Terrestrial sources and export of particulate organic carbon in the Waipaoa sedimentary system: Problems, progress and processes

The composition of particulate organic carbon in sedimentary systems can provide information concerning sources of materials and their history. The NSF-MARGINS Source to Sink study of the Waipaoa Sedimentary System in New Zealand provides a platform to calibrate the particulate organic carbon sedimentary record by linking chemical characteristics to terrestrial and marine sedimentary processes. This report is a preliminary evaluation of the available organic carbon concentration, elemental C/N ratio and C-isotopic composition data for the terrestrial portion of the system. Samples analyzed were rocks, soils, floodplain sediments and riverine suspended sediments. Measurements confirm previous conclusions that a portion of the riverine particulate organic carbon is derived from sedimentary rocks. The rock C (kerogen) concentration is relatively invariant (~ 0.25%) from rock source, to riverine and floodplain sediments. The non-rock C is derived from C3 vegetation in most of the watershed, the exception being the floodplain in which local C4 plant inputs are detected. The non-rock organic carbon has a mean 14C-age of approximately 1000 years, which reflects a mixture of modern and aged components. The presence of aged non-rock C, which is chronic in its supply to the river, indicates storage of particulates within the watershed. Remobilized colluvial and alluvial deposits are among the possible sources. Systematic differences are found between δ13C databases generated by various groups. While there may be natural explanations for these differences, methodological artifacts have to be considered. Standardization of methods that cross source-to-sink boundaries is recommended.