Sterane biomarkers as indicators of palaeozoic algal evolution and extinction events

Algae are among the oldest organisms known to inhabit the Earth's biosphere with a record dating back some 2.7 Ga. Only few marine algae produce hard parts that can be preserved over geologic time and consequently their fossil record is incomplete. Molecular fossils or biomarkers add complementary information to the fossil palynomorph record. Steroids are important constituents of eukaryotic cell-membranes and are preserved in sediments as steranes. C-28- and C-29-steranes are indicators for the presence of green and C-27-steranes for the presence of red algae, respectively. The relative abundance of steranes allows the investigation of the fossil record for Palaeozoic algal diversification and evolution. In this study, 500 Palaeozoic rock samples, representing a broad facies variety from the Upper Ordovician to the P/T-boundary were investigated for sterane compositions. In addition three profiles covering the Hirnantian, the F/F and the Hangenberg-Event were analyzed for possible shifts in the sterane distribution associated with extinction events. A sharp increase of the C-28/C-29-sterane ratio from < 0.55 to > 0.70 at the Devonian/Carboniferous boundary implies a fundamental change in the green algae assemblage from more primitive, mainly C-29-sterane-producing algae, to modem C-28-sterane-producing algae. For all investigated extinction events a pronounced but short-lived rise in the C-28-sterane content occurs that is attributed to an episodic increase in prasinophytes. The gradual radiation of algae may have been triggered by frequent mass extinctions in the Upper Devonian culminating with the massive decline of acritarchs at the D/C-boundary. The coeval rise in the C-28/C-29-sterane ratio indicates the presence of an non-encysting algal group and coincides with the global augmentation of numerous filamentous Codiacea (Siphonales) and the rise of euspondyle and metaspondyle Dasycladales. (c) 2006 Elsevier B.V. All rights reserved.