lessons from the younger dryas

7/28/2019 Lessons From the Younger Dryas

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Will Russack

Climate Change and Social Response

4/3/13

Lessons from the Younger Dryas

Introduction

In a modern world where our global climate is undergoing rapid change,

historical and paleoclimate records of abrupt climate change are critical to

understanding how these changes occur and what the repercussions may be. One

such event that has always captivated paleoclimate scientists is the Younger Dryas

(YD). The YD occurred in the middle of a warming trend, when temperatures

suddenly dropped up to 15C, including global decreases of 5-6C within the first

few decades (Berger, 1990). The proposed cause for the YD is the disruption of the

Atlantic Meridional Overturning Circulation (AMOC or MOC), a major ocean circuit

that plays a key role in global climate (Srokosz et al., 2012; Schmittner et al., 2007).

The changes accompanying the YD had both positive and negative impacts for

ancient humans, and it is considered a major turning point in human society (Goebel

et al., 2011; Moore & Hillman, 1992). Analysis of current climate trends indicates the

possibility of a similar AMOC disruption due to anthropogenic activity.

Understanding the outcomes of such a major event in the near future is critical as

we continue to respond to the threat of abrupt climate change.

About 14,500 years ago, the last glacial phase in Earths climate ended and a

warmer interglacial phase began. However, around 12,800 years ago this warming

trend was interrupted by the Younger Dryas, which pushed the Northern


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Hemisphere back into near-glacial conditions (NOAA; Murton et al., 2010; Carlson et

al., 2007). The process was quick, and within a few decades climate had changed

dramatically. Geological evidence from the Younger Dryas shows far-reaching

tundra conditions in Europe, vegetation changes in the American West, increased

aridity in the Middle East, and shifting monsoons in Southern Asia, among other

changes (Weber et al., 2011; Moore & Hillman, 1992; Goebel et al., 2011). Several

factors have been cited as possible causes of the Younger Dryas including volcanic

activity, irradiation from a nearby supernova, and extraterrestrial impact. However,

the most supported theory is that massive quantities of freshwater from glacial

lakes emptied into the ocean and interrupted important oceanic systems, including

the Atlantic meridional overturning circulation and therefore the creation of the

North Atlantic Deep Water (NADW) (McManus et al., 2004; Carlson et al., 2007;

Tarasov & Peltier, 2005; Rayburn et al., 2011).

The Atlantic Meridional Overturning Circulation & Global Climate

The Atlantic meridional overturning circulation is a global ocean circuit,

often referred to as the great ocean conveyor belt.1 It is responsible for the

transportation of warmer waters to the northern Atlantic Ocean off the coast of

Greenland and Norway, where the water cools and sinks, forming NADW. This deep

water returns slowly towards the equator, where it mixes with warmer water and

once again rises to the surface and moves northward, thus completing the cycle

(Figure 1. NOAA; Berger 1990; Srokosz et al., 2012). Besides heat, the AMOC is

1The MOC is sometimes referred to as the Thermohaline Circulation (THC). However, the two arenot synonymous; the MOC is what can be determined and roughly measured in practice, whereas the

THC is a theory used to explain global ocean transport of heat and salinity.


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responsible for transporting water, salt, carbon, and other nutrients across the

globe. In fact, the AMOC is accountable for about 25% of the total heat transport to

the northern latitudes, contributing to the temperate climate of northwest Europe

(Schmittner et al., 2007; Srokosz et al., 2012). The MOC and the creation of the

NADW are critical to the global climate, and alterations to the circuit have

potentially large consequences (Schmittner et al., 2007; Srokosz et al., 2012; Wood

et al., 2003).

Deglaciation is a complex process that occurs in a pulsating manner, where

periods of meltwater discharge are followed by times of lesser activity (Berger,

1990). At the end of the last glacial maximum (LGM), freshwater from the North

American Laurentide Ice Sheet (NAIS or LIS) drained through the Mississippi River

valley to the Gulf of Mexico (Rayburn et al., 2011; Berger, 1990; Tarasov & Peltier,

2005). However, as the LIS continued to recede with the warming climate, the

changing margin of the ice sheet opened up new drainage routes for glacial

meltwater (Rayburn et al., 2011; McManus et al., 2004). Scientists speculate that a

large quantity of freshwater entered the northern Atlantic Ocean and disrupted the

MOC and deep-water production. Specifically, a colossal glacial lake called Lake

Agassiz has attracted considerable attention as the leading candidate for such a

massive freshwater influx (Rayburn et al., 2011; Tarasov & Peltier, 2005). Although

Lake Agassiz has been generally agreed upon as the source of this flood, debate

exists over the supposed pathway. Early evidence seemed to identify an eastern

drainage route through the St. Lawrence river basin into the North Atlantic Ocean,

but recent research has instead shown support for the existence of a northwestern


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channel that emptied into the Arctic Ocean (Figure 2. Tarasov & Peltier, 2005;

Murton et al., 2010).

