Teaching Slides: Causation

Here are teaching slides for "Causation". You can see a full-size version by clicking on the thumbnail. You can then download that slide individually by simply dragging it to your desktop.

13.2: Postulated rise in atmospheric oxygen over geologic time, and its relationship to ice ages (blue bars) and biospheric evolution. If the step-like rises in oxygen were rapid, consequent destruction of atmospheric methane (a powerful and potentially major greenhouse gas in the early atmosphere) could account for the Makganyene and Sturtian snowball earths. If the oxygen rises were slow (millions of years), methane destruction might be offset by CO2 rise modulated by silicate weathering feedback. The Marinoan snowball earth, coming <75 Myr after the Sturtian, is more difficult to attribute to methane destruction, requiring a methane-oxygen see-saw.

13.3: Li et al. (2004) postulated that a polar mantle superplume, located beneath the India-Australia-South China part of the Rodinia supercontinent, rotated to the equator between 800 and 750 Ma as a result of true-polar-wander (TPW). They suggest that increased silicate weathering due to the paleogeographic change caused the global climate to cool. (TPW refers to a bodily rotation of the entire silicate shell (mantle and crust) of the Earth with respect to the rotation axis, which is assumed to be fixed in the celestial reference frame. TPW is a response to changes in mass distribution [inertial tensor figure] of the Earth associated with mantle convection).

13.4:Paleogeographic reconstruction of the Rodinia supercontinent at 800 Ma (Torsvik, 2004). It is widely accepted that a supercontinent centered on equatorial Laurentia existed from ~1000-800 Ma, but the relative locations of the extra-Laurentia continents remain uncertain.

13.5: Breakup and dispersal of the Rodinia supercontinent in a 750-Ma reconstruction (Torsvik, 2004). Paleomagnetic data from 800-750 Ma exist for Laurentia, Baltica, South China, Australia, India and Congo, but none for Kalahari, East Antarctica, Rio Plata, Amazonia, West Africa, North China and Siberia. Such data can constrain paleolatitude and paleodeclination, but not paleolongitude.

13.6: Coupled climate-geochemical model of the breakup and dispersal of Rodinia, 800-700 Ma, in which the 0° isotherm migrates equatorward in response to increased silicate weathering, primarily due to higher precipitation in previously arid regions (Donnadieu et al., 2004).

13.7: Same as the last slide, but with the addition of a flood basalt emplaced at 780 Ma in the subtropics and tectonically rotated into the tropics (Donnadieu et al., 2004). The additional weathering when the flood basalt entered the tropics caused the model to lapse into the snowball state. The 723-Ma Natkusiak flood basalt was in fact erupted near the equator, and could have played an immediate causative role in the Sturtian snowball earth without any tectonic rotation.

13.8: Tectonic settings (syn-rift or post-rift) over time of selected Neoproterozoic sedimentary successions hosting multiple glacigenic intervals.