Influence of Solar-Insolation Variation on Cyclic Sediments of the Monterey Formation, Analyzed with Microsoft Excel

Jacques Laskar and colleagues at l’Observatorie de Paris are the modern successors to Milankovich. They produced models of orbital parameters extending into the Eocene. However, the uncertainty of orbital solutions beyond 50 Myr led Laskar to conclude that “geological data are now required to discriminate between planetary orbital solutions beyond 50 Myr”. Laskar created Program Insola, which translates variations in orbital eccentricity, obliquity, and precession into calculations of solar insolation, the intensity of solar radiation (watts/sq.m.) received at the top of Earth’s atmosphere. Though Laskar used supercomputers to model orbital parameters, Program Insola is trigonometry and thus within the capabilities of Excel. I will supply you with a link to an Excel spreadsheet that calculates insolation back to 50 Myr. I will offer an example from the Miocene Monterey Formation of California, renowned for its rhythmic cyclicity of biogenic carbonate and siliceous sediment. Insola requires a precise age model to match rock layers to absolute age points on the insolation curve. Uranium–Lead ages from zircon crystals in volcanic ash layers provide age control in this example. Thin dolostone beds alternate with thicker organic mudstone layers in a succession that correlates with solar insolation peaks. The peaks are the product of constructive interference of obliquity and eccentricity cycles. Insolation peaks also drive the expansion of the monsoon belt within the intertropical convergence zone, leading to wet and dry phases evident in shallow water carbonates. Download the Insola spreadsheet, try it with your data, let Laskar know what you find.