The dilemma of Rwanda’s Lake Kivu

In 1986 the small, roughly circular Lake Nyos in the Cameroon highlands silently released a massive cloud of carbon dioxide. Being a dense gas it hugged the ground and flowed down valleys for up to 25 km. 1700 local people perished by suffocation, together with their livestock (See Geohazards 2000). Having a recent volcanic origin, the lake is fed by springs in its bed that contain dissolved CO2 emitted from the residual magma chamber below. At 200 m deep the bottom water is sufficiently pressurised to retain the dissolved gas so that signs of the potential hazard remain hidden until such a limnic eruption occurs. Far larger, with a surface area of 2700 km2, Lake Kivu bordered by Rwanda and The Democratic Republic of Congo, is even deeper (up to 470 m). It too lies within a volcanically active zone, in this case the western arm of the East African Rift System. Being one of the most nutrient-rich bodies of fresh water on Earth, its biological productivity is extremely high, so as well as bottom water enriched in dissolved CO2 – a staggering 256 km3 – methane (CH4) is also present in very large amounts (~65 km3). This comes partly from anaerobic decay of dead organisms and from microbial reduction of the magmatic CO2 passing through its bottom sediments. Sulfate-reducing bacteria also generate toxic hydrogen sulfide (H2S) in the anoxic bottom waters – Lake Nyos contains less dissolved salts and did not emit H2S.

So Kivu presents a far greater hazard than the volcanic lakes of Cameroon and an emission of a dense gas mixture might fill the rift valley in the area to a depth of about a hundred metres. Being highly fertile the valley around the lake has a high population (2 to 3 million), so the death toll from a limnic eruption could be huge. A further hazard stems from tsunamis generated by such gas bursts. Once bubbles form at depth the bulk density of water drops, so large masses of water surge to the surface rather than the gas itself; a phenomenon known to happen in the periodic eruptions of Lake Nyos. What might trigger such an event in Lake Kivu? The East African Rift System is seismically active, but recent earthquakes did not result in limnic eruptions. Subaqueous volcanic eruption is the most likely to set one off. A surface lava flow from the nearby Mount Nyiragongo entered the lake at the town of Goma in 2002 but, fortunately, did not reach the threatening deeper part of Kivu. Sediment samples from the lake reveal periodic transport of land vegetation to its deeper parts, roughly every thousand years. The sediments with plant fossils also contain abundant remains of aquatic animals, suggesting both tsunamis accompanied by toxic emissions.

KIVUWATT’s methane extraction rig on Lake Kivu. (Credit: Contour Global)

Mitigating the hazard of limnic eruptions at Lake Nyos was made possible in 2002 by linking its bottom waters to the surface by plastic piping. After initial pumping, the release of bubbles at shallower depths and the resulting fall in bulk water density set off something akin to a large soda siphon, slowly relieving the deeper layers of their load of dissolved CO2. This resulted in 50 m high fountains of what was effectively soda ‘pop’. In 2009 this was repeated on a far larger scale on Lake Kivu, the operation being paid for by separation and sale of methane. Yet even this attempt at mitigation has its risks: first of destabilising what may be a fragile equilibrium to trigger a limnic eruption; second by lifting nutrient-rich bottom water that would encourage algal blooms at the lake surface and potential deoxygenation. The current issue of the Journal of African Earth Sciences includes a detailed review of the issues surrounding such dual-purpose hazard mitigation (Hirslund, F. & Morkel, P. 2020. Managing the dangers in Lake Kivu – How and why. Journal of African Earth Sciences, v. 161, Article 103672; DOI: 10.1016/j.jafrearsci.2019.103672). By 2015 the Rwandan KivuWatt Methane Project had a capacity for 25 MW of electrical power generation.

