Category Archives: Economic and applied geology

A new explanation for banded iron formations (BIFs)

The main source for iron and steel has for more than half a century been Precambrian rock characterised by intricate interlayering of silica- and iron oxide-rich sediments known as banded iron formations or BIFs. They always appear in what were shallow-water parts of Precambrian sedimentary basins. Although much the same kind of material turns up in sequences from 3.8 to 0.6 Ga, by far the largest accumulations date from 2.6 to 1.8 Ga, epitomised by the vast BIFs of the Palaeoproterozoic Hamersley Basin in Western Australia. This peak of iron-ore deposition brackets the time (~2.4 Ga) when world-wide evidence suggests that the Earth’s atmosphere first acquired tangible amounts of free oxygen: the so-called ‘Great Oxidation Event’. Yet the preservation of such enormous amounts of oxidised iron compounds in BIFs is paradoxical for two reasons: the amount of freely available atmospheric oxygen at their acme was far lower than today; had the oceans contained much oxygen, dissolved ions of reduced Fe-2 would not have been able to pervade seawater as they had to for BIFs to have accumulated in shallow water. Iron-rich ocean water demands that its chemical state was highly reducing.

Oblique view of an open pit mine in banded iron formation at Mount Tom Price, Hamersley region Western Australia (Credit Google earth)

Oblique view of an open pit mine in banded iron formation at Mount Tom Price, Hamersley region Western Australia (Credit Google earth)

The paradox of highly oxidised sediments being deposited when oceans were highly reduced was resolved, or seemed to have been, in the late 20th century. It involved a hypothesis that reduced, Fe-rich water entered shallow, restricted basins where photosynthetic organisms – probably cyanobacteria – produced localised enrichments in dissolved oxygen so that the iron precipitated to form BIFs. Later work revealed oddities that seemed to suggest some direct role for the organisms themselves, a contradictory role for the co-dominant silica-rich cherty layers and even that another kind of bacteria that does not produce oxygen directly may have deposited oxidised iron minerals. Much of the research focussed on the Hamersley BIF deposits, and it comes as no surprise that another twist in the BIF saga has recently emerged from the same, enormous repository of evidence (Rasmussen, B. et al. 2015. Precipitation of iron silicate nanoparticles in early Precambrian oceans marks Earth’s first iron age. Geology, v. 43, p. 303-306).

The cherty laminations have received a great deal less attention than the iron oxides. It turns out that they are heaving with minute particles of iron silicate. These are mainly the minerals stilpnomelane [K(Fe,Mg)8(Si, Al)12(O, OH)27] and greenalite [(Fe)2–3Si2O5(OH)4] that account for up to 10% of the chert. They suggest that ferruginous, silica-enriched seawater continually precipitated a mixture of iron silicate and silica, with cyclical increases in the amount of iron-silicate. Being such a tiny size the nanoparticles would have had a very high surface area relative to their mass and would therefore have been highly reactive. The authors suggest that the present mineralogy of BIFs, which includes iron carbonates and, in some cases, sulfides as well as oxides may have resulted from post-depositional mineral reactions. Much the same features occur in 3.46 Ga Archaean BIFs at Marble Bar in Western Australia that are almost a billion years older that the Hamersley deposits, suggesting that a direct biological role in BIF formation may not have been necessary.

More on BIFs and the Great Oxidation Event

Bicentenary of the first national geological map

It’s good to know that the geosciences have had revolutionising developments to match those of the rest of science. Forget the Battle of Waterloo in 1815, which of course was ‘the nearest-run thing you ever saw in your life’ when the Brits were saved from defeat by the timely arrival of the Prussians: This year we can celebrate one that literally put geology on the map, kicked-off the systematic exploration for every kind of physical resource, thereby putting a great deal of money in the pockets of coal, petroleum and metal moguls and making geology a career rather than a pastime. In 1815 William Smith published A Delineation of the Strata of England and Wales with part of Scotland, which despite the title was a map showing the basic geology and structure of the whole of England and Wales: the first ever map showing accurately the distribution of rocks for an entire country. The original, at 2.6 by 1.8 m, dominates the main staircase at Burlington House, the home of the Geological Society of London.

