KEROGEN CHEMICAL-STRUCTURE AND SOURCE ORGANISMS IN A CENOMANIAN ORGANIC-RICH BLACK SHALE (CENTRAL ITALY) - INDICATIONS FOR AN IMPORTANT ROLE OF THE SORPTIVE PROTECTION PATHWAY

The Bonarelli horizon is an important Cenomanian/Turonian black shale in the Umbria-Marche Appenines of Central Italy. It appears as the fin al term of a cyclic deposition of ca. 40 thinner, laminated, clayey or siliceous organic-rich levels. The kerogen of the Cenomanian clayey b lack level located 12 m below the Bonarelli, termed NN 8c, was examine d via microscopic, spectroscopic and pyrolytic methods. The chemical s tructure of this kerogen is based on a network of long aliphatic chain s linked via ether bridges. However, in addition to the classical norm al chains, NN 8c comprises C-40 chains with a lycopane skeleton. Such chains have only been previously reported in kerogen pyrolysates of tw o samples derived from the microalga Botryococcus braunii. However, an y contribution of B. braunii can be ruled out in the case of NN 8c ker ogen since it is nanoscopically amorphous. Based on delta(13)C values of their pyrolysis products, a common algal origin is demonstrated for the above two types of chains and hence there is a major algal input to this kerogen. However, some incorporation of bacterial lipids and a low contribution of higher plant material are indicated by the nature of some pyrolysis products. Although a large contribution of pyrite i s noted in the NN 8c sample, thus revealing an intense sulphate-reduci ng activity, no significant sulphur incorporation took place in the or ganic matter. Accordingly, the vulcanization pathway is not implicated in the formation of NN 8c kerogen. The selective preservation and the degradation-recondensation pathways were possibly involved but they a re unlikely to have played a major role. In fact, the ''sorptive prote ction'' process, favoured by the clayey nature of the mineral matrix, was probably the main pathway implicated in the formation of NN 8c ker ogen and this process may have provided an efficient protection to var ious diagenetically sensitive constituents, especially lipids. (C) 199 7 Elsevier Science Ltd. All rights reserved.