Blog Extinction: a New and Tragic Phenomena

From animals.nationalgeographic.com

So this is my last post for a while, at least while I get through the last half of third year! Thanks to those of you have been reading along. But I wanted to end on a concluding point. While rewilding is an enticing idea the extinct megafauna are for now, gone. So can they help us save the wildness, particularly the megafauna, we have left?

The assessment of species vulnerability needs to be stepped up (Pearson et al., 2014), particularly in the face of climate change combined with increasing human populations and material wealth in the regions currently preserving much of the world's wildlife. These new &/ or increasing  (depending on how you see it) threats means that the IUCN Red List may need reviewing (Pearson et al., 2014). Looking at the megafauna we've lost, can future extinction be better predicted (and therefore mitigated)? The Ice Age megafauna were subject to increasing human populations and a shifting climate, with relative contributions of these factors in different regions (Stuart, 2014), much the same as today. Pearson et al. (2014) analysed the attributes that cause species to be at high risk of extinction specifically due to climate change. They found that occupied area and population size, actually already used in conservation assessments, were particularly important for predicting extinction risk. Other variables considered in this review included generation length, landscape connectivity and niche breadth (i.e., what ranges of temperatures and precipitation did their habitats include).

From Pearson et al. (2014). Variables were estimated for the year 2000. Y axes are scaled so that 0.0 is the mean value of the response

So essentially, it's pretty complicated even with modern species! And interactions between these variables can throw the whole thing off. For example, although extinction risk under climate change is highest when occupied area is small, the risk is lower when the species has a long generation time (i.e., a longer period between new generations) (Pearson et al., 2014). The papers that consider differential megafaunal extinction risk factors seem to do so as a side note, or as the last line of an otherwise largely incomprehensible (at least to me) sea of mathematical models (for example see Zuo et al., 2013). So this seems to be an area that warrants more research. Indeed, so is the topic of megafauna extinctions itself. A recent review of the current understanding of the extinctions concluded that assertions that we have resolved the debate are definitely premature (Stuart, 2014).

In the meantime, there are areas which have been sorely neglected in terms of data collection. For example Southern Asia, which has been studied much less than North America, Europe and Australia. However, Southern Asia contains an astonishing quantity and diversity of surviving megafauna, most of which is threatened to varying degrees.

The megafauna found here includes the Javan and Sumatran rhinos, Asian elephant, Malayan tapir, Indian water buffalo, banteng, gaur, Bornean orangutan, Sumatran orangutan, leopard, tiger, Asiatic lion, giant panda, sloth bear, saltwater crocodile and reticulated python. Both rhino species and the Sumatran orangutan are 'critically endangered' on the IUCN Red List. The Asian elephant, Indian water buffalo, banteng, Malayan tapir, Bornean orangutan, tiger and giant panda are 'endangered' (Stuart, 2014). That's the worst and second worst ratings. The Asiatic lion has been the focus of the Zoological Society of London's campaign Lions400 which is attempting to raise awareness of this little known species before the last 400 individuals disappear from the wild. A similar story can be told about the African continent, famous for it's megafauna.

In the face of so many megafaunal species predicted as likely to become extinct, it may be useful to look at the species that did not make it at the end of the Ice Age, and what set these survivors apart, if anything.

Overall, there is a need to focus on collecting reliable radiocarbon dates on megafaunal remains,
and lots of them (Stuart, 2014). In the case of Australia, the extinctions occurred beyond radiocarbon dating range. New dating methods will undoubtedly help to clear up uncertainties surrounding the mechanisms behind the megafauna extinctions here (Stuart, 2014).

Lutz like one Heck of a mistake

An interesting bit of news caught my eye today. The article itself can be found here. It relates nicely to my Conservation Cow post yesterday.

The Heck cattle have been adopted by European conservationists as a stand-in for the lost auroch megafauna. Heck cattle have a complicated history. However, this history seems to have been overly stressed compared to the disregard for life that the man in this interview has shown (in my humble opinion).

