Palaeoclimatic interprétation of clay minerals in marine deposits : an outlook from the continental origin.

International audienceAs distribution of clay minerals in modern oceans seems to be controlled by contemporary climates, marine clays in ancient deposits have been widely used to reconstruct palaeoclimates. In this review, we examine this question mainly from the continental origin of the clay minerals and show the difficulties of retrospective analysis of the influence of climate on common marine deposits. We examine successively how soils develop and behave, are reworked and palaeoweatherings are preserved, focusing the review on kaolinitic soils indicative of tropical wet climates. Several stages intervene between the time of clay mineral development in soils and their final deposition in the basin: 1 The persistence over geological times of huge amounts of kaolinitic palaeoweathering materials in the landscapes may seriously alter the palaeoclimatic signal of kaolinite in the sedimentary record. Although the kaolinite deposited in recent sediments of the world oceans is consistent with wet climatic zones, this is mainly because the major kaolinitic and bauxitic palaeosurfaces coincide with the tropical. areas where kaolinite is still developing at present. 2 Possible development of deep kaolinitic profiles at high latitudes and cold conditions, under CO -enriched palaeoatmospheres, or even by unusual acidic weathering linked with dry climates is 2 . misleading plaeoenvironments for palaeoclimatic reconstruction. 3 Erosion and transport processes always cloud the signal to some extent, because frequently, erosion products from the bedrock and different soil horizons will be mixed and major drainage systems may include soils of different climatic zones. 4 The arrival of the soil clay assemblages in the basin inevitably lags against their formation on the continent and may occur several million years after formation of the soils on the continent and climate may have considerably changed at that time. It is pointed out that sedimentologists and palaeoclimatologists should consider the complexity of the relations between soil and sedimentary clay minerals. Soil formation rates are slow and therefore the resolution of the palaeoclimatic record in marine clay may not be closer than 1 or 2 Ma. At present, the palaeoclimatic interpretations of marine clay assemblages are yielding, at best, nothing more than rather broad palaeoclimatic information. This approach cannot achieve the same degree of resolution as other techniques such as isotope or microfossil studies. Clay mineral assemblages may provide integrated records of overall climatic impacts, whereas other techniques are more likely to reveal local or temporary climates