Hydrogen sulphide plumes off the Namibian coast

By Arthur Capet (MAST - University of Liege), Charles Troupin and Aida Alvera-Azcárate (GHER, University of Liege), Ben Loveday (Innoflair UG) and Hayley Evers-King (EUMETSAT)

Milky turquoise threads might appear beautiful when seen from space (Figure 1), but they reflect a much less poetic reality onsite. Large numbers of stranded dead fish; smelly, corrosive gases released from the ocean; threats to local fisheries, aquaculture and tourism industries, the culprit of these torments has a name: hydrogen sulphide.

Hydrogen sulphide, or H₂S, is a chemical compound produced when large amounts of organic matter (eg. dead phytoplankton cells) are decomposed in oxygen-lacking conditions. When released in open water, H₂S is quickly oxidised. This process consumes massive amounts of the oxygen dissolved in seawater, and leads to the formation of colloidal sulphur, a precipitate that gives seawater this milky turquoise colour when present in large quantities (Ohde et al., 2007).

But why are such 'sulfur' plumes are almost exclusively observed along the Namibian coast and, more recently, off Peru (Ohde, 2018)? The reason is that only very specific conditions can lead to the accumulation of H₂S in seawater in such quantities that it can be spotted from space.

The Namibian shelves are extremely productive. Trade winds permanently blow along the Namibian shelves, and, therefore, sustain the so-called upwelling process. By pushing surface waters offshore, they 'upwell' deep waters along the shelf slope, bringing important amounts of nutrients in the well-lit surface layer. The upwelling can be clearly seen in cold water signatures observed by satellite instruments monitoring sea surface temperature (Figure 2), with the temperature difference between the coast and the offshore area larger than 6°C.

With both requirements for growth — nutrients and light — an exceptional growth of phytoplankton biomass can be triggered (Figure 3) (Note: this example is not directly related to the observed H₂S plume — these are caused over time by similar events and their resultant decomposition).

This is not specific to the Namibian coastal region. In fact, the Benguela upwelling system, that Namibia is part of, counts among the four major Eastern Boundary Upwelling Systems (EBUS) located at the eastern boundary of the Atlantic and Pacific oceans, at subtropical latitudes where regular equator-ward trade winds prevail. Yet, the resulting upwelling is particularly strong for the Benguela system, and so is the sustained phytoplankton growth.

As well as supporting bountiful fisheries and aquacultures, one consequence of this intense primary production is that EBUS are characterised as oxygen minimum zones (OMZs). Oxygen is required to degrade the large part of the produced biomass that is 'respired' locally. As incoming deep waters do not provide enough oxygen to counter-balance this process, oxygen concentration draws down to the point that it affects biogeochemical process rates and, in the worst cases, the survival of marine organisms.

As oxygen concentration draws down, a succession of chemical processes takes the lead to achieve the degradation of organic matter. This succession is conditioned by the availability of the oxidant (eg. oxygen, nitrate, iron oxide, etc) which provides the most energetically efficient way of oxidising organic matter. At the end of the line, in strongly oxygen-deficient conditions, sulphate reduction dominates, which, as a by-product, provides the culprit : hydrogen sulphide.

Typically, sulphate reduction occurs in the marine sediments, where sinking organic matters accumulates, and where restrained transport limits the external supply of oxygen (it’s more difficult for oxygen to be diffused within the sediment matrix than in the water) . In Namibia, the water column consumption of fresh organic cells by grazer organisms is limited (Weeks et al., 2004). As a consequence, a large fraction of the produced biomass reaches the seafloor in a relatively fresh state, and respiration occurs within the sediment, in anoxic conditions, leading to large accumulation of hydrogen sulphide in the interstitial pores of the marine sediments.

What retains this H₂S in the sediments? Why doesn’t it diffuse gently and regularly to the seawater, but instead get released sporadically, in burst events that cause much larger impacts? Current research is trying to answer this.

It is known that large bacterial mats, specialised in acquiring energy from the oxidation of hydrogen sulphide, play a role in limiting the upward diffusion from sediments to the water (Brüchert et al., 2003). It is also known that methane accumulation in the sediments may lead to bubble eruptions, disrupting the sediment water interface and releasing accumulated H₂S on its way (Weeks et al., 2002, 2004). To pinpoint specific external environmental factors that enhance the probability of such outburst releases is a current concern (Ohde & Dadou, 2018). Crucially, it is important to investigate whether climate change will increase the probability of such events (Bakun, 2017)?

This is precisely where remote sensing comes into play. For a long time, such events were thought to be occasional, or rare, in part due to the lack of sampling means for the region. Yet, the advent of multi-spectral ocean colour sensors on satellites allows us to build algorithmic procedures to distinguish sulphur plumes from other water-colouring events. Thanks to these advances, clear documentation of these events, their spatial extent and temporal frequencies can be made, and put into relation with other time series describing external conditions.

In addition to the direct toxic impact of H₂S, outburst release lead to the depletion of oxygen (consumed to oxidise H₂S) in bottom waters. Subsequent anoxic conditions can last for days, extend over a large area and, therefore, trap organisms into deadly conditions. In 1992-1993, a catastrophic loss of two billion young Cape Hake was associated with an anoxic outbreak, which amounts to about a thousand fish per Namibian inhabitant (Weeks et al., 2004)! In a country where fisheries represents about 20% of export earnings, assessing the future trends of sulfidic outbreaks’ intensity, frequency and extent clearly appears as a security issue in relation to UNDP SDGs 1 ,2 ,3 ,8 and 14. By providing the means to decipher the underlying mechanism, state-of-art remote sensing clearly contribute to address that issue.

References

Bakun, A. (2017). Climate change and ocean deoxygenation within intensified surface-driven upwelling circulations. Philosophical Transactions. Series A, Mathematical, Physical, and Engineering Sciences, 375(2102). https://doi.org/10.1098/rsta.2016.0327

Brüchert, V., Jørgensen, B. B., Neumann, K., Riechmann, D., Schlösser, M., & Schulz, H. (2003). Regulation of bacterial sulfate reduction and hydrogen sulfide fluxes in the central namibian coastal upwelling zone. Geochimica et Cosmochimica Acta, 67(23), 4505–4518

Ohde, T. (2018). Coastal sulfur plumes off Peru during El Niño, la Niña, and neutral phases. Geophysical Research Letters, 45(14), 7075–7083

Ohde, T., & Dadou, I. (2018). Seasonal and annual variability of coastal sulphur plumes in the northern Benguela upwelling system. PloS One, 13(2), e0192140

Ohde, T., Siegel, H., Reißmann, J., & Gerth, M. (2007). Identification and investigation of sulphur plumes along the Namibian coast using the MERIS sensor. Continental Shelf Research, 27(6), 744–756

Weeks, S. J., Currie, B., & Bakun, A. (2002). Massive emissions of toxic gas in the Atlantic. Nature, 415(6871), 493–494

Weeks, S. J., Currie, B., Bakun, A., & Peard, K. R. (2004). Hydrogen sulphide eruptions in the Atlantic Ocean off southern Africa: implications of a new view based on SeaWiFS satellite imagery. Deep Sea Research Part I: Oceanographic Research Papers, 51(2), 153–172