Category Archives: Climate change and palaeoclimatology

Evidence for North Atlantic current shut-down ~120 ka ago

Gulf stream map

Warming surface currents of the North Atlantic (credit: Wikipedia)

A stupendous amount of heat is shifted by ocean-surface currents, so they have a major influence over regional climates. But they are just part of ocean circulation systems, the other being the movement of water in the deep ocean basins. One driver of this world-encompassing system is water density; a function of its temperature and salinity. Cold saline water forming at the surface tends to sink, the volume that does being replaced by surface flow towards the site of sinking: effectively, cold downwellings ‘drag’ major surface currents along. This is especially striking in the North Atlantic where sinking cold brines are focused in narrow zones between Canada and Greenland and between Greenland and Iceland. From there the cold water flows southwards towards the South Atlantic at depths between 1 and 5 km. The northward compensating surface flow, largely from tropical seas of the Caribbean, is the Gulf Stream/North Atlantic Current whose warming influence on climate of western and north-western Europe extends into the Arctic Ocean.

Circulation in the Atlantic Ocean. the orange and red water masses are those of the Gulf stream and North Atlantic Deep Water (credit: Science,  Figure 1 in Galaasen et al. 2014)

Circulation in the Atlantic Ocean. the orange and red water masses are those of the Gulf stream and North Atlantic Deep Water (credit: Science, Figure 1 in Galaasen et al. 2014)

 

Since the discovery of this top-to-bottom ‘conveyor system’ of ocean circulation oceanographers and climatologists have suspected that sudden climate shifts around the North Atlantic, such as the millennial Dansgaard-Oeschger events recorded in the Greenland ice cores, may have been forced by circulation changes. The return to almost full glacial conditions during the Younger Dryas, while global climate was warming towards the interglacial conditions of the Holocene and present day, has been attributed to huge volumes of meltwater from the North American ice sheet entering the North Atlantic. By reducing surface salinity and density the deluge slowed or shut down the ‘conveyor’ for over a thousand years, thereby drastically cooling regional climate. Such drastic and potentially devastating events for humans in the region seem not to have occurred during the 11.5 thousand years since the end of the Younger Dryas. Yet their suspected cause, increased freshwater influx into the North Atlantic, continues with melting of the Greenland ice cap and reduction of the permanent sea-ice cover of the Arctic Ocean, particularly accelerated by global warming.

 

The Holocene interglacial has not yet come to completion, so checking what could happen in the North Atlantic region depends on studying previous interglacials, especially the previous one – the Eemian – from 130 to 114 ka. Unfortunately the high-resolution climate records from Greenland ice cores do not extend that far back. On top of that, more lengthy sea-floor sediment cores rarely have the time resolution to show detailed records, unless, that is, sediment accumulated quickly on the deep sea bed. One place that seems to have happened is just south of Greenland. Cores from there have been re-examined with an eye to charting the change in deep water temperature from unusually thick sediment sequences spanning the Eemian interglacial (Galaasen, E.V. and 7 others 2014. Rapid reductions in North Atlantic Deep Water during the peak of the last interglacial period. Science, v. 343, 1129-1132).

 

The approach taken by the consortium of scientiosts from Norway, the US, France and Britain was to analyse the carbon-isotope composition of the shells of foraminifers that lived in the very cold water of the ocean floor during the Eemian. The ratio of 13C to 12C, expressed as δ13C, fluctuates according to the isotopic composition of the water in which the forams lived. What show up in the 130-114 ka period are several major but short-lived falls in δ13C from the general level of what would then have been North Atlantic Deep Water (NADW). It seems that five times during the Eemian the flow of NADW slowed and perhaps stopped for periods of the order of a few hundred years. If so, then the warming influence of the Gulf Stream and North Atlantic Current would inevitably have waned through the same intervals. Confirmation of that comes from records of surface dwelling forams. This revelation should come as a warning: if purely natural shifts in currents and climate were able to perturb what had been assumed previously to be stable conditions during the last interglacial, what might anthropogenic warming do in the next century?

 

 

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Did ice-age climate changes across Europe happen at the same time?

Although the frigid conditions at the last glacial maximum, around 19 to 20 thousand years ago, gradually relinquished their grip through slow global warming, this amelioration came to sudden stop around 12 800 years before the present. Northern hemisphere ice-core and other climate records show that there was a return to glacial conditions over a period of a few decades at most, to launch what is known as the Younger Dryas stadial that lasted over a thousand years until about 11 500 years ago, with the onset of the warm, climatically more stable Holocene that launched the transformation of the human way of life. The start of the Younger Dryas had dramatic effects throughout the northern hemisphere, the cold conditions emerging suddenly from an immense oceanographic change; a weakening or the halt of the North Atlantic thermohaline circulation in which cold, very salty surface waters at the fringe of the Arctic Ocean sink to drag warmer water to high latitudes. In short, the Gulf Stream slowed or stopped its warming influence at high northern latitudes.  Current thoughts centre on a freshening of surface sea water following the collapse of the North American ice sheet to gush meltwater and icebergs into the North Atlantic to buoy-up surface waters.

