Abstract
Earth’s changing climate, due to anthropogenic forcing, has accelerated the melting
of polar ice caps and this has been observed for decades. Studying parallel climatic time
periods in the past can help scientists predict future repercussions of this melting. The
Younger Dryas cold period is one of particular interest because the cause for the onset of
this climatic phenomenon has yet to be confirmed. Although scientists are confident that
influx of fresh water into the ocean led to cold temperatures, the source of the fresh water
is still hotly debated and this paper reviews these possible candidates.
Introduction
Climate change research has grown continuously since the 1980’s when global warming
first became known. The need to foresee consequences of climate change has driven scientists to
research the Earth’s past condition in order to create predictive climate models. One of the most
pressing consequences we will soon face is the rising sea level due to melting glaciers and ice
shelves. To better understand the repercussions of melting, climate scientists are studying
changes the North Pole has undergone since the Last Glacial Maximum (LGM).
The Earth has warmed since the LGM with the exception of a few cold periods, one of
which will be the focus of this paper: the Younger Dryas period (YD). This time period is of
particular interest because the mechanism that usually instigates a cold period cannot explain the
YD. Although scientists are convinced that fresh water influx into the salty northern Atlantic and
Arctic oceans forced the onset of this cold period, the origin of the fresh water is debated. This
paper will explain the sources of evidence, salient ocean dynamics, historical context
surrounding the YD, and five theories for the source of the fresh water along with the evidence
1
supporting each theory. The theories include ablation of the Greenland Ice sheet, comet impact,
two different spatial drainages for Lake Agassiz, and paleocrystic ice over the Arctic Ocean.
Sources of Evidence
The data to support the existence of the YD cooling period is obtained from ice and
sediment core analysis. This practice involves drilling meter-long cylinders out of either the
ocean floor or an ice shelf. Scientists then sample the cores at sequential depths and relate their
findings to time periods based on climatic signifiers. In sediment cores, δ18O, 14C, U/Ca, and
87
Sr/86Sr isotopes, and foraminifera assemblages serve as climate proxies 1, 2, 4. Foraminifera are
microscopic shelled creatures whose presence and concentration indicate the condition of the
environment by assessing which species are abundant under various conditions4. If greater than
2.5 milligrams of a species is found within a single sample, 14C dating can be used to assess how
old these creatures are. By comparing the species present at each depth and their age, inferences
can be made about the climate. The heavy isotope of water is analyzed within ice cores and is
used as the primary indicator of historic temperature variation1. Along with planktic foraminifera
found in sediment cores, the relative concentration of heavy water informs scientists of the
environmental temperature associated with past climate.
Sediment core analysis by Jennings et al. (2013), shows the end of the Younger Dryas
and the beginning of the Holocene (the current time period) through concentrations of
Spiroplectammina biformis dating to 11,100 to 10,860 calibrated years before present (cal a bp).
Radiocarbon dating relies on the amount of carbon in the atmosphere, so the concentration of
carbon at the time of deposition is used to calibrate the age. This species, along with Steinforthia
feylingi, indicate “cold conditions of seasonal productivity in the marginal sea-ice zone.”4 We
then see that the abundance of Spiroplectammina biformis decreases as we move up in the core,
2
suggesting this cold period transitioned into a period of warming around 10,000 cal a bp. It is
important to point out that it is difficult to obtain an age preceding the termination of the YD due
to the nature of ice sheet growth and retreat. Cofaigh et al. (2012) and R. B. Alley (2007) discuss
the Disko trough west of Greenland to explain that ice sheets reposition the sediment as they
advance and retreat. If the ice sheet reworks any of the material, scientists lose the ability to
accurately date the foraminifera because sediment core analysis relies on a continuous deposition
of foraminifera and sediment. Identifying the cause of the YD is difficult under this
circumstance, which makes this cooling period intriguing to climate scientists.
Ocean Dynamics
Scientists agree that the YD cooling period was caused by hindering or shutting down of
the meridional overturning circulation (MOC), which is a circular current associated with the
thermohaline circulation (THC). The THC is described as large-scale ocean dynamics driven by
salinity and temperature gradients and it is responsible for exchanging heat between the equator
and the poles1. Climatic shifts toward warm temperatures can often be associated with
strengthening of the THC and the converse is true for relatively cold time periods7.
Tarasov and Peltier (2006) draw attention to the North Atlantic Deep Water (NADW) as
this is the location where freshening of surface waters would have the greatest impact on
weakening and potentially turning off the MOC. This is because freshening of the surface water
will destroy the pressure gradient created by the salinity difference between the pole and equator.
