Ecological hazards of ocean-floor mining

Spiralling prices for metals on the world market, especially those that are rare and involved in still-evolving technologies, together with depletion of onshore, high-grade reserves are beginning to make the opportunity of mining deep, ocean-floor resources attractive. By early 2018, fifteen companies had begun detailed economic assessment of one of the most remote swathes of the Pacific abyssal plains. In April 2018 (How rich are deep-sea resources?) I outlined the financial attractions and the ecological hazards of such ventures: both are substantial, to say the least. In Japan’s Exclusive Economic Zone (EEZ) off Okinawa the potential economic bonanza has begun, with extraction from deep-water sulfide deposits of zinc equivalent to Japan’s annual demand for that metal, together with copper, gold and lead. One of the most economically attractive areas lies far from EEZs, beneath the East Pacific Ocean between the Clarion and Clipperton transform faults. It is a huge field littered by polymetallic nodules, formerly known as manganese nodules because Mn is the most abundant in them. A recent article spelled out the potential environmental hazards which exploiting the resources of this region might bring (Hefferman, O. 2019. Seabed mining is coming – bringing mineral riches and fears of epic extinctions. Nature, v. 571, p. 465-468; DOI: 10.1038/d41586-019-02242-y).

ocean floor resources
The distribution of potential ocean-floor metal-rich resources (Credit: Hefferman 2019)

Recording of the ecosystem on the 4 km deep floor of the Clarion-Clipperton Zone (CCZ) began in the 1970s. It is extraordinarily diverse for such a seemingly hostile environment. Despite its being dark, cold and with little oxygen, it supports a rich and unique diversity of more than 1000 species of worms, echinoderms, crustaceans, sponges, soft corals and a poorly known but probably huge variety of smaller animals and microbes inhabiting the mud itself. In 1989, marine scientists simulated the effect on the ecosystem of mining by using an 8-metre-wide plough harrow to break up the surface of a small plot. A plume of fine sediment rained down to smother the inhabitants of the plot and most of the 11 km2 surrounding it. Four subsequent visits up to 2015 revealed that recolonisation by its characteristic fauna has been so slow that the area has not recovered from the disturbance after three decades.

The International Seabed Authority (ISA), with reps from 169 maritime member-states, was created in 1994 by the United Nations to encourage and regulate ocean-floor mining; i.e. its function seems to be ‘both poacher and gamekeeper’. In 25 years, the ISA has approved only exploration activities and has yet to agree on an environmental protection code, such is the diversity of diplomatic interests and the lack of ecological data on which to base it. Of the 29 approved exploration licences, 16 are in the CCZ and span about 20% of it, one involving British companies has an area of 55,000 km2. ISA still has no plans to test the impact of the giant harvesting vehicles needed for commercial mining, and its stated intent is to keep only 30% of the CCZ free of mining ‘to protect biodiversity’. The worry among oceanographers and conservationists is that ISA will create a regulatory system without addressing the hazards properly. Commercial and technological planning is well advanced but stalled by the lack of a regulatory system as well as wariness because of the huge start-up costs in an entirely new economic venture.

The obvious concern for marine ecosystems is the extent of disturbance and ecosystem impact, both over time and as regards scale. The main problem lies in the particles that make up ocean-floor sediments, which are dominated by clay-size particles. The size of sedimentary particles considered to be clays ranges between 2.0 and 0.06 μm. According to Stokes Law, a clay particle at the high end of the clay-size range with a diameter of 2 μm  has a settling speed in water of 2 μm s-1. The settling speed for the smallest clays is 1,000 time slower. So, even the largest clay particles injected only 100 m above the ocean floor would take 1.6 years to settle back to the ocean floor – if the water column was absolutely still. But even the 4,000 m deep abyssal plains are not at all stil, because of the ocean-water ‘conveyor belt’ driven by thermohaline circulation. An upward component of this flow would extend the time during which disturbed ocean-floor mud remains in suspension – if that component was a mere >2 μm s-1, even the largest clay particles would remain suspended indefinitely. Deepwater currents, albeit slow, would also disperse the plume of fines over much larger areas than those being mined. Moreover such turbidity pollution is likely to occur at the ocean surface as well, if the mining vessels processed the ore materials by washing nodules free of attached clay. Plumes from shipboard processing would be dispersed much further because of the greater speed of shallow currents. This would impact the upper and middling depths of the oceans that support even more diverse and, in the case of mid-depths poorly known, ecosystems Such plumes may settle only after decades or even centuries, if at all.

