Category Archives: Climate change and palaeoclimatology

Volcanism and sea level fall

Most volcanic activity stems from the rise of hot, deep rock, usually within the mantle. Pressure suppresses partial melting, so as hot rock rises the greater the chance that it will begin to melt without any rise in its temperature. That is the reason why mantle plumes are associated with many volcanic centres within plates. Extension at oceanic ridges allows upper mantle to rise in linear belts below rift systems giving rise to shallow partial melting, mid-ocean ridge basalts and sea-floor spreading. These aren’t the only processes that can reduce pressure to induce such decompression melting; any means of uplift will do, provided the rate of uplift exceeds the rate of cooling at depth. As well as tectonic uplift and erosion, melting of thick ice sheets and major falls in sea level may result in unloading of the lithosphere.

During Messinian Stage of the late Miocene up to 3 km of evaporitic salt was deposited in the deepest parts of the Mediterranean Basin. One mechanism might have been faster evaporation of seawater than its resupply from the Atlantic through the Straits of Gibraltar, similar to the way in which salts is deposited below the Dead Sea. But the salt layer beneath the modern Mediterranean Sea bed has interleaved riverine sediments containing fossils of land plants. The Straits had closed and the Mediterranean Sea evaporated away. From about 6 to 5.3 Ma ago sea level fell by 3 to 5 km, only returning to normal when the Straits reopened to launch the huge Zanclean flood, with which the Pliocene of southern Europe and North Africa commenced. A team from the Universities of Geneva, Orleans and Paris and the Instituto de Ciencias de la Tierra Jaume Almera in Barcelona has tested the hypothesis that the Messinian Crisis affected volcanic activity in the area (Sternai, P. et al. 2017. Magmatic pulse driven by sea-level changes associated with the Messinian salinity crisis. Nature Geoscience, v. 10 online; doi:10.1038/ngeo3032).

From the record of salt precipitation, Pietro Sternai and colleagues, reckon that the main phase of unloading of the Mediterranean Basin began at around 5.6 Ma. Allowing for loading by the thick evaporites they calculated that the effect of the loss of water mass was equivalent to an unloading of 15 MPa in the deeper Eastern Mediterranean and 10 MPa in the west. Using standard pressure-temperature melting curves for the upper mantle, they then estimated that any magma chambers affected by the decrease in pressure could yield up to 17% more melt. Radiometrically dated lavas and igneous dykes within the Mediterranean region became more frequent and the number of events more than doubled during the time of main salt deposition.

https://upload.wikimedia.org/wikipedia/commons/thumb/6/64/DenglerSW-Stromboli-20040928-1230x800.jpg/1200px-DenglerSW-Stromboli-20040928-1230x800.jpg

Stromboli, one of the most active volcanoes in the Mediterranean Basin (credit: Wikipedia)

In May 2017 a study of subglacial volcanoes in West Antarctica based on radar mapping of the solid surface identified 138, 91 of them previously unknown (van Wyk de Vries et al. 2017. A new volcanic province: an inventory of subglacial volcanoes in West Antarctica.  Geological Society, London, Special Publication 461) They lie within a buried rift system and are covered by thick ice. Only one volcano in Antarctica is known to be active, Erebus, which is part of the cluster. Most of the news items stemming from the publication mentioned the possibility that the buried volcanic tract could be adding to the instability of the West Antarctic Ice Sheet through heating up its base. The WAIS is the ice sheet most feared to collapse seawards leading to a rise of about 3 m in global sea level. If the 2 km thick WAIS did slide off its underlying crust it might possibly trigger reactivation of the volcanic cluster.

Advertisements

The late-Ordovician mass extinction: volcanic connections

The dominant feature of Phanerozoic stratigraphy is surely the way that many of the formally named major time boundaries in the Stratigraphic Column coincide with sudden shifts in the abundance and diversity of fossil organisms. That is hardly surprising since all the globally recognised boundaries between Eras, Periods and lesser divisions in relative time were, and remain, based on palaeontology. Two boundaries between Eras – the Palaeozoic-Mesozoic (Permian-Triassic) at 252 Ma and Mesozoic-Cenozoic (Cretaceous-Palaeogene) at 66 Ma – and a boundary between Periods – Triassic-Jurassic at 201 Ma – coincide with enormous declines in biological diversity. They are defined by mass extinctions involving the loss of up to 95 % of all species living immediately before the events. Two other extinction events that match up to such awesome statistics do not define commensurately important stratigraphic boundaries. The Frasnian Stage of the late-Devonian closed at 372 Ma with a prolonged series of extinctions (~20 Ma) that eliminated  at least 70% of all species that were alive before it happened. The last 10 Ma of the Ordovician period witnessed two extinction events that snuffed out about the same number of species. The Cambrian Period is marked by 3 separate events that in percentage terms look even more extreme than those at the end of the Ordovician, but there are a great many less genera known from Cambrian times than formed fossils during the Ordovician.

