Well preserved Palaeogene and Cretaceous biomarkers from the Kilwa area, Tanzania

摘要

We have investigated the biomarker composition of sediments ranging in age from 30 to 70 Ma from the Kilwa area of Tanzania. They are dominated by material of terrestrial (n-alkanols, n-alkanoic acids, n-alkanes and triterpenoids) and bacterial [hopanoids and a dipentadecyl glycerol diether (DGD)] origin with only a minor contribution of marine origin (crenarchaeol and some algal steroids). This indicates a marine setting dominated by higher plant input, which most likely stimulated sedimentary microbial activity, including sulfate reduction. The lipid composition, specifically the presence of bacteriohopanepolyols (BHPs) and the stereochemistry of hopanoids, suggests a low degree of thermal maturity. Many biomarkers have still retained functionality even after 30-70 million years. This is in good agreement with the exceptional preservation of foraminifera and nannofossils and can probably be attributed to the very low sediment permeability and burial depth. There is a clear distinction in the biomarker distributions between the Oligocene-age sediments collected at the end of the peninsula near Kilwa Masoko and the older sediments collected from elsewhere, indicating differences in depositional environment. Sediments from the end of the peninsula have higher branched and isoprenoid tetraether index values, indicating a relatively higher terrestrial input. They also contain two tentatively assigned C-28 hopenes (28,30-dinorneohop-13(18)-ene and 28,30-dinorhop-17(21)-ene), (aromatic) des-A-triterpenes, des-E-hopenes, aromatic pentacyclic triterpenoids, C-33 DGD, archaeol and BHPs, which are absent from or only present in small amounts in the other sediments. These differences likely reflect an evolution of the depositional area from a relative open shelf environment with substantial water depth to a shallower setting during the early Oligocene. (c) 2006 Elsevier Ltd. All rights reserved.