2.1 Data Sources of Marine Sedimentation
The relevant data and information required for analysis were collected from literature. This includes information on rock weathering processes and their products, biogenic sedimentation of organic particle and biogenic reef generated carbonate detrital materials, continental shelf and coastal sedimentation (i.e., estuary and delta, sandy and gravel beach, and tidal flat sedimentation), and deep-sea sedimentation (i.e., sediment gravity flow, contour current, and pelagic-hemipelagic sedimentation)
Rock weathering includes physical, chemical, and biological processes. Thermal expansion and contraction cause rocks to crack, while the introduction of air, water, and biological material triggers chemical reactions. Biological processes such as plant growth and animal burrowing can also contribute to the breakdown of rock structures (Tarbuck and Lutgens, 2006). Weathering products often contain newly formed components with lower density, such as clay minerals, resulting in loose sediment having a lower density than the original rock. The process of rock disintegration or decomposition into sediment primarily occurs in terrestrial environments, with relatively weaker rock weathering on the seafloor. The essence of weathering processes is energy conversion, particularly solar energy. Since organisms themselves are products of solar radiation, biological weathering is also an indirect result of solar energy conversion (Hall et al., 2012). The ultimate products of weathering are commonly gravel, sand and mud (a mixture of clay and silt). In the global inventory of sediments and the sedimentary rocks formed by their compaction, the ratio of sand / gravel to mud material is approximately 3:5 (Davis, 1983).
Particles formed by biological growth also fall under the category of sediment. Organic particulate matter from plant sources, skeletal remains from animals, shell fragments, and carbonate deposits on biogenic reefs are all examples of sediment particles that contain carbon. Carbonate deposits formed by biogenic reefs in the ocean are the largest part (Fagerstrom, 1987; Woodroffe and Webster, 2014). Global carbonate sediments contain approximately 50×1015 t of carbon, which is about five times the total amount of particulate organic carbon (Libes, 2009). Overall, sediment derived from rock weathering accounts for 80% of the total, while biogenic sources contribute to the remaining 20% (Davis, 1983). Undoubtedly, organic particulate matter primarily originates from photosynthesis, which relies on solar energy consumption. The formation of biogenic reefs involves the growth of benthic organisms and is also a result of solar energy transfer within the ecosystem.
Shelf and coastal deposition is the result of fluvial, tidal, wave, and shelf circulation transport processes (Dyer, 1986; Kim, 1992). Glaciers and the atmosphere also contribute material inputs, but their fluxes are an order of magnitude smaller than fluvial inputs (Hay, 1998). Weathered materials from watershed basins are transported by rivers, with some being trapped in estuarine bays and deltas, and much of the material being transported to shelf environments, where the gravelly shores of the inner shelf are characterized by beaches accreted by wave action, as well as tidal flats and tidal ridge deposits produced by tidal action (Woodroffe, 2002). On short time scales, a greater proportion of the land-sourced material is derived from erosional, accretionary cyclic processes (Clift and Jonell, 2021), but on long time scales it is all weathering products. The kinetic energy of rivers, tidal currents, waves, and shelf circulation is derived from solar and tidal energy, respectively.
Shelf and coastal sedimentation results from fluvial, tidal, wave, and shelf circulation transport processes (Dyer, 1986; Jin, 1992). Glaciers and the atmosphere also contribute to sediment input, but their fluxes are an order of magnitude smaller than those from rivers (Hay, 1998). Weathered sediments from the drainage basin are transported by rivers, with some being trapped in estuaries and deltas, while the majority is transported to the shelf environment. On the inner shelf, beach deposits are formed by wave action, and tidal flats and tidal ridge deposits are created by tidal processes (Woodroffe, 2002). On shorter time scales, a significant proportion of terrestrial sediments come from erosion and deposition cycles (Clift and Jonell, 2021). However, on longer time scales, they are primarily derived from weathering processes. The kinetic energy of rivers, tidal currents, waves, and shelf circulation is derived from solar energy and tidal energy.
