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Terrigenous Sediment

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Terrigenous Sediment Transport to the Ocean

In relation to Oceanography, terrigenous sediments are those derived from the erosion of continental rocks and soils that are transported by a fluid medium to the oceans. They are derived from terrestrial, as opposed to marine, environments.[1] This type of sediment consists of sand, mud, or silt and is related to its source rocks. The deposition of these sediments is largely limited to the continental shelf.[1] Sources of terrigenous sediments include weathering of rocks, wind-blown dust, grinding by glaciers, and sediment carried by rivers or icebergs. When sampled from marine depositional environments, the chemical and physical properties of these sediments can be traced back to their source rocks and formation processes at the surface. Terrigenous sediments play an important role in many biogeochemical processes, including carbon burial, iron fertilization, and the phosphorus cycle. Major historical changes in atmospheric oxygen and carbon dioxide concentrations may have been stimulated by terrigenous sediment fluxes.

Importance of terrigenous sediments

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Carbon burial

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Most of the terrigenous sediment delivered to the ocean by fluvial processes is deposited on continental margins.[2] This rapid sedimentation leads to high rates of organic carbon burial. Terrigenous sediments themselves often contain a significant amount of particulate organic carbon, which is deposited and buried in shallow continental shelves.[3] In addition to this effect, terrigenous sediment also serves to bury organic carbon that was produced in the ocean. Shallow, coastal waters are often regions of high primary productivity. Although continental margins make up only around 20% of the ocean's total surface area, studies have shown that continental margins play as large a role in the ocean biogeochemical carbon cycle as the rest of the ocean combined.[4] Organic matter produced in continental shelves is less likely to be consumed and respired than organic matter produced in surface layers of deep ocean basins because it has a shorter distance to travel before it is deposited on the ocean floor. This means that there is less opportunity for other organisms to consume this organic matter before deposition. When this organic matter reaches the ocean floor, it is trapped and buried due to high rates of sediment deposition from terrigenous sources.[5]

This in turn plays an important role on the control of global oxygen levels. Oxygen is produced during photosynthesis and consumed during respiration. If photosynthesis and respiration are balanced, oxygen levels will remain static. However, processes such as carbon burial driven by terrigenous sediment deposition provide mechanisms for net oxygen production. Photosynthesis produces both oxygen and organic matter. If the organic matter is not respired, the oxygen produced along with it will not be consumed, and oxygen accumulates.[6]

Iron dust fertilization

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Iron can be a limiting nutrient for many species of phytoplankton, especially in the Southern Ocean and many parts of the Pacific and Atlantic Oceans.[7] Because of this, iron dust transport has a large impact on ocean biogeochemical cycles. Winds can carry large quantities iron dust great distances into the open ocean, out of the reach of other sediment transport process like riverine input and iron transport. This means that iron dust can supply iron to regions of the ocean that would otherwise be iron-limited.[8] A major source of iron for the Atlantic Ocean is Saharan Dust.

The impact of iron dust on primary productivity has been debated. Some studies have found that upwelling and volcanic ash have historically had a stronger influence on ocean phytoplankton growth than iron dust, because most iron dust is insoluble.[9][10] However, there is evidence that some microorganisms may be capable of using organic ligands to increase the bioavailability of iron dust.[11] Other studies have identified high concentrations of atmospheric iron dust as a potential cause of the last ice age. It is possible that iron dust fertilization may have caused an increase a photosynthesis on a scale large enough to significantly lower global atmospheric carbon dioxide concentrations, weakening the influence of the greenhouse effect and thereby cooling the planet.[12]

Iron dust has an impact on other biogeochemical cycles as well. Nitrogen fixation by diazotrophs requires the enzyme nitrogenase, which contains iron.[13]

