Organic matter burried in sediments contains a multitude of palaeoceanographic and palaeoclimatic signals. With the major developments in instrumental analytical chemistry in the last 20 years, the decoding of these signals has become possible and molecular fossils have become an important tool in the assessment of palaeoenvironments. For instance, the determination of the number of double bonds in sedimentary long-chain ketones derived from coccolithophorid algae is now a widely applied technique in palaeoceanography to determine past sea-surface temperatures (Brassell, 1993). In this paper we will demonstrate the use of molecular fossils derived from the carotenoid isorenieratene to assess photic zone anoxia in ancient depositional environments. As an example the redox state of the North Atlantic during the Cenomanian/Turonian (C/T) anoxic event will be evaluated.
Photosynthetic green sulphur bacteria (Chlorobiaceae) are photoautotrophic organisms that are strictly anaerobic and require hydrogen sulphide. Their presence points to
an anoxic water layer that reaches into the photic zone. Chlorobiaceae produce a characteristic pigment, the diaromatic carotenoid isorenieratene, which can be used to trace such conditions. For example, in the recent Black Sea, Repeta et al. (1989) isolated isorenieratene from water samples at 80 m from the surface, indicating the coexistence of light and hydrogen sulphide at this depth. The diagenetically related C40 diaryl isoprenoid isorenieratane, which has been found in several recent and ancient sediments (e.g. Sinninghe Damsté et al., 1993), is therefore an excellent palaeoenvironmental indicator for "photic zone anoxia". Recognition of photic zone anoxia is, however, hampered by the various transformation reactions occurring with isorenieratene during dia- and catagenesis. We have discovered a wide range of novel diagenetic products of isorenieratene in rock samples from different geographical locations throughout the Phanerozoic which will be briefly discussed. Their identification was confirmed by their enriched d13C values, which relate to the biosynthesis of Chlorobiaceae via the reversed TCA-cycle.
The application of isorenieratene-derived products as a proxy for photic zone anoxia will be illustrated by analysis of black shales deposited during the C/T (ca. 91 Ma ago) oceanic anoxic events (OAE). These OAE's led to enhanced preservation of organic matter in the sedimentary record (deposition of black shales). Numerous models have been presented to explain the occurrences of black shales and a key factor in these models is the (partial) anoxicity of
the water column (Arthur and Sageman, 1994). With the molecular fossil isorenieratane (and derivatives thereof, see above) it is now possible to test this hypothesis. Therefore, a suite of black shales from the Cenomanian/Turonian boundary from DSDP/ODP cores from different sites in the North Atlantic was analysed. The results indicate that photic zone anoxia indeed occurred during the Cenomanian/ Turonian OAE but was restricted to the southern North Atlantic (palaeolatitude 0°-15°N). Interestingly, in this geographical area the organic carbon accumulation rates
are a factor 5-10 higher than at greater palaeolatitudes in
the Cretaceous North Atlantic. These data have important implications for the models explaining OAE's and seem to favour the restricted circulation model.
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Brassell, S.C., In Organic Geochemistry: Principles and Applications (Engel, M.H. & Macko, S. A., eds.). Plenum, New York, pp. 699-738 (1993)
Repeta, D.J., Simpson, D.J., Jørgensen, B.B. & Jannasch, H.W., Nature 342, 69-72 (1989).
Sinninghe Damsté, J.S., Wakeham, S.G., Kohnen, M.E.L., Hayes, J.M. & de Leeuw, J.W., Nature 362, 827-829 (1993).