Deep-sea sediments primarily originate from land and can be classified into three main types: sediment gravity flow, contour current, and pelagic-hemipelagic sedimentation, with transitional states between them (Rebesco et al., 2014; Stow and Smillie, 2020). Sediment gravity flow sedimentation are commonly found at continental margins and can take various forms, including slumping, submarine landslides, turbidity currents, grain flows, liquefied flows, and debris flows (Middleton and Hampton, 1976; Anderson, 1986; Pickering et al., 1989; Pickering and Hiscott, 2015; Sun et al., 2022). The underlying principle is the work done by gravity, and the potential energy associated with sediment at continental margins is primarily a result of the dissipation of internal heat within the Earth's geological history. Contourites and associated sediments are controlled by deep-water circulation processes; some of the sediments is originally derived from gravity flow deposits and re-enters the transport system through erosion processes (Stow and Lovell, 1979; Rebesco et al., 2014). Pelagic-hemipelagic sedimentation represents the settling of fine-grained sediments, sourced from either land or deep-sea areas of the continental margin, and influenced by gravity and/or ocean currents before deposition (Stow, 1981; Pickering and Hiscott, 2015; Mosher and Yanez-Carrizo, 2021). Therefore, these deposits are associated with gravity processes (which can be indirectly related to Earth’s thermal heat energy that causes tectonic movements and the resultant topographic differences) and solar energy-driven deep-sea circulation.
2.2 Energy Estimation of Land Weathering Processes
Land weathering processes transform rocks into sediments, but the energy consumption associated with these processes is rarely addressed in the literature and lacks quantitative descriptions. In terms of physical weathering, temperature variations, changes in overlying load, and biological growth are the main factors. Regarding chemical weathering, chemical reactions require the involvement of external substances, such as precipitation, and the reaction rate is temperature-dependent. The energy balance in chemical reactions is complex and specific to the reaction equation. In the case of biotic weathering, both physical and chemical processes may be involved. Although researchers have emphasized the consideration of energy conversion when rocks are transformed into sediments through weathering processes (Hall et al., 2012), previous studies have focused mainly on the products and indicators of weathering processes while overlooking energy conversion.
Due to the complexity of physical, chemical, and biological processes, it is challenging to summarize them with simple calculation formulas. Therefore, for the time being, the following order-of-magnitude estimation method is adopted. Starting with the process of rock weathering, the quantity of external substances required for weathering processes is calculated. Taking granite as a representative rock, assuming its composition consists of 30% quartz, 60% potassium feldspar, and 10% mica, the fundamental chemical reaction is the hydration of potassium feldspar, transforming it into clay minerals (Tarbuck and Lutgens, 2006):
2 K Al Si3 O8 + H2O + other substances → Al2 Si2 O5 (OH)4 + dissolved substances (1)
From Eq. (1), the required amount of water can be estimated. Then, based on the sediment yield of weathering processes, the total mass of water involved in the reaction can be estimated, leading to the estimation of the required power:
where P is the power of weathering processes, g is the acceleration due to gravity, Mw is the water requirement for the transformation reaction per unit time, H is the elevation difference for transporting the required water from the ocean to the weathering site.
In general, the water involved in weathering processes is transported through the global hydrological cycle, and its energy is derived from solar radiation. For the estimation of Mw, it is assumed that the flux of weathering products and sediments entering the ocean is consistent, which is approximately 2.0×1010 t/a. Then, based on the composition of granite and the chemical reaction Eq. (1), the proportion of clay minerals in the flux entering the ocean can be calculated to obtain the value of Mw. The assumption in Eq. (2) is that the energy consumption of physical, chemical, and biological weathering processes is of the same order of magnitude.
