Decoupling of bio- and geohopanoids in sediments of the Benguela Upwelling System (BUS) Martin Blumenberg a, * , Gesine Mollenhauer b , Matthias Zabel c , Andreas Reimer a , Volker Thiel a a Department of Geobiology, Geoscience Centre, Georg-August-University Göttingen, Goldschmidtstr. 3, 37077 Göttingen, Germany b Alfred Wegener Institute for Polar and Marine Research, Am Handelshafen 12, 27570 Bremerhaven, Germany c Marum – Center for Marine Environmental Sciences, University of Bremen, Leobener Str., 28359 Bremen, Germany article info Article history: Received 12 February 2010 Received in revised form 8 June 2010 Accepted 11 June 2010 Available online 16 June 2010 abstract Geohopanoids derive from bacteriohopanepolyol (BHP) precursors. However, knowledge of the specific sources and diagenetic fate of both compound classes in marine sediments is fragmentary. We studied the relationship between BHPs and geohopanoids in a sediment core from the central Benguela Upwell- ing System (BUS), including deposits over the last 75 ka. The BUS is a (palaeo)oceanographically stable setting, where primary production is largely controlled by variation in upwelling intensity and therefore nutrient supply. Marine production in the BUS was generally higher during glacial than during intergla- cial times, as reflected in the levels of sedimentary total organic carbon. Geohopanoids at the study site consisted mainly of extractable hopanes and hopanols and occurred in the highest amounts during glacial periods. A minor, but still considerable fraction of geohopanoids was sulfur-linked to geomacromolecular organic matter. Among these S-bound hopane moieties, 17b(H),21b(H)-trishomohopane (C 33 ) was the predominant compound, suggesting a diagenetic overprint of BHPs before linkage. Among the BHPs, a rare bacteriohopanetetrol with likely 22S configuration was observed. This structural feature was lacking in the geohopanoids, which all had the 22R configuration. Different behaviour of BHP and geohopanoid concentrations, as well as d 13 C discrepancies indicate a decoupling of bio- and geohopanoids. Geohopa- noids most likely reflect laterally-transported fossil organic matter, whereas BHPs are indigenous to the upper water column of the study site and may relate to past upwelling intensity/nutrient levels. Hence, care has to be taken using geohopanoids and other biomarkers for palaeoreconstruction in settings where sedimentation processes are not fully understood. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction Bacteriohopanepolyols (BHPs) are excellent biomarkers for bac- teria (Rohmer et al., 1984; Ourisson and Rohmer, 1992) and are the source of the ubiquitous diagenetic products, the geohopanoids (Ourisson and Albrecht, 1992). Various bacterial groups (e.g. met- hanotrophs, methylotrophs, cyanobacteria and purple non-sulfur bacteria) are known to produce BHPs with modifications in the side chain and ring system (e.g. Talbot and Farrimond (2007) for a recent review). About 60% of 270 bacterial taxa tested so far con- tain hopanoids (Talbot and Farrimond, 2007). However, this num- ber is obviously strongly biased towards specific taxonomic groups and is most likely an overestimate. In a recent approach, only 5– 10% of bacteria were found to contain the enzyme squalene hopene cyclase, which is necessary for hopanoid biosynthesis (Pearson et al., 2007). Hence, an essential function of hopanoids for most bacteria, comparable to sterols in eukaryotic membranes (Ourisson et al., 1982; Kannenberg and Poralla, 1999), is questionable. Inter- estingly, there is almost a complete overlap of bacteria containing nitrogenase enzymes with those containing squalene hopene cy- clase, suggesting a role of hopanoids in nitrogen fixation (Pearson et al., 2007). Such a function was already pointed out earlier (Berry et al., 1993), so increased nitrogen fixation served as an explana- tion for the high amounts of BHPs at a certain horizon in Holocene sediments from the Black Sea (Blumenberg et al., 2009). The latter study also demonstrated that the vast majority of sedimentary BHPs in the Black Sea originated from euphotic zone bacterio- plankton and not from bacteria dwelling deeper in the water col- umn and/or sediments. Many phylogenetic groups contain characteristic BHPs that are recognisable in recent and fossil sediments and rocks (e.g. Sum- mons et al., 1999). Hence, they are used for various palaeoenviron- mental reconstructions (Farrimond et al., 2000; Watson, 2002; van Dongen et al., 2006; Cooke et al., 2008a,b, 2009; Coolen et al., 2008; Blumenberg et al., 2009). However, knowledge about sedimentary BHPs, their sources and changes in the fossil record, particularly in marine settings, is limited. An excellent site for studying the fate of marine biomarkers is the Benguela Upwelling System (BUS), which has represented a 0146-6380/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.orggeochem.2010.06.005 * Corresponding author. Tel.: +49 (0)551 3913756; fax: +49 (0)551 397918. E-mail address: martin.blumenberg@geo.uni-goettingen.de (M. Blumenberg). Organic Geochemistry 41 (2010) 1119–1129 Contents lists available at ScienceDirect Organic Geochemistry journal homepage: www.elsevier.com/locate/orggeochem