Category Archives: Climate change and palaeoclimatology

The core’s influence on geology: how does it do it?

Although no one can be sure about the details of processes in the Earth’s core what is accepted by all is that changes in core dynamics cause the geomagnetic field to change in strength and polarity, probably through some kind of physical interaction between core and deep mantle at the core-mantle boundary (CMB). Throughout the last 73 Ma and especially during the Cenozoic Era geomagnetism has been more fickle than at any time since a more or less continuous record began to be preserved in the Jurassic to Recent magnetic ‘stripes’ of the world ocean floor. Moreover, they came in bursts: 5 in a million years at around 72 Ma; 10 in 4 Ma centred on 54 Ma; 17 over 3 Ma around 42 Ma; 13 in 3 Ma at ~24 Ma; 51 over a period of 12 Ma centring on 15 Ma. During the Late Jurassic and Early Cretaceous the core was similarly ‘busy’, the two time spans of frequent reversals being preceded by quiet ‘superchrons’ dominated by the same normal polarity as we have today i.e. magnetic north being roughly around the north geographic pole.

The Cenozoic history of magnetic reversals - black periods were when geomagnetic field polarity was normal and white when reversed. (credit: Wikipedia)

The Cenozoic history of magnetic reversals – black periods were when geomagnetic field polarity was normal and white when reversed. (credit: Wikipedia)

Until recently geomagnetic ‘flips’ between the two superchrons were regarded as random , perhaps suggesting chaotic behaviour at the CMB. But such a view depends on the statistical method used. A novel approach to calculating reversal frequency through time, however, shows peak-trough pairs recurring 5 times through the Cenozoic Era, approximately 13 Ma apart: maybe the chaos is illusory (Chane, J. et al. 2015. The 13 million year Cenozoic pulse of the Earth. Earth and Planetary Science Letters, v. 431, p. 256-263). So, here is a kind of yardstick to see if there may be any connection between core processes and those at the surface, which Chen of the Fujian Normal University, Fushou China and Canadian and Chinese colleagues compared with the very detailed Cenozoic oxygen-isotope (δ18O) record preserved by foraminifera in ocean-floor sediments, which is a well established proxy for changes in climate. Removing the broad trend of cooling through the Cenozoic resulted in a plot of more intricate climatic shifts that matches the geomagnetism record in both shape and timing of peak-trough pairs. It also turns out, or so the authors claim, that both measures correlate with changes in the rate of Cenozoic subduction of oceanic lithosphere (a measure of plate tectonic activity), albeit negative – peaks in magnetism and climate connecting with slowing in the pace of tectonics.

The analyses involved some complicated maths, but taken at face value the correlations beg the questions why and how? Long-term climate change contains an astronomical signal, encapsulated in the Milankovich hypothesis which has been tested again and again with little room for refutation. So is this all to do with gravitational influences in the Solar System. More exotic still is the possibility of 13 Ma cyclicity linking the Milankovich mechanism with the vaster scale of the Sun’s orbit oscillating through the disc of the Milky Way galaxy and theoretical hints of a mysterious role for dark matter in or near the galaxy. Or, is it a relationship in which climate and the magnetic field are modulated by plate tectonics through varying volcanic emissions of greenhouse gases and the deep effect of subduction on processes at the CMB respectively? To me that seems more plausible, but it is still as exceedingly complex as the maths used to reveal the correlations.

Fascinating glacial feature found on Mars

Many of the vast wastes of northern Canada and Scandinavia that were ground to a paste by ice sheets during the last glacial cycle show peculiar features that buck the general glacial striation of the Shield rocks. They are round-topped ridges that wind apparently aimlessly across the tundra. In what is now a gigantic morass, the ridges form well-drained migration routes for caribou and became favourite hunting spots for the native hunter gatherers: in Canada they are dotted with crude simulations of the human form, or inugoks, that the Innuit erected to corral game to killing grounds. Where eroded they prove to be made of sand and gravel, which has proved an economic resource in some areas lacking in building aggregate, good but small examples being found in the Scottish Midland Valley that have served development of Glasgow and Edinburgh. They were given the Gaelic name eiscir meaning ‘ridge of gravel’ (now esker) from a few examples in Ireland.

