Insight into Carbon Sources on An Active Continental Margin: Isotopic Signatures Over Contrasting Time Scales

As part of the MARGINS Source-to-Sink initiative, the Waipaoa River is the focus of an ongoing study identifying dynamic changes in sediment transfer from land to sea. The Waipaoa River has a small catchment (2200km²), and is a high-yield, mountainous river, which empties onto a tectonically active, energetic shelf and represents an important analog for shelf sedimentation in similar environments worldwide. Stable carbon isotopes and C/N from box cores (short-term) and giant piston cores (long-term) are presented from continental shelf sediments off the Waipaoa River. Pre- and post-Polynesian and European influences on sediment availability, transport and preservation of storm sequences on the shelf are explored as well as the transition from sediment bypassing to sediment trapping during the Holocene transgression. Frequent, large deviations from mean δ¹³C and C/N in the upper 40cm (box cores) of the seabed signify rapid changes in terrestrial input during recent wet and dry storm deposition. Two shelf depocenters (north and south) bounded by actively uplifiting anticlines retain sediment from different source pools and/or via different modes of transport to these locations. This is evidenced by a lack of δ13C overlap between the two depocenters, with the northern depocenter having a greater terrestrial δ¹³C signal. Giant piston cores (between 16 and 26 meters) retrieved aboard the R/V Marion Dufresne in February 2006 date to 18 ky bp and show variations in allochthonous and autochthonous carbon sources, throughout the Holocene. Shelf break, northern depocenter and slope cores have an increasingly marine δ¹³C signature towards the surface, revealing the transition between lowstand (late Pleistocene) and modern highstand conditions. The southern depocenter has a more variable δ¹³C signal than the north, perhaps revealing a greater capacity to record event sedimentation. Northern depocenter and shelf break cores both have significantly depleted δ¹³C signatures (i.e., more terrestrial) at their bases.