Late Miocene to Pliocene orbital and climatic forcing on marine productivity

The late Miocene to Pliocene was a geological time interval of global cooling, albeit in a warmer-than-present world, which is commonly used as a past analogue for future anthropogenic climate change. The investigation of marine sediments recovered by the International Ocean Discovery Program (IODP) sheds light on different paleoclimatic, paleoceanographic and paleobiological characteristics of this period. The NW Australian shelf represents an interesting area for such investigation, because it is strategically positioned near the only remaining equatorial warm-water valve on Earth. In Chapter I and Chapter II, focus is given on calcareous nannofossil time-series data and records of the element potassium (K), which is mainly delivered by rivers to the shelf, at IODP Sites U1463 and U1464. Results demonstrate that humid conditions were probably prevailing earlier than previously thought (at least since ~6 Ma), but that regional tectonics (basin subsidence) has complicated the identification of the exact onset. In addition, nannofossil assemblages data and paleotemperature gradients between the shelfal area and the eastern Indian Ocean reveal a shift in oceanographic and climatic regime that occurred between 5.4-5.2 Ma, as a likely result of an overall long-term increase in seasonality. Finally, an interval of decreasing nannofossil accumulation rates (fluxes) and a distinct change in the dominant nannoplankton species occurred between 4.6-4.4 Ma and is hypothesized to be part of broader changes in ocean nutrient availability. This hypothesis is further explored in Chapter III and Chapter IV through the investigation of a well-established period of globally elevated biogenic sedimentation (and related marine export productivity) known as the late Miocene to early Pliocene biogenic bloom. In Chapter III, age model accuracy and sample resolution of previously published biogenic sediment accumulation rate records are evaluated. The compilation of multiple records shows that an abrupt reduction in ocean paleoproductivity occurred between 4.6-4.4 Ma at (sub)tropical latitudes. This event coincided with a rather unique configuration of the Earth’s orbit, which could have led to a weakened Asian monsoon activity and therefore reduced river runoff and nutrient supply to the ocean. Chapter IV focuses on the comparison between the calcareous nannofossil assemblages at the NW Australian shelf sites and ODP Site 1264 in the South Atlantic Ocean, across the termination of the biogenic bloom. Although the overall decrease in paleoproductivity occurred around the same time, the shift in species dominance across the end of the biogenic bloom, as shown in the tropical Indian Ocean, is not observed at ODP Site 1264.