Eskers form from glacial meltwater that makes its way from surface chasms known as moulins to the very bottom of an ice sheet where water flows much in the manner of a river, except in tubes rather than channels. Where the ice base is more or less flat the tubes meander as do normal sluggish rivers, and like them the tubes deposit a proportion of the abundant sediment derived by melting glacial ice. Once the ice sheet melts and ablates away, the sediments lose the support of the tube walls and flop down to form the eponymous low ridges: the reverse of the sediment filled channels of streams that have either dried up or migrated. Eskers are one of the features that shout ‘glacial action’ with little room for prevarication.

The classic form of eskers in the Phlegra Montes  of Mars. (credit:  Figure 6 in Gallagher and Balme, 2015)

The classic form of eskers in the Phlegra Montes of Mars. (credit: Figure 6 in Gallagher and Balme, 2015)

Glacial terrains on Mars have been proposed for some odd looking surfaces, but other processes such as debris flows are equally attractive. To the astonishment of many, Martian eskers have now been spotted during systematic interpretation of the monumental archives of high-resolution orbital images of the planetary surface (Gallagher, C. & Balme, M. 2015. Eskers in a complete, wet-based glacial system in the Phlegra Montes region, Mars. Earth and Planetary Science Letters, v. 431, p. 96-109). The discovery is in a suspected glacial terrain that exhibits signs of something viscous having flowed on low ground around higher topographic features, bombardment stratigraphy suggests a remarkable young age for the terrain or about 150 Ma ago: the Amazonian. Ice and its effects are not too strange to suggest for Mars which today is pretty much frigid, except for a few suggestions of active flow of small watery streams. Eskers demand meltwater in abundance, and Gallagher and Balme attribute some of the other features in the Phlegra Montes to wet conditions. However, the eskers are a one-off, so far as they know. Consequently, rather than appealing to some climatic warm up to explain the evidence for wetness, they suggest that the flowing water tubes resulted from melting deep in the ice as a result of locally high heat flow through the Martian crust, which is a lot more plausible.

Pleistocene megafaunal extinctions – were humans to blame?

Australia and the Americas had an extremely diverse fauna of large beasts (giant wombats and kangeroos in Australia; elephants, bears, big cats, camelids, ground sloths etc in the Americas) until the last glaciation and the warming period that led into the Holocene interglacial. The majority of these megafauna species vanished suddenly during that recent period. To a lesser extent something similar happened in Eurasia, but nothing significant in Africa. Because the last glacial cycle also saw migration of efficient human hunter-gatherers to every other continent except Antarctica, many ecologists, palaeontologists and anthropologists saw a direct link between human predation and the mass extinction (see Earth-Pages of April 2012. Earlier humans had indeed spread far and wide in Eurasia before, and the crude hypothesis that the last arrivals in Australasia and the Americas devoured all the meatiest prey in three continents had some traction as a result: predation in Eurasia and Africa by earlier hominids would have made surviving prey congenitally wary of bipeds with spears. In Australia and the Americas the megafauna species would have been naive and confident in their sheer bulk, numbers, speed and, in some cases, ferocity. Other possibilities emerged, such as the introduction of viruses to which faunas had no immunity or as a result of climate change, but none of the three possibilities has gained incontrovertible proof. But the most popular, human connection has had severe knocks in the last couple of years. A fourth, that the extinctions stemmed from a comet impact proved to have little traction.

English: s were driven to extinction by and hu...

