PII S0016-7037(02)00864-5 The influence of oxic degradation on the sedimentary biomarker record I: Evidence from Madeira Abyssal Plain turbidites MARCEL J. L. HOEFS, 1 W. IRENE C. RIJPSTRA, 2 and JAAP S. SINNINGHE DAMSTE ´ 1,2, * 1 Utrecht University, Institute of Earth Sciences, Department of Geochemistry, P.O. Box 80.021, 3508 TA Utrecht, The Netherlands 2 Netherlands Institute for Sea Research (NIOZ), Department of Marine Biogeochemistry and Toxicology, P.O. Box 59, 1790 AB Den Burg, Texel, The Netherlands (Received March 6, 2001; accepted in revised form February 13, 2002) Abstract—Free and ester-bound lipid biomarkers were analysed in oxidised and unoxidised parts of four distinct turbidites from the Madeira Abyssal Plain (MAP), which contained 1 to 2% organic carbon homogeneously distributed throughout the turbidites at the time they were deposited. These turbidites are well suited to study the effects of oxic degradation on lipid biomarkers without the complicating influence of varying organic matter sources, sedimentation rates, or bioturbation. One sample from the oxidised turbidite was compared with two samples from the unoxidised part of each turbidite. Postdepositional oxic degradation decreased concentrations of biomarkers by several orders of magnitude. The ester-bound lipids were degraded to a far lesser extent than their free counterparts were. The extent of degradation of different compounds differed substantially. Within a specific class of biomarkers, degradation also took place to a different extent, altering their distributions. This study shows that oxic degradation of the organic matter may have a profound effect on the biomarker fingerprint and may result in a severe bias in, for example, the interpretation of organic matter sources and the estimation of the palaeoproductivity of specific groups of phytoplankton. Copyright © 2002 Elsevier Science Ltd 1. INTRODUCTION Lipid biomarkers are extensively used for reconstruction of depositional environments, and there is a growing interest in applying them to quantify marine processes, e.g., in determin- ing palaeoproductivity of specific phytoplanktonic species (e.g., Schubert et al., 1998; Grimalt et al., 2000; Werne et al., 2000). As pointed out by Hedges and Prahl (1993), early diagenesis, however, has a strong control on biomarker con- centrations in sediments, leading to a number of often implicit assumptions in the application of biomarkers. Oxic degradation of biomarkers is an important process in this respect and can be divided into two phases based on the environment and resi- dence time. The first phase is the degradation of the original biologically derived lipid pool in the water column and at the sediment-water interface. This compartment is characterised by relatively short residence times of the compounds and a rela- tively intense remineralisation by heterotrophes, e.g., zooplank- ton, bacteria, and benthos (for review, see Wakeham and Lee, 1993). The second phase is the degradation of the remaining biolipids in the sediment. This process of postdepositional alteration is generally slower and when compared on the same time scale as the first phase, less intense. However, as an effect of the long residence times in this compartment, the ultimate resulting sedimentary biomarker content can be significantly altered compared to the input from the water column. Ulti- mately, anoxic degradation of biolipids may also occur (e.g., Grossi et al., 2001). For both the water column and the sediment, the influence of oxygen on organic matter (OM) preservation, and thus on biomarker preservation, has been the subject of intense scien- tific debate for a long time. Generally, two schools of thought can be distinguished. One argues that the preservation of OM is more or less independent of the presence of oxygen in the water column and sediment but depends primarily on supply, i.e., rain rate (e.g., Henrichs and Reeburgh, 1987; Pedersen and Calvert, 1990; Calvert and Pedersen, 1992, Calvert et al., 1992). In contrast, the other school claims a dominant control of oxygen concentrations (e.g., Demaison and Moore, 1980; Pratt, 1984; Canfield, 1989, 1993; Paropkari et al., 1992). Recently, Hart- nett et al. (1998) and Hedges et al. (1999) showed that organic carbon burial efficiency is strongly inversely correlated with the length of time accumulating particles are exposed to mo- lecular oxygen in the sedimentary pore waters (i.e., oxygen exposure time, or OET). OET also seems to be a major variable determining the degradation of individual lipid biomarkers during early diagen- esis. Studies of surface sediments have indicated a large vari- ability in the degradation rates of biomarkers, depending on structure, packaging, and oxygen level (e.g., Cranwell, 1981; Sun and Wakeham, 1994; Canuel and Martens, 1996). Such studies are often complicated by varying input in natural envi- ronments. This can be partially overcome by the use of labeled biomarkers in incubation studies (e.g., Sun et al., 1997; Sun and Wakeham, 1998). Alternatively, biomarker degradation pro- cesses can be studied by controlled laboratory experiments (e.g., Harvey and Macko, 1997a,b; Rieley et al., 1997; Teece et al., 1998). These laboratory studies show that lipids are rela- tively labile components, which can be degraded to a different extent. It is, however, often hard to extrapolate results from these types of laboratory stability studies to the natural envi- ronment due to, in part, the relatively short OET. In open ocean sediments, where sedimentation rates are low and oxygen pen- etration is relatively deep, exposure times to oxidative degra- * Author to whom correspondence should be addressed (damste@nioz.nl). Pergamon Geochimica et Cosmochimica Acta, Vol. 66, No. 15, pp. 2719 –2735, 2002 Copyright © 2002 Elsevier Science Ltd Printed in the USA. All rights reserved 0016-7037/02 $22.00 + .00 2719