this problem the relative paucity of suitable sedimentary successions and the difficulty of obtaining hard geochronologic constraints with which to calibrate the tempo of sedimentary rhythms, and the hurdles seem substantial. While we know that Milankovitch cycles encode information about our ancient Solar System, we need some means to provide reliable hypothesis tests.

Analysis of rhythmic strata from the Phanerozoic and Proterozoic has demonstrated how the geological record of Milankovitch cycles can be used to reconstruct the underlying history of the Earth–Moon system [e.g., Earth’s precession constant (10–12)] and Solar System motions [known as the fundamental secular frequencies (10, 13)] that give rise to the cycles and even identify chaotic resonance transitions (10, 14, 15). These studies appeal to geochronologic constraints and the distinctive fingerprint of the astronomical signal: the existence of hierarchies of cyclicity (“frequency ratios”) such as the relationship between 405,000- and ∼100,000-y eccentricity cycles and the expected amplitude modulation of astronomical cycles. Fortunately, the astronomical signal provides a rich and complex fingerprint, with a well-known physical basis, which can be harnessed for hypothesis testing even if the precise details of that history are poorly constrained.

Lantink et al. (2) utilize the astronomical fingerprint, in combination with geochronologic constraints, to evaluate lithologic rhythmicity in the Joffre Member of the Brockman Iron Formation, a Proterozoic BIF in northwestern Australia (5). The primary basis for this evaluation is a detailed measurement of visual bedding patterns at Joffre Falls in Karijini National Park where the unit outcrops (and in a rock core 150 km away), with new U-Pb zircon ages, supplemented by elemental geochemical data. Their quantitative analysis reveals evidence of eccentricity and precession-scale BIF cycles, from which they constrain Earth–Moon history, using the known physics that underlies the precession cycle. The result is an estimated Earth–Moon distance of 321,800 ± 6,500 km and a length of day of 16.9 ± 0.2 h at 2.46 Ga, a value that agrees with recent tidal evolution modeling results (16). This astronomically based assessment dramatically reduces the uncertainty of prior estimates derived from tidalites of a similar age (17). Indeed, astronomical cycles are clearly overcoming the ambiguity of estimates from ancient tidalites and bioarchives, which can have a range of plausible interpretations and can suffer missing laminae that obfuscate their assessment, with large uncertainties. The results of Lantink et al. (2) provide stunning constraints that are capable of discerning between multiple possible models for the Earth–Moon system.

Lantink et al. (2) focused their evaluation on estimating Earth–Moon parameters, but additional information about the evolution of the Solar System is likely embedded in the finer details of these astronomical-BIF rhythms, waiting to be discovered. In particular, interactions of the fundamental secular “g” frequencies, which characterize the frequency of rotation of the location of orbital perihelia (g₁ = Mercury, g₂ = Venus, g₃ = Earth, g₄ = Mars, g₅ = Jupiter, etc.), generate multiple eccentricity cycles of differing period, and the interaction of different g-frequencies with Earth’s precession constant is the source of multiple observed precession periods (8,9). The methodology implemented by Lantink et al. (2) relies on astronomical models spanning the past 80 million y to make assumptions about the evolution of the fundamental secular frequencies that underlie the eccentricity and precession cycles and amalgamates the observed BIF precession signal into one average period. This approach inhibits a reliable assessment of the g-frequencies, but recently developed methodologies do provide the means to evaluate both the fundamental frequencies (10, 13) and the Earth–Moon history (10) in such ancient strata. Equally important are recent advances in the theoretical modeling of the evolution of the fundamental frequencies (18) and the tidal modeling of the precession constant (16, 19), which provide important constraints that can be formally incorporated as prior information in Bayesian inversions (10). However, these approaches require high-quality stratigraphic datasets, ideally with high-precision geochronology, to reconstruct Solar System evolution. The remarkable 2.46-billion-y-old BIFs evaluated by Lantink et al. (2) are exactly that type of record, and their analysis heralds what is possible in terms of constraints on the evolution of our Solar System, which could possibly be extended to Earth’s earliest BIFs deposited ∼3.8 billion y ago. Thus, while direct observation of the prior states of our Solar System is not possible from the vantage point of Earth, the signatures of planetary and lunar orbits, as well as the history of life and environment, are encoded in our planet’s sedimentary cover. Whether we look up toward the stars or down into the depths of the Earth, a rich history is there, waiting to be read.