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Millennial-scale variations in climate forcing are recognized to drive changes in terrestrial processes, and, by extension, impart controls on fluvial sediment loads (e.g., weathering and erosion). However, the impact of decadal- to centennial- scale climate fluctuations on downstream coastal sedimentation patterns and landscape evolution remains unclear. Specifically, the connection between long-term (decades or more) precipitation intensity/seasonality and sediment export from river systems has not been established. This study examines the manner in which sub-millennial-scale fluctuations in precipitation within a river catchment in southern Brazil are recorded in a coastal sedimentary archive. The 5-km wide Tijucas Strandplain formed over the last 5800 years through the rapid reworking of sediment discharged from the Tijucas River in a regime of falling sea level. Within a beach-ridge plain characterized by an overall shift from sand- to mud- dominance (linked to a long-term reduction in wave energy caused by bay shoaling) are nearly 70 distinct transitions between shore-parallel sand- and mud- dominated facies. to assess the potential role of climate forcings (e.g., precipitation patterns) in controlling the delivery of sediment to this coastal system, changes in bulk organic and inorganic characteristics, as well as terrestrial vascular plant wax fatty acid stable hydrogen (δD) and carbon (δ13C) isotopic values, were measured from samples collected across sandy and muddy segments of the plain, and from the modern river, estuary, and bay. Bulk δ13C measurements from modern system samples increase by 3.3 ‰ from the most upstream sampling location to the estuary, indicating considerable mixing and/or replacement of terrestrial with marine organic material prior to sediment preservation in the strandplain. However, C28 fatty-acid δ13C data indicate that the plain faithfully records the terrestrial component of the organic matter pool. Concurrent and equal magnitude shifts in δD values (tracking precipitation source/amount changes) between 2015 and 2017 of both river and beach sediments indicate that river sediments are rapidly transported from the river-estuary interface, onto the modern beach, and preserved within the strandplain. This interannual variability was absent from bulk and biomarker δ13C values, reflecting the slower response of vegetation dynamics to precipitation changes. Modern isotopic data from rainfall characterized by seasonally alternating northeast (distal) and southeast (proximal) sources indicates that relatively isotopically depleted average annual rainfall reflects a higher ratio of austral summer to austral winter precipitation, and thus enhanced seasonality. Long-term changes in these precipitation patterns are observed in strandplain biomarker data: δD values become ~10 ‰ more depleted over the last ca. 2000 years, reflecting a gradual moistening and/or decreased seasonality of regional climate. This is supported by strandplain biomarker δ13C values, which record a long-term shift towards more C3-dominated continental vegetation. Moreover, sand-dominated strandplain segments have biomarker δD values indicative of enhanced seasonality (or aridity), as compared with mud-dominated strandplain segments. It is concluded that drier and/or more seasonal rainfall allows for deeper erosion of soils and enhanced export of sand from the Tijucas River, and that changes in the balance of precipitation source and amounts can force substantial changes in the texture and rate of sediment delivery to the coast.

