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

Abstract

The Bonarelli horizon is an important Cenomanian/Turonian black shale in the Umbria-Marche Appenines of Central Italy. It appears as the final term of a cyclic deposition of ca. 40 thinner, laminated, clayey or siliceous organic-rich levels. The kerogen of the Cenomanian clayey black level located 12 m below the Bonarelli, termed NN 8c, was examined via microscopic, spectroscopic and pyrolytic methods. The chemical structure of this kerogen is based on a network of long aliphatic chains linked via ether bridges. However, in addition to the classical normal chains, NN 8c comprises C 40 chains with a lycopane skeleton. Such chains have only been previously reported in kerogen pyrolysates of two samples derived from the microalga Botryococcus braunii. However, any contribution of B. braunii can be ruled out in the case of NN 8c kerogen since it is nanoscopically amorphous. Based on δ 13C 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 is noted in the NN 8c sample, thus revealing an intense sulphate-reducing activity, no significant sulphur incorporation took place in the organic 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 are unlikely to have played a major role. In fact, the “sorptive protection” process, favoured by the clayey nature of the mineral matrix, was probably the main pathway implicated in the formation of NN 8c kerogen and this process may have provided an efficient protection to various diagenetically sensitive constituents, especially lipids.