That fact that Lutz Heck, (a member of the Nazi elite, thoroughly bad person and a terrible scientist at that) created the Heck cattle breed should be in my opinion disregarded. They exist, and despite his blundering method and less than pure motives Heck created a hardy and less docile breed of cattle, one that stands a chance of standing in for the lost aurochsen and contributing to restoring European ecosystems. So when you see something like this, off the back of the high spirits I had raised yesterday for the Conservation Cow, it’s a bit disheartening.

The farmer has bought around 30 Heck cattle, to add to his collection of rare cattle breeds. ‘Collection’ right away has me on edge. Animals, especially fierce ones like the Heck cattle, should not be collected. The only one who stands to benefit there is him if he makes a tidy profit on what I think of as the current ‘indie food’ market. If you haven’t noticed already, you will probably see some purple carrots or lumpy potatoes soon. Lidl has even brought out kangaroo steaks. It smacks of the Victorian zoophagy obsession. The rarer it was, the better. Costly signalling anyone? 

The reason Heck cattle have a chance of being a conservation cow for is that they may have some of the wits that the aurochsen possessed. Cows were not always the blundering, slightly dim animals we see today. We made them that way in our selection for meek, placid animals that gave us no trouble (Levy, 2011). So was purchasing a breed of cattle designed to not be kept domestically, to live on a farm, really a great idea? No. It wasn't. Predictably, the animals paid the price and the man profited. He killed 20 of the most aggressive bulls because they would happy have “wiped [him] from the face of this earth”. The 6 that remain are the ones that gave him least trouble. The 20 dead have been turned into burgers. Yet still the main focus seems to be on the word ‘Nazi’.

The video also features a man giving his professional opinion of the Nazi party’s use for the Heck Cattle. While I agree with what he says, I feel he has left out a large part of their story. 

Lutz Heck was obsessed with lost beasts and believed they could be brought back to life. He assumed that the biological legacy of aurochs lived on, divided up among domestic breeds. He thought that by crossing various breeds of domestic cattle, he could recreate the aurochsen (see, I told you about the bad science). Obviously, he was completely wrong. He created a new breed of domestic cattle. aurochsen were giants compared to Heck cattle, standing 6.5 feet at the shoulder (Levy, 2011). They were smarter, faster, more graceful and fearless than domestic cattle. A few years of breeding by Heck couldn't undo thousands of years of selection for docile, meat-heavy cows (Levy, 2011).

For Nazi officials, the goal of purging people they deemed inferior from society went hand in hand with resurrecting Europe’s megafauna. They wanted an imagined ‘pristine’ state of nature (Levy, 2011). In fact, this is an issue for modern conservationists. This ideology is a myth, there is no one ideal state, and in any case, humans have been altering our world for thousands of years before we came to decide what was pristine and what was not.

Phew, thank goodness for science blogs, or it would have taken me weeks to find someone who would listen to that rant! 

The Conservation Cow

From carbonsoultionsglobal,com

Cows get a lot of bad press. Many people have heard that cows produce greenhouse gasses, including methane. According to a report in 2006 by the United Nations Food and Argiculture Organisation (FAO) our diets, and the meat in them release more carbon into the atmosphere than transportation or industry (Scientific American: The Greenhouse Hamburger, 2009). Methane is responsible for 20% of the warming created by long-lived green house gasses since pre-industrial times (Kirschke et al., 2013).

From autonomousmind.wordpress

Garnett (2009), and many other scientists, seem to agree that domestic livestock are a problem for both atmospheric carbon and valued ecosystems such as rainforest, which is cleared to make way for cattle ranches. In fact the post on Conservation by George Monbiot that I discussed in my last post, he mentions that domestic livestock are a major cause of global warming. He also points out that, in human-impacted systems, wild herbivores can be as well.

In part, I disagree with this view. I think that a change in how we use our livestock could turn them from an ecological nightmare to a key part of healthy ecosystem. A carbon source to a carbon sink. This has been suggested before, often in terms of altering their diets (For example, see Aquerre et al., 2011). But I think it needs to go further than that.

