Global assessment of marine paleoproductivity proxies
- Autores: Maria Raja Sanchez
- Directores de la Tesis: Antoni Rosell i Melé (dir. tes.), Eric Galbraith (codir. tes.)
- Lectura: En la Universitat Autònoma de Barcelona ( España ) en 2019
- Idioma: español
- Tribunal Calificador de la Tesis: Vincent Grossi (presid.), P. Graham Mortyn (secret.), James Bendle (voc.)
- Programa de doctorado: Programa de Doctorado en Ciencia y Tecnología Ambientales por la Universidad Autónoma de Barcelona
- Materias:
- Enlaces
- Tesis en acceso abierto en: TESEO
- Resumen
Ocean primary productivity (PP) is a key driver of the global marine carbon cycle and thus, the climate of our planet. Obtaining quantitative information on past PP relies on biogeochemical models since current paleoproductivity proxies are qualitative at best. Moreover, the global scale applicability of such proxies and their constraints remains unclear.
The objective of this thesis is to evaluate the applicability and constraints of some of the most frequently used organic proxies to obtain information on biomass export and surface nutrient conditions, and appraise their potential to deliver quantitative information on global spatial scales.
The central approach followed in this thesis relies on the comparison of remote sensing sea-surface ocean chlorophyll-a (SSchla) concentration, as an indicator of marine PP, with global surface sediment data of chlorins, alkenones and total organic carbon (TOC) concentration. In addition, the study on nutrient proxies is based on the comparison of nitrogen isotopic signal (δ15N) derived from sedimentary bulk and chlorophyllic pigments.
A key result is that the global spatial distribution of chlorins and alkenones is mainly related to SSchla concentration rather than depositional factors (e.g. oxygen and sedimentation rate). However, sedimentary chlorins concentration is linked to SSchla by a non-linear relationship, while alkenones and TOC by a linear equation. Global TOC presents higher scatter than biomarkers on its correlation with global SSchla. Depositional factors do play a role in decomposing organic matter, and we estimate that only 0.33% of SSchla accumulates in sediment. However, we also postulate that the rate of degradation is spatially constant. Hence, our results indicate that chlorins and alkenones can be used to quantitative reconstruct past PP at global scale. These export proxies would deliver lower errors on past PP quantitative estimation than TOC. Regarding to nutrient proxies, our data show a linear relationship between δ15N of bulk and chlorophyllic pigments, which indicate these proxies globally provide equivalent information on past nutrient conditions and past PP. Furthermore, δ15N presents considerable differences between pigments, which suggests chlorins might not track chlorophyll-a signal, and complicates the application of individual chlorins as nutrient proxies.
This study highlights the limitations on the applicability of TOC as a quantitative paleoproductivity proxy at global scale, and the potential constraints on the use of individual chlorins for reconstructing past nutrient conditions and past PP. Our findings also pave the way for the use of biomarkers as global quantitative proxies of past PP using globally applicable correlations in ocean sediments.
