Nature and origin of variations in pelagic carbonate production in thetropical ocean since the mid-Miocene (ODP Site 927)

Marine plankton is an important component of the global carbon cycle.Whereas the production and seafloor export of organic carbon produced by theplankton, the biological pump, has received much attention, the long-termvariability in plankton calcification, controlling the carbonate counterpump, remains less well understood. However, it has been shown that changes in pelagic calcification (biologicalcompensation) could affect the ocean's buffering capacity and thus regulateglobal carbon budget ongeological timescales. Here we use Neogene pelagic sediments deposited on theCeara Rise in the tropical Atlantic to characterize the variability inpelagic carbonate production with a focus on warm climates. A re-evaluation ofpublished records of carbonate accumulation at the Ceara Rise reveals asystematic increase in sedimentation rates since the late Miocene, but thecarbonate accumulation rate does not show a clear trend. Instead, we observesubstantial orbital timescale variability in carbonate accumulation,combined with a trend towards less carbonate on average at sites locatedbelow 4 km, likely due to the effect of carbonate dissolution. To evaluatelong-term changes against possible orbital-scale variability, we generatednew high-resolution records of carbonate accumulation rate at Ocean Drilling Program (ODP) Site 927across two Quaternary interglacials (MIS 5 and MIS 9), the Pliocene warmperiod (MIS KM5) and the Miocene Climatic Optimum (MCO). We observe that thehighest carbonate accumulation rates occurred during the Pliocene but thateach of the studied intervals was characterized by large-magnitude orbitalvariability. Prominent variations in carbonate accumulation prior to theQuaternary preservation cycles appear to follow Earth obliquity andeccentricity. These results imply that pelagic carbonate accumulation in thetropical ocean, buffered from large temperature changes, varied on orbitaltimescales. The magnitude of the orbital-scale variability was similar oreven higher than the long-term mean differences among the studied intervals.Since preservation can be excluded as a driver of these changes prior to theQuaternary, the observed variations must reflect changes in the export fluxof pelagic biogenic carbonate. We conclude that the overall carbonateproduction by pelagic calcifiers responded to local changes in light,temperature, and nutrients delivered by upwelling, which followed longorbital cycles, as well as to long-term shifts in climate and/or oceanchemistry. The inferred changes on both timescales were sufficiently largesuch that when extrapolated on a global scale, they could have played a rolein the regulation of the carbon cycle and global climate evolution duringthe transition from the Miocene warm climates into the Quaternary icehouse.