Wild herbivores can turn an ecosystem from a sink to a source of carbon in the presence of human interference. An example of this can be seen in the Serengeti. Pre-1960's, wildebeest numbers there fell from 1.2 million to 300,000 (Monbiot, 2014). The result echoed the hypothesised mechanism behind the loss of Australia's rainforest, which covered much of the country in Pleistocene times (Levy, 2011) (Monbiot, 2014). In the Serengeti, the decline in wildebeest caused dry vegetation to accumulate. This abundance of fuels resulted in wildfires, which burned around 80% of the Serengeti every year. Wildebeest numbers have since increased with conservation efforts, and now more vegetation is eaten, and their dung in incorporated into the soil. This represents a transformation of the Serengeti from a source of carbon to a sink. This shift is equivalent to the entire (current) emissions of carbon dioxide from burning of fossil fuels in the whole of east Africa (Monbiot, 2014).

In the US, cattle ranches occupy huge areas of arid land. They trample and munch up the strips of vegetation, collapse stream banks and drive out native animal populations. The deterioration of these habitats fueled a campaign to remove cattle from federal rangelands (Levy, 2011). Naturally the ranchers were unhappy. But was this the right thing to do?

This land has supported large herds of herbivores since long before the end of the Ice Age (Levy, 2011). These included native horse, camel and bison. The horse and camel went extinct in the end-Pleistocene, but the bison survived up until only a century before the conflict over the rangeland domestic cattle began (Levy, 2011). The bison numbered in the tens of millions. From 1830 to 1880, a deliberate effort to slaughter them reduced their numbers to only a few thousand (Knapp et al., 1999). At the same time, much of their habitat was ploughed up by farmers.

More recently, conservationists working on native prairie land began to experiment with introducing bison back into the mix. The study by Knapp et al. (1999) on bison ecology highlights the way the bison shape the landscape. Their faeces rich in nitrogen increases plant growth while their differential grazing (cattle are picky eaters) creates a diverse plant community (containing more species) in different stages of growth depending on when the bison were last there (Knapp et al., 1999).

From americanprairie.org
A bison herd on american prairie

This diversity increases the habitats available to the native wildlife, particularly the prairie birds (Knapp et al., 1999). For example, the lesser prairie chicken needs tall grass to hide its nest in, but short grass for its courtship displays (Levy, 2011). So bison play an important role in a diverse prairie ecosystem (Knapp et al., 1999). But they way they do so is through behaviours that they share with the domestic cow (Levy, 2011). So then, why do environmentalists dislike the domestic cow? And why are ranchers so afraid of the bison?

Cattle graze during spring and summer. In the autumn they are fenced into paddocks and fed hay. Bison on the other hand are free to roam and aren't supplemented in the winter months (Levy, 2011). The long-term study by Knapp et al. (1999) showed that when managed in the same sustainable way, both cows and bison increase the diversity of native prairie plants. The way the cow is used by people has a greater effect on its environmental impact than any behavioural or other traits (Knapp et al., 1999).

The best ranchers in the US find ways to keep their cattle on the move. In places like the UK, it's more difficult to imagine how this would be achieved, since we have very few large open spaces. Sadly, the open spaces remaining in the US are under threat from being sold to create suburban ranchettes, with far lower potential for sustainable use (Levy, 2011).

During the Pleistocene, grazers wandered wherever the fresh grass was, allowing the soil time to absorb the nutrients and the plants to grow. Dire wolves and other predators helped by creating a landscape where herbivores kept on the move and avoided certain areas out of fear (Levy, 2011). Domestic cows need to be moved by humans, requiring more effort than other methods. But the rewards are healthier grasslands, cattle and wildlife (Levy, 2011).  

Whale Poo and Gothenburg Natural History Museum

Hej! I'm in Gothenburg, Sweden this week, and as a Palaeobiology student naturally I made a trip to the Natural History Museum. I have to admit, I had been slightly blinded by London-centrism and forgot that, despite what the London equivalent would have you think, most museums don't require several days to walk around meaningfully. So I was a teensy bit underwhelmed by the size of this museum at first glance. But once inside, I began to see it's charm (not that it's not charming on the outside - set in a huge wooded park within the city). Gothenburg's Natural History museum has the title of holding the world's only mounted blue whale (reportedly - feel free to correct me if I'm wrong). Not only that, once I was stood in front of the thing, I could see that it was mounted on a wooded interior 'skeleton' and contained benches, accessible presumably by the hinged top of it's former skull. Now you can't see that in London.