William Smith's A Delineation of the Strata of England and Wales with part of Scotland (1815)

William Smith’s A Delineation of the Strata of England and Wales with part of Scotland (1815)

Tom Sharpe has nicely summarized the key facts surrounding Smith’s masterpiece (Sharpe, T. 2015. The birth of the geological map. Science, v. 347, p. 230-232). One feature that I certainly did not know was that the colour scheme for the different stratigraphic units was based on the dominant colour of the rocks themselves, such as purples for the abundant slates of the Lower Palaeozoic, brown and red for the Old- and New Red Sandstone, greys and blacks for the Coal Measures and green for the Greensand, which until quite recently remained widely used to signify Cambrian, Ordovician and Silurian; Devonian and Permian; Upper Carboniferous and Cretaceous.

Although celebrated today, Smith’s map was panned by the gentlemen geologists of the Geol Soc, who attempted to do a better job, but failed ignominiously. William Smith was not a leisured chap of the Enlightenment, but worked for a living surveying coal mines, navigating canals and draining fens. Despite their antipathy, the Fellows of the Geological Society of London knew a good earner when they saw one and plagiarized Smith’s work and undercut his regular price for his map. As a result he ended up in a London debtors’ prison. Even on the day of his release in 1819, bailiffs seized his house and its contents. The Geol Soc eventually did honour Smith with its Wollaston Medal in 1831, its then president Adam Sedgwick dubbing him ‘the Father of English Geology’: by that time geology had become a profession…

Serious groundwater depletion in western US

The 2300 km long Colorado River whose catchment covers most of Arizona and parts of the states of Colorado, California, Nevada, Utah, New Mexico and Wyoming is one of the world’s most harvested surface water resources. So much so that barely a trickle now ends up in Baja California where the huge river once flowed into the sea. The lower reaches of the river system cross arid lands and it is the water source for several major cities and areas of intensive agriculture, serving as many as 40 million people and 16 thousand km2 of irrigated fields. It has been nicknamed the US Nile because of its economic importance, but Egypt’s Nile has far less pressure put on it, although its exit flow to the Mediterranean is also hugely reduced from its former peak volume. The water crisis affecting the Colorado River and the areas that it serves has peaked during the 14-year drought over its lower reaches. To ease conditions in the former wet lands of Mexico near the river’s outlet 2014 saw deliberate major releases from giant reservoirs higher in the Colorado’s course.

English: New map of the Colorado River watersh...

The Colorado River Basin (credit: Wikipedia)

Surface abstraction is not the only drain on water resources of the Colorado River basin: groundwater pumping from the sediments beneath has grown enormously for both irrigation and urban use. That it is possible to play golf at many courses in the desert and to see monstrous musical fountains in Las Vegas is down largely to groundwater exploitation. There have been concerns about depletion of underground reserves once abstraction outpaced natural recharge by infiltration of rainfall and snow melt, but highlighting the magnitude of the problem required a rather dramatic discovery: so much water has been lost from aquifers that the missing mass has reduced the Earth’s gravitational field over the south-west US (Castle, S.L. et al. 2014. Groundwater depletion during drought threatens future water security of the Colorado River Basin. Geophysical Research Letters, doi: 10.1002/2014GL061055).

Global Gravity Anomaly Animation over land fro...

Global Gravity Anomaly Animation over land from GRACE (credit: Wikipedia)

The evidence comes from the Gravity Recovery and Climate Experiment (GRACE), jointly funded by NASA and Germany’s DLR and launched in March 2002. GRACE uses two satellites that follow the same orbit with a spacing of 220 km between them.  Range finders on each measure their separation distance, and so their ups and downs as gravity varies, with far greater accuracy than any other method.  Measuring the Earth’s entire gravitational field at their orbital height takes about a month. Groundwater depletion beneath the Gangetic Plains of northern India, to the tune of 109 km3, was detected in 2009  and the same approach has been applied to the Colorado Basin for nine years between 2004 and 2013. It shows that during this part of one of the longest droughts in the history of the south-west US 50 km3 have been lost from beneath, as a rate of about 5.5 km3 per year. Though the total is half the loss from beneath northern India, it should be remembered that more than ten times as many people depend on the Ganges Basin. Moreover, there is no monsoon recharge in the south-western states.

Fracking in the UK; will it happen?