Iceage time 18kyr

Major climate shifts in Europe since 18 ka (credit: Wikipedia)

Most of the data about this climatic shock can only be dated accurately to within a few centuries: it is clear that the initial cooling was very rapid, on the scale of a few years, as was the warming that closed the Younger Dryas and marked the start of the Holocene, but the ‘when’ is known only to within a few hundred years. To resolve the start and stop ages needs records that include several indicators: clear signs of the beginning and end of the episode, an accurate means of dating them and confirmation from other sites, which presupposes a cast-iron means of correlating the records over large distances. The most reliable markers for correlation are volcanic ashes that can be dated radiometrically and which drift on the wind to be deposited over very large areas. If sedimentary sequences that accumulated continuously preserve such ashes, contain clear signs of climatic change and clearly record the passage of time in great detail, there is a chance of resolving climatic events very accurately; but they are no common.  A British-German team have located and analysed two such promising sites (Land, C.S. et al. 2013. Volcanic ash reveals time transgressive abrupt climate change during the Younger Dryas. Geology, v. 41, p. 1251-1254). One of them is from the bed of a lake that formed by a single volcanic eruption (Meerfelder Maar) in the Eifel region of western Germany. Quiet sediment accumulation has occurred there continuously to form very narrow, alternating dark and light layers, the variegation being due to sedimentation under ice in winter and open water in summer respectively. Twelve thousand of these annual varves provide a means of dating potentially with a precision of ± 1 year, but calibration to absolute time is necessary. The maar sediments contain three ash layers, two of which are from small local eruptions; the older having an age of 12 900 years before 2000 AD, the other being 11 000 years old, showing that the entire Younger Dryas is spanned by the Meerfelder Maar sediments. The third was dated by varve counting, showing the eruption had taken place 12 140 years ago. That age coincides closely with that of major eruption in Iceland.

Panorama Weinfelder Maar oder Totenmaar, Eifel

A typical volcanic maar in Eifel Region of Germany (credit: Wikipedia)

One prominent climatic feature of the Younger Dryas of Europe is a shift around halfway through: it started with the fiercest cold and then ameliorated. This change shows up in the Meerfelder Maar record as a reduction in mean varve thickness and an increase in the titanium content of the clays, the latter taking place in about a year (12 250 years ago) some 100 years before the Icelandic ash was deposited. The same kind of change occurs in records from lakes as far north as the Arctic Circle. One of the core records from Kråkenes in Northern Norway also contains the tell-tale Icelandic ash (as do ice cores from Greenland), but in its case it occurs 20 years before the abrupt climate shift. This clearly shows that major climate changes at the end of the last ice age occur at different times from place to place. The authors ascribe the 120 year difference between the two records to the times when prevailing, warm westerly winds began to affect central and northern Europe, linked to a gradual northward migration of the polar front. The data from both lakes also suggest that the Younger Dryas ended about 20 years earlier in Norway than in Germany, although Lane et al. do not comment..

Hitherto, correlation between climate records has been based on an assumption that major climate changes were at the same time, so that climate proxies such those discussed here have been ‘wiggle-matched’. Quite probably a lot of subtleties have thereby been missed.

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Greening the Earth, Devonian forest fires and a mass extinction

Land plants begin to appear in the fossil record as early as the late Ordovician (~450 Ma), show signs of diversification during the Silurian and by the end of the Devonian Period most of the basic features of plants are apparent. During the Carboniferous Period terrestrial biomass became so high as to cause a fall in atmospheric carbon dioxide, triggering the longest period of glaciation of the Phanerozoic, and such a boost to oxygen in the air (to over 30%) that insects, huge by modern standards, were able to thrive and the risk of conflagration was perhaps at its highest in Earth’s history. Yet surprisingly, the first signs of massive forest fires appear in the Devonian when vegetation was nowhere near so widespread and luxuriant as it became in the Carboniferous (Kaiho, K. et al. 2013. A forest fire and soil erosion event during the Late Devonian mass extinction. Palaeogeography, Palaeoclimatology, Palaeoecology, v. 392, p. 272-280). Moreover, Devonian oxygen levels were well below those of the present atmosphere and CO2 was more than 10 times even the post-industrial concentration (387 parts per million in 2013). Such atmospheric chemistry would probably have suppressed burning.

