Dispersal and transformation of organic carbon across an episodic, high sediment discharge continental margin, Waipaoa Sedimentary System, New Zealand

Hannah L. Brackley*, Neal E. Blair, Noel A. Trustrum, Lionel Carter, Elana L. Leithold, Elizabeth A. Canuel, James H. Johnston, Kevin R. Tate

*Corresponding author for this work

Research output: Contribution to journalArticle

28 Scopus citations

Abstract

The rivers that drain active, collisional margins of the southwest Pacific deliver up to 35% of particulate organic carbon (POC) to the world ocean, and are a key component of the global organic carbon flux. However, knowledge of the fate of terrestrial POC in the ocean is both limited and necessary for quantifying terrestrial and coastal ocean carbon budgets. Here, the fate of terrestrial POC is determined off the high discharge, Waipaoa River (sediment yield 15 Mt y- 1) based on a transect of seven cores from the river floodplain to the adjacent continental shelf and slope. Total organic carbon (%TOC), δ13C, 14C, C/N ratios and lipid biomarker compounds were used to determine biogeochemical characteristics of surface sediments from terrestrial source to marine sink, and how these characteristics vary with river discharge. Complementary to surface sediments, down-core characteristics of three multi-cores covering the shelf and slope regions were used to identify perturbations in sediment supply via major floods. The presence of flood deposits allows us to compare their OC characteristics with non-flood sediment, thereby helping address the question of how flood events in the river catchment affect the transfer and fate of terrestrial OC through the marine environment. Results from this study show that as surface sediments are physically and biologically processed across the continental margin, they gain a marine signature. Biomarker analyses of surface samples show decreases in terrigenous vascular plant sources with increasing distance offshore. Biomarkers also demonstrate 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. In contrast to ambient conditions represented by surface sediments, rapid delivery by floods allows for more complete transfer of terrestrial carbon to the marine environment. A 1-10 cm thick flood layer preserved from Cyclone Bola (March 1988) contains a significant amount of terrestrially-sourced OC which subsequently was rapidly buried by sediments delivered during less extreme conditions.

Original languageEnglish (US)
Pages (from-to)202-212
Number of pages11
JournalMarine Geology
Volume270
Issue number1-4
DOIs
StatePublished - Apr 15 2010

Keywords

  • New Zealand
  • Waipaoa River
  • carbon isotopes
  • organic carbon
  • sediments

ASJC Scopus subject areas

  • Oceanography
  • Geology
  • Geochemistry and Petrology

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