Category Archives: Climate change and palaeoclimatology

Closure for the K-Pg extinction event?

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More on the Younger Dryas causal mechanism

Colour-coded subglacial topography from radar sounding over the Hiawatha Glacier of NW Greenland (Credit: Kjaer et al. 2018; Fig. 1D)

Read about new data from lake-bed sediments, which suggest that a major impact around 12.8 thousand years ago may have triggered a return to glacial conditions at the start of the Younger Dryas.


What followed the K-Pg extinction event?

Reconstruction of the 35 kg early Palaeocene mammal Taeniolabis (credit: Wikipedia)

Read about processes connected with the Chicxulub impact that may have influenced the K-Pg mass extinction and the evolution of mammalian survivors during the first million years of the Palaeocene, as revealed by a unique sedimentary sequence near Denver, Colorado, USA.

Chaos and the Palaeocene-Eocene thermal maximum

Read how chaotic behaviour in the Solar System may have affected Milankovich cycles in the late Palaeocene

Ediacaran glaciated surface in China

Read about a unique confirmation of Snowball Earth conditions during the Ediacaran Period at Earth-logs.

29 Ma old striated pavement beneath the Carboniferous Dwyka Tillite in South Africa (credit: M.J Hambrey)

Geochemical background to the Ediacaran explosion

The first clear and abundant signs of multicelled organisms appear in the geological record during the 635 to 541 Ma Ediacaran Period of the Neoproterozoic, named from the Ediacara Hills of South Australia where they were first discovered in the late 19thcentury. But it wasn’t until 1956, when schoolchildren fossicking in Charnwood Forest north of Leicester in Britain found similar body impressions in rocks that were clearly Precambrian age that it was realised the organism predated the Cambrian Explosion of life. Subsequently they have turned-up on all continents that preserve rocks of that age (see: Larging the Ediacaran, March 2011). The oldest of them, in the form of small discs, date back to about 610 Ma, while suspected embryos of multicelled eukaryotes are as old as the very start of the Edicaran (see; Precambrian bonanza for palaeoembryologists, August 2006).


Artist’s impression of the Ediacaran Fauna (credit: Science)

The Ediacaran fauna appeared soon after the Marinoan Snowball Earth glaciogenic sediments that lies at the top of the preceding Cryogenian Period (650-635 Ma), which began with far longer Sturtian glaciation (715-680 Ma). A lesser climatic event – the 580 Ma old Gaskiers glaciation – just preceded the full blooming of the Ediacaran fauna. Geologists have to go back 400 million years to find an earlier glacial epoch at the outset of the Palaeoproterozoic. Each of those Snowball Earth events was broadly associated with increased availability of molecular oxygen in seawater and the atmosphere. Of course, eukaryote life depends on oxygen. So, is there a connection between prolonged, severe climatic events and leaps in the history of life? It does look that way, but begs the question of how Snowball Earth events were themselves triggered.

There are now large amounts of geochemical data from Neoproterozoic sedimentary rocks that bear on processes in the atmosphere, seawater, continental crust and the biosphere of the time. Some are indicative of the reducing/oxidising (redox) potentials of ocean water in which various sediments were deposited. Carbon isotopes chart organic burial and the abundance of CO2 in the oceans and atmosphere. Strontium isotopes give details of the rates of continental erosion. The age statistics of zircon grains in sediments are useful; the proportion of zircons close in age to the time of sediment deposition relative to older grains is a proxy for the rate of continental-arc volcanism and thus for subduction rates. Joshua Williams of Britain’s University of Exeter and colleagues from the universities of Edinburgh and Leeds have used complex modelling to assess the pace at which oxygen was added to the surface environment through the Ediacaran Period (Williams, J.J. et al. 2019. A tectonically driven Ediacaran oxygenation eventNature Communications, v.  10 (1); DOI: 10.1038/s41467-019-10286-x).

They estimate a 50% increase in atmospheric oxygen during the Ediacaran to about 0.25 % of the present concentration, which would be sufficient to support large, mobile animals. They attribute this primarily to a boost in the supply of CO2 to the atmosphere as a result of increased volcanic activity. This would have warmed the surface environment so that exposed rock on the continents underwent accelerated chemical weathering. By freeing from continental crust increased amounts of nutrients, such as phosphorus and potassium, the boost to photosynthesis would have increased the oceanic biomass, thereby emitting oxygen. Multicelled animals would have been beneficiaries of such a transformation. The trend continued into the Cambrian, thereby unleashing the explosion of animals and their evolution that continued through the Phanerozoic. Ultimately, the trigger was increased Late-Neoproterozoic tectonic activity that drove the massive Pan-African orogeny and the accretion of the Gondwana supercontinent.