The MOC is one of the largest ocean circulations and a millennium must pass before returning to
its steady state strength7 once shut down. The YD cold period spans roughly a thousand years
and the similar lifetime for restoring the MOC is what convinces scientists that this is the oceanic
mechanism associated with the YD cooling1. The theories presented in this paper attempt to
3
identify the origin of fresh water responsible for destroying the NADW and consequently turning
off the MOC.
Historical Context
Ice core data show that the Earth periodically experiences ice ages, the most recent of
which is intuitively named the Last Glacial Maximum (LGM). The LGM time period dates from
24 kcal a bp to 19 kcal a bp1. Since then, the trend of warming has been interrupted by abrupt of
periods of cooling1, 2. The convention is to refer to periods of relative warming (interstadial) as
Bolling/Allerod events and relative cooling (stadial) as Heinrich events. The Heinrich events are
surface freshening (lowering salinity) and cooling of the ocean due to the release of icebergs into
the open water. As the air temperature increases, glaciers and ice shelves melt and this process
can release enough icebergs to affect ocean circulation.
Depths in the sediment cores with high concentration of ice-rafted detritus (IRD) are
indicative of Heinrich events2 because these icebergs carry and release sediments (IRD) as they
melt in the ocean. Although the YD is a period of cooling, it is not classified as a Heinrich event.
The YD occurred roughly 10,000 years after the LGM and the glaciers/ ice shelves were not
large enough by this time to release a sufficient number of icebergs to instigate cooling of the
ocean1. Without a Heinrich event as a possibility, the cause of cooling for the YD is left to be
debated.
Fresh Water Source Theories
Ablation of the Greenland Ice Sheet
Jennings et al. (2006) discuss the possibility of the fresh water forcing produced by the
ablation (melting) of the Greenland Ice Sheet. They argue that a pronounced light isotope event
found in sediment cores is the signature of an introduction of fresh water into the Kangerlussauq
4
Trough. A light isotope event entails a reduction in δ18O (the heavy isotope of water) found in
the foraminifera species Neogloboquadrina pachyderma. The light isotope event during the YD
is explained by the occurrence of the same ocean dynamics that are present today (Figure 1): the
Atlantic Intermediate Water entering the Kangerlussauq Trough via the Irminger current5, 8.
The introduction of this warm Atlantic water, as described by Jennings et al. (2006),
ablated the Greenland Ice Sheet. Other explanations of the light isotope event fail to account for
the spike in population of the foraminifera species C. neotertis that coincides with δ18O
reduction8. This species prefers a stratified water column, which would have been present given
the dynamics suggested by Jennings et al (2006). Tarasov and Peltier (2006) assert that
significant THC response is to be expected only when melt water is added directly to the surface
of the ocean where deep convection would occur. The region they describe is off the coast of
Greenland, exactly where Jennings et al. (2006) proposed the ablation of the Greenland Ice
Sheet. This region of “ deep convection” is the NADW.
Bolide Impact
An interesting, yet less accepted, trigger for the onset of the YD is the so-called bolide
impact theory (meaning comet that explodes upon entry into the atmosphere). Carlson (2010)
discusses this possibility by explaining that comets break down N2 to produce atmospheric
nitrate. A model constructed by Melott et al. (2010) demonstrated that a comet could produce
observed concentrations of nitrate discovered in ice cores obtained from Greenland believed to
relate to the Tunguska event in Russia, another period of time characterized by climatic shifts.
This theory cannot, however, account for the YD cooling because nitrate depositions are six
orders of magnitude smaller than the concentrations found by ice core analysis of nitrate during
the YD3.
5
Figure 1. Modern surface currents in the north Atlantic. The Irminger current (IC) acts to ablate the
southeastern GIS. NAC = North Atlantic Current; EGC = East Greenland Current; EIC = East Iceland
Current; KT = Kangerlussuaq Trough; GISP2 = Greenland Ice Sheet Project 2. Jennings et al. (2006)
Drainage into the Gulf of St. Lawrence
Broecker (2003) asserts his claim, the prevailing hypothesis for the trigger of the YD, that
there was a release of ~9500 km3 of water into the northern Atlantic before the onset of the YD.
He hypothesizes that this happened as a result of the retreating Laurentide ice sheet opening an
Atlantic outlet for the drainage of Lake Agassiz into the Gulf of St. Lawrence. This lake had
previously been draining into the Gulf of Mexico via the Mississippi river2. This theory is
extremely attractive as Tarasov and Peltier (2006) point out because this new drainage outlet
would be in close proximity to the formation of the NADW. However, Tarasov and Peltier
(2006) also point out that this theory directly contradicts paleo observations of sea surface
6
salinity reconstruction for the Gulf of St. Lawrence, which shows that fresh water influx dropped
in the time period that the YD is believed to have begun.
