Stable isotope (delta C-13 and delta O-18) records of bulk marine sediment carry information on past carbon cycling and oceanography, but origins and interpretations remain uncertain because such signals represent mixtures of different biogenic components, each with potential offsets from primary parameters. Studies of Neogene sediment from the eastern equatorial Pacific (EEP) exemplify this issue, because stable isotope records of bulk sediment and foraminifera at different sites exhibit similarities and differences in absolute value that somehow relate to depositional age. Here we measure delta C-13 and delta O-18 of bulk carbonate, two fine-grained fractions (<63 and <20 mu m), mixed-species planktic and benthic foraminifera, and foraminifera fragments from sediments deposited over four time intervals within the last 7 Ma at ODP Site 851. These data are compared to published delta C-13 and delta O-18 records of multiple single-species planktic foraminifera from the same site and benthic foraminifera from an adjacent site. Bulk sediment delta C-13 and delta O-18 records represent a mixed signal dominated by reticulofenestrid coccolith calcite but modified by variable amounts of different foraminifera. Similarities and differences between stable isotope records result from temporal changes in water chemistry and temperature, depths of calcite precipitation, and vital effects that impact fractionation of various biogenic components. The remarkable correlation of bulk stable isotope records within the EEP suggests that several factors change collectively over time across a broad oceanographic region. Ideally, multiple stable isotope records coupled with other proxy measurements might lead to an internally consistent paleoceanographic perspective of the EEP since the late Miocene.

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.

To improve the understanding and utility of bulk carbonate stable carbon and oxygen isotope measurements, we examine sediment from cores in the eastern equatorial Pacific that span the last 8Ma. We measured C-13 and O-18 in 791 samples from Integrated Ocean Drilling Program Site U1338 and Deep Sea Drilling Project Site 573, both located close to the Pacific equator. In 100 samples, we measured C-13 and O-18 on isolated <63 mu m and <38 mu m fractions, which concentrates calcareous nannofossil carbonate and progressively excludes foraminiferal carbonate. Bulk carbonate C-13 and O-18 records are similar to published records from other sites drilled near the equator and seem to reflect mixed layer conditions, albeit with some important caveats involving the precipitation of calcite by coccolithophores. The comparatively lower C-13 and O-18 of the <63 mu m and <38 mu m fractions in sediments younger than 4.4Ma is attributed to an increase in deep-dwelling planktic foraminifera material in bulk carbonate, shifting the bulk isotopic signals toward higher values. Bulk carbonate C-13 is similar over 2500km along the Pacific equator, suggesting covarying concentrations and C-13 of dissolved inorganic carbon within surface waters since 8Ma. Greater bulk sediment C-13 and O-18, higher sedimentation rates, and low content of coarse material suggest intensified wind-driven upwelling and enhanced primary productivity along the Pacific equator between 8.0 and 4.4Ma, although a full understanding of bulk carbonate records will require extensive future work.

Many studies have investigated the evolution of the Eastern Equatorial Pacific (EEP) Ocean paleoceanography since the late Neogene because of its role in regulating the Earth’s climate and because global climate has undergone major changes during this time period. However, current understanding of the EEP paleoceanography encounters two basic problems. First, this region is characterized by steep east-west and meridional gradients in Sea Surface Temperatures (SSTs) and biological productivity, which can vary from one year to another. Second, the seafloor lies below the lysocline in most of the area, leading to poor preservation of biogenic carbonate. As a result, sediments from the EEP exhibit major changes in their physical and compositional properties over short time, and hence sediment depth, increments. Therefore, the study of bulk sediment properties is crucial to further understand the paleoceanography of EEP during the past 8 Myr. Bulk sediment properties are highly appropriate proxies for achieving highly resolved and time comparable SST and biological productivity data because changes in such properties can be traced across the EEP over large distances.This thesis presents two manuscripts. In Paper 1 we examine the positive second-order relationship between wet bulk density (WBD) and carbonate content of sediments from the EEP. This relationship is important because it can be used to determine high-resolution carbonate content records. In Paper 2 we examine bulk sediment stable isotopes records from the EEP to understand what these records actually represent and whether these records are robust across the EEP. The main conclusions of these studies are (1) a single two-component equation cannot be used to determine carbonate content from the WBD. Instead, the WBD-carbonate relationship can only be described by an infinite series of curves representing the mixing of different components; (2) bulk sediment stable isotope records are robust across the EEP and changes in grain-size are not the primary cause of variations of these records.

Sediment cores collected from the Eastern Equatorial Pacific Ocean display a clear positive second-order relationship between wet bulk density (WED) and carbonate content. This has long interested the paleoceanography community because detailed Gamma Ray Attenuation Porosity Evaluator (GRAPE) measurements, which approximate WBD, might be used to determine records of carbonate content at very high temporal resolution. Although general causes for the relationship are known, they have not been presented and discussed systematically on the basis of first principles. In this study, we measure the mass and carbonate content of 50 sediment samples with known WBD from Site U1338, before and after rinsing with de-ionized water; we also determine the mass related proportion of coarse (>63 mu m) material. Samples exhibit clear relationships between WBD, carbonate content, mass loss upon rinsing, and grain size. We develop a series of mathematical expressions to describe these relationships, and solve them numerically. As noted by previous workers, the second-order relationship between WBD and carbonate content results from the mixing of biogenic carbonate and biogenic silica, which have different grain densities and different porosities. However, at high carbonate content, a wide range in WBD occurs because samples with greater amounts of coarse carbonate have higher porosity. Moreover compaction impacts carbonate particles more than biogenic silica particles. As such, a single two-component equation cannot be used to determine carbonate content accurately across depth intervals where both the porosity and type of carbonate vary. Instead, the WBD-carbonate relationship is described by an infinite series of curves, each which represents mixing of multiple sediment components with different densities and porosities. Dissolved ions also precipitate from pore space during sample drying, which adds mass to the sediment. Without rinsing samples, simple empirical relationships between WBD and carbonate content are further skewed by salt dilution.

The oceanographic evolution of the eastern equatorial Pacific Ocean (EEP) since the late Neogene is still under debate. One school of thought proposes weaker than modern equatorial upwelling and sustained warm sea surface temperatures (SSTs) during the late Miocene to early Pliocene, while another suggests stronger than modern upwelling and cool SSTs over this period. These opposing theories appear to be proxy dependent and new perspectives are needed. Bulk carbonate stable isotopes signals have shown potential to carry information on EEP mixed layer water properties including temperature but the relative contribution of different biogenic carbonate components, and thus the origin of these signals, remains uncertain. Here we measured δ¹³C and δ¹⁸O of bulk carbonate, and several finer fractions that concentrate coccoliths, from ODP Site 851 sediments over the last 7 Ma. These data are compared to a series of new and published planktic and benthic foraminifera and foraminifera fragment δ¹³C and δ¹⁸O records to help disentangle coccolith ecological and ocean signals, and refine the use of bulk carbonate d¹³C and d¹⁸O as palaeoceanographic proxies. Our results imply that, once coccolith vital effects are accounted for, bulk δ¹³C and δ¹⁸O records mixed layer signals shallower than the depth of the planktonic foraminifera Globigerinoides sacculifer. Higher bulk carbonate δ¹³C, δ¹⁸O, sedimentation rate and opal content, combined with lower CaCO₃ and >63 µm content during the late Miocene and until 4.6 Ma imply enhanced upwelling and cool SSTs along the EEP at this time, supporting the biogenic bloom hypothesis.