Parisian summer months of 1783 were heady days in more ways than one, as Benjamin Franklin noted. The first diplomatic representative of the newly liberated United States of America to newly republican France, Franklin was both a scientist and a revolutionary. He was in France as a US delegate for the Treaty of Paris that on 3 September 1783 recorded the British Crown’s acknowledgement that the USA was sovereign and independent. Despite the solemnity of his mission Franklin, as was his habit, observed and recorded all around him.
‘During several of the summer months of the year 1783, when the effects of the Sun’s rays to heat the Earth in these northern regions should have been at its greatest, there existed a constant fog over all Europe, and a great part of North America. This fog was of a permanent nature; it was dry, and the rays of the Sun seemed to have little effect towards dissipating it, as they easily do a moist fog, arising from water. They were indeed rendered so faint in passing through it that when collected in the focus of a burning glass, they would scarce kindle brown paper.’
Records for winter 1783-4 show that temperatures in the northern hemisphere plummeted lower than ever measured before or since. A general cooling of 1°C seems to have taken place in the northern hemisphere.
In Iceland the fog was blue and acrid, and devastated that year’s crops. About 75% of its livestock were poisoned, it now seems, by flurorides bound to fine volcanic ash that covered the grass. As a result famine killed 25% of Iceland’s population. The ground-hugging, blue fog was probably an aerosol of sulfuric acid, formed when sulfur dioxide gas reacted with moisture in the air. Made of tiny fluid droplets, too small to fall out of turbulent air on their own and able to scatter light in all directions, such aerosols are highly reflective. They reduce the input of solar energy. That year one of Iceland’s volcanoes, Laki, erupted 14 km3 of lava along a 27 km chain of small craters following the line of a fissure, now known to lie close to the onshore extension of the mid-Atlantic Ridge. Fluid basalt steadily filled valleys, and pumped its volatile contents to the atmosphere. Because the lava and gas issued quietly by comparison with the more familiar cone-shaped volcanoes above subduction zones, the natural pollution entered only the lower atmosphere. Eventually the acid fell in rain to halt its effects.
Mount Pinatubo in the Phillipines is such a cone-shaped volcano. June 1991 saw it produce the second largest eruption of the 20th century, a spectacle of terrifying violence but one involving less arupted material than at Laki. Gas and fine ash jetted high into the upper atmosphere. The stratosphere produces no rain and water vapour is at a premium there. Sulfur dioxide consumes it in producing aerosols of sulfuric acid. As well as generally scattering radiation to reduce incoming solar energy, aerosols can affect some wavelengths more than others, depending on how tiny their droplets are. Fine dust gives much the same effect. Aerosols and dust trapped in the stratosphere scatter the red part of white light, and large violent eruptions, such as Pinatubo usually result in spectacular red sunsets. Sunset in the northern hemisphere was a notable affair throughout late 1991. It is worth noting that molecules of water vapour and of other atmospheric gases scatter light too, but they are so small that the effect shifts to the blue end of the spectrum. That is why clear skies and distant mountains are blue.
Weather satellites provided platforms for monitoring Pinatubo’s full effects. By late July stratospheric aerosols spread to high latitudes in both hemispheres. About 3% of normal solar warming failed to reach the tropics. Globally, the mean surface temperature for 1992 fell by about half a degree from average levels. Not so much as in Laki’s case, but instructive information nonetheless. Volcanic jetting of gas and dust to the stratosphere can trigger global cooling, but averaging conceals larger, more local effects. Laki was followed by the two coldest winters recorded in North America, but London, despite having the acid fog, experienced an exceptionally hot summer in 1783. The ground-hugging emissions there probably trapped long-wave radiation. Other volcanic events for which climate records exist produced perturbations rather than any clear downward trend – perhaps cool summers yet exceptionally warm winters, and so on.
