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.
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.
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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.
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.
Radar microwaves are able to penetrate easily through several kilometres of ice. Using the arrival times of radar pulses reflected by the bedrock at glacial floor allows ice depth to be computed. When deployed along a network of flight lines during aerial surveys the radar returns of large areas can be converted to a grid of cells thereby producing an image of depth: the inverse of a digital elevation model. This is the only means of precisely mapping the thickness variations of an icecap, such as those that blanket Antarctica and Greenland. The topography of the subglacial surface gives an idea of how ice moves, the paths taken by liquid water at its base, and whether or not global warming may result in ice surges in parts of the icecap. The data can also reveal topographic and geological features hidden by the ice (see The Grand Greenland Canyon September 2013).
Such a survey over the Hiawatha Glacier of NW Greenland has showed up something most peculiar (Kjaer, K.H. and 21 others 2018. A large impact crater beneath Hiawatha Glacier in northwest Greenland. Science Advances, v. 4, eaar8173; DOI: 10.1126/sciadv.aar8173). Part of the ice margin is an arc, which suggests the local bed topography takes the form of a 31km wide, circular depression. The exposed geology shows no sign of a structural control for such a basin, and is complex metamorphic basement of Palaeoproterozoic age. Measurements of ice-flow speeds are also anomalous, with an array of higher speeds suggesting accelerated flow across the depression. The radar image data confirm the presence of a subglacial basin, but one with an elevated rim and a central series of small peaks. These are characteristic of an impact structure that has only been eroded slightly; i.e. a fairly recent one and one of the twenty-five largest impact craters on Earth.. Detailed analysis of raw radar data in the form of profiles through the ice reveals that the upper part is finely layered and undisturbed. The layering continues into the ice surrounding the basin and is probably of Holocene age (<11.7 ka), based on dating of ice in cores through the surrounding icecap. The lower third is structurally complex and shows evidence for rocky debris. Sediment deposited by subglacial streams where they emerge along the arcuate rim contain grains of shocked quartz and glass, as well as expected minerals from the crystalline basement rocks. Some of the shocked material contains unusually high concentrations of transition-group metals, platinum-group elements and gold; further evidence for impact of extraterrestrial material – probably an iron asteroid that was originally more than 1 km in diameter. The famous Cape York iron meteorite, which weighs 31 t – worked by local Innuit to forge harpoon blades – fell in NW Greenland about 200 km away.
The central issue is not that Hiawatha Glacier conceals a large impact crater, but its age. It certainly predates the start of the Holocene and is no older than the start of Greenland glaciation about 2.6 Ma ago. That only Holocene ice layers are preserved above the disrupted ice that rests immediately on top of the crater raises once again the much-disputed possibility of an asteroid impact having triggered the Younger Dryas cooling event and associated extinctions of large mammals in North America at about 12.9 ka (see Impact cause for Younger Dryas draws flak May 2008). Only radiometric dating of the glassy material found in the glaciofluvial sediments will be able to resolve that particular controversy.
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.
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.
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.
On 11 May 2011, the Geological Society of London hosted a conference, co-sponsored by the British Geological Survey, to discuss evidence for the dawn of a new geological Epoch: the Anthropocene, supposed to mark the impact on the Earth of our species. The Society, and no doubt others internationally, is interested in gathering thoughts, reflections and observations about the Anthropocene. There is indeed a a powerful and vocal, though not necessarily large, lobby directed at the International Commission on Stratigraphy (ICS) to enshrine this new division. That lobby has been active since 2000 (see: No escape from global warming; Changing the world; Epoch, Age, Zone or Nonsense in EPN issues of November 2000, April 2005 and March 2008 respectively)
We currently live in the Holocene (‘entirely recent’), an Epoch with ICS imprimatur. Yet the last 11.7 ka has been but one of very many interglacials since about 2.6 Ma ago; the start of the Pleistocene Epoch and the Quaternary Period – Arduino’s last surviving division of geological time, and lately resurrected from an untimely demise! The ‘golden spike’ for the Pleistocene/Holocene boundary is at the agreed combination of signals – ‘deuterium excess values, accompanied by more gradual changes in 18O, dust concentration, a range of chemical species, and annual layer thickness’ – of the end of the Younger Dryas/Greenland Stadial 1 in a single Greenland ice core (NGRIP) held in a cold store in Copenhagen.