Mounted blue whale at Gothenburg Natural History Museum. You can just about make out the two rows of blue benches inside.

The importance of whale poo

Recently there's been a lot of news coverage about UK marine megafauna and the 17 key megafauna sites that the Wildlife Trust wants protected along our coastline. (For more on this, you can read this blog post on Ocean Commocean). But what exactly is so newsworthy about these marine megafauna? These animals were fairly untouched in comparison to their contemporaries on land during the Pleistocene extinctions, but over more recent history large marine animals have suffered comparatively unnoticed, due in part to their apparently low commercial value (Lewison et al., 2004). Despite this, they have a big impact on both the marine and terrestrial environment. Whale poo even helps to slow climate change, as discussed in George Monbiot's Conservation blog last month. 

Here's what caught my attention from the post... 

Whales return to the surface to breath and defecate (Lavery et al., 2014). The faeces fertilises the surface water, known as the photic zone - where sunlight penetrates and where photosynthesising plankton live. This iron-rich faeces helps them to grow and multiply.  

In the 1970's, people who wanted more krill suggested that the decline in large whale numbers in the southern oceans would lead to an increase in krill populations. This is called the surplus-yield model (Lavery et al., 2014). They were wrong. Instead krill populations have declined steadily along with the whales. Bad for the whales, bad for the krill and bad for us. So what happened?

By feeding at depth when they dive, then defecating at the surface, the whales are transporting nutrients into the photic zone. Iron, part of the nutrient boost contained in whale poo, is a limiting factor in the southern oceans, and the phytoplankton require it to grow (Smith, 2013). The krill feed on the plankton, and so increased plankton growth supports more krill... On top of this, these photosynthesising plankton collect carbon from the atmosphere, and lock it up for thousands of years when they die and fall to the sea floor. So before their populations began to decline, it's likely whales made a small but significant contribution to the removal of carbon dioxide from the atmosphere. 

From Sciencedaily
Whales are ecosystem engineers. 

Lesson to be learned (again)?

It seems in ecology more and more often we are discovering that we have underestimated the complexity of ecological interactions, and as such our fiddling with parts of the system often results in loss on both sides.

As discussed in my post Trophic Cascades in the Cascade Mountains... , megafauna are key to ecosystem functioning . In the past, ecologists studied ecosystems and found them to be controlled by abiotic factors (climate, geology, nutrients...) rather than by biotic (living) components, such as megafauna. Modern ecology recognises the importance of living components. Large carnivores are particularly vital due to their impact on the populations and behaviour of large herbivores (Monbiot, 2014). These in turn alter the plant community structure and composition, which affects soil erosion, river movement, carbon storage and other processes. So nutrient availability, the shape of the land and atmosphere composition are powerfully affected by living system components.

It seems that the old view came about because the ecologists were studying degraded ecosystems affected by mankind. In a human-impacted system, abiotic factors increasingly rule (Monbiot, 2014). 

Ecosystem connectivity

When humans tip the balance, whales can become part of the problem we created. Roman et al. (2014) suggest that the decline in the great whales (baleen and sperm whales) triggered orcas to switch from feeding on them to seals and sea lions. Then the decline in seals due to (take a guess) human hunting in the Aleutian archipelago, near Alaska, seems to have caused the orcas to switch their diet again to sea otters. 

From seaotters.com
The effect of sea otters on the kelp forest ecosystem. Kelp forests have been described as the rainforests of the oceans. If you're interested there's more info available on the seaotters website

Sea otters, megafauna in their own right, are famous for their part in the kelp forest/ sea urchin trophic cascade disaster, with huge impacts on atmospheric carbon dioxide. 

Sea otters have been heavily hunted along North American coastlines, causing urchins to increase and kelp forests to die. Whales preying on the sea otters, which are now a focus of international conservation efforts, creates another challenge in the restoration of this ecosystem (Monbiot, 2014). 

The video below is from the Planet Earth series, showing the sea urchins in action

Ecosystems are connected more intricately than we often appreciate. This understanding can not only explain some of the mystery behind the megafaunal extinctions of the past, but also help us to understand the ecosystems we have today, and to better protect them to ensure their long time survival as well as our own.

