太陽系のカオスと地球の炭素循環の相互作用を明らかに（Junggar Basin Sediments Reveal Interplay Between Solar System Chaos and Earth’s Carbon Cycle）

The solar system is chaotic and the motions of the planets are unpredictable beyond about 60 Ma. Depositional environments, organic carbon content, and stable carbon isotopes ratios from paleo-high-latitude lake sequences in northwestern China show that the sensitive Mars–Earth orbital eccentricity cycle had a period of about 1.6 Myr during the late Early Jurassic (193 to 175 Ma), very different from today. One cycle at about 183 Ma, embraces the global Jenkyns Event triggered by the Karoo-Ferrar large igneous province causing global warming and migration of low-latitude plants into the region. Combined with older records from elsewhere, these data provide a 45 Myr window constraining possible solutions of planetary motions in the deep past.

Solar system gravitational interactions are embedded in Earth’s record of climate, providing a way to bypass the 60 Myr limit imposed by chaos. Presently with a 2.4 Myr period, the Mars–Earth beat cycle of orbital perihelion frequencies is particularly sensitive to chaotic diffusion, potentially varying by more than a million years. Early Mesozoic (252 to 145 Ma) strata provide some constraints on this cycle, with evidence of a swing through most of the solution space from 1.8 Myr at 210 Ma to 2.5 Myr at 190 Ma and back to 1.6 Myr at 180 Ma. However, only the 1.8 Myr cycle is corroborated by geochronologic data and the 1.6 Myr period is disputed. Here, we show that variations in land-plant-dominated stable carbon isotopic ratios (δ¹³C_org) from the lacustrine, paleo-high-latitude Sangonghe Formation (Junggar Basin, northwestern China), reveal at least three 1.6 Myr Mars–Earth beat cycles centered at 183 Ma, tracking atmospheric CO₂ isotopic composition in Earth’s exchangeable carbon reservoirs. Furthermore, the middle cycle includes the famous Jenkyns Event, expressed here by poleward migration of cheirolepidaceous conifers driven by CO₂ warming from the Karoo-Ferrar large igneous province (LIP). Our data do not, however, support major, LIP-triggered input of isotopically light carbon and instead support CO₂ amplification of local processes via warming and ecosystem change. Although requiring additional independent geochronological support, Sangonghe data help provide empirical constraints for filtering orbital solutions, tightening initial conditions, and testing gravitational models, as well as showing how extrinsic cyclical processes interact with a tectonic event, the Karoo-Ferrar LIP.