Molecular evidence for basin-scale photic zone euxinia in the Permian Zechstein Sea

Previous studies of maleimides (1-H-pyrrole-2,5-diones), oxidative degradation products of chlorophylls and bacteriochlorophylls, and aromatic compounds derived from certain carotenoids (i.e., isorenieratene) indicate that photic zone euxinia (PZE) occurred at least intermittently during deposition of the German Kupferschiefer (Late Permian). However, that work was restricted to a stratigraphic profile through the Kupferschiefer of the Lower Rhine Basin, a shallow lagoon setting, and it was not established if photic zone euxinia prevailed in the larger and deeper basins of the Permian Zechstein Sea. Here, we present maleimide and porphyrin distributions in sediments from the Northern and Southern Permian basins. In samples from both basins, methyl,iso-butyl (Me,i-Bu) maleimide was recovered from the free extractable fraction and upon oxidation of vanadyl and nickel alkyl porphyrins. On structural grounds, Me,i-Bu maleimide derives ultimately from bacteriochlorophyll (bchl) c, d, or e and is thus indicative of the presence of green sulfur bacteria in the ancient Zechstein Sea. Consistent with this, C34 and C35 porphyrins apparently derived from bacteriochlorophyll d were also identified in the Northern Basin samples. Because green sulfur bacteria require both light and free H2S, these results indicate that photic zone euxinia occurred at least periodically during the early history of the Zechstein Sea.