See also:

Note added, 26 June 2019: Roger Mason has referred me to the carbon-isotope record during the Ediacaran. It shows some of the stratigraphic record’s largest negative δ13C excursions in carbonate rocks (Tahata, M. and 10 others 2013. Carbon and oxygen isotope chemostratigraphies of the Yangtze platform, South China: Decoding temperature and environmental changes through the EdiacaranGondwana Research, v.23, p. 333-353; DOI: 10.1016/ Such isotopic excursions went on throughout the Ediacaran, along with sudden fossil appearances and disappearances – so-called ‘Strangelove’ oceans – plus fluctuations in sediment types and climate. The Ediacaran was a wild time in most respects.


Geochemical changes recorded in the complete Ediacaran sedimentary sequence of the Three Gorges of the Yangtze River, China (credit: Tahata et al. 2013; Fig. 4)

Soluble iron, black smokers and climate


Phytoplankton bloom in the Channel off SW England (Landsat image)

At present the central areas of the oceans are wet deserts; too depleted in nutrients to support the photosynthesising base of a significant food chain. The key factor that is missing is dissolved divalent iron that acts as a minor, but vital, nutrient for phytoplankton. Much of the soluble iron that does help stimulate plankton ‘blooms’ emanates from the land surface in wind blown dust (Palaeoclimatology September 2011) or dissolved in river water. A large potential source is from hydrothermal vents on the ocean floor, which emit seawater that has circulated through the basalts of the oceanic crust. Such fluids hydrate the iron-rich mafic minerals olivine and pyroxene, which makes iron available for transport. The fluids originate from water held in the muddy, organic-rich sediments that coat the ocean floor, and have lost any oxygen present in ocean-bottom water. Their chemistry is highly reducing and thereby retains soluble iron liberated by crustal alteration to emanate from hydrothermal vents. Because cold ocean-bottom waters are oxygenated by virtue of having sunk from the surface as part of thermohaline circulation, it does seem that ferrous iron should quickly be oxidised and precipitated as trivalent ferric compounds soon after hydrothermal fluids emerge. However, if some was able to rise to the surface it could fertilise shallow ocean water and participate in phytoplankton blooms, the sinking of dead organic matter then effectively burying carbon beneath the ocean floor; a ‘biological pump’ in the carbon cycle with a direct influence on climate. Until recently this hypothesis had little observational support.

The Southern Ocean surrounding Antarctica is iron-starved for the most part, but it does host huge phytoplankton blooms that are thought to play an important role in sequestration of CO2 from the atmosphere. Oceanographic research now benefits from semi-autonomous buoys set adrift in the deep ocean. The most sophisticated (Argo floats ) are able to dive to 2 km below the surface, measuring variations of physical and chemical conditions with depth for long periods. There are 4,000 of them, owned by several countries. Two of them drifted with surface currents across the line of the Southwest Indian Ridge through waters thought to be depleted in phytoplankton, despite having high nitrate, phosphate and silica contents – major ‘fertilisers’ in water. They showed up ‘spikes’ in chlorophyll concentrations in the upper levels of the Southern Ocean (Ardyna, M. and 11 others 2019. Hydrothermal vents trigger massive phytoplankton blooms in the Southern Ocean. Nature Communications, 5 June 2019, online; DOI: 10.1038/s41467-019-09973-6). Their location relative to a large cluster of hydrothermal vents on the Southwest Indian Ridge was ‘downstream’ of them in the circum-Antarctic Current, but remote from any known terrestrial source of iron (continental shelves, dust deposition melting sea ice). Earlier oceanographic surveys that detected anomalous helium isotope, typical of emanations from the mantle, show that hydrothermal-vent water moves through the two areas. Although the Argo floats are equipped for neither helium nor iron measurements, it is likely that the blooms benefitted from hydrothermal iron. Modelling of the likely current dispersion of material in the hydrothermal plumes also outlines a large area of ocean where iron fertilisation may encourage regular blooms where they would otherwise be highly unlikely. Unfortunately, the study does not include any direct evidence for elevated soluble iron.

