DURING THE MIDDLE AND LATE DEVONIAN HOTHOUSE, NITROGEN LIMITATION MADE JUICY ROCKS

Organic-rich shales deposited during Middle and Late Devonian hothouse climate conditions are characterized by uniquely elevated C/P ratios that indicate high rates of organic P regeneration and suggest that primary production was chronically nitrogen-limited. Under N-limited growth conditions, modern green algal prasinophytes, the dominant primary producers in Paleozoic black shale-forming environments, have been observed to increase their cellular lipid content by as much as three times. This stoichiometric plasticity may have permitted Devonian prasinophytes to outcompete N-fixing prokaryotes that might otherwise have ameliorated the relative N deficit in the photic zone. Because lipids are the least labile component of algal biomass, especially where oxygen is absent, the increase in algal lipid content had two direct consequences: First, the burial efficiency of organic matter within the sediment increased as the fraction of recalcitrant lipids increased. Second, the carbon use efficiency of both aerobic and anaerobic heterotrophs within the water column declined as they metabolized a less nutritious diet. This resulted in greater O2 demand and CO2 production above the chemocline and greater H2S and CO2 production below the chemocline, reinforcing climate-driven stratification of the water column. Data from Devonian black shales of the Appalachian, Illinois, and Madre de Dios Basins exhibit a strong positive correlation between organic carbon preservation and sediment N/P suggesting that as P was regenerated preferentially to N, fatty biomass resulted in juicier rocks.