Earth chemical gradients

Chemical gradients Across Earth s layers of core solids Sea depth Atmosphere height Concentrations in liquids behind barriers... 

So metabolism was propelled into being on Earth billion years ago by chemical, electrochemical and thermal disequilibria. It emerged at the semi-lithified physical front separating an alkaline hydrothermal mound from the acidulous ocean (72). This is where chemical, redox, pH, and temperature gradients were at their steepest, yet were commensurate with the requirements of metabolic processes. The chemical gradients—CO2 on the outside and H2 on the inside—provided the ultimate chemical potential for their interaction, catalyzed... 

The planets nearest the Sun have a high-temperature surface while those further away have a low temperature. The temperature depends on the closeness to the Sun, but it also depends on the chemical composition and zone structures of the individual planets and their sizes. In this respect Earth is a somewhat peculiar planet, we do not know whether it is unique or not in that its core has remained very hot, mainly due to gravitic compression and radioactive decay of some unstable isotopes, and loss of core heat has been restricted by a poorly conducting mainly oxide mantle. This heat still contributes very considerably to the overall temperature of the Earth s surface. The hot core, some of it solid, is composed of metals, mainly iron, while the mantle is largely of molten oxidic rocks until the thin surface of solid rocks of many different compositions, such as silicates, sulfides and carbonates, occurs. This is usually called the crust, below the oceans, and forms the continents of today. Water and the atmosphere are reached in further outward succession. We shall describe the relevant chemistry in more detail later here, we are concerned first with the temperature gradient from the interior to the surface (Figure 1.2). The Earth s surface, i.e. the crust, the sea and the atmosphere, is of... 

Rapid increases in the interstitial water concentrations of dissolved strontium with increasing burial depth of deep-sea carbonate sediments have been interpreted as evidence of the recrystallization reaction (Baker et al., 1982 Elderfield et al., 1982 Gieskes, 1983). Figure 8.17 shows an example of interstitial-water profiles of dissolved alkaline-earth species from a carbonate nanno-fossil ooze from the Ontong Java Plateau (DSDP site 288 5°58 S, 161°50 E). At this site calcium and magnesium concentrations are linearly correlated, and their gradients are governed by chemical reactions deep in the sediment column. 

The remote setting of the Earth s core tests our ability to assess its physical and chemical characteristics. Extending out to half an Earth radii, the metallic core constitutes a sixth of the planet s volume and a third of its mass (see Table 1 for physical properties of the Earth s core). The boundary between the sihcate mantle and the core (CMB) is remarkable in that it is a zone of greatest contrast in Earth properties. The density increase across this boundary represents a greater contrast than across the crust-ocean surface. The Earth s gravitational acceleration reaches a maximum (10.7 m s ) at the CMB and this boundary is also the site of the greatest temperature gradient in the Earth. (The temperature at the base of the mantle (—2,900 °C) is not well established, and that at... 

Reductants for chemoautotrophs are generally deep in the Earth s crust. Vent fluids are produced in magma chambers connected to the Atheno-sphere. As such, the supply of vent fluids is virtually unlimited. While the chemical disequili-bria between vent fluids and bulk seawater provides a sufficient thermodynamic gradient to continuously support chemoautotrophic metabolism in the contemporary ocean, in the early Earth the oceans would not have had a sufficiently large thermodynamic energy potential to support a pandemic outbreak of chemoautotrophy. 

---