Geochemical fluxes and weathering of volcanic terrains on high standing islands: Taranaki and Manawatu-Wanganui regions of New Zealand

Abstract

Sediment fluxes from high standing oceanic islands (HSIs) such as New Zealand are some of the highest known [Milliman J. D. and Syvitski J. P. M. (1992) Geomorphic/tectonic control of sediment discharge to the ocean: the importance of small mountainous rivers. J. Geol. 100, 525–544]. Recent geochemical work has suggested that along with their extremely high physical weathering yields, many New Zealand watersheds also have very high chemical weathering yields. In New Zealand, the magnitude of both the physical and chemical weathering yields is related to the lithology of the watershed. Most of the previous work on this topic has been undertaken in Southern Alps watersheds of schist and greywacke and in East Cape watersheds of semi-consolidated marine sediments and greywacke. We recently sampled North Island watersheds in the Taranaki and Manawatu-Wanganui regions which have been subjected to volcanism since the Miocene. We sampled watersheds that contain both volcanic and sedimentary rocks. A series of water and sediment samples was collected and analyzed for major, minor and trace elements. This was done to quantify the weathering intensities in the watersheds and to establish the relationship between physical and chemical weathering yields in volcanic lithologies. Our results reveal distinct chemical signatures for the different regions. Waters draining the Taranaki region volcanics are significantly enriched in K +, and depleted in Ca 2+ and Sr 2+ compared to waters draining the Manawatu-Wanganui region volcanics, which also traverse expanses of sedimentary siltstones and mudstones. The Ca 2+ and Sr 2+ depletions may reflect the relative absence of CaCO 3 in the Taranaki region watersheds. In addition, sediment samples from the Taranaki region show significant enrichment in Ti, Al, Ca, Fe, Mn, Mg, Ca, and P and depletion in Si and Rb compared to those of the Manawatu-Wanganui region. From total dissolved solids concentrations and mean annual water discharge, we calculate chemical weathering yields of 60–240 tons km −2 a −1. These weathering yields fall within the middle to upper range of those previously documented for the Southern Alps (93–480 tons km −2 a −1) and East Cape (62–400 tons km −2 a −1). Calculated silicate weathering yields of 12–33.6 tons km −2 a −1 and CO 2 consumption of 852–2390 × 10 3 mol km −2 a −1 for the rivers draining the Taranaki volcanic region are higher than those previously reported for watersheds hosted in sedimentary and metamorphosed rock terrains on HSIs. CO 2 consumption is found to be within the range previously measured for the basaltic terrains of the Deccan Traps (580–2450 × 10 3 mol km −2 a −1) and Réunion Island(1300–4400 × 10 3 mol km −2 a −1). Our calculated chemical weathering yields demonstrate the importance of HSIs, particularly those with volcanic terrains, when considering global geochemical fluxes.