Megafauna in a late-Pleistocene landscape including woolly mammoths and rhinoceroses, horses, and cave lions with a carcass. (credit: Wikipedia)

Since the amazing success of analysing the bulk DNA debris in sea water – environmental DNA or eDNA – to look at the local diversity of marine animals, the analytical and computing techniques that made it possible have been turned to ancient terrestrial materials: soils, permafrost and glacial ice. One of the first attempts revealed mammoth and pre-Columbian horse DNA surviving in Alaskan permafrost, thanks to the herds’ copious urination and dung spreading. Several articles in the 24 July 2015 issue of Science review ancient DNA advances, including eDNA from soils that chart changes in both fauna and flora over the last glacial cycle (Pennisi, E. 2015. Lost worlds found. Science, v. 349, p. 367-369). Combined with a variety of means of dating the material that yield the ancient eDNA, an interesting picture is emerging. The soil and permafrost samples potentially express ancient ecosystems in far more detail than would fossil animals or pollens, many of which are too similar to look at the species level and in any case are dominated by the most abundant plants rather than showing those critical in the food chain.

Nunavut tundra

Plants of the Arctic tundra in Nunavut, Canada (Photo credit: Wikipedia)

The first major success in palaeoecology of this kind came with a 50-author paper using eDNA ‘bar-coding’ of permafrost from 242 sites in Siberia and Alaska IWillerslev, E. and 49 others 2014. Fifty thousand years of Arctic vegetation and megafaunal diet. Nature, v. 506, p. 47-51. doi:10.1038/nature12921). Dividing the samples into 3 time spans – 50-25, 25-15 (last glacial maximum) and younger than 15 ka – the team found these major stages in the last glacial cycle mapped an ecological change from a dry tundra dominated by abundant herbaceous plants (forbs including abundant anemones and forget-me-not), to a markedly depleted Arctic steppe ecosystem then moist tundra with woody plants and grasses dominating. They also analysed the eDNA of dung and gut contents from ice-age megafauna, such as mammoths, bison and woolly rhinos, where these were found, which showed that forbs were the mainstay of their diet. Using bones of large mammals 6 member of the team also established the timing of extinctions in the last 56 ka (Cooper, A. et al. 2015. Abrupt warming events drove Late Pleistocene Holarctic megafaunal turnover. Science, DOI: 10.1126/science.aac4315), showing 31 regional extinction pulses linked to the rapid ups and downs of climate during Dansgaard-Oeschger cycles in the run-up to the last glacial maximum. By the end of the last glacial maximum, the megafauna were highly stressed by purely climatic and ecological factors. Human predation probably finished them off.

Are coral islands doomed by global warming?

Among the most voluble and persistent advocates of CO2 emissions reduction are representatives of islands in the tropics that are built entirely of reef coral. All the habitable land on them reaches only a few metres above high-tide level, so naturally they have more cause to worry about global warming and sea-level rise than most of us. Towns and villages on some atolls do seem to be more regularly inundated than they once were. So a group of scientists from New Zealand and Australia set out to check if there have been losses of land on one Pacific atoll, Funafuti, during the century since tidal observatories first recorded an average 1.7 mm annual rise in global sea level and a faster rate (~3 mm a-1) since 1993 (Kench, P.S. et al. 2015. Coral islands defy sea-level rise over the past century: Records from a central Pacific atoll. Geology, v. 43, p.515-518).

English: Funafuti (Tuvalu) from space Magyar: ...

Funafuti atoll (Tuvalu) from space (credit: Wikipedia)

Funafuti atoll comprises 32 islands that make up its rim, with a range of sizes, elevations, sediment build-ups and human modifications. The atoll was first accurately surveyed at the end of the 19th century, has aerial photographic cover from 1943, 1971 and 1984 and high-resolution satellite image coverage from 2005 and 2014, so this is adequate to check whether or not sea-level rise has affected the available area and shape of the habitable zone. It appears that there has been no increase in erosion over the 20th century and rather than any loss of land there has been a net gain of over 7%. The team concludes that coral reefs and islands derived from their remains and debris are able to adjust their size, shape and position to keep pace with sea level and with the effects of storms.

English: Looking west from a beach on Fongafal...