Kunio Kaiho of Tohoku University in Japan and colleagues from Japan, the US and Belgium analysed organic molecules in Belgian marine sediments from the time of the late-Devonian mass extinction (around the Frasnian-Famennian boundary at 372 Ma). A range of compounds produced by hydrocarbon combustion show marked ‘spikes’ at the F-F boundary. The thin bed that marks the extinction boundary also shows sudden increase then decrease in δ13C and total organic carbon, indicative of increase burial of organic material and a likely increase in atmospheric oxygen levels. Another biomarker that is a proxy for soil erosion follows the other biogeochemical markers, perhaps signifying less of a binding effect on soil by plant colonisation: a likely consequence of large widlfires. Unlike the biomarkers, magnetic susceptibility of the boundary sediments is lower than in earlier and later sediments. This is ascribed to a decreased supply of detrital sediment to the Belgian marine Devonian basin, probably as a result of markedly decreased rainfall around the time of the late-Devonian mass extinction. But the magnetic data from 3 metres either side of the boundary also reveal the influence of the 20, 40, 100 and 405 ka Milankovich cycles.

Juan Ricardo Cortes , a placoderm from the Dev...

Dunkleosteus, a giant (10 m long) placoderm fish from the Devonian, which became extinct in the late Devonian along with all other placoderms (credit: Wikipedia)

This set of environmentally-related data encourages the authors to suggest a novel, if not entirely plausible, mechanism for mass extinction related to astronomically modulated dry-moist climate changes that repeatedly killed off vegetation so that dry woody matter could accumulate en masse during the Frasnian while atmospheric oxygen levels were too low for combustion. A mass burial of organic carbon at the end of that Age then boosted oxygen levels above the burning threshold to create widespread conflagration once the wood pile was set ablaze. Makes a change from continental flood basalts and extraterrestrial impacts… Yet it was about this time that vertebrates took it upon themselves to avail themselves of the new ecological niche provided by vegetation to haul themselves onto land.

Earth’s first major glacial epochs

The global glaciations of the Neoproterozoic that reached low latitudes – the so-called ‘Snowball Earth’ events have dominated accounts of ancient glaciations since the start of the 21st century. Yet they are not the oldest examples of large-scale effects of continental ice sheets. Distinctive tillites or diamictites that contain large clasts of diverse, exotic rocks occur in sedimentary sequences of Archaean and Palaeoproterozoic age. The oldest are dated at around 2.9 Ma in the Pongola Supergroup of Swaziland, South Africa and formed at an estimated palaeolatitude of 48°; within the range of the equatorward extent of Pleistocene ice sheets. No evidence has turned up for glaciation of that age in other regions, and therefore for a ‘Snowball Earth’ at that time. The surprise is not the antiquity of the Pongola glaciation but the fact that tillites formed by glaciers are not more common in the early part of geological history. The sun has increased in its warming effect since the Earth formed so that the very absence of glaciations over huge spans of early Precambrian time points strongly towards an early atmosphere far richer in greenhouse gases than it is now.

Evidence for Palaeoproterozoic glaciation is more widespread, important tillites occurring in the Great Lakes region of North America and in the Transvaal and Griqualand regions of South Africa. Those of South Africa formed at a latitude of around 10°, suggesting ‘Snowball’ conditions, and in each region there are multiple tillites in the stratigraphic column. Accurate dating of volcanic ash horizons in the sequences of both areas (Rasmussen, B. et al. 2013. Correlation of Paleoproterozoic glaciations based on U-Pb zircon ages for tuff beds in the Transvaal and Huronian Supergroups. Earth and Planetary Science Letters, v. 382, p. 173-180) has made it possible to correlate three glacial deposits precisely between the two now widely separated areas. The dating also reveals that four glacial events occurred over a period of 200 Ma between 2.45 and 2.22 billion years ago: longer than the duration of the Mesozoic Era of the Phanerozoic and about the same as the time span during which 3 or 4  ‘Snowball’ events plastered the planet with ice in the Cryogenian and Ediacaran Periods of the Neoproterozoic.

Diamictite from the Palaoproterozoic Gowganda Formation in Ontario Canada (credit: Candian Sedimentology Research Group)

Diamictite from the Palaeoproterozoic Gowganda Formation in Ontario Canada (credit: Canadian Sedimentology Research Group)

This episode of the first large-scale glaciations neatly brackets the first appearance of significant amounts of oxygen in the Earth’s atmosphere during the Great Oxidation Event from 2.45 to 2.2 Ga. It is hard to avoid the conclusion that the two were connected as an increase in oxygen in the air must have influenced the concentration of greenhouse gases, especially that of methane, the most powerful of several that delay loss of heat to space by radiation from the surface. Once oxygen production by photosynthetic organisms exceeded a threshold atmospheric methane would very rapidly have been oxidized away to CO2 plus water vapour, leaving excess oxygen in the air to prevent the build-up of methane thereafter as is the case nowadays. But what pushed atmospheric composition beyond that threshold? A key piece of evidence lies in the record of different carbon isotopes in seawater of those times, which emerges from their study in Precambrian limestones.

