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.