Earth secular evolution

Albarede, F. Brouxel, M. (1987). The Sm/Nd secular evolution of the continental crust and depleted mantle. Earth Planet. Sci. Letters, 82, 25-35. 

The atmosphere, ocean, and biosphere leave their record in sedimentary rocks. It is likely that this record reflects both secular and cyclic evolutionary processes. The cyclic processes involve chemical mass transfer of materials in and out of global reservoirs like the atmosphere, ocean, and sedimentary rocks. If inputs and outputs of these reservoirs are nearly balanced so that over long periods of geologic time the mass and composition of the reservoirs remain constant, a quasi-steady state is maintained. Hand-in-hand with this cyclicity go changes in Earth s surface environment reflecting secular evolution of the planet, an aging process. 

It is perhaps not surprising therefore that an enthusiastic application of the plate tectonic paradigm to Archaean time has delivered mixed results (e.g. see critique by Hamilton 1993, 1998) and, currently, impedes an equally exciting research avenue into what is different about the Archaean Eon and the early Earth , and what this tells us about secular evolution of our planet. A broader view, one naturally promoted by planetary science, is that Earth is just one of several... 

Earth s secular evolution what is different about the Archaean Eon ... 

We also know that the Earth reservoirs have changed in composition over time. Such changes have been documented in this book. See for example - the isotopic evolution of the mantle (Chapter 3, Section 3.2.3), the secular evolution of the continental crust (Chapter 5, Section 5.3), the evolution of the composition of the atmosphere (Chapter 5, Section 5.3) and oceans (Table 5.5). Secular change in the biosphere, a process which we otherwise call evolution, is discussed in Chapter 6. Charting these changes and identifying the precise character of the systems of the early Earth is a task which is well underway. 

One implication of Earth - degassing models of metallogenesis is that there should be links between the formation of the Earth s resources, secular changes in architecture and geochemistry of the planet over some 4.5 billion years of evolution and phenomena such as mass extinction events, global anoxia, and atmospheric evolution. 

Because sedimentary carbonates represent primarily chemical and biochemical precipitates from seawater, and because they make up 20% of the common sedimentary rock record, these rock types have been particularly good sources of chemical and mineralogical data for interpretation of the secular and cyclic evolution of the Earth s surface environment. This carbonate rock record as a function of geological age is now explored as are age trends in other rock types and sediment properties. With this information as background material, we can then discuss what these relationships tell us about the history of carbonates and the exogenic system throughout geologic dme. 

These trends in lithologic features of the sedimentary rock mass can be a consequence of both evolution of the surface environment of the planet and recycling and post-depositional processes. It has been argued (e.g., Mackenzie, 1975 Veizer, 1988) that both secular and cyclic evolutionary processes have played a role in generating the lithology-age distribution we see today. For the past 1.5-2.0 billion years, the Earth has been in a near present-day steady state, and the temporal distribution of rock types since then has been controlled primarily by... 

Third, it will be argued in this book that many of the principal Earth reservoirs have experienced gradual changes over time. These are not changes which are cyclical, rather they are unidirectional, on a timescale of billions of years. This type of long-term, unidirectional change is known as secular change. It is seen, for example, in the chemical evolution of the atmosphere and the oceans and probably also took place in the mantle. 

Lower limit of the evolution field When the evolution field approaches the earth field, i.e. the field when the magnet is switched off, a compensation of this zero-field with the aid of correction coils becomes necessary. In this way, flux densities less than 5 X 10 T can reliably be reached. Another limit is due to the local field within the sample. The local field arises because of secular spin interactions. It can therefore not be compensated by external correction coils. If the local field exceeds the external evolution field, two situations can arise. Firstly, the local field is approached by adiabatically switching off the polarization field. That is, the local magnetization vectors follow the instantaneous field direction which is finally given by the local field. In this case, dipolar or, in the case of quadrupole nuclei, quadru-polar order is produced. The relaxation time measured under such circumstances is the dipolar-order spin-lattice relaxation time. In the opposite case, the... 

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