Processes controlling the stanol/stenol ratio in Black Sea seawater and sediments

Abstract

STEROIDAL ALCOHOLS^1–10, ketones¹¹ and hydrocarbons^12,13 have been used as tracers for the transformation processes of biogenic organic matter in Recent sediments. Several of these studies have been concerned with one of the sedimentary transformation reactions of steroidal alcohols, the reduction of stenols to stanols. However, little attention has been directed towards the role of the overlying water column in controlling sedimentary stenol/stanol distributions. The input of stanols from surface-water organisms coupled with relatively different degradation rates of stenols and stanols^9,14 could also contribute to observed sediment distributions. To evaluate further the relative importance of these processes in controlling stanol/stenol distributions in anoxic water and surface sediments, we undertook a cruise to the Black Sea. Because water below 125 m is anoxic in the two central 2,000-m basins of the Black Sea, it is an ideal environment for observing any microbially or chemically mediated reduction reactions in the water column or surface sediments^15,16. In addition, since the surface sediments are anoxic, the effects of macrobenthos on these reactions are minimal. Indeed, sterol transformation reactions have been postulated to be microbially mediated in sediments^{1,2,7–9,11,13}, and in seawater¹⁷. However, very few, if any, bacteria are thought to biosynthesise desmethyl stenols^17,18. If stenol reduction reactions are chemically mediated, they should also be observed in this highly reducing (low E_H) environment. We discuss here the major processes controlling a specific transformation reaction (stenol to stanol conversion) of a class of biogenic organic compounds. These conclusions are based on results obtained by sampling both the sediments and the overlying water column from two stations in the Black Sea.