There is so much more in the post by George Monbiot, it's 2000 words long but an easy read so I would encourage you to take a look if you're interested. 

Younger Dryas Impact Hypothesis: Part 3

Phew, finally at Part 3! These are the last two areas of debate surrounding the YDIH I'm going to cover, but there are a few more that go deeper into the geology. The Holliday et al (2014) has quite a bit more on the geology as does the Pinter et al., (2011) paper. 

Megafauna extinctions

Does the YDIH explain the megafauna losses better than human or climate models? As mentioned in Part 1 of this post, the YDIH is claimed to overcome many of the more uncertain aspects of the climate or overkill hypotheses (Firestone et al., 2007). The YDIH, however, also runs into some issues when it is applied to the megafaunal extinctions.

The problem is that a giant impact in North America should, intuitively, cause the highest number of extinctions in North America. Even on the American continent this is not true, with around 50 mammal genera lost in South America and 33 in North America  (Barnosky et al., 2004). 

This problem gets bigger when you consider the extinctions that occurred world wide. Europe, Africa and Australia suffered megafaunal losses (see my Causes of Late Pleistocence Continental Extinctions post!). You could imagine that the impact was big enough to cause these widespread extinctions, but then how did any megafauna survive in North America? The cougar, grey wolf, bison, musk-oxen, elk and tapirs are all end-Pleistocene survivors, to name a few (Holliday et al., 2014). 

These aren't ecologically confined examples either. The survivors from North and South America present a wide range of life histories and ecological niches (Holliday et al., 2014). So it can't be that one environment was less affected, allowing a certain group to survive. 

Moreover, the fact that megafauna survived on Wrangel island, Russia and St. Paul island, Alaska while megafauna on the neighbouring continents did not, doesn't make sense in the context of an impact event (Holliday et al., 2014). 

Clovis decline and cultural shift:

If the megafauna were affected by an impact, then the humans should have been too (note: humans are considered part of the mammalian megafauna group (Barnosky, 2008)). Humans in North America at that time belonged to the Clovis culture (Haynes, 2008). The YDIH suggests that this group underwent an adaptive shift combined with a population decline (Firestone et al., 2007). This is claimed to explain the shift in culture at 12,900 BP, as well as an archaeological gap immediately following the Clovis period in which no human artefacts are found (Firestone et al., 2007).

YDIH proponents point out that sites containing both Clovis and post-Clovis are rare (Firestone et al., 2007), implying a disruption in settlement or landscape use, as the result of an impact. Interpreting this as a population collapse, however, is problematic because most Palaeoindian sites were not re-used (Holliday et al., 2014), and so the majority of these sites, including Clovis, also lack immediately succeeding occupations/ land use. Where multiple occupations do occur at such sites, stratigraphic gaps between them are readily explained by geomorphic processes (Holliday et al., 2014). 

Also, there is an issue with defining exactly what is and what isn't Clovis. Clovis culture is generally defined by the characteristic shape of the arrow heads its people made (Howard, 1990), but even within this (somewhat arbitrary) group there is considerable variation, e.g. between different populations (Smallwood, 2010). 

From Simthsonianmag
Variation in Clovis points

Where Clovis and post-Clovis sites are well defined chronologically (that is, people assign definite time boundaries to the two groups), the archeological and stratigraphic records fail to provide evidence of a population collapse (Holliday et al., 2014). On top of this, calibrated radiocarbon ages show continuous occupation across the time of the impact event rather (Holliday et al., 2014). 

Finally, the apparent end of the Clovis culture is probably actually an evolution of parts of a tool assemblage (a common occurrence in the global archaeological record). It might therefore be hard to argue that a change is tool types is due to an environmental disaster (Holliday et al., 2014). 

So the YDIH may be an unnecessary solution for archaeological problems that don't exist  (Holliday et al., 2014). (And it doesn't really help with the environmental or megafauna problems either)... In summary, a large proportion of the lines of evidence suggested have been non-reproducible (a very bad thing for a scientific theory) (Pinter et al., 2011). The evidence left over seem to be a result of non-catastrophic mechanisms and terrestrial processes (Pinter et al., 2011). There's a fair bit of bad science surrounding the YDIH, so I, personally, am unconvinced. But read around, and decide for yourself. 

