Fusing the thermohaline circulation - the 8200-Year Event

Having concluded the Younger Dryas in my last post, it is time to move onto the second major abrupt cooling event within my time-series. 8400 cal yr BP, an abrupt cooling occurred before temperatures rebounded to 8000 cal yr BP. Today's post will consider the causes of the 8200-yr Event by looking at one paper in particular, a review by Barber et al (1997) (1).

Although the reasons for this abrupt change are still unknown, several papers have attributed the event to the final retreat of the Laurentide Ice Sheet releasing a series of outbursts from Lake Aggasiz through the Hudson Strait to the Northern Atlantic. This freshwater flux has been estimated at >10¹⁴m³ occurring c. 8470 cal yr BP. Just like in the case of the Younger Dryas, such a deposit of freshwater into the North Atlantic would prevent the formation of North Atlantic Deep Water preventing heat transfer to the high-latidudes of the Northern Hemisphere.

Evidence for this event can be found through many sources. Alley et al. (1997) (2) present a useful diagram as shown in Figure 1. Across many proxies, the 8200 year downward spike can be seen through ice accumulation, a decrease in methane, temperature and other substances. Further evidence from Klitgaard-Kristensen et al., (1998) (3) present data from marine records in the North Sea and dendrochronology in Germany which show evidence for a 2 C drop in temperature in correlation with the Greenland ice core data.

Figure 1. - Holocene climate clearly showing 8200-yr Event (2)

Today's short post has continued to explain abrupt climate events within this blog's time-series. My next post will prevent further evidence of the event found in the literature. From then on, I will look at a few less pronounced climate changes, including the Medieval Warm Period and the Little Ice Age, before relating the changes in the thermohaline circulation to what could potentially occur in our future.

1. Barber et al., 1997 doi: 10.1038/22504
2. Alley et al., 1997 doi: 10.1130/0091-7613(1997)​025<0483:HCIAPW>2.3.CO;2
​3. Klitgaard-Kristensen 1998 doi: 10.1002/(SICI)1099-1417(199803/04)13:2<165::AID-JQS365>3.0.CO;2-#

Point to Ponder...Perhaps?

Afternoon All, 

The last few posts have generally accepted that the Younger Dryas (YD) was caused by freshwater outbursts disrupting the thermohaline circulation. A few theories have been proposed, some have been rejected but to date I have not posted about the end of the stadial.  

This post will produce evidence by three studies which suggest that the YD stadial event abruptly ended within 3-50 years. In 1989, a paper reviewing the evidence to support the abrupt termination of the YD was written by Dansgaard et al (1). In this paper, the late glacial and the early Holocene is recorded from the ice core taken by the Greenland Ice Sheet Project in South Greenland. The Younger Dryas is recorded in Delta Oxygen-18 isotope analyses as shown in Figure 1, b (Dansgaard et al). 

Figure 1: Greenland ice core evidence showing δO-18, deuterium excess and dust variation supporting a rapid end to the Younger Dryas

In Figure 1, d-f parts of the image zoom in on the end of the Younger Dryas and start of the Pre-Boreal warmer stage. d shows a sharp increase in delta Oxygen-18 (δO-18) as the over a 50 year period. When an increase in δO-18 is seen in ice cores, compared with the concentrations of standard mean ocean water (SMOW), it is assumed that this precipitation will have been enriched in δO-18 which requires greater heat energy, and thus higher surface and air temperatures, to evaporate it before falling as precipitation. There is also less likelihood that the δO-18 will fall on the ice core as it is heavier. Therefore, a higher proportion of δO-18 will increase the chances of it falling at higher latitudes. 

Part e of Figure 1 shows evidence of a decrease in deuterium excess, defined as δDeuterium - 8* δO-18, over just 20 years. Dansgaard and his colleagues attribute this decrease to the northward movement of the Polar front and sea-ice margin causing greater moisture supply to the ice cores leading to a lower deuterium signal.

Part f provides evidence for a decrease in dust in the ice core over just 20 years. The prevalence of dust in an ice core is correlated with storminess (to transport the dust to the ice core) and dry climatic conditions (to allow for increased dust entrainment). The dust signal is negatively correlated with δO-18 and so cooler conditions give rise to more dust. Such a decrease in dust over just 20 years shows that temperatures increased rapidly over this period.

Looking at the evidence, the three proxies of δO-18 isotope analysis, deuterium excess and dust concentrations all point to higher temperatures over a very rapid time-series. Dansgaard estimates the 5‰ increase in δO-18 over 50 years to be equivalent to a 7°C warming. Another paper by Alley et al (1993) (2) has found evidence for increased ice accumulation over an abrupt period of just 3 years at the onset of the YD's termination. Figure 2 (Alley et al) shows ice accumulation in the Late Glacial into the early Holocene.

Figure 2: Ice accumulation rates in Greenland showing evidence of rapid shifts during abrupt climate changes

The areas of abrupt change are focussed on each showing an abrupt change in ice accumulation as temperatures varied. Alley et al suggest that a near doubling of ice accumulation rates would require a c. 7°C warming in line with Dansgaard's predictions. Ice accumulation rate increases with temperature as more snow is able to fall when temperatures allow for greater air humidity increasing snowfall.

If the previous two articles weren't enough evidence for rapid abrupt climatic change at the end of the YD, the paper by Steffensen et al (2008) which I posted in my second post "Late Glacial and Holocene climate variability" details a deuterium excess shift occurring over just 1-3 years. 

Today's post has provided some ice core evidence to suggest that the Younger Dryas ended very abruptly. It has also shown that abrupt climate shifts can occur in human life expectancy time-scales. Towards the end of this blog, I will be assessing past changes and how they could be realised in the future. Although the YD was a significant event in Earth's recent history, a large increase in freshwater from ice-cap melting could feasibly occur again over an alarmingly rapid time-period, possible within our life-time. Next time, I will wrap up my discussion of the Younger Dryas before moving on to another large abrupt climate shift which occurred 8.2 ka BP.

(1) Dansgaard et al (1989) doi: 10.1038/339532a0
(2) Alley et al (1993) doi: 10.1038/362527a0
(3) Steffensen et al. (2008) doi: 10.0.4.102/science.1157707

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