Tempo and Modes of Climate Variability: Perspectives From Deep-Time Rift Basins

Olsen, Paul E.¹ (1) Lamont-Doherty Earth Observatory of Columbia University, Palisades, NY

Lacustrine Triassic-Jurassic rift lakes strata in eastern North America and Morocco show the effects of climate on a very broad hierarchy of timescales. The longest of these records are cores from the 32 m.y., NSF-funded Newark basin coring project from the Newark rift basin that display the full spectrum of precession-related periods expressed as lake-level cycles, in a tectonically evolving basin, itself drifting north from the equator to about 20° N during a major hothouse interval. Varves, bundles of varves (7 - ~2000 yr? solar?), half-period precession (~10 ky), precession (~20 ky), short-eccentricity (~100 ky), long eccentricity (~400 ky), and grand eccentricity (1.75 and 3.5 m.y.) cycles influenced climate and the rift lakes. The 1.75 m.y. grand eccentricity cycle, corresponds to the present 2.4 m.y. g3-g4 eccentricity cycle differing from its present value because of chaotic diffusion of planetary orbits. The longest period cycles bundle the shorter ones into more organic-rich vs organic-poor intervals, but where the richest units occur was geographically and tectonically contingent, deposited during the initial pulses of extension while the basin was closest to the equator. Viewed over different latitudes at the same time, paleoequatorial basins the largest thicknesses of organic-rich strata, while those deposited in the paleosubtropics have none at all. Eastern North American and Moroccan basins show trends towards shallower water and more arid environments through the Triassic, but this was due to a combination of waning extension rates and northward drift and not Triassic aridification as usually assumed.