Calculation of atmospheric effects of these kinds from dust thrown up by air-burst nuclear weapons of different magnitudes coined the idea of ‘nuclear winters’. Scattering and reflection of solar radiation by volcanic aerosols and dust make explosive volcanism an obvious candidate for decisive climate change, for which researchers have combed the distant past of volcanoes and temperature records in marine sediments and cores through polar ice caps. The Indonesia island of Sumatra possesses the largest surviving volcanic crater, Toba, which is 100 km long. Toba’s eruption 75 thousand years dwarfed any volcanic event that has occurred since. It blasted out 2500 km3 of lava and 3 billion tonnes of sulfur dioxide, enough to have consumed all water in the stratosphere in volcanic aerosols. Sure enough, cores through the Greenlandic ice cap record Toba’s ash and a drop in local temperature. But the cooling was subsumed in an already developing downward trend in global temperature en route to the last glacial maximum. The climate-volcanic aerosol connection, if it appears at all in palaeoclimatic records, is one of short-lived blips. The many temperature downswings since the first stone tools were discarded by early humans more than 2.5 Ma ago – there have been about 50 large drops – fail to link at all with volcanism. Toba gives the clue as to why fogging of the stratosphere has an upper limit. Once sulfuric acid formation has consumed the water there sulfur dioxide builds up. Although far less effective than carbon dioxide, it too has a ‘greenhouse’ effect. Slow as it is, exchange of air between the stratosphere and the lower atmosphere mixes natural pollutants in periods of years. Once below the tropopause, SO2 is quickly extracted by rainfall.
Choking, deadly and climatically active as it is, sulfur dioxide is but one of several gases emitted from the mantle by volcanoes. In total volcanic gases amount to only around 1% of the mass erupted, the bulk being silicate-rich magma. Water dominates, but it adds very little to the normal load generated continuously by evaporation from the ocean surface. There are traces of nitrogen, hydrochloric and hydrofluoric acids too. Nitrogen oxides are partially rained out as nitric acid, the remainder being extracted by nitrogen-fixing bacteria living symbiotically in plant rootlets. The second most abundant volcanic gas is carbon dioxide, and that builds up in air because it is not so easily removed as the others. As you know, CO2 is the greenhouse gas and does form one of the main controls on climate.
Earth’s continual volcanic action is the main reflection of its internal driving forces. Major volcanic eruptions are obvious, but there is hidden activity at least equal to that from tangible volcanism. Ocean floor spreads by quiet effusion of magma at oceanic ridges. Averaged out from the sea-floor magnetic stripes over the last 150 Ma, ocean-floor volcanism exudes between 16 to 26 km3 of magma every year; but in a continuous fashion rather than in isolated episodes. Only a proportion actually spills onto the ocean bed, the rest building new crust down to about 8 km below the surface. Ocean crust reacts with sea water, to lock water into the minerals produced by these changes. Most returns to the mantle at subduction zones, thereby maintaining a balance. As explained in Chapter 2, pressure during this descent induces the altered crust to transform to a water-poor assembly of minerals, and so drives watery fluids into the overriding wedge of mantle, inducing it to partially melt. Volcanism of a second kind results, that pervades island arcs and those continental margins above subduction zones. Because the rate of sea-floor spreading seems to have fluctuated on time scales of tens to hundreds of million years, so too has the pace of sub-aerial volcanic activity above subduction zones. Third, and least important at present, are isolated volcanoes such as Hawaii puffing away in the middle of tectonic plates. Their magma emanates from narrow plumes of mantle that rise from as deep as the core’s outer limit. Whether or not gas emerges from lava depends on the pressures that act when it erupts.
Atmospheric pressure is lower than that which keeps gas dissolved in magma. Any lava erupting on the land surface adds its gas content directly to the atmosphere in a manner that depends on the lava’s fluidity. Basalts are sufficiently fluid that gas bubbles out with little explosive effect, so to remain around ground level. Volcanoes above subduction zones sometimes erupt fluid lavas not far removed from the composition of basalt. But as you saw in Part II, the magma has a long way to travel to reach the surface. En route iron-, calcium- and magnesium-rich minerals crystallise to remain deep in the crust, so that the left-over magma is richer in silica and lower in density. This is the main manner in which continental crust forms and becomes destined to defy resorption back to the mantle. Given enough silicon, oxygen and aluminium, magma tends to structure itself before crystallizing with temporary but all-enveloping bonds linking those elements. That gives it considerably more resistance to flow than basalt magmas, in which the 3-D network of bonding fails to develop when other metals consume electrons in simpler bonds. Evolved, sticky magma dominates island-arc volcanoes and those of continental margins. Gas is less able to bubble out, and so eruptions are frequently explosive. The gas blasts upwards, perhaps to punch its way into the stratosphere.