Salt marshes provide valuable ecosystem services to human society, but are currently under threat from accelerating sea level rise and nutrient enrichment. Carbon (C) and mineral accumulation allow salt marshes to maintain elevation above sea level and survive. Anthropogenic nitrogen (N) loading is increasing in many salt marshes, causing negative impacts on marsh resilience such as increased decomposition and decreased below-ground production. However, increasing N may also have simultaneous positive effects such as increased primary production and above-ground biomass, surface sediment accretion, and denitrification rates, which remove excess N from coastal waters. Many studies have been conducted to determine the effect of fertilization on salt marsh resilience; however, inconsistent conclusions across studies may result from varying physical and chemical characteristics across salt marsh locations that impact responses to fertilization. In this dissertation we performed experiments to determine how C cycling, C accumulation, N cycling, and microbial communities vary in both natural and fertilized salt marsh locations at Marine Corps Base Camp Lejeune, North Carolina, USA. Here we show that edge marsh with a high elevation berm had lower pore water sulfide, ammonium, dissolved organic C (DOC), and dissolved organic C (DIC) concentrations than interior marsh, which displayed longer pore water residence time and flooding duration with high pore water sulfide, ammonium, DOC, and DIC concentrations. Respiration and primary production were higher in the edge marsh compared to the interior marsh but net ecosystem CO2 exchange (NEE) was nearly balanced at all sites. Fertilization had a much greater impact on edge than interior NEE, shifting edge NEE toward net CO2 emission. Net ecosystem carbon balance (NECB), based on the mass balance of NEE, lateral C export, and sediment C deposition for edge and interior sites was calculated to examine the effect of fertilization on net C accumulation. NECB displayed a net C gain in the interior marsh but a large net C loss on the edge; fertilization stimulated more C loss on the edge than in the interior. When extrapolating NECB to the entire marsh, C loss on the edge greatly impacted the whole marsh C budget, causing the marsh to have a net loss of 53 kg C yr-1 under natural conditions and a five-fold increase in C loss with fertilization. N removal through denitrification was greater on the edge and increased with fertilization, but was not affected by fertilization at the site with highest sulfide concentrations. DNRA, which retains N in the marsh, dominated over denitrification only during summer, and varied widely across locations. Fertilization generally decreased DNRA rates. Microbial community composition was distinct on the edge vs. interior, with differences driven by the differences in pore water sulfide, ammonium, DOC, and DIC. The edge was a hotspot for nitrifying microbial communities. The processes of respiration and denitrification were positively correlated to the relative abundance of sulfate reducers and ammonia oxidizers, respectively. Thus, we conclude that fertilization had an overall negative effect on marsh resilience with especially large impacts on edge marsh.

Methane contained in gas hydrate is a significant component of the global organic carbon inventory. Describing the methane sources supporting these systems and the mechanisms that control the distribution of methane in marine sediments are critical elements in evaluating the resource potential, climate change implications and geologic hazards associated with gas hydrate. The northern Cascadia margin (offshore Vancouver Island, Canada) is a convergent margin with gas hydrate-bearing cold seeps composed of both thermogenic (Barkley Canyon) and microbial (Bullseye vent) gas sources. Gas hydrate and sediment cores were collected from each of these settings to examine the sources that sustain the gas hydrate and the biogeochemical processes that control the flux and cycling of methane carbon within the cold seep system. Gas hydrate bound methane from Barkley Canyon, Bullseye vent and four other oceanic locations was primarily synthesized from "fossil" organic matter devoid of radiocarbon. Recognizing that the global gas hydrate reservoir is comprised of fossil carbon may alter our understanding of how gas hydrate methane influences the radiocarbon content of other ocean carbon pools (e.g., dissolved organic carbon). Analysis of gas and oil samples from the thermogenic gas hydrate site, Barkley Canyon, indicated the thermogenic hydrocarbons were generated from a source rock with a low hydrocarbon producing potential. The pore water and sediment geochemistry from Barkley Canyon and Bullseye vent were evaluated to identify fluid flux regimes and the metabolic pathways that control how methane carbon was cycled by the microbial consortium. Evidence for coupling between anaerobic oxidation of methane (AOM) and methanogenesis was inferred from the stable carbon isotope composition of the dissolved inorganic carbon (DIC) and methane at both locations. Lipid biomarker analysis further indicated sulfate reducing bacteria consumed organic matter sources in addition to methane. This study demonstrated the value of integrating geochemical, chronological and geophysical data to understand the sources and cycling of methane in cold seep systems and provided a more comprehensive model for how the northern Cascadia margin is interconnected and how the gas hydrate system has evolved.

