The Sensitivity of the Phanerozoic Inorganic Carbon System to the Onset of Pelagic Sedimentation

Abstract

The onset of pelagic sedimentation attending the radiation of pelagic calcifiers during the Mesozoic was an important divide in Earth history, shifting the locus of significant carbonate sedimentation from the shallow shelf environments of the Paleozoic to the deep sea. This shift would have impacted the CO2 cycle, given that decarbonation of subducted pelagic carbonate is an important return flux of CO2 to the atmosphere. Coupled with the fact that the mean residence time of continental platform and basin sedimentary carbonate exceeds that of the oceanic crust, it thus becomes unclear whether carbon cycling would have operated on a substantially different footing prior to the pelagic transition. Here, we examine this uncertainty with sensitivity analyses of the timing of this transition using a coupled model of the Phanerozoic atmosphere, ocean, and shallow lithosphere. For purposes of comparison, we establish an age of 250 Ma (i.e., after the Permo-Triassic extinctions) as the earliest opportunity for deposition of extensive biogenic pelagic carbonate on the deep seafloor, an age that predates known occurrences of pelagic calcifiers (and intact seafloor). Although an approximate boundary, we do show that attempts to shift this datum either significantly earlier or later in time produce model results that are inconsistent with observed trends in the mass–age distribution of the rock record and with accepted trends in seawater composition as constrained by proxy data. Significantly, we also conclude that regardless of the timing of the onset of biogenic pelagic carbonate sedimentation, a carbon sink involving seawater-derived dissolved inorganic carbon played a critical role in carbon cycling, particularly in the Paleozoic. This CaCO3 sink may have been wholly abiogenic, involving calcium derived either directly from seawater (thus manifest as a direct seafloor deposit), or alternatively from basalt–seawater reactions (represented by precipitation of CaCO3 in veins and fissures within the basalt). Despite the uncertainty in the source and magnitude of this abiogenic CaCO3 flux, it is likely a basic and permanent feature of global carbon cycling. Subduction of this CaCO3 would have acted as a basic return circuit for atmospheric CO2 even in the absence of biogenically derived pelagic carbonate sedimentation. Lastly, model calculations of the ratio of dissolved calcium to carbonate ion (Ca2+/CO32−) show this quantity underwent significant secular evolution over the Phanerozoic. As there is increasing recognition of this ratio’s role in CaCO3 growth and dissolution reactions, this evolution, together with progressive increases in nutrient availability and saturation state, may have created a tipping point ultimately conducive to the appearance of pelagic calcifiers in the Mesozoic.