Freshwater Forcing as the Cause of the Younger Dryas

The first to postulate a pathway for this catastrophic flood was Broecker et

al. who proposed that the Younger Dryas was initiated by a diversion of meltwater

from the Mississippi drainage to the St. Lawrence drainage system (1989: 318).

Using oxygen isotopes from planktonic foraminifera in cores from the Gulf of

Mexico, Broecker demonstrated that that discharge from glacial Lake Agassiz ceased

to flow southward at the onset of the Younger Dryas about 13,000 years ago.

The retreat of the LIS exposed new bedrock in the St. Lawrence valley,

allowing the relationship between river geochemistry and underlying bedrock

lithology and resulting changes in foraminifera to be used as tracers of glacial runoff

from different areas (Figure 3. Carlson et al., 2007). Results show that increases in

Mg/Ca and U/Ca at the onset of the YD indicate an increase in runoff over Mg-rich

and U-rich bedrock as freshwater drained through the St. Lawrence basin (Carlson

et al., 2007).

More recently, data from varve chronology, sedimentation rates, proglacial

lake volumes, and sediment cores demonstrated evidence for the occurrence of two

large, closely spaced freshwater outflows through the St. Lawrence valley near the

start of the YD event (Rayburn et al., 2011). Foraminifera fossils and chemical

changes from cores in the Champlain Sea reveal multiple shifts between freshwater

and marine scenarios, indicating several large influxes of glacial meltwater (Figure

4.Rayburn et al., 2011).These shifts to temporary lacustrine conditions align


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chronologically with the beginning of the Younger Dryas, leading the authors to

conclude that We can think of no better candidate than glacial Lake Agassiz for the

great volume of fresh water that must have passed through the Champlain Sea at

this time (Rayburn et al., 2011: 550).

Despite such evidence, doubt regarding the existence of a St. Lawrence

channel persists. Recent research shows evidence of a channel to the northwest

through which Lake Agassiz would have drained to the Arctic Ocean (Tarasov &

Peltier, 2005; Murton et al., 2010). Although Tarasov & Peltier believe that a

massive freshwater discharge disrupted the MOC, they maintain that in order for

freshwater to significantly influence the MOC it must be directly deposited into the

region of North Atlantic Deep Water formation, which occurs in the Greenland-

Iceland-Norwegian (GIN) Seas. Planktonic 18O data from sedimentary cores in the

western Fram Strait exhibit evidence of a major freshwater discharge into the GIN

seas during most of, if not all of the YD (Tarasov & Peltier, 2005).

Additional evidence of flooding in the northwest Mackenzie River valley

comes from geological unconformities due to fluvial erosion from major discharge

events (Murton et al., 2010). Large quantities of glacial till and aeolian sand deposits

have been stripped and immediately covered by large gravelly beds at unusually

high elevations, indicating high-energy fluvial episodes (Figure 5. Murton et al.,

2010: 740). The presence of two gravel beds indicates two flooding events, the first

of which occurred just before the Younger Dryas (~ 13,000 years B.P) therefore

providing compelling evidence that such a flood was responsible for the YD. This

physical geologic evidence is the kind that many critics claim is missing from the


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supposed St. Lawrence channel (Murton et al., 2010; Tarasov & Peltier, 2005).

However, it does not eliminate the prospect of freshwater discharge to the east. In

fact, a major discharge event in the northwest combined with modest but consistent

flooding through the St. Lawrence basin could be responsible for the continuation of

the YD until the LIS readvanced and obstructed the channels (Tarasov & Peltier,

2005; Murton et al., 2010; Clark et al., 2001).