Younger Dryas Impact Hypothesis: Part 2

So, following on from my last post, what are some other potential issues with the Younger Dryas Impact Hypothesis? 

Craters

There are no well-dated craters from the end-Pleistocene. However, the impact proponents have argued for: 

A) an impact on the Laurentide ice sheet, which didn't leave a crater due to hitting the ice, or 

B) an airburst explosion that affected the whole continent (and beyond) and left no craters, or 

C) a combination of the two.

The Carolina Bays and playa basins of the Great Plains have been suggested as evidence for an impact around 12,900 BP (Holliday et al., 2014). The bays are elliptical depressions found along the Atlantic Coastal Plain, North America. However, dating now indicates that the bays formed over a period of time throughout the late Pleistocene, but before 12,900 BP (Holliday et al., 2014).

The playas are smaller, and contain water at some times of year. There are over 20,000 of these, and only one has been confirmed as the result of an impact. All others have a roughly horizontal erosional unconformity (a break in the rock layer succession as result of erosion wearing down top layers. Over time, new layers are deposited on top. The surface where these contact is the unconformity) between the playa layer and the older, underlying layers. This is not the geology of an impact site, it's the result of terrestrial geomorphic processes (Holliday et al., 2014).

Ice sheet impact

The impactor may have broken up to produce a scattering of airbursts, starting wildfires across the continent and destabilising the ice sheet (without leaving any crater/s). However, a 4 km wide comet (the size calculated to be needed to cause the widespread environmental changes Firestone et al. (2007) hitting an ice sheet, would shock the rock layers beneath the ice, leaving behind an impact structure of some kind but the record of landforms and sediments left at the ice margin provide no evidence for an impact around 12,900 BP (Holliday et al., 2014).

From: Quaternary Geology
Laurentide Ice Sheet: the larger ice sheet on the right. The one on the left is the Cordilleran Ice Sheet, with the ice free corridor between. This corridor is one of the hypothesised routes of humans into North America

Holliday et al., (2014) state 'the basic physics of the YDIH does not agree with the physics of impacts nor the basic laws of physics'. (Ouch!) The mechanisms suggested for the explosion do not conserve energy or momentum (Holliday et al., 2014). On top of this, they argue that no mechanism is known to create an airburst that would affect an entire continent. 

[This may seem like gobbledygook but I’m sure the physics readers will be nodding along, (at least I hope so). As a non-physicist, you may have to take a leap of faith here along with me: conserving energy and momentum are important laws that things like comets normally are expected to follow.]  

The rest of this argument will be in the final Part 3, since the last part is actually the most interesting in my opinion and it'd be a shame if you were asleep by the time you got there...

Younger Dryas Impact Hypothesis

From: The Subversive Archeologist

(I've tried to link jargon terminology to google definitions of those words, just click on them as you would a reference)

The Younger Dryas impact hypothesis (YDIH) proposes that at around 12,900 cal a BP (calibrated date), North America was subject to an extraterrestrial impact event. This event is hypothesised to be responsible for the end Pleistocene environmental changes such as the Younger Dryas cooling, huge wildfires, the extinction of late Pleistocene megafauna, and the end of the Clovis culture (Holliday et al., 2014). 

[Younger Dryas cooling: a period of rapid cooling of the North Atlantic and a weakening of the Northern Hemisphere monsoon. The reduction in heat moving north resulted in a warmer Southern Hemisphere. The cooling is widely thought to be a result of a slowing of the Atlantic meridional overturning circulation, AMOC. However, the forcing behind this is debated] (Carlson, 2010)

The YDIH is argued to overcome many of the shortfalls of the overkill and abrupt environmental change hypotheses - see my previous posts! (Firestone et al., 2007) (Holliday et al., 2014). For example, the lack of kill sites for 33 genera of now extinct megafauna, including camels and ground sloths, has been identified as a problem with the overkill hypothesis. The fact that similar environmental shifts occurred throughout the Pleistocene but the extinctions only occur at the end of this period has likewise been highlighted as a flaw in the climate shift theory (Firestone et al., 2007). 

