Browsing by Author "Coffey, Nicole R."
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Item Controls on the chemical composition of the surface microlayer of Delaware Bay(University of Delaware, 2020) Coffey, Nicole R.The surface microlayer (SML) is a thin (10s-100s μm) layer at the top of the water column which serves as a boundary between bodies of water and the atmosphere that mediates fluxes across the air-sea interface. The SML’s enrichment in organic matter (OM) is well-documented, and is thought to contribute to the SML’s physical properties and mediation of exchanges. Mechanisms behind material enrichments and their relationship to SML properties in space and time are still debated though, which complicates prediction of air-sea exchange rates. By sampling both the SML and subsurface (SUB, 8-15 cm) water at four stations throughout Delaware Bay to encompass different influences (marine, terrestrial/fluvial, salt marsh) on the system, this study aims to deconvolute relationships between SML and SUB OM composition, microbial community composition, and air-sea exchange relevant physical parameters, specifically surface tension. SML samples collected between December 2018 and October 2019 show consistent enrichments of dissolved organic carbon (DOC) as high as 4.42 times the SUB concentration and as low as 0.87 times the SUB concentration. Compositional assessment, using Fourier-transform ion cyclotron resonance mass spectrometry (FTICR-MS), reveals marine site samples to be compositionally distinct from the three inner-bay sites. This separation is due to an abundance of CHON compounds found in marine samples. Principal component analyses performed on individual sites reveal a consistent difference in the OM composition in the samples from the SML relative to their corresponding SUB sample. OM in the SML is characterized by higher abundances of compounds with H/C ratios above 1.7 and formulas with O/C ratios below 0.2, including an abundance of sulfur-containing compounds, CHO3S compounds, in particular. These traits are indicative of surfactant-like molecules, which can change the physical properties of the air-sea interface. Unsaturated aliphatic compounds, identified from FTICR data, significantly (p < 0.05) correlate with surface tension depression (SUB surface tension – SML surface tension) across the dataset. The sources and structures of these candidate surfactant molecules should be a focus of future work. Assessments of DOC concentration or enrichments in the microlayer alone showed no association with surface tension. The lack of relationship between DOC and surface tension speaks to the need for a compositional approach to estimating properties of the SML and the exchanges it regulates.Item Organic Signatures of Surfactants and Organic Molecules in Surface Microlayer and Subsurface Water of Delaware Bay(ACS Earth and Space Chemistry, 2022-12-15) Burdette, Tret C.; Bramblett, Rachel L.; Deegan, Ariana M.; Coffey, Nicole R.; Wozniak, Andrew S.; Frossard, Amanda A.Surface-active species are present in marine waters and can accumulate in the surface microlayer (SML). Surfactants are amphiphilic organic compounds that reduce surface tension at an interface. Current work is investigating the link between the molecular composition of surfactants in the SML and those in subsurface waters, as well as the differences in surfactant molecular composition across different water types. In this study, subsurface water and SML samples were collected on three sampling days at three sites in the Delaware Bay: the Mouth of the Bay, Mid-Bay, and the marsh-influenced Broadkill River. Organic matter was extracted from the SML and subsurface water using two solid-phase extractions (graphitized carbon and C18) and then analyzed using tensiometry and high-resolution mass spectrometry, in positive and negative ionization modes. Here, we show that molecules with high H/C are preferentially enriched in the SML compared to the subsurface waters. We demonstrate that the measured organic extracts contribute to lower surface tensions in the SML. A rainfall event led to increased terrestrial runoff and mixing that altered the composition of the organic molecules in the SML and subsurface waters of the Broadkill River site, and the composition of the extracted organic molecules varied across sampling days. These results imply that the surfactant compositions are not uniform across sampling regions or from the subsurface to the SML.