Sedimentary C org:P ratios, paleocean ventilation, and Phanerozoic atmospheric pO 2

Abstract

The C org:P ratios of organic-rich facies (TOC > 1%) exhibit considerable variation through the Phanerozoic, from < 10:1 in the Permian Phosphoria Formation to > 1000:1 in some Devonian black shales. Relative to the composition of phytoplankton (C:P ∼ 106:1), which provide the bulk of organic C and P to organic-rich marine sediments, the range of Phanerozoic sedimentary C org:P ratios largely reflects the influence of benthic redox conditions on diagenetic pathways of C and P remobilization. Enhanced preservation of organic C at lower redox potentials combined with enhanced sedimentary retention of remineralized organic P at higher redox potentials results in a strong relationship between benthic redox conditions and sedimentary C org:P ratios. The strong secular coherence of the Phanerozoic C org:P record (i.e., limited variation within narrow time slices) suggests that these ratios are controlled primarily by factors influencing benthic redox conditions on a global scale, such as atmospheric pO 2. Inversion and scaling of the Phanerozoic C org:P record yields a new atmospheric pO 2 model that has strong similarities to existing models based on elemental and isotopic mass balances, yet differs in some potentially significant respects. In particular, the new model suggests that atmospheric pO 2 was markedly lower during the Early to Middle Paleozoic than previously inferred. During most of the Devonian, atmospheric O 2 levels may have been sufficiently low (< 13%) as to prevent the sustained combustion of plant material. This feature of the model may account for an anomalous gap in the fossil charcoal record during the Devonian. The redox-dependent burial of P in the ocean results in positive and negative feedback mechanisms that tend to stabilize atmospheric oxygen concentrations on geologic timescales. Temporal variations in the burial of P relative to C suggest that the strength of these feedback mechanisms varied over the Phanerozoic.