The Wearing Away of Continents

Abstract

Denudation is a substantial part of a cycle in which flowing water associates the terrestrial sector of the global hydrological cycle with continental wearing down, of which it is a chief agent. Tectonic uplift stimulates fluvial erosion, on contact with the water cycled by solar energy and clustered in surface channels by catchment processes. Progressive sediment transfers occur between upper and lower catchments, and subsequently between lower catchments and the marine environment. Big river flood plains store sediment in larger systems for longer periods, where reworking continues mechanical and chemical sorting before reaching the onward transfer of mature sediments to the coastal zone. This is a continuous process where coarse, raw fluvial sediments are eventually swept as molasse into trenches and back-arc basins, close to orogens. About 19.1 Gt of sediment and 3.8 Gt of dissolved phases are annually transferred to coastal oceans by river discharge. This huge amount of material is, directly or indirectly, the result of the action of weathering, which is a significant link in the cycling of carbon and, hence, a participant in controlling the Earth’s climate. A significant portion of this material is, however, an indirect product of weathering because it is rock debris that has been recycled, having passed two or more times through the Earth’s exogenous cycle. Also, anthropogenic activities are responsible for opposing actions, which increase the denudation rate through soil erosion, on one hand, and sequester sediments in human-made reservoirs, on the other.

Glossary

Bed load:

The term is used to describe the transport of sand, gravel, boulders, or other debris by flowing water by rolling or sliding along the bottom of a stream.

Colloid:

It is a solid or liquid substance microscopically dispersed throughout another substance (e.g., water). The dispersed-phase particles have a diameter between ~1 and ~1,000 nm (1 nm = 10⁻⁹ m). The dispersed-phase particles or droplets are affected largely by their surface chemistry.

Comminute:

From comminution; the process in which solid materials are reduced in size, by crushing, grinding, and other processes. It occurs naturally during faulting in the upper part of the Earth’s crust, and it is an important unit operation in mineral processing, ceramics, electronics, and other fields.

Continuous wavelet transform (CWT):

It provides a redundant but detailed description of a signal in terms of both time and frequency. It is used to divide a continuous-time function into wavelets. Contrasting with Fourier transform, the CWT is characterized by the ability to construct a time–frequency representation of a signal that offers very good time and frequency confinement.

Deseasonalize:

In statistics, data are deseasonalized when the regular seasonal fluctuations are removed from a time series.

El Niño-Southern Oscillation (ENSO):

It is a band of anomalously warm ocean water temperatures that occasionally develops off the western coast of South America and can cause climatic changes across the Pacific Ocean. The Southern Oscillation refers to variations in air surface pressure in the tropical western Pacific and in the temperature of the surface of the tropical eastern Pacific Ocean (warming and cooling known as El Niño and La Niña, respectively). The two variations are attached: the warm oceanic phase, El Niño, occurs with high air surface pressure in the western Pacific, while the cold phase, La Niña, accompanies low air surface pressure in the western Pacific.

Epsilon (ε) notation:

It is an alternative way of expressing isotope ratios which allows greater flexibility in the way in which isotopic data are presented; the value is a measure of the deviation of a sample or sample suite from the expected value in a uniform reservoir and may be used as a normalizing parameter for samples of different age. It is normally calculated for ¹⁴³Nd/¹⁴⁴Nd ratio and is used to represent parts in 10,000 by the following equation:$\varepsilon^{143} {\text{Nd}} = \left[ {\frac{{\left( {\frac{{143{\text{Nd}}}}{{144{\text{Nd}} }}} \right){\text{sample}}}}{{\left( {\frac{{143{\text{Nd}}}}{{144{\text{Nd}} }}} \right){\text{standard}}}} - 1} \right] \times 10,000$

Exogenous cycle:

It is a set of events or processes which are completed, returning to its beginning and then repeating itself in the same sequence. It describes the fluxes of materials, water, and gasses that occur at the intersection of the lithosphere, biosphere, hydrosphere, and atmosphere.

Harmonic analysis:

A branch of mathematics concerned with the representation of functions or signals as the superposition of basic waves, and the study of and the generalization of the notions of Fourier series and Fourier transforms, harmonic functions, trigonometric series, almost periodic functions, and others.

Isostasy:

The term is used in geology to refer to the state of gravitational equilibrium of all large portions of Earth’s lithosphere as though they were floating on the denser underlying layer, the asthenosphere, a section of the upper mantle composed of plastic rock that is about 110 km below the surface.

Molasse:

The term refers to the sandstones, shales, and conglomerates formed as terrestrial or shallow marine deposits in front of rising mountain chains. These deposits are typically the nonmarine alluvial anyd fluvial sediments of lowlands, as compared to deep-water sediments.

Phyllosilicates:

A class of silicate minerals that form parallel sheets of silicate, where a central silicon atom is surrounded by four oxygen atoms at the corners of a tetrahedron. Three of the oxygen atoms of each tetrahedron are shared with other tetrahedrons. Examples are the clay minerals, kaolinite, and illite.

Total dissolved solids (TDS):

Are a measure of the combined content of all inorganic and organic substances contained in water, as molecular, ionized, or micro-granular (colloidal sol) suspended form. The operational definition is that the solids must be small enough to survive filtration through a filter with 2 μm nominal size pores (or smaller). TDS are usually discussed only for freshwater systems.

Total suspended sediment (TSS):

The portion of the sediment that is carried by a fluid flow, such as in a river or coastal current. It is maintained in suspension by turbulence in the flowing water and consists of particles generally of the fine sand, silt, and clay size.