Characterization of the sedimentary organic matter preserved in Messel oil shale by bulk geochemistry and stable isotopes

Highlights

• The Messel oil shale was deposited in a small maar lake under anoxic conditions.

• Bulk geochemical and isotope data suggest a complex aquatic ecosystem.

• Stable isotopes indicate microbial reworking of organic matter (OM).

• High C_org/N_tot ratios suggest a comparatively high contribution of terrestrial OM.

Abstract

We investigated a 150 m thick drill core section of Messel oil shale using bulk geochemical and stable isotope techniques in order to determine the organic matter sources and the environmental conditions that prevailed during the deposition of the lacustrine sequence. High C_org values (on average 27%) indicate that the Messel oil shale has likely been deposited under highly productive conditions and/or in an environment largely free of oxygen, which suggests a permanent stratification of the paleolake and prolonged periods of bottom water anoxia. Low stable nitrogen isotope values (ca. + 1 to + 2‰), observed at the transition from holomictic to meromictic conditions, suggest a brief period of an increased contribution of diazotrophic, possibly heterocystous, cyanobacteria that proliferated under the stagnant conditions. The basal oil shale unit (Lower Messel-Formation) is characterized by generally high HI values (> 570 mg HC/g C_org) and molar C_org/N_tot ratios (> 35) that evidence an increased loading of terrestrial organic matter to the lake system, which is hypothesized to be dominated by the lipid-rich constituents of vascular plants. The oil shale of the mid-section (Middle Messel-Formation) is characterized by carbon isotope excursions towards comparatively heavy δ¹³C_org values of − 24‰ that together with slightly lower C_org/N_tot ratios (ca. 30) of this interval are taken as evidence for a higher loading of aquatic-derived organic matter to the paleolake. The uppermost part of the Middle Messel-Formation displays decreasing δ¹³C_org and concomitantly high δ¹⁵N_tot values, which is interpreted to indicate an increased importance of bacterial driven processes (such as methanogenesis, methanotrophy and denitrification) in the paleolake system. Our results thus indicate that the Eocene maar lake received organic matter from various autochthonous and allochthonous sources with contributions of each source varying significantly over the lake's existence.

Introduction

The Eocene Messel oil shale is a finely laminated, organic-rich lacustrine deposit that accumulated at the bottom of a small maar lake characterized by a permanent stratification of the water column and persistent bottom water anoxia (Franzen and Schaal, 2000). The outcropping oil shale, covering the uppermost 60 m of the 210 m thick lacustrine sequence (Felder and Harms, 2004), has been studied in great detail using geochemical as well as paleontological means and there is ample information on the environmental conditions that prevailed during the deposition of the oil shale and the primary producers that contributed to the exceptional high C_org content (on average 27%) of the lacustrine deposit (Rullkötter et al., 1988). Macrofossil remains and palynological data indicate that the lake was surrounded by a dense rainforest-type of vegetation that grew under a paratropical climate with members of the Juglandaceae and Fagaceae being particularly dominant (Thiele-Pfeiffer, 1988, Franzen and Schaal, 2000, Lenz et al., 2011). Among the aquatic primary producers Tetraedron minimum seemed to be abundant and seasonal blooms of this coccal green alga are considered to be the principal source of the OM preserved in the Messel oil shale (Goth et al., 1988). Other algae, including Botryococcus sp. (Jankowski and Littke, 1986), the chlorococcal alga Coelastrum sp. (Richter and Baszio, 2001), various diatom species including Melosira (Goth, 1990) and representatives of the Chrysophyceae (Liebig, 1998), have also been reported in highly variable abundances from the outcropping oil shale based on microscopical observations.

