The short-term diagenetic fate of 5~t-cholestan-3fl-ol: in situ radiolabelled incubations in algal mats K. L. H. EDMUNDS, S. C. BRASSELL and G. EGLINTON Organic Geochemistry Unit, University of Bristol, School of Chemistry, Cantock's Close, Bristol BS8 ITS, England Abstract--Analysis of the lipid products from three one-month incubations of 4-1*C-5~t-cholestan-3fl-ol in algal mats at Laguna Guerrero Negro, Baja California, Mexico, shows the formation of 14C-5:t-cholestan-3-one in each case (1%, 0.49o and 0.4~, ~, conversionl (identified by radio-TLC, radio-GC, capillary GC and capillary GC-MS). A likely explanation for the presence of radioactivity (2.1'~,, 1.6~,,, and l.l"i, respectively) in the non-solvent-extractable residues is the incorporation of the stanol into the kerogen or humic acids, or from adsorption to particulate material. INTRODUCTION An important area of organic geochemical research is the investigation of early-stage diagnetic processes in Recent sediments. Direct information on such conversions may be obtained by incubation experiments using radiolabelled compounds, and this method has been used in recent years to study the fate of oleic acid, phytol, and cholesterol in estuarine and lacustrine environments (RHEAD et al., 1971b and 1972; GASKELL et al., 1976; BROOKSand MAXWELL, 1974; GASKELLand EGLINTON, 1975). The sedimentary environment selected for the present study is that of the algal mat, which possesses a unique combination of factors making it a particularly appropriate type of sediment for incubation studies. The microbial communities in algal mats are distinctly stratified (GOLUBIC, 1976), enabling the selection of a specific zone of biological activity for investigation. They receive relatively simple inputs so that their lipid compo- sition (CARDOSO et al., 1976, 1978) is less complex than that of other Recent sediments. Algal mats occur in hypersaline and hot spring environments (CASTENHOLZ, 1969; DOE- MEL and BROCK, 1974, 1977), which are often remote from the pollution introduced by major urban and industrial areas. Their high organic carbon content makes them amen- able to organic geochemical analysis. Finally, they are of considerable geological signifi- cance since they have existed since the Precambrian and algae are believed to have been important contributors to various oil shales, such as the Green River shale (HENDERSON et al., 1969) and the Kimmeridgean (GALLOIS, 1976). 5ct-Cholestan-3fl-ol was chosen for incubation for two main reasons: (1) The conversion of cholesterol to 5~- and 5fl-cholestan-3fl-ol has been demonstrated for a sediment by GASKELL and EGLINTON (1975), using laC-labelling and is shown in Fig. 1 which is a composite of the results of several investigations (see Legend). This incubation experiment verified the hypothesis of stenol to stanol conversion put forward by GASKELL and EGLINTON (1974) to account for the parallel carbon number distribu- tions of sterols and stanols and for the increase in stanol/stenol concentration ratio with depth in Rostherne Mere (Cheshire) sediment. However, in a study of another lacustrine environment (Lake Suwa, Japan), evidence was given for a significant direct biological input of stanol, with the increase in stanol/stenol ratio with depth being explained in terms of the greater resistance of stanols to degradation (NISHIMURA and KOYAMA,1977). Further incubation experiments are needed to determine whether cholestanol undergoes short term diagnetic alteration. (2) Similarities between the stanol and sterene distributions in algal ooze from Laguna Mormona, Baja California, Mexico, led CARDOSO et al. (1976) to suggest that stanol to A2-sterene conversion may occur, either by biochemical means or by some non-biological acid or clay-catalysed process. Stanol to sterene conversions have been 427