MOLECULAR INDICATORS FOR PALEOENVIRONMENTAL CHANGE IN A MESSINIAN EVAPORITIC SEQUENCE (VENA DEL GESSO, ITALY) .2. HIGH-RESOLUTION VARIATIONS IN ABUNDANCES AND C-13 CONTENTS OF FREE AND SULFUR-HOUND CARBON SKELETONS IN A SINGLE MARL BED

The extractable organic matter of 10 immature samples from a marl bed of one evaporitic cycle of the Vena del Gesso sediments (Gessoso-solfi fera Fm., Messinian, Italy) was analyzed quantitatively for free hydro carbons and organic sulphur compounds. Nickel boride was used asa desu lphurizing agent to recover sulphur-bound lipids from the polar and as phaltene fractions. Carbon isotopic compositions (delta vs PDB) of fre e hydrocarbons and of S-bound hydrocarbons were also measured. Relatio nships between these carbon skeletons, precursor biolipids, and the or ganisms producing them could then be examined. Concentrations of S-bou nd lipids and free hydrocarbons and their delta values were plotted vs depth in the marl bed and the profiles were interpreted in terms of v ariations in source organisms, C-13 contents of the carbon source, and environmentally induced changes in isotopic fractionation; The overal l range of delta values measured was 24.7 parts per thousand, from -11 .6 parts per thousand for a component derived from green sulphur bacte ria (Chlorobiaceae) to -36.3 parts per thousand for a lipid derived fr om purple sulphur bacteria (Chromatiaceae). Deconvolution of mixtures, of components deriving from multiple sources (green and purple sulphu r bacteria, coccolithophorids, microalgae and higher plants) was somet imes possible because both quantitative and isotopic data were availab le and because either the free or S-bound pool sometimes appeared to c ontain material from a single source. Several free n-alkanes and S-bou nd lipids appeared to be specific products of upper-water-column prima ry producers (i.e. algae and cyanobacteria). Others derived from anaer obic photoautotrophs and from heterotrophic protozoa (ciliates), which apparently fed partly on Chlorobiaceae. Four groups of n-alkanes prod uced by algae or cyanobacteria were also recognized based on systemati c variations of abundance and isotopic composition with depth. For hyd rocarbons probably derived from microalgae, isotopic Variations are we ll correlated with those of total organic carbon. A resistant aliphati c biomacromolecule produced by microalgae is, therefore, probably an i mportant component of the kerogen. These variations reflect changes in the depositional environment and early diagenetic transformations. Ch anges in the concentrations of S-bound lipids induced by variations in conditions favourable for sulphurization were discriminated from thos e related to variations in primary producer assemblages. The water col umn of the lagoonal basin was stratified and photic zone anoxia occurr ed during the early and middle stages of marl deposition. During the l ast stage of the marl deposition the stratification collapsed due to a significant shallowing of the water column. Contributions from anaero bic photoautotrophs were apparently associated with variations in dept h of the chemocline.