Sources and diagenesis of organic matter in tidal flat sediments from the German Wadden Sea

The sources and diagenesis of organic matter in a sediment core from the Sw innplate backbarrier area near Spiekeroog Island in the northwest German Wa dden Sea have been examined using stable carbon isotopes, C-14-ages and lip id biomarker data. Twenty-two core sections were analysed from the surface to a depth of 90 cm, representing sedimentation over the past approximately 200 years. Total organic carbon (TOC) contents were highly variable (0.1-1 %), as was the grain size with some core sections containing up to 50% of c lay and silt (mud fraction). These data indicate a highly variable depositi onal regime in which organic matter is extensively degraded both before and after incorporation into the sediments. The TOC content was strongly corre lated with the abundance of the mud fraction, indicating the importance of organic matter sorption onto particles for preservation of both marine and terrestrial organic matter. Sediments near the top of the core were enriche d in marine organic matter, but terrestrial organic matter predominated in most core sections. Some samples showed higher TOC contents than might be p redicted from the TOC-grain size relationship. Isotope and biomarker studie s showed that these contained additional terrestrial organic matter from pe ats, possibly eroded from areas to the west of the investigated area. The o rganic matter in these layers had the lightest values of delta(13)C (about -26 parts per thousand compared with a more typical mixed marine-terrestria l value of -24 parts per thousand). Most of the Iz-alkane distributions sho w a strong predominance of odd-carbon-number alkanes typical of the distrib utions found in higher plant waxes. All core sections contained abundant lo ng-chain alcohols and triterpenoid alcohols such as alpha-amyrin, beta-amyr in, lupeol, taraxerol, taraxerone and friedelin from higher plants. The dih ydroxy triterpenoid betulin was particularly abundant confirming that erode d peats are a major source of the lipids. Further confirmation was obtained from AMS-determined C-14-ages of fatty acid and neutral lipid fractions fr om 46.5 to 60 cm deep core sections which were 5570 and 6240 yr BP, respect ively. These data attest to the importance of recycled older organic materi al (eroded peat) in the carbon cycle of this coastal environment. (C) 2000 Elsevier Science Ltd. All rights reserved.