More on the Younger Dryas causal mechanism

The divergence of opinion on why a millennium-long return to glacial conditions began 12.8 thousand years recently deepened. The Younger Dryas stadial was an unprecedented event that halted and even reversed the human recolonisation of mid- to high northern latitudes after the end of the last ice age. Its inception was phenomenally rapid, taking a couple of decades to as little as perhaps a few years. The first plausible explanation was put forward by Wallace Broecker in 1989, who looked to explosive release of meltwater trapped in glacial lakes astride the Canadian-US border along the present St Lawrence River Valley, effectively flooding the source of NADW with a surface layer of low-density, low-salinity water. This, he suggested, would have shut down the thermohaline circulation in the North Atlantic. This is currently driven by cooling of salty surface water brought from the tropics to the Arctic Ocean by the Gulf Stream so that the resulting increase in density causes it to sink and thereby drive this part of the ocean water ‘conveyor’ system. A massive freshwater influx would prevent sinking and shut down the Gulf Stream, with the obvious effect of cooling high northern latitudes allowing ice caps to return to the surrounding continents. Yet Broecker’s St Lawrence flood mechanism was flawed by lack of evidence and the knowledge that a well-documented flood along that valley a thousand years before had raise se level by 20 m with no climatic effect. In 2005 clear evidence was found for a huge glacial outburst flood directly to the Arctic Ocean at around 12.8 ka that had followed Canada’s MacKenzie River; a route that would force low-density seawater to the very source of North Atlantic Deep Water through the Fram Straits, thereby stopping thermohaline circulation.

The year 2007 saw the emergence of a totally different account (see Whizz-bang view of Younger Dryas, July 2007; Impact cause for Younger Dryas draws flak, May 2008) centring on evidence for a 12.8 ka major impact in the form of excess iridium; spherules; fullerenes and evidence for huge wildfires in soils directly above the last known occurrences of the superbly crafted tools known as Clovis points – the hallmark of the earliest known humans in North America. Later (see Comet slew large mammals of the Americas?, March 2009) the same team reported minute diamonds from the same soils along with evidence for extinction of the Pleistocene megafauna; a view that was panned unmercifully.  Like the yet-to-be-found ‘end-Permian impact’ previously proposed by the same team, no crater of Younger Dryas age was then known. However, in 2018, ice-penetrating radar surveys revealed a convincing, 31 km wide subglacial impact structure beneath the Greenland ice cap, that is directly overlain by ice of Holocene (<11.7 ka) age. This reopened the case for an extraterrestrial origin for the Younger Dryas, followed by evidence from Chile for 12.8 ka wildfires presented by a team that includes academics who first made claims of an impact cause.

Untitled-2
Colour-coded subglacial topography from radar sounding over the Hiawatha Glacier of NW Greenland (Credit: Kjaer et al. 2018; Fig. 1D)

Last week, the impact-hungry team provided further evidence in lake-bed sediments from South Carolina, USA, which they have dated using an advanced approach to the radiocarbon method (Moore, C.R. and 16 others 2019. Sediment Cores from White Pond, South Carolina, contain a Platinum Anomaly, Pyrogenic Carbon Peak, and Coprophilous Spore Decline at 12.8 ka. Nature Scientific Reports, v. 9, online 15121; DOI: 10.1038/s41598-019-51552-8). This centres on a large spike in platinum and palladium, which they date to 12,785 ± 58 years before present; i.e. the start of the Younger Dryas. Preceding it is a peak in soot with a distinctive δ13C value attributed to wildfires (12, 838 ± 103 years b.p), and is followed by a peak in nitrogen isotopes (δ15N), indicating environmental changes, and a sharp decline in spores (12,752 ± 54 years b.p) attributed to fungi that consume herbivore dung – a sign of a decline in the local megafauna. In other words, a confirmation of previous findings at the Clovis site– but no diamonds. The variations in different parameters are based on 30 to 35 samples (each about 2 cm long) from about 0.8 m of sediment core, so it is curious that most of the data are presented as continuous curves. That issue may become the focus of criticism, as may the need for confirmation from other lake-bed cores from a wider number of localities. With such polarised views on a crucial episode in recent geological and biological history critical scrutiny is sure to come.

Younger Dryas impact trigger: evidence from Chile

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.

407458aa.2
Temperature fluctuations over the Greenland ice cap during the past 17,000 years, showing the abrupt cooling during the Younger Dryas. (credit: Don Easterbrook)

Continue reading “Younger Dryas impact trigger: evidence from Chile”

Subglacial impact structure: trigger for Younger Dryas?

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).

Untitled-2
Colour-coded subglacial topography from radar sounding over the Hiawatha Glacier of NW Greenland (Credit: Kjaer et al. 2018; Fig. 1D)

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.

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.

Enhanced by Zemanta

A sign of the times; the ‘Anthropocene’

Alternative version of image:Wooden hourglass ...
Image via Wikipedia

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.

Related articles