Drainage into the Arctic Ocean
Taking into consideration this contradiction, Tarasov and Peltier assert their own modelbased theory (supported by sediment cores) that Lake Agassiz drained through a northwest outlet
and into the western Arctic Ocean. Their model shows that the THC is much more sensitive to
Arctic fresh water discharge than the northern Atlantic. This implies that the MOC can be turned
off more readily by introducing fresh water into the Arctic Ocean versus introducing the fresh
water into the NADW. The authors claim their most significant result of the model is a 1-2 dSv
over 100 years melt water discharge into the Arctic ocean at 12.8 cal a bp, approximately the
same time the YD began (1 dSv = 106 m3s-1). However, Tarasov and Peltier admit their model
predicted up to 1.5 dSv drainage into the Gulf of St. Lawarence at about this same time, which
has been contradicted with paleo observations7. Despite the inconsistency in their model,
Tarasov and Peltier assert that drainage of Lake Agassiz into the Arctic Ocean could have
hindered the thermohaline circulation enough to have triggered the YD.
Paleocrystic Ice
Bradley and England (2008) propose a theory for the origin of the fresh water that does
not discount any of the others, but rather asserts that melting of paleocrystic ice from the Arctic
Ocean would have resulted in a much larger freshwater influx than any of the theories claim.
Paleocrystic ice is particularly hard and thick ice that is believed to have formed during the
LGM6. They assert that an environment that would be conducive to formation of paleocrystic ice
would also be one that would heavily reduce sedimentation rates (i.e., reduce presence of IRD
thus ruling out Heinrich events) and also not leave a heavy oxygen signature in the ice cores.
7
Figure 26. The Trans-Polar Drift would have brought lower salinity water through the Fram Strait and into
the NADW resulting in hindering the MOC.
Bradley and England state that the LGM would have allowed for the formation of ~50m
thick paleocrystic ice that rapidly melted as a result of re-establishing the Trans-Polar Drift just
before the YD. Figure 2 shows the rising sea level has led to three major effects: (1) allowed the
Northern Atlantic Current to traverse the Arctic Basin; (2) resubmerged the Bering strait
allowing lower salinity water into the Arctic; (3) reestablished the Trans-Polar Drift. The
combination of these three effects, and appropriate assumption of the volume of paleocrystic ice
(~50m thick and 99% fresh water), would have discharged 10.2 dSv into the Fram Strait. Bradley
8
and Enlgand argue that the sensitivity of the thermohaline circulation in this region, combined
with 10.2 dSv of fresh water introduction, is more than enough forcing to have halted the MOC
and brought about the YD.
Conclusion
Ice and sediment cores have allowed scientists to characterize climatic shifts through
analyzing foraminifera, oxygen isotope ratios, chemical concentration, and radiocarbon dating.
With these associations, scientists are able to not only derive theories about the environmental
conditions that produced the layers, but are also able to make assertions on what could have
triggered climatic shifts. The YD cold period is intriguing to scientists because the there are
several theories that could explain the contributing factors to its onset. Scientists agree that fresh
water influx to the NADW is the most probably trigger, however the origin of this water is still
uncertain. Present theories suggest Lake Agassiz and ablation of the Greenland and the
paleocrystic ice sheet as the most probable sources. Studying fresh water influence on the MOC
aids in foreseeing the repercussions of climate change as our own ice sheets are ablating every
day.
9
Reference style:
http://www.sciencemag.org/site/feature/contribinfo/prep/res/refs.xhtml
[1] R. B. Alley, Wally was right: Predictive ability of the north Atlantic “conveyor
belt” hypothesis for abrupt climate change. Annual Review of Earth and Planetary
Science. 35, 241-272 (2007)
[2]W. S. Broecker, Does the trigger for abrupt climate change reside in the ocean or
in the atmosphere? Science. 300: 1519-1522 (2003)
[3] A. E. Carlson, What cause the Younger Dryas cold event? Geology
. 38: 383-384 (2010)
[4] A. E. Jennings et al., Paleoenvironments during Younger Dryas- early Holocene
retreat of the Greenland ice sheet form outer Disko trough, central west Greenland.
Journal of Quaternary Science. 1-14 (2013)
[5] C. O. Cofaigh et al., An extensive and dynamic ice sheet on the West Greenland
shelf during the last glacial cycle. Geology. (2012)
[6] R. S. Bradley and J. H. England, The Younger Dryas and the Sea of Arctic Ice. Quaternary
Research, 1-10 (2008)
[7] L. Tarasov and W. R. Peltier, A Calibrated Deglacial Drainge Chronology for the North
American continent: Evidence of an Arctic Trigger for the Younger Dryas. Quaternary Research,
1-23 (2006)
[8] A. E. Jennings et al., Freshwater forcing from the Greenland Ice Sheet during the Younger
Dryas: evidence from southeastern Greenland shelf cores. Journal of Quaternary Science, 282298 (2006)
10