Beach on Fongafale Islet part of Funafuti Atoll, Tuvalu. (credit: Wikipedia)

This is an observation of just one small community in the vastness of the Pacific Ocean, so is unlikely to reassure islanders elsewhere who live very close to sea level and are anxious. It is a finding that bears out longer-term evidence that atolls remained stable during the major sea-level changes of the post-glacial period until about 7 thousand years ago when land glaciers stabilised. Since coral grows at a surprisingly rapid rate, that growth and the local redistribution of debris released by wave action keep pace with sea-level change; at least that taking place at rates up to 3 mm per year. But the study leaves out another threat from global warming. Corals everywhere are starting to show signs of ill thrift, partly resulting from increasing acidity of seawater as more CO2 dissolved in it and partly from increases in sea-surface temperature, as well a host of other implicated factors. This manifests itself in a phenomenon known as coral bleaching that may presage die-off. Should coral productivity decrease in the Pacific island states then the material balance shifts to land loss and sea level will begin an irresistible threat.

Flourishing life during a Snowball Earth period

That glacial conditions were able to spread into tropical latitudes during the late Neoproterozoic, Cryogenian Period is now well established, as are the time spans of two such events. But what were the consequences for life that was evolving at the time? That something dramatic was occurring is signalled by a series of perturbations in the carbon-isotope composition of seawater. Its relative proportion of 13C to 12C (δ13C) fell sharply during the two main Snowball events and at other times between 850 to 550 Ma. Since 12C is taken up preferentially by living organisms, falls in δ13C are sometimes attributed to periods when life was unusually suppressed. It is certain that the ‘excursions’ indicate that some process(es) must have strongly affected the way that carbon was cycled in the natural world.

English: Earth, covered in ice.

Artist’s impression of a Snowball Earth as it would appear with today’s continental configuration adjacent to the East Pacific Ocean. (Photo credit: Wikipedia)

The further sea ice extended beyond landmasses during Snowball events the more it would reduce the amount of sunlight reaching the liquid ocean and so photosynthesis would be severely challenged. Indeed, if ice covered the entire ocean surface – the extreme version of the hypothesis – each event must have come close to extinguishing life. An increasing amount of evidence, from climate- and oceanographic modelling and geological observation, suggests that a completely icebound Earth was unlikely. Nevertheless, such dramatic climate shifts would have distressed living processes to the extent that extinction rates were high and so was adaptive radiation of survivors to occupy whatever ecological niches remained or came into being: evolution was thereby speeded up. The roughly half-billion years of the Neoproterozoic hosted the emergence and development of multicellular organisms (metazoan eukaryotes) whose cells contained a nucleus and other bodies such as mitochondria and the chloroplasts of photosynthesisers. This hugely important stage of evolution burst forth shortly after – in a geological sense – the last Snowball event, during the Ediacaran and the Cambrian Explosion. But recent investigations by palaeontologists in glaciogenic rocks from China unearthed a rich diversity of fossil organisms that thrived during a Snowball event (Ye, Q. et al. 2015. The survival of benthic macroscopic phototrophs on a Neoproterozoic snowball Earth. Geology, v. 43, p. 507-510).

The Nantuo Formation in southern China contains glaciogenic sedimentary rocks ascribed to the later Marinoan glaciation (640 to 635 Ma). Unusually, the pebbly Nantuo glaciogenic rocks contain thin layers of siltstones and black shales. The fact that these layers are free of coarse fragments that floating ice may have dropped supports the idea that open water did exist close to glaciated landmasses in what is now southern China. Palaeomagnetic measurements show that the area was at mid-latitudes during the Marinoan event. The really surprising feature is that they contain abundant, easily visible fossils in the form of carbonaceous ribbons , disks, branching masses and some that dramatically resemble complex multi-limbed animals, though they are more likely to be part of an assemblage of algal remains. Whatever their biological affinities, the fossils clearly signify that life happily flourished beneath open water where photosynthesis provided a potential base to a food chain, though no incontrovertible animals occur among them.

See also: Corsetti, F.A. 2015. Live during Neoproterozoic Snowball Earth. Geology, v. 43, p. 559-560.