Untitled-1

Faunal extinctions during the Phanerozoic in relation to the Stratigraphic Column.

Empirical coincidences between the precise timing of several mass extinctions with that of large igneous events – mainly flood basalts – suggest a repeated volcanic connection with deterioration of conditions for life. That is the case for four of the Famous Five, the end-Ordovician die-off having been ascribed to other causes; global cooling that resulted in south-polar glaciation of the Gondwana supercontinent and/or an extra-solar gamma-ray burst (predicated on the preferential extinction of Ordovician near-surface, planktonic fauna such as some trilobite families). Neither explanation is entirely satisfactory, but new evidence has emerged that may support a volcanic trigger (Jones, D.S. et al. 2017. A volcanic trigger for the Late Ordovician mass extinction? Mercury data from south China and Laurentia. Geology, v. 45, p. 631-634; doi:10.1130/G38940.1). David Jones and his US-Japan colleagues base their hypothesis on several very strong mercury concentrations in thin sequences in the western US and southern China of late Ordovician marine sediments that precede, but do not exactly coincide with, extinction pulses. They ascribe these to large igneous events that had global effects, on the basis of similar Hg anomalies associated with extinction-related LIPs. Yet no such volcanic provinces have been recorded from that time-range of the Ordovician, although rift-related volcanism of roughly that age has been reported from Korea. That does not rule out the possibility as LIPs, such as the Ontong Java Plateau, are known from parts of the modern ocean floor that formed in the Mesozoic and Cenozoic. Ordovician ocean floor was subducted long ago.

The earlier Hg pulses coincide with evidence for late Ordovician glaciations over what is now Africa and eastern South America. The authors suggest that massive volcanism may then have increased the Earth’s albedo by blasting sulfates into the stratosphere. A similar effect may have resulted from chemical weathering of widely exposed flood basalts which draws down atmospheric CO2. The later pulses coincide with the end of Gondwanan glaciation, which may signify massive emanation of volcanic CO2 into the atmosphere and global warming. Despite being somewhat speculative, in the absence of evidence, a common link between the Big Five plus several other major extinctions and LIP volcanism would quieten their popular association with major asteroid and/or comet impacts currently being reinvigorated by drilling results from the K-Pg Chicxulub crater offshore of Mexico’s Yucatan Peninsula.

Gas hydrates: a warning from the past

Detailed acoustic imaging above the Troll gas field in the northern North Sea off western Norway has revealed  tens of thousands of elliptical pits on the seabed. At around 10 to 20 per square kilometre over an area of about 15,000 km2 there are probably between 150 to 300 thousand of them. They range between 10 to 100 m across and are about 6 m deep on average, although some are as deep as 20 m. They are pretty much randomly distributed but show alignment roughly parallel to regional N-S sea-floor currents. Many of the world’s continental shelves display such pockmark fields, but the Troll example is among the most extensive. Almost certainly the pockmarks formed by seepage of gas or water to the surface. However, detailed observations suggest they are inactive structures with no sign of bubbles or fluid seepage. Yet the pits cut though glacial diamictites deposited by the most recent Norwegian Channel Ice Stream through which icebergs once ploughed and which is overlain by thin Holocene marine sediments. One possibility is that they record gas loss from the Troll field, another being destabilisation of shallow gas hydrate deposits.

Troll pockmarks

Parts of the Troll pockmark field off Norway. A: density of pockmarks in an area of 169 square km. B: details of a cluster of pockmarks. (Credit: Adriano Mazzini, Centre for Earth Evolution and Dynamics (CEED) University of Oslo)

Norwegian geoscientists have studied part of the field in considerable detail, analysing carbonate-rich blocks and foraminifera in the pits (Mazzini, A. and 8 others 2017. A climatic trigger for the giant Troll pockmark field in the northern North Sea. Earth and Planetary Science Letters, v. 464, p. 24-34; http://dx.doi.org/10.1016/j.epsl.2017.02.014). The carbonates show very negative δ13C values that suggest the carbon in them came from methane, which could indicate either of the two possible means of formation. However, U-Th dating of the carbonates and radiocarbon ages of forams in the marine sediment infill suggest that they formed at around 10 ka ago; 1500 years after the end of the Younger Dryas cold episode and the beginning of the Holocene interglacial. Most likely they represent destabilisation of a once-extensive, shallow layer of methane hydrates in the underlying sediments, conditions during the Younger Dryas having been well within the stability field of gas hydrates. Sporadic leaks from the deeper Troll gas field hosted by Jurassic sandstones is unlikely to have created such a uniform distribution of gas-release pockmarks. Adriano Mazzini and colleagues conclude that rapid early Holocene warming led to sea-floor temperatures and pressures outside the stability field of gas hydrates. There are few signs that hydrates linger in the area, explaining the present inactivity of the pockmarks – all the methane and CO2 escaped before 10 ka.