Whether or not one has read the Tractatus Logico-Philosophicus of Ludwig Wittgenstein, there can be little doubt that one of his most famous quotations can be applied to much of the furore over hydraulic fracturing (fracking) of hydrocarbon-rich shale in south-eastern Britain: ‘Whereof one cannot speak, one must remain silent’ (more pithily expressed by Mark Twain as ‘Better to remain silent and be thought a fool than to speak and remove all doubt’). A press release by the British Geological Survey  in late May 2014 caused egg to appear on the shirts of both erstwhile ‘frackmeister’ David Cameron (British Prime Minister) and anti-fracking protestors in Sussex. While there are oil shales beneath the Weald, these Jurassic rocks have never reached temperatures sufficient to generate any significant gas reserves (see: Upfront, New Scientist, 31 May 2014 issue, p. 6). Yet BGS estimate the oil shales to contain a total of 4.4 billion barrels of oil. That might sound a lot, but the experience of shale fracking companies in the US is that, at best, only about 5% can be recovered and, in cases that are geologically similar to the Weald, as little as 1% might be expected. Between 44 and 220 million barrels is between two and six months’ worth of British oil consumption; and that is only if the entire Wealden shales are fracked.

Areas where petroleum-rich shales occur at the surface in Britain. (credit: British Geological Survey)

Areas where petroleum-rich shales occur at the surface in Britain. (credit: British Geological Survey)

Why would any commercial exploration company, such as Cuadrilla, go to the trouble of drilling wells, even of an ‘exploratory nature’, for such meager potential returns? Well, when there is sufficient hype, and the British Government has gushed in this context for a few years, bigger fish tend to bite and cash flows improve. For instance, Centrica the owner of British Gas forked out $160 million to Cuadrilla in June 2013 for a quarter share in the well-publicised licence area near Blackpool in Lancashire; the grub stake to allow Cuadrilla to continue exploration in exchange for 25% of any profit should commercial quantities of shale-gas be produced.

Sedimentary rock sequences further north in Britain whose geological evolution buried oil shales more deeply are potential gas producers through fracking; an example is the Carboniferous Bowland Shale beneath the Elswick gasfield in west Lancashire, targeted by Cuadrilla. Far greater potential may be present in a large tract of the Pennine hills and lowlands that flank them where the Bowland Shale occurs at depth.

Few people realize just how much detail is known about what lies beneath their homes apart from maps of surface geology. That is partly thanks to BGS being the world’s oldest geological survey (founded in 1835) and partly the sheer number of non-survey geologists who have prowled over Britain for 200 years or more and published their findings. Legally, any excavation, be it an underground mine, a borehole or even the footings for a building, must be reported to BGS along with whatever geological information came to light as a result. The sheer rarity of outcropping rock in Britain is obvious to everyone: a legacy of repeated glaciation that left a veneer of jumbled debris over much of the land below 500m that lies north of the northern outskirts of the London megalopolis. Only highland areas where glacial erosion shifted mullock to lower terrains have more than about 5% of the surface occupied by bare rock. Of all the data lodged with BGS by far the most important for rock type and structure at depth are surveys that used seismic waves generated by vibrating plates deployed on specialized trucks. These and the cables that connected hundreds of detectors were seen along major and minor roads in many parts of Britain during the 1980s during several rounds of licenced onshore exploration for conventional petroleum resources. That the strange vehicles carried signs saying Highway Maintenance lulled most people apart from professional geologists as regards their actual purpose. Over 75 thousand kilometers of seismic sections that penetrated thousands of metres into the Earth now reside in the UK Onshore Geophysical Library (an Interactive Map at UKOGL allows you to see details of these surveys, current areas licenced for exploration and the locations of various petroleum wells).

Seismic survey lines in northern England (green lines) from the interactive map at the UK Onshore Geophysical Library

Seismic survey lines in northern England (green lines) from the interactive map at the UK Onshore Geophysical Library