Whether a massive flood from Lake Agassiz emptied to the northwest, east,

or both, studies indicate that over 9,500 km3 of freshwater was released (Murton et

al., 2010; Rayburn et al., 2011). Such a large quantity of freshwater entering any

combination of the North Atlantic and Arctic Oceans would decrease NADW

formation, thereby greatly reducing the MOC (Rayburn et al., 2011; Murton et al.,

2010; McManus et al., 2004; Srokosz et al., 2012; Tarasov & Peltier, 2005; Clark et

al., 2001). Critics claim that for thousands of years before the Younger Dryas, Lake

Agassiz discharged large amounts of freshwater into the Gulf of Mexico without

affecting the MOC (Tarasov & Peltier, 2005; Carlson et al., 2007). However, as

Tarasov & Peltier write, The mostimportant factor in determining the effect of

freshwater forcing upon the MOC is not the total amount of meltwater, but rather its

regional distribution (2005: 663). It is unlikely that freshwater deposited in the

Gulf of Mexico would make it intact to the North Atlantic, thereby explaining the

stability of the MOC before the YD (Tarasov & Peltier, 2005)2. However, when

freshwater from Agassiz was rerouted to the northwest or east it was deposited

directly in the area of the North Atlantic Deep Water formation, slowing the MOC to

2In fact, Clark et al. postulates that freshwater injection to the Gulf of Mexico actually has thepotential to energize the MOC.


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a crawl and plunging the Earth into the brief but intense cold period of the YD.

Human Activity During the Younger Dryas

The Younger Dryas had positive and negative impacts on ancient humans

who had to adapt to the sudden cooling and increased aridity (Burdukiewicz, 2011;

Moore & Hillman, 1992; Weber et al., 2011). Near-glacial temperatures, higher wind

speeds, and decreased vegetation made survival more difficult as hunter-gatherer

populations decreased across northern Europe. The increase in migratory reindeer

forced groups to become more mobile, and bow-and-arrow artifacts become more

plentiful (Burdukiewicz, 2011; Weber et al., 2011). In southwest Asia, where many

growing sedentary populations relied on local plants and fruits, the increased

aridity was so disruptive that it forced the abandonment of several sites (Moore &

Hillman, 1992). In addition, archaeological evidence shows the possibility of dogs

being used by YD-era hunters in northern Europe, indicating a sophisticated level of

planning and organization (Weber et al., 2011). The findings of long-blade

technology near the end of YD also indicate a major technological advancement

(Burdukiewicz, 2011; Weber et al., 2011). Lastly, the shortages in resources lead

some researchers to suggest an increased level of cooperation and communication

between hunter-gatherer groups (Burdukiewicz, 2011; Weber et al., 2011)

While the YD caused significant disruptions in culture and settlement in

Europe and southwest Asia, humans in the Great Basin of the U.S experienced

favorable conditions (Moore & Hillman, 1992: 482; Goebel et al., 2011)

Interestingly, the YD brought wetter conditions to the western U.S, refilling lakes

and creating ample marshes and meadows rich with life (Goebel et al., 2011). Like


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their European counterparts, humans in the Great Basin lived a mobile lifestyle,

arriving at shelters with prepared toolkits including scrapers, arrows, blades, and

even eyed needles made from bone (Figure 6. Goebel et al., 2011). The additional

finding of awls, needles, and cordage indicate the production of nets used to catch

birds and perhaps fish (Goebel et al., 2011).

Overall, humans in both Europe and North America demonstrated increased

mobility during the YD and the maintenance of a rich stone, bone, and perishable

tool kit which they carried with them over long distances (Goebel et al., 2011: 498;

Weber et al., 2011). Human populations on both sides of the Atlantic showed a

remarkable ability to adapt to the sudden climate change by increasing inter-group

communication and developing new technology and hunting techniques. In fact,

many archaeologists cite the YD as being a contributing factor in the subsequent

development of agriculture across much of Europe (Moore & Hillman, 1992; Srokosz

et al., 2012).

