We stand on the shoulders of … ancient oceans

Did you know, as we float atop the waters of the Subtropical North Atlantic, we also sit on layers and layers of ocean from all over the globe, and that these can be decades old?

The ocean here is around 4 kilometres deep, with different layers having originated in different places and made their way here by deep convection – the process by which mixed surface layers of the ocean travel to it’s interior. In this way, a water mass’ ‘age’ refers to the last time it was in contact with the air, and we can divide the Atlantic into roughly four vertical layers to help us characterise them.

The first couple of hundred metres we call the upper thermocline, the region where our phytoplankton are living and interfacing with the atmosphere, and where my samples are coming from. Below us, between 200-500 metres, lies North Atlantic Subtropical Mode Water, which is formed 300 miles north of us every winter, and subducted here through deep water currents. This water, once it reaches us here, hasn’t been in contact with the surface for anywhere between 6 months and 5 years.

Below the upper layer, we have an intermediate layer (~500-1000 m) which consists of Antarctic Intermediate Water (AAIW), Subarctic Intermediate Water (SAIW) and Mediterranean Water (MW). Then we have the deep and overflow layer, which predominantly consists of North Atlantic Deep Water (NADW), originating from Labrador Sea Water (LSW), Iceland-Scotland Overflow Water (ISOW) and Denmark Strait Overflow Water (DSOW). This water can be 10 - 20 years old! Finally, we have Antarctic Bottom Water (AABW), which here in the North Atlantic we call Northeast Atlantic Bottom Water (NEABW) [Liu and Tanhua (2021)].

The calm waters of the subtropical North Atlantic.

So, how do we know what these waters are and trace where they came from? Well, first we need to know the properties of the ‘source water type’, defined as ‘the original properties of water masses in their formation areas’, in order to characterise a ‘water mass’, ie ‘a body of water with a common formation history, having its origin in a particular region of the ocean’ [Tomczak (1999)].

The difference between these two things is that a water mass refers to a specific ‘package’ of water, whereas a source water type defines only mathematical properties of seawater. In order to characterise source water types and define water masses, we use 6 properties of water, delineated into two types – conservative and non-conservative. Conservative properties are those which can only be affected by abiotic (non-biological) factors, such as temperature and salinity. Non-conservative properties are influenced by the ocean’s physics and biogeochemistry; oxygen concentration and nutrients such as silicate, phosphate and nitrate. By identifying specifics of these 6 properties, we can essentially ‘fingerprint’ different water masses, tracing them back to their source water even if they’ve travelled hundreds of miles away over many decades.

It’s crazy to think, whilst we’re sailing over this one point in the North Atlantic, we’re also sitting on all those layers of water, from such different places, that have travelled so far so be here. Isn’t the ocean amazing?