PICKING APART THE ORGANIC GEOCHEMICAL STRATIGRAPHIC RECORD ON CONTINENTAL MARGINS—AN APPROACH TO DECIPHERING THE SIGNALS OF TERRESTRIAL ENVIRONMENTAL CHANGE

As the primary repositories for particulate matter eroded from land, river-dominated continental margins contain a stratigraphic record that may include abundant information about terrestrial environmental change. The organic geochemical stratigraphic record on continental shelves in particular can provide a detailed archive of geomorphic processes operative in adjacent watersheds. Interpretation of that record is complicated, however, by the selective manner in which mineral particles and associated organic matter are transported en route from terrestrial sources to marine sinks. Because the source, composition, and concentration of organic matter vary among sediment grain size fractions, organic geochemical signals can be difficult to decipher in continental margin sediments of variable texture.

Physical fractionation is one approach to resolving the sources of particulate organic matter in heterogeneous samples. We have taken this approach in studying Holocene sediments deposited on the continental shelf offshore from the Waipaoa River, North Island of New Zealand. The overarching goal of our investigation was to investigate how environmental signals were transferred from terrestrial environments to the marine stratigraphic record. Our results reveal that different size and density fractions carry those signals with different degrees of clarity. Sand-sized wood fragments isolated from these deposits provide information about the composition and age of vascular plant debris transported through the sedimentary system and, based on evidence for their short residence time in the watershed, were most likely derived from forest litter. Charcoal fragments, isolated along with the wood debris, evince long-term sediment storage in watershed soils and carry a signal of disturbance following a major volcanic eruption and again following human colonization. The clay-sized fraction, on the other hand, provides evidence for changing dominance of various geomorphic processes in the watershed over time, including a mid-Holocene interval of increased bedrock erosion, a shift to greater contributions from soil, and the well documented recent dominance of gully erosion due to anthropogenic deforestation.