Discussion

Extensive modeling and current research strongly indicates that

anthropogenic activity is directly contributing to the rapidwarming of our global

climate (Solomon et al., 2007). The main source of this warming is the creation of

greenhouse gases by burning fossil fuels (Solomon et al., 2007). Among the many

consequences of this activity is the rapid melting of glacial ice in Greenland and the

Arctic, which has already begun (Wood et al., 2003; Solomon et al., 2007). As the YD

demonstrates, a rapid influx of freshwater in the north Atlantic and Arctic Oceans

has the potential to drastically weaken the AMOC and trigger climate changes on a


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global scale. In 2007, the Intergovernmental Panel on Climate Change (IPCC)

released a detailed report on the state of climate change and its potential effects,

which stated, Based on current simulations, it is very likely that the AMOC will slow

down during the course of the 21stcentury. (Srokosz et al., 2012: 1669)

Would a current or future weakening of the MOC cause a similar cold snap

like the Younger Dryas? Certainly the less informed might see such an outcome as a

positive change that could combat global warming. However, the MOC is not a

simple mechanism but an intricately complex system that we do not fully

understand (Srokosz et al., 2012; Wood et al., 2003). In most models, a weakening of

the MOC does result in a cooling period in similar geographic regions as the YD.

However, such a cooling would also result in an increased warming in the southern

hemisphere, as the AMOC would no longer be removing heat from that area (Wood

et al., 2003). In addition to these temperature shifts, major precipitation changes

would occur, notably a weakening of the East Pacific and Indian monsoons (Srokosz

et al., 2012; Wood et al., 2003). An anthropogenic forcing of freshwater would also

strengthen storms in the north Atlantic, and some researchers point to recent

events such as Hurricane Sandy as evidence that this has already begun (Srokosz et

al., 2012).

Although our human ancestors managed to adapt to the sudden climate shift

of the YD relatively well, our population today is several magnitudes larger and still

growing. As population grows, available agricultural land continues to shrink, and

several countries face water shortages. The precipitation changes accompanying a

MOC weakening would affect primary productivity and therefore vegetation growth


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around the world. Models run by Wood et al. predict the following decreases in

primary productivity: 16% in Europe, 36% in the Indian subcontinent, and 109% in

Central America (a decrease over 100% indicates the current vegetation would be

unsustainable). Such decreases would have drastic impacts on agriculture, livestock,

and water availability. The danger of widespread famine and drought, especially in

areas like the Indian subcontinent where populations are growing rapidly, would be

incredibly high.

Part of the reason YD-era humans were capable of adapting to the climate

changes was their increased cooperation. However, given current world politics, it is

not unlikely that substantial food and water shortages would lead to invasions and

conflict between multiple countries, possibly leading to large scale war. On the other

hand, an YD event in the near future could also lead to major innovations in food

production, greenhouse gas emissions, or other areas in the same way the YD is

credited with the agricultural revolution.

Perhaps the most troubling repercussion of a MOC weakening would be its

effect on CO2 levels. The North Atlantic is a major sink for atmospheric carbon

dioxide, and a shutdown of NADW formation would mean anthropogenic CO2 has

one less place to go besides the atmosphere (Srokosz et al., 2012). Human activity

has already greatly distorted the global carbon balance, and the loss of such a major

storage unit could create a vicious cycle of increased global warming and a further

shutdown of the AMOC.

Conclusion

Study of the last deglaciation demonstrates that the rapid discharge of


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freshwater from melting glaciers in the north Atlantic and Arctic Ocean can lead to a

weakening of the AMOC and therefore the cessation of NADW formation, which can

in turn trigger a rapid decrease in temperature along with several other changes.

Anthropogenic release of greenhouse gases has instigated a current period of rapid

warming that has the potential to trigger another weakening of the AMOC, which

could have drastic implications for meeting human societys growing food and water

needs. The bottom line, however, is that the AMOC is a complex system that we do

not fully understand, and even our most sophisticated models cannot predict every

change that would accompany an AMOC shutdown. The best scenario would be one

in which we never have to find out what the consequences would be. By focusing on

current techniques for mitigating greenhouse gas emissions and ways to improve

our food security, such a catastrophe can be adverted.


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Supplementary Figures

Figure 1. Global overturning circulation model. From Schmittner et al., 2007.

Figure 2. Possible drainage routes from Lake Agassiz. From Murton et al., 2010


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Figure 3. Identification of exposed bedrocks in St. Lawrence Valley and

corresponding geochemical tracers. From Carlson et al., 2007.


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Figure 4. Cores depicting evidence of alternation between lacustrine and marine

environments in the Champlain Sea. From Rayburn et al., 2011.


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Figure 5. Stratigraphic sections depicting flood events near the mouth of the

Mackenzie River. From Murton et al., 2010.

Figure 6. Human artifacts, including eyed needles (b,c), found at a Great Basin site.

From Goebel et al., 2011.


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