Snowball Earth events pinned down

The Period that lasted from 850 to 635 million years ago, the Cryogenian, takes its name from evidence for two and perhaps three episodes of glaciation at low latitudes. It has been suggested that, in some way, they were instrumental in the decisive stage of biological evolution from which metazoan eukaryotes emerged: the spectacular Ediacaran fossil assemblages follow on the heels of the last such event Although controversies about the reality of tropical latitudes experiencing ice caps have died away, there remains the issue of synchronicity of such frigid events on all continents, which is the central feature of so-called ‘Snowball Earth’ events. While each continent does reveal evidence for two low latitude glaciations – the Sturtian (~710 Ma) and the later Marinoan (~635 Ma) – in the form of diamictites (sediments probably dropped from floating ice and ice caps) it has proved difficult to date their start and duration. That is, the cold episodes may have been diachronous – similar conditions occurring at different localities at different times. Geochronology has, however, moved on since the early disputes over Snowball Earths and more reliable and precise dates for beginnings and ends are possible and have been achieved in several places (Rooney, A.D. et al. 2015. A Cryogenian chronology: Two long-lasting synchronous Neoproterozoic glaciations. Geology, v. 43, p. 459-462).

One computer simulation of conditions during a...

Computer simulation of conditions during a Snowball Earth period. (credit: Macmillan Publishers Ltd: Hyde et al., Nature 405:425-429, 2000)

Rooney and colleagues from Harvard and the University of Houston in the USA used rhenium-osmium radiometric dating in Canada, Zambia and Mongolia. The Re-Os method is especially useful for sulfide minerals as in the pyritic black shales that occur extensively in the Cryogenian, generally preceding and following the glacial diamictites and their distinctive carbonate caps. Combined with a few ages obtained by other workers using the Re-Os method and U-Pb dating of volcanic units that fortuitously occur immediately beneath or within diamictites, Rooney et al. establish coincident start and stop dates and thus durations of both the Sturtian and Marinoan glacial events: 717 to 660 Ma and 640 to 635 Ma respectively on all three continents. Their data is also said to refute the global extent and even the very existence of an earlier, Kaigas glacial event (~740 Ma) previous recorded from diamictites in Namibia, the Congo, Canada and central Asia. This assertion is based on the absence of diamictites with that age in the area that they studied in Canada and their own dating of a diamictite in Zambia, which is one that others assigned to the Kaigas event

The dating is convincing evidence for global glaciation on land and continental margins in the Cryogenian, as all the dates are from areas based on older continental crust. But the concept of Snowball Earth, in its extreme form, is that the oceans were ice-capped too as the name suggests, which remains to be convincingly demonstrated. That would only be achieved by suitably dated diamictites located on obducted oceanic crust in an ophiolite complex. Moreover, there are plenty more Cryogenian diamictites on other palaeo-continents and formed at different palaeolatitudes that remain to be dated (see here)

Wet spells in Arabia and human migration

In September 2014, Earth Pages  reported how remote sensing had revealed clear signs of extensive fossil drainage systems and lakes at the heart of the Arabian Peninsula, now the hyper-arid Empty Quarter (Rub al Khali). Their association with human stone artifacts dated as far back as 211 ka, those with affinities to collections from East Africa clustering between 74-90 ka, supported the sub-continent possibly having been an early staging post for fully modern human migrants from Africa. Member of the same archaeological team based at Oxford University have now published late Pleistocene palaeoclimatic records from alluvial-fan sediments in the eastern United Arab Emirates that add detail to this hypothesis (Parton, A. ­et al. 2015. Alluvial fan records from southeast Arabia reveal multiple windows for human dispersal. Geology, advance online publication doi:10.1130/G36401.1).

The eastern part of the Empty Quarter is a vast bajada formed from coalesced alluvial fans deposited by floods rising in the Oman Mountains and flowing westwards to disappear in the great sand sea of dunes. Nowadays floods during the Arabian Sea monsoons are few and far between, and restricted to the west-facing mountain front. Yet, older alluvial fans extend far out into the Empty Quarter, some being worked for aggregate used in the frantic building boom in the UAE. In one of the quarries, about 100 km south of the Jebel Faya Upper Palaeolithic tool site , the alluvial deposit contains clear signs of cyclical deposition in the form of 13 repeated gradations from coarse to fine waterlain sediment, each capped by fossil soils and dune sands. The soils contain plant remains that suggest they formed when the area was colonized by extensive grasslands formed under humid conditions.