Gas hydrates are thought to be present beneath shallow seas today, wherever sea-floor sediments have a significant organic carbon content and within the pressure-temperature window of stability of these strange ice-like materials. Mazzini et al.’s analysis of the Troll pockmark field clearly has profound implications for the possible behaviour of gas hydrates at a time of global climatic warming. As well as their destabilisation adding to methane release from onshore peat deposits currently locked by permafrost and a surge in global warming, there is an even more catastrophic possibility. The whole of the seaboard of the southern North Sea was swept by a huge tsunami about 8000 years ago, which possibly wiped out Mesolithic human occupancy of lowland Britain, the former land mass of Doggerland, and the ‘Low Countries’ of western Europe. This was created by a massive submarine landslide – the Storegga Slide just to the north of the Troll field – which may have been triggered by destabilisation of submarine gas hydrates during early Holocene warming of the oceans.

Odds and ends about Milankovitch and climate change

It is some 40 years since the last explosive development in understanding the way the world works. In 1976 verification of Milutin Milanković’s astronomical theory to explain cyclical climate change as expressed by surface processes has had a similar impact as the underpinning of internal processes by the emergence of plate tectonics in the preceding decade. Signals that match the regularity of changes in the Earth’s orbital eccentricity and the tilt and precession of its axis of rotation, with periods of roughly 96 and 413 ka, 41 ka, 21 and 26 ka respectively, were found in climate change proxies in deep-sea sediment cores (oxygen isotope sequences from benthonic foraminifera) spanning the last 2.6 Ma. The findings seemed as close to proof as one might wish, albeit with anomalies. The most notable of these was that although Milanković’s prediction of a dominant 41 ka effect of changing axial tilt, the strongest astronomical forcing, had characterised cooling and warming cycles in the early Pleistocene, since about a million years ago a ~100 ka periodicity took over – that of the weakest forcing from changing orbital obliquity. Analysis of sedimentary cycles from different episodes in earlier geological history, as during Carboniferous to Permian global frigidity, seemed to confirm that gravitational fluctuations stemming from the orbits of other planets, Jupiter and Saturn especially, had been a continual background to climate change.

All manner of explanations have been offered to explain why tiny, regular and predictable changes in Earth’s astronomical behaviour produce profound changes in the highly energetic and chaotic climate system. Much attention has centred on the mathematically based concept of stochastic resonance. That is a phenomenon where weak signals may be induced to show themselves if they are mixed with a random signal – ‘white noise’ spanning a great range of frequencies. The two resonate at the hidden frequencies thereby strengthening the weak, non-random signal. Noise is already present in the climate system because of the random and highly complex nature of the components of climate itself and the surface processes that it induces.

The latest development along these lines suggests that something quite simple may be at the root of inner complexities in the climatic history of the Pleistocene Epoch: the larger an ice sheet becomes and the longer it lasts the easier it is to cause it to melt away (Tzedakis, P.C. et al. 2017. A simple rule to determine which insolation cycles lead to interglacials. Nature, v. 542, p. 427-432; doi:10.1038/nature21364). The gist of the approach taken in the investigation lies in analysing the degree to which the onsets of major ice-cap melting match astronomically predicted peaks in summer insolation north of 65° N. It also subdivides O-isotope signals of periods of sea level rise into full interglacials, interstadials during periods of climate decline and a few cases of extended interglacials. Through time it is clear that there has been an  increase in the number of interstadials that interrupt cooling between interglacials. Plotting the time of peaks in predicted summer warming closest to major glacial melting events against their insolation energy is revealing.

Before 1.5 Ma the peak energy of summer insolation in the Northern Hemisphere exceeded a threshold leading to full interglacials rather than interstadials more often than it did during the period following 1 Ma. Although Milanković’s 41 ka periodicity remained recognisable throughout, from about 1.5 Ma ago more and more of the energy peaks resulted in only the partial ice melting of interstadial events. The energy threshold for the full deglaciation of interglacials seems to have increased between 1.5 to 1.0 Ma and then settled to a ‘steady state’. The balance between glacial growth and melting increasingly ‘skipped’ 41 ka peaks in insolation so that ice caps grew bigger with time. Deglaciation then required additional forcing. But considering the far larger extent of ice sheets, the tiny additional insolation due to shifts in  orbital eccentricity every ~100 ka surprisingly tipped truly savage ice ages into warm interglacials.

