Molecule of the Month - Gammacerane

This unusual hopane-like compound is recognised together with the hopanes in the m/z 191 mass chromatogram and was for many years used as a biomarker of hypersalinity due to its common high abundance in sediments deposited under such conditions.  However, a key paper by Sinninghe-Damsté et al. (1995) showed that gammacerane derives from the precursor tetrahymanol which is biosynthesised by ciliate protozoa when their diet is deprived of sterols. 

This occurrence is found at or just beneath the chemocline or thermocline in stratified water bodies, where ciliates feed on dense populations of bacteria (which lack sterols), showing that gammacerane is thus an indicator of water column stratification.  This explains why abundant gammacerane can be found in many oils and source rocks deposited under non-hypersaline conditions, including many examples of lake sediments (as such water bodies are frequently seasonally stratified).  In fact, small amounts of gammacerane are very common in marine sediments and have no significance regarding water column stratification.

Reference:

Sinninghe Damsté J.S., Kenig F., Koopmans M.P., Köster J., Schouten S., Hayes J.M. & de Leeuw J.W. (1995). Evidence for gammacerane as an indicator of water column stratification. Geochimica et Cosmochimica Acta 59, 1895-1900.