Relict Cryogenic Mounds in the UK as Evidence of Climate Change

Abstract

Relict perennial cryogenic mounds, (the remains of pingos or paisas) have long been identified in the UK and have been attributed to the Last Glacial or Younger Dryas cold periods. These features take the form of circular to oval depressions surrounded completely or partially by a raised rim or rampart. The depressions very often contain a wetland with an organic soil or peat. At depth within the depressions there can be up to several metres of soft sediments (very often a silt-clay). The ramparted depressions are usually found in clusters where individual features may overlap such that they share ramparts or even where the depressions merge to produce a ‘figure-of-eight’ shape in plan. Such relict periglacial features are extremely useful for palaeoclimatic reconstruction, since they can indicate the average thermal condition of the ground during the period in which they were actively forming. Many relict cryogenic mounds in the UK have been interpreted as the remains of hydraulic pingos, features which in contemporary cold climate areas indicate either discontinuous permafrost (mean annual air temperature (MAAT) of ≤ -3° C) or continuous permafrost (MAAT of ≤ -7° C). Some of these features, however, have now been re-interpreted as the remains of ‘mineral-cored’ paisas, features that are indicative of only sporadic permafrost (MAAT of ≤ -1° C) or discontinuous permafrost (MAAT of ≤ 3°C). Thus the varying interpretation and classification of these geomorphological features can lead to a wide disparity in their palaeoclimatic significance. Clearly, to obtain the correct palaeoclimatic information, such features must be accurately identified and appropriate modern analogues utilised in their interpretation.