Cybertectonic Earth and Gaia's weak hand: sedimentary geology, sediment cycling and the Earth system

摘要

Tectonics, climate and sea level are dominant controls on the nature and distribution of sedimentary environments. By inverse logic, recognition of influences on sedimentation from the stratigraphic record as a result of perturbations in these variables leads to clarification of past environmental and structural change. The succession of sedimentary environments in time and place is determined by a wider combination of variables; namely, sediment flux, biological evolution, atmosphere and water body productivity and chemistry. The feedbacks and interrelationships between all variables cause the complexity Seen in sedimentation processes over geological time. The processes of global tectonics that cause widespread Mountain belt and continental plateau uplift have produced numerous 'severe' events during Earth history: these often random workings-out of the plate tectonic cycle define the state of 'Cybertectonic Earth' (from cyber, after the Greek kubernan: to steer or govern). This has worked within a Usually zonally arranged series of climatic belts and within the framework provided by biological evolution. At certain times in the geological past, a combination of factors arose as a response to continental uplift and have acted to cause certain Earth Surface conditions and variables (notably mean global surface temperature, atmospheric pCO(2), pO(2)) to have varied by very large amounts (X2-X10) compared with present values. These large fluctuations, such as those responsible for the Neoproterozoic, Late Palaeozoic and late Tertiary glaciations, lead one to doubt the reality of homeostatic control of surface conditions as proposed in Lovelock's Gaia hypothesis. The Gaian kubernetes (steersman) had a weak hand on the helm, frequently unable to prevent vast areas of the globe experiencing rapid fluctuations in environmental conditions and inimical conditions to life for very long periods during Neoproterozoic and Phanerozoic times. At the same time, biogenic and abiogenic processes have proved capable Of returning Earth to states of mean stability, although biogeochemical cycling models seem alarmingly ad hoc and largely untestable as scientific hypotheses in any true geological sense.