Isotopic compositions and probable origins of organic molecules in the Eocene Messel shale

Abstract

The sediments that now comprise the Messel shale accumulated 47 +/- 2 million years ago in anaerobic waters at the bottom of a lake. Subsequent depths of burial have not exceeded 300 m, nor has the temperature of the shale exceeded 40 degrees C. Contents of organic carbon reach 25%, and preservation of molecular structures has been excellent. Sixteen different geoporphyrins, including three derived from bacteriochlorophylls of the d series and thus indicative of the existence in the lake of an anaerobic photic zone, have been isolated and identified. Here, we show that the carbon isotopic compositions of these and other biomarkers allow identification of specific sources for some materials and reconstruction of carbon flows within the lake and its sediments. The 13C content of organic matter synthesized by lacustrine primary producers can be estimated from the observed 13C content of the geoporphyrins derived from their chlorophylls. Total organic material in the shale is depleted in 13C by six parts per thousand relative to that input. This difference cannot be explained by selective loss of components enriched in 13C, nor, as shown by isotopic compositions of other biomarkers, by inputs from land plants surrounding the lake or from methanogenic bacteria.

Factors controlling 13C contents of sedimentary organic compounds: Principles and evidence

“…The same paper reports that amino acids, which share biosynthetic precursors with the porphyrins, have on average Aaa = 1.3%o. If Apo r were -5%0, indicated values of 6p in many sedimentary systems (Hayes et al, 1987;Freeman et al, 1990;Boreham et al, 1989) would fall below the ~ values of coexisting lipids of probable phytoplanktonic origin (pristane, phytane, steranes) and enormous heterotrophic shifts (A,) would be required to explain observed values of 6a-oc. There are numerous elements of consistency which can be sought in isotopic data sets.…”

The importance of diazotrophic cyanobacteria as primary producers during Cretaceous Oceanic Anoxic Event 2

“…A1; e.g., Treibs, 1934;Baker and Louda, 1986;Callot and Ocampo, 2000). Since production of chloropigments in natural environments were estimated to be overwhelmingly large relative to those of other compounds, the nitrogen isotopic composition of geoporphyrins should mainly reflect those of chloropigments (e.g., Baker and Louda, 1986;Hayes et al, 1987;Callot and Ocampo, 2000). Stable nitrogen isotopic composition provides significant information for paleoceanographic reconstruction.…”

δ13Corgchemostratigraphy of the Permian‐Triassic boundary in the Maitai Group, New Zealand: Evidence for high‐latitudinal methane release

“…13 C values in this range are usuall associated with carbon derived from isotopically ligh methane sources (Hayes et al 1987;Freeman et al 1990Hayes 1994. The trend toward more I3 C-depleted value in high compared to lower paleolatitudes may suggest releas< from methane sources in high latitudes.…”