Resolving this paradox may lie with three simple, purely terrestrial factors associated with great ice caps: thicker and more extensive ice becomes warmer at its base and more prone to flow; climate above and around large ice caps becomes progressively colder and drier, so reducing their growth rate; the more sea level falls as land ice builds up, the more the vertical structure and flow of ocean water change. The first of these factors leads to periodic destabilisation when ice sheets surge outwards and increase the rate of iceberg calving into the surrounding oceans. Such ‘iceberg armadas’ characterised the last Ice Age to result in sudden irregularly spaced changes in ocean dynamics and global climate to return to metastable ice coverage, as did earlier ones of similar magnitude. The second factor results in dust lingering at the surface of ice caps that reduced the ability of ice to reflect solar radiation back to space, which enhances summer melting. The third and perhaps most profound factor reduces the formation of ocean bottom water into which dissolved carbon dioxide has accumulated from thermohaline sinking of surface water. This leads to more CO2 in the atmosphere and a growing greenhouse effect. Comforting as finding simplicity within huge complexity might seem, that orbital eccentricity’s weak effect on climatic warming – an order of magnitude less than any other astronomical forcing – can tip climate from one extreme to the other should be a grave warning: climate is chaotic and responds unpredictably to small changes …

Ancient CO2 estimates worry climatologists

Concerns about impending, indeed actual, anthropogenic climate change brought on by rapidly rising levels of the greenhouse gas carbon dioxide have spurred efforts to quantify climates of the distant past. Beyond the CO2 record of the last 800 ka established from air bubbles trapped in glacial ice palaeoclimate researchers have had to depend on a range of proxies for the greenhouse effect. Those based on models linking plate tectonic and volcanic CO2 emissions with geological records of the burial of organic matter, weathering and limestone accumulation are imprecise in the extreme, although they hint at considerable variation during the Phanerozoic. Other proxies give a better idea of the past abundance of the main greenhouse gas, one using the curious openings or stomata in leaves that allow gases to pass to and fro between plant cells and the atmosphere. Well preserved fossil leaves show stomata nicely back to about 400 Ma ago when plants first colonised the land.

Stomata on a rice leaf (credit: Getty images)

Stomata draw in CO2 so that it can be combined with water during photosynthesis to form carbohydrate. So the number of stomata per unit area of a leaf surface is expected to increase with lowering of atmospheric CO2 and vice versa. This has been observed in plants grown in different air compositions. By comparing stomatal density in fossilised leaves of modern plants back to 800 ka allows the change to be calibrated against the ice-core record. Extending this method through the Cenozoic, the Mesozoic and into the Upper Palaeozoic faces the problems of using fossils of long-extinct plant leaves. This is compounded by plants’ exhalation of gases to the atmosphere – some CO2 together with other products of photosynthesis, oxygen and water vapour. Increasing stomatal density when carbon dioxide is at low concentration risks dehydration. How extinct plant groups coped with this problem is, unsurprisingly, unknown. So past estimates of the composition of the air become increasingly reliant on informed guesswork rather than proper calibration. The outcome is that results from the distant past tend to show very large ranges of CO2 values at any particular time.

An improvement was suggested some years back by Peter Franks of the University of Sydney with Australian, US and British co-workers (Franks, P.J. et al. 2014. New constraints on atmospheric CO2 concentration for the Phanerozoic. Geophysical Research Letters, v. 41, p. 4685-4694; doi:10.1002/2014GL060457). Their method included a means of assessing the back and forth exchange of leaf gases with the atmosphere from measurements of the carbon isotopes in preserved organic carbon in the fossil leaves, and combined this with stomatal density and the actual shape of stomata. Not only did this narrow the range of variation in atmospheric CO2 results for times past, but the mean values were dramatically lessened. Rather than values ranging up to 2000 to 3000 parts per million (~ 10 times the pre-industrial value) in the Devonian and the late-Triassic and early-Jurassic, the gas-exchange method does not rise above 1000 ppm in the Phanerozoic.

The upshot of these findings strongly suggests that the Earth’s climate sensitivity to atmospheric CO2 (the amount of global climatic warming for a doubling of pre-industrial CO2 concentration) may be greater than previously thought; around 4° rather than the currently accepted 3°C. If this proves to be correct it forebodes a much higher global temperature than present estimates by the Intergovernmental Panel on Climate Change (IPCC) for various emission scenarios through the 21st century.

See also: Hand, E. 2017. Fossil leaves bear witness to ancient carbon dioxide levels. Science, v. 355, p. 14-15; DOI: 10.1126/science.355.6320.14.

Amazonian forest through the last glacial maximum

Accelerated evolution may occur when a small population of a species – whose genetic variability is therefore limited – becomes isolated from all other members. This is one explanation for the rise of new species, as in the Galapagos archipelago. Creation of such genetic bottlenecks encourages rapid genetic drift away from the main population. It has been suggested to explain sudden behavioural shifts in anatomically modern humans over the last hundred thousand years or so, partly through rapid and long-distance migrations and partly through a variety of environmental catastrophes, such as the huge Toba eruption around 74 ka. Another example has been proposed for the teemingly diverse flora and fauna of the Amazon Basin, particularly among its ~7500 species of butterflies, which has been ascribed to shrinkage of the Amazonian rain forest to isolated patches that became refuges from dry conditions during the last glacial maximum.