Such is the detail of geological knowledge that estimates of any oil and gas, conventional or otherwise, residing beneath many areas of Britain are a lot more reliable than in other parts of the world which do not already have or once had a vibrant petroleum industry. So you can take it that when the BGS says there is such and such a potential for oil or gas beneath this or that stretch of rural Britain they are pretty close to the truth. Yet it is their raw estimates that are most often publicized; that is, the total possible volumes. Any caveats are often ignored in the publicity and hype that follows such an announcement. BGS recently announced that as much as 38 trillion cubic metres of gas may reside in British shales, much in the north of England. There followed a frenzy of optimism from Government sources that this 40 years’ worth of shale gas would remove at a stroke Britain’s exposure to the world market of natural gas, currently dominated by Russia, and herald a rosy economic future to follow the present austerity similar to the successes of shale-gas in North America. Equally, there has been fear of all kinds of catastrophe from fracking on our ‘tight little island’ especially amongst those lucky enough not to live in urban wastelands. What was ignored by both tendencies was reality. In the US, fracking experience shows that only 10% at most of the gas in a fractured shale can be got out; even the mighty Marcellus Shale of the NE US underlying an area as big as Britain can only supply 6 years of total US gas demand. Britain’s entire shale-gas endowment would serve only 4 years of British gas demand.

To tap just the gas in the upper part of the Bowland basin would require 33 thousand fracking wells in northern Britain. Between 1902 and 2013 only 19 onshore petroleum wells were drilled here in an average year. To make any significant contribution to British energy markets would require 100 per annum at a minimum. Yet, in the US, the flow rate from fracked wells drops to a mere zephyr within 3 years. Fracking on a large scale may well never happen in Britain, such are the largely unstated caveats. But the current hype is fruitful for speculation that it will, and that can make a lot of cash sucked in by the prospect – without any production whatsoever.

Enhanced by Zemanta

Fresh offshore groundwater resources

There are paradoxes with groundwater: while over-use of coastal aquifers may draw in seawater to become undrinkable, on reef islands with no surface water adequate supplies may be had from fresh groundwater ‘floating’ on deeper, denser salt water. Seemingly even more odd, there are places several kilometres off some coastlines where freshwater rises in large volumes to the surface from springs on the sea floor.

Despite this and the fact that onshore aquifers extend far out to sea on continental shelves, hydrogeologists have paid scant attention to the potential water supplies that they might offer. Indeed, around the Persian Gulf where many submarine fresh springs are known petrodollars have poured into desalination rather than cheaper drilling and pipelines to the aquifers feeding the springs.

Reviewing the known potential of offshore groundwater, which occurs seawards of most continental shores, Vincent Post of Flinders University, Australia and colleagues from Holland, the US and Britain, consider that the global potential might be as high as half a million cubic kilometres (Post, V.E.A. et al. 2013. Offshore fresh groundwater reserves as a global phenomenon. Nature , v. 504, p. 71-78), around one tenth that of shallow (<750 m deep) groundwater onshore . It should be noted that the maximum safe level of salts dissolved in drinking water is about 1 gram per litre, and double that for irrigation water. The best prospects are where aquifers are trapped beneath impermeable sedimentary layers that prevent downward contamination by salt water.

The key to explaining such huge reserves is dating the water. In those places where that has been done the water is older than the Holocene (i.e. > 11 ka), which suggests infiltration when sea level was as much as 130 m lower than in interglacial periods, due to storage of evaporated seawater in major ice sheets. That would have exposed vast areas of what is now the sea floor to recharge. Modelling downward diffusion of seawater as sea level rose suggests that interglacials have too short to fully flush fresh water from the now submarine aquifers. Nevertheless, recharge is not continual, so that exploiting the resource is akin to ‘mining’ water. Yet the potential may prove essential in some coastal regions, and the authors caution against contamination by human activities offshore, such as exploration drilling for petroleum and carbon dioxide sequestration.

The review points out that submarine hydrogeology is one of the last great challenges in analysis of sedimentary basins.

Electricity from ‘black smokers’

English: Black smoker at a mid-ocean ridge hyd...

Hydrothermal vent at the mid-Atlantic Ridge (credit: Wikipedia)

Occasionally, journals not usually associated with mainstream geosciences publish something startling, but easily missed. Nature of 12 September 2013 alerted me to just such an oddity. It seems that the chemistry of sea-floor hydrothermal vents potentially can generate electrical power (Yamamoto, M. et al. 2013. Generation of electricity and illumination by an environmental fuel cell in deep-sea hydrothermal vents. Angewandte Chemie, online DOI: 10.1002/ange.201302704).

The team from the Japan Agency for Marine-Earth Science and Technology, the Riken Centre for Sustainable Resource Science and the University of Tokyo used a submersible ROV to suspend a fuel cell based on a platinum cathode and iridium anode in hydrothermal vents that emerge from the Okinawa Trough off southern Japan at a depth of over 1 km. It recorded a tiny, but significant power generation of a few milliwatts.

