Palaeo-environment i formation from deep water siderite (Lake of

S U M M A RY: A comparative study from modern and ancient sediments from the Lake of Laach was performed by means of chemical and stable isotope analyses. In the modern environment siderite is precipitated around CO2-spring vents. In contrast, siderite from the Pleistocene/Holocene boundary occurs as mall layers in a varved sequence. The stable isotope data provide strong evidence for an analogous formation of both types of siderite. The different CO2 sources of the siderites allow reconstruction of a distinct rise of productivity at the Pleistocene/Holocene transition. It is proposed that siderite may be developed as a useful tool for tracing both palaeo-environmental and early diagenetic processes. Siderite isa common mineral phase in oil shales (Weber & Hofmann 1982; Negendank et al., 1982), coal-bearing sequences (Curtis 1967) and other potential petroleum source rocks. Up to now, stable isotope analysis of siderite has only been applied in order to solve diagenetic problems (e.g., Gautier 1982). As such it may be used to identify marine or fresh pore waters, and often to trace certain diagenetic sources of CO2. Recent work on siderites from the German coal measures and from different Eocene oil shales (Bahrig & Conze 1986) gives evidence for the value of stable isotope and chemical investigations on siderite for the reconstruction of the conditions of accumulation and preservation of different types of organic matter. Isotopic ratios of the siderite presented in this study stress the idea that palaeo-environmental information also may be preserved in the stable isotope composition of this mineral, because of the strong linkage of early diagenetic and sedi-mentary conditions. algal blooms induce an open water precipitation of low-Mg-calcite. The lower water mass is characterized by O2-depletion (Matterne & Scharf 1977; Bahrig 1985). The most important features of the modern lake are CO2 springs at the lake margins and on the lake bottom, where volcanic gas bubbles rise (Fig. 1)