One thing that the study does foster is renewed interest in deliberate iron-fertilisation of the oceans to speed up the ‘biological pump’ as a means of managing global warming (Boyd, P. & Vivian, C. 2019. Should we fertilize oceans or seed clouds? No one knows. Nature, v. 570, p. 155-157; doi: 10.1038/d41586-019-01790-7). Small scale pilots of such ‘geoengineering’ have been tried, but raised outcries from environmental groups. Other than detecting, or hinting at, soluble iron from a deep natural source, scientific research has provided scanty evidence of what iron-seeding at the surface might do. There could be unexpected consequences, such as methane emission from decay of the blooms – a worse greenhouse gas than carbon dioxide.

See also: An iron age for climate engineering? (Palaeoclimatology, July 2007); Dust in the wind: North Pacific Ocean fertilized by iron in Asian dust ( National Science Foundation 2019)

Anthropocene edging closer to being ‘official’

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The issue of erecting a new stratigraphic Epoch encompassing the time since humans had a global effect on the Earth System has irked me ever since the term emerged for discussion and resolution by the scientific community in 2000. An Epoch in a chronostratigraphic sense is one of several arbitrary units that encompass all the rocks formed during a defined interval of time. The last 541 million years (Ma) of geological time is defined as an Eon – the Phanerozoic. In turn that comprises three Eras – Palaeozoic, Mesozoic and Cenozoic. The third level of division is that of Periods, of which there are 11 that make up the Phanerozoic. In turn the Periods comprise a total of 38 fourth-level Epochs and 85 at the fifth tier of Ages. All of these are of global significance, and there are even finer local divisions that do not appear on the International Chronostratigraphic Chart . If you examine the Chart you will find that no currently agreed Epoch lasted less than 11.7 thousand years (the Holocene) and all the others spanned 1 Ma to tens of Ma (averaged at 14.2 Ma). Indeed, even Ages span a range from hundreds of thousands to millions of years (averaged at 6 Ma).


The Vattenfall lignite mine in Germany; the Anthropocene personified

In the 3rd week of May 2019 the 34-member Anthropocene Working Group (AWG) of the International Commission on Stratigraphy (ICS) sat down to decide on when the Anthropocene actually started. That date would be passed on up the hierarchy of the geoscientific community  eventually to meet the scrutiny of its highest body, the executive committee of the International Union of Geological Sciences, and either be ratified or not. In the meantime the AWG is seeking a site at which the lower boundary of the Anthropocene would be defined by the science’s equivalent of a ‘golden spike’; theGlobal boundary Stratotype Section and Point (GSSP).

Several options were tabled for discussion and decision, summarised by a 2015 paper in Nature. A case against the erection of an official Anthropocene Epoch on stratigraphic grounds appeared in a GSA Todaypaper in 2016. Despite the fact that there is evidence for the start of human geological, geochemical and biological influences as far back as 8 000 years ago (in effect the Holocene is the Epoch of rapid human growth and transformations), the 2015 paper concludes that there are two candidates for the base of the Anthropocene. The earliest is the decline in atmospheric CO2 that began around 1570 CE and its recovery around 1620 CE recorded in Greenland ice cores. This is suggested to mark a fall in the indigenous population of the Americas from ~60 to ~6 million that followed the completion of European conquest, as a result of genocide, disease and famine. Regeneration of the American forest lands (~5 x 107 hectare) that the dead had once occupied drew down CO2.  However this overlaps with the coolest part of the Little Ice Age which may also have resulted in absorption of the greenhouse gas by cooled ocean water. The beginning of the industrial revolution was discounted on the grounds that it was diachronous as well as being difficult to define, having arisen first in Europe at some time in the 18th century. The second candidate was the period when ~500 nuclear weapons were tested above-ground, beginning in 1945 and ending by treaty between the then nuclear powers in 1963. These distributed long-lived plutonium globally, which resides in sediments as a ‘spike’. Around 1963 there are also clear signs that plastics, aluminium, artificial fertilisers, concrete and lead from petrol began to increase in sediments. It is this last option upon which the AWG settled, with 29 members for and 5 against, and is to forward up the ‘chain of command’ in the geoscientific bureaucracy. A detailed and sometimes amusing account of the AWG’s deliberations appeared in the online Guardian newspaper on 30 May 2019.

