Ocean Layers & Mixing

Now that we’ve quickly discussed some basics of how our atmosphere works, it’s time to get down to business and start discussing how our oceans work! The oceans are a key component to discussing and understanding climate change. On this page, we will talk about the main layers of the ocean, since understanding this material is essential to understanding the next few pages (Ocean Chemistry & Acidification) that will go into more detail about our ocean systems.

Main Points

  • The ocean has three main layers: the surface ocean, which is generally warm, and the deep ocean, which is colder and more dense than the surface ocean, and the seafloor sediments.
  • The thermocline separates the surface from the deep ocean.
  • Due to density differences, the surface and deep ocean layers do not easily mix. This means that the gases dissolved in surface seawater, such as CO2, also get mixed into deep seawater on longer timescales.
  • The seafloor is made up of 4 main types of sediments: Biogenic, Lithogenic, Hydrogenous, and Cosmogenic.
  • Biogenic sediments are very important, and include both siliceous and carbonaceous sediments made up of the hard parts of marine organisms.
  • Carbonaceous sediments are those that contain calcite, or calcium carbonate (CaCO3).

Our Layered Ocean

There are three main ‘layers’ to the ocean that we will focus on: the surface ocean, the deep ocean, and the seafloor sediments (sediments that are still in contact with seawater).  Each plays a vastly important role in the absorption of carbon dioxide from the atmosphere, and in ocean circulation.

Hypothetical cross section of a continental shelf to ocean transect with different layers of the ocean showing timescales on which different layers of the ocean mix completely, along with how long it takes for the biosphere to take in CO2 from the atmosphere, and how long it takes for CO2 that is buried on the seafloor to be part of the rock record. Blue whale not to scale.

Surface Ocean and Thermocline

The surface ocean can be thought of as the upper ‘skin’ of the world’s ocean, and is often referred to as the mixed layer. When scientists refer to the surface ocean/mixed layer, we’re referring to the top 200 meters of the ocean, on average, but the depth of the mixed layer changes based on the seasons and latitude. The mixed layer is defined as the layer in which there is active turbulence and mixing of oceanic waters due to winds, heat fluxes, evaporation, and salinity fluxes. Turbulence in this sense is different from the turbulence you may think of when flying in an airplane. In ocean studies, turbulence is a physical process in the transfer of heat and momentum which disperses small particles within the water column (Thorpe, 2007).

The surface ocean is important because it is constantly exchanging gases with the atmosphere. In other words, the mixed layer surface ocean remains in approximate equilibrium with the atmosphere with respect to gases. Much of the CO2 that is put into the atmosphere is also absorbed into the surface ocean. This exchange of gases between the atmosphere and the surface ocean takes place on the scale of 10-100 years. This is how long it takes for the atmosphere and surface ocean to come into equilibrium, or a state in which the amount of CO2 in the atmosphere and in the surface ocean are balanced.

Ocean temperature profile within the mid-latitude regions. The surface ocean is relatively warm, but with depth into the ocean, temperature begins to decrease until it stabilizes around or slightly above freezing (32˙F or 0˙C). The area where temperature changes from the surface ocean to the deep ocean is called the thermocline. Image modified from NOAA.

In general, the surface ocean is much warmer than the deep ocean, and the bottom of the surface ocean is determined via measurements of water temperature and density. The transition into the deep ocean happens when the temperature of the water drops and the density increases. This zone of falling seawater temperatures and increased density is called the thermocline. The depth and strength of the thermocline changes at different latitudes and seasonally. In the tropics, the thermocline is relatively stable because it is consistently warm year round. At the mid latitude regions, the thermocline deepens in the summer and shallows in the winter. Because surface temperatures are already so cold in polar regions, the thermocline in these areas is almost non-existent.

Deep Ocean

The deep ocean is all the seawater that is colder (generally 0-3°C or 32-37.4°F), and thus more dense, than mixed layer waters. Here, waters are deep enough to be away from the influence of winds. In general, deep ocean waters, which make up approximately 90% of the waters in the ocean, are homogenous (they are relatively constant in temperature and salinity from place to place) and non-turbulent.

Mixing of the Surface and Deep Oceans

Because the surface and deep ocean layers are of very different densities (due to salt content and temperature), these layers of the ocean do not mix easily. The resistance of two water bodies with different densities to mix is called stratification. This is a natural phenomenon of our oceans, and can even occur in lakes. Winds and storms, such as hurricanes and typhoons, are the mechanisms by which the surface ocean is mixed with the deep ocean. On average, it takes the waters in the ocean’s mixed layer and deep ocean about 1,000 years (or more) to completely mix. We’ll talk more about ocean stratification on the next page.

Check out this YouTube video to see what happens when two bodies of water with different temperatures (and thus different densities) are mixed together.

Seafloor Sediments

Seafloor sediments are sediments that can be found at the bottom of the seafloor that are still in contact with seawater. This means that these sediments can still be influenced by the water around them. Sediments located deeper below the sediment-water interface (the contact zone between sediments and seawater) are insulated, or cut off, from seawater.  There are several different types of sediments that make up the seafloor, and a few of the most important are discussed briefly below.

