Rapid hydrogenation of sterols in a contemporary lacustrine sediment

Abstract

THE steroid carbon skeleton has been widely used as an indicator of biological origin of the organic material in ancient sediments^1–3 and petroleum⁴. Sterols, presumed precursors of the geolipid steranes, and stanols, their saturated analogues, have been identified in Recent^5–7 and ancient^8,9 sediments. In view of their generally low abundance in living organisms, the presence of stanols in Recent sediments suggests the operation of a sterol hydrogenation process early in the geological time scale. We have sought to confirm this hypothesis by examination of the contemporary sediments of Rostherne Mere, Cheshire, England. The sterol composition of the sediment, and its variation with depth, provide circumstantial evidence for the hydrogenation of Δ⁵-sterols¹⁰. Here we describe the application of radiochemical techniques^11,12 to the study of the early diagenesis of cholesterol (cholest-5-en-3β-ol) in Rostherne sediment and report the first direct evidence of rapid sterol hydrogenation in a geological environment. The sediment chosen for study is that of a highly productive lake. Significant allochthonous (land-derived) organic input to Rosterne Mere has been exceeded in recent years by an increasing autochthonous production of blue-green algae^13,14. The sediments in the deepest part of the lake (from where samples were taken) remain severely depleted in oxygen throughout the year¹⁵. As a National Nature Reserve, Rostherne Mere is protected from gross pollution, though there is some chemical evidence to suggest a recent input of petroleum-derived material¹⁴.