After introducing D/O cycles and Heinrich events, we now turn to review the mechanisms associated with millennial-scale climatic processes. In a nutshell, there are two competing theories in the literature; externally forced global climate change and internal ice sheet dynamics (Maslin et al., 2001 in Seidov et al., 2001). The former shall be the subject of an upcoming post, as mechanisms of iceberg discharges are the subject of the current post.
The prevailing hypothesis is the Binge-Purge hypothesis caused by internal stability of the Laurentide Ice Sheet, formulated by MacAyeal (1993) (Figure 1). Essentially, ice sheets on unconsolidated sediments when frozen acts analogous to cement, supporting the weight of the growing ice sheet. Upon ice sheet expansion, geothermal heat from the earth’s crust in conjunction with that from ice-ice friction, trapped by the overlying ice sheet, causes an insulating effect. This causes the temperature of the sediment to increase until a critical threshold is reached. Here, sediment becomes soft causing the base of the ice sheet to allow ice overflow through the Hudson strait to the North Atlantic. This in turn, can lead to a sudden loss of ice mass and reduce the effect of refreezing to the point when ice reverts a phase of slow-build up. MacAyeal (1993) observes that this system of progressive ice build-up, melting and surge followed by renewed build up have a periodicity of approximately 7,000 years when compared to intervals between the last Heinrich events (outlined in the last post).
Figure 1. A simple kitchen oscillator as an analogy to describe the Heinrich event cycle of the Laurentide Ice Sheet. Initially, the container sits upright as the centre of mass is assumed to position between the bottom of the container and the axle. As water drips slowly (binge phase), the centre of mass rises to a point where it exceeds the axle, causing the container to become unstable and purge the contents onto the floor (purge phase). Once the container has been emptied, it reverts back to the initial upright position and slowly fills with water, thus repeating the cycle.
Source: MacAyeal (1993)
Buttressing ice shelves and ocean forcing can also be attributed as another mechanism (Hulbe, 2010). This can be seen in the form of a pervasive coupled ocean, ice-shelf and ice-sheet mechanism of mass flow (Alvarez- Solas et al., 2010). Snow accumulation on ice sheet encourages glacial flow via an ice shelf, and is lost via melting. When this is forced to occur against ocean-driven changes, the ice sheet responds with iceberg discharge. The frequency of this response is modulated by the rate of snow-accumulation and ice-stream sliding. Changes in the rate of snow-accumulation could attribute for the production of irregular Heinrich events.
These concepts highlight the complexity of debate in internal ice sheet dynamics as processes of millennial-scale climate variability. We now turn to questions of impact and one of the most remarkable findings in the study of millennial-scale climate events…..
