R.I.P.- Younger Dryas impact event hypothesis

Avid followers, 

Last time I posted on some hypothesis behind the Younger Dryas stadial. A large part of that post described the theories of Firestone, Kennett and their colleagues who suggested (implored readers) that an impact event may have caused rapid melting of the Laurentide Ice sheet. After largely being discredited by subsequent literature, Anson Mackay has pointed me to a new article (1), still in press, which seeks to end the debate on the impact event hypothesis. I thought it was necessary to include a short post here to review that paper. 

Pinter and his colleagues, including Daulton, a previously cited opponent of the hypothesis, consider the evidence for the impact hypothesis in two sections. The first section involves those signatures of an impact event which have already been largely rejected in the literature. These include the magnetics within bones found at the YD onset, concentration variations in radioactivity, iridium and Helium isotopes. 

A second section involves the evidence such as carboniferous spheroids, magnetism in particles, wildfire evidence and nanodiamonds pushed so far by Kennett et al (See my previous post). Within this second section, Pinter et al. reject the origin of each materials supposedly attributed to an impact event. The carbon spheres, attributed to large-scale wildfires across North America, have been re-identified as terrestrial deposits from fungal and arthropod fauna. The paper also rejects the discovery of grains which have magnetism which could only have been produced by a meteorite as the authors, in their own work and the work of colleagues, have yet to reproduce these findings. The absence of a large fire event around the onset of the YD is also seen suggesting that there is no evidence to suggest an impact event causing fires and melting the Laurentide Ice sheet. Finally, the identification of nanodiamonds is called into question. This paper confirms evidence in my last post that the nanodiamonds were likely incorrectly identified and that they should have been identified as graphene and graphane compounds.

Overall, Pinter and his colleagues say that 7 of the 12 evidence arguments for the YD impact hypothesis are non-reproducible. This sounds to me as if they are undermining the work of Firestone et al. Indeed, further to my last post when I suggested that the hypothesis proponents may have been embarrassed, take a look at the conclusion of Pinter's paper to find a rejection of the hypothesis and a belittling of the proponents whom 'will continue their quest until the hypothesis is confirmed'.

Pinter et al. have called this hypothesis now a requiem, defined as a celebratory ceremony for the dead, hoping to end any further research on the subject. However, from this paper's utter rejection of the hypothesis, I would suggest that a less decorated ceremonial event should take place.

(1) Pinter et al (2011) doi: 10.1016/j.earscirev.2011.02.00

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

Short circuiting the thermohaline circulation

Ladies and Gentlemen,

For this post, I will look at how the Younger Dryas (YD) the most-well researched of the Late Glacial abrupt climate shifts, supposedly cut-off the thermohaline circulation (THC) causing an abrupt cooling event. The THC, also known as the Atlantic Meridional Overturning Circulation (AMOC), is the method by which the ocean regulates global energy budgets by transporting heat and water across the globe and through the water column. By transporting heat from the equator polewards, together atmospheric circulation, heat is transported to mid- and high-latitude areas. The THC is driven by ocean currents which travel as a factor of sea-water density, affected by temperature and salinity. 

Cold, saline water at high latitudes is transported to low latitudes via the oceans' deep currents, warmer water is then transported from low-latitudes to replace this deficit. Water from the Northern Atlantic sinks and flows to the Southern Hemisphere and eventually to the conveyors circulating the Antarctic continent. Here more cold, saline water joins and is transported to the Indian Ocean before interactions with the Pacific basin. In areas of upwelling, especially in the Pacific, cold deep water rises to the surface and is heated and evaporated leaving saltier water behind. Such water flows North to join up with the Gulf Stream which travels from the Gulf of Mexico along the North American Eastern Seaboard and eventually towards NW Europe. The evaporated heat from this maintains the relatively mild British climate for its latitude. Evaporation, sea-ice formation and cooling within this process leaves very cool, saline water behind which sinks to the deep to re-start the process.  Since the THC relies on the sinking of cold, saline water in the polar regions, if a large volume of freshwater was dumped into the system, the water would become too light to sink. In this case, no warmer water would replace the regular sinking cold water and so the heat transfer to the polar regions would cease from the THC causing a rapid return to glaciation. Please see Figure 1 for a visual representation of the Earth's ocean currents.