The fluids issuing from the vents are at over 300°C while seawater is around 4°C, creating a very high thermal gradient. More importantly, the fluid-seawater interface is also a boundary between geochemically very different conditions. The fluids are highly acidic (pH 4.8) compared with the slight alkalinity of seawater, and contain high concentrations of hydrogen and hydrogen sulfide but undetectable oxygen (sea water is slightly oxygenated).

The fuel cell was designed so that iridium in the anode speeds up the oxidation of H2S at the geochemical interface which yields the electrons necessary in electrical currents. The experiment neatly signified its success by lighting up three light-emitting diodes.

Does this herald entirely new means of renewable power generation? Perhaps, if the fuel cell is scaled-up enormously. Yet, the very basis of oxidation and reduction is expressed by the mnemonic OILRIG (Oxidation Is Loss Reduction Is Gain – of electrons) and any potential redox reaction in nature has potential, even plants can be electricity producers. In fact all fuel cells exploit oxidation reactions of one kind or another.

Review of fracking issues

The release and exploitation of natural gas from shales using the unconventional means of in situ hydraulic fracturing – ‘fracking’ – has had plenty of bad press, including some hammering in Earth Pages. Now, what seems to be a balanced academic review has appeared on-line in Science magazine (Vidic, R.D. et al. 2013. Impact of shale gas development on regional water quality. Science, v. 340, DOI: 10.1126/science.1235009). The review focuses on hazards to groundwater resources from a variety of environmental effects, primarily gas migration, contaminant transport through induced and natural fractures, wastewater discharge, and accidental spills.

English: Protests against shale gas drilling i...

Protests against shale gas drilling in Bulgaria (credit: Wikipedia)

Much attention has centred on faulty seals put in place to stop gas escaping from drill targets. Yet fewer than 3% of seals are said to have proved problematic, with some finger-pointing at natural gas leakage from the hydrocarbon-rich shales. After all, there are plenty of natural fractures and completely ‘tight’ stratigraphic sequences are rare. in fact toxic effects of natural gas leakage on surface vegetation have been widely used as exploration indicators for conventional petroleum. The review does point out that there are so few pre-drilling studies of natural leakage that this controversy – including widely publicised blazing household water supplies – can not yet be resolved. Obviously more independent monitoring of areas above prospective shales are essential; but who will fund them? The one well-documented before-and-after study, from 48 water wells in Pennsylvania, USA, showed no change, though it seems that monitoring after fracking was short-lived.

The chemically-charged water used to induce the hydrofracturing obviously leaves an unmistakable mark when leaks occur, and there have been cases of considerable environmental release. The fluids are indeed a wicked brew of acids, organic thickeners, biocides, alkalis and inorganic surfactants, to name but a few infredients. To some extent re-use of such fluids, which are costly, ought to mitigate risks. However, once a shale-gas field is fully developed, large volumes of the fracking fluids remain in the subsurface and may leak into shallow groundwater sources. But what pathways do these fluids follow when they are pumped into shales under very high pressure? The review warns of the lesson of toxic fluid leakage from underground coal mines.

The University of Pittsburgh team who compiled the review usefully outline why shale gas is both profitable and feasible. They deal with what methane does in an environmental chemistry sense. It isn’t a solvent, so carries no other materials such as toxic ions, but its interaction with bacteria creates reducing conditions. A now well-known hazard of subsurface reduction is dissolution of iron hydroxide, naturally an important component of many rocks, that can adsorb a great range of dangerous ions at potentially high concentrations, including those involving arsenic. Reductive dissolution lets such ions loose into natural waters, even at shallow depths. Yet methane is emitted by a host of sources other than hydrocarbon-rich shale: landfill; swamps; other bacterial action; conventional petroleum fields both active and abandoned; and even deep water boreholes themselves. A recent study of groundwater geochemistry in relation to fracking in Arkansas, USA (Warner, N.R. et al. 2013. Geochemical and isotopic variations in shallow groundwater in areas of the Fayetteville shale development, north-central Arkansas. Applied Geochemistry, v. 33, doi/10.1016/j.apgeochem.2013.04.013) does address changes in groundwater chemistry, but not for all the ions cited by the WHO as potential hazards.

