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

As distribution of clay minerals in modern oceans seems to be controlled by contemporary climates, marine clays in ancient deposits have been widely u sed to reconstruct palaeoclimates. In this review, we examine this question mainly from the continental origin of the clay minerals and show the diffi culties of retrospective analysis of the influence of climate on common mar ine deposits. We examine successively how soils develop and behave, are rew orked and palaeoweatherings are preserved, focusing the review on kaoliniti c soils indicative of tropical wet climates. Several stages intervene betwe en 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 alte r the palaeoclimatic signal of kaolinite in the sedimentary record. Althoug h the kaolinite deposited in recent sediments of the world oceans is consis tent with wet climatic zones, this is mainly because the major kaolinitic a nd bauxitic palaeosurfaces coincide with the tropical areas where kaolinite is still developing at present. (2) Possible development of deep kaoliniti c profiles at high latitudes and cold conditions, under CO2-enriched palaeo atmospheres, or even by unusual acidic weathering linked with dry climates is misleading palaeoenvironments for palaeoclimatic reconstruction. (3) Ero sion and transport processes always cloud the signal to some extent, becaus e frequently, erosion products from the bedrock and different soil horizons will be mixed and major drainage systems may include soils of different cl imatic zones. (4) The arrival of the soil clay assemblages in the basin ine vitably lags against their formation on the continent and may occur several million years after formation of the soils on the continent and climate ma y have considerably changed at that time. It is pointed out that sedimentol ogists and palaeoclimatologists should consider the complexity of the relat ions between soil and sedimentary clay minerals. Soil formation rates are s low and therefore the resolution of the palaeoclimatic record in marine cla y may not be closer than 1 or 2 Ma. At present, the palaeoclimatic interpre tations 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 microfossi l studies. Clay mineral assemblages may provide integrated records of overa ll climatic impacts, whereas other techniques are more likely to reveal loc al or temporary climates. (C) 2000 Elsevier Science B.V. All rights reserve d.