CLIMATIC SIGNALS IN CHEMICAL WEATHERING AND SOIL PRODUCTION IN NEW ZEALAND’S TARARUA RANGE (Invited Presentation)

Weathering transforms rock into soil, liberates nutrients for biota, and influences global carbon cycles over geologic timescales. Understanding the interactions between climate, tectonics, erosion rates, biota, and weathering has been a recent focus of much research, but disentangling these complex relationships remains a challenge. Climate should influence chemical reaction rates, and previous workers have suggested a weathering “speed limit” based on reaction kinetics. Recent work from New Zealand’s Southern Alps clearly exceed this speed limit, but the relative importance of climate and tectonic uplift in driving rapid weathering remain unclear. Here we present a new dataset from the Tararua Range of New Zealand’s North Island, which experiences similar climate to the Southern Alps but are uplifting more slowly. Erosion and weathering rates are significantly slower in the Tararuas (60-320 and 48-160 t/km²/yr, respectively) than in the western Southern Alps (230-4330 and 80-2060 t/km²/yr), suggesting a strong tectonic control on hillslope weathering rates overall. The intensity of weathering in the Tararuas is roughly invariant across the landscape, suggesting that weathering keeps pace with uplift and erosion. However, spatial patterns in weathering in saprolite and soils differ markedly across an altitudinal transect. The balance between soil and saprolite weathering shifts dramatically across the landscape: at low (warm) elevations, saprolite weathering dominates, while at high (cold) elevations weathering occurs almost exclusively in soil. This pattern suggests that climate influences saprolite weathering, even where soil weathering keeps pace with erosion. This work highlights the importance of considering deep saprolite weathering independently from soils, and may explain why soil-focused empirical studies have not observed strong trends in chemical weathering across climate gradients.