Running at full capacity, degassing the depths of Lake Kivu would provide the economic benefit of low-cost electricity for Rwanda and the DRC, at a maximum generating capacity of 300 mW using the most efficient power plant, as well as removing the risk of a catastrophic gas release. Yet the release of CO2 from the lake and from methane burning would increase atmospheric greenhouse warming significantly, albeit less than if the methane was simply released, for CH4 has 25 times the potential for trapping outgoing heat. Hence the dilemma. Either way, there remains the risk of turning Kivu’s surface water into an anoxic algal ‘broth’ with devastating effects on its fishery potential. Burial of the dead phytoplankton, however, might generate more methane by bacterial decay; a possible source of renewable biofuel that ‘recycles’ the atmospheric CO2 consumed by algal photosynthesis. The geohazards, according to Hirslund and Morkel, are really the ultimate driver for development of Lake Kivu’s fossil fuel potential, now that they are better understood as a real and present danger to millions of people. The authors calculate that a catastrophic gas release may be on the cards in the late 21st century. Yet there are other resource issues bound up with the health of the lake’s surface waters. Preserving the layered structure of the lake water to some extent is also important. Until the rates of natural infiltration of volcanic CO2 and biogenic production of methane are known, a minimum rate of gas extraction to make the lake safe is impossible to calculate. Perhaps matching those rates with gas removal should govern future operation. The total methane content of Lake Kivu is just 1.5 times the annual production from the UK sector of the North Sea. It is sufficient for power generation at 300 MW, at most, for 50 years, which would roughly double Rwanda’s current installed generation capacity – mainly from hydropower. Although Kivu is shared equally between Rwanda and the DRC even half of the short term power potential would be a significant benefit to Rwanda’s ~11 million people, though considerably less to the ~81 million living in the DRC; if access was equitable.

Nickel, life and the end-Permian extinction

The greatest mass extinction of the Phanerozoic closed the Palaeozoic Era at the end of the Permian, with the loss of perhaps as much as 90% of eukaryote diversity on land and at sea. It was also over very quickly by geological standards, taking a mere 20 thousand years from about 252.18 Ma ago. There is no plausible evidence for an extraterrestrial cause, unlike that for the mass extinction that closed the Mesozoic Era and the age of dinosaurs. Almost all researchers blame one of the largest-ever magmatic events that spilled out the Siberian Traps either through direct means, such as climate change related to CO2, sulfur oxides or atmospheric ash clouds produced by the flood volcanism or indirectly through combustion of coal in strata beneath the thick basalt pile. So far, no proposal has received universal acclaim. The latest proposal relies on two vital and apparently related geochemical observations in rocks around the age of the extinctions (Rothman, D.H. et al. 2014. Methanogenic burst in the end-Permian carbon cycle. Proceedings of the National Academy of the United States, v. 111, p. 5462-5467).

Siberian flood-basalt flows in Putorana, Taymyr Peninsula. (Credit: Paul Wignall; Nature http://www.nature.com/nature/journal/v477/n7364/fig_tab/477285a_F1.html)
Siberian flood-basalt flows in Putorana, Taymyr Peninsula. (Credit: Paul Wignall; Nature http://www.nature.com/nature/journal/v477/n7364/fig_tab/477285a_F1.html)

In the run-up to the extinction carbon isotopes in marine Permian sediments from Meishan, China suggest a runaway growth in the amount of inorganic carbon (in carbonate) in the oceans. The C-isotope record from Meishan shows episodes of sudden major change (over ~20 ka) in both the inorganic and organic carbon parts of the oceanic carbon cycle. The timing of both ‘excursions’ from the long-term trend immediately follows a ‘spike’ in the concentration of the element nickel in the Meishan sediments. The Ni almost certainly was contributed by the massive outflow of basalt lavas in Siberia. So, what is the connection?

Some modern members of the prokaryote Archaea that decompose organic matter to produce methane have a metabolism that depends on Ni, one genus being Methanosarcina that converts acetate to methane by a process known as acetoclastic methanogenesis. Methanosarcina acquired this highly efficient metabolic pathway probably though a sideways gene transfer from Bacteria of the class Clostridia; a process now acknowledged as playing a major role in the evolution of many aspects of prokaryote biology, including resistance to drugs among pathogens. Molecular-clock studies of the Methanosarcina genome are consistent with this Archaea appearing at about the time of the Late Permian. A burst of nickel ‘fertilisation’ of the oceans may have resulted in huge production of atmospheric methane. Being a greenhouse gas much more powerful than CO2, methane in such volumes would very rapidly have led to global warming. Before the Siberian Traps began to be erupted nickel would only have been sufficiently abundant to support this kind of methanogen around ocean-floor hydrothermal springs. Spread globally by eruption plumes, nickel throughout the oceans would have allowed Methanosarcina or its like to thrive everywhere with disastrous consequences. Other geochemical processes, such as the oxidation of methane in seawater, would have spread the influence of the biosphere-lithosphere ‘conspiracy’. Methane oxidation would have removed oxygen from the oceans to create anoxia that, in turn, would have encouraged other microorganisms that reduce sulfate ions to sulfide and thereby produce toxic hydrogen sulfide. That gas once in the atmosphere would have parlayed an oceanic ‘kill mechanism’’ into one fatal for land animals.

