Honors College Thesis


Distribution and origin of organic matter preserved in modern surface sediments throughout Coastal SE Alaska Public Deposited

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  • Twenty seven modern (top 1-2cm) sediment samples from multicores retrieved from throughout coastal SE Alaska were analyzed for their organic matter content and the source composition. Total organic carbon (TOC) as well as biogenic silica (bioSi), and calcium carbonate (CaCO3) content were analyzed to evaluate the organic matter content and its context with respect to total sediment mass. Terrestrial and marine contribution to TOC content was assessed from interpretation of the spatial distribution of elemental (C/N), stable isotopic (13C, 15N) and a variety of lipid biomarker measurements. Results showed that TOC was highly enriched in the region of the study area south of 58oN off the coast of the Queen Charlotte and Baranof Islands. TOC was highly correlated with bioSi content (r = +0.98), indicating that the majority of the TOC originates from diatom production in this area. C/N, 13C and 15N results indicated higher terrestrial contribution to TOC deposited north of 58°N and higher marine contribution to the south. Lipid biomarkers of marine origin, such as long chain (C37-39) alkenones and highly branched isoprenoid (HBI) C25 and C30 hydrocarbons, were most enriched in the sediments south of 58°N, consistent with results from bulk analyses. However, brassicasterol and dinosterol, two sterols indicative of diatoms and dinoflagellates, respectively, displayed no recognizable spatial pattern. Concentrations for homologous series of long chain (>C20) n-alkanes, n-alcohols and n-acids of terrestrial origin were distributed in a pattern largely counter to that of the bulk analyses. Long chain n-alkane signatures isolated from northern sediments were not typical of higher plants but rather displayed a lack of odd carbon number predominance. Those in southern sediments were typical of higher plants and accompanied by a major relative amount of the pentacyclic triterpene, 17β(H),21β(H)-hop-22(29)-ene (diploptene). These biomarker observations suggest that the terrestrial component of TOC deposited in sediments north of 58oN is fossil organic matter associated with glacial outwash of major rivers in that area, while that in sediments depositing to the south is derived from more modern soil erosion processes.
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