Climate Induced Geochemical Variations in Dutch Overbank Deposits During the Lower Pleistocene and Pliocene

B. J. H. van Os Rijks Geologische Dienst, 157, 2000 AD Haarlem, The Netherlands

D. J. Huisman Agricultural University Wageningen, 37 6700 AA Wageningen, The Netherlands

G. Th. Klaver Rijks Geologische Dienst, 157, 2000 AD Haarlem, The Netherlands

M. W. van den Berg Rijks Geologische Dienst, 157, 2000 AD Haarlem, The Netherlands

W. E. Westerhoff Rijks Geologische Dienst, 35, 5670 AA Nuenen, The Netherlands

In the continental/marine succesion from the Dutch Pliocene towards the Lower Pleistocene, fine grained sediments are found with intercalated organic rich layers (brown coal). The sediments form a record of Pliocene (Brunssumian) to Early Pleistocene (Tiglien) times. During our sampled period of the Brunssumian, the climate changed from relative temperate to warm (sub)tropical. From the Reuverian to the early Pleistocene, cooling took place to temperate with glaciations. A lowering of the sealevel is associated with this cooling trend. In this study, we investigate the effects of climate change on the geochemical composition in these sediments.

In the Brunssumian high Ti (TiO2 1.5-2%) and Al (Al2O3 15-22%) contents suggest large input of mainly tropical soil weathering input. Clay mineralogical data shows that the main clay mineral in these sediments is kaolinite. The K in these sediments is present in illite or micas. The clays intercalate with organic rich layers (brown coal) containing up to 1% pyrite, suggesting marine influence. Low K/Al ratios can be associated with periods of (sub)tropical weathering wheras high K/Al ratios correlate with a less extensive weathering of the hinterland. This is due to the amount of kaolinite that is formed and transported
relative to the amount of illite. High Ti contents can be explained by the formation of clay-sized secondary anatase in the soils of the source area.

In the younger sediments of Reuverian age, lower Ti and Al, and higher K/Al ratios are found, suggesting less intensive weathering than during the Brunssumian. This trend confirms the cooling recorded by the pollen data. In addition, the absence of pyrite and organic matter in the sampled section indicates that marine influence diminished during the Reuverian. In the
overlying Tiglian sediments Ti and Al decrease even further while increased K/Al ratios are found compared with the Reuverian sediments. In these Early Pleistocene sediments only low levels of kaolinite are found. The Ti/Al in this stage is at a value that is normal for the Ti content in clay minerals. This confirms that no secondary anatase was formed and residual enrichment of Ti minerals did not take place in this stage.This indicates that climate changed to temperate which correlates with the occurence of the first Pleistocene ice ages.

Our results show that large scale climate change like the one at the end of the Pliocene are closely matched by geochemical changes in fluvial sediments. This geochemical fingerprinting can be used for refinement of existing (pollen) records and allow for high resolution stratigraphic correlations in terrestrial sequences.