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In this video, we will look closely at the sediment that makes the top layer of ocean crust. Where does it come
from? What does it look like? Feel like? Let’s start with the vocabulary used to
describe sediment size. Though there are many terms used to describe sediments of all sizes,
we’ll simplify them for this class into three categories: muds, sands,
and gravels. Technically, gravels are sediment grains larger than 2 mm in diameter. Muds are less
than 1/16 mm. Sands are everything in between. Where is each of these grain sizes found in the
ocean? The largest grains, the heaviest, settle quickly in still water and require high-energy
water to be picked up and carried. The smallest grains require perfectly still, unmoving water to
settle out. Ocean environments are separated into two major geographic zones – neritic,
over the continental shelf – and oceanic over the slope, rise, abyssal plains,
mid ocean ridges and trenches. Neritic waters tend to have faster-moving currents interacting
with surface waters and the shallow bottom. These waters can pick up and shift around
larger sediment grains. Rivers that enter here will retain mud-sized sediments in suspension
and can carry them quite a distance offshore. Gravels tend to stick close to shore. Fine
sands collect in medium-energy coves and can be carried out to the edge of the shelf. Muds settle
out only in closed-off no-energy lagoons. Oceanic waters are deep with no currents that touch the
bottom. Gravels and sands cannot be transported out here by anything other than floating icebergs,
so the sediment that falls here is only the suspended muds. Where do these muds come from
that get out to the oceanic zone? Nearby rivers that have a lot of suspended sediment. Airborne
dust and ash that settles on the surface. And the shells and debris of organisms that
live in the surface waters and settle to the seafloor when those organisms die.
The oceanic zone represents 4/5 of the world’s ocean environments. However, the sediment that
collects in the margin environment is the thickest in the world – up to 9 km thick,
compared with the oceanic sediment thickness, which increases from 0 at the ridges to 0.6 km at
the thickest oldest edges of the abyssal plains. Even though the sediment that collects in the
margin environment represents only 1/5 the ocean’s surface, its sediment volume represents almost
90% of the total ocean sediment. Why? Sediment collects here much faster – at rates from 0.5
to 800 m per 1000 years. For comparison, in the oceanic zones, sediment collects, at its fastest,
1 cm per 1000 years.
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What makes up all these sediments? There are four major sources of sediment to the ocean,
and of course each gets its own name. We call sediment that originated from rock weathering on
land lithogenous or terrigenous sediment. What is it made of? Bits of rocks and minerals. When next
to a weathering cliff, like along an eroding coastline, the material is made of bits of
the rock in the cliff, and gravity carries them towards the oceans. When dropped by glaciers,
these sediments can be quite large and come, relatively preserved, from the backcountry far
inland. We call both of these types of sediments immature, because they haven’t been modified
much by travel. They can be angular and poorly sorted, consisting of grains of all compositions,
shapes, and sizes -- mostly gravels. When lithogenous sediment is carried by rivers,
on the other hand, it becomes mature. Grains decrease in size to fine sands and muds. Why?
During transit, the grains are knocked around and hit each other. They physically break down into
smaller pieces. They also break down chemically. Most of the material dissolves or is turned into
clays and rust. The most resistant materials, predominantly quartz grains, which don’t dissolve
or turn into clay or rust, remain. Clay minerals are mud sized and are the most common byproducts
of rock weathering on earth’s surface. Some of these clay minerals make it out to the oceanic
zones, either by being held in suspension near large river deltas, or by being carried
by winds off desert and mountain areas. We call those clays abyssal clays (as they are found in
abyssal plains). We’ll return to them shortly. As lithogenous sediment dominates the coastal
margins and accumulates at the fastest rates of all sediments there, and also reaches out
in its mud-sized form to the deeper parts of the oceans predominantly as abyssal clay, lithogenous
sediments make up the largest percentage by volume of marine sediments. However, because most of this
volume stays close to the margins and piles up there thickly, they represent only 45% of the
total seafloor surface coverage. The sediment that covers most of the seafloor surface is biogenous,
which consists of shells of dead organisms and dominates the oceanic zones. The most important
contributors to biogenous sediment are the single-celled autotrophic and heterotrophic
plankton that live in the surface waters. Billions of these organisms live and die with lifetimes of
days to weeks. Their shells collect on the bottom of the seafloor and contribute to siliceous and
calcareous oozes, depending upon whether the shells are made of silica (siliceous oozes)
or calcium carbonate (calcareous oozes). The term ooze tells us a lot about the consistency of these
sediments. They are all mud sized – a very fine flour-like material. Larger, gravel-sized shells
contribute to neritic nearshore sediments where there are large reefs – either tropical coral
reefs or colder-water reefs made of mollusks, like mussels and limpets. We’ll return to the oceanic
oozes shortly to describe them in more detail. Another type of sediment, called hydrogenous,
represents minerals that are produced from from ions that were originally dissolved in water. We
call that process precipitation. Seawater has many dissolved ions in it, and in numerous locations,
those ions reach a concentration that is higher than can be dissolved by the water anymore,
either because of evaporation of water or because of an influx of dissolved ions.