Top: Arid ice age climate Middle: Atlantic Per...

Potential forest cover inferred from global climate models for the last glacial maximum (top) the Holocene thermal maximum and at present.. (credit: Wikipedia)

A great deal of evidence suggests that during glacial maxima global climate became considerably drier than that in interglacials, low-latitude deserts and savannah grasslands expanding at the expense of humid forest. Yet the emerging complexity of how climate change proceeds from place to place suggests that evidence such continental drying from one well-documented region, such as tropical Africa, cannot be applied to another without confirming data. Amazonia has been the subject of long-standing controversy about such ecological changes and formation of isolated forest ‘islands’ in the absence of definitive palaeoclimate data from the region itself. A multinational team has now published data on climatic humidity changes over the last 45 ka in what is now an area of dense forest but also receives lower rainfall than most of Amazonia; i.e. where rolling back forest to savannah would have been most likely to occur during the last glacial maximum (Wang, X. et al. 2017. Hydroclimate changes across the Amazon lowlands over the past 45,000 years. Nature, v. 541, p. 204-207; doi:10.1038/nature20787).

Their study area is tropical karst, stalagmites from one of whose caves have yielded detailed oxygen-isotope time series. Using the U/Th dating technique has given the data a time resolution of decades covering the global climatic decline into the last glacial maximum and its recovery to modern times. The relative abundance of oxygen isotopes (expressed by δ18O) in the calcium carbonate layers that make up the stalagmites is proportional to that of the rainwater that carried calcium and carbonate ions dissolved from the limestones. The rainwater δ18O itself depended on the balance between rainfall and evaporation, higher values indicating reduced precipitation. Relative proportions of carbon isotopes in the stalagmites, expressed by δ13C, record the balance of trees and grasses, which have different carbon-isotope signatures. Rainfall in the area did indeed fall during the run-up to the last glacial maximum, to about 60% of that at present, then to rise to ~142% in the mid-Holocene (6 ka). Yet δ13C in the stalagmites remained throughout comparable with those in the Holocene layers, its low values being incompatible with any marked expansion of grasses.

English: View of Amazon basin forest north of ...

Amazonian rain forest north of Manaus, Brazil. (credit: Wikipedia)

One important factor in converting rain forest to grass-dominated savannah is fire induced by climatic drying. Tree mortality and loss of cover accelerates drying out of the forest floor in a vicious circle towards grassland, expressed today by human influences in much of Amazonia. Fires in Amazonia must therefore have been rare during the last ice age; indeed sediment cores from the Amazon delta do not reveal any significant charcoal ‘spike’.

See also: Bush, M.B 2017. The resilience of Amazonian forests. Nature, v. 541, p. 167-168; doi:10.1038/541167a

When did the Greenland ice cap last melt?

The record preserved in cores through the thickest part of the Greenland ice cap goes back only to a little more than 120 thousand years ago, unlike in Antarctica where data are available for 800 ka and potentially further back still. One possible reason for this difference is that a great deal more snow falls on Greenland so the ice builds up more quickly than in Antarctica. Because ice flows under pressure this might imply that older ice on Greenland long flowed to the margins and either melted or calved off as icebergs. So, although it is certain that the Antarctic ice cap has not melted away, at least in the last million years or so, we cannot tell if Greenlandic glaciers did so over the same period of time. Knowing whether or not Greenland might have shed its carapace of ice is important, because if ever does in future the meltwater will add about 7 metres to global sea level: a nightmare scenario for coastal cities, low-lying islands and insurance companies.

Margin of the Greenland ice sheet (view from p...

Edge of the Greenland ice sheet with a large glacier flowing into a fjiord at the East Greenland coas  (Photo credit: Wikipedia)

One means of judging when Greenland was last free of ice, or at least substantially so, is based on more than a ice few metres thick being opaque to cosmic ‘rays’. Minerals, such as quartz, in rocks bared at the surface to ultra-high energy, cosmogenic neutrons accumulate short-lived isotopes of beryllium and aluminium – 10Be and 26Al with half-lives of 1.4 and 0.7 Ma. Once rocks are buried beneath ice or sediment, the two isotopes decay away and it is possible to estimate the duration of burial from the proportions of the remaining isotopes. After about 5 Ma the cosmogenic isotopes will have decreased to amounts that cannot be measured. Conversely, if the ice had melted away at any time in the past 5 Ma and then returned it should be possible to estimate the timing and duration of exposure of the surface to cosmic ‘rays’. Two groups of researchers have applied cosmogenic-isotope analysis to Greenland. One group (Schaefer, J.G. et al. 2016. Greenland was nearly ice-free for extended periods during the Pleistocene. Nature, v. 540, p. 252-255) focused on bedrock, currently buried beneath 3 km of ice, that drilling for the ice core finally penetrated. The other systematically analysed the cosmogenic isotope content of mineral grains at different depths in North Atlantic seafloor sediment cores, largely supplied from East Greenland since 7.5 Ma ago (Bierman, P.R. et al. 2016. A persistent and dynamic East Greenland Ice Sheet over the past 7.5 million years Nature, v. 540, p. 256-260). As their titles suggest, the two studies had conflicting results.

