Use of Stable Isotopes and GIS Modeling to Study Late Pleistocene to Holocene Environmental Change in the Waipaoa Sedimentary System, New Zealand


The source to sink investigation of complex sedimentary systems necessitates chemical (stable isotope and elemental) and physical (modeling) analyses to elucidate temporal changes in volume and provenance of sediment supply. The stable isotopic composition of organic matter in continental margin sediments provides a useful, long-term record of environmental change. The Waipaoa River watershed, New Zealand, represents a system of interest due to its location on an active margin, very large sediment supply, and well known, relatively recent history of anthropogenic disturbance. Radiocarbon measurements of three continental shelf cores taken aboard the RV Marion Dufresne in January 2006 offshore from the river mouth suggest a record extending into the late Pleistocene, dating as far back as 14,000 years. Geographic information systems (GIS) modeling suggests large increases in erosion with reduction in landcover due to natural volcanic events and anthropogenic disturbances. Carbon and nitrogen isotope analyses of terrestrial sources including soil profiles, sedimentary rocks and riverine sediments from within the watershed suggest terrestrial processes such as shoreline progradation, hillslope erosion and gully incision, volcanic eruptions, and the capture of river tributaries are possibly influencing isotopic ratios and impacting the marine stratigraphic record. Unique isotopic signatures of soil profiles from disparate areas of the watershed could explain some isotopic variation seen in the cores as deviation in delivery volume from certain tributaries. Within the marine record in all three cores exists a distinct excursion of carbon isotopes to more positive values. Possible explanations for this include: 1) an increase in the flux of an isotopically heavy terrestrial fraction from the erosion of kerogen and 2) a decrease in the proportion of terrestrial organic matter due to dilution of river sediments with volcanic ash. Increasing nitrogen isotope values also within the excursion could suggest an increase in marine organic matter, however this could be the result of increased input of degraded refractory terrigenous organic matter or contributions of inorganic nitrogen.



stable isotopes, GIS, carbon, source to sink





Marine, Earth and Atmospheric Sciences