NITROGEN ISOTOPIC SIGNALS IN AMINO ACIDS REVEAL THE ORIGINS AND ALTERATION PATHWAYS OF SINKING AND SEDIMENTED ORGANIC MATTER

Abstract

The sinking and sedimentation of marine particulate organic matter (POM) plays a critical role in the biogeochemical carbon cycling and hence helps to regulate global climate. From surface production, POM undergoes various biogeochemical transformations during water column sinking and ultimate deposition at the seafloor. Organic carbon (OC) and nitrogen (N) are two major nutrients in the ocean, their marine cycling can regulate the biological carbon pump and, together with light, they provide the building blocks for POM. Continental margins are some of the most productive depositional environments in the ocean, thus understanding the environmental factors controlling the export and preservation of POM in these environments over climate-relevant timescales is a fundamental concern. However, the molecular and isotopic composition of POM, its degree of preservation and degradation state are important factors to consider for paleoclimate applications. This thesis aims to improve the understanding of organic N transformation/alteration processes during sinking, sedimentation and ultimate burial of POM. Chapter 2 examines two sediment traps and underlying pushcore sediments to investigate temporal and depth related changes in the export flux in Jordan Basin, Gulf of Maine. Compound-specific N isotope analysis of amino acids (δ15N-AA) reveals depth-related differences in the relative contributions of particles with different processing histories. Significant microbial degradation in the deeper trap and sediments emphasize the key roles of resuspension-deposition cycles that are mainly (but not exclusively) facilitated by a thick year-around benthic nepheloid layer in the region. Chapter 3 discusses a two-year preservation experiment using filter seawater and marine sediments. Formalin is a common preservative used in both ecology and marine (sediment trap) studies, and also known to induce isotopic fractionation in bulk carbon isotopes and potentially bulk nitrogen isotopes. Therefore, evaluating potential isotopic alterations in δ15N-AA data is essential for future AA work on marine materials. Chapter 4 expands this work by analyzing surface sediments from continental margins in 13 different locations worldwide that span a large range in depositional environments and conditions. δ15N-AA parameters show various levels of POM degradation between locations while the bulk N isotopic signatures are remarkably similar. These findings suggest a uniform ‘mode’ of preservation of OM of the AA fraction of sedimentary OM, globally, independent of surface ocean conditions, sediment age or sedimentary environments. This would suggest that variable POM preservation is not a dominant control on OC contents in marine sediments. With this work, I demonstrate the utility and limitations of δ15N-AA analysis in a range of sedimentary environments for tracking the origins and alteration of detrital POM. These findings have important implications for marine biogeochemistry, paleoceanography, and other disciplines.