After the end of the Archaean Eon at 2.5 Ga the proportion of marine 13C to 12C increased dramatically. Its accepted measure (δ13C) changed rapidly from the near-zero values that had previously characterised the Archaean to more than 10; an inflated value that lingered for much of the half-billion years that spanned the Great Oxidation Event and the Palaeoproterozoic glaciations (Martin, A.P et al. 2013. A review of temporal constraints for the Palaeoproterozoic large, positive carbonate carbon isotope excursion (the Lomagundi–Jatuli Event). Earth-Science Reviews, v. 127, p. 242-261). Later times saw δ13C return to hovering between slightly negative and slightly positive values either side of zero until the Neoproterozoic when once more ‘spikes’ affected the C-isotope record during the period of the better known ‘Snowball’ events. What lay behind this very broad carbon-isotope anomaly?

To increase 13C at the expense of 12C requires to removal from seawater of very large amounts of the lighter isotope. The only likely mechanism is the prolonged and permanent burial of masses of organic material, the only substances that selectively take up 12C. In turn, that implies a huge increase in biological productivity and its efficient burial without being oxidised to CO2 plus water. There are three possibilities: oxygen was absent from the ocean floor; sedimentation was too fast for oxidising bacteria to keep pace or such bacteria did not evolve until the end of the Lomagundi–Jatuli Event. It seems likely that such a dramatic change in the biosphere may have marked some fundamental shift in biological evolution not long after the close of the Archaean. Whichever, the biosphere somehow increased its capacity to generate oxygen. Since oxygen is anathema to many kinds of anaerobic bacteria and archaea, probably the only kinds of organism at the outset of these events, it is possible to imagine continual extinctions yet to maintain high biological productivity new organisms may have emerged to replace those that vanished. By 2.0 Ma, the first putative eukaryote cells (those with nuclei and a variety of organelles) had appeared.

New approach to the Milankovitch mystery

Melting pond on the ice sheet

Melting pond on the Greenland ice sheet (credit: Photo by Leif Taurer)

Milutin Milankovitch’s astronomical theory to account for glacial – interglacial cycles is based on 3 gravitational influences on the Earth that change the way it spins and orbits the Sun. Each is cyclic but with different periods: the angle of axial tilt every 41 ka; precession of its rotation axis on a 23 ka pacing; the change in shape of the orbit around the Sun over 100 ka. Each subtly affects the amount of solar energy, their influences combining to produce a seemingly complex, but predictable variation through time of solar heating for any point on the Earth’s surface. Milankovitch’s work was triumphantly confirmed when analysis of oxygen-isotope time series from sea-floor sediments revealed precisely these periods in the record of continental ice cover. Specifically, astronomical pacing of midsummer insolation at 65°N matches the real climatic pattern through time.

Yet the periods between glacial maxima have not stayed constant over the last 2 Ma or so (Figure showing Phanerozoic climate changes). About 0.8 to 1 Ma ago a sequence with roughly 41 ka spacing was replaced by another about every 100 ka, i.e. both overall climate periods matched one of the astronomical forcings. What is a puzzle is that the current periodicity seems to follow the very weakest influence in energy terms; that from orbital eccentricity. The energy shifts from changes in orbit shape are, in fact, far too weak to drive the accumulation and eventual melting of ice sheets on land. Climatologists have suggested a variety of processes that might be paced by eccentricity but which act to amplify is climatic ‘signal’. None have been especially convincing.

In an attempt to resolve the mystery Ayako Abe-Ouchi of the University of Tokyo and Japanese, US and Swiss colleagues linked a climate model driven by Milankovitch insolation and variations in CO2 recorded in an Antarctic ice core with a model of how land-ice forms and interacts with the underlying lithosphere (Abe-Ouchi, A. et  al. 2013. Insolation-driven 100,000-year glacial cycles and hysteresis of ice-sheet volume. Nature, v. 500, p. 190-193).

CLIMAP map of ice sheets, sea temperature chan...