In shallow water, quietly bubbling volcanic gas dissolves quickly. But at depths greater than a few hundred metres the pressure of overlying water effectively seals escape. Gases remain in the volcanic rock to enter crystallizing minerals. However, this does not seal them away for ever. Hydrothermal circulation alters igneous minerals, thereby liberating part of their complement of elements. Carbon and sulfur oxides locked in igneous minerals can add CO32-, HCO3– and SO42- ions to sea water. Water and new igneous crust achieve an equilibrium that balances the exchange. Debris settling to the ocean floors slows the hydrothermal exchange, and water circulation finally draws off the magmatic heat that set it in motion. Carbonate-secreting organisms enter the process by taking up some of the carbon-oxygen ions released from volcanic rocks. When they die their fossilised remains accumulate to keep the magmatic CO2 from the atmosphere. The fact that such carbonates may re-dissolve complicates matters still further, either because of increases in ocean acidity or when they pass into the highest pressure, deepest water layers before having the chance to settle as sediment. So far as oceanographers can judge, the usual outcome of the tangle of linkages is that ocean-floor volcanism adds little carbon dioxide directly to the atmosphere, despite continual eruptions. Whatever does emerge has to pass through the ocean-water circulation system, and that recycles on thousand-year time scales. Volcanic carbonates on the ocean floor, together with shelly carbonates and carbon from undecayed soft tissue in sediments are not permanent residents. As with water locked in minerals, subduction processes drive a proportion as CO2 towards the surface. Second- or third-hand, it can emerge to the atmosphere from volcanoes that rise above sea level. It is that direct venting that is affected by changes in rates of sea-floor spreading that accelerate or slow the downward part of the tectonic conveyor.
Mantle plumes that underpin within-plate volcanism buoy up the Earth’s solid surface, because their creation of more or less fixed hot spots lowers the density of the lithosphere. Oceanic island volcano systems, such as Hawaii and Iceland, vent gases to the air because of this and, of course, so would plume-driven systems on the continents. Oceanic hot-spot volcanoes build on moving lithosphere, so with time the piles that they build are dragged ‘off the hob’, as it were. They extinguish, cool, shrink and subside to form a sunken chain of seamounts pointing towards their source. Today, gas emission from ocean-island volcanoes is tiny by comparison with that from plate-related systems. Apart from those associated with the East African Rift, there are few volcanoes deep within continental land masses. At isolated times in the geological past things were very different indeed, and huge volcanic events took place within plates. After establishing a few more general aspects of the Earth’s emissions, we shall explore their implications.
Geology and the ‘greenhouse’
Carbon dioxide is far and away the most geologically significant volcanic gas. It pervades all aspects of the surface processes of weathering. Plants and eventually animals take it up in order to live and grow. Their death, when accompanied by burial and fossilisation, returns part in the form of solid carbon or carbonate for long-term geological storage. Not only is CO2 the ‘greenhouse’ gas par excellence, the climate that it helps warm is bound up with the circulation and ultimate fate of carbon dioxide. More than any other gas, CO2 tracks much of geological history. It lingers while others quickly move from residence to residence. That it does not build up from volcanic emissions is down largely to life’s presence. The fact that this is by now a point burned into your consciousness(!) is amplified by the fate of Venus. Without life, CO2 has become the dominant atmospheric gas there, so that lead would melt on the Venusian surface.
