ALGAL CELL STOICHIOMETRY AND ORGANIC MATTER PRESERVATION IN LATE DEVONIAN BLACK SHALES

Green algal prasinophytes were the dominant eukaryotic primary producers in the Paleozoic and the remains of their reproductive cysts are plentiful in the Late Devonian black shales. Like other green algae, modern prasinophytes exhibit a cellular stoichiometry distinctly different from more recently evolved algal types. The ratio of C to P in green algae averages ~200:1 as opposed to diatoms, the dominant modern form, with a C:P of ~80:1. A consequence of this difference is that biomass could be purchased more cheaply in terms of P in the Paleozoic; however, because the C:N composition of all eukaryotic algae is similar (~8:1), green algae-dominated Paleozoic marine systems were much more likely to become N limited, especially where P was abundant. Phosphorus was abundant in Devonian dysoxic/euxinic marine systems because P cannot complex with Fe oxyhydroxides in the absence of O₂. Under N limiting conditions, green algae shift biomass production from proteins to lipids and may double or triple their lipid content. An increase in the lipid content of primary producers would likely have had two direct consequences: The burial efficiency of organic matter would increase because lipids are the least labile organic fraction; and, the carbon use efficiency by heterotrophs both above and below the chemocline would diminish as the nutritional quality of their food source diminished. This would have the effect of increasing O₂ demand and CO₂ production above the chemocline, and increasing the production of CO₂ and H₂S below the chemocline, serving in both cases to reinforce euxinic conditions and to promote organic matter preservation. In this study, we examined the elemental and isotopic composition of Frasnian and Famennian black shales from the Indiana and Appalachian basins.