Three 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 variable contents of three series of rearranged hopanes: 17 alpha-diahopanes (C-27 and C29-34; and trace C-35), 18 alpha-neohopanes (C-27 and C29-30) and an unidentified early-eluting series (C-27 and C29-35) Principal component analysis (PCA) shows the 17 alpha-diahopanes and the early-eluting rearranged hopanes to closely covary, suggesting a related diagenetic mechanism for the formation of their rearranged skeletons. A general covariance with diasteranes is also noted. The 18 alpha-neohopanes show no covariance with diasteranes or the other two series of rearranged hopanes. We also report 2 alpha-methyl analogues of the 17 alpha-diahopanes and early-eluting rearranged hopanes. We propose that the neohopanes may be at least partly derived from different biohopanoid precursors than the other two series of rearranged hopanes, and that the diagenetic origin of the neohopane structure differs from that of the 17 alpha-diahopanes and the unidentified early-eluting series. (C) 1997 Elsevier Science Ltd.