Tracking Organic Matter from Source to Sink in the Waiapu River Watershed, New Zealand: A Geochemical Perspective

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Title: Tracking Organic Matter from Source to Sink in the Waiapu River Watershed, New Zealand: A Geochemical Perspective
Author: Thompson, Catherine Elizabeth
Advisors: Paul Liu, Committee Member
Dave DeMaster, Committee Member
Neal Blair, Committee Chair
Elana Leithold, Committee Co-Chair
Elizabeth Nichols, Committee Member
Abstract: The significant contribution of small mountainous river systems, including the Waiapu River on New Zealand’s East Cape, to the global fluvial sediment supply motivates investigation into the processes that influence the character and composition of the organic carbon that they carry. Organic matter preserved in continental margin sediments originates from terrestrial sources such as kerogen, fresh and aged soil carbon, as well as marine sources. Due to the reactivity of marine carbon in the seabed, terrestrial sources of carbon are preferentially preserved unaltered. Therefore, identification of specific terrestrial sources of sediment from the watershed preserved on the continental margin can facilitate interpretation of the organic geochemical record and enable reconstruction of the watershed history. Carbon and nitrogen isotopic analyses and polycyclic aromatic hydrocarbons have been used to apportion terrestrial carbon sources preserved on the shelf, specifically to resolve aged soil contributions. Spatial geochemical patterns on the margin appear to be complicated by structural deformation of the shelf and its influence on marine sediment dynamics. Resolution of riverine sources suggests that gullying, bank failure, and sheetwashing are the chronic geomorphic process delivering most of the organic carbon to the Waiapu sedimentary system. The middle and outer Waiapu continental shelf buries an average of 59 Gg C/y. Relative to the 200 Gg C/y delivered by the Waiapu River to the ocean, approximately 23% of the riverine carbon is retained on the mid- to outer shelf, matching the sediment inventory. Radiocarbon analysis of DIC indicates that there is no apparent oxidation of kerogen in the seabed. Stable carbon isotopic signatures suggest minimal oxidation of modern terrestrial carbon, signifying that the 77% of the riverine carbon not accounted for has likely been retained and potentially oxidized on the inner shelf or escaped to the slope or beyond the established boundaries of the mid- to outer shelf.
Date: 2010-03-04
Degree: PhD
Discipline: Marine, Earth and Atmospheric Sciences

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