| Summary: | Nitrogen (N) holds a central position in ocean biogeochemistry due to its role as a limiting nutrient for biological productivity in the ocean and its resultant influence on the marine carbon cycle. Nitrogen isotopes represent a powerful tool to investigate changes in the marine N-cycle across different timescales. However, their use in geochemical studies has been traditionally limited by the potential diagenetic artifact related to changes in organic matter preservation through geologic time. Over the past ten years, the analysis of the isotopic com- position of organic matter protected within the mineral structure of planktonic foraminifera shells (foraminifera bound, FB) has emerged as a way to circumvent diagenetic overprints of classical techniques. The objective of this thesis is to use this novel technique to study the evolution of the N-cycle over previously unexplored periods of the Cenozoic, with a particular focus on the late Pleistocene glacial cycles (Chapter 3), the Mid Miocene (Chapter 4), and the Mid Eocene Climate Optimum (Chapter 7). Foraminifera-bound N isotope measurements (FB-d15N) are complemented by the development of a new method for organic biomarker extraction (Chapter 5), which has allowed the analysis of a significant number of samples, and provided new constraints on climate evolution across the studied time periods (Chapters 6 and 8). In Chapter 3, we show a pervasive coupling between low-latitude N-fixation and ocean circulation changes that control the supply of excess phosphorous (P) to the surface ocean in the Atlantic Ocean. Our data suggest decreased N-fixation in the North and South Atlantic oligotrophic gyres during periods when the supply of excess P by Antarctic Intermediate Water (AAIW) is suppressed by Glacial North Atlantic Intermediate Water (GNAIW). In contrast, precessional changes in the strength of equatorial upwelling - which in turn drive the supply of excess P - appear to be particularly important to N-fixation in the Caribbean Sea, noticeably weaker in the ...
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