The paleoclimatic interpretation of clay minerals in soils and weathering profiles

Abstract

The interpretation of paleoclays for paleoclimatic purposes is based on five major assumptions: (1) clay mineral formation is directly related to climatic parameters; (2) once formed in the weathering milieu, clay minerals are stable and do not change any more as long as the climate remains stable (pre-burial stability); (3) clay mineral assemblages are uniform throughout the weathering profile; (4) once formed or deposited and buried, clay minerals are stable (post-burial stability); (5) the sensitivity of clay minerals towards environmental factors is uniform. All these assumptions have only a limited validity. Clay mineral formation is in few cases directly related to climatic parameters, nor do clay minerals always represent the stable end products in equilibrium with environmental factors. The vertical distribution pattern of authigenically formed clay minerals is seldom monomineralic. Post-depositional changes are not infrequent. The sensitivity of clay minerals to environmental factors is variable. The use of paleoclays occurring in paleosols and weathering profiles is reviewed. Paleoclays occurring in paleosols or weathering profiles are, in well-defined situations, suitable for paleoclimatic interpretation. At our present state of knowledge, references as to the nature of climates of the past that are based solely on the interpretation of paleoclays are warranted only in exceptional cases. Authigenic occurrences of clay minerals with limited stability fields that can be used as ‘marker minerals’ and the isotopic composition of paleoclay minerals promise, with future research, to increase the value of paleoclays as paleoclimatic indicators.