Major types of seafloor sediments and their distribution on the seafloor. Siliceous ooze and carbonate ooze are both types of biogenic sediments. Image from ‘Physical Geology‘ by Steven Earle used under a CC-BY 4.0 international license.

Terrigenous Sediments

Terrigenous sediments are those that are derived from the Earth, mostly from the continents themselves. They include pieces of debris from land, from volcanoes, and even wood. These sediments can be dust blown into the oceans from deserts (as well as from other places on Earth), small particles of clay carried out to sea by rivers, and larger pieces carried into the ocean by glaciers and icebergs. The greatest amount of terrigenous sediments are found near the continents, but smaller particles, such as clay and dust, can be deposited into the middle of some ocean basins.

Clay

Very small particles of sediment that settle onto the seafloor are considered clay. Clay is composed of terrigenous sediment, such as dust from deserts that is carried far into the ocean by wind. Clay minerals are dominant in the deepest parts of the ocean.

Cosmogenic Sediments
One of the famous K/Pg cores that records the Chicxulub impact.

Cosmogenic sediments are small pieces of rocks and dust that come from outer space, and can be found in any region of the seafloor. They are included here under ‘Clay’ because these particles are generally very small, and can occur in the deepest parts of the ocean. These particles are usually derived from broken-down meteors, or small pieces from meteorites. These sediments are usually hard to detect because they are so rare on the seafloor. Their presence can be indicated through extensive lab testing of sediments, and if found in high abundance, can tell paleoclimatologists some important information about the Earth.

For example, in the late 1990s, scientists drilled into the seafloor sediments off the coast of the Yucatan Peninsula. Earlier, other scientists had detected a large, circular feature below the seafloor which they thought to be a crater created by an ancient meteorite impact. One of the recovered cores contained sediments that indicated that there had indeed been an impact in the geologic past. Detailed analysis of the sediment in this core, pictured at left, revealed a thick layer of ash and rock, called ejecta, that was deposited weeks to months after the meteorite struck Earth. At the top of this layer was a thinner layer that contained a high amount of iridium, which has been detected in cosmogenic sediments and on other meteorites. Although the scientists didn’t exactly find pieces of cosmogenic sediments in the core, mostly because the meteorite was vaporized upon impact; they did find iridium in the sediments from a cosmogenic source. From this famous horizon, scientists were able to determine that the Chicxulub impactor is what caused the non-avian dinosaurs to go extinct, as well as tsunamis and global wildfires.

More recently, an IODP expedition drilled into the peak ring of the Chicxulub crater off the Yucatan Peninsula. The team recovered basement rock, or igneous/metamorphic rock that is underneath the ocean sediments, for several different reasons. First, they wanted to know exactly what happened when the meteor hit, how the rock underneath behaved, and how quickly life returned to ‘ground zero’.

Click here to read a nice write-up in the Nautilus about the K/Pg impactor, and here to learn about what the scientists from the IODP expedition have learned from their Chicxulub core.

Biogenic Sediments

A radiolarian, an example of a siliceous microfossil.

Biogenic sediments are derived from hard parts of marine organisms. These sediments are derived from plankton, phytoplankton, corals, sponges, bivalves (such as clams), and other marine animals that have hard parts, such as echinoderms (in today’s oceans, animals like starfish and sea urchins, and in the geologic past, organisms like blastoids).

There are two main types of biogenic sediments that paleoclimatologists are concerned with: silicious and carbonate sediments. Siliceous sediments are those that are formed from microfossils that form their skeletons from silica (SiO2), such as radiolarians and diatoms.

Carbonaceous sediments are made from ancient marine organisms that made their skeletons out of calcite (CaCO3), such as foraminifera, ostracods, and coccolithophores. In some areas of the ocean, the sediments are made of almost 100% siliceous or carbonaceous sediments. In these cases, geoscientists call the sediments ‘siliceous ooze‘ or ‘carbonate ooze‘. When drilled from the seafloor, these sediments look like snow due to their very white to light grey appearance.

Biogenic sediments are very important sediment types, as their presence on the seafloor through time tells paleoclimatologists important information about how our ocean operated in the geologic past. In addition, the fossils themselves can be used in isotope analyses. For example, in today’s oceans, siliceous sediments (and siliceous ooze) are found at very high latitudes (cold areas) and places where the surface ocean is rich in nutrients. Therefore, when geoscientist drill sediment cores from the seafloor and find intervals in time when the ocean at that location contained siliceous ooze, they know that the surface ocean in the geologic past was colder and/or contained lots of nutrients. 

Hydrogenous Sediments

Hydrogenous sediments are ocean sediments that are formed in place from chemical reactions with the surrounding seawater. Often, these types of sediments come in the form of nodules, or large chunks of particles, such as manganese and iron oxides. They can be made up of several different materials, such as carbonates (similar to calcite/calcium carbonate), salts (mostly from evaporation of sesawater), and nodules (mostly found in the deepest parts of the oceans).

To learn more about ocean layers and seafloor sediments, visit the below sites:

Proceed to ‘Ocean Chemistry & Acidification’

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