Exit of carbon dioxide from long-term geological storage as carbonates is achieved through the dissolution of exhumed limestones by the weak acid (H2CO3) in rainwater (Chapter 2), but the gas does not emerge through fizzing as it would in the case of limestone being attacked by a strong acid. Instead it is released as the bicarbonate ion (HCO3–), an increase in whose concentration may reduce the potential of seawater to dissolve CO2 gas from the atmosphere or be taken up in the reprecipitation of carbonate (Equation 3.4). Depending on the efficiency of reprecipitation, solution of limestones may or may not contribute to increase in ‘greenhouse’ conditions. There is another way for entombed carbon dioxide to rise like the undead, which owes more to its deep burial than to circulation of sedimentary material. Even though firing limestone in a lime kiln liberates the gas to produce quicklime (CaO), no matter how deep and hot limestone becomes during burial or subduction, rock pressure prevents that reaction. Much the same goes for buried elemental carbon in coal, for instance. It cannot burn within the Earth because there is no free oxygen. Both are immensely enduring geological materials. But not all carbonate is pure. Some sediments mix it with silicates, mainly quartz. Given high enough temperatures, the two combine:
SiO2 + CaCO3 = CaSiO3 + CO2 (Eqn 15.1)
We know that this happens from clusters of calcium silicates, sometimes seen in metamorphic rocks, which grow from mixtures of the two components. Whether such mixed sediments form on the deep ocean floors, or are blended when basins onshore or at continental margins swallow products of erosion, tectonics can plunge them to depths where such reactions do proceed. This is one of the most vexing issues relating to internal controls on climate. Theoretically, metamorphism should blow off carbon dioxide, and that suggests ultimately that continental collisions bring on flatulence, because that is when lots of crust thickens and descends. There are two problems in assessing its importance. The first is this: the bulk of carbonate deposited on or around continents is secreted by animals like corals and various shelly creatures, and they cannot live in muddy water. Gravel, sand, silt or clay help sterilise shallow sea beds of the main carbonate builders. Geological records of continental shelves contain little of the crucial mixture. But it is there aplenty in deep ocean sediments, since little organisms die and their shells fall together with fine clays. Such handy mixes undergo metamorphism in subduction zones to add their half-penn’orth of CO2 to volcanism, and we know all about that. But where does this hypothetical gas escape when continents collide? Some say up large faults in the thickened crust in the form of naturally carbonated mineral water. Spas offering such kidney-cleansing beverages sit on faults where the Earth is habitually flatulent, but as yet little is known of global emissions of this kind
Despite some recycling by silicate-carbonate reactions, as well as the long-term sedimentary storage of CO2, the mantle ingests both elemental carbon and pure carbonate. Hundreds of million years are needed for it to return to the surface, if at all. Today’s volcanoes directly add about 70 million tons of CO2 to the atmosphere each year. Temper that with the knowledge that humanity mobilises 7 billion tons, of which three quarters comes from burning fossil fuels and most of the rest from deforestation. Left to themselves, other Earth processes in the carbon cycle balance volcanic additions; not a stable balance in the sense of constancy, but nonetheless regulated in the long term. Human emissions place an enormous load on the carbon-climate-rest-of-the-world relationship, the prognosis for which awaits coverage of modern times in Part VII. For now the focus is the full scope of geological time, at least as far back as we have precise means of relating climate to time.
Have volcanic emissions always remained the same? The answer is several types of ‘no’! Radioactive decay generates the Earth’s internal heat, and it must slacken regularly with time, so too the overall activity within the planet. Although some heat is lost by conduction through the lithosphere, most emerges in the form of hot magma. At Year Zero, 4.5 billion years ago, the internal engine had to rid itself of about five times as much heat as it does now. Assuming much the same gas composition in magma, that added 350 million tons of CO2 per year. Stripped of any earlier atmosphere, devoid of life, slowly building an ocean from volcanic water vapour and beginning the air-water-rock interchanges of mobile gases, the early Earth probably built up an atmosphere dominated by CO2. Nitrogen would grow more slowly, and air would recycle an ephemeral load of water vapour and highly soluble acid gases. That much we established in Chapter 11, together with the balancing of emission and recycling. Most specialists agree on a CO2 partial pressure between one tenth to 7 times that of modern atmospheric pressure at sea level. Very uncertain, but considerably less than that on Venus today. Remember, the argument to arrive at that crude estimate comes from an astronomical assessment that the young Sun emitted less heat, and without a strong ‘greenhouse’ effect Earth would have been a perpetually icy world. The oldest sedimentary rocks contain signs of liquid water’s sculpting role. They also contain evidence that evaporation precipitated a particular form of calcium sulfate. Surface temperatures higher than 58°C would have made that impossible.
