The Younger Dryas: Part 2- Murton et al. (2010)

Following on from the last post which offered a conventional take on the Younger Dryas,  a paper I’ve recently read by Murton et al. (2010) offers a fascinating new insight into the relationship between ocean circulation and abrupt climate change. The conventional view to recall, is that an outburst from Lake Agassiz led to freshwater discharge via the St. Lawrence valley into the North Atlantic ocean, suppressing the Atlantic meridional overturning circulation (AMOC), which in turn caused abrupt climate change via cooling experienced in the North Atlantic, during the Younger Dryas (Broecker et al., 1989). However, to gauge an accurate understanding of the link between freshwater input and abrupt climate change in the context of the changes in ocean circulation hypothesised during the Younger Dryas case requires all the potential pathways and timing of freshwater discharges to be identified.  

Murton et al. (2010) conducted a field investigation on northeast Richards Island, an intervalley between the Channel- Kugmallit Trough valley (east) and Middle Channel- Mackenzie Trough systems (west) . They estimated the age of intervalley erosion and gravel deposition through optical stimulated luminescence dating eleven samples above and below erosion surface at six locations.  Through determination of the age of the Mackenzie River flood into the Arctic Ocean after 13,000 yr ago near the onset of the Younger Dryas, the authors attribute the flood to a boulder terrace near Fort McMurray with radiocarbon dates of 11,500 yr ago. The geomorphological evidence presented is intriguing; the Lake Agassiz overflow via St. Lawrence route into the North Atlantic Ocean appears to not be the only flow at the time (Figure 1).

Figure 1: Large proglacial lakes along the retreating Laurentide Ice Sheet at 12.65-12.75 cal.kyr BP, near the start of the Younger Dryas. Three outlets are shown; northwest along the Mackenzie River to the Arctic Ocean, east via the St. Lawrence River to the North Atlantic Ocean and south along the Mississippi River to the Gulf of Mexico (Glacial ice= white, Proglacial lakes=dark blue and land= grey).

Source: Murton et al. (2010)

They postulated that a shift of approximately 50 km in the Laurentide Ice sheet (LIS) at 12.9 cal. kyr BP would open a corridor from Lake Agassiz to the Arctic Ocean during the Younger Dryas (Figure 3). In turn, the advance of the LIS at approximately 11.5 cal. kyr BP closed this corridor and the glacier dam subsequently retreated. A second outburst from Lake Agassiz embedded a boulder terrace in Athabasca valley, depositing younger and boulder terraces, supported by fluvial gravels downstream in Richards islands dated between 11.7 kyr – 9.3 kyr by optically simulated luminescence.

Figure 3: Modelled palaeotopgraphy of the Fort McMurray region at the Herman beach stage of Lake Agassiz.  Stage age= 10.9 ¹⁴C kyr BP- 12.9 cal.kyr BP with selected radiocarbon dates shown.  The following Glacial boundaries are shown; 11.0 ¹⁴C kyr BP= 12.9 cal.kyr BP, 10.5 ¹⁴C kyr BP= 12.5 cal. kyr BP, 10.25 ¹⁴C kyr BP= 12.0 cal. kyr BP and 10.0 ¹⁴ C kyr BP= 11.5 cal. kyr BP. Yellow-red= areas above Lake Agassiz level at 50m intervals. Blue colours= areas below level of Lake Agassiz at 50m intervals to -600m. Overflow from Lake Agassiz is shown to have occurred into Clearwater Valley across the ‘Modern Divide’, and flowed west to Fort McMurray, where it entered Athabasca Valley, flowed north to Lake Athabasca and to the Mackenzie River and Arctic Ocean.

Source: Murton et al. (2010)

A point to note is that several dates exceed 10¹⁴ Ckyr BP, equivalently 11.5 cal. kyr BP, around the onset of the Younger Dryas. This has led the authors to suggest a Younger Dryas link between Lake Agassiz and the Arctic Ocean.

The study offers an atypical conclusion; that the Younger Dryas was triggered along Artic route contrary to previous thought. It can be acknowledged that this study extends our understanding of freshwater forcing and abrupt climate change.  Murton et al. (2010) suggest their hypothesis would consolidate the fact that overflow (via this route) would cause a suppression of the AMOC, triggering a negative feedback leading to rapid cooling.

Interestingly, I think the key point is that this debate (between the papers discussed in part 1 and 2) over a particular event in the paleoclimate record raises the issue of thresholds, defined in this sense as the level of freshwater needed to cause a suppression of the AMOC. This has telling implications as we shall see in future blogs to come,  for predictions of abrupt climate change.