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

Abstract

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 p CO 2 , p O 2 ) to have varied by very large amounts (×2–×10) 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 kuberne0te0s (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.

Tectonic sedimentology: sediment systems deciphering global to local tectonics

“…As abundant Ni is a key metal cofactor in several methanogenic bacterial enzymes, the subsequent demise of these at the expense of oxygen‐producing cyanobacteria may be explained. The hypothesis adds support to notions that the Earth’s oceanic and atmospheric elemental biogeochemical cycles are subject more to the vagaries of ‘internal’ solid Earth tectonic controls rather than expressions of a Gaia‐like driver (Leeder, 2007).…”

“… (A) Highly speculative time series of denudation rates expressed as tonnes per year of sediment supply to the oceans over geological time (after Leeder, 2007). Note the logarithmic abscissa scale.…”

“…At this time, C‐drawdown from silicate‐weathering was probably tectonically‐enhanced by swamp/forest C‐burial in numerous huge equatorial intracratonic, thermal sag and foreland basins that developed along the margins of the Appalachian–Variscan–Tianshan orogen (Fig. 3C; Leeder, 2007).…”

Human impact on fluvial regimes and sediment flux during the Holocene: Review and future research agenda

“…The previous section looked at the long-term quantification of sediment budgets, however, it is also important to note that the same processes of sediment mobilisation, transport and deposition represent a key component of the global biogeochemical cycle through the transfer of sediments from continents to oceans (Meybeck, 1993;Leeder, 2007;Dürr et al, 2009). In this context it is important to recognise that the carbon and nutrients associated with the sediment loads transported by rivers commonly represent a large proportion of the total flux of those elements (Martin and Meybeck, 1979;Meybeck, 2003;Battin et al, 2008).…”

Geoarchaeology, the four dimensional (4D) fluvial matrix and climatic causality

“…The interaction of the 'Cybertectonic Earth' (tectonically steered, sensu Leeder, 2007) and biological evolution are now becoming a theme in Palaeolithic archaeology (King and Bailey, 2006;Basell, in press). Whilst not at the epicentre of this interaction, which is Africa and particularly East Africa, the British Isles illustrate the dramatically changing environmental context for human incursions to the north west of Eurasia over the last million years.…”