Quantifying decadal-scale sediment flux using the historic sediment record in a range-front catchment, South Westland, New Zealand

Abstract

Accurately quantifying sediment flux from mountain catchments is important for understanding the processes that drive mountain landscape dynamics and evolution. In this study, total sediment yield and erosion rates were calculated for the small, range-front Potters Creek catchment on the western Southern Alps over 68 years, from 1948 to 2016 using the historic sediment record. Volumetric estimates of two sediment sinks, a lake basin and a Gilbert-type delta representing the respective fine and coarse portions of the total sediment flux enabled the calculation of sediment yield and the proportion of fine to coarse sediment over a decadal scale under inter-seismic conditions. This study demonstrates the feasibility of using the historic sediment record to accurately quantify decadal sediment fluxes. The decadal-timescale connects short-term studies that have used instrumental approaches to quantify sediment flux at the daily to yearly scale and studies that focus on the centennial to millennial scale of sediment flux using geochemical techniques. Aerial photographs were used to map the changes to the delta surface at the boundaries of three epochs; 1948, 1965, 1981 and 2014 to calculate volumetric growth of coarse sediment deposition. Fine sediment flux was assessed with a network of sediment cores extracted from Lake Mapourika. An inter-annual chronology of the fine sediment record was developed using bomb-spike radiocarbon dating and Bayesian modelling and was used to calculate flux rates between 1948 and 2016. Average denudation of 3.03 mm yr-1 and total sediment flux of 7220.32 t km-2 yr-1 showed that coarse sediment contribution was ~90% of the total sediment flux, a finding that contradicts previous estimates of the proportion of fine to coarse sediment in total sediment loads. Comparison to the rainfall record showed that hyperpycnal deposits could be linked to high magnitude rainfall events. Variation in rainfall did not account for all fluctuations in sediment flux rate, leading to the conclusion that sedimentation in the catchment was primarily supply-limited rather than transport-limited. In the context of the Southern Alps as a system in steady-state equilibrium, inter-seismic sedimentation accounts for a relatively low proportion of denudation over a seismic cycle contributing to knowledge of the role of inter-seismic sedimentation in overall mountain landform dynamics.