There is one aspect that puzzles me: the Siberian Traps probably involved many huge lava outpourings every 10 to 100 ka while the magma lasted, as did all other flood basalt events. Why then is the nickel from only such eruption preserved in the Meishan sediments, and if others are known from marine sediments is there evidence for other such methanogen ‘blooms’ in the oceans?

Short fuse on clathrate bomb?

Structure of a gas hydrate (methane clathrate)...
Gas hydrate (methane clathrate) block embedded in seabed sediment (Photo credit: Wikipedia)

The biggest tsunami to affect inhabitants of Britain, mentioned in the earlier post Landslides and multiple dangers, emanated from the Storegga Slide in the northern North Sea west of Norway. That submarine debris flow was probably launched by gas hydrates beneath the sea bed breaking down to release methane thereby destabilising soft sediments on the continental slope. Similar slides were implicated in breaking Europe-America communications in the 20th century, such as the Grand Banks Slide of 1929 that severed submarine cables up to 600 km from the source of the slide. Even now, much Internet traffic is carried across oceans along optic-fibre cables, breakages disrupting and slowing services. A more mysterious facet of clathrate breakdown is its possible implication in unexplained and sudden losses of ships. When gas escapes to the surface, the net density of seawater decreases, the more so as the proportion of bubbles increases. Ship design and cargo loading rests on an assumed water density range from fresh to salt water and for different temperatures at high and low latitudes.

Gulf stream map
Gulf stream map (credit: Wikipedia)

The Atlantic seaboard of the USA hosts some of the best-studied accumulations of clathrates in the top 100-300 m of seabed sediments. Since their discovery these ‘cage complexes’ of mainly methane and carbon dioxide trapped within molecules of water ice have been studied in detail. Importantly, the temperatures at which they form and the range over which they remain stable depend on pressure and therefore depth below the sea surface. At atmospheric pressure solid methane hydrate is unstable at any likely temperature and requires -20°C to form at a pressure equivalent to 200 m water depth. Yet is stable at temperatures up to 10°C 500 m down and 20°C at a depth of 2 km. Modern sea water cools to around 0°C at depths greater than 1.5 km, so gas hydrates can form virtually anywhere that there is a source of methane or CO2 in seafloor sediment. In the sediments temperature increases sharply with depth beneath the seabed due to geothermal heat flow thereby limiting the clathrate stability zone to the top few hundred metres.

Two factors may lead to clathrate instability: falling sea level and sea-floor pressure or rising sea-floor temperature. Many gas-hydrate deposits, especially on the continental shelf and continental edge are likely to be close to their stability limits, hence the worries about destabilisation should global warming penetrate through the water column. The western North Atlantic is an area of especial concern because the Gulf Stream flows northward from the Caribbean to pass close to the US seaboard off the Carolinas: that massive flow of tropical warm water has been increasing during the last 5 thousand years so that its thermal effects are shifting westwards.

Geophysicists Benjamin Phrampus and Matthew Hornbach of the Southern Methodist University in Dallas, Texas have used thermal modelling to predict that gas-hydrate instability is imminent across 10 thousand square kilometres of the Caroline Rise (Phrampus, B.J. & Hornbach, M.J. 2012. Recent changes to the Gulf Stream causing widespread gas hydrate destabilization. Nature, v. 490, p. 527-530). As a test they analysed two seismic reflection profiles across the Carolina Rise, seeking anomalies known as bottom-simulating reflectors that signify free gas in the sediments. These are expected at the base of the gas-hydrate zone and their presence helps assess sediment temperature. At depths less than 1 km the base of the gas-hydrates modelled from the present temperature profile through the overlying seawater lies significantly above the base’s signature on seismic lines. The deeper levels probably formed under cooler conditions than now – probably eight degrees cooler – and may be unstable. If that is correct, the Caroline Rise area seems set to release around 2.5 Gt of methane to add to atmospheric greenhouse warming. The Storegga Slide also lies close to the northern track of the Gulf-Stream – North Atlantic Drift…