Geochemical characterization of Malm Zeta laminated carbonates from the Franconian Alb, SW-Germany (II) L. SCHWARK 1 *, M. VLIEX 1 and P. SCHAEFFER 2 1 Geological Institute, University of Cologne, ZuÈlpicher Str. 49a, 50674 Cologne, Germany and 2 Institut de Chimie, Universite Louis Pasteur, 1 rue Blaise Pascal, 67000 Strasbourg, France AbstractÐDuring Jurassic times, especially within the Malm z stage, local depressions formed on the Eastern Bavarian Carbonate Platform that were surrounded by wall reefs. This created a unique deposi- tional environment, where in an open-marine setting stagnant and anoxic bottom waters developed in an intra-reef depression. Anoxic conditions were stabilized below wave base by enhanced salinity in the bottom waters and establishment of a density strati®cation. Biomarker analysis was applied to charac- terize palaeosalinity and redox conditions, utilizing organic sulfur compound, methylated chroman, hopanoid and saturated isoprenoid distributions. Prevention of terrigenous in¯ux by protecting reef walls lead to iron de®ciency in intra-reefal sediments causing early diagenetic sulfurization of functiona- lized lipids. Absence of clay-mineral catalysts and early sulfurization favored an unusual steroid distri- bution lacking rearranged analogues but providing coexistence of saturated steranes with baa-, aaa- and abb-con®guration, D 13(17) -spirosterenes, unknown sterenes and steradienes as well as mono-, di- and triaromatic steroids. Desulfurization of the polar fraction was carried out for two samples to verify that palaeoenvironment reconstruction based on free hydrocarbon and heterocompound distribution was not invalidated by sulfur quenching of selected compounds. Gammacerane was found to be the only component to occur exclusively in the sulfur-bound fraction. Only minor amounts of hydrocar- bons were released upon desulfurization implying that free bitumen analysis was applicable for organo- facies characterization. Organic petrological investigation of organic mats revealed the presence of two dierent types of such structures. Comparison of organoclast ¯uorescence spectra with those of extracted porphyrin fractions indicate an origin of porphyrins exclusively from cyanobacterial mats whereas algal dominated mats yield no porphyrins. This aims towards a better localization of speci®c biomarker origin in geological samples. # 1998 Elsevier Science Ltd. All rights reserved Key wordsÐJurassic, Malm, Solnhofen Lithographic Limestone, marls, cherts, limestones, palaeoenvir- onment reconstruction, palaeosalinity, organic sulfur compounds, aromatic steroids, sterenes, spiroster- enes, steradienes, hopenes, porphyrins, ¯uorescence properties INTRODUCTION The Jurassic age lithographic limestone of the Solnhofen area, Franconian Alb in SW-Germany (Fig. 1), is well known for its excellent preservation of fossils. Vertebrates like Archeopterix, the earliest bird and Compsognathus, known to be the smallest dinosaur, were found in the well laminated micritic limestones of the Solnhofen and Jachenhausen area. The exquisite preservation of fossils and the extre- mely ®ne lamination of the matrix rocks indicate exceptional conditions of sedimentation for the car- bonates, pointing towards a low energy and poss- ibly anoxic environment. Several palecological models have been established, based on micro- and macrofossil faunal assemblages to de®ne, for example, water circulation and strati®cation pat- terns, autochthonous productivity and terrigenous in¯ux, redox and salinity conditions (von Freyberg, 1966, 1968; Barthel, 1970, 1978; Keupp, 1977, 1994; Keupp et al., 1993). Reconstruction of palaeoenvir- onmental conditions by means of organic geochem- istry, however, is usually hindered because the entire area was subjected to uplift and erosion during the Early Cretaeous and late Tertiary, allow- ing groundwater to penetrate into the limestones and thus causing oxidation of the organic matter, especially in the Solnhofen area (Meyer and Schmidt-Kaler, 1993). In order to overcome the problem of weathering, we investigated a core from the Rennertshofen Trough, south of Solnhofen, where the organic mat- ter embedded in the Malm limestone was preserved, allowing for reliable organofacies characterization. Molecular indicators for palaeoenvironmental reconstruction in carbonatic/evaporitic depositional settings, previously developed during phases I and II of the ENOG programme (ENOG: European Network of Organic Geochemists; see Huc and Sinninghe Damste , 1993; Leythaeuser and Sinninghe DamsteÂ, 1995) were applied to character- ize organic matter incorporated in the Malm z sedi- ments, in order to verify or re®ne and improve existing palecological models. This is also regarded Org. Geochem. Vol. 29, No. 8, pp. 1921±1952, 1998 # 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0146-6380/98 $ - see front matter PII: S0146-6380(98)00192-2 *To whom correspondence should be addressed. E-mail: lorenz.schwark@uni-koeln.de 1921