Eastern equatorial Pacific bulk sediment properties and paleoceanography since the late Neogene

Abstract: Reconstructing eastern equatorial Pacific (EEP) oceanography since the late Neogene (about 8 million years ago, Ma) is a central topic in current paleoceanographic investigations. The reason for this is two-fold. First, the EEP exerts a strong control on global climate because steep gradients in sea surface temperature (SST) and biological productivity linked to equatorial wind-driven upwelling affect global climate and carbon cycling, including the exchange of upwelled CO2 to the atmosphere. Second, during the last 8 Ma global climate underwent major changes before arriving at its current state, evolving through the last period of widespread global warmth (the early Pliocene) to the colder ‘mean’ global climate state of the pre-industrial world. Deciphering the dynamics of the EEP system since the late Neogene is thus important for understanding how this ocean area works under changing climatic conditions. Despite the large numbers of studies devoted to the EEP, its paleoceanographic evolution since 8 Ma is still debated and contrasting scenarios have often been proposed. This is in part because paleoceanographic reconstructions are challenging in the EEP due to the high environmental, and thus sedimentary, heterogeneity, as well as the extreme seafloor depth, which compromises the preservation of useful foraminiferal archives in many regions. Moreover, some existing legacy data sets are confounded by some basic issues with the way in which the data were collected. Yet, reconstructing properties of surface mixed-layer remains a crucial requirement for deciphering EEP paleoceanography. Fossil foraminifera tests are typically not available for tracing EEP surface ocean properties because of strong sea floor diagenetic alteration. However, calcareous nannofossil carbonate, also produced in the surface mixed layer and accessible in the form of the bulk sedimentary carbonate or sediment ‘fine fraction’, is available. The challenge is to understand what these bulk geochemical signals mean. Bulk sediment comprises a mixture of different, mostly biogenic particles. The information carried by its properties, including physical and geochemical signals, comprises a mixed signal reflecting different ecological, metabolic and depositional processes associated with the formation and sedimentation of the various calcite particles. The purpose of this thesis is to understand what bulk sediment records represent in terms meaningful for deciphering the paleoceanographic history of the EEP. The results add to, and significantly improve the paleoceanographic “tool box” available for developing proxy records in this complex oceanic region. This thesis comprises a kappa, two published papers and a manuscript, in review as this is written (April 2019). Paper I examines the factors linking sediment composition and physical properties in the EEP through analysis of the relationship between sediment carbonate content and sediment density. Paper II and Paper III focuses on improving the understanding of bulk carbonate carbon and oxygen stable isotope signals by disentangling the contribution of different carbonate components. In these studies, a multiproxy approach was adopted to reconstruct ocean evolution of the EEP since the late Miocene. A major conclusion is that bulk carbonate stable isotopes reflect the isotopic composition of calcareous nannofossils, and therefore surface water conditions, despite complication by several factors. The ideas and findings of the latter two papers have been further tested in an unpublished inter-basin comparison presented in the Chapter 6 of this kappa. The findings of this doctoral thesis demonstrate that bulk sediment is more than just a correlation tool and can provide a reliable indicator of surface ocean conditions that can be used to decipher EEP oceanographic history since 8 Ma.