The widespread occurrence of extended hopanoids in sediments and petroleums illustrates the importance of bacterial lipid contributions to geological materials. In archaebacteria, however, hopanoids are absent; their role as structural components of biomembranes is fulfilled by acyclic isoprenoids. Recent studies of the lipid constituents of archaebacteria have greatly extended the range of acyclic isoprenoid skeletons known in organisms (Fig. 1). In particularly, isoprenoids with head-to-head linkages have been identified, and such compounds (for example, 3,7,11,15,18,22,26,30-octamethyldotriacontane, I) have been recognized in petroleum and as degradation products of Messel shale kerogen. Here we report the first recognition of 2,6,10,15,19-pentamethyleicosane (II), a known component of methanogens, in marine sediments of Recent to Cretaceous age (Table 1) and suggest that it and certain other acyclic isoprenoids may be used as biological markers for methanogens.

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Stream sediments were sampled systematically in nine areas totalling 1100 sq. miles" underlain in part by marine black shales ranging in age from Ordovician to Cretaceous. Regional patterns of molybdenum in the sediment were related to the underlying bedrock. Soils developed from the black shale parent material together with associated pasture herbage in eight of the areas contained above normal concentrations of molybdenum (>3 parts/million).Detailed studies in four of the areas confirmed the high molybdenum content of stream sediment, rock, soil and pasture. Examples are shown of the importance of both glacial drift and of specific soil factors such as pH in the interpretation of geochemical reconnaissance data for agriculture. Clinical hypocuprosis in cattle is already recognised over parts of two of the molybdenum-anomalous areas. In the other two, copper deficiency disorders are not recognised but may well occur at a sub-clinical level. It is suggested that appreciable acreages of the United Kingdom underlain by marine black shales may be enriched in molybdenum and be potentially limiting to livestock performance.