Monday, February 20, 2006
The mysterious case of the missing loess
An intriguing paper concerning periglacial loess (aeolian silt) has been published in the February 2006 issue of Earth-Science Reviews. One of the problems discussed in this review is that much less loess is deposited nowadays in periglacial areas than during the last ice age (apart from, possibly, Alaska). Although we might expect a belt of loess to be deposited in front of the present-day ice caps, hardly any significant recent loess is to be found. The standard uniformitarian principle (“the present is the key to the past”) appears not to apply to loess formation. A second problem raised by the review is the almost complete lack of loess deposits from ice ages before the last one (although there are claims of some pre-Pleistocene loessites, not necessarily deposited under periglacial conditions). Removal by erosion during interglacial episodes is said to be inadequate to explain their almost universal absence. Further evidence, perhaps, that there was only one ice age in recent Earth history, consistent with the young-age creation model?
van Loon AJ, ‘Lost loesses’, Earth-Science Reviews 2006;74(3-4):309-316.
Abstract: Loesses form wide belts in front of previously glaciated areas. Their thicknesses may be considerable, changing in Eurasia from maximally a few metres in the west to a hundred metres or more in the east. The Eastern (particularly Chinese) loesses are mostly unrelated to glaciations. The periglacial loesses from China and elsewhere predominantly date from the last Pleistocene glaciation: relatively few comparable occurrences are known from earlier Quaternary glaciations. As it is difficult to imagine that the conditions in front of the land-ice masses during the earlier glaciations differed fundamentally from those of the last one, considerable quantities of loess must have disappeared. This disappearance, which is commonly ascribed to fluvial and eolian erosion, is not easily explained as equivalent deposits that may have the older loesses as a source, are practically absent. A possible explanation might be that loess is recycled during successive glaciations. Some loess disappears during interglacials by erosion, but this quantity is more than compensated by the formation of new silt particles. The implication would be that the loess deposits increase in volume for each new glaciation.
van Loon AJ, ‘Lost loesses’, Earth-Science Reviews 2006;74(3-4):309-316.
Abstract: Loesses form wide belts in front of previously glaciated areas. Their thicknesses may be considerable, changing in Eurasia from maximally a few metres in the west to a hundred metres or more in the east. The Eastern (particularly Chinese) loesses are mostly unrelated to glaciations. The periglacial loesses from China and elsewhere predominantly date from the last Pleistocene glaciation: relatively few comparable occurrences are known from earlier Quaternary glaciations. As it is difficult to imagine that the conditions in front of the land-ice masses during the earlier glaciations differed fundamentally from those of the last one, considerable quantities of loess must have disappeared. This disappearance, which is commonly ascribed to fluvial and eolian erosion, is not easily explained as equivalent deposits that may have the older loesses as a source, are practically absent. A possible explanation might be that loess is recycled during successive glaciations. Some loess disappears during interglacials by erosion, but this quantity is more than compensated by the formation of new silt particles. The implication would be that the loess deposits increase in volume for each new glaciation.
# posted by Nemagraptus @ 2:25 AM 
