Funds are provided to the PIs to collect data on the Waipaoa continental shelf to build on results of prior work in the area. The objective of the study is to use bottom tripods to record data relevant to sediment processes along an apparent transport route, and to collect cores and, at some sites, measure critical shear-stress. The study will consider locations where cores were collected during a prior study, and one major focus of the study is the comparison of cores collected in different years to determine if additional layers have been deposited. Radiochemical studies will also be undertaken at these sites to help quantify the deposition of sediments. The bottom tripods will be placed along a line on the shelf, near the shelf break and on the upper rise in areas that appear to be transport pathways or sites of deposition. The tripods would be deployed for a year. The third component of the study is the creation and use of a model with the objective hindcast sedimentation events in the area.
The study includes student involvement and training and will lead to an understanding of the relationship of shelfal processes to resultant stratigraphic architecture which is an important challenge in stratigraphy and has the potential to aid exploration for resources in the future.
Most humans live on and utilize the continental margin, the surface of which changes continually in response to environmental conditions such as weather, climate change, tectonism, earthquakes, volcanism, sea level, and human settlement and land use. Some areas of the margin erode, while others accumulate sediment, and it is these accumulating areas of margins that record past events, and can be sampled to interpret the geologic history of a region. However, in order to make interpretations we must understand the marine sedimentary processes and the dynamics of the deposits formed on the seabed over modern timescales. In this study, a small mountainous river system on the North Island of New Zealand was selected because of the large and episodic sedimentary signals generated by the Waipaoa River, and the comprehensive studies that had preceded this study in the Waipaoa Sedimentary System. In this collaborative program, we deployed bottom-boundary-layer tripods in three locations on the continental shelf seaward of the Waipaoa River, to monitor sediment and water movement from January 2010 to February 2011. Four research cruises were conducted at approximately four-month intervals during this time period to service and repair instrumentation, as well as to collect short (<50 cm) sediment cores at repeat locations across the shelf. Cores were examined for their textural and radiochemical properties as evidence of recent sediment deposition. Typical of other small mountainous rivers, the water and sediment discharge from the Waipaoa River is highly episodic. Unlike some of the rivers studied to date (for example on the US West Coast), both storms and river floods can occur year round, although a seasonal pattern is evident, and storms and floods may or may not occur concurrently. Events that caused seabed erosion tended to be of shorter duration, with low wave height but long wave period, whereas aggradation tended to occur with longer duration tall, steep waves. Sediment from river floods needed approximately 120 hours of energetic, "event-level" conditions to be observed on the mid shelf, and these 120 hours could occur as an infrequent single event, or a series of shorter events. When flood and storm events occur concurrently, and with enough duration, wave-supported fluid muds can move sediment from the inner shelf to the mid shelf, and potentially off the continental shelf. These dense, negatively buoyant layers move downslope under the force of gravity. Observations of a wave-supported fluid mud and numerical experiments of the flows showed that although the slope of the Waipaoa shelf is very low and thus gravity-driven flows are slow, the fluid muds formed were a dominant process in moving material in the across-isobath direction. The deposits associated with this flow were clearly identified in x-radiographs of the seabed, and the flow was found to be capable of moving sediment to the continental slope, towards a location where previous studies had identified modern accumulation. A graduate student based his PhD dissertation on this component of the collaborative project, and other students used the data to support their research both within and outside the collaboration. In addition, results from this study have been incorporated into educational materials at the University of Washington. A special issue of Continental Shelf Research was led by guest editors from this collaboration, "Formation and preservation of sedimentary strata from coastal events: Insights from measurements and modeling", and research studies from this effort are included. Conclusions from this study are being incorporated into a Earth Science Reviews article focusing on Source to Sink perspectives in the Waipaoa system.