Map of ice sheets, sea temperature changes, and changes in the outline of coastal regions during the last glacial maximum. (credit: Wikipedia)

Their key discovery is that the ice-sheets that repeatedly formed on the Canadian Shield and extended further south than Chicago had such a huge mass that they changed the shape of the land surface beneath them so much it had an effect on climate as a whole. The reason for this is that glacial loading forces the lithosphere down by displacing the more ductile asthenosphere sideways. But when melting begins rebound of the rock surface lags a long time behind the shrinking ice volume – well displayed today in Britain and Scandinavia by continued rise of the land to form raised beaches. In the case of the North American ice sheet, what had become an enormous ice bulge at glacial maxima developed into a huge basin up to 1 km deep as the ice began to melt. Simply by virtue of its low elevation this sub-continental basin would have warmed up more and more rapidly as the ice-surface fell because of this ‘isostatic’ lag.

Another feature to emerge from the model was the interaction between the 100 ka eccentricity ‘signal’ and that of precession at 23 ka. For long periods that kept summer temperature low enough for snow to pile up and become glacial ice, but on a roughly 100 ka time scale both acted together to increase summer temperatures at high northern latitudes. Melting that instantaneously removed some ice load each summer brought into play the sluggish isostatic  response that helped even more warming the following year. As well as convincingly accounting for the 100 ka mystery, the model explains the far more rapid deglaciations in that mode than in the preceding 41 ka cycles, which were almost symmetrical compared with the more recent slow accumulation of continental ice sheets over ~90 ka followed by almost complete melting in a mere 10 ka.

If true, the model seems to imply that before 800 ka the positions, thicknesses and extents of continental ice sheets were different from those in later times. Or perhaps it reflects a steady increase in the overall volume of ice being produced over northern North America, or that glacial erosion thinned the crust until changing isostatic influences could ‘trip’ sufficient additional warming.

Arctic climate in the run-up to the Great Ice Age

Around 3.6 Ma ago a large extraterrestrial projectile slammed into the far north-east of Siberia forming crater 16 km across. The depression soon filled with water to form Lake El’gygytgyn, on whose bed sediments have accumulated up to the present. A major impact close to the supposed start of Northern Hemisphere glacial conditions was a tempting target for coring: possibly two birds with one stone as the lowest sediments would probably be impact debris and boreal lake sediments of this age are as rare as hens’ teeth. The sedimentary record of Lake El’gygytgyn has proved to be a climate-change treasure trove (Brigham-Grette, J and 15 others 2013. Pliocene warmth, polar amplification, and stepped Pleistocene cooling recorded in NE Arctic Russia. Science, v. 340, p. 1421-1426).

El'gygytgyn, Russia, is a impact crater with a...

Lake El’gygytgyn impact crater. (credit: Wikipedia)

The team of US, Russian, German and Swedish scientists discovered that the sedimentary record was complete over a depth of 318 m and so promised a high resolution climate record. The striking feature of the sediments is that they show cyclical variation between five different facies, four of which are laminated and so preserve intricate records of varying weathering and sediment delivery to the lake. The sediments also contain pollens and diatom fossils, and yield good magnetic polarity data. The last show up periods of reversed geomagnetic polarity, which provide age calibration independent of relative correlation with marine isotope records.

A host of climate-related proxies, including pollen from diverse tree and shrub genera, variations in silica due to changes in diatom populations and organic carbon content in the cyclically  changing sedimentary facies are correlated with global climate records based on marine-sediment stable isotope. These records reveal intricate oscillations between cool mixed forest, cool coniferous forest, taiga  and cold deciduous forest, with occasional frigid tundra conditions through the mid- to late Pliocene. Compared with modern conditions NE Siberia was much warmer and wetter at the start of the record. Around the start of the Pleistocene sudden declines to cooler and drier conditions appear, although until 2.2 Ma ago average summer conditions seem to have been higher that at present, despite evidence from marine proxies of the onset of glacial-interglacial cycles in the Northern Hemisphere.

In detail, Lake El’gygytgyn revealed some surprises including rapid onset of a lengthy cold-dry spell of tundra conditions between 3.31 to 3.28 Ma. The first signs that the lake was perennially frozen appear around 2.6 Ma, well before evidence for the first continental glaciation in North America, presaged by signs around 2.7 Ma that winters consistently became colder than present ones. Overall the lake record presents a picture of a stepped shift in climate in the run-up to the Great Ice Age. Lake El’gygytgyn seems set to become the standard against which other, more patchy records around the Arctic Ocean are matched and correlated. Indeed it is the longest and most detailed record of climate for the Earth’s land surface, compared with 120 and 800 ka for the Greenland and Antarctic ice-caps.

