AN INVESTIGATION OF SPATIAL AND TEMPORAL VARIABILITY OF SURFACE SHELF SEDIMENTS ON THE WAIPAOA RIVER MARGIN, NEW ZEALAND

The Waipaoa River Margin, located off the northeast coast of the North Island of New Zealand, affords the opportunity to assess the fidelity of the stratigraphic record and sediment dynamics in a coastal setting characterized by a narrow shelf (~20 km) and ample sediment supply (15 Mt y^-1). Sediments are delivered to this margin via the Waipaoa River which drains a small mountainous catchment (2205 km²) comprised of highly erodible lithologies. As part of an NSF-funded project, time-series analysis of surface seabed properties is being used as a foundation to evaluate spatial and temporal changes in strata formation and sediment dynamics on the adjacent margin. Samples were collected on three cruises taking place in 2010 (January, May, and September) with a fourth scheduled for mid-February 2011.

Preliminary results show that both erodibility measurements of the seabed and radioisotope inventories vary in a complex manner through space and time. Erodibility measurements (n=14) of mid-shelf depocenter muds, determined using a Gust microcosm, had a mean value (AVG) of 0.42 kg m^-2, with a standard deviation (σ) of 0.10 in comparison to measurements (n=12) made at coarser-grained sites on the margin (AVG = 1.44 kg m^-2, σ = 0.94), indicating depocenter muds are less erodible. Variations in physical seabed properties (e.g., grain size, total organic carbon, porosity) and amount of biological influence (e.g., bioturbation, biofilms) are likely responsible for the complexity of erodibility variations across the margin. The distribution of short-lived radionuclides, specifically ⁷Be, is also spatially and temporally complex. ⁷Be inventories suggest that short-term deposition does not necessarily mimic long-term sediment accumulation. Mid-shelf sediment depocenter data from January and September 2010 are characterized by high ⁷Be inventories in comparison to other areas of the margin which is consistent with previous research. Conversely, sediments collected in May 2010 from “Poverty Gap”, a suspected sediment bypass region on the shelf, are characterized by high ⁷Be inventories, suggesting ephemeral deposition. The complete annual dataset combined with in situ sediment-transport measurements and modeling efforts of collaborators will give valuable insight into sedimentation in this complex system.