Although there is thus a great deal of information available on the lake's primary producers and the paleoenvironmental conditions that prevailed during the deposition of the uppermost oil shale unit, our current knowledge on the geochemical composition of the remaining 150 m thick sedimentary sequence and the depositional environment of the initial lake phase is still incomplete. Several palynological studies performed on a drill core obtained in summer 2001, covering most of the lacustrine sequence, indicated that the paleolake experienced brief periods of significant primary production by the dinoflagellate Messelodinium thielepfeifferae and the green algae Botryococcus sp. (Lenz et al., 2007b). Based on pollen data, it was also deduced that changes in the local climate from humid to more arid conditions in the later lake stage resulted in a turnover of the terrestrial vegetation surrounding the paleolake (Lenz et al., 2011). Previous studies on the organic–geochemical properties of the deeper oil shale section are comparatively few and were conducted on drill cores not intersecting the complete lacustrine sequence (Michaelis et al., 1988, Rullkötter et al., 1988). Results of these analyses showed that the oil shale of the initial lake phase received variable contributions from prokaryotic organisms (possibly methanogens) as well as dinoflagellates and land plants based on biomarker compositions. In an attempt to investigate the bulk geochemical and isotopic signature of the deeper oil shale succession at a higher resolution, we studied the uppermost 150 m of a drill core taken in the profundal facies of the Messel oil shale. The results obtained were used to (1) infer the origin, and variations therein, of the preserved OM, (2) integrate changes in the relative contribution of autochthonous and allochthonous OM sources over the lake's history and (3) to reconstruct the depositional conditions that prevailed during the formation of the Messel oil shale.

The Messel oil shale is located in the center of the Sprendlinger Horst, which represents the northern lengthening of the crystalline Odenwald. It covers an area of approximately 1.7 km² at a distance of 9 km northeast of Darmstadt (Fig. 1a). The organic-rich sediments were deposited at the bottom of a small maar lake that was formed by a phreatomagmatic eruption (Schulz et al., 2002, Felder and Harms, 2004) with the distribution of the currently outcropping oil shale roughly reflecting the surface area of the former lake (Fig. 1b), which had a diameter of ca. 900 m and an estimated water depth of several tens to a hundred of meters (Franzen and Schaal, 2000). The organic-rich facies is well known as the Messel-Formation and according to Matthess (1966) is divided into three distinct lithostratigraphical units. The Lower Messel-Formation consists of an alternation of breccias, tuffs, sand and silts that in the center of the paleolake deposit equal a total thickness of ca. 130 m (Felder and Harms, 2004). These coarse-grained sediments gradually turn into finely laminated, dark-colored oil shale at a core depth of ca. 141 m. Frequently interrupted by mm- to cm-thick intercalations of sand and silts, the oil shale constitutes the main form of sedimentation in the uppermost part of the Lower Messel-Formation and indicates the transition from holomictic to meromictic conditions within the paleolake (Schulz et al., 2002, Felder and Harms, 2004, Lenz et al., 2007a). At a core depth of ca. 110 m, the oil shale is overlain by a series of white-colored sands and silts that in their uppermost part contain intercalations of re-deposited oil shale debris. This 16 m thick event deposit of coarse-grained sediments spreads across the entire lake basin and is considered to represent the boundary between the Lower and the Middle Messel-Formation (Matthess, 1966). The sedimentation of the latter is again composed of finely laminated and organic-rich oil shale, which in contrast to its underlying homologue, contains only few intercalations of coarse-grained sediments derived from the crater rim and transported to the profundal via slope destabilization and slumping. Sediments of the Upper Messel-Formation are composed of 1 to 5 m thick clay and sand beds that are accompanied by lignite intercalations (Matthess, 1966). These sediments, however, have almost completely been removed as a result of the extensive mining activities performed in the Messel pit over the past century and are only accessible in three distinct depressions in the northern part of the oil shale deposit. In marginal areas, the Messel-Formation is in fault contact with the diatreme wall, which consists of igneous and metamorphic rocks of Paleozoic age. The use of palynological methods points to a Middle Eocene age for the Messel sediments (Thiele-Pfeiffer, 1988), which based on mammal stratigraphy can be attributed to the MP11 zone of European mammal stratigraphy (Harms, 2001). This age was more recently confirmed using ⁴⁰Ar/³⁹Ar dating of volcanic fragments occurring at the base of the lacustrine deposit revealing an age of 47.8 ± 0.2 Ma (Mertz and Renne, 2005).