Modelling their findings against likely atmospheric CO2 levels the authors provide grist to the media mill which focuses on how the late Pliocene may be a model for a future warm Earth if emissions are not curtailed, with visions of dense polar forests

Yes, it was hot during the Permian

For those of us living in what was the heart of Pangaea – Europe and North America – more than 250 Ma ago this item’s title might seem like the ultimate truism. However, despite our vision of desert dune sands and evaporating inland seas, glaciation blanketed much of the Gondwana part of the supercontinent until the Middle Permian then lying athwart the South Pole. That would go a long way to accounting for extreme dryness at low to mid-latitudes, especially in the deep interior of Pangaea, but just how hot might tropical climates have been? The deglaciation of Gondwana was abrupt and has been touted as an analogue for a possible anthropogenic closure to the Cenozoic glacial epoch that began around 34 Ma in Antarctica and has periodically gripped land at northern latitudes as low as 40°N for the last 2.5 Ma. Since the present distribution of continents is totally different from the unique pole-to-pole shape of Pangaea, that is probably a view that is not widely held by palaeoclimatologists. Nonetheless, getting hard data on Permian conditions has an intrinsic interest for most geoscientists.

The bottom of Death Valley, USA

Playa lake in Death Valley, USA (credit: Wikipedia)

One of the best ways of measuring past temperatures, whether surficial or deep within the crust, almost directly is based on fluids trapped within minerals formed at the time of interest. In Permian strata there is no shortage of suitable material in the form of evaporite minerals, especially common salt or halite.  A distinctive chevron-like texture develops in halite that forms at the water-atmosphere interface in playa lakes that dry out every year. When thin sections of samples that contain fluid inclusions are slowly heated the air bubbles trapped in salt during crystallisation gradually homogenise with the other trapped fluids. Based on samples that have formed at the present day under a range of air temperatures, the temperature of homogenisation indicates the prevailing air temperature accurately. So well, in fact, that it is possible to assess diurnal temperature variations in suitable halite crystals.

Results have been obtained from Middle Permian halites in Kansas, USA (Zambito, J.J. & Benison, K.C. 2013. Extremely high temperatures and paleoclimate trends recorded in Permian ephemeral lake halite. Geology, v. 41, p. 587-590). In part of the section studied air temperatures reached 73°C, compared with a modern maximum of 57°C recorded in halites from the playas of Death Valley. Moreover, they exhibit changes of more than 30°C during daily cycles. But that kind of weather is common in other hot dry areas today, such as the Dasht-e Lut in eastern Iran. Also, the full data show crystallisation at lower temperatures (maxima of 30-40°C) in part of the sequence. What is noteworthy is that these data are the first quantitative indicators of weather before the last 2.5 Ma. Since evaporites extend back into the Precambrian, the method will undoubtedly extend accuracy and precision to paleoclimate  where only proxies and a modicum of guesswork were previously available.

Tiny shrinking horses

English: This reproduction of a painting of an...

Reconstruction of Sifrhippus. Image via Wikipedia

The earliest known ancestors of modern horses occur in Palaeogene mammal-rich terrestrial sediments of the northwestern US, particularly those of the Wind and Bighorn Basins. The first with clear horse-like features was Sifrhippus (formely Eohippus, or Hyracotherium), but famously it had four hoofed toes and was about the size of a household cat. Subsequent development to a single load-bearing toe has long formed one of the classic cases for evolution. Sifrhippus lived at the end of the Palaeocene. From the large numbers of well-preserved skeletons, this was a herding animal. The large numbers of fossils have also made it a candidate for testing a hypothesis that individuals of a mammal and bird species become smaller as climate warms: Bergmann’s Rule. The background to this view is that in modern warm-blooded or endothermic animal species individuals tend to be smaller the closer they are to the Equator.

The end of the Palaeocene was marked by a now well-documented rise in global surface temperature that left a marked sign of increased 13C in sediments spanning the Palaeocene-Eocene boundary, which is widely believed to have resulted from massive exhalations of methane from the seafloor. Bergmann’s Rule arose because there appears to be a general decrease in size of most mammal fossils through the P-E Thermal Maximum.  Sifrhippus lived through the event and indeed did undergo 30% decrease in size at the start of the carbon-isotope shift marking the PETM. Moreover, after the isotopic excursion its fossils indicate a 70% increase in size (Secord, R. and 8 others 2012. Evolution of the earliest horses driven by climate change in the Paleocene-Eocene Thermal Maximum. Science, v. 335, p. 959-962).