Dating the sequence reveals that 6 of the cycles formed over a 10 thousand-year period between 158 to 147 ka, which coincides with a peak in monsoon intensity roughly between 160 and 150 ka during the glacial period that preceded the last one. Three later cycles formed at times of monsoon maxima during the last interglacial and in the climatic decline leading to the last glacial maximum, at ~128 to 115 ka, 105 to 95 ka, 85 to 74 ka. So, contrary to the long-held notion that the Arabian Peninsula formed a hostile barrier to migration, from time to time it was a well watered area that probably had abundant game. Between times, though, it was a vast, inhospitably dry place.

English: SeaWiFS collected this view of the Ar...

Satellite view of the Arabian Peninsula. The Oman mountains sweep in a dark arc south eastwards from the Staits of Hormuz at the mouth of the Persian Gulf. The brownish grey area to the south of the arc is the bajada that borders the bright orange Empty Quarter (credit: NOAA)

The authors suggest that the climatic cyclicity was dominated by a 23 ka period. As regards the southern potential migration route out of Africa, via the Straits of Bab el Mandab, which has been highly favoured by palaeoanthropologists lately, opportunities for migration in the absence of boats would have depended on sea-level lows. They do not necessarily coincide with wet windows of opportunity for crossing the cyclically arid Arabian peninsula that would allow both survival and proceeding onwards to south and east Asia. So far as I can judge, the newly published work seems to favour a northward then eastward means of migration, independent of fluctuations in land-ice volume and sea level, whenever the driest areas received sufficient water to support vegetation and game. In fact most of NE Africa is subject to the Arabian Sea monsoons, and when they were at their least productive crossing much of Ethiopia’s Afar depression and the coastal areas of Eritrea, Sudan and Egypt would have been almost as difficult as the challenge of the Empty Quarter.

Glacial cycles and sea-floor spreading

The London Review of Books recently published a lengthy review (Godfrey-Smith, P. 2015. The Ant and the Steam Engine. London Review of Books, v. 37, 19 February 2015 issue, p. 18-20) of the latest contribution to Earth System Science by James Lovelock, the man who almost singlehandedly created that popular paradigm through his Gaia concept of a self-regulating Earth (Lovelock, J. A Rough Ride to the Future. Allen Lane: London; ISBN 978 0 241 00476 0). Coincidentally, on 5 February 2015 Science published online a startling account of the inner-outer-inner synergism of Earth processes and climate (Crowley, J.W. et al. 2015. Glacial cycles drive variations in the production of oceanic crust. Science doi:10.1126/science.1261508). In fact serendipity struck twice: the following day a similar online article appeared in a leading geophysics journal (Tolstoy, M. 2015. Mid-ocean ridge eruptions as a climate valve. Geophysical Research Letters, doi:10.1002/2014GL063015)

Both articles centred on the most common topographic features on the ocean floor, abyssal hills. These linear features trend parallel to seafloor spreading centres and the magnetic stripes, which chart the progressive additions to oceanic lithosphere at constructive margins. Abyssal hills are most common around intermediate- and fast-spreading ridges and have been widely regarded as fault-tilt blocks resulting from extensional forces where cooling of the lithosphere causes it to sag towards the abyssal plains. However, some have suggested a possible link with variations in magma production beneath ridge axes as pressure due to seawater depth varied with rising and falling sea level through repeated glacial cycles. Mantle melting beneath ridges results from depressurization of rising asthenosphere: so-called ‘adiabatic’ melting. Pressure changes equivalent to sea-level fluctuations of around 100-130 m should theoretically have an effect on magma productivity, falls resulting in additional volumes of lava erupted on the ocean floor and thus bathymetric highs.

English: A close-up showing mid-ocean ridge to...

