|Laurentide ice sheet during Last Glacial Maximum (LGM)|
Sunday, 14 August 2011
The Denton model is essentially climatically driven as it based on global cooling leading to greater snowfall and thus a rapid expansion of ice-sheets/shelves and then marine ablation calving the icebergs from the expanding ice-shelf. Fundamentally the Denton model lays blame with the external cause of ice-sheet/shelf expansion as a result of global cooling as the forcing factor.
- Evidence supports cooling in the North Atlantic before the Heinrich events
- The model explains the surging of South American glaciers and other global responses without the need for complicated teleconnections, during the same period of time
- Around 1.5ka climate cycles are known to have occured during the last 10ka and, despite them being an order of magnitude smaller than Dansgaard-Oeschger events, they too suggest external forcing over internal forcing
- The model cannot explain the rapidity of events
- Heinrich events do not have consistent cyclicity as early on in the last glacial period they occured every 13ka but then later on they occured every 7ka
The MacAyeal 'binge-purge' model for iceberg armadas
- The model is able to explain the rapid initiation and termination of Heinrich events - something that the Denton model fails to do
- Due to its dependency on the size of the ice sheet, it can explain the irregular cyclicity of Heinrich events
- Provides an explanation of the large amount of ice-rafted debris, which forms the Heinrich layer, found in the North Atlantic
- Cannot explain the cooling known to have occured before each Heinrich event
- It needs a mechanism, such as the NADW, to transport the 'signal' around the world
Both models have their advantages and disadvantages and as a result there is a lack of consistency in which is favoured by scientists. For some time though, the 'binge-purge' model was favoured by most but then it was revealed that some of the Heinrich layers contained material that could have only came from other ice sheets, other than Laurentide. Attempts to generate a combined model have failed to paint a clearer picture of the mechanism provoking iceberg armadas but have led to further research into the conditions surrounding such events. Some of the sediment in the Heinrich layers has been linked to areas other than those covered by the Laurentide ice-sheet, such as Iceland due to basaltic glass fragments. This suggests that separate ice sheets surged simultaneously, something unlikely unless climatically driven or as a result of increased marine ablation due to eustatic sea-level rise thanks to Laurentide ice-sheet melt. The westward thickening of Heinrich layers, across the Atlantic, and its continuation towards Hudson Bay, point to the latter being correct and it is possible that its break-up triggered a response in other ice-sheets. Evidence insinuates that three gradual advances and rapid retreats of the Laurentide ice-sheet occurred towards the end of the last glacial, with glacial advances culminating before Heinrich events; thus provoking rapid ice discharge into the Atlantic, reducing southward ice flow and resulting in rapid retreat of LIS. There also appears to be synchronicity between the North Atlantic ice-rafting events and ice-sheet growth/ collapse in the Andes and New Zealand; something which supports the idea of strong inter-hemispheric coupling of changes in temperature and therefore global forcing of climate change. Research into the abundance of left-hand coiling in foraminiferid populations in ocean floor sediment cores, accompanied with studies into the origins of lithic sediments, have indicated that iceberg-calving events have occurred more frequently than first believed (intervals of 2-3ka), albeit on a smaller scale than the six originally identified Heinrich events. Of greater importance, is that many of the fragment peaks coincided with >90% proportions of left-hand coiling foraminiferid, thus revealing that the launching of iceberg armadas corresponded with low North Atlantic SST’s, symbolising stadial periods followed by the prompt warming leading into an interstadials. The use of d18O variations as a proxy record for eustatic sea-level rise and a lack of coherence between evidence of temperature rises in ice-cores from Greenland and Antarctica imply that meltwater discharge pulses, during the above events, originate from the Northern Hemisphere; an inference only endorsed by changes in salinity that are known to have occurred in the North Atlantic. As a result it can be said with confidence that Heinrich events influenced alterations in ocean circulation.