Sulphur plumes off Namibia 'seen' by Sentinel-3

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Using Sentinel-3 satellite data to explain bright waters and unusual smells during a cruise to monitor carbon.

Sulphur plumes off Namibia
Date & Time
21 February 2018 08:49 UTC
Ocean Colour

By Hayley Evers-King, Ben Taylor, Ben Loveday (PML) and Stephanie Henson and Filipa Carvalho (National Oceanography Centre).

The colour of the ocean is influenced by microscopic plant-like organisms known as phytoplankton. They play many important roles in the marine ecosystem, are responsible for half of the oxygen in the air we breath, and they absorb the carbon dioxide that human activities emit.

Carbon is the topic of the European Research Council (ERC)-funded Gauging Ocean organic Carbon fluxes using Autonomous Robotic Technologies (GOCART) project. As part of this project, two gliders (Doombar and Grease) were deployed to gain better understanding of temporal variability in the ocean’s biological carbon pump (Figure 1, top right, click to expand).

Particles sinking through the water carry organic carbon from phytoplankton into the deep ocean, but how deep the particles get determines how long that carbon can potentially be locked away (sequestered) for. GOCART is using new approaches to determine the daily to seasonal changes in carbon sequestration depth at sites in the Southern Ocean and off Namibia. The gliders are equipped with temperature, conductivity, oxygen, PAR, and a bespoke optical sensor measuring chlorophyll fluorescence and two wavelengths of optical backscatter.

On the recent cruise to deploy the gliders, scientist Filipa Carvalho noticed a strange smell coming from the sea. On the other side of the world, PML scientist Ben Taylor was preparing satellite data to support the glider project, and also noticed something strange — a bright patch near to Walvis Bay.

Figure 2: Sentinel-3, 21 Feb, 08:49 UTC
OLCI Level-2 NRT False Colour
Figure 3: Bright blue waters of Walvis Bay. Photo credit: Filipa Carvalho @GOCARTcarbon

Figure 2 shows a false colour composite image made from images taken by the Ocean Land and Colour Instrument (OLCI) onboard the Sentinel-3 satellite. You can see the bright patches near Walvis Bay in the southern coastal area. The actual colour of the sea in this area can be seen in Figure 3.

These bright patches, and the smell noticed on the cruise, are the result of sulphur plumes that occur regularly in this region.

The highly productive waters ultimately create a lot of decomposition when the phytoplankton die. This decomposition strips the deep waters of oxygen leading to the formation of hydrogen sulphide. Plumes of hydrogen sulphide can have huge impacts on the marine ecosystem and associated industries in the region.

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