The study was of Sifrhippus and other mammals over a period representing several thousand generations. It broke new ground in two ways: it used the size of the horses’ teeth to estimate body mass, and teeth of a variety of mammals afforded systematic measurements of both carbon and oxygen isotopes. The carbon isotopic analyses pin-pointed the span of the PETM locally, while oxygen isotopes charted local changes in average temperature. The results show remarkable coherence with Bergmann’s Rule, but reveal other interesting aspects of the PETM in North America. Oxygen-isotope in the teeth of different mammal species give some idea of their diet and habitat. Sifrhippus shows the highest enrichment of 18O in its teeth, which suggests that it ate leaves from which water evaporation selectively removed the lighter 16O, i.e. in open, dry areas. Another ubiquitous fossil, Coryphodon, consistently has lower 18O than other mammals, signifying that it was water-loviong and ate aquatic plants, i.e. not subject to evaporation. Matching O-isotopes for the two species across the PETM shows a greater shift in 18O for Sifrhippus than for Coryphodon, which suggests that hidden in the O-isotope record of temperature is information about rainfall variations during the PETM. To further support Bergmann’s Rule, changes in the size of Sifrhippus, do not correlate with the aridity index. So it seem that heat alone was responsible for dwarfing – the other possibility considered by the researchers was that decreased availability or quality of diet could have been responsible.

Coryphodon

Reconstruction of Coryphodon. Image via Wikipedia

Dust: heating or cooling?

In the left image, thin martian clouds are vis...

Mars: with and without dust storms in 2001. Image via Wikipedia

Once every 13 years on average dust blots out most of the surface of Mars turning it into an orange ball. The last such planet-encircling dust storm occurred in 2001, but lesser storms spring up on a seasonal basis. Yet Martian seasons have very different weather from terrestrial ones because of the greater eccentricity of Mars’s orbit, as well as the fact that its ‘weather’ doesn’t involve water. When Mars is closest to the Sun solar heating is 20% greater than the average, for both hemispheres. The approach to that perihelion marks the start of the dust season which last a half the Martian year. Unsurprisingly, the sedimentary process that dominates Mars nowadays is the whipping up and deposition of sand and dust, though in the distant past catastrophic floods – probably when subsurface ice melted – sculpted a volcanic landscape pockmarked with impact craters up to several thousand kilometres across. Waterlain sediments on early Mars filled, at least in part, many of the earlier craters and probably blanketed the bulk of its northern hemisphere that is the lowest part of the planet and now devoid of large craters. Erosion and sedimentation since that eventful first billion years has largely been aeolian. Some areas having spectacular dunes of many shapes and sizes, whereas more rugged surfaces show streamlined linear ridges, or yardangs (http://earth-pages.co.uk/2011/05/08/winds-of-change/), formed by sand blasting. Most of the dust on Mars is raised by high winds in the thin atmosphere sweeping the great plains and basins, and, by virtue of Stokes’s law, the grains are very much smaller than on Earth.

The dustiest times on Earth, which might have blotted out sizeable areas from alien astronomers, in the last million years have been glacial maxima, roughly every 100 ka with the latest 20 ka ago. Layering in the Antarctic ice core records such dust-dominated frigid periods very precisely. Less intricate records formed away from the maximum extent of ice sheets as layers of fine sediment known as loess, whose thickness variations match other proxy records of palaeoclimate nicely. Loess, either in place or redeposited in alluvium by rivers, forms the most fertile soil known – when the climate is warm and moist. The vast cereal production of lowland China and the prairies of North America coincides with loess: it may seem strange but a large proportion of 7 billion living humans survive partly because of dust storms during glacial periods of the past.

Being derived from rock-forming minerals dust carries with it a diverse range of chemical elements, including a critical nutrient common on land but in short supply in ocean water far offshore: iron in the form of oxide and hydroxide coatings on dust particles – the dust coating your car after rain often has a yellow or pinkish hue because of its iron content. Even when the well-known ‘fertilizer’ elements potassium, nitrogen and phosphorus are abundant in surface ocean water, they can not encourage algal phytoplankton to multiply without iron. Today the most remote parts of the oceans have little living in their surface layers because of this iron deficiency. Yet oceanographers and climatologists are pretty sure that this wasn’t always the case. They are confident simply because reducing the amount of atmospheric carbon dioxide and its greenhouse effect to levels that would encourage climate cooling and glacial epochs needed more carbon to be buried on the ocean floors than happens nowadays, and lifeless ocean centres would not help in that.

Dust plume off the Sahara desert over the nort...