Biogeological processes

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Shallow coastal habitats play a huge role in biogeochemical cycles. As transport of terrigenous sediment to the ocean often results in a direct supply of organic matter to the surface waters ecosystems in regions near the deposits are often affected. Depending on the size of a sediment load the coastal system can be affected in different ways. If the terrigenous sediment delivered contains plenty of organic matter it can contribute to phytoplankton blooms.[14] Terrigenous sediment transported by wind is often high in Iron (Fe), contributing to such blooms in areas like Gulf of Alaska. [14] If the load of terrigenous sediment is sudden, very large and composed of refractory food benthic fauna can suffer due to degradation of their habitat.[15] Even sustained smaller inputs of terrigenous sediment over time can cause the benthic diversity of macrofauna to decrease within a coastal habitat. This is caused by alterations to nutrient cycling and primary production in within the benthic communities. [16] The sediment input causes increased turbidity which reduces light availability and alters the nutrient supply. This effect is more so elevated with input from land use change.[17] Sand flat organisms are significantly impacted by large fluxes of terrigenous sediment input. If these benthic organisms community becomes degraded by increased input, sediment reworking from bioturbation and bioirrigation will also decrease.[18]. Therefore, losses of these organisms would result in overall changes in carbon burial, organic matter composition, nutrient cycling, and primary production

Important elements

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Several chemical elements necessary for the development of life are delivered to the ocean via runoff and weathering, including carbon, calcium, silicon, and phosphorus.[19] Although much of this is in dissolved phase rather than particulate phase, the physical and chemical processes that deliver these dissolved element species are similar to those that transport sediment.

Rivers deliver an estimated 9 x 1014 g of carbon to the ocean each year.[20] In particular, bicarbonate is the most common ion in river water.[21] Bicarbonate reacts with calcium to form calcium carbonate, which is the primary component of the shells of marine organisms.

Approximately 80% of the silica in the oceans is derived from river runoff.[22] Silica makes up the cell walls of diatoms, which produce between 20 and 50% of the world’s oxygen.[23][24]

River and dust inputs provide the world’s oceans with approximately 5 x 1011 mol phosphorus each year.[25] Phosphate groups are a key ingredient in DNA, which is present in the cells of all living things.[26]

Furthermore, terrigenous sediments may have played an important role in limiting global oxygen levels before the great oxygenation event. Weathering of pyrite may have delivered large amounts of sulfide to the world’s oceans, possibly leading to euxinic conditions in the photic zone, and allowing for the persistence of anoxygenic photosynthesis. Euxinia describes water which is anoxic and highly sulfidic. In anoxygenic photosynthesis, carbon dioxide reacts with hydrogen sulfide rather than water. This reaction produces sulfur instead of oxygen. It has been hypothesized that euxinic conditions due to pyrite weathering facilitated anyoxygenic photosynthesis, which may explain the delay in the oxygenation of Earth's oceans.[27]

Classification of terrigenous particles

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Fig 1.) The ternary diagram of sand-silt-clay grain-size distribution in percentage,  USDA soil taxonomy.
Table 1.) Grain-size scales and textural classification following the Udden-Wentworth US standard. The phi values (ϕ) is according to Krumbein (1934)[28] and Krumbein and Graybill (1965)[29]; ϕ = -log2(d/d0), where d0 is the standard grain diameter.

The source of terrigenous sediment is from insoluble material, primarily rock and soil particles. It is the result of constant material erosion. The erosion is caused by broken, crushed, and polished by both physical and chemical processes such as a river, weathering, heating, dissolving in water, etc. Sediment does not only settle down on the nearshore shallow-water or deltaic beds but also accumulates in shelf seas and continental margins.[30]. Sediment will be accumulated by gravity transport. Moreover, it will end in redistribution to the deep sea by marine animal activities and ocean currents. It is still debatable to have a scientific applicable for classification on terrigenous sediment. However, a large number of ideas have been put in action for classification and proposed in the literature. Those sediments can be sorted by many components or how it forms such as chemical and mineralogical components, origin, size, etc. However, the widely-known classification is sort by a variety of grain sizes for the sediment on the continental shelf which is from coarse sand to fine-grained material. Furthermore, those originate from silica-rich sources, which scientists called siliciclastic sedimentary rocks[30]. 