Processing on land, obviously, presents the same risk for near-shore waters. It may be said that such pollution could be controlled easily by settling ponds, as used in most conventional mines on land. But the ‘fines’ produced by milling hard ores are mainly silt-sized particles (2.0 to 60 μm) of waste minerals, such as quartz, whose settling speeds are proportional to the square of their diameter; thus a doubling in particle size results in four-times faster settling. The mainly clay-sized fines in deep-ocean ores would settle far more slowly, even in shallow ponds, than the rate at which they are added by ongoing ore processing; chances are, they would eventually be released either accidentally or deliberately

A mining code is expected in 2020, in which operating licences are likely to be for 30 years. Unlike the enforced allowance of environmental restoration once a land-based mining operation is approved, the sheer scale, longevity and mobility of fine-sediment plumes seem unlikely to be resolvable, however strong such environmental-protection clauses are for mining the ocean floor.

Frack me nicely?

‘There’s a seaside place they call Blackpool that’s famous for fresh air and fun’. Well, maybe, not any more. If you, dear weekender couples, lie still after the ‘fun’ the Earth may yet move for you. Not much, I’ll admit, for British fracking regulations permit Cuadrilla, who have a drill rig at nearby Preston New Road on the Fylde coastal plain of NW England, only to trigger earthquakes with a magnitude less than 0.5 on the Richter scale. This condition was applied after early drilling by Cuadrilla had stimulated earthquakes up to magnitude 3. To the glee of anti-fracking groups the magnitude 0.5 limit has been regularly exceeded, thereby thwarting Cuadrilla’s ambitions from time to time. Leaving aside the view of professional geologists that the pickings for fracked shale gas in Britain [June 2014] are meagre, the methods deployed in hydraulic fracturing of gas-prone shales do pose seismic risks. Geology, beneath the Fylde is about as simple as it gets in tectonically tortured Britain. There are no active faults, and no significant dormant ones near the surface that have moved since about 250 Ma ago; most of Britain is riven by major fault lines, some of which are occasionally active, especially in prospective shale-gas basins near the Pennines. When petroleum companies are bent on fracking they use a drilling technology that allows one site to sink several wells that bend with depth to travel almost horizontally through the target shale rock. A water-based fluid containing a mix of polymers and surfactants to make it slick, plus fine sand or ceramic particles, are pumped at very high pressures into the rock. Joints and bedding in the shale are thus forced open and maintained in that condition by the sandy material, so that gas and even light oil can accumulate and flow up the drill stems to the surface. Continue reading “Frack me nicely?”

Gravity signals of earthquakes

A sign that an earthquake is taking place is pretty obvious: the ground moves. Seismometers are now so sensitive that they record significant seismic events at the far side of the world. The Richter magnitude scale commonly used to assign the power of an event is logarithmic, and the difference between each unit represents an approximately 32-fold change in the energy released at the source, so that a magnitude 6.0 earthquake is 32 times more powerful than one rated as magnitude 5.0. Because seismic motion affects a mass of rock it also perturbs the gravitational field. So, theoretically, gravimeters should also be able to detect an earthquake. Seismic waves travel at a maximum speed of about 6 to 8 km s-1 about 20 times the speed of sound, yet changes in the gravitational field propagate at the speed of light, i.e. almost instantaneously by comparison. The first ground disturbances of the magnitude 9.0 Tohoku-Oki earthquake of NE Japan on 11 March 2011 hit Tokyo about 2 minutes after the event began offshore. Although that is a quite short time it would be sufficient for people to react and significantly reduce the earthquake’s direct impact on many of them. A seismic gravity signal would give that warning. The full horror of Tohoku-Oki was unleashed by the resulting tsunami waves, whose speed in the deep ocean water off Japan was about 800 km hr-1 (0.22 km s-1). An almost real-time warning would have allowed 40 times more time for evasion.

