Quantifying sediment production and storage dynamics in the tectonically active Southern Alps of New Zealand

Understanding the interactions between Earth's surface processes, climate, and tectonic forcing remains challenging due to the inherently stochastic nature of erosion and sedimentation. Landslides exemplify this stochasticity while dominating sediment production in steep mountain landscapes. Here, we investigate landsliding and sediment-storage dynamics in the Southern Alps of New Zealand, where differences in tectonic advection and a strong orographic precipitation gradient create a pronounced west-to-east contrast in erosion rates that differ by approximately an order of magnitude across the Alpine drainage divide. These contrasts, largely driven by landslide activity, provide an exceptional opportunity for evaluating how distinct process domains regulate sediment production, storage, and evacuation.

We combined terrestrial cosmogenic radionuclide measurements (¹⁴C and ¹⁰Be) with single-grain K-feldspar post-infrared infrared stimulated luminescence (post-IR IRSL) signal analysis of modern fluvial sediments to constrain sediment sources and residence times. Our preliminary results indicate that while the western catchments exhibit ¹⁴C/¹⁰Be ratios of ~11 to 69, well above the nuclide production rate ratio of ~3, indicating landslide-dominated sediment supply, eastern catchments show lower ratios of 2 to 8, reflecting mixed sediment dynamics: landslide contributions in some catchments (with ratios >3) whereas significant sediment buffering in others (with ratios <3). Two-isotope plots (¹⁴C-¹⁰Be) further reveal minimal storage (negligible to a couple of hundred years of burial) for landslide-dominated western catchments, indicating rapid evacuation of eroded materials and complex exposure histories in eastern catchments record burial durations of ~2000 to 5000 years, demonstrating substantial sediment storage, reworking, and a correspondingly buffered erosional signal.

Unexpectedly, our post‑IR IRSL results do not reflect these contrasting process domains very clearly. Mean equivalent doses show substantial overlap between western (~45 to 177 Gy) and eastern (~40 to 128 Gy) catchments, with no systematic distinction between landslide-dominated and storage-dominated systems. We propose that high doses in western catchments reflect incomplete bleaching of landslide-evacuated sediment coupled with rapid transport, while elevated doses from eastern catchments result from competing processes such as partial or incomplete bleaching of episodic landslide-derived materials from steep slopes and progressive signal accumulation during prolonged storage. These observations thus warrant further systematic investigation of topographic controls, including landslide frequency-magnitude analysis to fully resolve luminescence behavior across erosional process domains.