The glacigenic sediment grains contained no more than 1 atom of 10Be per gram compared with the 5000 to 6000 in grains deposited and exposed to cosmic rays along the shores of Greenland since the end of the last ice age. These results challenge the possibility of any significant deglaciation and exposure of bedrock in the source of seafloor sediment since the Pliocene.  The bedrock from the base of Greenland’s existing ice cap, however, contains up to 25 times more cosmogenic isotopes. The conclusion in that case is that there must have been a protracted, >280 ka, exposure of the rock surface in what is now the deepest ice cover at 1.1 Ma ago at most. Allowing for the likelihood of some persistent glacial cover in what would have been mountainous areas in an otherwise substantially deglaciated Greenland, the results are consistent with about 90% melting suggested by glaciological modelling.

Clearly, some head scratching is going to be needed to reconcile the two approaches. Ironically, the ocean-floor cores were cut directly offshore of the most likely places where patches of residual ice cap may have remained. Glaciers there would have transported rock debris that had remained masked from cosmic rays until shortly before calved icebergs or the glacial fronts melted and supplied sediment to the North Atlantic floor. If indeed the bulk of Greenland became ice free around a million years ago, under purely natural climatic fluctuations, the 2° C estimate for global warming by 2100 could well result in a 75% glacial melt and about 5-6 m rise in global sea level.

Read more about glaciation here and here.

Impact linked to the Palaeocene-Eocene boundary event

The PalaeoceneEocene (P-E) boundary at 55.8 Ma marks the most dramatic biological changes since the mass extinction at the Cretaceous-Palaeogene boundary 10 million years earlier. They included the rapid expansions of mammals and land plants and major extinction of deep-water foraminifera.  It was a time of sudden global warming (5-10°C in 10-20 ka) superimposed on the general Cenozoic cooling from the ‘hothouse’ of the Cretaceous Period. It coincided with a decrease in the proportion of 13C in marine carbonates.  Because photosynthesis, the source of organic carbon, favours light 12C, such a negative δ13C “spike” is generally ascribed to an unusually high release of organic carbon to the atmosphere.  The end-Palaeocene warming may have resulted from a massive release of methane from gas-hydrate buried in shallow seafloor sediments. But another process may yield such a signature; massive burning of organic material at the land surface. Since its discovery, the P-E thermal maximum has been likened to the situation that we may face should CO2 emissions from fossil-fuel burning continue to rise without control. Unsurprisingly, funds are more easily available for research on this topic than, say, ‘Snowball Earth’ events.

Climate change during the last 65 million year...

Climate change during the last 65 million years. The Paleocene–Eocene Thermal Maximum is labelled PETM. (Photo credit: Wikipedia)

Three seafloor sediment cores off the east coast of the US that include the P-E boundary have been found to contain evidence for an impact that occurred at the time of the δ13C “spike” (Schaller, M.F. et al. 2016. Impact ejecta at the Paleocene-Eocene boundary. Science, v. 354, p. 225-229). The evidence is dominated by tiny spherules and tear-shaped blobs of glass, some of which contain tiny crystals of shocked and high-temperature forms of silica (SiO2). These form part of the suite of features that have been used to prove the influence of asteroid impacts. Two other onshore sites have yielded iridium anomalies at the boundary, so it does look like there was an impact at the time. The question is, was it large enough either to cause vast amounts of methane to blurt out from shall-water gas hydrates or set the biosphere in fire? Two craters whose age approximates that of the P-E boundary are known, one in Texas the other in Jordan, with diameters of 12 and 5 km respectively; far too small to have had any global effect. So either a suitably substantial crater of the right age is hidden somewhere by younger sediments or the association is coincidental – the impact that created the Texan crater could conceivably have flung glassy ejecta to the area of the three seafloor drilling sites.

Almost coinciding with the spherule-based paper’s publication another stole its potential thunder. Researchers at Southampton University used a mathematical model to investigate how a methane release event might have unfolded (Minshull, T.A. et al. 2016. Mechanistic insights into a hydrate contribution to the Paleocene-Eocene carbon cycle perturbation from coupled thermohydraulic simulations. Geophysical Research Letters, v. 43, p. 8637-8644, DOI: 10.1002/2016GL069676). Their findings challenge the hypothesized role of methane hydrates in causing the sudden warming at the P-E boundary. But that leaves out the biosphere burning, which probably would have neded a truly spectacular impact.