Tsunami Punx
Devastation in NE Japan caused by the Tohoku-Oki tsunami in March 2011

Japan is particularly well endowed with advanced geophysical equipment because of its notorious seismic and volcanic hazards. The first data to be analysed after Tohoku-Oki were understandably those from Japan’s large array of seismometers. The records from two super-sensitive gravimeters, between 436 and 515 km from the epicentre, were examined only recently. These instruments measure variations in gravity as small as a trillionth of the average gravitational acceleration of the Earth using a superconducting sphere suspended in a magnetic field, capable of detecting snow being cleared from a roof. Masaya Kimura and colleagues from Tokyo University and other geoscientific institutes in Japan undertook the analyses of both seismic and gravity data (Kimura, M. et al. 2019. Earthquake‑induced prompt gravity signals identified in dense array data in Japan. Earth Planets and Space, v. 71, online publication. DOI: 10.1186/s40623-019-1006-x). The gravimeter record did show a statistically significant perturbation at the actual time of the earthquake, albeit after complex processing of both gravity and seismographic data.

That only 2 superconducting gravimeters detected the event in real-time is quite remarkable, despite the need for a great deal of processing. It amounts to a test of the concept that such instruments or others based on different designs and deployed more widely may eventually be deployed to give prompt warnings of seismic events that could save thousands.

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Volcanism and the Justinian Plague

Between 541 and 543 CE, during the reign of the Roman Emperor Justinian, bubonic plague spread through countries bordering the Mediterranean Sea. This was a decade after Justinian’s forces had had begun to restore the Roman Empire’s lost territory in North Africa, Spain, Italy and the present-day Balkans by expeditions out of Byzantium (the Eastern Empire). At its height, the Plague of Justinian, was killing 5000 people each day in Constantinople, eventually to consume 20 to 40% of its population and between 25 to 50 million people across the empire. Like the European Black Death of the middle 14th century. The bacterium Yersinia pestis originated in Central Asia and is carried in the gut of fleas that live on rats. The ‘traditional’ explanation of both plagues was that plague spread westwards along the Silk Road and then with black rats that infested ship-borne grain cargoes. Plausible as that might seem, Yersinia pestis, fleas and rats have always existed and remain present to this day. Trade along the same routes continued unbroken for more than two millennia. Although plagues with the same agents recurred regularly, only the Plague of Justinian and the Black Death resulted in tens of million deaths over short periods. Some other factor seems likely to have boosted fatalities to such levels.

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Monk administering the last rites to victims of the Plague of Justinian

Five years before plague struck the Byzantine historian Procopius recorded a long period of fog and haze that continually reduced sunlight; typical features of volcanic aerosol veils. Following this was the coldest decade in the past 2300 years, as recorded by tree-ring studies. It coincides with documentary evidence of famine in China, Ireland, the Middle East and Scandinavia.. A 72 m long ice core extracted from the Colle Gnifetti glacier in the Swiss Alps in 2013 records the last two millennia of local climatic change and global atmospheric dust levels. Sampled by laser slicing, the core has yielded a time series of data at a resolution of months or better. In 536 an Icelandic volcano emitted ash and probably sulfur dioxide over 18 months during which summer temperature fell by about 2°C. A second eruption followed in 540 to 541. ‘Volcanic winter’ conditions lasted from 536 to 545, amplifying the evidence from tree-ring data from the 1990’s.

The Plague of Justinian coincided with the second ‘volcanic winter’ after several years of regional famine. This scenario is paralleled by the better documented Great Famine of 1315-17 that ended the two centuries of economic prosperity during the 11th to 13th centuries. The period was marked by extreme levels of crime, disease, mass death, and even cannibalism and infanticide. In a population weakened through malnutrition to an extent that we can barely imagine in modern Europe, any pandemic disease would have resulted in the most affected dying in millions. Another parallel with the Plague of Justinian is that it followed the ending of four centuries of the Medieval Warm Period, during which vast quantities of land were successfully brought under the plough and the European population had tripled. That ended with a succession of major, sulfur-rich volcanic eruption in Indonesia at the end of the 13th century that heralded the Little Ice Age. Although geologists generally concern themselves with the social and economic consequences of a volcano’s lava and ash in its immediate vicinity– the ‘Pompeii view’ – its potential for global catastrophe is far greater in the case of really large (and often remote) events.

Chemical data from the same ice core reveals the broad economic consequences of the mid-sixth century plague. Lead concentrations in the ice, deposited as airborne pollution from smelting of lead sulfide ore to obtain silver bullion, fell and remained at low levels for a century. The recovery of silver production for coinage is marked by a spike in glacial lead concentration in 640; another parallel with the Black Death, which was followed by a collapse in silver production, albeit only for 4 to 5 years.

Related article: Gibbons, A. 2018. Why 536 was ‘the worst year to be alive’. Science, v. 362,p. 733-734; DOI:10.1126/science.aaw0632

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