In Chapter 14 we saw that life and geology have pulled down CO2 levels. In doing so they reduced atmospheric pressure, but did not add much oxygen in return until 2.4 billion years back at the earliest. The Great Oxidation Event coincided with a CO2 partial pressure of about 10% that of current atmospheric pressure. That life and geological processes did temper the early greenhouse is just as well, for the Sun slowly heated. Without the life-rock coalition in regulating CO2, Earth would have followed the path of Venus to high-temperature sterility long ago. Volcanic emissions have slowed with tectonics, but not so much as to have prevented overheating without life’s crucial intervention. With the appearance of multicelled eukaryote life at the start of the Phanerozoic (542 Ma) the partial pressure of carbon dioxide had fallen by almost two orders of magnitude to around 0.2% of present atmospheric pressure. Equivalent to about 6000 parts per million, the principal greenhouse gas still had a way to go to reach modern, pre-industrial levels (180 ppm during glacial epochs and 280 during interglacials).
But, such is the scarcity of data on CO2 for the Precambrian and even the early part of the Phanerozoic Eon, this is peering back in time with an ‘on average’ perspective, and averages cover a multitude of sins. There is a way to unmask deviations from constant tectonic and volcanic rates, to which we now turn. Sadly, the key time series holds in detail only for the times since fossils linked with radiometric dating became available for study, which is since 542 Ma ago.
A proxy for tectonics and climate
Changes in sea level that are global in extent (Chapter 5) have links to three main Earth processes: daily tides; growth and melting of ice caps on continents; changes in the volume of the ocean basins. The last almost certainly governs broad sea-level changes through the Phanerozoic (Figure 5.5, included here in Figure 15.1). The warmer the lithosphere sitting beneath the oceans, the less dense it is and the higher it stands, so reducing the volume of the ocean basins and displacing water to flood low-lying continental areas. Ocean lithosphere forms at boundaries between plates from which it spreads sideways. It is hottest there, and so such constructive plate margins stand high as great ridge systems. As lithosphere spreads away from its hot source it cools and so subsides. That explains the broad shape of today’s ocean floors – axial ridges giving way to abyssal flanks. Matching the different shapes of this bathymetry to the rates at which different ridge systems have been spreading over the last few million years (from the width of the magnetic stripes), shows that the faster the spreading the broader the axial ridge is. Faster spreading results in a greater volume of ocean lithosphere retaining some heat and buoyancy. If the total amount of sea-floor spreading in the past was greater, such broad ridges would be a general feature of all ocean basins. Overall, ocean basins would have been shallower and water level would rise relative to the continents. Should sea-floor spreading slacken, then the opposite tendency would come into play, and water would withdraw into basins with a greater holding capacity. So, the grey part of Figure 15.1 is a rough guide to the varying pace of sea-floor spreading for the last 542 Ma. I say ‘rough’ because there are other factors than changing basin volume that affect global sea level. For instance, the volume of water in the oceans is climatically controlled, being much reduced when huge ice caps develop on land.
Figure 15.1 Global mean sea-level changes during the Phanerozoic Eon, estimated from the stratigraphic record of major sedimentary basins of the continents, relative to that at present (shown in grey, from Figure 5.5). Coloured lines show trends in global climate based on oxygen isotope data from marine sediments (the thick line shows broad climatic trends). Blue bars show major periods of glaciation, the lilac bar indicating anomalously cool conditions from mid-Jurassic to mid-Cretaceous times.
The faster sea-floor spreading is the faster magma is generated in volcanoes above subduction zones. Subaerial volcanoes generally vent gas directly to the atmosphere. So, in a roundabout, though straightforward, way the sea-level record should roughly chart the pace of plate tectonics, volcanic additions of CO2 to the atmosphere and so the potential for ‘greenhouse’ warming. We can calculate variations in the global sea-floor spreading rate directly from the width of magnetic stripes as far back around 200 Ma ago (the oldest ocean floor). That younger record confirms the longer-term sea-level link to tectonics.