Whereas the mechanisms involved in vertical and lateral migration of subsurface fluids are well understood there is little knowledge of natural structural features such as deep jointing, fractures and fault fragmentation that control actual migration from area to area. The use of natural seepage as an exploration guide was largely abandoned when many studies showing apparently high-priority targets proved to be far removed from the actual source of the moving fluids. The most easily investigated route for leakage is the actual ‘plumbing’ that fracking uses. This is held together by cement that high pressures can disrupt before it sets, resulting in leaks. A lot depends on ‘due diligence’ deployed by the contractors, whose regulation can leave a lot to be desired. Vidic and colleagues devote most space to the matter of wastewater and deep formation water, yet make little if any case for routine geochemical monitoring of domestic groundwater supplies in shale-gas fields. Much is directed at the industry itself rather than independent surveys.

Resource snippets

Wasted natural gas

Much attention has centred on fracking shales to release otherwise locked-in gas, while production of liquid petroleum by the same kind of process is also increasing with little publicity, especially in the US. From a purely economic standpoint wells that yield oil and gas from fractured shale might seem to be quite a boon. Well, they probably are, if the gas can be sold. One of the biggest shale-oil targets is the Late Devonian to Early Carboniferous Bakken Shale in the Williston Basin that stretches across 360 thousand km2‑ beneath parts of the Dakotas, Wyoming and Montana in the US and Saskatchewan in Canada. This shale is the source rock for most of the conventional oil production from the Williston basin since the 1940s. At the start of the 21st century direct production of oil from the Bakken began in North Dakota, unleashing a major drilling boom and a ten-fold increase in land-leases for production. The state is now the second largest US oil producer after Alaska warranting a major feature National Geographic. Trouble is North Dakota is not well served by pipelines of any kind and oil is shipped by rail, much as it was in the early days of the US oil industry.

Flame at PTT (ปตท.) (Map Ta Phut, Rayong, Thai...

Typical natural gas flare with black-carbon plume (credit: Wikipedia)

The natural gas released by fracking is simply wasted, partly by flaring at the wellhead but an unknown volume of pure methane is simply vented to the atmosphere. At rough 25 times the greenhouse warming capacity of CO2 the perverted economics of waste methane is, unsurprisingly, becoming scandalous and increasingly dangerous. Such is the magnitude of shale-gas production in the US the price of natural gas has fallen dramatically so that from the Williston Basin simply carries no profit and therefore has nowhere to go except up in flames or directly to the air. The US Environmental Protection Agency apparently can do little to halt the venting. British onshore source rocks, such as the Upper Jurassic Kimmeridge Shale,  which has a hydrocarbon content up to 70% and is regarded as the most important rock in Europe being the source for much of the petroleum beneath the North Sea and other oil provinces, are likely targets for fracking now the UK government has given the go-ahead in a new ‘dash for gas’. Chances are it may become a dash for onshore shale-oil .

Manganese nodules finally tagged for production

Manganese nodules taken from the bottom of the...

Manganese nodules taken from the bottom of the Pacific. (credit: Wikipedia)

Almost 40 years ago my desk was almost buried under tomes of information about dull black nodules looking like blighted potatoes as I worked on the now abandoned Level-2 Open University course on The Earth’s Physical Resources. Made mainly out of manganese and iron minerals they also contain ore-grade amounts of nickel, copper and cobalt together with other metals. Were they beneath the crust they would be mined eagerly, but such manganese nodules litter vast areas at the surface of the oceans’ abyssal plains. Such was their potential that around half a billion dollars was spent on oceanographic and geochemical surveys to map the richest nodule fields. Part of the attraction at a time when the non-renewable nature of conventional metal deposits was touted as a threat to civilisation as we know it, as in The Limits to Growth, was that the nodules were zoned and clearly growing: they appear to be renewable metal resources.

