Lower Tertiary Black Shales near the North Pole: Organic-carbon Sources, Paleoenvironment and Source-rock Potential (IODP Expedition 302 - ACEX)
Ruediger Stein1 and Jan Backman2
1Geosciences, Alfred Wegener Institute for Polar and Marine Research, Bremerhaven, Germany.
2Department of Geology and Geochemistry, University of Stockholm, Stockholm, Sweden.
In late summer 2004, the Integrated Ocean Drilling Program (IODP) conducted one of the most transformational missions in the almost 40 year history of scientific ocean drilling: the IODP Expedition 302 or Arctic Coring Expedition (ACEX) (Backman, Moran et al., 2006; Backman and Moran, 2008). This technically challenging expedition to the Lomonosov Ridge near 88°N recovered the first long-term Cenozoic sediment record from the permantly ice-covered central Arctic Ocean-extending previous records from ~1.5 Ma to an unprecedented ~56 Ma. Some of the ACEX highlights include a Paleocene/Eocene Thermal Maximum (appr. 55 Ma) and Early Eocene Climate Optimum (EECO) substantially warmer than previously estimated; an episodic freshening of Arctic surface waters reflected in the major occurrence of the freshwater fern Azolla near 49 Ma; and ice-rafted debris and sea-ice diatoms having started to occur in the middle Eocene epoch (~46-47 Ma), i.e., some 35 myr earlier than previously thought.
During ACEX, in total a 430 m thick sequence of upper Cretaceous to Quaternary sediments has been drilled. The lower 230 m of the ACEX sequence consist of unique, very dark gray biosiliceous oozes and mudstones (“black shales“ in a broader sense) of Campanian and Paleogene (late Paleocene to middle Eocene) age, which are distinctly enriched in organic carbon reaching values of about 1 to 14% (Stein et al., 2006). Significant amounts of the organic matter preserved in these sediments is of algae-type origin and accumulated under anoxic/euxinic conditions. Detailed data on the source-rock potential of these black shales indicate that most of the Eocene sediments have a (fair to) good source-rock potential, prone to generate a gas/oil mixture. The source-rock potential of the Campanian and upper Paleocene sediments, on the other hand, is rather low. The presence of oil or gas already generated in-situ from the Campanian and Paleogene ACEX sediments at this part of Lomonosov Ridge, however, can be ruled out due to the immaturity of the ACEX sediments. If these sediments are buried more deeply, however, in-situ hydrocarbon formation is possible. This situation might occur in the more southern part of Lomonosov Ridge closer to the Eurasian continental margin, where sedimentation rates are significantly higher. The results of this study does also not mean that in the underlying deeper (Mesozoic) sedimentary rocks from the Lomonosov Ridge belonging to the rifted continental crustal block of the Eurasian continental margin, hydrocarbons could not have been generated.
Backman, J. and Moran, K., 2008. Editorial Summary: The Arctic Ocean Unplugged. Paleoceanography 23.
Backman, J., Moran, K., McInroy, D.B., Mayer, L.A., and the Expedition 302 Scientists, 2006. Proceedings IODP Exp 302, doi:10.2204/iodp.proc.302.104.2006.
Stein, R., Boucsein, B., and Meyer, H., 2006. Geophys. Res. Lett. 33, doi:10.1029/2006GL026776.
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