By sorting in the rage of grain-size distribution and the derived sediment characteristics can easily distinguish what we are looking for. Also, throughout the sorting process provide information about the history of sediment. According to the Wentworth scale(or Udden–Wentworth scale), there are four subdivisions of sedimentary particles: gravel (> 2 mm), sand (2 - 0.0625 mm), silt (0.0625 - 0.0039 mm), and clay (< 0.0039 mm), each further divided into several subcategories (Table 1). Plotting the percentages of these grain sizes can have a basic and clear classification of the terrigenous sediments (Fig.1). Still, both of the subdivisions and classifications do not have an universal accepted. Furthermore, the ternary diagram contains variable amounts of biogenic components may also be present in the sediment, which is clay. Accordingly, sediment classifications are likely to become confusing. However, a certain degree of standardization has developed owing to the frequent and identical usage of terms descriptive of sediment cores, as employed in the international Ocean Drilling Program (ODP).[31]


Origins, transportation & sedimentation

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This figure illustrates the distribution of sediment types across the seafloor of the worlds oceans.

Terrigenous sediment, before it is transported to the world’s oceans, originates in various land-based locations around the world. This sediment type dominates continental margins as inputs from land, such as rock weathering, wind-blown dust, grinding by glaciers, and river input are the sole contributing input processes. [32] This also suggests that the initial buildup of this sediment type is much greater near continents with larger continental shelves.[33]

Rivers

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East China and East China Sea tmo 2017313

Continent erosion followed by river runoff are the major contributing processes to this type of sediment in the ocean.[34] River transport is the largest contributing mode of transport providing about 85% of the terrigenous sediment found in the ocean.[34] However, different rivers have various flow rates, topography, and climate all which affect the amount of sediment transported in a specific area. In areas with high annual rainfall, larger amounts of sediment that have been eroded away can be picked up and transported downstream. This increased discharge allows for large plumes of sediment to be deposited to the shorelines. Some rivers do, however, connect to estuaries; this creates an obstacle for sediment transport. Sea level and tidal ranges then dictate how much sediment can be transported directly to the oceans. In some cases, sediment can be trapped in these locations, never making it to the ocean. In the United States, the Mississippi River has the largest sediment discharge load of about 3.49 x 108 t.[35] The western side of the United States has larger sediment loads than the eastern border, but increasing alterations by humans such as the building of dams or the manipulations of rivers alters these fluxes over time. Sediment loads have been adequately monitored in the United States in our past history, but most values in literature are considered estimates as fluxes are heavily altered by the aforementioned process and by human impacts. In South America similar measurement hurdles arise. While this continent holds three of the worlds largest rivers, their sediment loads are still relatively under documented. The sediment load of the Amazon River is estimated by a few sources with discharges ranging from 9 x 108 t to 5 x 108 t.[35] Overall, the total sediment load deposited into the ocean basins from our worlds rivers is adequately documented depending on the location of the river and the tools available to make detailed measurements. With increasing human alterations to major rivers, terrigenous sediment loads from smaller streams and rivers have recently been found to greatly contribute to the overall input.[36] Flash floods and earthquakes can cause massive amounts of sediment to be deposited from these smaller rivers.[36]

Precipitation and glacial melt cause the initial weathering of rock and transport sediment grains through river/stream networks to the oceans. This process emphasizes the transport of smaller and lighter sediment grains over larger, heavier grains.[37] The size and mass distributions are also dictated by the rate and turbulence of the flow.[30] Faster, more turbulent river flows are able to suspend more sediment and larger grains, then weaker, smooth-flowing rivers. Generally, grains 30 µm or less are supported in a river’s suspension load, while large sizes are moved more slowly in the traction load.[30] Given an adequate flow velocity, almost any grain size can be transported by a river's traction load.