Figure 1: Thermohaline circulation (Source: 1. TSC)

Interposed between the start of the Holocene and the Allerod/Bolling warming stages, the YD cooled the Earth from c.12,800 cal yr BP before coming to an abrupt stop c.11,500 cal yr BP. As temperatures rose through the Allerod and Bolling warm stages, the Laurentide ice sheet over North America retreated creating the largest North American lake by volume, Lake Agassiz. In these warm stages, the Lake periodically released water to the North (Arctic Ocean), South (Gulf of Mexico) and East (North Atlantic Ocean) as shown by Figure 2. 

Figure 2: Suggested overflow routes from Lake Aggasiz causing the Younger Dryas (2. Broecker, 2006). Note: the axes show latitude (y) and longitude (x)

However, it is widely believed that a large outburst of freshwater from Aggasiz into the North Atlantic caused the YD. This disrupted the THC plunging the Northern Hemisphere, and especially Europe into a period of cooling once more. The evidence for such large outbursts affecting the THC and causing the YD is well summarised by Teller et al. (2002). Teller et al suggest that freshwater inputs to the THC as low as a 0.1 Sv  flux (where 1 Sverdrup = 1 x 10⁶ m³s^-1) may interrupt the formation of North Atlantic Deep Water (NADW) which drives the cool, saline water in the deep THC of the North Atlantic. Data from Lake Agassiz outbursts suggest that a 0.3 Sv flood flux of 9500km³, the second largest recorded outburst, occurred 12.9 ka cal yr BP in line with the beginning of the Younger Dryas. Its route was through the Great-Lakes to the East and the St. Lawrence River flowing NW into the North Atlantic. If this flux was seen over a period of 1 year, it would be at least 6 times higher than the regular flow into the St. Lawrence from Agassiz.

Other authors have proposed different reasons for the inception of the YDs which will be explored next time. Following this, I will look at evidence for the YD's abrupt termination.

1. TSC: thermohaline circulation
2. Broecker (2006) doi:10.1126/science.1123253

Late Glacial and Holocene climate variability

Morning All, 

Today I thought a little bit more justification for studying abrupt climate change was needed before pressing on with the Younger Dryas explanation. Therefore, this post reports a brief summary of the climatic changes, mainly recorded in ice cores, since the Last Glacial Maximum.

There are several dating uncertainties inherent in the calculation of the last glacial maximum (LGM), however, as suggested by 37Cl dating by Bowen et al. (2002) (1) the LGM, occurred approximately 22 ka before a period of deglaciation occurred 21.4 ± 1.3 cal yr ka. Global temperatures fell again during the Oldest Dryas, identified in one of the many Greenland ice cores archives, GISP2, by Stuiver et al. (1995) (2), between c. 15.1 to 14.5 cal yr ka. 

Further climate variability occurred in the lead up to the Holocene as summarised by Figure 1 in Steffensen et al. (2008) (3). The Allerod/Bolling warming periods are often separated by a period of cooling known as the Older Dryas, before the Younger Dryas period appeared approximately 12,800 years ago. After 1200 years of cooled climate, an abrupt warming event ended the Younger Dryas and began the present Holocene. The climate of the Holocene has been more stable with fewer abrupt shifts. However, an event at 8200 cal yr BP, among others, will be discussed in the coming months.

Figure 1. Temperatures derived from deuterium analyses on Greenland NGRIP ice core from 14760 - 11660 cal yr BP displaying the major abrupt climate shifts within (Steffensen et al., 2008) (3).

The grey areas on Figure 1 show how quickly abrupt warming can take place. Although the rapid shift of the Atlantic meridional overturning circulation in the Day After Tomorrow is unlikely at the weekly time-scale, shifts in deuterium excess, a proxy for sea surface temperatures (SSTs) in Greenland ice cores, as reported by Steffensen et al. (2008), can take place over 1-3 years.

I think now we are fully set-up to look at the Younger Dryas stadial event, or more colloquially termed as the "Big Freeze" by Berger (1990) (4). To refresh your memory, take a look at the youtube video I posted last week which I've again added below. The Younger Dryas is coming, figuratively speaking.

(1) Bowen et al. (2002) doi: 10.1016/S0277-3791(01)00102-0
(2) Stuiver et al. (1995) doi:10.1006/qres.1995.1079
(3) Steffensen et al. (2008) doi:10.1126/science.1157707

(4) Berger (1990) doi:10.1016/0921-8181(90)90018-8