The Holocene itself was based on anthropocentric grounds; i.e. it roughly coincides with the transition from human foraging to sedentary life, agriculture, the relentless development of exploitation of the majority of humans and the commodification of the physical and organic environments following the Younger Dryas stadial. I guess that paraphrases how the ‘Anthropocene’ is proposed to be defined – a signal of the beginning of irreversible global change due to human activities whose future we cannot predict.
Even if it was possible to agree on some definitive signal of the onset of human-induced global change in the geological record there remains the formal difficulty for the ICS of agreeing on the location as well as the age and likely durability of the GSSP that would mark the beginning of the ‘Anthropocene’. The originator of the idea, Nobel Prize winning chemist Paul Crutzen, argued vaguely for the ‘start of the Industrial Revolution’. Recently it has been proposed by some to be 6 August 1945 marked by long-lived radioactive fallout from the atomic massacres of civilians at Hiroshima and Nagasaki. Quaternary researchers decided some time back that the ‘present’ (as in ‘before present’ or b.p.) should be the year 1950 when atmospheric testing of thermonuclear weapons created excess 14C that will make radiocarbon dating of the next 50 ka somewhat more uncertain than it otherwise would have been. The ICS may well have a lengthy debate on its hands if the proposal ever reaches its deliberations.
Furthermore, the advocates are concerned that we are living in the transition into their ‘Anthropocene’ and that it will be so rapid and biologically disastrous as to manifest itself in stratigraphic sections of the future as a mass-extinction event. No previous mass extinction event has been allocated epochal status, being so brief, though never so brief (~10 ka) as the Holocene or any other interglacial of the past 2.6 Ma.
All that I can conclude is that should there still be geologists in, say, a million years time, who will be living in conditions and possessing intellects about which we would be ill advised to guess, they will still be in awe of the vast tracts of geological time and their stratigraphic and tectonic records over the last 4.55 Ga. Consequently, it is possible that they may well regard the then ancient proposal for an ‘Anthropocene’ as premature, hubristic and not a little reminiscent of the fable of Chicken Little; a humorous legacy of their somewhat startled predecessors. By all means let us be concerned about and take action to halt adverse human influences on the planet, but sloganeering to climb aboard a bandwagon does neither. At the Geological Society meeting, Paul Crutzen observed “… it will probably take another 20 years before it is formally accepted.” Thank goodness for a sense of reality: we may all be extinct by then…
Added 12 August 2011: Between 11.5 and 3.5 ka the greatest event in the evolution of modern humans took place on all continents except Australia and Antarctica; a foraging lifestyle gave way to settlement and the domestication of both plants and animals – the Neolithic Agricultural Revolution. The production of surplus value, stored in the form of livestock herds and grain, marked by this transition set humanity on the road to its current social, ecological and economic crisis. Interestingly, William Ruddiman of the University of Virginia in 2005 noted a shift in the CO2 content of glacial ice around 8 ka, which he ascribed to intense farming and suggested that if there were to be an Anthropocene Epoch it should coincide with the start of agriculture. Combining geological and societal factors points unerringly to the start of the Holocene, so there is little need for a new Epoch. That sensible view receives support from a palaeo-demographic survey of 133 burial sites in the Northern Hemisphere: some before the local transition to agriculture, others following it (Boquet-Appel, J.-P. 2011. When the world’s population took off: the springboard of the Neolithic demographic transition. Science, v. 333, p. 560-561). The proportion of 5 to 19 year-old remains in the cemeteries shows a marked rise in the thousand years after the first local signs of agriculture thereafter to stabilise at a new higher level. This indicates a significant increase in female fertility, perhaps by as much as two births per woman. That would set in train the relentless, 1200-fold rise in world population from the estimated 6 million at the start of the Holocene to 7 billion at present.