Higher plant biomarkers reflect palaeovegetation changes during Jurassic times

The relative abundances of three higher-plant-derived biomarkers, retene, c adalene and ip-iHMN, have been measured in marine sedimentary rocks from th e northwest margin of Australia. It is thought that each biomarker represen ts input from a different plant type. The distributions of these three comp ounds form a fingerprint, representing higher plant input (HPF). Variations in HPF in Oxfordian sediments were nearly identical in all three locations , with retene becoming very abundant relative to the other two compounds wi th decreasing age of the sediment. This finding strongly suggests that the composition of terrestrial input during deposition largely determines HPF a nd that the possible effects of diagenesis and catagenesis on the distribut ion of the three biomarkers are relatively unimportant. The marked increase in the abundance of retene relative to that of cadalene during the Oxfordian is interpreted to reflect an increase in the contribu tion of plants that produced precursors for retene, i.e., conifers, brought about by a significant change in climate. This was exemplified by measurin g the distributions of retene and cadalene, expressed in the higher plant p arameter (HPP) for a suite of sediments from the Carnarvon Basin, Western A ustralia, covering the complete Jurassic period. The HPP profile displays t hree major cycles, each covering a period of at least 10 million years. Thi s profile not only compared well with published palaeoclimate data, but als o showed a remarkable similarity with second order cycles in the global sea level curve, thus strongly supporting the proposal that variations in HPF and HPP are indications of changes in palaeoclimate. The relation with glob al sea level further suggests that global factors, e.g., the atmospheric ca rbon dioxide concentration, may play a major role in determining the observ ed variations in the distributions of these higher-plant-derived biomarkers . Copyright (C) 2000 Elsevier Science Ltd.