We call that state supersaturated, and you can learn more about this in the seawater chemistry
video tutorials. For now, we’ll focus on where to find these precipitated hydrogenous sediments. At
shallow inland seas, where rains are limited and evaporation rates are high, minerals can
precipitate in high amounts. These minerals consist of halite (table salt), gypsum (dry
wall material), and calcite. In some deep areas of the abyssal plains, dissolved phosphates and
manganese become supersaturated and precipitate as nodules that grow bigger over time as new
material precipitates atop the older material. Any other place in the ocean where sediment
precipitates from solution? Something we discussed in the plate tectonics and earth
formation video tutorials? Hydrothermal vents! Hot waters rich in dissolved ions leached from
the ocean crust enter the colder seawater. As that hot water cools, it becomes supersaturated
with dissolved ions (cold water can dissolve less than warm water; that’s why when cooking,
we heat up our water when we want to dissolve things like sugar or salt in it!). Hydrogenous
sediments are found in many places across the oceans, but they represent less than 1% of the
surface area, as they are largely scattered. The most uncommon sediment found in the ocean,
but still found in abundances greater than on land, is cosmogenous sediment – meteorites
that fall from space – and tektites, which are bits of Earth’s crust that have been ejected
into space after a meteorite impact excavates a crater. This ejected excavated crustal material
melts as it exits the atmosphere and solidifies as it falls back down – producing a spherical,
tear-drop-shaped glass nodule, with eroded pits created by its journey back down through the
atmosphere. Why are these found in the oceans in more abundance than on land? Simply because of
the slow sedimentation rate of the oceans. Where sediment collects at rates as low as
1 cm per 1000 years, it’s easier to find the tektites and meteorites that fall among them.
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Now let’s return to the oceanic zone and look
more closely at the mud-sized abyssal clays and siliceous and calcareous oozes that form there.
Are all three found in the same location? In most cases, one is the dominant. Why? Abyssal clays,
remember, are lithogenous or terrigenous will dominate near high-volume rivers with large
deltas. Without rivers nearby, abyssal clays come from the settling of airborne dust and ash that’s
been blown from the land. If there are nearby deserts with high wind activity, the rates of
abyssal clay settling can be comparable to that of calcareous or siliceous shells. So the settling
rate of abyssal clays in the deep oceans can be anywhere from 1 mm to 1 cm per 1,000 years.
We call the organisms that live in the surface waters plankton (free-floating, can’t swim faster
than currents). If they are autotrophic (make their own food), they are the bottom of the food
chain, and are referred to as phytoplankton. Heterotrophic plankton are referred to as
zooplankton. Diatoms are phytoplankton with silica shells (SiO2). Coccolithophores are phytoplankton
with calcium carbonate shells (CaCO3). Radiolaria are the most common zooplankton with SiO2 or
silica shells. Foraminifera are the most common with CaCO3 shells. These four organisms play very
important roles in the world’s oceans, and it’s valuable to memorize these names and shell types
now, as they will appear many times throughout future video tutorials. We’ll review one more
time, then you’ll take a quiz. Autotrophs: Diatoms and Coccolithophores. Heterotrophs:
Radiolaria and Foraminifera. Silica shells: Diatoms and Radiolaria. Calcareous shells:
Coccolithophores and Foraminifera.
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So where do these organisms thrive and hence their shells dominate the oceanic muds on the seafloor?
Calcareous-shelled creatures are the most abundant organisms in the world’s oceans, so theoretically,
their shells should dominate all deep–sea muds. That would be true, except for two things. First,
in areas where deep water rises to the surface, a process called upwelling, silica-shelled organisms
thrive. These waters are cold and filled with nutrients. Nutrients are dissolved ions that
autotrophs pull from the water to use for building cells and shell material. All autotrophs require
that the water have nutrients. Heterotrophs get their cell-building and shell-building nutrients
from their food. Since autotrophs make their own food, they need to get their nutrients from the
surrounding water. Silica-shelled autotrophs are adapted to cold nutrient-rich water,
while calcareous-shelled autotrophs tend to prefer warmer waters and don’t require as high a nutrient
content. So siliceous oozes dominate under areas of upwelling. Though some calcareous shells might
also be found in these oozes, along with abyssal clays, the silica shells dominate. Where are these
areas? Upwelling can occur anywhere at any time, but it is typically found year round in the waters
surrounding Antarctica, and along the eastern edges of the ocean basins along the equator.
We’ll learn more about why that’s true in the video tutorials that deal with ocean currents.
So everywhere else there should be calcareous oozes? Not quite. There is one remaining issue
to take into account. Calcareous shells will dissolve in acidic waters. And as water gets deep,
cold temperatures and high pressure mean more carbon dioxide is dissolved. More carbon dioxide
means more acidic. For more information, refer to the video tutorial on Carbonated Oceans. For now,
we focus on the result – though calcareous creatures might exist in the surface waters,
their shells will dissolve en route to the seafloor, if the seafloor is deeper than the
calcium carbonate compensation depth otherwise known as the CCD. The CCD is the depth at which
the acidity of the seawater is high enough to dissolve these shells in their normal abundance,
and thus no calcareous shells accumulate below this depth unless they collect faster than they
can dissolve. For our purposes, that means they should be absent. This depth varies in the oceans,
but is, on average, around 4500 meters. That means the deep abyssal plains will not have calcareous
shells accumulating. Only siliceous oozes or abyssal clays can collect at these depths. So
let’s use this information now to describe the distribution of oceanic muds across the world’s
oceans. With what seafloor feature do these calcareous oozes correspond? Mid-ocean ridges,
the shallowest parts of the oceanic zone. Siliceous oozes dominate at the
eastern equatorial zones and in polar regions as previously discussed. The rest of the oceans is
dominated by abyssal clays.
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