Tracing the marine nitrogen cycle in breathless seas: a lipid biomarker approach

The ocean is a key driver of global biogeochemical cycles and the Earth’s climate. Over the past century, anthropogenic climate change and ocean fertilization have reduced marine dissolved oxygen (DO) levels and have expanded oxygen minimum zones. Changes in the oceanic oxygen content may lead to shifts in biogeochemical processes, in particular those associated with the redox dependent nitrogen (N) cycle. These biogeochemical changes may, in turn, provide a feedback on ocean deoxygenation. Under anoxic conditions, bioavailable N can be removed via anaerobic ammonium oxidation (anammox) and denitrification. N2-fixation is the only biological mechanism that can fix atmospheric N2 gas back again into forms available for life. These processes can provide a feedback on marine deoxygenation; By regulating the availability of N in the sea, the abundance and subsequent remineralisation of phytoplankton is controlled, which influences oxygen consumption rates. Yet, the strength and direction of marine N cycling feedbacks in response to changing redox conditions are still poorly understood. Fortunately, the Earth has experienced many previous warming periods, which may act as analogues for modern climate change. In this thesis, biogeochemical shifts in the marine N cycle in response to changing DO concentrations have been investigated across globally distributed seas, in both a modern marine system and during various periods of Quaternary climate change. At each site, a multi-proxy approach was applied, in which lipid biomarker information was combined with a range of (geo)chemical environmental tracers. This allowed for unique insights into N cycling processes in ‘breathless seas’ and their feedbacks on marine deoxygenation. Moreover, this has provided novel information for the application and constraint of various lipids as biomarkers to trace biogeochemical cycling. In Chapter 2, we compared distributions of anammox biomarkers (ladderanes) and 16S rRNA genes with a recently proposed anammox biomarker (BHT-x), alongside physiochemical characteristics of the water column in the Benguela upwelling system. In Chapter 3, shifts in the N cycle in response to orbital-induced changes in the redox conditions of the Eastern Mediterranean Sea were reconstructed. Chapter 4 reconstructs N-loss in the California Current System throughout the late Quaternary. In Chapter 5, a class of bacterial membrane lipids called bacteriohopanepolyols were analysed in a Holocene record from Prydz Bay, the Southern Ocean. Collectively, these chapters provide a deeper understanding of the intricate feedbacks between the marine N cycle and redox conditions during the Quaternary. They particularly emphasise the importance of anammox as an N-loss process, as anammox bacteria showed a temporally persistent presence across globally distributed breathless seas. It is proposed that an anammox contribution to N-loss can thereby contribute to a negative N-loss feedback on marine deoxygenation, a mechanism which is overlooked in many biogeochemical models. Furthermore, we demonstrate how N cycle feedbacks can be region-specific and depend on local conditions beyond O2 concentrations. Additionally, the multi-proxy approach applied throughout the various chapters enhances our understanding of lipid biomarker distributions and their potential applications, thus expanding our capacity to trace biogeochemical processes in the marine environments over a range of redox conditions.