Formation of mid-ocean ridge topography, including abyssal hills that parallel the ridge axis. (credit: Wikipedia)

A test of this hypothesis would be see how the elevation of the sea floor adjacent to spreading axes changes with the age of the underlying crust. John Crowley and colleagues from Oxford and Harvard Universities and the Korea Polar Research Institute analysed new bathymetry across the Australian-Antarctic Ridge, whereas Maya Tolstoy of Columbia University performed similar work across the Southern East Pacific Rise. In both studies frequency analysis of changes in bathymetry through time, as calibrated by local magnetic stripes, showed significant peaks at roughly 23, 41 and 100 ka in the first study and at 100 ka in the second. These correspond to the well known Milankovitch periods due to precession, changing axial tilt and orbital eccentricity: persuasive support for a glacial control over mid-ocean ridge magmatism.

Enlarged by 100% & sharpened file with IrfanView.

Periodicities of astronomical forcing and global climate over the last million years (credit: Wikipedia)

An interesting corollary of the observations may be that pulses in sea-floor eruption rates emit additional carbon dioxide, which eventually percolates through the ocean to add to its atmospheric concentration, which would result in climatic warming. The maximum effect would correspond to glacial maxima when sea level reached its lowest, the reduction in pressure stimulating the greatest magmatism. One of the puzzling features of glacial cycles over the last million years, when the 100 ka eccentricity signal dominates, is the marked asymmetry of the sea-level record; slowly declining to a glacial maximum and then a rapid rise due to warming and melting as the Earth changed to interglacial conditions. Atmospheric CO2 concentrations recorded by bubbles in polar ice cores show a close correlation with sea-level change indicated by oxygen isotope data from oceanic sediments. So it is possible that build-up of polar ice caps in a roundabout way eventually reverse cooling once they reach their greatest thickness and extents, by modulating ocean-ridge volcanism and thereby the greenhouse effect.

Reconstructing the structure of ancient vegetation canopies

One of the central measures used to describe modern ecosystems is the ratio of foliage area to that of the ground surface – the leaf area index (LAI) – which expresses the openness of vegetation canopies. A high LAI helps to retain moisture in the soil, partly by shading and cooling the surface to reduce evaporation and partly by stopping surface soil from being battered to a concrete-like consistency by heavy rain, which reduces the amount of water that can infiltrate. It is possible to estimate LAI across today’s entire land area using satellite image data but a proxy for palaeoecological LAI has remained hard to find.

English: Creative Commons attribution "ph...

Hemispherical photograph used to calculate modern canopy cover. (credit: Wikipedia; photo by S.B. Weiss)

The outer coating of leaves in well-shaded (high LAI) areas tends to have protective or pavement cells that are larger and have more complicated shapes than does that of leaves in more open canopies. The framework of leaf cells is silica-based and made up of structures known as phytoliths whose morphologies vary in much the same way as the cells that they support. So theoretically it is possible to use fossil phytoliths in terrestrial sediments to estimate LAI variations through time in local canopies, but first the approach needs a means of calibration from living ecosystems. The vegetation of Central American Costa Rica varies through the entire range of possible LAI values, which leads to varying amounts of sunlight available to the leaves of cover plants. Measuring the area and the degree of shape-complexity of phytoliths in modern soils there shows that each is positively correlated with LAI.

Lowland Paca near Las Horquetas, Costa Rica. F...

A modern herbivorous mammal (lowland paca) from dense forest in Costa Rica. (Photo credit: Wikipedia)

Putting this approach to use in the Cenozoic terrestrial sediments of Patagonia, US and Argentinean palaeoecologists aimed to examine how the evolution of the teeth of herbivorous mammals – a major feature in their speciation – linked to changes in vegetation structure (Dunn, R.E. et al. 2015. Linked canopy, climate and faunal change in the Cenozoic of Patagonia. Science, v. 347, p. 258-261). Using phytoliths they were able to show that in the Eocene the area was covered by dense, closed forest canopies that gradually became more open towards the end of the Eocene to be replaced by open forest and shrubland habitats in the Oligocene and Miocene, with a brief period of regreening. It was during the period of more open vegetation that tooth structure underwent the most change. Chances are that the vegetation shifts began in response to the onset of Antarctic glaciation at the beginning of the Oligocene Epoch and related climate change at the northern margin of the Southern Ocean. Changes in the herbivore teeth may have been in response to the increasing amount of dust adhering to leaves as canopies became more open and soil increasingly dried out.