The grey part of Figure 15.1 suggests that the rate at which CO2 entered the surface environment to play its role as one control over climate was broadly higher than now from 542 to about 320 Ma ago. It dropped sharply during the late Carboniferous to remain low until the end of the Jurassic, and then peaked around 100 Ma ago to fall again to modern rates. Interestingly, that sequence roughly ties in with fragmentation of the late Precambrian supercontinent, continental drift eventually to regroup all land as one mass in the Carboniferous, then a new round of break-up and drift, which seems to be reassembling continents today (Chapter 6). More of this and its climatic repercussions in Chapters 16 and 17. Unsurprisingly, there is a hitch in this neat idea. During the Cretaceous the oceanic magnetic record does not show a rate of sea-floor spreading anywhere near fast enough to account for the biggest ever recorded rise in sea level (around 300 m). But, unless somehow the Earth acquired a great deal more surface water (and that is highly improbable) the volume of the ocean basins must have shrunk at that time. There must have been another means for them to buoy upwards so much.
Volcanic super events
In Chapter 7 you encountered the most obvious signs that the past has not always been like the present; flood basalt events emerged in pulses roughly 30 Ma apart to vent huge volumes of magma with extraordinary speed. Their most obvious expression is in the stepped piles of lava flows that characterise the Deccan of India, the Columbia Plateau of the north-west USA, and the Inner Hebrides of Scotland, each situated on old continental crust. Basalt floods seem likely to link to larger versions of the plume-like upwellings in the mantle that seismic tomography shows beneath modern, within-plate volcanic islands (Figs 2.11 and 15.2a). The prodigious amounts of magma that emerge in tightly restricted areas, together with their distinctive chemistry point to unusually large amounts of partial melting in the mantle associated with such superplumes. That requires a great deal more heat than drives standard sea-floor spreading. The plumes must be much hotter than normal upper mantle temperatures, and various lines of evidence suggest an excess of maybe 150 to 250°C. Seismic tomography has resolved features close to the core-mantle boundary (CMB) where dense dregs of subduction appear to bottom-out and accumulate. One possibility is that this downwelling of cold dense mantle displaces hot material upwards to form plumes (Figs 2.10c, 15.2b). There are small plumes active today, beneath Hawaii, Iceland and northern Ethiopia, so this sort of mantle-wide circulation may be a continuous feature. However, that does not explain repetition of such massive events every 30 Ma or so.
Figure 15.2b Artistic impression of plume formation at the core-mantle boundary. Material from a subducted and metamorphosed slab of oceanic lithosphere accumulates at the CMB to displace older, hot material into an upwelling plume. Complex mineralogical changes are likely to occur, such as the transformation of a high-pressure form of MgSiO3 (perovskite or Pv) in relics of ocean crust to a different molecular structure (post-perovskite or PPv). The relics may melt and mix with mantle material to induce its rise. After Hirose & Lay (2008).
There are two very different lines of evidence that throw light on superplume events. The first stems from the magnetic record associated with some of them. For lengthy periods there are no reversals and so no stripes above the ocean floor corresponding to those times. Terrestrial magnetism links to the circulation within the liquid iron-nickel outer core, reversals perhaps showing shifts in that behaviour. Superplumes seem to coincide with quiet, settled behaviour in the core. Since motion is inseparable from energy, perhaps these big events involve the core disposing of some excess of heat. If so, then that implies that the core has its own source of energy. That might be radioactive 40K, for potassium can form sulfides and the core probably contains some iron sulfide, potassium tagging along chemically. The other line comes from another planet. Studies of the cratering record of Venus using radar images that penetrate its dense cloudy atmosphere show an unexpected feature by comparison with other planet. There are very few impact craters on Venus. Those that are present affect smooth plains that seem not to have been drastically modified by processes akin to the Earth’s continual tectonic activity. Using the Moon’s cratering history to interpret that of the Venusian surface suggests that it has been bombarded for only the last 500 Ma of that planet’s history. The implication is that the whole of Venus’s surface was volcanically repaved 500 Ma ago and then went into a sort of geological hibernation so that later craters remain intact.