Saharan dust carried over the Atlantic Ocean by a tropical cyclone. Image via Wikipedia

At present, the greatest source of atmospheric dust is the Sahara Desert (bartholoet, J. 2012. Swept from Africa to the Sahara. Scientific American, v. 306 (February 2012), p. 34-39). Largely derived from palaeolakes dating from a Holocene pluvial episode, Saharan dust accounts for more than half the two billion metric tonnes of particulate atmospheric aerosols dispersed over the Earth each year. Located in the SE trade-wind belt, the Sahara vents dust clouds across the Atlantic Ocean, most to fall there and contribute dissolved material to the mid-ocean near-surface biome but an estimated 40 million t reaches the Amazon basin, contributing to fertilising the otherwise highly leached tropical rain-forest soils. While over the ocean the high albedo of dust adds a cooling effect to the otherwise absorbent sea surface. Over land the fine particles help nucleate water droplets in clouds and hence encourages rainfall. The climatic functions of clouds and dusts are probably the least known factors in the climatic system, a mere 5% uncertainty in their climatic forcing may mean the difference between unremitting global warming ahead or sufficient cooling by reflection of solar radiation to compensate for the cumulative effects of industrial CO2 emissions.

Recording amounts of dust from marine sediments quantitatively is very difficult and impossible in terrestrial sediments, but superb records tied accurately to time at annual precision exist in ice sheets. Low dust levels in Greenland and Antarctic ice tally well with the so-called ‘Medieval Climate Anomaly’ (a warm period) whereas through the 13th to 19th centuries (the ‘Little Ice Age’) more dust than average circulated in the atmosphere. Crucially, for climate change in the industrial era, there has been a massive spike in dust reaching near-polar latitudes since the close of the 18th century during the period associated with signs of global warming: a counterintuitive relationship, but one that is difficult to interpret. The additional dust may well be a result of massive changes in land use across the planet following industrialised agricultural practices and growing population. There are several  questions: does the additional dust also reflect global warming with which it is correlated, i.e. evaporation of the huge former lakes in the Sahara (e.g. Lake Chad); is the dust preventing additional greenhouse warming that would have taken place had the atmosphere been clearer; is it even the ‘wrong kind of dust’, which may well reflect short-wave solar radiation away but also absorbs the longer wavelength thermal radiation emitted by the Earth’s surface, i.e. an aerosol form of greenhouse warming. Needless to say, neither clouds nor dust can be factored into climate prediction models with much confidence.

Dust tied to climate

This TOMS image shows a record-setting Asian d...

Dust moving in April 2001 from arid areas in Central Asia and North Africa to the oceans. From NASA's Nimbus-7 satellite. Image via Wikipedia

At present the central areas of the oceans are wet deserts; too depleted in nutrients to support the photosynthesising base of a significant foot chain. Oddly, even when commonly known nutrients are brought to the ocean surface far from land by deep-sourced upwellings the effect on near-surface biomass is far from that expected. The key factor that is missing is dissolved divalent iron that acts as a minor nutrient for phytoplankton: even in deep ocean waters any such ferrous iron is quickly oxidised and precipitated as trivalent ferric compounds. One of the suggested means of geoengineering away any future climatic warming is to seed the far-off oceans reaches with soluble iron in the hope of triggering massive planktonic blooms, dead organisms sinking to be buried along with the their carbon content in the ocean-floor oozes. Retrospectively, it has been suggested that the slight mismatch between changes in atmospheric CO2 concentration and climate changes may be linked to fluctuating availability of iron dissolved from dust in ocean-surface waters, but so far that hypothesis has not been robustly tested. It is well known, however, that global cooling is accompanied by drying of continental climates and thereby an increase in the delivery of dust, even to polar ice caps where cores have shown dustiness to fluctuate with temperature.

Recently an ocean-floor sediment core from around 42° S has revealed a high-resolution record of the deposition of dust and iron at that location over the last 4 Ma (Martinez-Garcia, A. et al. 2011. Southern Ocean dust-climate coupling over the past 4 million years. Nature, v. 476, p. 312-315). In it one proxy for dust is the amount of organic compounds known as n-alkanes that are a major component of the waxes shed from plant leaves. Others are iron, titanium and thorium concentrations in the ooze. Dust proxies tally with land-ice volumes shown by the fluctuating d18O measured in bottom-dwelling foraminifera found as fossils in the core to form a convincing link between dust and climate over the Southern Ocean. Those proxies also match nicely the record of dust delivered to Antarctica that emerged from the 0.8 Ma Dome C ice core that was extracted and analysed by the EPICA consortium. The record shows boosts in iron and dust deposition at 2.7 Ma, when ice first took hold of northern high latitudes, and at 1.25 Ma when larger ice sheets began to develop and climate shifts switched to 100 ka cyclicity. Although the match between marine and glacial dust accumulation in the latter part of this mid-Pleistocene Transition is an important step forward in palaeoclimatology, it is a surprise that the new ocean-floor data is not plotted with the record of atmospheric CO2 in Antarctic ice bubbles: if there was a clear relationship that would have iced the cake.