2003 Seattle Annual Meeting (November 2–5, 2003)

Paper No. 1
Presentation Time: 8:00 AM-12:00 PM


MORRISON, Roger B., 7500 N. Calle Sin Envidia #8104, Tucson, AZ 85718, rbmorrison@earthlink.net

This presentation describes periodicity in Quaternary surficial processes influenced by earth-orbital (Milankovitch) mechanisms. Four ranks of geomorphic cycles operate globally, quasiperiodic in time but varying in timing and amplitude from region to region due to latitude, orography, tectonism, ice cover, changing ocean-current systems, etc. These ranks are: microcycles of ~102 years duration, mesocycles of 103 to 104 years, Macrocycles of 82+35 kyr, and Megacycles lasting ~ 400 kyr. Macrocycles are equivalent to glacial-interglacial cycles and seem to correspond to the short (~100 kyr) eccentricity cycle in the earth-orbital system modulated by the precession and orbital-tilt cycles. Megacycles include four to six glacial-interglacial cycles, perhaps influenced by the long eccentricity (~ 413 kyr) cycle. Climate change is the fundamental control for all ranks of these geomorphic cycles. In terms of fluvial processes, each cycle tends to progress through four successive phases: (1) stream downcutting, (2) fluvial planation (pedimentation), (3) deposition (alluviation), and (4) landscape stability, commonly with pedogenesis (soil development). Here, these are termed “erosion-deposition-stability” (EDS) cycles. All four ranks of EDS cycles involve thresholds in geomorphic processes induced by climatic change, between one phase of a cycle and the next phase, and requiring a larger change between one rank and a higher rank. The thresholds are times of rapid change in surficial processes, whose frequency and magnitude depend on the rank of the cycle. Microcycles are crossed frequently with relatively small climate change, but Macrocycle thresholds are crossed only by a large climate change every ~100 kyr I propose that the last Megacycle threshold event occurred right after 18O-stage 5e about 100,000 years ago. It induced an erosion episode resulting in widespread land-scape reshaping by fluvial and colluvial processes throughout much of temperate North America and Europe. The next Macrocycle threshold, from the Holocene interglacial into the coming glacial, is due within the next few thousand years, but likely will not have such severe geomorphic effects.