Of course, the geographic location, and thereby the type of rock/soil being weathered, controls the compositions of the sediment grains. For example, agricultural regions are far more prone to soil loss than rock weathering,[38] and therefore supply nearby water networks with more sediment grains to transport. Regional climates also control the intensity of the local water cycle and therefore control the relative amount of sediment transport from one location to another. Generally, most sediment grains transported via rivers are deposited in coastal zones, while only the smallest and lightest grains make it far enough offshore to be carried further by ocean currents. [30][39][40] Transportation by river flow allows the formation of submerged sediment fans in nearshore continental zones (discussed more below). [30]

Ice

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Ventisquero negro (Black Glacier) on spills and breaks down Cerro Tronador.

Icebergs transport only about 10% of terrigenous sediment to the ocean in a process called ice-rafting.[34] As sediment is deposited or trapped in ice it can then be carried out into the ocean. Once the ice melts those particles are then deposited. Pack ice and icebergs are known for depositing sediment in this way. Particle sizes from fine material to boulders can be trapped in or atop glaciers.[41] As they break free from landfast ice their source material is transported out to sea. This ice-rafted debris (IRD) is commonly used in science to determine where and when an iceberg may have originated from.[41] The main source areas for this type of terrigenous sediment transport is the Arctic, Antarctic, and Greenland.[42] Both the Antarctic and Greenland hold massive continental glaciers. As the extents of these glaciers calve into the ocean or generate icebergs the sediment it previously resided on is transported with or within the ice. In the Arctic, river pack ice that makes its way to the coast lines contributes terrigenous sediment in the same process. In most cases the volume of sediment within the ice is only 1-2% of the total mass.[42]

Glacial processes may or may not be fully involved in the transportation of sediments to the ocean. That is, some sediment, suspended in ice, is transported to the ocean entirely by glacial processes (glacial flow, caving, iceberg migration, and melting).[30] Otherwise, glacier transportation may comprise most of the process, carrying the grains from the initial source to the glacial terminus, where glacial melt takes over and the remainder of the transportation occurs through a liquid water medium. It should be noted that sediments are not always sourced from the land beneath the glaciers. Fine aeolian dust can be deposited on glacial surfaces and over time will become encased as more snow accumulated.[43] Whether in the northern or southern hemisphere, icebergs usually migrate to the mid-latitudes before melting completely, dispersing sediment along the way. Iceberg transport in the northern hemisphere is restricted to the North Atlantic due to the limited iceberg sources.[30] Sediments tend to not be chemically or physically altered, being encased and protected in ice. The IRD are then deposited in a wide variety of sizes and shapes.[30] Iceberg transportation speed is entirely dependent on the ocean currents the ice is suspended in, while the deposition of the sediment (loosened from the ice due to melting) is entirely driven by when the sediment became encased in ice and melting rates.[30] Generally, the largest grains, up to meter-thick boulders,[30] occur along the bottom of the glaciers where the glacier scraped across the underlying land as it flowed.

Wind

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Sahara dust plume Nov 1998

In the same way that water moves particles by its flow, wind also transports sediment. Finer particles such as silts and clays are more readily transported by wind. Aeolian, or wind-blown dust, contributes about 3% of terrigenous sediment transport to the ocean.[34]  In regions with more arid conditions wind-blown dust transport can be more dominant.[44] In these regions, typically between 30 N and 30 S, wind patterns tend to be much stronger and constant over time resulting in larger masses of sediment being carried by the wind. Specifically regions such as Australia, Arabia, and North Africa are important for significant aeolian dust contributions as they contain large deserts.[34]  For the North Atlantic, Mediterranean, and Caribbean seas, African dust transport supplies 5 Tg to 25 Tg of dust per year to these oceanic regions.[45] This huge reservoir of African dust acts as a major transport for terrigenous sediment to the ocean. Transport occurs almost year around, but depending on the season, and thus the strength of the prevailing winds, increased transport is also known to occur during these times.[45] The Sahara is the 3rd largest desert in the world has been frequently imaged transporting plumes of dust into the Atlantic.[46] While winds transport this terrigenous sediment to the continental shelves, this method of transport also easily reaches miles and even continents away at times.[47] This dust has the ability to travel so far due to a temperature inversion of the air mass that typically carries this sediment from the desert. As the Sahara gains heat during the day so does the air above it, then as it is blown toward the coast this warm air mass meets the cool west African coastal air and an inversion happens.[47] This inversion prevents mixing of the air masses and this allows the intact air mass transporting the sediment to travel further.[47]