Firestone et al. (2007) suggest evidence for the YDIH includes: 

1. The discovery of markers, including nanodiamonds, aciniform soot, high-temperature melt-glass, and magnetic microspherules; all attributed to cosmic impacts/ mid-air explosions.
2. A carbon rich deposit overlying Clovis-age sites in North America.
3. The impact's apparent coincidence with the onset of Younger Dryas cooling, caused by the destabilisation of the Laurentide ice sheet as a result of the impact.
4. Bones of megafauna and Clovis tool assemblages have been reported to occur below this layer only.

Despite this, the YDIH has fallen out of favour more recently. Here I'll outline some arguments against the markers and carbon deposit lines of evidence.

LeCompte et al. (2012) state that morphological and geochemical analyses of the microspherules (see this post on the Earth Science Eratics blog for more about spherules) suggest they are not cosmic, volcanic, authigenic (a deposit formed where it's found), or anthropogenic in origin. The spherules were found to be similar in composition to terrestrial metamorphic rocks and very different from those formed by cosmic or authigenic processes. 

They appear to have formed by the rapid melting, then quenching of terrestrial materials. LeCompte et al. (2012) describe spherules occurring above the Clovis artifacts with a significant drop below, implying that they were deposited on top of the artifacts at the surface. This implies an impact/ explosion event left behind a spherule layer, after which the humans and megafauna are absent.   

From LeCompte et al. (2012) 

Spherules from the Younger Dryas boundary, at three sites sampled. Topper and BWD (Blackwater Draw) sites are discussed further below. White numbers indicate the diameter of the spherules in microns

Holliday et al., (2014) pose a strong argument against the YDIH. They point out that the stratigraphic, depositional and pedogenic (soil related) contexts of the YDIH have rarely been addressed. But they probably have a significant effect on the record of indicators of impacts (Holliday et al., 2014)

The sedimentological and geochemical data used in support of the YDIH are: 

• Changes in the rates of sedimentation (magnetic microspherules, nanodiamonds and other features of cosmic dust regularly fall on Earth) (Holliday et al., 2014)
• The nature of the depositional environments
• Discontinuities/ breaks in the rock record created by erosion. The carbon rich layer, according to Holliday et al., (2014) represents stability following more rapid/ energetic sedimentation. 

Translocation is a common soil process, where water moving through a soil moves particulates and solutes. These can then accumulate, the accumulation increasing with depth (Holliday et al., 2014, see p.523 for an explanation of this).

Spherules were present in all samples collected from Blackwater Draw and Topper (LeCompte et al., 2012, see figure above). Samples collected below and above the highest concentration of spherules still contained high numbers of spherules (Holliday et al., 2014) (see also Firestone et al., 2007, figure 1). 

Magnetic microspherules and magnetic grains are <500mm to <2mm (Firestone et al., 2007) and nanodiamonds are 2–300 nm (typical of translocated materials). These particulates increase in frequency with depth in the carbon rich layer at Blackwater Draw and Topper (LeCompte et al., 2012, figures 3 and 4) 

The spherules, magnetic grains and nanodiamonds may therefore be affected by translocation and accumulation in the soil. If these markers used to identify the YDB (Younger Dryas Boundary) are present in other sediments as well, then they probably can't be used as reliable indicators of an impact event. They may have accumulated in a layer due to translocation rather than deposition at the surface after an impact/ explosion.

Also, the layer of spheres was 4 cm thick and buried by only 50 cm of sand. This suggests that either: 

1. All the sand from just below the Clovis artifacts to near the surface was deposited with spheres and the amount of spheres depends on the rate of sand deposition, or
2. The spheres were translocated downward and accumulated at the lithologic break created by the artifacts.

Therefore, the Younger Dryas boundary zones used to support the YDIH are in depositional environments that either A) select for the microscopic indicators by being low energy environments (lakes and marshes) compared with immediately underlying high-energy alluvium (riverine) or, B) the indicators are from soils that represent landscape stability over a long time, therefore concentrating those materials

Next post: the arguments against the ice sheet destabilisation, human cultural shifts/ population decline and the link to megafaunal extinctions