The ‘8.2-kyr’ Event

The Younger Dryas is not the only episode of abrupt climate change documented in the transition to the Early Holocene (Oldfield. 2005). High-latitude records widely show a sharp dip in temperature at 8200 yr BP, temperature decreases by 4-8 °C in c. Greenland and 1.5- 3°C at marine and terrestrial sites around N. E. North Atlantic Ocean (Barber et al., 1999)

Barber et al. (1999) argue the cooling may hint that ocean-atmosphere heat transfer was reduced in the North Atlantic via a freshwater outburst from  Lake Agassiz and Ojibway as the Hudson Bay Ice mass disintegrated (Figure 1).

Figure 1. Northeast Canada and adjacent seas. Former ice-sheet margins are shown for 8.9 cal. kyr ago and 8.2 cal. kyr ago (vertical hatched line and thick grey line respectively), before and after disintegration of ice in Central Hudson Bay. At the same time, northward drainage is shown via the Hudson Bay and Hudson Strait. Horizontal hatching shows Lake Agassiz and Ojibway.

Source: Barber et al. (1999: 345).

Based on determining of ice-core layers using radiocarbon dating, they present evidence of a freshwater pulse at 8.470 cal. yr BP which they believe coincided with the 8,400 cal. yr BP climate cooling observed in Greenland and around the North Atlantic (Figure 2).

Figure 2.  Proxy records of the 8.2 kyr event. ¹⁴C (top) and calendar (lower) timescales are shown. Upper curve shows Cariaco basin greyscale record, with reduced values indicating high zonal-speed wind attributed to high-latitude cooling. Lower curve shows bidecadal ¹⁸ O values of ice from Greenland Ice Sheet Project 2 (GISP 2), to reflect temperature of precipitation, with negative values indicating cooler temperature. The age for lake drainage event is shown; 8.470 cal. yr BP is shown (vertical dashed line).

Source: Barber et al. (1999: 347).

   

The logic being that the freshwater pulse reduced sea surface salinity in the North-West Atlantic; reducing Intermediate Water in Labrador seas and North Atlantic Deep Water, causing reduced northward heat transport associated with the meridional overturning circulation in the North Atlantic. The idea that abrupt climate change caused a shift in North Atlantic freshwater balance represents a significant point for our understanding of ocean circulation.

From a paleoclimatic perspective, significance of this event for future predictions lies in the attribution and  proof that such a large freshwater pulse can disrupt ocean circulation and climate under interglacial conditions (Oldfield, 2005). Incidentally. as we will see in the next post and  through examination of ‘Heinrich Events’ and ‘Dansgaard- Oescheger Cycles’ in future posts, events have been documented whereby changes in freshwater content cause episodic changes in patterns of ocean circulation. However, it is the difficulty of attributing the mechanics whereby a large freshwater pulse can disrupt ocean circulation that is a key issue in the science of climate change.