Mining them is likely to be hugely costly: they will have to be dredged or sucked-up from the deep ocean basins; intricate metallurgical methods are needed to separate and smelt the paying metals and the risks of deep-sea pollution are obvious. As with shale gas, the UK Tory premier David Cameron has leapt onto Lockheed Martin UK’s announcement that it is finally profitable to get at the nodules, in the manner of the proverbial ‘rat up a drainpipe’. Cameron believes that the venture to harvest one of the most metalliferous patches on the east Pacific floor off Mexico may rake the UK’s economic potatoes out of the fire to the tune of US$60 billion over the next 30 years. Lockheed Martin is an appropriate leader in this scramble having designed some of the equipment aboard a ship financed by Howard Hughes, the 50 thousand tonne Glomar Explorer. A curious vessel, the Glomar Explorer was widely publicised in the mid-70s as the flagship for a manganese nodule pilot project. In fact it was built to snaffle a Soviet submarine (K-129) and its contents of codebooks, technical equipment and nuclear missiles that sank to the abyssal plains in the Pacific about 2500 km to the north-west of Hawaii. It did grapple the submarine, some cryptographic equipment, a couple of nuclear tipped torpedoes and six of the dead crew members. It is still operational, but as an ultra-deep water drill rig.

We will have to wait to see if nodule mining is a ‘go-er’, and very little information has emerged about methodology. The target metal is probably nickel with its importance in rechargeable batteries, plus rare-earth metals that are in notoriously short supply. Whether or not raking, dredging or sucking-up the nodules will have insupportable environmental impact depends on the amount of on-board processing; the nodules themselves are pretty much insoluble. Extracting and separating the metals will probably involve some kind of solution chemistry rather than the beneficiation common in most on-shore metal mines. Such hydrometallurgy has considerable potential for pollution, unless the raw nodules are shipped to shoreline facilities, at a hefty cost. One thing occurred to me while writing about manganese nodules as a major resource was that their blends of metals would not match the proportions actually required in commerce. On a grand scale their exploitation could well play havoc with currently booming metal prices and drive on-shore mining to the wall. But, to be frank, I think this is a bit of tropical sea-bed bubble fraught with legal tangles connected with the United Nations Convention on the Law of the Sea.

Geochemistry and economic history

At first reading this item’s title might seem to convey nonsense, yet there is an interesting relationship between these two very different disciplines. It concerns the pillaging of South and Central America by conquistadors who followed Columbus’s pioneering route across the North Atlantic in 1492. Aside from glory their motive was profit, and that was most conveniently concentrated in the form of gold and silver, to be found in abundance among the native people of what came to be known as the Americas. Once such plunder declined silver ores were soon discovered in Peru and Mexico, thereby maintaining the supply. Bullion or plate – so named from the fact that precious metal was most often transported in the form of sheets – was the major cargo of the great treasure ships in the period from 1515 to 1650. It is remembered in such geographic names as the Rio de la Plata separating modern Argentina and Uruguay.

Werner Herzog and Klaus Kinski shooting "...

Klaus Kinski, well into his role as an insane conquistador, disputes the script with director Werner Herzog while shooting “Aguirre, The Wrath Of God” (credit: Flickr p373)

It might seem that when such a vast amount of loot entered Europe the buying power of silver in particular would have fallen to result in inflation in the price of basic commodities, much as printing paper money may have that result nowadays. Indeed, over those roughly 150 years prices increased by as much as five times. Another factor was a tendency for silver supply to be augmented simply by debasing newly minted currency with other metals. Yet another is that over the same period China adopted silver as a money commodity increasing demand and so spurring exploration and advances in metallurgical extraction from new ores. Furthermore, the entire fabric of economy in Europe began to shift as feudalism began to be supplanted by capitalism at the close of Medieval times. The sheer complexity of competing factors has made the so-called ‘Price Revolution’ of the 16th and 17th centuries a thorny issue for economic historians. This is where geochemists found that they had a ‘shout’ in what Thomas Carlisle dubbed the ‘dismal science’.

Silver ores also contain lead and copper, which inevitably contaminate silver metal extracted from them. Depending on the processes involved in mineralisation the abundances of both metals vary from mine to mine. More tellingly, so do the relative proportions of the different Pb and Cu isotopes, Pb isotopes reflecting the age of the rocks in which ores are found. Inherited by coinage, the isotopes can be used to assess provenance of coins (Desaulty, A.-M. & Albarede, F. 2013. Copper, lead and silver isotopes solve a major economic conundrum of Tudor and early Stuart Europe. Geology, v. 41, p. 135-138), while the dates embossed on coins at the mint potential chart the course of the bullion trade. Desaulty and Albarede show that silver from the vast Potosí mine in modern Bolivia opened by conquistadors barely shows up in British coinage of the period, which is dominated with Mexican isotopic signatures as well as those from European mines. The latter account almost exclusively for the coinage of the late Medieval period. The conclusion is that the huge potential of Potosí served the needs of Spanish entrepreneurs though a trans-Pacific Spanish trade in which Bolivian silver bought goods from China, including gold. Spanish coins, on the other hand, show little of either Bolivian or Mexican silver, suggesting that Spanish world trade may well have used American bullion directly to purchase goods throughout its sphere of influence centred on the Philippines, while Mexican silver engaged in European trade and also found its way into the British economy by way of the slave trade.

