Histories and Mechanisms of Change in the Development of Shore Platforms at Kaikoura and Rodney New Zealand: Application of Cosmogenic Nuclides and Numerical Modelling on Exposed Coastal Surfaces

Global sea level rise is contributing to the acceleration of cliff erosion rates in New Zealand, where it surpasses rates of uplift. A significant challenge facing scientists and managers is that we have no method for reliably extracting past rates of coastal erosion along harder rock cliffs over the time-scales that significant sea level change occurs (100s-1000s of years). This gap in knowledge is limiting efforts to model and understand the relationship between sea level rise and cliff erosion rates and what form of that relationship takes. Cosmogenic Beryllium-10 analysis has been applied on two low angle shore platforms in New Zealand to produce chronologies of sea cliff retreat during the late-Holocene. Surface exposure ages were attained on a tectonically active platform at Kaikoura, Canterbury and a tectonically quiescent platform at Cape Rodney, Auckland. This is the first application of cosmogenic nuclides to a shore platform study in New Zealand and adds two new data-sets to the very small group of global shore platform chronologies. Exposure ages show New Zealand platforms have developed in the late-Holocene. Long-term platform surface erosion rates at Kaikoura (0.4mm a-1), potentially due to uplift driven positive feedback such as altered sea level position, driving up weathering rates on the tidally inundated platform. Nuclide concentrations at Okakari Point, Rodney, reveal a significant role of recent sea level fall after ~4000yrs BP, driving surface denudation (0.1mm a-1). The long-term cliff back-wearing rate at Okakari point was found to be 24.66mm a-1. Patterns in cosmogenic nuclide concentrations in New Zealand’s shallow platforms differ from global examples recorded on steeper platforms. Exploratory numerical modelling was applied with the coupled Rocky Profile CRN model (RPM_CRN) to identify process relationships between key drivers within platform coastal systems and scenarios of sea level change and active tectonics. This combined geochemical and numerical modelling study has shown that shore platforms in New Zealand have complex histories, with different potential driving forces at Kaikoura and Okakari. This highlights the local variability in platform development and cliff retreat, suggesting that estimates of future shoreline erosion will need to take local contingencies into account.