Spatial and Temporal Variability in Surficial Seabed Character, Waipaoa River Margin, New Zealand

Abstract

The Waipaoa River margin, located off the northeast coast of the North Island of New Zealand, provides an opportunity to examine shelf-wide sediment dynamics in a coastal setting. The region is characterized by a narrow shelf (~20 km) and ample sediment supply (15 Mt y⁻¹, ideal precursors to assess the fidelity of the stratigraphic record. Sediments are delivered to this margin via the Waipaoa River which drains a small mountainous catchment (2205 km²) comprised of highly erodible fine-grained lithologies. As part of an NSF-funded project, a time-series of surface seabed properties were used as a foundation to evaluate spatial and temporal changes in sediment dynamics and strata formation on the adjacent margin. Samples were collected on four cruises over 13 months; January, May, and September 2010; and February 2011. Site deposition was assessed using short-lived radionuclides, X-radiography, and through measurement of surface seabed grain-size distribution and organic content. Seabed erodibility was measured with a Gust microcosm device and presented as the total eroded mass (kg m⁻²) for a given experiment. Generally, sediment deposition was variable, in both space and time and dependent on pre-sampling fluvial and oceanographic conditions. Sediment erodibility also varied; generally sediments were more erodible in water depths <40 m in comparison to deeper areas. Pronounced temporal variation in sediment erodibility was evident at sites located in <40 m water depth. X-radiographs collected from these sites generally show interbedded muds and sands, whereas images of sediments collected from more distal shelf locations are more homogenous, the result of efficient biological mixing in the surface seabed. The temporary and periodic emplacement of flood layers on the Waipaoa margin was most likely responsible for sediment erodibility variation in sites <40 m water depth. X-radiography coupled with grain-size and porosity measurements indicated fluid-mud deposition in Poverty Bay and the region immediate to the bay mouth. Radioisotope measurements suggest that recently deposited layers are more erodible than the average erodibility level calculated for the shelf. Other published research from the York River estuary, VA indicated that the seasonal emplacement of thick sediment deposits following river discharge resulted in higher measured bed erodibility. Post-depositional alteration of the seabed likely also resulted in reduced low erodibility measured on the Waipaoa margin. In summary, recently deposited sediments in the nearshore were prone to erosion and transported to more quiescent and deeper shelf regions, where high rates of modern sediment accumulation have been shown by previous researchers. These processes affected surficial seabed properties which in turn influenced subsequent sediment remobilization.