Although Desaulty and Albarede claim to have solved a ‘conundrum’ it seems more likely that their revelations will make historians of post-Medieval economics scratch their heads even more.

Global groundwater depth

The single most vital resource for human survival is clean, fresh drinking water. For a large proportion of the world’s population that right is not guaranteed, with harrowing consequences especially for children under 5-years old. Without careful processing surface water can only rarely be assumed fit to drink, especially in areas with dense populations of people, livestock or wildlife. Groundwater, on the other hand, has generally passed through aerated upper soil layers before it ended up below the water table in an aquifer. In that passage it is filtered and subject to various oxidising processes, both chemical and organic, that renders it a great deal more free of pathogens than standing or running surface water. Remarkably, a common mineral in any oxidised soil horizon is goethite, an iron hydroxide, which is capable of adsorbing a variety of potentially damaging ions.  So, of all fresh water that stored beneath the surface is the safest for people to drink.

By its very nature groundwater is hidden and requires both geological exploration and the drilling or digging of wells before it can become a resource. Areas underlain by simple stratiform sediments or lava flows present far less of a challenge than do geological settings with complex structures or that are dominated by ancient crystalline basement rocks. Time and again, however, crises in water supply arise from drought or sudden displacements of  populations a great deal faster than the pace of groundwater exploration or development needed to cope with shortages. Were the potential for subsurface supplies known beforehand relief would be both quicker and more effective than it is at present.

Image of simulated depth to water table for Africa (Courtesy of Y. Fan, Rutgers University, USA)

Image of simulated depth to water table for Africa (Courtesy of Y. Fan, Rutgers University, USA)

Thanks to three geoscientists from Rutgers University, USA and the University of Santiago de Compostela, Spain, (Fan, Y et al. 2013. Global patterns of groundwater table depth. Science, v. 339, p. 940-943) a start has been made in quantifying the availability of groundwater worldwide. They have modelled how the likely depth of the water table may vary beneath the inhabited continents. As a first input they digitised over 1.5 million published records of water table depths. Of course, that left huge gaps, even in economically highly developed areas. There is also bias in hydrogeological data towards shallow depths as most human settlements are above easily accessible groundwater.

To fill in the gaps and assess the deeper reaches of groundwater Fan et al. adapted an existing model that assumes groundwater depth to be forced by climate, topography and ultimately by sea-level. It is based on algorithms that predict groundwater flow after its infiltration from the surface. Such an approach leaves out drawdown by human interference and is at a spatial resolution that removes local complexities. The influence of terrain relies on the near-global elevation data acquired by NASA’s Shuttle Radar Topography Mission (SRTM) in February 2000, resampled to approximately 1 km spatial resolution, supplemented by the less accurate Japan/US ASTER GDEM produced photogrammetrically from stereo- image pairs. Other input data are assumptions about variation in hydraulic conductivity, which is reduced to a steady decrease with depth, models of infiltration from the surface based on global rainfall and evapotranspiration patterns and those of surface drainage and slopes. No attempt was made to input geological information

The results have been adjusted using actual water-table depths as a means of calibration across climate zones on all inhabited continents. The article itself is not accessible without a Science subscription, but the supplementary materials that detail how the work was done are available to the public, and include remarkably detailed maps of simulated water table depths for all continents except Antarctica.  The detail is much influenced by terrain to create textures that override climate, which might suggests that the results flatter to deceive. Yet the modelling does result in valleys and broad basins of unconsolidated sediment showing shallower depths that tallies with the tendency for less infiltration where slopes are steep and run-off faster. The fact that the degree of fit between model and known hydrogeology is high does suggest that at the regional scale the maps are very useful points of departure for more detailed work that brings in lithological and structural information.