The Younger Dryas: Part 1

The transition from the last glacial maximum (C. 1800 ¹⁴c yrs ago) to present interglacial (10,000 ¹⁴C yrs ago), involved episodes of abrupt climate change (Oldfield, 2005). An example of this is the extended cold period, termed the Younger Dryas, which, led to key changes in global ocean circulation and a decline in temperatures in the North Atlantic from 11,000 to 10,000 yr BP (Oldfield, 2005). 

Rooth (1982) hypothesised that the Younger Dryas was caused by a diversion of water from glacial Lake Agassiz from the Mississippi drainage to St. Lawrence drainage system. Prior to 11,000 yr BP, meltwater to Lake Agassiz overflowed to Gulf of Mexico via Mississippi drainage system (Broecker et al., 1989) (Figure 1a.)

Figure 1a: Laurentide ice sheet and overflow route from Lake Agassiz basin to the Gulf of Mexico pre- Younger Dryas. (note: the area outlined is enlarged in Figure 1b)

Source: (Broecker et al., 1989)

By 11,000 yr BP, the Laurentide ice sheet retreated to create channels to Lake Superior basin via Great Lakes and St. Lawrence valley to N. Atlantic with a discharge estimated at 30,000 m³ (Broecker et al., 1989) (Figure 1b.)

Figure 1b: Overflow route from Lake Agassiz through the Great Lakes to the St. Lawrence valley and Northern Atlantic during the Younger Dryas.

Source: Broecker et al. (1989)

Broecker et al. (1989) postulated that a meltwater pulse would have affected ocean circulation by causing cooling of adjacent land and atmosphere over the North Atlantic. As recalled in post two, freshwater input will lead to a reduction in sea surface salinity (SSS) and density of water, leading to a decline in North Atlantic Deep Water (NADW) formation, thus suppressing the Atlantic meridional overturning circulation (Wunsch, 2002).  Given the difference in heat capacity between the oceans and atmosphere (due to the chemical properties of air and water), this would create a delay before cooling in the North Atlantic, causing the  climate in  Western Europe to cool by several degrees (Broecker et al., 1989).

In a classic paper, Broecker et al. (1989) investigated the link between the freshwater outburst and ocean circulation. Data concerning O¹⁸ and radiocarbon on planktonic foraminifera were taken from the Orca Basin core GW21- PC4 (Broecker et al., 1989). Negative spikes in cores support the concept of major influx in meltwater via Mississippi river to Gulf of Mexico 11,200- 10,000 radiocarbon yr ago, appear broadly consistent with the Lake Agassiz record (although the authors were unclear at the how much of the change in the white form of G. ruber is due to meltwater) (Broecker et al.1989) (Figure 2).

 Figure 2: Oxygen isotope changes in two species of Globigenniodes ruber(white and pink, left and right) from core EN32-PC4. Ages are accelerator radiocarbon ages. Both show a major meltwater spike from 320-200 cm. 

Source: (Broecker et al., 1989)

Broecker et al. (1989)’s observation, in relation to the relationship between ocean circulation and abrupt climatic change has been described as the ‘hosing scenario’ (Wunsch, 2010: 7). This is the view that that the North Atlantic largely controls the climate system, with freshwater inputs largely contributing to a shutdown of the North Atlantic thermohaline circulation (NATHC) (Wunsch, 2010). However, re-diversion of meltwater from Mississippi to St. Lawrence after 10,000 yr BP failed to produce a cold event comparable to that observed in the Younger Dryas (Broecker et al., 1989). Broecker et al. (1989) and subsequently, Broecker (2006) acknowledged the possibility of alternative meltwater diversion routes. 

As shown in Part 2, this has led to the traditional view of the Younger Dryas to come under scrutiny. This highlights the problems that begin to emerge when examining links between past changes in ocean circulation and climate. Stay tuned to find out further….