Nitrogen (N) is one of the essential building blocks for all life and is available in the form of dissolved N in aquatic ecosystems. It is important to understand how this N can support primary and secondary production mediated by phytoplankton and bacteria, respectively, as it can affect both microbial loop biogeochemistry and the higher trophic levels of food webs. Nitrogen studies have traditionally focused on dissolved inorganic N (DIN) as a labile N source. Dissolved organic N (DON), while still often considered refractory, has been increasingly recognized as an important N source supporting primary and secondary production. However, the inclusion of DON into uptake studies is still limited. Expanding N research to encompass DON will be important as researchers continue to assess how nutrient cycles respond to a changing climate. The goal of this dissertation was to expand the understanding of how phytoplankton and bacteria use N by investigating uptake rates of a suite of DIN and DON substrates in two different ecosystems. Research for this dissertation was conducted in the York River, VA and the coastal Alaskan Arctic. In both systems, nutrient uptake rates were measured using 13C and 15N stable isotopes for N and carbon (C) substrates. In the York River, N uptake (>0.3 µm size class) was investigated in alternating months during a period of elevated precipitation. Ammonium (NH4+) uptake was found to be the greatest, but urea uptake was elevated relative to other substrates in late fall. Rates of NH4+ regeneration were lower than measured uptake rates, which indicates that autochthonous production was insufficient and allochthonous sources were needed to meet the N demand. Finally, this study also reported the rates of NH4+ release from urea, finding that urea provided minimal NH4+, averaging <1% of NH4+ needed to support measured NH4+ uptake rates. Further study in the York River used 16S rDNA sequencing to determine if wastewater effluent with different DIN and DON content affected the composition and diversity of the microbial communities in receiving waters. Overall, addition of minimally treated effluent with high DIN lowered microbial diversity, while exposure to more heavily treated effluents resulted in communities that were more similar to the control community without effluent addition. In the Alaskan Arctic, late season N uptake was investigated through 15N and 13C substrate incubation experiments in the Chukchi and Beaufort Seas over two summers for the >0.3 µm size class. During these experiments, urea uptake was often greater than nitrate, but NH4+ was taken up at the highest rate. Differing sea-ice conditions were also found to support different rates of NH4+ regeneration and C uptake. Collectively, the results of this dissertation demonstrate that while DIN is the form of N primarily used in coastal and marine ecosystems, DON can be an important nutrient source to aquatic microbial communities. Future studies should aim to incorporate DON substrates as both nutrient cycling and community composition will likely continue to shift as anthropogenic activity alters ecosystems.

Benthic-pelagic coupling is defined as the deposition of organic matter from the water column to the sediments, and the subsequent remineralization of this organic matter and release of inorganic nutrients back to the water column. This process plays an important role in determining the magnitude of benthic net community production (NCP), a metric that reflects the balance between gross primary production and respiration. Environmental factors, such as the presence or absence of intense phytoplankton blooms can influence the direction and magnitude of benthic-pelagic coupling and determine if benthic NCP is net autotrophic or heterotrophic. The objective this thesis was to quantify the feedbacks among intense phytoplankton blooms, benthic NCP, net nutrient fluxes, and nitrogen cycling in the York River Estuary (YRE). Sediment cores were collected from shoal and channel stations across the YRE in January June, July, August, and October 2020 and April 2021 to characterize spatiotemporal variability in benthic NCP, net fluxes, and nitrogen cycling rates. To characterize these parameters in the context of intense phytoplankton blooms, sediment cores were also collected in the lower YRE (LYRE) in July and September 2020 before and after the occurrence of a bloom. The cores were incubated at ambient temperatures in both the light and dark to determine fluxes of dissolved oxygen and organic and inorganic carbon and nitrogen. Rates of denitrification and dissimilatory nitrate reduction to ammonium (DNRA) were determined via the isotope pairing technique. Results indicated that the benthos was net heterotrophic, with increased heterotrophy observed after the bloom. Dissolved organic carbon uptake increased post bloom, fueling benthic respiration and net heterotrophy. There was primarily net uptake of nitrate and both uptake and release of ammonium. Rates of DNRA were higher than denitrification, especially after the occurrence of the bloom. Overall, results indicated that the presence of intense phytoplankton blooms have the potential to dramatically alter benthic NCP and nutrient cycling, with increased heterotrophy and rates of DNRA having the potential to recycle inorganic nutrients that can further promote blooms.

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