The Younger Dryas

19 06 2012

We have an interesting seminar here at VUW this Thursday (21/6/12), where Professor James Kennett will be speaking about the ‘Younger Dryas Impact Hypothesis’ (YDIH). In short, the YDIH suggests that approximately 12900yrs ago, an extraterrestrial impact or ‘fireburst’ in the North American region caused widespread fires, the extinction of the Clovis culture and contributed to the rapid, short-lived regional cooling event observed in climate records covering that time frame. I was an undergraduate student on an exchange at a Norwegian university when this theory was announced at and I remember watching the press conference (see video below) with my lecturers.

This is a controversial theory that challenges the longer held beliefs that the Younger Dryas resulted from the breakup of the last North American ice sheet. It is based on the discovery of a widespread carbon layer in sedimentary deposits that date to the start of the Younger Dryas at around 12900yrs ago. This is thought to represent the widespread burning that resulted from the extraterrestrial impact. The theory has sparked much debate and has also been widely criticized since it was announced at the AGU meeting in 2007. This went so far as a ‘requiem‘ being published for the YDIH. It will therefore be very interesting to here one of the original authors of the study talk to us firsthand about the evidence upon which this theory is based.

I will post another blog about the seminar next week, and if I remember to bring my generic tablet device I may even do one of those ‘live-tweet’ things on my twitter feed. In the mean time here’s a bit of background to the Younger Dryas….

The Younger Dryas

Following the peak of the last glaciation between 19000-26000years ago (commonly referred to as the ‘Last Glacial Maximum’ or ‘LGM’, the Earth’s climate was warming and the large ice sheets that had covered much of northern Eurasia and North America were retreating northwards. Similarly in the Southern Hemisphere, glaciers that had extended out from the mountain ranges such as the Southern Alps and the Andes were also retreating in response to the global increase in temperature. These climatic changes are thought to be caused by changes in the Earth’s position relative to the sun, which varies over known timescales.

Temperature change in Greenland from 23000-8000 years ago (Source:climateshifts.org)

Following the LGM, a number of more rapid (c. 1000-2000 yr long) cooling-warming cycles occurred, until global climate stabilised at around 11000yrs ago. These rapid events were first  identified in polar ice cores, where high-resolution records of past temperature changes are preserved in the layers of snow accumulation. The Younger Dryas represents one of these climatic events and covers a  period of time  approximately 1300 years long, between 12900 and 11600 years ago. It was first defined in fossil pollen assemblages in Scandinavia in the 1970s, where the relative abundance of pollen from plant species that inhabit cooler climatic zones increased, suggesting a decrease in local temperature. Subsequent work has shown that the Younger Dryas represents a period of dramatic cooling in the North Atlantic region, where a cooling of approximately 10degC caused glaciers to readvance in many parts of Europe and North America. One of these species is called Dryas octopetala, which is where the Younger Dryas gets its name (there is an Older Dryas, but that’s another story). The ice core records show that the Younger Dryas ended very abruptly, with rapid warming of several degrees occurring in a matter of decades. The fact that these changes occurred so rapidly and the fact that this occurred relatively recently (geologically speaking), has meant that the Younger Dryas is one of the most studied time-periods of the whole Quaternary. The main question surrounding this event is, ‘what caused it?’. The timescale over which it occurred is too short for it to have been caused by changes in the Earth’s orbit of the sun – as was the case at the LGM, therefore another explanation is needed.

Early work (and work still being undertaken today) focused on trying to establish the geographic extent of this cooling event (i.e. was it restricted to the North Atlantic, or was it a global event?), in order to try to identify the likely cause.  In the mid-1990s, radiocarbon dates of a glacial deposit suggested glacier advance in New Zealand during the Younger Dryas. The author’s of this influential study (cited 192 times and counting), interpreted this as meaning that the Younger Dryas was a time of global cooling and suggested that this was driven by changes in the atmosphere, which are rapidly translated around the globe. However, refined dating techniques, such as surface exposure dating, and a greater range of climate records now suggest that the opposite is the case and that the mid- to high-latitudes of the Southern Hemisphere actually experienced warming during the Younger Dryas, whilst the North Atlantic experienced cooling. This is also seen in polar ice cores where Antarctica is seen to experience warming whilst Greenland cools during this period.

 

The leading hypothesis for the observed differences in climate between the hemispheres during this time is known as the ‘bi-polar seesaw’ and was developed by eminent scientist Wally Broecker. Broecker recognised the role of the oceans in distributing thermal energy around the globe via density driven currents. In the early 1990s, following the development of the polar ice cores climate records (mentioned above), a correlation was made between these rapid cooling events and large inputs of freshwater (as recognised by marine deposits showing increased iceberg discharge) to the North Atlantic from the decaying North American and Eurasian ice sheets. It was suggested that these freshwater inputs altered the density of oceanic waters in the North Atlantic to an extent where the circulation slowed, therefore reducing the heat flux from the South to the North and causing cooling the North Atlantic region and warming in the Southern Ocean. This reconciles well with the interhemispheric asynchrony observed in glacier advance records (outlined above). No iceberg deposits are found to be associated with the onset of the Younger Dryas and initially Broecker hypothesised that the freshwater input associated with this event was sourced from Lake Agassiz, a large body of terrestrial water that was dammed by the former North American (Laurentide) ice sheet. However, later work  could not reconcile the timing of the main drainage of this lake with the onset of of the Younger Dryas. Other studies have identified a different route for the lake drainage, that appears to coincide with the onset of cooling.

In summary, despite the abundance of research that has focused on this time period, great uncertainty still surrounds the precise mechanisms that caused this dramatic climatic reversal. The ‘impact hypothesis’ presents another theory of the potential mechanism(s) that caused or contributed to the climatic changes seen at this time. Recently, Broecker and others have suggested that the Youngas Dryas is not a ‘freak event’ that it has long thought to be. They identify similar events in the ice-core temperature records of previous glacial-interglacial transitions and suggest that these short but intense climatic reversals are an integral part of the deglacial process. They conclude that “there is no need to call upon a one-time catastrophic event to explain the YD” which is a statement clearly leveled at the supporters of the impact hypothesis. Of course, it does not mean that there was not a meteorite impact at this time, but Broecker and others believe that the climate changes during the YD were driven by internal mechanisms. I look forward to hearing Prof. Kennett’s views on the whole subject…