Palynological and organic geochemical characterization of marine and terrestrial Early Pleistocene climate in northwest Europe

Abstract

Ever since the Eocene Thermal Maximum, ~52-50 Ma, Cenozoic long-term climate evolution is characterized by a global cooling trend. In the Northern Hemisphere, this development eventually let to large scale glaciations from the Pliocene-Pleistocene transition (~2.6 Ma) onward. The exact rate and characteristics of this key transition on both land and sea in the Northern Hemisphere is yet unclear. Two marine cores, A15-3 and A15-4, from the northwestern part of the Dutch North Sea basin contain extensive Eridanos river delta sediments covering this transition. Paleomagnetic data, dinocyst and pollen records and foraminiferal proxies have provided a solid integrated timeframe. Gamma ray and magnetic susceptibility logs show presumed obliquity-length cycles, expressed in alternating clay and coarser grained sand layers, that were tentatively correlated with Marine Isotope Stages 103 to 94. Therefore, these cores are well suited for a high resolution coupled climate reconstruction of the terrestrial and marine environments during the Early Pleistocene. This climate reconstruction was obtained by generation of high resolution pollen records and records of several (organic) geochemical proxies. The main focus of this thesis lies on the application of novel paleotemperature proxies based on the distribution of glycerol dialkyl glycerol tetraethers, membrane lipids produced by microorganisms. Petrochemical proxies based on the distribution of long chain n-alkanes in combination with the C/N ratio and the Branched vs. Isoprenoid Tetraether (BIT) index generally show a dominant terrestrial derived source for the organic matter in the studied sediments, although there are episodes with increased marine influence coupled to sea level variations. High resolution pollen records and dinocyst assemblage scans show a dominance of “warm taxa”, e.g. Osmunda spores and Operculodinium israelianum cysts, in the coarser grained intervals. The clay intervals, on the contrary, are dominated by “cool taxa”, e.g. Ericaceae pollen and Filisphaera cysts. The organic geochemical paleothermometers TEX86 and MBT/CBT, used for the reconstruction of sea surface temperatures (SSTs) and annual mean air temperatures (MATs), respectively, show high temperatures in the clay intervals, with SSTs up to ~17°C and MATs up to ~21°C. For the coarser intervals lower temperatures are reconstructed, with SST minima close to 9°C and MAT minima around 6.5°C. Amplitudes in temperature reconstructions are comparable with results of previous studies, absolute temperatures are higher than expected. The organic geochemical proxies contradict the palynology, and thus the previously established tentative correlation to the Marine Isotope Stages. A possible explanation for the difference in phasing between palynology and the organic geochemical proxies is found in the two different source areas were the deposited sediments are transported from, in combination with reworking by deltaic and fluvial dynamics. The different marine and terrestrial climate proxies are generally in phase with each other, from which it is concluded that there was a strong climate coupling of marine and terrestrial environments in the Early Pleistocene of northwest Europe.