2.3 Energy Estimation of Sediment Transport and Accumulation
For the products of gravitational processes, the power resulting from the elevation difference between the deposition site and the source area can be calculated (when there is a slope, accompanied by horizontal movement, the energy is from the same source and does not need to be calculated separately):
where P is the power consumed by sediment accumulation, g is the acceleration due to gravity, M is the total mass of sediment (kg), H is the average vertical elevation difference from the source area to the deposition area (m), and T is the duration of the deposition (s).
When the potential energy of sediment increases, Eq. (3) does not represent the work done by gravity but rather reflects the effect of external forces. For example, when the increase in sediment potential energy is caused by tidal flow transport, the energy is derived from tidal energy dissipation. Similarly, when the potential energy of sediment increases due to ocean circulation, the energy is sourced from solar radiation.
For the products of fluvial processes, it is necessary to consider the horizontal displacement caused by water transport:
where k is the acceleration due to the force acting on the water flow, primarily to overcome bed resistance, and L is the horizontal transport distance (m).
Studies in sediment dynamics have shown that only a small portion of the kinetic energy of water flow is used for net sediment transport. Submarine gravity flows rely on sediment potential energy for transport, and the transport distance can reach magnitudes of up to 103 km for a vertical drop of 100 km. Comparing Eqs. (3) and (4), it can be inferred that k should be much smaller than the acceleration due to gravity, approximately on the order of 10− 3 of the acceleration.
Therefore, in the estimation of this study, it is assumed that the energy consumption defined by Eqs. (3) and (4) is of the same order of magnitude, representing the conversion of potential energy. When this potential energy is derived from the work done by gravity, it is attributed to the dissipation of geothermal energy within the Earth's interior. When water flow is caused by tidal action or solar energy conversion (in the form of river runoff, waves, ocean circulation, etc.), it is attributed to the energy dissipation effects of tides or solar radiation.
2.4 Estimation of Solar Radiation Energy Fixed by Coral Reef Sedimentation
Plants convert solar energy into chemical energy through photosynthesis, but the specific energy consumption data for the chemical reactions involved in photosynthesis are generally not provided. If the biomass of plants contained in the sediment (expressed as organic matter mass) is known, the power of solar radiation conversion can be represented as:
where Co is the organic matter content of the sediment (%), M is the mass of the sediment (kg), and Hp is the heating value of unit mass of organic matter (J kg− 1).
Biological reefs, especially coral reefs, have a more complex relationship with primary production. For example, coral polyps and zooxanthellae have a symbiotic relationship, and the growth of coral polyps is linked to primary production, secondary production, and even higher trophic levels (Fagerstrom, 1987). In this study, an indirect approach is used to estimate biomass based on the carbon fixation capacity of zooxanthellae, and then calculate the solar radiation conversion energy or power using the heating value of plants:
$$P=A Mc Hp /\left(R T\right)$$
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where A is the area of the coral reef (m2), Mc is the carbon fixation capacity of zooxanthellae per unit area (kg C m− 2), R is the carbon content of organic matter (%).
In an ecosystem, the energy conversion ratio from primary production to secondary production may be approximately 7:1. However, coral polyps do not strictly fall into the category of secondary production, and coral reefs may have up to six trophic levels with lower conversion efficiency (Fagerstrom, 1987). Therefore, the ratio for coral reefs should be smaller than this value, and here it is assumed to be 10:1. In coral reef sediments, the organic matter content is low, and the carbon is primarily contained in the coral skeleton rather than organic matter. It is assumed that the energy consumption of this production is equivalent to the conversion from primary production to coral polyp energy. Thus, the total sediment mass and corresponding power of coral reef sediments are given by:
$$P=Ks A Mc Hp /\left(R T\right)$$
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In Eq. (7), Ms represents the total sediment mass of the coral reef, Gs is the average coral skeleton production rate, and the integration is performed over the growth period of the coral reef. In Eq. (8), Ks represents the energy conversion ratio from primary production to coral polyps. Globally, coral reefs are the main component of bioherms, so Eqs. (7) and (8) approximate the situation for the entire bioherm.