More on mechanisms for ancient climate change

Out of Africa: a little less blurred?

DNA from the mitochondria of humans who live on all the habitable continents shows such a small variability that all of us must have had a common maternal ancestor, and she lived in Africa about 160 ka ago. Since this was first suggested by Rebecca Cann, Mark Stoneking and Allan Wilson of the University of California, Berkeley in 1987 there has been a stream of data and publications – subsequently using Y-chromosome DNA and even whole genomes – that both confirm an African origin for Homo sapiens and illuminate it. Analyses of the small differences in global human genetics also chart the routes and – using a ‘molecular clock’ technique – the timings of geographic and population branchings during migration out of Africa. As more and better quality data emerges so the patterns change and become more intricate: an illustration of the view that ‘the past is always a work in progress’. The journal Nature published four papers online in the week ending 25 September 2016 that demonstrate the ‘state of the art’.

Three of these papers add almost 800 new, high-quality genomes to the 1000 Genomes Project that saw completion in 2015. The new data cover 270 populations from around the world including those of regions that have previously been understudied for a variety of reasons: Africa, Australia and Papua-New Guinea. All three genomic contributions are critically summarized by a Nature News and Views article (Tucci, S & Akey, J.L. 2016. A map of human wanderlust. http://dx.doi.org/10.1038/nature19472). The fourth paper pieces together accurately dated fossil and archaeological findings with data on climate and sea-level changes derived mainly from isotopic analyses of marine sediments and samples from polar ice sheets (Timmermann, A & Friedrich, T. 2016. Late Pleistocene climate drivers of early human migration. Nature, doi:10.1038/nature19365). Axel Timmermann and Tobias Friedrich of the University of Hawaii have attempted to simulate the overall dispersal of humans during the last 125 ka according to how they adapted to environmental conditions; mainly the changing vegetation cover as aridity varied geographically, together with the opening of potential routes out of Africa via the Straits of Bab el Mandab and through what is now termed the Middle East or Levant. They present their results as a remarkable series of global maps that suggest both the geographic spread of human migrants and how population density may have changed geographically through the last glacial cycle. Added to this are maps of the times of arrival of human populations across the world, according to a variety of migration scenarios. Note: the figure below estimates when AMH may have arrived in different areas and the population densities that environmental conditions at different times could have supported had they done so. Europe is shown as being possibly settled at around 70-75 ka, and perhaps having moderately high densities for AMH populations. Yet no physical evidence of European AMH is known before about 40 ka. Anatomically modern humans could have been in Europe before that time but failed to diffuse towards it, or were either repelled by or assimilated completely into its earlier Neanderthal population: perhaps the most controversial aspect of the paper.

timmermann

Estimated arrival time since the last continuous settlement of anatomically modern human migrants from Africa (top); estimated population densities around 60 thousand years ago. (Credit: Axel Timmermann University of Hawaii)

The role of climate change and even major volcanic activity – the 74 ka explosion of Toba in Indonesia – in both allowing or forcing an exodus from African homelands and channelling the human ‘line of march’ across Eurasia has been speculated on repeatedly. Now Timmermann and Friedrich have added a sophisticated case for episodic waves of migration across Arabia and the Levant at 106-94, 89-73, 59-47 and 45-29 ka. These implicate the role of Milankovich’s 21 ka cycle of Earth’s axial precession in opening windows of opportunity for both the exodus and movement through Eurasia; effectively like opening and closing valves for the flow of human movement. The paper is critically summarised by a Nature News and Views article (de Menocal, P.B. & Stringer, C. 2016. Climate and peopling of the world. Nature, doi:10.1038/nature19471.

This multiple-dispersal model for the spread of anatomically modern humans (AMH) finds some support from one of the genome papers (Pangani, L. and 98 others 2016. Genomic analyses inform on migration events during the peopling of Eurasia. Nature (online). http://dx.doi.org/10.1038/nature19792). A genetic signature in present-day Papuans suggests that at least 2% of their genome originates from an early and largely extinct expansion of AMH from Africa about 120 ka ago, compared with a split of all mainland Eurasians from African at around 75 ka. It appears from Pangani and co-workers’ analyses that later dispersals out of Africa contributed only a small amount of ancestry to Papuan individuals. The other two genome analyses (Mallick, S. and 79 others 2016. The Simons Genome Diversity Project: 300 genomes from 142 diverse populations. Nature (online) http://dx.doi.org/10.1038/nature18964; Malaspinas, A.-S. and 74 others 2016. A genomic history of Aboriginal Australia. Nature (online). http://dx.doi.org/10.1038/nature18299) suggest a slightly different scenario, that all present-day non-Africans branched from a single ancestral population. In the case of Malaspinas et al. an immediate separation of two waves of AMH migrants led to settlement of Australasia in one case and to the rest of Mainland Eurasia. Yet their data suggest that Australasians diverged into Papuan and Australian population between 25-40 ka ago. Now that is a surprise, because during the lead-up to the last glacial maximum at around 20 ka, sea level dropped to levels that unified the exposed surfaces of Papua and Australia, making it possible to walk from one to the other. These authors appeal to a vast hypersaline lake in the emergent plains, which may have deterred crossing the land bridge. Mallick et al. see an early separation between migrants from Africa who separately populated the west and east of Eurasia, with possible separation of Papuans and Australians from the second group.  These authors also show that the rate at which Eurasians accumulated mutations was about 5% faster than happened among Africans. Interestingly, Mallick et al. addressed the vexed issue of the origin of the spurt in cultural, particularly artistic, creativity after 50 ka that characterizes Eurasian archaeology. Although their results do not rule out genetic changes outside Africa linked to cultural change, they commented as follows:

‘… however, genetics is not a creative force, and instead responds to selection pressures imposed by novel environmental conditions or lifestyles. Thus, our results provide evidence against a model in which one or a few mutations were responsible for the rapid developments in human behaviour in the last 50,000 years. Instead, changes in lifestyles due to cultural innovation or exposure to new environments are likely to have been driving forces behind the rapid transformations in human behaviour …’.

Variations in interpretation among the four papers undoubtedly stem from the very different analytical approaches to climate and genomic data sets, and variations within the individual sets of DNA samples. So it will probably be some time before theoretical studies of the drivers of migration and work on global human genomics and cultural development find themselves unified. And we await with interest the pooling of results from all the different genetics labs and agreement on a common data-mining approach.

Salt and Earth’s atmosphere

It is widely known that glacial ice contains a record of Earth’s changing atmospheric composition in the form of bubbles trapped when the ice formed. That is fine for investigations going back about a million years, in particular those that deal with past climate change. Obviously going back to the composition of air tens or hundreds of million years ago cannot use such a handy, direct source of data, but has relied on a range of indirect proxies. These include the number of pores or stomata on fossil plant leaves for CO2, variations in sulfur isotopes for oxygen content and so on. Variation over time of the atmosphere’s content of oxygen has vexed geoscientists a great deal, partly because it has probably been tied to biological evolution: forming by some kind of oxygenic photosynthesis and being essential for the rise to dominance of eukaryotic animals such as ourselves. Its presence or absence also has had a large bearing on weathering and the associated dissolution or precipitation of a variety of elements, predominantly iron. Despite progressively more clever proxies to indicate the presence of oxygen, and intricate geochemical theory through which its former concentration can be modelled, the lack of an opportunity to calibrate any of the models has been a source of deep frustration and acrimony among researchers.

Yet as is often said, there are more ways of getting rid of cats than drowning them in butter. The search has been on for materials that trap air in much the same way as does ice, and one popular, if elusive target has been the bubbles in crystals of evaporite minerals. The trouble is that most halite deposits formed by precipitation of NaCl from highly concentrated brines in evaporating lakes or restricted marine inlets. As a result the bubbles contain liquids that do a grand job of preserving aqueous geochemistry but leave a lot of doubt as regards the provenance of gases trapped within them. For that to be a sample of air rather than gases once dissolved in trapped liquid, the salt needs to have crystallized above the water surface. That may be possible if salt forms from brines so dense that crystals are able to float, or perhaps where minerals such as gypsum form as soil moisture is drawn upwards by capillary action to form ‘desert roses’. A multinational team, led by Nigel Blamey of Brock University in Canada, has published results from Neoproterozoic halite whose chevron-like crystals suggest subaerial formation (Blamey, N.J.F. and 7 others, 2016. Paradigm shift in determining Neoproterozoic atmospheric oxygen. Geology, v. 44, p. 651-654). Multiple analyses of five halite samples from an ~815 Ma-old horizon in a drill core from the Neoproterozoic Canning Basin of Western Australia contained about 11% by volume of oxygen, compared with 25% from Cretaceous salt from China, 20% of late-Miocene age from Italy, and 19 to 22% from samples modern salt of the same type.

Salar de Atacama salt flat in the Chilean puna

Evaporite salts in the Salar de Atacama Chile (credit: Wikipedia)

Although the Neoproterozoic result is only about half that present in modern air, it contradicts results that stem from proxy approaches, which suggest a significant rise in atmospheric oxygenation from 2 to about 18% during the younger Cryogenian and Ediacaran Periods of the Neoproterozoic, when marine animal life made explosive developments at the time of repeated Snowball Earth events. Whether or not this approach can be extended back to the Great Oxygenation Event at around 2.3 Ga ago and before depends on finding evaporite minerals that fit stringent criteria for having formed at the surface: older deposits are known even from the Archaean.