Wind transportation is most predominant in dry climates with sources of finer sediment grains.[48][49] Transportation of various grain sizes is determined by the wind speed and ability to keep particles suspended through turbulence.[48] Grains are restricted to a size of 80 µm or less.[30] However, for the extensive suspension needed for transport to the ocean, a grain size of only 20 µm or less is generally observed.[48] This size preference is dictated by the fact that wind velocities and turbulence are not continuously elevated. Therefore, when the winds weaken, the larger particles are lost, and only the smallest are maintained aloft by the winds. Regional climatic wind patterns and the hydro-meteorological tendencies place further control on transportation potential. Though some source regions may meet the criteria for wind transportation, the prevailing wind patterns may not necessarily lead to ocean deposition.[49] Regions like the Sahara and the Middle East are prone to direct wind transportation from land to ocean.[50] Under certain conditions, suspended sediment grains act as cloud condensation nuclei and may be precipitated back on land or directly on the oceans. These sediments are crucial components in the development and organization of severe tropical storms. Once entering the ocean, these very fine grains see further transport by currents before being permanently deposited.

Controls

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Terrigenous sediments are transported through several physical processes and the transportation method inherently controls sediment grain size, shape, and/or composition.[51]

Size controls

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The transportation mechanism determines the size of the grains and the speed at which they are transported. Therefore, the composition of a sediment sample, with respect to grain size, is controlled by the most predominant transportation method that created it. Weaker processes, such as wind, tend to carry only the finest grains and generally transport them further and more rapidly than other processes. Rivers tend to move grains of larger sizes (along with fine grains) but at a slower rate. Fine grains are transported quickly in the suspension load, while larger grains move slowly in the traction load. River runoff usually does not have as much of a regional expanse as other processes. Glacial and iceberg transportation move the largest and heaviest grains, but this occurs at a variety of rates. Transportation fully by glacial flow is extremely slow, however, the transportation speed immediately increases after iceberg calving and melting. A sediment sample sourced primarily by icebergs tends to be very coarse, where a sample sourced by wind processes is very fine. Grain sizes and descriptions are commonly categorized with the Udden-Wentworth scale.[52]

Shape controls

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Grain geometry is largely influenced by the method in which the sediment was transported to the oceans. Liquid water transportation methods tend to yield rounded, smooth grains. This is driven by the combined effects of both impacts and further erosion while submerged.[30] This additional erosion and dissolution enhances ocean primary production by providing dissolved macronutrients.[19] Wind transportation methods tend to yield rounded but unsmooth grains,[51] where shape is only driven by impacts. While smoothing of aeolian grains does occur, this is a function of chemical weathering and is not related to the transportation mechanism. Lastly, glacial/iceberg transportation methods have almost no controls on grain shape; this is largely due to the lack of weathering and encasement the grains.

Sedimentation controls

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The location of deposition can depend on several variables. Once introduced into the ocean, grain size and ocean turbulence determine when a grain is deposited, therefore also determining where the sedimentation occurs. Smaller, lighter grains tend to be transported across great geographical extents by ocean currents, though at slower speeds. Conversely, larger and heavier grains tend to sink rapidly and thereby are deposited near where they first entered the ocean. Otherwise, and as mentioned earlier, the sedimentation location depends on the transportation mechanism. River-sourced sediment is most common around coastlines. Aeolian grains, being the smallest, are easily deposited everywhere. While, glacial/iceberg sediments are generally deposited between the high and mid-latitudes, with almost no deposition in the North Pacific due to the lack of iceberg sources.[30] These ocean sedimentation location controls are subsequent and in addition to the location controls dictated by the primary transport mechanism.

