Victoria University

Land to Ocean Transfer of Erosion-Related Organic Carbon, Waipaoa Sedimentary System, East Coast, New Zealand

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dc.contributor.advisor Trustrum, Noel
dc.contributor.advisor Johnston, Jim
dc.contributor.advisor Tate, Kevin
dc.contributor.advisor Carter, Lionel Brackley, Hannah Lema 2008-06-30T05:02:26Z 2008-06-30T05:02:26Z 2006 2006
dc.description.abstract Mountainous islands of the Pacific Rim (such as New Zealand) purportedly deliver up to 40% of the suspended sediment load and up to 35% of the riverine particulate organic carbon (POC) load to the world's oceans. On the east coast of New Zealand's North Island, the Waipaoa River drains a steep, 2205 km2 catchment located on the active collisional East Coast Continental Margin. It has an annual suspended sediment load of 15 Tg (15 x 1012 g), making up ~7% of New Zealand's total yield to the Pacific Ocean, and a mean annual POC discharge to the Pacific Ocean of 86.7 Gg (86.7 x 109 g). The annual loss of OC to the floodplain is ~9% of this annual POC discharge (~7.8 Gg). A range of analyses (including organic carbon content (%OC), stable carbon isotopes (Delta 13C), radiocarbon (14C), carbon to nitrogen ratios (C/N)a and carbon loadings (OC:SA)) were performed on correlative sediments from a transect of 7 cores from depositional sites located on the Waipaoa River floodplain and adjacent continental shelf and slope. Results were used to determine biogeochemical characteristics of organic carbon (OC) at a range of depositional sites during its transfer from terrestrial source to marine sink, and how large floods impact OC transfer to the marine environment. The high temporal variability in OC content (0.2 to 3.5%) and different source signatures (Delta 13C of -26.7 to -20.6% degrees) of Waipaoa River floodplain deposits prevented the establishment of a clear benchmark signature for flood deposits that may be recognisable in the marine sedimentary record. The high spatial and temporal variability of floodplain sediment OC, combined with the areal extent of floodplains within the catchment, indicates the appreciable modulating effect the floodplain has on OC transfers to the ocean. Since extensive stopbanks were constructed on the main floodplain since the 1940' s, sequestration of OC in floodplain sediments has reduced by about half, increasing the overall efficiency of the Waipaoa River in transferring terrestrial OC directly to the marine environment. Flood layers are preserved in the marine sedimentary record. Continental shelf sediments indicate that during Cyclone Bola (March 1988, a rainfall event with a >100 year return period), the extreme river discharge produced a hyperpycnal (negatively buoyant) plume, preserved as a ~10 cm thick layer on the inner shelf and a ~1 cm thick layer on the mid-shelf. The flood layer contains a significant amount of terrestrially-sourced OC (up to 86% of total OC in >25 Mu m fraction) which subsequently was rapidly buried by normal marine deposits (in which ~60% of OC in >25 Mu m fraction is terrestrial), thereby preserving its strong terrestrial source signature. As sediments are physically and biologically processed at various depositional sites across the continental shelf and slope, they lose some of their modern terrestrial OC, and the concurrent addition of marine sourced OC results in the sediments gaining a stronger marine biogeochemical signature (Delta 13C values increasing from -26.2% degrees for floodplain sediments to -21.6% degrees for upper continental slope sediments). Carbon loading (OC:SA) and 14C data revealed the contributions of kerogen, modern terrestrial OC and modern marine OC to the total OC of continental shelf and slope surface sediments. Sediments retain about 40% of their terrestrial OC following transport to the continental slope, of which a significant amount consists of kerogen. Because of high erosion rates within the catchment, kerogen associated with the particles escapes oxidation, and therefore makes up a large part of the POC flux. Kerogen is preserved across the margin to the mid-slope, where only 8% of the bulk sediment OC consists of modern terrestrial OC, 58% is modern marine OC and 34% is kerogen. Biomarker analyses of surface samples also support findings that terrestrial OC is being transferred across the continental margin, with plant sterols, long chain alcohols and long chain fatty acids (biomarkers indicative of vascular plants) persisting as far offshore as the mid-continental slope. Results presented verify and add to the understanding of OC transfers and transformations at a range of depositional sites from terrestrial source to marine sink. This study provides the first quantitative assessment of land to ocean OC transfers from New Zealand. These findings, together with information on sediment budgets and depositional rates of OC in terrestrial and marine depositional environments, could provide a vital step toward establishing global OC budgets for small mountainous island environments. en_NZ
dc.language.iso en_NZ
dc.publisher Victoria University of Wellington en_NZ
dc.subject Sediment systems en_NZ
dc.subject Marine sediment en_NZ
dc.subject Sedimentary transfer en_NZ
dc.title Land to Ocean Transfer of Erosion-Related Organic Carbon, Waipaoa Sedimentary System, East Coast, New Zealand en_NZ
dc.type Text en_NZ
vuwschema.contributor.unit School of Chemical and Physical Sciences en_NZ
vuwschema.subject.marsden 260104 Sedimentology en_NZ
vuwschema.type.vuw Awarded Doctoral Thesis en_NZ Chemistry en_NZ Victoria University of Wellington en_NZ Doctoral en_NZ Doctor of Philosophy en_NZ
vuwschema.subject.anzsrcfor 049999 Earth Sciences not elsewhere classified en_NZ

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