At ~662 gigatonnes of carbon (GtC), marine dissolved organic matter (DOM) is the largest reduced pool of actively cycling carbon and nitrogen in the oceans1. Operationally defined as smaller than 0.1µm in size, this carbon reservoir comprises all non-living organic matter smaller than a bacterial cell and comprises organic colloids and molecules spanning as a continuum of sizes ranging from marine viruses and large macromolecules (e.g. DNA, enzymes) to small organic molecules (e.g. polymers and monomers)2. With deep apparent 14C-ages ranging between 4900-6400 ybp 3,4, marine DOM is anomalously old given timescales of global ocean ventilation (1000-1500 years). The great age of DOM has remained one of the most elusive lines of scientific inquiry in Chemical Oceanography for decades. The size and molecular composition of DOM has been shown to be a key variable in determining its biological reactivity (e.g. cycling rate) and long-term persistence in the deep ocean5,6.
Despite the importance of DOM in the marine carbon and nitrogen cycles, we lack a detailed understanding of the molecular composition of DOM. Due to the high concentration of salts in seawater relative to DOM, it is difficult to analyze the molecular composition of seawater with conventional chemical- or size- fractionation methods without introducing bias (i.e. isolating only hydrophobic and/or high molecular weight DOM). In fact, it is commonly reported that >80% of DOM remains uncharacterized at the molecular level (e.g. not readily identifiable as an individual known biomolecule)5. Nuclear magnetic resonance (NMR) spectroscopy has been used as a tool for several decades to describe the composition of marine DOM isolates7. For example, 13C-NMR of major high molecular weight DOM functional groups at the molecular-level demonstrated that DOM is largely made up of reactive polysaccharides with low aromaticity compared to terrestrial DOM8. To date, all marine DOM NMR measurements have been made on size-fractionated DOM or chemically-fractionated (e.g. solid phase extracted) DOM isolates. In this thesis, I report the first Proton (1H) NMR composition of total seawater DOM from seawater samples collected from 10 stations in Baffin Bay aboard the CCGS Amundsen (2019). Samples were measured using 1H-NMR at uOttawa following a novel water suppression method established by Lam and Simpson9.
The use of this method has allowed for the first molecular composition assessment of total seawater DOM to be measured (e.g. without any chemical or size fractionation). I report the % relative abundance of individual biomarkers and determine molar concentrations of two compound classes of interest. These results are shown in Ocean Data View section plots, and are listed within appendix tables, to provide a comprehensive depiction of the changing concentrations of dissolved organic carbon (DOC), total carbohydrates (TCHO), and carboxyl-rich alicyclic molecules (CRAM). In this thesis, I explore changes in the abundance of these unique DOM compound classes and discuss how the composition of DOM directly determines its bioavailability and thus cycling in Baffin Bay 5. The core objective of my thesis was to measure DOM concentrations for TCHO and CRAM, as well as to calculate the production and removal of these key DOM compounds in Baffin Bay due to either physical and/or biological processes. We found that the concentration of both TCHO and CRAM decreased with depth throughout Baffin Bay. This is consistent with previous work suggesting the rapid cycling of carbohydrates, however it contradicts the current paradigm of CRAM cycling. Our results indicate between 21-43% of CRAM produced in the surface is subsequently removed at depth. Rapid cycling of a surface CRAM population suggests that not all CRAM can be considered recalcitrant DOM
We live in a time of unprecedented global change. The Arctic Ocean is warming at a rate at least four times faster than the global average10. The impact of a rapidly warming, freshening and increasingly acidified Arctic Ocean on the biogeochemistry of DOM remains unknown. It is imperative that more DOM research be conducted as early as possible in order to better understand these impacts and inform future research directions. The distribution and cycling of CRAM in Baffin Bay provide novel and fundamental knowledge of DOM cycling in a key Arctic region, but could also potentially occur throughout the global ocean. Such data will no doubt be of use in informing future iterations of Earth System Climate models seeking to forecast how the marine carbon cycle will respond to global change.
| Identifer | oai:union.ndltd.org:uottawa.ca/oai:ruor.uottawa.ca:10393/45146 |
| Date | 13 July 2023 |
| Creators | McKee, Kayla |
| Contributors | Walker, Brett |
| Publisher | Université d'Ottawa / University of Ottawa |
| Source Sets | Université d’Ottawa |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | application/pdf |
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