Graph of terrigenous sediment grain size gradient at a river outflow

Further comments on aqueous sedimentation controls

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Coastal sedimentation processes can be complex due to the turbulence caused by tidal activity, river outflow, and near-shore ocean currents.[53] However, the processes are even more complex in estuarine zones. In these mixing zones, between ocean and river, sedimentation rates are heavily influenced by the turbulence of ocean waves, tidal periods, river outflow, and wind forcing.[53] Remote sensing observations have empirically highlighted the role of these physical processes on near-shore turbidity.[54] Again, these processes are highly nonlinear and are very difficult to decompose. In non-estuarine locations, there tends to be a sediment grain size gradient around river mouths driven by the flow velocity gradient. As the flow velocity lessens upon reaching the river-ocean interface, progressively larger sediment grains can no longer be transported by the suspension or traction loads.[39] This allows courser sedimentation near the river outlet and finer sedimentation offshore.[39] Several factors (such as the river’s flow velocity, tidal patterns, and coastal currents) at any given time further dictate this sedimentation gradient’s spatial pattern.[39] It has been found that high river discharges can re-suspend near-shore sediments and export them into the open coastal ocean, where larger ocean currents dictate their transport and sedimentation.[55] Although it does not actually change grain size, flocculation (coalescence of small suspended sediments) can make suspended particles more prone to sedimentation than they individually were.[37] Generally, clay sediments are the primary catalyst for this phenomenon.[37] This can lead to unique sedimentation patterns that do not reflect the character of the physical process(es) that transported the grains.

Relationship to surface processes

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A Bouma sequence in a turbidite with coarser grained sediment underlying more fine sediments.
A turbidite showing normal grading, where a coarse-grained sediment layer is under progressively more fine sediment layers. Turbidity currents may deposit terrigenous sediments further from a river mouth such that a sudden change in sediment grain size indicates the base.[56][57] A flood carrying terrigenous sediment may show reverse grading.[58]

Through X-ray fluorescence (XRF), oceanic sediments such as those in the Gulf of California can be linked back to their terrigenous origins.[59] The elements present in a sample and the sample's radioactivity and magnetic properties are all used to narrow down when and where a mineral was formed according to known lithogenic processes. Argon-argon dating has been used to match ages of terrigenous sediments to their source rocks in the Yukon River Basin.[60]

Before 2019, academic articles in geochemistry indicated that a majority mafic continental crust was present 3.0 billion years ago (Gya) and changed rapidly on geologic timescales to a primarily felsic composition around 2.5 Gya.[61] Inverse mixing models, where mineral compositions are traced back through geologic processes,  were used on fine-grained terrigenous sediments from the Paleoarchean. Their current-day composition best fits the beginning conditions of sediments eroded from a continental crust that was greater than 50% felsic minerals.[62]  Samples of terrigenous sediments from the Paleoarchean era have a chemical composition that is approximately 65% by weight felsic. This indicates that subduction and magmatism, which are features of plate tectonics, were required to start before this era, about 3.25 Gya.[61]

A change in the ratio of zircon (Zr) to rubidium (Rb) and distinct changes in coarseness or graded bedding of sediments between sediments layers indicate stronger than average weather events such as hurricanes or floods.[59] The Zr/Rb ratio corresponds to the grain size of sediments at the time they are deposited.[63] The 1981 Tropical Storm Lidia was identified in a sediment core sample using the Zr/Rb ratio in Gulf of California sediments. Three other zones of higher Zr/Rb in the same sediment core were linked to Fuerte River flood events.[59]

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