Ants and carbon sequestration

Aside from a swift but highly unlikely abandonment of fossil fuels, reduction of greenhouse warming depends to a large extent, possibly entirely, on somehow removing CO2 from the atmosphere. Currently the most researched approach is simply pumping emissions into underground storage in gas permeable rock, but an important target is incorporating anthropogenic carbon in carbonate minerals through chemical interaction with potentially reactive rocks. In a sense this is a quest to exploit equilibria involving carbon compounds that dominate natural chemical weathering and to sequester CO2 in solid, stable minerals.

The two most likely minerals to participate readily in weathering that involves CO2 dissolved in water are plagioclase feldspar, a calcium-rich aluminosilicate and olivine, a magnesium silicate. Both are abundant in mafic and ultramafic rocks, such as basalt and peridotite, which themselves are among the most common rocks exposed at the Earth’s surface. The two minerals, being anhydrous, are especially prone to weathering reactions involving acid waters that contain hydrogen ions, and in the presence of CO2 they yield stable carbonates of calcium and magnesium respectively. Despite lots of exposed basalts and ultramafic rocks, clearly such natural sequestration is incapable of absorbing emissions as fast as they are produced.

One means of speeding up weathering is to grind up plagioclase- and olivine-bearing rocks and spread the resulting gravel over large areas; as particles become smaller their surface area exposed to weathering increases. Yet it doesn’t take much pondering to realise that a great deal of energy would be needed to produce sufficient Ca- and Mg-rich gravel to take up the approximately 10 billion tonnes of CO2 being released each year by burning fossil fuels: though quick by geological standards the reaction rates involved are painfully slow in the sense of what the climatic future threatens to do. So is there any way in which these reactions might be speeded up?

Two biological agencies are known to accelerate chemical weathering, or are suspected to do so: plant roots and animals that live in soil. Ronald Dorn of Arizona State University set out to investigate the extent to which such agencies do sequester carbon dioxide, under the semi-arid conditions that prevail in Arizona and Texas (Dorn, R.I. 2014. Ants as a powerful biotic agent of olivine and plagioclase dissolution. Geology, v. 42, p. 771-774). His was such a simple experiment that it is a wonder it had not been conducted long ago; but it actually took more than half his working life. Spaced over a range of topographic elevations, Dorn used an augur at each site to drill five half-metre holes into the root mats of native trees, established ant and termite colonies and bare soil surfaces free of vegetation or animal colonies, filling each with sand-sized crushed basalt.

Empire of the Ants (film)

Film poster for Empire of the Ants (starring Joan Collins) (credit: Wikipedia)

Every five years thereafter he extracted the basalt sand from one of the holes at each site and each soil environment. To assess how much dissolution had occurred he checked for changes in porosity, and heated the samples to temperatures where carbonates break down to discover how much carbonate had been deposited. That way he was able to assess the cumulative changes over a 25 year period relative to the bare-ground control sites. The results are startling: root mats achieved 11 to 49 times more dissolution than the control; termites somewhat less, at 10 to 19 times; while ants achieved 53 to 177 times more dissolution. While it was certain that the samples had been continuously exposed to root mats throughout, the degree of exposure to termites and ants is unknown, so the animal enhancements of dissolution are probably minima.

Microscopic examination of mineral grains exposed to ant activity shows clear signs of surface pitting and other kinds of decay. Chemically, the samples showed that exposure to ants consistently increased levels of carbonate in the crushed basalt sand compared with controls, with levels rising by 2 to 4% by mass, with some variation according to ant species. Clearly, there is some scope for a role for ants in carbon sequestration and storage; after all, there are estimated to be around 1013 to 1016 individual ants living in the world’s soils. In the humid tropics the total mass of ants may be up to 4 times greater than all mammals, reptiles and amphibians combined. There is more to learn, but probably a mix of acid secretions and bioturbation by ants and termites is involved in their dramatic effect on weathering. One interesting speculation is that ants may even have played a role in global cooling through the Cenozoic, having evolved around 100 Ma ago.