The decision, in my opinion, signifies that the Anthropocene is an Epoch that includes the future, which is somewhat pessimistic as well as being scientific nonsense. Yet, coinciding as it does with rapidly escalating efforts, mainly by young people, to end massive threats to the Earth System, that can only be welcomed. It is an essentially political statement, albeit with a learned cloak thrown over it.  The only way to erase the exponentially growing human buttock print on our home world is for growth-dependent economics to be removed too. That is the only logical basis for the ‘green’ revolt that is unfolding. If that social revolution doesn’t happen, there will be a mass extinction to join the ‘Big Five’, and society in all its personifications will collapse. That is known as barbarism…


Younger Dryas impact trigger: evidence from Chile

Note: Earth-Pages will be closing as of early July, but will continue in another form at Earth-logs

A sudden collapse of global climate around 12.8 ka and equally brusque warming 11.5 ka ago is called the Younger Dryas. It brought the last ice age to an end. Because significant warming preceded this dramatic event palaeoclimatologists have pondered its cause since it came to their attention in the early 20th century as a stark signal in the pollen content of lake cores – Dyas octopetala, a tundra wild flower, then shed more pollen than before or afterwards; hence the name. A century on, two theories dominate: North Atlantic surface water was freshened by a glacial outburst flood that shut down the Gulf Stream [June 2006]; a large impact event shed sufficient dust to lower global temperatures [July 2007]. An oceanographic event remains the explanation of choice for many, whereas the evidence for an extraterrestrial cause – also suggested to have triggered megafaunal extinctions in North America – has its supporters and detractors. The first general reaction to the idea of an impact cause was the implausibility of the evidence [November 2010], yet the discovery by radar of a major impact crater beneath the Greenland ice cap [November 2018] resurrected the ‘outlandish’ notion. A recent paper in Nature: Scientific Reports further sharpens the focus.

Temperature fluctuations over the Greenland ice cap during the past 17,000 years, showing the abrupt cooling during the Younger Dryas. (credit: Don Easterbrook)

Since 2007, a team of Chilean and US scientists has been working on a rich haul of late Pleistocene fossil mammals from Patagonian Chile that turned up literally in someone’s suburban back garden in the town of Osorno. The stratigraphy has been systematically dated using the radiocarbon method. A dark layer composed of peat with abundant charcoal gave an age of about 12.8 ka, thereby marking both the local base of the Younger Dryas episode and a cap to the rich mammalian fossil assemblage. Similar beds have been found at more than 50 sites elsewhere in the world at this stratigraphic level, including a site in Arizona carrying Clovis artifacts. Steadily, such ‘black mats’ have been yielding magnetised spherules, elevated concentrations of platinum-group metals, gold, native iron, fullerenes and microscopic diamonds, plus convincing signs of wild fires at some sites; the very evidence that most researchers had panned when first reported. The Chilean example contains much the same pointers to an extraterrestrial cause, attributed to air-burst impacts (Pino, M. and 14 others 2019. Sedimentary record from Patagonia, southern Chile supports cosmic-impact triggering of biomass burning, climate change, and megafaunal extinctions at 12.8 ka. Scientific Reports, v. 9, article 4413; DOI: 10.1038/s41598-018-38089-y)

A larger team of researchers, to which several of the authors of the Chilean paper are affiliated, claim the evidence supports some kind of impact event 12.8 ka ago, possibly several produced by the break-up of a comet. Yet the criticisms persist. For instance, had there been wildfires on the scales suggested, then there ought to be a significant peak in the proportion of charcoal in lake bed sediments from any one region at 12.8 ka. In fact such data from North America show no such standalone peak among many from the age range of the Younger Dryas. The fossil record from the last few millennia of the Pleistocene does not support a sudden extinction, but a decline. The Clovis-point culture, thought by many to have wrought havoc on the North American megafauna, may have come to an end around 12.8 ka, but was quickly succeeded by an equally efficient technology – the Folsom point.  As regards the critical evidence for impacts, shocked mineral grains, none are reported, and some of the reported evidence of microspherules and nanodiamonds is not strongly supported by independent analysis – and nor are they unique to impact events. How about the dating? The evidence from ice cores strongly suggests that the Younger Dryas began with an 8° C temperature decline over less than a decade, and the end was equally as sudden. Is radiocarbon dating capable of that time resolution and accuracy? Certainly not

Related articles: Gramling, C. 2018. Why won’t this debate about an ancient cold snap die? (Science News); Easterbrook, D.L. 2012.The Intriguing Problem Of The Younger Dryas—What Does It Mean And What Caused It? (Watts Up With That); Wolbach, W.S. and 26 others 2018.  Extraordinary Biomass-Burning Episode and Impact Winter Triggered by the Younger Dryas Cosmic Impact ∼12,800 Years Ago. 1. Ice cores and Glaciers. Journal of Geology, v. 126, p. 165-184; DOI: 10.1086/695703; Wolbach, W.S. and 30 others 2018. Extraordinary Biomass-Burning Episode and Impact Winter Triggered by the Younger Dryas Cosmic Impact ∼12,800 Years Ago. 2. Lake, Marine, and Terrestrial Sediments. Journal of Geology, v. 126, p. 185-205; DOI: 10.1086/695704.