Clearly, Venus is a very different world from ours, but there may be a general planetary process revealed by its geological record. Maybe objects the size of planets do not efficiently lose their internal heat production. It builds up only to blurt out periodically as massive episodes of volcanism. Venus seems particularly prone to this planet-scale disorder and is wracked by activity only rarely. The Earth is more efficient, thanks probably to plate tectonics, but some energy does build up internally, to emerge as superplumes on a much shorter cycle than that of Venus. This helps to explain the significant variations in sea-floor spreading rates. Radioactive decay goes on willy-nilly, so producing heat at a regular rate. If the Earth had a thermally efficient engine, then loss of heat by volcanism would be similarly regular – still a complex place, but more easily predictable. A great deal of difficult research is needed to grasp such deep matters, but we do see the outcomes of superplume events. Measuring them gives more input to understanding aspects of both climate and sea-level changes.
Should a superplume rise beneath ocean floor, whatever the age of that lithosphere heat would reduce its density to make it buoyant. Ocean-floor plateaus of flood basalt (Figure 7.3) can explain rises in sea-level when increases in sea-floor spreading rates are simply insufficient to explain the flooding of continents. Because only yet-to-be-subducted ocean floor younger than 200 Ma is available to seek proper measures of past spreading rates, only one such major event is available as a test; the Ontong-Java plateau of the West Pacific (Figure 7.4) implicated in the very high sea level of Cretaceous times (Figure 15.1), of which more in Chapter 17. Such a connection draws attention to earlier upsurges of sea-level. That spanning 300 to 542 Ma, which shows two or three pulses (Figure 15.1), may have a connection with huge ocean-floor plateaus, direct evidence for which long ago vanished into the mantle. Then there is the extraordinary break-out of Laurentia from the billion-year old Rodinia supercontinent (Chapter 6). Maybe a superplume is implicated there … The trail goes cold for want of evidence in both cases. Turning to continental expressions of such mantle upheavals provides more concrete estimates of their potential influence over surface events.
The biggest of around 20 well-documented continental flood-basalt provinces is that which blankets parts of Siberia. In it are just 45 separate lava flows, but each is prodigiously thick, from 400 metres to 3.5 kilometres. The Siberian Traps erupted in about a million years that bracket the age of the great boundary between the ‘ancient life’ of the Palaeozoic and the ‘middle life’ of the Mesozoic, 251 Ma ago. These flood basalts coincide with the Earth’s greatest mass extinction that came close to sterilizing the Earth (Chapter 5 and Part VI). Quarter of a billion years of erosion have left 1.5 million cubic kilometres of basalt intact, and there must have been more. Their estimated original volume of 3 million km3 is sufficient to bury the whole of western Europe beneath more than 1 km of basalt. Based on estimates of the gas content of the magma, back-of-envelope calculations gives a total CO2 emission of 5 x 1015 kg and maybe twice as much sulfur dioxide. The CO2 mass is equivalent to twice that in the present atmosphere. Even allowing for the fact that life and geology conspire to balance the climate by burying carbon, 45 short, massive pulses in less than a million years might overwhelm that balancing. Besides which, Siberia also exhaled an even greater mass of sulfur dioxide. Because fluid, basalt lavas release gas quietly, most of that gas would probably have hugged the ground in the same way as did those from Laki in 1783, eventually to rain out as sulfuric acid to the oceans. Due to the mass extinction there were few living things to balance sudden addition to the ‘greenhouse’ effect. Carbon dioxide has a residence time in the atmosphere of about 30 years in today’s amply vegetated and thriving biosphere, but it would linger far longer in a world rapidly losing most living things. The Siberian events may have marked a major climatic change, or accelerated one with other driving forces that was already in motion (Chapter 17). Their matching in time to the end-Palaeozoic mass extinction was by no means a coincidence (Part VI).
Check out Further Reading for Part V to see source material and links to articles
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