3 SERIES OF REARRANGED HOPANES IN TOARCIAN SEDIMENTS (NORTHERN ITALY)

A suite of 36 shales and marls from the Toarcian Oceanic Anoxic Event section at three localities in northern Italy was found to contain var iable contents of three series of rearranged hopanes: 17 alpha-diahopa nes (C-27 and C29-34; and trace C-35), 18 alpha-neohopanes (C-27 and C 29-30) and an unidentified early-eluting series (C-27 and C29-35) Prin cipal component analysis (PCA) shows the 17 alpha-diahopanes and the e arly-eluting rearranged hopanes to closely covary, suggesting a relate d diagenetic mechanism for the formation of their rearranged skeletons . A general covariance with diasteranes is also noted. The 18 alpha-ne ohopanes show no covariance with diasteranes or the other two series o f rearranged hopanes. We also report 2 alpha-methyl analogues of the 1 7 alpha-diahopanes and early-eluting rearranged hopanes. We propose th at the neohopanes may be at least partly derived from different biohop anoid precursors than the other two series of rearranged hopanes, and that the diagenetic origin of the neohopane structure differs from tha t of the 17 alpha-diahopanes and the unidentified early-eluting series . (C) 1997 Elsevier Science Ltd.