Tectonics and glacial epochs

Because the configuration of continents inevitably affects the ocean currents that dominate the distribution of heat across the face of the Earth, tectonics has a major influence over climate. So too does the topography of continents, which deflects global wind patterns, and that is also a reflection of tectonic events. For instance, a gap between North and South America allowed exchange of the waters of the Pacific and Atlantic Oceans throughout the Cenozoic Era until about 3 Ma ago, at the end of the Pliocene Epoch, although the seaway had long been shallowing as a result of tectonics and volcanism at the destructive margin of the eastern Pacific. That seemingly minor closure transformed the system of currents in the Atlantic Ocean, particularly the Gulf Stream, whose waxing and waning were instrumental in the glacial-interglacial cycles that have persisted for the last 2.5 Ma. This was partly through its northward transport of saltier water formed by tropical evaporation that cooling at high northern latitudes encouraged to sink to form a major component of the global oceanic heat conveyor system.   Another example is the rise of the Himalaya following India’s collision with Eurasia that gave rise to the monsoonal system  dominating the climate of southern Asia. The four huge climatic shifts to all-pervasive ice-house conditions during the Phanerozoic Eon are not explained so simply: one during the late-Ordovician; another in the late-Devonian; a 150 Ma-long glacial epoch spanning much of the Carboniferous and Permian Periods, and the current Ice Age that has lasted since around 34 Ma. Despite having been at the South Pole since the Cretaceous Antarctica didn’t develop glaciers until 34 Ma. So what may have triggered these four major shifts in global climate?

Five palaeoclimatologists from the University of California and MIT set out to find links, starting with the most basic parameter, how atmospheric greenhouse gases might have varied. In the long term CO2 builds up through its emission by volcanoes. It is drawn down by several geological processes: burial of carbon and carbonates formed by living processes; chemical weathering of silicate minerals by CO2 dissolved in water, which forms solid calcium carbonate in soil and carbonate ions in seawater that can be taken up and buried by shell-producing organisms. Rather than comparing gross climate change with periods of orogeny and mountain building, mainly due to continent-continent collisions, they focused on zones that preserve signs of subduction of oceanic lithosphere – suture zones (Macdonald,F.A. et al. 2019. Arc-continent collisions in the tropics set Earth’s climate state. Science, v. 363 (in press); DOI: 10.1126/science.aav5300 ). Comparing the length of all sutures active at different times in the Phanerozoic with the extent of continental ice sheets there is some correlation between active subduction and glaciations, but some major misfits. Selecting only sutures that were active in the tropics of the time – the zone of most intense chemical weathering – results in a far better tectonic-climate connection. Their explanation for this is not tropical weathering of all kinds of exposed rock but of calcium- and magnesium-rich igneous rocks; basaltic and ultramafic rocks. These dominate oceanic lithosphere, which is exposed to weathering mainly where slabs of lithosphere are forced, or obducted, onto continental crust at convergent plate margins to form ophiolite complexes. The Ca- and Mg-rich silicates in them weather quickly to take up CO2 and form carbonates, especially in the tropics. Through such weathering reactions across millions of square kilometres the main greenhouse gas is rapidly pulled out of the atmosphere to set off global cooling.


Top – variation in the total length of active, ophiolite-bearing sutures during the Phanerozoic; middle – length of such sutures in the tropics; bottom – extent of Phanerozoic glaciers. (Credit: Macdonald et al. 2019; Fig.2

Rather than the climatic influence of tectonics through global mountain building, the previous paradigm, Macdonald and colleagues show that the main factor is where subduction and ophiolite obduction were taking place. In turn, this very much depended on the configuration of continents on which ophiolites can be preserved. The most active period of tectonics during the Mesozoic, as recorded by the global length of sutures, was at 250 Ma – the beginning of the Triassic Period – but they were mainly outside the tropics, when there is no sign of contemporary glaciation. During the Ordovician, late-Devonian and Permo-Carboniferous ice-houses active sutures were most concentrated in the tropics. The same goes for the build-up to the current glacial epoch.

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