Organic Matter Sources, Transformations, and Fates in Northern High-Latitude Regions on the Forefront of Climate Change

Climate change is dramatically altering the global carbon cycle. The warming caused by climate change is approximately twice the global average in Arctic and contiguous northern high-latitude regions, which are thus especially susceptible to ecosystem shifts (e.g. permafrost thaw, wetland drying/wetting cycle exacerbation). Rivers process and transport organic matter (OM) from land where warming is destabilizing previously stabilized carbon stocks to the ocean. This dissertation examines how fluvial OM in northern high latitudes responds to climate change, and how the fate of such OM may influence the global carbon cycle. Specifically, it focuses on how the source and processing of dissolved and particulate organic matter (DOM and POM) interact to dictate OM fate. To do so, Fourier-transform ion cyclotron resonance mass spectrometry, carbon isotopes (Δ14C, δ13C), and data on OM concentrations and carbon to nitrogen (C:N) ratios are used. Across the pan-Arctic (Chapters 2 and 3) and northeast Pacific coastal temperate rainforest (NPCTR; Chapters 4 and 5), sources appear to influence the fate of DOM (Chapters 2, 4, and 5) and POM (Chapter 3). Both landscape-scale factors like permafrost or agricultural extent (Chapters 2 and 3) and watershed slope and wetland extent (Chapters 3, 4, and 5), as well as specific source types like tree canopy or bark and soil layer (Chapters 4 and 5) impact OM molecular, isotopic, and elemental compositions and thus processing. Further, seasonality varies between the pan-Arctic watershed (Chapters 2 and 3) and the NPCTR (Chapters 4 and 5), leading to differential timing of the strongest terrestrial carbon source impacts on the system.In large pan-Arctic rivers, spring freshet contains DOM primarily sourced from terrestrial material but also includes a latent high-energy subsidy that explains the historical paradox of freshet DOM's bulk terrestrial composition but apparent high biolability. Winter riverine DOM is mostly sourced from old, microbially degraded groundwater DOM. This work uncovered a stable core Arctic riverine fingerprint (CARF) present in all samples from six years. The CARF may contribute to a carbon sink of aged DOM that appears to persist in the global ocean. Further, this dissertation uses a decade of compositional data on POM to untangle multiple allochthonous and autochthonous pan-Arctic and watershed-specific POM endmembers sourced from literature (for the full list of literature values used in endmembers, see supplementary file "Table B5.csv"). The three tracers used (C:N ratio, δ13C, and Δ14C) appear to reveal a substantial, previously overlooked aquatic biomass signal in POM (~53% of the annual pan-Arctic POM flux which averages 4391 Gg/y of exported particulate organic carbon). Splitting soil endmember values into shallow and deep soil pools (mean ± standard deviation: -228 ± 211 v. -492 ± 173‰) rather than the less representative traditional active layer and permafrost categories (-300 ± 236 v. -441 ± 215‰) also improved endmember resolution. In the NPCTR, dissolved organic carbon concentrations as high as 167 mgCL-1 were found in tree DOM (which proved highly biolabile in this study; >50%). Since the NPCTR is hypermaritime, tree DOM fluxes may be substantial. Condensed aromatic formulae comprised as much as 38% relative abundance of spruce and hemlock bark leachate DOM. Coniferous trees could thus be major condensed aromatic sources. Soil wetness determined DOM composition and response to microbial processing. Source-specific marker formulae were discovered in multiple soil and tree sources, and though the majority were consumed during bioincubations certain marker formulae persisted. All of these source-specific marker formulae (of which >90% were from spruce stemflow) were then located downstream in the watershed demonstrating that tree DOM may escape biodegradation. Summertime in the NPCTR yields the most terrestrial DOM, which then appears to degrade during the colder, wetter, fall-spring period. Despite these seasonal trends two metrics of fresh terrestrial DOM contribution (marker formulae and modified aromaticity index) did not show any significant relationships with hydrologic or temperature variables. Antecedent conditions (rather than precipitation alone) thus seems to control molecular terrestrial connectivity. Climate change is modifying both the sources and seasons that this dissertation shows to influence DOM composition and fate in northern regions. Increasing precipitation intensity in the NPCTR may tighten land-ocean linkages, while warming, permafrost thaw, and increasing CO2 concentrations could enhance allochthonous and autochthonous POM fluxes and the contribution of old, microbially processed, stable DOM to the Arctic Ocean. This work shows that an improved understanding of current controls on OM composition and fate will be necessary to predict future changes in northern high-latitude ecosystem carbon cycling and its role in furthering global climate change.