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…