Dissertations
Permanent URI for this collectionhttps://hdl.handle.net/1969.6/1139
Browse
Browsing Dissertations by Author "Abdulla, Hussain"
Now showing 1 - 2 of 2
- Results Per Page
- Sort Options
Item Impact of subsurface methane transport on shallow marine sediment geochemistry(2020-08) Abdullajintakam, Sajjad; Abdullajintakam, Sajjad; Coffin, Richard B.; Coffin, Richard B.; Abdulla, Hussain AN; Reese, Brandi K; Naehr, Thomas H; Lyons, Timothy W; Abdulla, Hussain; Reese, Brandi K.; Naehr, Thomas H; Lyons, Timothy WMarine sediments host a vast amount of methane, a potent greenhouse gas, in the subsurface. Transport of this subsurface methane towards the seafloor creates unique biogeochemical interactions which result in important consequences for the chemical and biological composition of the oceans at present and over the Earth’s geological history. This dissertation studied the impact of subsurface methane venting to shallow marine sediment geochemistry with a goal to quantify the role of methane induced biogeochemical processes in marine carbon cycling and to recognize geochemical proxies that will enable better reconstruction of these processes from the geological record. Key results suggest the following: (i) Globally, diffusive methane charged sediments are significantly contributing to the oceanic dissolved inorganic carbon (DIC) pool (comparable to ~20% global riverine DIC flux to oceans) and sedimentary carbonate accumulation (comparable to ~15% of carbonate accumulation on continental shelves), primarily due to microbially induced carbon-sulfur (C-S) coupling. (ii) C-S coupling induced by methane seeps and crude oil seeps can be distinguished from the sediment records using a combined stable carbon (δ13C) and sulfur (δ34S) analysis of authigenic carbonate and sulfide mineral phases formed in seep settings. (iii) Molecular fossil records of methane metabolizing archaea in the sediment column involve unique isomer patterns of Isoprenoid Glycerol dialkyl glycerol tetraether (GDGT) lipids, which can serve as an important proxy to study paleo-methane flux records. These results will substantially contribute to our existing coastal and geological carbon models as well as enhance our existing inventory of geochemical proxies to characterize the methane venting systems in the geological past.Item The importance of submarine groundwater discharge on nutrient fluxes and dissolved organic matter molecular composition in a semi-arid, highly disturbed estuary(2020-12) Douglas, Audrey; Murgulet, Dorina; Montagna, Paul; Abdulla, Hussain; Felix, Joseph D.; Peterson, RichardThis dissertation advances our understanding of the temporal and spatial variability of submarine groundwater discharge (SGD) rates, SGD-derived nutrient fluxes and its role in the nitrogen budget, and the dissolved organic matter (DOM) molecular composition of surface water and porewater in semi-arid, hydrological disturbed estuaries. Nueces Bay was selected as the study area because it is highly disturbed both anthropogenically (e.g., dredging, oil/gas development, and reduced freshwater inflows) and naturally (e.g., drought, flooding) and the nitrogen budget remains unbalanced after multiple attempts. This study collected surface water and porewater samples quarterly for 2 years and employed radon and radium mass balances and Darcy estimates to assess SGD rates, principal components analysis and a partial hierarchical two-way ANOVA to evaluate the water quality and relative importance of SGD-derived nutrient fluxes to the overall nutrient budget, and PPL-SPE and UPLC-Orbitrap Fusion Tribrid mass spectrometry to assess DOM molecular composition. This study found significant spatial and temporal variability in SGD rates that vary over 1-2 orders of magnitude depending on method (i.e., radon or radium mass balance, Darcy estimate), location, and groundwater endmember. The high SGD rates, compared to literature, are driven by steeper gradients nearshore, shortcircuiting of confining layers due to substantial oil/gas development and dredging, sediment heterogeneity, and reduced confining layer integrity favoring vertical advective flux. The observed spatiotemporal variability in SGD was related to nutrient fluxes and nitrogen budgets, suggesting that SGD brings 2-4 orders of magnitude more nitrogen and other nutrients into the system than surface runoff under all hydroclimatic conditions. Thus, the average SGD flux supplies more nitrogen to the system than previously accounted for creating an excess of vi 370.6x109 g N⸱yr-1 . Finally, a molecular characterization of the DOM indicates that surface water DOM composition is significantly different from flooding and baseflow conditions (6- and 9- months post flooding) during flood recession (3-months post flooding) with more heteroatom compounds detected. Surface water and porewater are most similar during flooding recession, indicating greater SGD-derived DOM. While other studies have suggested that semi-arid systems receive significant SGD, this dissertation further suggests that SGD in highly anthropogenically disturbed systems may be derived from both shorter (shallower) and longer (deeper) flowpaths and lag the climatic conditions by weeks (surficial aquifers) and months or longer (deeper aquifers). As anthropogenic disturbances continue to increase along with a changing climate, the groundwater-surface water interactions will be impacted and the long-term effects of these changes on nutrient and DOM composition might be significant, though potential consequences remain largely unknown.