Alternative theories to explain the Younger Dryas

Last week, I blogged on the thermohaline circulation (THC) and the well-accepted cause of the Younger Dryas (YD). Today, I will look at two alternative causes of the YD, both of which suggest that extra-terrestrial factors forced the onset of the YD on Earth.

One such cause to consider suggests that the Earth was hit by one or many comets in the decades leading up to the beginning of the Younger Dryas 12.9 ka cal yr BP. First theorised by Firestone et al (2007) (1), this paper suggested that a black carbon-rich layer in many North American sediment cores may have been formed by a meteorite or series of meteorite events. This theory is supported by Kennett et al (2009) (2) who found evidence for nanodiamonds in the ground boundary layer dated to the YD, termed the 'black mat', in several North American sites. This layer was supposedly caused by continent-wide wildfires. The paper suggests that the nanodiamonds are left behind after comet impact events having been prevalent in cores dated to the Chicxulub impact event which is thought to have caused the Cretaceous-Tertiary extinction event. Both papers suggest that such an impact event would have destabilised the Laurentide ice sheet causing rapid melting and therefore freshwater floods to the Atlantic.

However, this theory has since been, essentially, destroyed. Other studies have found evidence for nanodiamonds in a YD dated layer in other locations including Tian et al (2010) (3). However, Tian and his colleagues found that the nanodiamonds present were consistent in all layers of their Lommel core in Belgium and therefore not attributable to an impact event. No evidence to support Kennett et al's theory was found by Surovell et al (2009) (4) who found no peak in magnetic minerals or substances which could have been left by a cosmic impact event. If this theory required any further rebuttal, Daulton et al (2010) (5) cast further doubt on the impact hypothesis by suggesting that what Firestone et al and Kennett et al identified as nanodiamonds, were actually graphene - naturally occurring single planes of graphite. Daulton et al found no evidence for nanodiamonds posing strong challenges to the impact hypothesis. I believe this may have caused some embarrassment for the supporting authors and they have yet to respond to support their theory.

In another theory, Renssen et al (2000) (6) argue that the main freshwater outburst occurred 1,000 years before the onset of the YD. Therefore the THC shutdown must have been helped by another factor which they suggest could be decreased solar activity.

This paper compiled several studies of cosmogenic isotope identification, the prevalence of Beryllium-10, from the GISP2 ice core, and Carbon-14, from dendrochronology, both produced by cosmic rays in the upper atmosphere and therefore an indication of solar activity. During the late glacial, the abundance of Beryllium-10 and Carbon-14 are well correlated pointing to three possible forcing mechanisms. Firstly, strong increases in both isotopes around the YD would suggest a decrease in solar activity. Such a sharp Carbon-14 increase can be seen in Figure 1 around the onset of the YD. Secondly, there is evidence for a c.2500 year cold periods which correlates with the Carbon-14 record. Such a sudden decline in Carbon-14, and therefore solar activity could have triggered the YD. Finally, Renssen and his colleagues argue that the current THC shutdown does not explain the evidence found for the YD in the tropics and the mid-latitudes of the Southern Hemisphere. A solar minimum could explain the global effect of the YD although the global nature of the YD is well disputed.

Figure 1. Link between Carbon-14 and Oxygen-13 (Renssen et al, 2000).

The mechanisms behind this theory are further explained by Renssen et al by suggesting that reduced solar activity could have caused a decrease in ozone content possibly resulting in a reduced latitudinal effect of the Hadley cell. This would cause cooling in all non-tropical areas and would shift precipitation belts. It is then explained that the precipitation shift may destabilise the polar ice sheets, increasing icebergs and increasing the freshwater flux to the THC causing a shutdown. Another possible mechanism suggests that cloud cover would increase from enhanced cosmic rays. More cloud cover would increase reflection of incoming radiation thus cooling Earth. Greater cloud cover may also increase precipitation and therefore freshwater input into the Atlantic Ocean.

Today's two additions to the THC shutdown theory posted last week both largely support a shutdown of the THC. However, the discredited cosmic impact event suggests that the THC shutdown is an effect of a series of meteorite whereas the theory of depressed solar activity seeks to operate alongside the freshwater outbursts from Lake Agassiz. It is still unknown what caused the THC shutdown but, having reviewed a wide-range of literature, this mechanism is still best positioned to explain the YD.

Next time, I will consider the abrupt termination of the YD before leaving the YD behind to look at another abrupt climate change event.

(1) Firestone et al (2007) doi: 10.1073/pnas.0706977104

(2) Kennett et al (2009) doi:10.1126/science.1162819

(3) Tian et al (2010) doi: 10.1073/pnas.1007695108

(4) Surovell et al (2010) doi: 10.1073/pnas.0907857106

(5) Daulton et al (2010) doi: 10.1073/pnas.1003904107

(6) Renssen et al (2000) doi:10.1016/S1040-6182(00)00060-4