Sediment in the deep ocean. Part 2: thermohaline currents that shape the seafloor

In Part 1 we differentiated between (1) shallow-marine tide-related currents from (2) purely gravitational sediment-laden currents. We could add that the former are periodic, as they are controlled by the effect of the Moon and Sun gravitational fields on the oceanic water as the Earth revolves, while the latter are sort of ‘spontaneous’ currents driven by the Earth gravitation field on sediment and rock, with an unpredictable duration and spatial impact.

Both current types erode and deposit sediment on the seafloor, but these are not the only current types in the ocean that can do that. Thermohaline currents can do that also. These currents are movements of ‘huge’ masses of seawater driven by gravity. As the name suggests, the temperature (thermo) and the salinity (haline) of the seawater are the key elements that promote the water to move. Simply spoken – the colder and the more salt in water, the denser the water will be and eventually the Earth gravitation field will move it to a position closer to the Earth centre; this is, the seafloor (see the key aspects of thermohaline currents in Table 1).

Thermohaline currents occur at any water depth. They can start at the sea surface, like the so-called “dense shelf water cascading process” (Canals et al., 2006) and they have been observed the deepest parts of the ocean like in subduction zone trenches (Thomson et al., 2010). Thermohaline currents are still not well understood as they are connected to a complex system that integrates the atmosphere and the ocean dynamics. The combined action of wind stress and the Coriolis effect on the sea surface produces a series of large eddies called ocean gyres. These gyres are also connected with the formation of global upwelling and downwelling currents that reach the seafloor (see Figure 1).

Figure 1 Global thermohaline circulation
This Mollweide projection of the Earth shows a simplification of surface and subsurface global currents and areas where water sinks (Deep water formation) or rises (upwelling) to the surface because of density differences. Source: Rebesco et al. 2014.

If we were to dive to abyssal depths where thermohaline currents occur, we would see ripples and scours of any type (see Table 1). However, in a much larger scale these currents create strata geometries that can be similar to some submarine channel systems formed by sporadic gravity-driven currents caused by collapses of sediment and rock in areas relatively close to coastlines. Thermohaline currents also create channels and overbank deposits analog to levees of channels connected to continental margins. These are comparatively much larger and typically formed by finer sediments.

Figure 2 The Thermohaline current pathways of the Gulf of Cadiz.
The figure on the left shows continents in black and the ocean in blue. In dark blue, the thermohaline channels where currents from the Mediterranean Sea move to the west. The figure on the right shows a seismic section of the northernmost channel and the levee-like deposit created by the thermohaline current, named Faro Drift (A-B in the figure above).

Some thermohaline channels are connected to both continental-margin channels and controlled by regional tidal pathways. One of the most studied areas of this type is the Gulf of Cadiz (Figure 2).

Anyone can imagine how important these thermohaline currents may be in the global climate evolution. These currents are thought to play a crucial role in controlling global climate. However, their impact on oceanic physical and chemical processes are not well known. It is therefore still under discussion what the effects of anthropogenic climate change on thermohaline currents are, and how they interact with deep sea geological and biological systems. Research is the key.

More on links between thermohaline currents and the global climate:

https://easac.eu/publications/details/a-sea-of-change-europes-future-in-the-atlantic-realm/

References

Alonso, B., Ercilla, G., Casas, D., Stow, D.A., Rodríguez-Tovar, F.J., Dorador, J. and Hernández-Molina, F.J., 2016. Contourite vs gravity-flow deposits of the Pleistocene Faro Drift (Gulf of Cadiz): Sedimentological and mineralogical approaches. Marine Geology, 377, pp.77-94.

Canals, M., Puig, P., de Madron, X.D., Heussner, S., Palanques, A. and Fabres, J., 2006. Flushing submarine canyons. Nature, 444(7117), pp.354-357.

Integrated Ocean Drilling Program - Expedition 339 Scientists, 2012. Mediterranean outflow: environmental significance of the Mediterranean Outflow Water and its global implications. IODP Prel. Rept., 339. doi:10.2204/ iodp.pr.339.2012.

Rebesco, M., Hernández-Molina, F.J., Van Rooij, D. and Wåhlin, A., 2014. Contourites and associated sediments controlled by deep-water circulation processes: State-of-the-art and future considerations. Marine Geology, 352, pp.111-154.

Thomson, R.E., Davis, E.E., Heesemann, M. and Villinger, H., 2010. Observations of long‐duration episodic bottom currents in the Middle America Trench: Evidence for tidally initiated turbidity flows. Journal of Geophysical Research: Oceans, 115(C10).